libgo: update to Go1.10beta1

    
    Update the Go library to the 1.10beta1 release.
    
    Requires a few changes to the compiler for modifications to the map
    runtime code, and to handle some nowritebarrier cases in the runtime.
    
    Reviewed-on: https://go-review.googlesource.com/86455

gotools/:
	* Makefile.am (go_cmd_vet_files): New variable.
	(go_cmd_buildid_files, go_cmd_test2json_files): New variables.
	(s-zdefaultcc): Change from constants to functions.
	(noinst_PROGRAMS): Add vet, buildid, and test2json.
	(cgo$(EXEEXT)): Link against $(LIBGOTOOL).
	(vet$(EXEEXT)): New target.
	(buildid$(EXEEXT)): New target.
	(test2json$(EXEEXT)): New target.
	(install-exec-local): Install all $(noinst_PROGRAMS).
	(uninstall-local): Uninstasll all $(noinst_PROGRAMS).
	(check-go-tool): Depend on $(noinst_PROGRAMS).  Copy down
	objabi.go.
	(check-runtime): Depend on $(noinst_PROGRAMS).
	(check-cgo-test, check-carchive-test): Likewise.
	(check-vet): New target.
	(check): Depend on check-vet.  Look at cmd_vet-testlog.
	(.PHONY): Add check-vet.
	* Makefile.in: Rebuild.

From-SVN: r256365

31 files changed, 4262 insertions, 1468 deletions

diff --git a/libgo/go/archive/tar/common.go b/libgo/go/archive/tar/common.go
index d49c5c3..4a2c173 100644
--- a/libgo/go/archive/tar/common.go
+++ b/libgo/go/archive/tar/common.go
@@ -3,20 +3,22 @@
 // license that can be found in the LICENSE file.
 
 // Package tar implements access to tar archives.
-// It aims to cover most of the variations, including those produced
-// by GNU and BSD tars.
 //
-// References:
-//   http://www.freebsd.org/cgi/man.cgi?query=tar&sektion=5
-//   http://www.gnu.org/software/tar/manual/html_node/Standard.html
-//   http://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html
+// Tape archives (tar) are a file format for storing a sequence of files that
+// can be read and written in a streaming manner.
+// This package aims to cover most variations of the format,
+// including those produced by GNU and BSD tar tools.
 package tar
 
 import (
 	"errors"
 	"fmt"
+	"math"
 	"os"
 	"path"
+	"reflect"
+	"strconv"
+	"strings"
 	"time"
 )
 
@@ -24,42 +26,500 @@ import (
 // architectures. If a large value is encountered when decoding, the result
 // stored in Header will be the truncated version.
 
-// Header type flags.
+var (
+	ErrHeader          = errors.New("archive/tar: invalid tar header")
+	ErrWriteTooLong    = errors.New("archive/tar: write too long")
+	ErrFieldTooLong    = errors.New("archive/tar: header field too long")
+	ErrWriteAfterClose = errors.New("archive/tar: write after close")
+	errMissData        = errors.New("archive/tar: sparse file references non-existent data")
+	errUnrefData       = errors.New("archive/tar: sparse file contains unreferenced data")
+	errWriteHole       = errors.New("archive/tar: write non-NUL byte in sparse hole")
+)
+
+type headerError []string
+
+func (he headerError) Error() string {
+	const prefix = "archive/tar: cannot encode header"
+	var ss []string
+	for _, s := range he {
+		if s != "" {
+			ss = append(ss, s)
+		}
+	}
+	if len(ss) == 0 {
+		return prefix
+	}
+	return fmt.Sprintf("%s: %v", prefix, strings.Join(ss, "; and "))
+}
+
+// Type flags for Header.Typeflag.
+const (
+	// Type '0' indicates a regular file.
+	TypeReg  = '0'
+	TypeRegA = '\x00' // For legacy support; use TypeReg instead
+
+	// Type '1' to '6' are header-only flags and may not have a data body.
+	TypeLink    = '1' // Hard link
+	TypeSymlink = '2' // Symbolic link
+	TypeChar    = '3' // Character device node
+	TypeBlock   = '4' // Block device node
+	TypeDir     = '5' // Directory
+	TypeFifo    = '6' // FIFO node
+
+	// Type '7' is reserved.
+	TypeCont = '7'
+
+	// Type 'x' is used by the PAX format to store key-value records that
+	// are only relevant to the next file.
+	// This package transparently handles these types.
+	TypeXHeader = 'x'
+
+	// Type 'g' is used by the PAX format to store key-value records that
+	// are relevant to all subsequent files.
+	// This package only supports parsing and composing such headers,
+	// but does not currently support persisting the global state across files.
+	TypeXGlobalHeader = 'g'
+
+	// Type 'S' indicates a sparse file in the GNU format.
+	TypeGNUSparse = 'S'
+
+	// Types 'L' and 'K' are used by the GNU format for a meta file
+	// used to store the path or link name for the next file.
+	// This package transparently handles these types.
+	TypeGNULongName = 'L'
+	TypeGNULongLink = 'K'
+)
+
+// Keywords for PAX extended header records.
 const (
-	TypeReg           = '0'    // regular file
-	TypeRegA          = '\x00' // regular file
-	TypeLink          = '1'    // hard link
-	TypeSymlink       = '2'    // symbolic link
-	TypeChar          = '3'    // character device node
-	TypeBlock         = '4'    // block device node
-	TypeDir           = '5'    // directory
-	TypeFifo          = '6'    // fifo node
-	TypeCont          = '7'    // reserved
-	TypeXHeader       = 'x'    // extended header
-	TypeXGlobalHeader = 'g'    // global extended header
-	TypeGNULongName   = 'L'    // Next file has a long name
-	TypeGNULongLink   = 'K'    // Next file symlinks to a file w/ a long name
-	TypeGNUSparse     = 'S'    // sparse file
+	paxNone     = "" // Indicates that no PAX key is suitable
+	paxPath     = "path"
+	paxLinkpath = "linkpath"
+	paxSize     = "size"
+	paxUid      = "uid"
+	paxGid      = "gid"
+	paxUname    = "uname"
+	paxGname    = "gname"
+	paxMtime    = "mtime"
+	paxAtime    = "atime"
+	paxCtime    = "ctime"   // Removed from later revision of PAX spec, but was valid
+	paxCharset  = "charset" // Currently unused
+	paxComment  = "comment" // Currently unused
+
+	paxSchilyXattr = "SCHILY.xattr."
+
+	// Keywords for GNU sparse files in a PAX extended header.
+	paxGNUSparse          = "GNU.sparse."
+	paxGNUSparseNumBlocks = "GNU.sparse.numblocks"
+	paxGNUSparseOffset    = "GNU.sparse.offset"
+	paxGNUSparseNumBytes  = "GNU.sparse.numbytes"
+	paxGNUSparseMap       = "GNU.sparse.map"
+	paxGNUSparseName      = "GNU.sparse.name"
+	paxGNUSparseMajor     = "GNU.sparse.major"
+	paxGNUSparseMinor     = "GNU.sparse.minor"
+	paxGNUSparseSize      = "GNU.sparse.size"
+	paxGNUSparseRealSize  = "GNU.sparse.realsize"
 )
 
+// basicKeys is a set of the PAX keys for which we have built-in support.
+// This does not contain "charset" or "comment", which are both PAX-specific,
+// so adding them as first-class features of Header is unlikely.
+// Users can use the PAXRecords field to set it themselves.
+var basicKeys = map[string]bool{
+	paxPath: true, paxLinkpath: true, paxSize: true, paxUid: true, paxGid: true,
+	paxUname: true, paxGname: true, paxMtime: true, paxAtime: true, paxCtime: true,
+}
+
 // A Header represents a single header in a tar archive.
 // Some fields may not be populated.
+//
+// For forward compatibility, users that retrieve a Header from Reader.Next,
+// mutate it in some ways, and then pass it back to Writer.WriteHeader
+// should do so by creating a new Header and copying the fields
+// that they are interested in preserving.
 type Header struct {
-	Name       string    // name of header file entry
-	Mode       int64     // permission and mode bits
-	Uid        int       // user id of owner
-	Gid        int       // group id of owner
-	Size       int64     // length in bytes
-	ModTime    time.Time // modified time
-	Typeflag   byte      // type of header entry
-	Linkname   string    // target name of link
-	Uname      string    // user name of owner
-	Gname      string    // group name of owner
-	Devmajor   int64     // major number of character or block device
-	Devminor   int64     // minor number of character or block device
-	AccessTime time.Time // access time
-	ChangeTime time.Time // status change time
-	Xattrs     map[string]string
+	Typeflag byte // Type of header entry (should be TypeReg for most files)
+
+	Name     string // Name of file entry
+	Linkname string // Target name of link (valid for TypeLink or TypeSymlink)
+
+	Size  int64  // Logical file size in bytes
+	Mode  int64  // Permission and mode bits
+	Uid   int    // User ID of owner
+	Gid   int    // Group ID of owner
+	Uname string // User name of owner
+	Gname string // Group name of owner
+
+	// If the Format is unspecified, then Writer.WriteHeader rounds ModTime
+	// to the nearest second and ignores the AccessTime and ChangeTime fields.
+	//
+	// To use AccessTime or ChangeTime, specify the Format as PAX or GNU.
+	// To use sub-second resolution, specify the Format as PAX.
+	ModTime    time.Time // Modification time
+	AccessTime time.Time // Access time (requires either PAX or GNU support)
+	ChangeTime time.Time // Change time (requires either PAX or GNU support)
+
+	Devmajor int64 // Major device number (valid for TypeChar or TypeBlock)
+	Devminor int64 // Minor device number (valid for TypeChar or TypeBlock)
+
+	// Xattrs stores extended attributes as PAX records under the
+	// "SCHILY.xattr." namespace.
+	//
+	// The following are semantically equivalent:
+	//  h.Xattrs[key] = value
+	//  h.PAXRecords["SCHILY.xattr."+key] = value
+	//
+	// When Writer.WriteHeader is called, the contents of Xattrs will take
+	// precedence over those in PAXRecords.
+	//
+	// Deprecated: Use PAXRecords instead.
+	Xattrs map[string]string
+
+	// PAXRecords is a map of PAX extended header records.
+	//
+	// User-defined records should have keys of the following form:
+	//	VENDOR.keyword
+	// Where VENDOR is some namespace in all uppercase, and keyword may
+	// not contain the '=' character (e.g., "GOLANG.pkg.version").
+	// The key and value should be non-empty UTF-8 strings.
+	//
+	// When Writer.WriteHeader is called, PAX records derived from the
+	// the other fields in Header take precedence over PAXRecords.
+	PAXRecords map[string]string
+
+	// Format specifies the format of the tar header.
+	//
+	// This is set by Reader.Next as a best-effort guess at the format.
+	// Since the Reader liberally reads some non-compliant files,
+	// it is possible for this to be FormatUnknown.
+	//
+	// If the format is unspecified when Writer.WriteHeader is called,
+	// then it uses the first format (in the order of USTAR, PAX, GNU)
+	// capable of encoding this Header (see Format).
+	Format Format
+}
+
+// sparseEntry represents a Length-sized fragment at Offset in the file.
+type sparseEntry struct{ Offset, Length int64 }
+
+func (s sparseEntry) endOffset() int64 { return s.Offset + s.Length }
+
+// A sparse file can be represented as either a sparseDatas or a sparseHoles.
+// As long as the total size is known, they are equivalent and one can be
+// converted to the other form and back. The various tar formats with sparse
+// file support represent sparse files in the sparseDatas form. That is, they
+// specify the fragments in the file that has data, and treat everything else as
+// having zero bytes. As such, the encoding and decoding logic in this package
+// deals with sparseDatas.
+//
+// However, the external API uses sparseHoles instead of sparseDatas because the
+// zero value of sparseHoles logically represents a normal file (i.e., there are
+// no holes in it). On the other hand, the zero value of sparseDatas implies
+// that the file has no data in it, which is rather odd.
+//
+// As an example, if the underlying raw file contains the 10-byte data:
+//	var compactFile = "abcdefgh"
+//
+// And the sparse map has the following entries:
+//	var spd sparseDatas = []sparseEntry{
+//		{Offset: 2,  Length: 5},  // Data fragment for 2..6
+//		{Offset: 18, Length: 3},  // Data fragment for 18..20
+//	}
+//	var sph sparseHoles = []sparseEntry{
+//		{Offset: 0,  Length: 2},  // Hole fragment for 0..1
+//		{Offset: 7,  Length: 11}, // Hole fragment for 7..17
+//		{Offset: 21, Length: 4},  // Hole fragment for 21..24
+//	}
+//
+// Then the content of the resulting sparse file with a Header.Size of 25 is:
+//	var sparseFile = "\x00"*2 + "abcde" + "\x00"*11 + "fgh" + "\x00"*4
+type (
+	sparseDatas []sparseEntry
+	sparseHoles []sparseEntry
+)
+
+// validateSparseEntries reports whether sp is a valid sparse map.
+// It does not matter whether sp represents data fragments or hole fragments.
+func validateSparseEntries(sp []sparseEntry, size int64) bool {
+	// Validate all sparse entries. These are the same checks as performed by
+	// the BSD tar utility.
+	if size < 0 {
+		return false
+	}
+	var pre sparseEntry
+	for _, cur := range sp {
+		switch {
+		case cur.Offset < 0 || cur.Length < 0:
+			return false // Negative values are never okay
+		case cur.Offset > math.MaxInt64-cur.Length:
+			return false // Integer overflow with large length
+		case cur.endOffset() > size:
+			return false // Region extends beyond the actual size
+		case pre.endOffset() > cur.Offset:
+			return false // Regions cannot overlap and must be in order
+		}
+		pre = cur
+	}
+	return true
+}
+
+// alignSparseEntries mutates src and returns dst where each fragment's
+// starting offset is aligned up to the nearest block edge, and each
+// ending offset is aligned down to the nearest block edge.
+//
+// Even though the Go tar Reader and the BSD tar utility can handle entries
+// with arbitrary offsets and lengths, the GNU tar utility can only handle
+// offsets and lengths that are multiples of blockSize.
+func alignSparseEntries(src []sparseEntry, size int64) []sparseEntry {
+	dst := src[:0]
+	for _, s := range src {
+		pos, end := s.Offset, s.endOffset()
+		pos += blockPadding(+pos) // Round-up to nearest blockSize
+		if end != size {
+			end -= blockPadding(-end) // Round-down to nearest blockSize
+		}
+		if pos < end {
+			dst = append(dst, sparseEntry{Offset: pos, Length: end - pos})
+		}
+	}
+	return dst
+}
+
+// invertSparseEntries converts a sparse map from one form to the other.
+// If the input is sparseHoles, then it will output sparseDatas and vice-versa.
+// The input must have been already validated.
+//
+// This function mutates src and returns a normalized map where:
+//	* adjacent fragments are coalesced together
+//	* only the last fragment may be empty
+//	* the endOffset of the last fragment is the total size
+func invertSparseEntries(src []sparseEntry, size int64) []sparseEntry {
+	dst := src[:0]
+	var pre sparseEntry
+	for _, cur := range src {
+		if cur.Length == 0 {
+			continue // Skip empty fragments
+		}
+		pre.Length = cur.Offset - pre.Offset
+		if pre.Length > 0 {
+			dst = append(dst, pre) // Only add non-empty fragments
+		}
+		pre.Offset = cur.endOffset()
+	}
+	pre.Length = size - pre.Offset // Possibly the only empty fragment
+	return append(dst, pre)
+}
+
+// fileState tracks the number of logical (includes sparse holes) and physical
+// (actual in tar archive) bytes remaining for the current file.
+//
+// Invariant: LogicalRemaining >= PhysicalRemaining
+type fileState interface {
+	LogicalRemaining() int64
+	PhysicalRemaining() int64
+}
+
+// allowedFormats determines which formats can be used.
+// The value returned is the logical OR of multiple possible formats.
+// If the value is FormatUnknown, then the input Header cannot be encoded
+// and an error is returned explaining why.
+//
+// As a by-product of checking the fields, this function returns paxHdrs, which
+// contain all fields that could not be directly encoded.
+// A value receiver ensures that this method does not mutate the source Header.
+func (h Header) allowedFormats() (format Format, paxHdrs map[string]string, err error) {
+	format = FormatUSTAR | FormatPAX | FormatGNU
+	paxHdrs = make(map[string]string)
+
+	var whyNoUSTAR, whyNoPAX, whyNoGNU string
+	var preferPAX bool // Prefer PAX over USTAR
+	verifyString := func(s string, size int, name, paxKey string) {
+		// NUL-terminator is optional for path and linkpath.
+		// Technically, it is required for uname and gname,
+		// but neither GNU nor BSD tar checks for it.
+		tooLong := len(s) > size
+		allowLongGNU := paxKey == paxPath || paxKey == paxLinkpath
+		if hasNUL(s) || (tooLong && !allowLongGNU) {
+			whyNoGNU = fmt.Sprintf("GNU cannot encode %s=%q", name, s)
+			format.mustNotBe(FormatGNU)
+		}
+		if !isASCII(s) || tooLong {
+			canSplitUSTAR := paxKey == paxPath
+			if _, _, ok := splitUSTARPath(s); !canSplitUSTAR || !ok {
+				whyNoUSTAR = fmt.Sprintf("USTAR cannot encode %s=%q", name, s)
+				format.mustNotBe(FormatUSTAR)
+			}
+			if paxKey == paxNone {
+				whyNoPAX = fmt.Sprintf("PAX cannot encode %s=%q", name, s)
+				format.mustNotBe(FormatPAX)
+			} else {
+				paxHdrs[paxKey] = s
+			}
+		}
+		if v, ok := h.PAXRecords[paxKey]; ok && v == s {
+			paxHdrs[paxKey] = v
+		}
+	}
+	verifyNumeric := func(n int64, size int, name, paxKey string) {
+		if !fitsInBase256(size, n) {
+			whyNoGNU = fmt.Sprintf("GNU cannot encode %s=%d", name, n)
+			format.mustNotBe(FormatGNU)
+		}
+		if !fitsInOctal(size, n) {
+			whyNoUSTAR = fmt.Sprintf("USTAR cannot encode %s=%d", name, n)
+			format.mustNotBe(FormatUSTAR)
+			if paxKey == paxNone {
+				whyNoPAX = fmt.Sprintf("PAX cannot encode %s=%d", name, n)
+				format.mustNotBe(FormatPAX)
+			} else {
+				paxHdrs[paxKey] = strconv.FormatInt(n, 10)
+			}
+		}
+		if v, ok := h.PAXRecords[paxKey]; ok && v == strconv.FormatInt(n, 10) {
+			paxHdrs[paxKey] = v
+		}
+	}
+	verifyTime := func(ts time.Time, size int, name, paxKey string) {
+		if ts.IsZero() {
+			return // Always okay
+		}
+		if !fitsInBase256(size, ts.Unix()) {
+			whyNoGNU = fmt.Sprintf("GNU cannot encode %s=%v", name, ts)
+			format.mustNotBe(FormatGNU)
+		}
+		isMtime := paxKey == paxMtime
+		fitsOctal := fitsInOctal(size, ts.Unix())
+		if (isMtime && !fitsOctal) || !isMtime {
+			whyNoUSTAR = fmt.Sprintf("USTAR cannot encode %s=%v", name, ts)
+			format.mustNotBe(FormatUSTAR)
+		}
+		needsNano := ts.Nanosecond() != 0
+		if !isMtime || !fitsOctal || needsNano {
+			preferPAX = true // USTAR may truncate sub-second measurements
+			if paxKey == paxNone {
+				whyNoPAX = fmt.Sprintf("PAX cannot encode %s=%v", name, ts)
+				format.mustNotBe(FormatPAX)
+			} else {
+				paxHdrs[paxKey] = formatPAXTime(ts)
+			}
+		}
+		if v, ok := h.PAXRecords[paxKey]; ok && v == formatPAXTime(ts) {
+			paxHdrs[paxKey] = v
+		}
+	}
+
+	// Check basic fields.
+	var blk block
+	v7 := blk.V7()
+	ustar := blk.USTAR()
+	gnu := blk.GNU()
+	verifyString(h.Name, len(v7.Name()), "Name", paxPath)
+	verifyString(h.Linkname, len(v7.LinkName()), "Linkname", paxLinkpath)
+	verifyString(h.Uname, len(ustar.UserName()), "Uname", paxUname)
+	verifyString(h.Gname, len(ustar.GroupName()), "Gname", paxGname)
+	verifyNumeric(h.Mode, len(v7.Mode()), "Mode", paxNone)
+	verifyNumeric(int64(h.Uid), len(v7.UID()), "Uid", paxUid)
+	verifyNumeric(int64(h.Gid), len(v7.GID()), "Gid", paxGid)
+	verifyNumeric(h.Size, len(v7.Size()), "Size", paxSize)
+	verifyNumeric(h.Devmajor, len(ustar.DevMajor()), "Devmajor", paxNone)
+	verifyNumeric(h.Devminor, len(ustar.DevMinor()), "Devminor", paxNone)
+	verifyTime(h.ModTime, len(v7.ModTime()), "ModTime", paxMtime)
+	verifyTime(h.AccessTime, len(gnu.AccessTime()), "AccessTime", paxAtime)
+	verifyTime(h.ChangeTime, len(gnu.ChangeTime()), "ChangeTime", paxCtime)
+
+	// Check for header-only types.
+	var whyOnlyPAX, whyOnlyGNU string
+	switch h.Typeflag {
+	case TypeReg, TypeChar, TypeBlock, TypeFifo, TypeGNUSparse:
+		// Exclude TypeLink and TypeSymlink, since they may reference directories.
+		if strings.HasSuffix(h.Name, "/") {
+			return FormatUnknown, nil, headerError{"filename may not have trailing slash"}
+		}
+	case TypeXHeader, TypeGNULongName, TypeGNULongLink:
+		return FormatUnknown, nil, headerError{"cannot manually encode TypeXHeader, TypeGNULongName, or TypeGNULongLink headers"}
+	case TypeXGlobalHeader:
+		h2 := Header{Name: h.Name, Typeflag: h.Typeflag, Xattrs: h.Xattrs, PAXRecords: h.PAXRecords, Format: h.Format}
+		if !reflect.DeepEqual(h, h2) {
+			return FormatUnknown, nil, headerError{"only PAXRecords should be set for TypeXGlobalHeader"}
+		}
+		whyOnlyPAX = "only PAX supports TypeXGlobalHeader"
+		format.mayOnlyBe(FormatPAX)
+	}
+	if !isHeaderOnlyType(h.Typeflag) && h.Size < 0 {
+		return FormatUnknown, nil, headerError{"negative size on header-only type"}
+	}
+
+	// Check PAX records.
+	if len(h.Xattrs) > 0 {
+		for k, v := range h.Xattrs {
+			paxHdrs[paxSchilyXattr+k] = v
+		}
+		whyOnlyPAX = "only PAX supports Xattrs"
+		format.mayOnlyBe(FormatPAX)
+	}
+	if len(h.PAXRecords) > 0 {
+		for k, v := range h.PAXRecords {
+			switch _, exists := paxHdrs[k]; {
+			case exists:
+				continue // Do not overwrite existing records
+			case h.Typeflag == TypeXGlobalHeader:
+				paxHdrs[k] = v // Copy all records
+			case !basicKeys[k] && !strings.HasPrefix(k, paxGNUSparse):
+				paxHdrs[k] = v // Ignore local records that may conflict
+			}
+		}
+		whyOnlyPAX = "only PAX supports PAXRecords"
+		format.mayOnlyBe(FormatPAX)
+	}
+	for k, v := range paxHdrs {
+		if !validPAXRecord(k, v) {
+			return FormatUnknown, nil, headerError{fmt.Sprintf("invalid PAX record: %q", k+" = "+v)}
+		}
+	}
+
+	// TODO(dsnet): Re-enable this when adding sparse support.
+	// See https://golang.org/issue/22735
+	/*
+		// Check sparse files.
+		if len(h.SparseHoles) > 0 || h.Typeflag == TypeGNUSparse {
+			if isHeaderOnlyType(h.Typeflag) {
+				return FormatUnknown, nil, headerError{"header-only type cannot be sparse"}
+			}
+			if !validateSparseEntries(h.SparseHoles, h.Size) {
+				return FormatUnknown, nil, headerError{"invalid sparse holes"}
+			}
+			if h.Typeflag == TypeGNUSparse {
+				whyOnlyGNU = "only GNU supports TypeGNUSparse"
+				format.mayOnlyBe(FormatGNU)
+			} else {
+				whyNoGNU = "GNU supports sparse files only with TypeGNUSparse"
+				format.mustNotBe(FormatGNU)
+			}
+			whyNoUSTAR = "USTAR does not support sparse files"
+			format.mustNotBe(FormatUSTAR)
+		}
+	*/
+
+	// Check desired format.
+	if wantFormat := h.Format; wantFormat != FormatUnknown {
+		if wantFormat.has(FormatPAX) && !preferPAX {
+			wantFormat.mayBe(FormatUSTAR) // PAX implies USTAR allowed too
+		}
+		format.mayOnlyBe(wantFormat) // Set union of formats allowed and format wanted
+	}
+	if format == FormatUnknown {
+		switch h.Format {
+		case FormatUSTAR:
+			err = headerError{"Format specifies USTAR", whyNoUSTAR, whyOnlyPAX, whyOnlyGNU}
+		case FormatPAX:
+			err = headerError{"Format specifies PAX", whyNoPAX, whyOnlyGNU}
+		case FormatGNU:
+			err = headerError{"Format specifies GNU", whyNoGNU, whyOnlyPAX}
+		default:
+			err = headerError{whyNoUSTAR, whyNoPAX, whyNoGNU, whyOnlyPAX, whyOnlyGNU}
+		}
+	}
+	return format, paxHdrs, err
 }
 
 // FileInfo returns an os.FileInfo for the Header.
@@ -92,63 +552,43 @@ func (fi headerFileInfo) Mode() (mode os.FileMode) {
 
 	// Set setuid, setgid and sticky bits.
 	if fi.h.Mode&c_ISUID != 0 {
-		// setuid
 		mode |= os.ModeSetuid
 	}
 	if fi.h.Mode&c_ISGID != 0 {
-		// setgid
 		mode |= os.ModeSetgid
 	}
 	if fi.h.Mode&c_ISVTX != 0 {
-		// sticky
 		mode |= os.ModeSticky
 	}
 
-	// Set file mode bits.
-	// clear perm, setuid, setgid and sticky bits.
-	m := os.FileMode(fi.h.Mode) &^ 07777
-	if m == c_ISDIR {
-		// directory
+	// Set file mode bits; clear perm, setuid, setgid, and sticky bits.
+	switch m := os.FileMode(fi.h.Mode) &^ 07777; m {
+	case c_ISDIR:
 		mode |= os.ModeDir
-	}
-	if m == c_ISFIFO {
-		// named pipe (FIFO)
+	case c_ISFIFO:
 		mode |= os.ModeNamedPipe
-	}
-	if m == c_ISLNK {
-		// symbolic link
+	case c_ISLNK:
 		mode |= os.ModeSymlink
-	}
-	if m == c_ISBLK {
-		// device file
+	case c_ISBLK:
 		mode |= os.ModeDevice
-	}
-	if m == c_ISCHR {
-		// Unix character device
+	case c_ISCHR:
 		mode |= os.ModeDevice
 		mode |= os.ModeCharDevice
-	}
-	if m == c_ISSOCK {
-		// Unix domain socket
+	case c_ISSOCK:
 		mode |= os.ModeSocket
 	}
 
 	switch fi.h.Typeflag {
 	case TypeSymlink:
-		// symbolic link
 		mode |= os.ModeSymlink
 	case TypeChar:
-		// character device node
 		mode |= os.ModeDevice
 		mode |= os.ModeCharDevice
 	case TypeBlock:
-		// block device node
 		mode |= os.ModeDevice
 	case TypeDir:
-		// directory
 		mode |= os.ModeDir
 	case TypeFifo:
-		// fifo node
 		mode |= os.ModeNamedPipe
 	}
 
@@ -176,33 +616,16 @@ const (
 	c_ISSOCK = 0140000 // Socket
 )
 
-// Keywords for the PAX Extended Header
-const (
-	paxAtime    = "atime"
-	paxCharset  = "charset"
-	paxComment  = "comment"
-	paxCtime    = "ctime" // please note that ctime is not a valid pax header.
-	paxGid      = "gid"
-	paxGname    = "gname"
-	paxLinkpath = "linkpath"
-	paxMtime    = "mtime"
-	paxPath     = "path"
-	paxSize     = "size"
-	paxUid      = "uid"
-	paxUname    = "uname"
-	paxXattr    = "SCHILY.xattr."
-	paxNone     = ""
-)
-
 // FileInfoHeader creates a partially-populated Header from fi.
 // If fi describes a symlink, FileInfoHeader records link as the link target.
 // If fi describes a directory, a slash is appended to the name.
-// Because os.FileInfo's Name method returns only the base name of
-// the file it describes, it may be necessary to modify the Name field
-// of the returned header to provide the full path name of the file.
+//
+// Since os.FileInfo's Name method only returns the base name of
+// the file it describes, it may be necessary to modify Header.Name
+// to provide the full path name of the file.
 func FileInfoHeader(fi os.FileInfo, link string) (*Header, error) {
 	if fi == nil {
-		return nil, errors.New("tar: FileInfo is nil")
+		return nil, errors.New("archive/tar: FileInfo is nil")
 	}
 	fm := fi.Mode()
 	h := &Header{
@@ -265,6 +688,12 @@ func FileInfoHeader(fi os.FileInfo, link string) (*Header, error) {
 			h.Size = 0
 			h.Linkname = sys.Linkname
 		}
+		if sys.PAXRecords != nil {
+			h.PAXRecords = make(map[string]string)
+			for k, v := range sys.PAXRecords {
+				h.PAXRecords[k] = v
+			}
+		}
 	}
 	if sysStat != nil {
 		return h, sysStat(fi, h)
@@ -282,3 +711,10 @@ func isHeaderOnlyType(flag byte) bool {
 		return false
 	}
 }
+
+func min(a, b int64) int64 {
+	if a < b {
+		return a
+	}
+	return b
+}
diff --git a/libgo/go/archive/tar/format.go b/libgo/go/archive/tar/format.go
index c2c9910..6e29698 100644
--- a/libgo/go/archive/tar/format.go
+++ b/libgo/go/archive/tar/format.go
@@ -4,38 +4,133 @@
 
 package tar
 
+import "strings"
+
+// Format represents the tar archive format.
+//
+// The original tar format was introduced in Unix V7.
+// Since then, there have been multiple competing formats attempting to
+// standardize or extend the V7 format to overcome its limitations.
+// The most common formats are the USTAR, PAX, and GNU formats,
+// each with their own advantages and limitations.
+//
+// The following table captures the capabilities of each format:
+//
+//	                  |  USTAR |       PAX |       GNU
+//	------------------+--------+-----------+----------
+//	Name              |   256B | unlimited | unlimited
+//	Linkname          |   100B | unlimited | unlimited
+//	Size              | uint33 | unlimited |    uint89
+//	Mode              | uint21 |    uint21 |    uint57
+//	Uid/Gid           | uint21 | unlimited |    uint57
+//	Uname/Gname       |    32B | unlimited |       32B
+//	ModTime           | uint33 | unlimited |     int89
+//	AccessTime        |    n/a | unlimited |     int89
+//	ChangeTime        |    n/a | unlimited |     int89
+//	Devmajor/Devminor | uint21 |    uint21 |    uint57
+//	------------------+--------+-----------+----------
+//	string encoding   |  ASCII |     UTF-8 |    binary
+//	sub-second times  |     no |       yes |        no
+//	sparse files      |     no |       yes |       yes
+//
+// The table's upper portion shows the Header fields, where each format reports
+// the maximum number of bytes allowed for each string field and
+// the integer type used to store each numeric field
+// (where timestamps are stored as the number of seconds since the Unix epoch).
+//
+// The table's lower portion shows specialized features of each format,
+// such as supported string encodings, support for sub-second timestamps,
+// or support for sparse files.
+//
+// The Writer currently provides no support for sparse files.
+type Format int
+
 // Constants to identify various tar formats.
 const (
-	// The format is unknown.
-	formatUnknown = (1 << iota) / 2 // Sequence of 0, 1, 2, 4, 8, etc...
+	// Deliberately hide the meaning of constants from public API.
+	_ Format = (1 << iota) / 4 // Sequence of 0, 0, 1, 2, 4, 8, etc...
+
+	// FormatUnknown indicates that the format is unknown.
+	FormatUnknown
 
 	// The format of the original Unix V7 tar tool prior to standardization.
 	formatV7
 
-	// The old and new GNU formats, which are incompatible with USTAR.
-	// This does cover the old GNU sparse extension.
-	// This does not cover the GNU sparse extensions using PAX headers,
-	// versions 0.0, 0.1, and 1.0; these fall under the PAX format.
-	formatGNU
+	// FormatUSTAR represents the USTAR header format defined in POSIX.1-1988.
+	//
+	// While this format is compatible with most tar readers,
+	// the format has several limitations making it unsuitable for some usages.
+	// Most notably, it cannot support sparse files, files larger than 8GiB,
+	// filenames larger than 256 characters, and non-ASCII filenames.
+	//
+	// Reference:
+	//	http://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06
+	FormatUSTAR
+
+	// FormatPAX represents the PAX header format defined in POSIX.1-2001.
+	//
+	// PAX extends USTAR by writing a special file with Typeflag TypeXHeader
+	// preceding the original header. This file contains a set of key-value
+	// records, which are used to overcome USTAR's shortcomings, in addition to
+	// providing the ability to have sub-second resolution for timestamps.
+	//
+	// Some newer formats add their own extensions to PAX by defining their
+	// own keys and assigning certain semantic meaning to the associated values.
+	// For example, sparse file support in PAX is implemented using keys
+	// defined by the GNU manual (e.g., "GNU.sparse.map").
+	//
+	// Reference:
+	//	http://pubs.opengroup.org/onlinepubs/009695399/utilities/pax.html
+	FormatPAX
+
+	// FormatGNU represents the GNU header format.
+	//
+	// The GNU header format is older than the USTAR and PAX standards and
+	// is not compatible with them. The GNU format supports
+	// arbitrary file sizes, filenames of arbitrary encoding and length,
+	// sparse files, and other features.
+	//
+	// It is recommended that PAX be chosen over GNU unless the target
+	// application can only parse GNU formatted archives.
+	//
+	// Reference:
+	//	http://www.gnu.org/software/tar/manual/html_node/Standard.html
+	FormatGNU
 
 	// Schily's tar format, which is incompatible with USTAR.
 	// This does not cover STAR extensions to the PAX format; these fall under
 	// the PAX format.
 	formatSTAR
 
-	// USTAR is the former standardization of tar defined in POSIX.1-1988.
-	// This is incompatible with the GNU and STAR formats.
-	formatUSTAR
-
-	// PAX is the latest standardization of tar defined in POSIX.1-2001.
-	// This is an extension of USTAR and is "backwards compatible" with it.
-	//
-	// Some newer formats add their own extensions to PAX, such as GNU sparse
-	// files and SCHILY extended attributes. Since they are backwards compatible
-	// with PAX, they will be labelled as "PAX".
-	formatPAX
+	formatMax
 )
 
+func (f Format) has(f2 Format) bool   { return f&f2 != 0 }
+func (f *Format) mayBe(f2 Format)     { *f |= f2 }
+func (f *Format) mayOnlyBe(f2 Format) { *f &= f2 }
+func (f *Format) mustNotBe(f2 Format) { *f &^= f2 }
+
+var formatNames = map[Format]string{
+	formatV7: "V7", FormatUSTAR: "USTAR", FormatPAX: "PAX", FormatGNU: "GNU", formatSTAR: "STAR",
+}
+
+func (f Format) String() string {
+	var ss []string
+	for f2 := Format(1); f2 < formatMax; f2 <<= 1 {
+		if f.has(f2) {
+			ss = append(ss, formatNames[f2])
+		}
+	}
+	switch len(ss) {
+	case 0:
+		return "<unknown>"
+	case 1:
+		return ss[0]
+	default:
+		return "(" + strings.Join(ss, " | ") + ")"
+	}
+}
+
 // Magics used to identify various formats.
 const (
 	magicGNU, versionGNU     = "ustar ", " \x00"
@@ -50,6 +145,12 @@ const (
 	prefixSize = 155 // Max length of the prefix field in USTAR format
 )
 
+// blockPadding computes the number of bytes needed to pad offset up to the
+// nearest block edge where 0 <= n < blockSize.
+func blockPadding(offset int64) (n int64) {
+	return -offset & (blockSize - 1)
+}
+
 var zeroBlock block
 
 type block [blockSize]byte
@@ -63,14 +164,14 @@ func (b *block) Sparse() sparseArray { return (sparseArray)(b[:]) }
 
 // GetFormat checks that the block is a valid tar header based on the checksum.
 // It then attempts to guess the specific format based on magic values.
-// If the checksum fails, then formatUnknown is returned.
-func (b *block) GetFormat() (format int) {
+// If the checksum fails, then FormatUnknown is returned.
+func (b *block) GetFormat() Format {
 	// Verify checksum.
 	var p parser
 	value := p.parseOctal(b.V7().Chksum())
 	chksum1, chksum2 := b.ComputeChecksum()
 	if p.err != nil || (value != chksum1 && value != chksum2) {
-		return formatUnknown
+		return FormatUnknown
 	}
 
 	// Guess the magic values.
@@ -81,9 +182,9 @@ func (b *block) GetFormat() (format int) {
 	case magic == magicUSTAR && trailer == trailerSTAR:
 		return formatSTAR
 	case magic == magicUSTAR:
-		return formatUSTAR
+		return FormatUSTAR | FormatPAX
 	case magic == magicGNU && version == versionGNU:
-		return formatGNU
+		return FormatGNU
 	default:
 		return formatV7
 	}
@@ -91,19 +192,19 @@ func (b *block) GetFormat() (format int) {
 
 // SetFormat writes the magic values necessary for specified format
 // and then updates the checksum accordingly.
-func (b *block) SetFormat(format int) {
+func (b *block) SetFormat(format Format) {
 	// Set the magic values.
-	switch format {
-	case formatV7:
+	switch {
+	case format.has(formatV7):
 		// Do nothing.
-	case formatGNU:
+	case format.has(FormatGNU):
 		copy(b.GNU().Magic(), magicGNU)
 		copy(b.GNU().Version(), versionGNU)
-	case formatSTAR:
+	case format.has(formatSTAR):
 		copy(b.STAR().Magic(), magicUSTAR)
 		copy(b.STAR().Version(), versionUSTAR)
 		copy(b.STAR().Trailer(), trailerSTAR)
-	case formatUSTAR, formatPAX:
+	case format.has(FormatUSTAR | FormatPAX):
 		copy(b.USTAR().Magic(), magicUSTAR)
 		copy(b.USTAR().Version(), versionUSTAR)
 	default:
@@ -128,12 +229,17 @@ func (b *block) ComputeChecksum() (unsigned, signed int64) {
 		if 148 <= i && i < 156 {
 			c = ' ' // Treat the checksum field itself as all spaces.
 		}
-		unsigned += int64(uint8(c))
+		unsigned += int64(c)
 		signed += int64(int8(c))
 	}
 	return unsigned, signed
 }
 
+// Reset clears the block with all zeros.
+func (b *block) Reset() {
+	*b = block{}
+}
+
 type headerV7 [blockSize]byte
 
 func (h *headerV7) Name() []byte     { return h[000:][:100] }
@@ -187,11 +293,11 @@ func (h *headerUSTAR) Prefix() []byte    { return h[345:][:155] }
 
 type sparseArray []byte
 
-func (s sparseArray) Entry(i int) sparseNode { return (sparseNode)(s[i*24:]) }
+func (s sparseArray) Entry(i int) sparseElem { return (sparseElem)(s[i*24:]) }
 func (s sparseArray) IsExtended() []byte     { return s[24*s.MaxEntries():][:1] }
 func (s sparseArray) MaxEntries() int        { return len(s) / 24 }
 
-type sparseNode []byte
+type sparseElem []byte
 
-func (s sparseNode) Offset() []byte   { return s[00:][:12] }
-func (s sparseNode) NumBytes() []byte { return s[12:][:12] }
+func (s sparseElem) Offset() []byte { return s[00:][:12] }
+func (s sparseElem) Length() []byte { return s[12:][:12] }
diff --git a/libgo/go/archive/tar/reader.go b/libgo/go/archive/tar/reader.go
index 9abe888..f4eeb55 100644
--- a/libgo/go/archive/tar/reader.go
+++ b/libgo/go/archive/tar/reader.go
@@ -4,33 +4,23 @@
 
 package tar
 
-// TODO(dsymonds):
-//   - pax extensions
-
 import (
 	"bytes"
-	"errors"
 	"io"
 	"io/ioutil"
-	"math"
 	"strconv"
 	"strings"
 	"time"
 )
 
-var (
-	ErrHeader = errors.New("archive/tar: invalid tar header")
-)
-
-// A Reader provides sequential access to the contents of a tar archive.
-// A tar archive consists of a sequence of files.
-// The Next method advances to the next file in the archive (including the first),
-// and then it can be treated as an io.Reader to access the file's data.
+// Reader provides sequential access to the contents of a tar archive.
+// Reader.Next advances to the next file in the archive (including the first),
+// and then Reader can be treated as an io.Reader to access the file's data.
 type Reader struct {
 	r    io.Reader
-	pad  int64          // amount of padding (ignored) after current file entry
-	curr numBytesReader // reader for current file entry
-	blk  block          // buffer to use as temporary local storage
+	pad  int64      // Amount of padding (ignored) after current file entry
+	curr fileReader // Reader for current file entry
+	blk  block      // Buffer to use as temporary local storage
 
 	// err is a persistent error.
 	// It is only the responsibility of every exported method of Reader to
@@ -38,68 +28,21 @@ type Reader struct {
 	err error
 }
 
-// A numBytesReader is an io.Reader with a numBytes method, returning the number
-// of bytes remaining in the underlying encoded data.
-type numBytesReader interface {
+type fileReader interface {
 	io.Reader
-	numBytes() int64
-}
+	fileState
 
-// A regFileReader is a numBytesReader for reading file data from a tar archive.
-type regFileReader struct {
-	r  io.Reader // underlying reader
-	nb int64     // number of unread bytes for current file entry
-}
-
-// A sparseFileReader is a numBytesReader for reading sparse file data from a
-// tar archive.
-type sparseFileReader struct {
-	rfr   numBytesReader // Reads the sparse-encoded file data
-	sp    []sparseEntry  // The sparse map for the file
-	pos   int64          // Keeps track of file position
-	total int64          // Total size of the file
-}
-
-// A sparseEntry holds a single entry in a sparse file's sparse map.
-//
-// Sparse files are represented using a series of sparseEntrys.
-// Despite the name, a sparseEntry represents an actual data fragment that
-// references data found in the underlying archive stream. All regions not
-// covered by a sparseEntry are logically filled with zeros.
-//
-// For example, if the underlying raw file contains the 10-byte data:
-//	var compactData = "abcdefgh"
-//
-// And the sparse map has the following entries:
-//	var sp = []sparseEntry{
-//		{offset: 2,  numBytes: 5} // Data fragment for [2..7]
-//		{offset: 18, numBytes: 3} // Data fragment for [18..21]
-//	}
-//
-// Then the content of the resulting sparse file with a "real" size of 25 is:
-//	var sparseData = "\x00"*2 + "abcde" + "\x00"*11 + "fgh" + "\x00"*4
-type sparseEntry struct {
-	offset   int64 // Starting position of the fragment
-	numBytes int64 // Length of the fragment
+	WriteTo(io.Writer) (int64, error)
 }
 
-// Keywords for GNU sparse files in a PAX extended header
-const (
-	paxGNUSparseNumBlocks = "GNU.sparse.numblocks"
-	paxGNUSparseOffset    = "GNU.sparse.offset"
-	paxGNUSparseNumBytes  = "GNU.sparse.numbytes"
-	paxGNUSparseMap       = "GNU.sparse.map"
-	paxGNUSparseName      = "GNU.sparse.name"
-	paxGNUSparseMajor     = "GNU.sparse.major"
-	paxGNUSparseMinor     = "GNU.sparse.minor"
-	paxGNUSparseSize      = "GNU.sparse.size"
-	paxGNUSparseRealSize  = "GNU.sparse.realsize"
-)
-
 // NewReader creates a new Reader reading from r.
-func NewReader(r io.Reader) *Reader { return &Reader{r: r} }
+func NewReader(r io.Reader) *Reader {
+	return &Reader{r: r, curr: &regFileReader{r, 0}}
+}
 
 // Next advances to the next entry in the tar archive.
+// The Header.Size determines how many bytes can be read for the next file.
+// Any remaining data in the current file is automatically discarded.
 //
 // io.EOF is returned at the end of the input.
 func (tr *Reader) Next() (*Header, error) {
@@ -112,18 +55,26 @@ func (tr *Reader) Next() (*Header, error) {
 }
 
 func (tr *Reader) next() (*Header, error) {
-	var extHdrs map[string]string
+	var paxHdrs map[string]string
+	var gnuLongName, gnuLongLink string
 
 	// Externally, Next iterates through the tar archive as if it is a series of
 	// files. Internally, the tar format often uses fake "files" to add meta
 	// data that describes the next file. These meta data "files" should not
 	// normally be visible to the outside. As such, this loop iterates through
 	// one or more "header files" until it finds a "normal file".
+	format := FormatUSTAR | FormatPAX | FormatGNU
 loop:
 	for {
-		if err := tr.skipUnread(); err != nil {
+		// Discard the remainder of the file and any padding.
+		if err := discard(tr.r, tr.curr.PhysicalRemaining()); err != nil {
 			return nil, err
 		}
+		if _, err := tryReadFull(tr.r, tr.blk[:tr.pad]); err != nil {
+			return nil, err
+		}
+		tr.pad = 0
+
 		hdr, rawHdr, err := tr.readHeader()
 		if err != nil {
 			return nil, err
@@ -131,43 +82,58 @@ loop:
 		if err := tr.handleRegularFile(hdr); err != nil {
 			return nil, err
 		}
+		format.mayOnlyBe(hdr.Format)
 
 		// Check for PAX/GNU special headers and files.
 		switch hdr.Typeflag {
-		case TypeXHeader:
-			extHdrs, err = parsePAX(tr)
+		case TypeXHeader, TypeXGlobalHeader:
+			format.mayOnlyBe(FormatPAX)
+			paxHdrs, err = parsePAX(tr)
 			if err != nil {
 				return nil, err
 			}
+			if hdr.Typeflag == TypeXGlobalHeader {
+				mergePAX(hdr, paxHdrs)
+				return &Header{
+					Name:       hdr.Name,
+					Typeflag:   hdr.Typeflag,
+					Xattrs:     hdr.Xattrs,
+					PAXRecords: hdr.PAXRecords,
+					Format:     format,
+				}, nil
+			}
 			continue loop // This is a meta header affecting the next header
 		case TypeGNULongName, TypeGNULongLink:
+			format.mayOnlyBe(FormatGNU)
 			realname, err := ioutil.ReadAll(tr)
 			if err != nil {
 				return nil, err
 			}
 
-			// Convert GNU extensions to use PAX headers.
-			if extHdrs == nil {
-				extHdrs = make(map[string]string)
-			}
 			var p parser
 			switch hdr.Typeflag {
 			case TypeGNULongName:
-				extHdrs[paxPath] = p.parseString(realname)
+				gnuLongName = p.parseString(realname)
 			case TypeGNULongLink:
-				extHdrs[paxLinkpath] = p.parseString(realname)
-			}
-			if p.err != nil {
-				return nil, p.err
+				gnuLongLink = p.parseString(realname)
 			}
 			continue loop // This is a meta header affecting the next header
 		default:
 			// The old GNU sparse format is handled here since it is technically
 			// just a regular file with additional attributes.
 
-			if err := mergePAX(hdr, extHdrs); err != nil {
+			if err := mergePAX(hdr, paxHdrs); err != nil {
 				return nil, err
 			}
+			if gnuLongName != "" {
+				hdr.Name = gnuLongName
+			}
+			if gnuLongLink != "" {
+				hdr.Linkname = gnuLongLink
+			}
+			if hdr.Typeflag == TypeRegA && strings.HasSuffix(hdr.Name, "/") {
+				hdr.Typeflag = TypeDir // Legacy archives use trailing slash for directories
+			}
 
 			// The extended headers may have updated the size.
 			// Thus, setup the regFileReader again after merging PAX headers.
@@ -177,9 +143,15 @@ loop:
 
 			// Sparse formats rely on being able to read from the logical data
 			// section; there must be a preceding call to handleRegularFile.
-			if err := tr.handleSparseFile(hdr, rawHdr, extHdrs); err != nil {
+			if err := tr.handleSparseFile(hdr, rawHdr); err != nil {
 				return nil, err
 			}
+
+			// Set the final guess at the format.
+			if format.has(FormatUSTAR) && format.has(FormatPAX) {
+				format.mayOnlyBe(FormatUSTAR)
+			}
+			hdr.Format = format
 			return hdr, nil // This is a file, so stop
 		}
 	}
@@ -197,105 +169,86 @@ func (tr *Reader) handleRegularFile(hdr *Header) error {
 		return ErrHeader
 	}
 
-	tr.pad = -nb & (blockSize - 1) // blockSize is a power of two
+	tr.pad = blockPadding(nb)
 	tr.curr = &regFileReader{r: tr.r, nb: nb}
 	return nil
 }
 
 // handleSparseFile checks if the current file is a sparse format of any type
 // and sets the curr reader appropriately.
-func (tr *Reader) handleSparseFile(hdr *Header, rawHdr *block, extHdrs map[string]string) error {
-	var sp []sparseEntry
+func (tr *Reader) handleSparseFile(hdr *Header, rawHdr *block) error {
+	var spd sparseDatas
 	var err error
 	if hdr.Typeflag == TypeGNUSparse {
-		sp, err = tr.readOldGNUSparseMap(hdr, rawHdr)
-		if err != nil {
-			return err
-		}
+		spd, err = tr.readOldGNUSparseMap(hdr, rawHdr)
 	} else {
-		sp, err = tr.checkForGNUSparsePAXHeaders(hdr, extHdrs)
-		if err != nil {
-			return err
-		}
+		spd, err = tr.readGNUSparsePAXHeaders(hdr)
 	}
 
 	// If sp is non-nil, then this is a sparse file.
-	// Note that it is possible for len(sp) to be zero.
-	if sp != nil {
-		tr.curr, err = newSparseFileReader(tr.curr, sp, hdr.Size)
+	// Note that it is possible for len(sp) == 0.
+	if err == nil && spd != nil {
+		if isHeaderOnlyType(hdr.Typeflag) || !validateSparseEntries(spd, hdr.Size) {
+			return ErrHeader
+		}
+		sph := invertSparseEntries(spd, hdr.Size)
+		tr.curr = &sparseFileReader{tr.curr, sph, 0}
 	}
 	return err
 }
 
-// checkForGNUSparsePAXHeaders checks the PAX headers for GNU sparse headers. If they are found, then
-// this function reads the sparse map and returns it. Unknown sparse formats are ignored, causing the file to
-// be treated as a regular file.
-func (tr *Reader) checkForGNUSparsePAXHeaders(hdr *Header, headers map[string]string) ([]sparseEntry, error) {
-	var sparseFormat string
-
-	// Check for sparse format indicators
-	major, majorOk := headers[paxGNUSparseMajor]
-	minor, minorOk := headers[paxGNUSparseMinor]
-	sparseName, sparseNameOk := headers[paxGNUSparseName]
-	_, sparseMapOk := headers[paxGNUSparseMap]
-	sparseSize, sparseSizeOk := headers[paxGNUSparseSize]
-	sparseRealSize, sparseRealSizeOk := headers[paxGNUSparseRealSize]
-
-	// Identify which, if any, sparse format applies from which PAX headers are set
-	if majorOk && minorOk {
-		sparseFormat = major + "." + minor
-	} else if sparseNameOk && sparseMapOk {
-		sparseFormat = "0.1"
-	} else if sparseSizeOk {
-		sparseFormat = "0.0"
-	} else {
-		// Not a PAX format GNU sparse file.
-		return nil, nil
-	}
-
-	// Check for unknown sparse format
-	if sparseFormat != "0.0" && sparseFormat != "0.1" && sparseFormat != "1.0" {
-		return nil, nil
-	}
-
-	// Update hdr from GNU sparse PAX headers
-	if sparseNameOk {
-		hdr.Name = sparseName
-	}
-	if sparseSizeOk {
-		realSize, err := strconv.ParseInt(sparseSize, 10, 64)
+// readGNUSparsePAXHeaders checks the PAX headers for GNU sparse headers.
+// If they are found, then this function reads the sparse map and returns it.
+// This assumes that 0.0 headers have already been converted to 0.1 headers
+// by the the PAX header parsing logic.
+func (tr *Reader) readGNUSparsePAXHeaders(hdr *Header) (sparseDatas, error) {
+	// Identify the version of GNU headers.
+	var is1x0 bool
+	major, minor := hdr.PAXRecords[paxGNUSparseMajor], hdr.PAXRecords[paxGNUSparseMinor]
+	switch {
+	case major == "0" && (minor == "0" || minor == "1"):
+		is1x0 = false
+	case major == "1" && minor == "0":
+		is1x0 = true
+	case major != "" || minor != "":
+		return nil, nil // Unknown GNU sparse PAX version
+	case hdr.PAXRecords[paxGNUSparseMap] != "":
+		is1x0 = false // 0.0 and 0.1 did not have explicit version records, so guess
+	default:
+		return nil, nil // Not a PAX format GNU sparse file.
+	}
+	hdr.Format.mayOnlyBe(FormatPAX)
+
+	// Update hdr from GNU sparse PAX headers.
+	if name := hdr.PAXRecords[paxGNUSparseName]; name != "" {
+		hdr.Name = name
+	}
+	size := hdr.PAXRecords[paxGNUSparseSize]
+	if size == "" {
+		size = hdr.PAXRecords[paxGNUSparseRealSize]
+	}
+	if size != "" {
+		n, err := strconv.ParseInt(size, 10, 64)
 		if err != nil {
 			return nil, ErrHeader
 		}
-		hdr.Size = realSize
-	} else if sparseRealSizeOk {
-		realSize, err := strconv.ParseInt(sparseRealSize, 10, 64)
-		if err != nil {
-			return nil, ErrHeader
-		}
-		hdr.Size = realSize
+		hdr.Size = n
 	}
 
-	// Set up the sparse map, according to the particular sparse format in use
-	var sp []sparseEntry
-	var err error
-	switch sparseFormat {
-	case "0.0", "0.1":
-		sp, err = readGNUSparseMap0x1(headers)
-	case "1.0":
-		sp, err = readGNUSparseMap1x0(tr.curr)
+	// Read the sparse map according to the appropriate format.
+	if is1x0 {
+		return readGNUSparseMap1x0(tr.curr)
 	}
-	return sp, err
+	return readGNUSparseMap0x1(hdr.PAXRecords)
 }
 
-// mergePAX merges well known headers according to PAX standard.
-// In general headers with the same name as those found
-// in the header struct overwrite those found in the header
-// struct with higher precision or longer values. Esp. useful
-// for name and linkname fields.
-func mergePAX(hdr *Header, headers map[string]string) (err error) {
-	var id64 int64
-	for k, v := range headers {
+// mergePAX merges paxHdrs into hdr for all relevant fields of Header.
+func mergePAX(hdr *Header, paxHdrs map[string]string) (err error) {
+	for k, v := range paxHdrs {
+		if v == "" {
+			continue // Keep the original USTAR value
+		}
+		var id64 int64
 		switch k {
 		case paxPath:
 			hdr.Name = v
@@ -320,17 +273,18 @@ func mergePAX(hdr *Header, headers map[string]string) (err error) {
 		case paxSize:
 			hdr.Size, err = strconv.ParseInt(v, 10, 64)
 		default:
-			if strings.HasPrefix(k, paxXattr) {
+			if strings.HasPrefix(k, paxSchilyXattr) {
 				if hdr.Xattrs == nil {
 					hdr.Xattrs = make(map[string]string)
 				}
-				hdr.Xattrs[k[len(paxXattr):]] = v
+				hdr.Xattrs[k[len(paxSchilyXattr):]] = v
 			}
 		}
 		if err != nil {
 			return ErrHeader
 		}
 	}
+	hdr.PAXRecords = paxHdrs
 	return nil
 }
 
@@ -348,7 +302,7 @@ func parsePAX(r io.Reader) (map[string]string, error) {
 	// headers since 0.0 headers were not PAX compliant.
 	var sparseMap []string
 
-	extHdrs := make(map[string]string)
+	paxHdrs := make(map[string]string)
 	for len(sbuf) > 0 {
 		key, value, residual, err := parsePAXRecord(sbuf)
 		if err != nil {
@@ -366,58 +320,13 @@ func parsePAX(r io.Reader) (map[string]string, error) {
 			}
 			sparseMap = append(sparseMap, value)
 		default:
-			// According to PAX specification, a value is stored only if it is
-			// non-empty. Otherwise, the key is deleted.
-			if len(value) > 0 {
-				extHdrs[key] = value
-			} else {
-				delete(extHdrs, key)
-			}
+			paxHdrs[key] = value
 		}
 	}
 	if len(sparseMap) > 0 {
-		extHdrs[paxGNUSparseMap] = strings.Join(sparseMap, ",")
+		paxHdrs[paxGNUSparseMap] = strings.Join(sparseMap, ",")
 	}
-	return extHdrs, nil
-}
-
-// skipUnread skips any unread bytes in the existing file entry, as well as any
-// alignment padding. It returns io.ErrUnexpectedEOF if any io.EOF is
-// encountered in the data portion; it is okay to hit io.EOF in the padding.
-//
-// Note that this function still works properly even when sparse files are being
-// used since numBytes returns the bytes remaining in the underlying io.Reader.
-func (tr *Reader) skipUnread() error {
-	dataSkip := tr.numBytes()      // Number of data bytes to skip
-	totalSkip := dataSkip + tr.pad // Total number of bytes to skip
-	tr.curr, tr.pad = nil, 0
-
-	// If possible, Seek to the last byte before the end of the data section.
-	// Do this because Seek is often lazy about reporting errors; this will mask
-	// the fact that the tar stream may be truncated. We can rely on the
-	// io.CopyN done shortly afterwards to trigger any IO errors.
-	var seekSkipped int64 // Number of bytes skipped via Seek
-	if sr, ok := tr.r.(io.Seeker); ok && dataSkip > 1 {
-		// Not all io.Seeker can actually Seek. For example, os.Stdin implements
-		// io.Seeker, but calling Seek always returns an error and performs
-		// no action. Thus, we try an innocent seek to the current position
-		// to see if Seek is really supported.
-		pos1, err := sr.Seek(0, io.SeekCurrent)
-		if err == nil {
-			// Seek seems supported, so perform the real Seek.
-			pos2, err := sr.Seek(dataSkip-1, io.SeekCurrent)
-			if err != nil {
-				return err
-			}
-			seekSkipped = pos2 - pos1
-		}
-	}
-
-	copySkipped, err := io.CopyN(ioutil.Discard, tr.r, totalSkip-seekSkipped)
-	if err == io.EOF && seekSkipped+copySkipped < dataSkip {
-		err = io.ErrUnexpectedEOF
-	}
-	return err
+	return paxHdrs, nil
 }
 
 // readHeader reads the next block header and assumes that the underlying reader
@@ -445,7 +354,7 @@ func (tr *Reader) readHeader() (*Header, *block, error) {
 
 	// Verify the header matches a known format.
 	format := tr.blk.GetFormat()
-	if format == formatUnknown {
+	if format == FormatUnknown {
 		return nil, nil, ErrHeader
 	}
 
@@ -454,59 +363,86 @@ func (tr *Reader) readHeader() (*Header, *block, error) {
 
 	// Unpack the V7 header.
 	v7 := tr.blk.V7()
+	hdr.Typeflag = v7.TypeFlag()[0]
 	hdr.Name = p.parseString(v7.Name())
+	hdr.Linkname = p.parseString(v7.LinkName())
+	hdr.Size = p.parseNumeric(v7.Size())
 	hdr.Mode = p.parseNumeric(v7.Mode())
 	hdr.Uid = int(p.parseNumeric(v7.UID()))
 	hdr.Gid = int(p.parseNumeric(v7.GID()))
-	hdr.Size = p.parseNumeric(v7.Size())
 	hdr.ModTime = time.Unix(p.parseNumeric(v7.ModTime()), 0)
-	hdr.Typeflag = v7.TypeFlag()[0]
-	hdr.Linkname = p.parseString(v7.LinkName())
-
-	// The atime and ctime fields are often left unused. Some versions of Go
-	// had a bug in the tar.Writer where it would output an invalid tar file
-	// in certain rare situations because the logic incorrectly believed that
-	// the old GNU format had a prefix field. This is wrong and leads to
-	// an outputted file that actually mangles the atime and ctime fields.
-	//
-	// In order to continue reading tar files created by a buggy writer, we
-	// try to parse the atime and ctime fields, but just return the zero value
-	// of time.Time when we cannot parse them.
-	//
-	// See https://golang.org/issues/12594
-	tryParseTime := func(b []byte) time.Time {
-		var p parser
-		n := p.parseNumeric(b)
-		if b[0] != 0x00 && p.err == nil {
-			return time.Unix(n, 0)
-		}
-		return time.Time{}
-	}
 
 	// Unpack format specific fields.
 	if format > formatV7 {
 		ustar := tr.blk.USTAR()
 		hdr.Uname = p.parseString(ustar.UserName())
 		hdr.Gname = p.parseString(ustar.GroupName())
-		if hdr.Typeflag == TypeChar || hdr.Typeflag == TypeBlock {
-			hdr.Devmajor = p.parseNumeric(ustar.DevMajor())
-			hdr.Devminor = p.parseNumeric(ustar.DevMinor())
-		}
+		hdr.Devmajor = p.parseNumeric(ustar.DevMajor())
+		hdr.Devminor = p.parseNumeric(ustar.DevMinor())
 
 		var prefix string
-		switch format {
-		case formatUSTAR:
+		switch {
+		case format.has(FormatUSTAR | FormatPAX):
+			hdr.Format = format
 			ustar := tr.blk.USTAR()
 			prefix = p.parseString(ustar.Prefix())
-		case formatSTAR:
+
+			// For Format detection, check if block is properly formatted since
+			// the parser is more liberal than what USTAR actually permits.
+			notASCII := func(r rune) bool { return r >= 0x80 }
+			if bytes.IndexFunc(tr.blk[:], notASCII) >= 0 {
+				hdr.Format = FormatUnknown // Non-ASCII characters in block.
+			}
+			nul := func(b []byte) bool { return int(b[len(b)-1]) == 0 }
+			if !(nul(v7.Size()) && nul(v7.Mode()) && nul(v7.UID()) && nul(v7.GID()) &&
+				nul(v7.ModTime()) && nul(ustar.DevMajor()) && nul(ustar.DevMinor())) {
+				hdr.Format = FormatUnknown // Numeric fields must end in NUL
+			}
+		case format.has(formatSTAR):
 			star := tr.blk.STAR()
 			prefix = p.parseString(star.Prefix())
 			hdr.AccessTime = time.Unix(p.parseNumeric(star.AccessTime()), 0)
 			hdr.ChangeTime = time.Unix(p.parseNumeric(star.ChangeTime()), 0)
-		case formatGNU:
+		case format.has(FormatGNU):
+			hdr.Format = format
+			var p2 parser
 			gnu := tr.blk.GNU()
-			hdr.AccessTime = tryParseTime(gnu.AccessTime())
-			hdr.ChangeTime = tryParseTime(gnu.ChangeTime())
+			if b := gnu.AccessTime(); b[0] != 0 {
+				hdr.AccessTime = time.Unix(p2.parseNumeric(b), 0)
+			}
+			if b := gnu.ChangeTime(); b[0] != 0 {
+				hdr.ChangeTime = time.Unix(p2.parseNumeric(b), 0)
+			}
+
+			// Prior to Go1.8, the Writer had a bug where it would output
+			// an invalid tar file in certain rare situations because the logic
+			// incorrectly believed that the old GNU format had a prefix field.
+			// This is wrong and leads to an output file that mangles the
+			// atime and ctime fields, which are often left unused.
+			//
+			// In order to continue reading tar files created by former, buggy
+			// versions of Go, we skeptically parse the atime and ctime fields.
+			// If we are unable to parse them and the prefix field looks like
+			// an ASCII string, then we fallback on the pre-Go1.8 behavior
+			// of treating these fields as the USTAR prefix field.
+			//
+			// Note that this will not use the fallback logic for all possible
+			// files generated by a pre-Go1.8 toolchain. If the generated file
+			// happened to have a prefix field that parses as valid
+			// atime and ctime fields (e.g., when they are valid octal strings),
+			// then it is impossible to distinguish between an valid GNU file
+			// and an invalid pre-Go1.8 file.
+			//
+			// See https://golang.org/issues/12594
+			// See https://golang.org/issues/21005
+			if p2.err != nil {
+				hdr.AccessTime, hdr.ChangeTime = time.Time{}, time.Time{}
+				ustar := tr.blk.USTAR()
+				if s := p.parseString(ustar.Prefix()); isASCII(s) {
+					prefix = s
+				}
+				hdr.Format = FormatUnknown // Buggy file is not GNU
+			}
 		}
 		if len(prefix) > 0 {
 			hdr.Name = prefix + "/" + hdr.Name
@@ -523,21 +459,22 @@ func (tr *Reader) readHeader() (*Header, *block, error) {
 // The Header.Size does not reflect the size of any extended headers used.
 // Thus, this function will read from the raw io.Reader to fetch extra headers.
 // This method mutates blk in the process.
-func (tr *Reader) readOldGNUSparseMap(hdr *Header, blk *block) ([]sparseEntry, error) {
+func (tr *Reader) readOldGNUSparseMap(hdr *Header, blk *block) (sparseDatas, error) {
 	// Make sure that the input format is GNU.
 	// Unfortunately, the STAR format also has a sparse header format that uses
 	// the same type flag but has a completely different layout.
-	if blk.GetFormat() != formatGNU {
+	if blk.GetFormat() != FormatGNU {
 		return nil, ErrHeader
 	}
+	hdr.Format.mayOnlyBe(FormatGNU)
 
 	var p parser
 	hdr.Size = p.parseNumeric(blk.GNU().RealSize())
 	if p.err != nil {
 		return nil, p.err
 	}
-	var s sparseArray = blk.GNU().Sparse()
-	var sp = make([]sparseEntry, 0, s.MaxEntries())
+	s := blk.GNU().Sparse()
+	spd := make(sparseDatas, 0, s.MaxEntries())
 	for {
 		for i := 0; i < s.MaxEntries(); i++ {
 			// This termination condition is identical to GNU and BSD tar.
@@ -545,25 +482,22 @@ func (tr *Reader) readOldGNUSparseMap(hdr *Header, blk *block) ([]sparseEntry, e
 				break // Don't return, need to process extended headers (even if empty)
 			}
 			offset := p.parseNumeric(s.Entry(i).Offset())
-			numBytes := p.parseNumeric(s.Entry(i).NumBytes())
+			length := p.parseNumeric(s.Entry(i).Length())
 			if p.err != nil {
 				return nil, p.err
 			}
-			sp = append(sp, sparseEntry{offset: offset, numBytes: numBytes})
+			spd = append(spd, sparseEntry{Offset: offset, Length: length})
 		}
 
 		if s.IsExtended()[0] > 0 {
 			// There are more entries. Read an extension header and parse its entries.
-			if _, err := io.ReadFull(tr.r, blk[:]); err != nil {
-				if err == io.EOF {
-					err = io.ErrUnexpectedEOF
-				}
+			if _, err := mustReadFull(tr.r, blk[:]); err != nil {
 				return nil, err
 			}
 			s = blk.Sparse()
 			continue
 		}
-		return sp, nil // Done
+		return spd, nil // Done
 	}
 }
 
@@ -571,28 +505,27 @@ func (tr *Reader) readOldGNUSparseMap(hdr *Header, blk *block) ([]sparseEntry, e
 // version 1.0. The format of the sparse map consists of a series of
 // newline-terminated numeric fields. The first field is the number of entries
 // and is always present. Following this are the entries, consisting of two
-// fields (offset, numBytes). This function must stop reading at the end
+// fields (offset, length). This function must stop reading at the end
 // boundary of the block containing the last newline.
 //
 // Note that the GNU manual says that numeric values should be encoded in octal
 // format. However, the GNU tar utility itself outputs these values in decimal.
 // As such, this library treats values as being encoded in decimal.
-func readGNUSparseMap1x0(r io.Reader) ([]sparseEntry, error) {
-	var cntNewline int64
-	var buf bytes.Buffer
-	var blk = make([]byte, blockSize)
-
-	// feedTokens copies data in numBlock chunks from r into buf until there are
+func readGNUSparseMap1x0(r io.Reader) (sparseDatas, error) {
+	var (
+		cntNewline int64
+		buf        bytes.Buffer
+		blk        block
+	)
+
+	// feedTokens copies data in blocks from r into buf until there are
 	// at least cnt newlines in buf. It will not read more blocks than needed.
-	var feedTokens = func(cnt int64) error {
-		for cntNewline < cnt {
-			if _, err := io.ReadFull(r, blk); err != nil {
-				if err == io.EOF {
-					err = io.ErrUnexpectedEOF
-				}
+	feedTokens := func(n int64) error {
+		for cntNewline < n {
+			if _, err := mustReadFull(r, blk[:]); err != nil {
 				return err
 			}
-			buf.Write(blk)
+			buf.Write(blk[:])
 			for _, c := range blk {
 				if c == '\n' {
 					cntNewline++
@@ -604,10 +537,10 @@ func readGNUSparseMap1x0(r io.Reader) ([]sparseEntry, error) {
 
 	// nextToken gets the next token delimited by a newline. This assumes that
 	// at least one newline exists in the buffer.
-	var nextToken = func() string {
+	nextToken := func() string {
 		cntNewline--
 		tok, _ := buf.ReadString('\n')
-		return tok[:len(tok)-1] // Cut off newline
+		return strings.TrimRight(tok, "\n")
 	}
 
 	// Parse for the number of entries.
@@ -626,80 +559,67 @@ func readGNUSparseMap1x0(r io.Reader) ([]sparseEntry, error) {
 	if err := feedTokens(2 * numEntries); err != nil {
 		return nil, err
 	}
-	sp := make([]sparseEntry, 0, numEntries)
+	spd := make(sparseDatas, 0, numEntries)
 	for i := int64(0); i < numEntries; i++ {
-		offset, err := strconv.ParseInt(nextToken(), 10, 64)
-		if err != nil {
-			return nil, ErrHeader
-		}
-		numBytes, err := strconv.ParseInt(nextToken(), 10, 64)
-		if err != nil {
+		offset, err1 := strconv.ParseInt(nextToken(), 10, 64)
+		length, err2 := strconv.ParseInt(nextToken(), 10, 64)
+		if err1 != nil || err2 != nil {
 			return nil, ErrHeader
 		}
-		sp = append(sp, sparseEntry{offset: offset, numBytes: numBytes})
+		spd = append(spd, sparseEntry{Offset: offset, Length: length})
 	}
-	return sp, nil
+	return spd, nil
 }
 
 // readGNUSparseMap0x1 reads the sparse map as stored in GNU's PAX sparse format
 // version 0.1. The sparse map is stored in the PAX headers.
-func readGNUSparseMap0x1(extHdrs map[string]string) ([]sparseEntry, error) {
+func readGNUSparseMap0x1(paxHdrs map[string]string) (sparseDatas, error) {
 	// Get number of entries.
 	// Use integer overflow resistant math to check this.
-	numEntriesStr := extHdrs[paxGNUSparseNumBlocks]
+	numEntriesStr := paxHdrs[paxGNUSparseNumBlocks]
 	numEntries, err := strconv.ParseInt(numEntriesStr, 10, 0) // Intentionally parse as native int
 	if err != nil || numEntries < 0 || int(2*numEntries) < int(numEntries) {
 		return nil, ErrHeader
 	}
 
 	// There should be two numbers in sparseMap for each entry.
-	sparseMap := strings.Split(extHdrs[paxGNUSparseMap], ",")
+	sparseMap := strings.Split(paxHdrs[paxGNUSparseMap], ",")
+	if len(sparseMap) == 1 && sparseMap[0] == "" {
+		sparseMap = sparseMap[:0]
+	}
 	if int64(len(sparseMap)) != 2*numEntries {
 		return nil, ErrHeader
 	}
 
 	// Loop through the entries in the sparse map.
 	// numEntries is trusted now.
-	sp := make([]sparseEntry, 0, numEntries)
-	for i := int64(0); i < numEntries; i++ {
-		offset, err := strconv.ParseInt(sparseMap[2*i], 10, 64)
-		if err != nil {
-			return nil, ErrHeader
-		}
-		numBytes, err := strconv.ParseInt(sparseMap[2*i+1], 10, 64)
-		if err != nil {
+	spd := make(sparseDatas, 0, numEntries)
+	for len(sparseMap) >= 2 {
+		offset, err1 := strconv.ParseInt(sparseMap[0], 10, 64)
+		length, err2 := strconv.ParseInt(sparseMap[1], 10, 64)
+		if err1 != nil || err2 != nil {
 			return nil, ErrHeader
 		}
-		sp = append(sp, sparseEntry{offset: offset, numBytes: numBytes})
+		spd = append(spd, sparseEntry{Offset: offset, Length: length})
+		sparseMap = sparseMap[2:]
 	}
-	return sp, nil
+	return spd, nil
 }
 
-// numBytes returns the number of bytes left to read in the current file's entry
-// in the tar archive, or 0 if there is no current file.
-func (tr *Reader) numBytes() int64 {
-	if tr.curr == nil {
-		// No current file, so no bytes
-		return 0
-	}
-	return tr.curr.numBytes()
-}
-
-// Read reads from the current entry in the tar archive.
-// It returns 0, io.EOF when it reaches the end of that entry,
-// until Next is called to advance to the next entry.
+// Read reads from the current file in the tar archive.
+// It returns (0, io.EOF) when it reaches the end of that file,
+// until Next is called to advance to the next file.
 //
-// Calling Read on special types like TypeLink, TypeSymLink, TypeChar,
-// TypeBlock, TypeDir, and TypeFifo returns 0, io.EOF regardless of what
+// If the current file is sparse, then the regions marked as a hole
+// are read back as NUL-bytes.
+//
+// Calling Read on special types like TypeLink, TypeSymlink, TypeChar,
+// TypeBlock, TypeDir, and TypeFifo returns (0, io.EOF) regardless of what
 // the Header.Size claims.
 func (tr *Reader) Read(b []byte) (int, error) {
 	if tr.err != nil {
 		return 0, tr.err
 	}
-	if tr.curr == nil {
-		return 0, io.EOF
-	}
-
 	n, err := tr.curr.Read(b)
 	if err != nil && err != io.EOF {
 		tr.err = err
@@ -707,116 +627,229 @@ func (tr *Reader) Read(b []byte) (int, error) {
 	return n, err
 }
 
-func (rfr *regFileReader) Read(b []byte) (n int, err error) {
-	if rfr.nb == 0 {
-		// file consumed
-		return 0, io.EOF
+// writeTo writes the content of the current file to w.
+// The bytes written matches the number of remaining bytes in the current file.
+//
+// If the current file is sparse and w is an io.WriteSeeker,
+// then writeTo uses Seek to skip past holes defined in Header.SparseHoles,
+// assuming that skipped regions are filled with NULs.
+// This always writes the last byte to ensure w is the right size.
+//
+// TODO(dsnet): Re-export this when adding sparse file support.
+// See https://golang.org/issue/22735
+func (tr *Reader) writeTo(w io.Writer) (int64, error) {
+	if tr.err != nil {
+		return 0, tr.err
 	}
-	if int64(len(b)) > rfr.nb {
-		b = b[0:rfr.nb]
+	n, err := tr.curr.WriteTo(w)
+	if err != nil {
+		tr.err = err
 	}
-	n, err = rfr.r.Read(b)
-	rfr.nb -= int64(n)
+	return n, err
+}
 
-	if err == io.EOF && rfr.nb > 0 {
-		err = io.ErrUnexpectedEOF
+// regFileReader is a fileReader for reading data from a regular file entry.
+type regFileReader struct {
+	r  io.Reader // Underlying Reader
+	nb int64     // Number of remaining bytes to read
+}
+
+func (fr *regFileReader) Read(b []byte) (n int, err error) {
+	if int64(len(b)) > fr.nb {
+		b = b[:fr.nb]
+	}
+	if len(b) > 0 {
+		n, err = fr.r.Read(b)
+		fr.nb -= int64(n)
+	}
+	switch {
+	case err == io.EOF && fr.nb > 0:
+		return n, io.ErrUnexpectedEOF
+	case err == nil && fr.nb == 0:
+		return n, io.EOF
+	default:
+		return n, err
 	}
-	return
 }
 
-// numBytes returns the number of bytes left to read in the file's data in the tar archive.
-func (rfr *regFileReader) numBytes() int64 {
-	return rfr.nb
+func (fr *regFileReader) WriteTo(w io.Writer) (int64, error) {
+	return io.Copy(w, struct{ io.Reader }{fr})
 }
 
-// newSparseFileReader creates a new sparseFileReader, but validates all of the
-// sparse entries before doing so.
-func newSparseFileReader(rfr numBytesReader, sp []sparseEntry, total int64) (*sparseFileReader, error) {
-	if total < 0 {
-		return nil, ErrHeader // Total size cannot be negative
-	}
+func (fr regFileReader) LogicalRemaining() int64 {
+	return fr.nb
+}
 
-	// Validate all sparse entries. These are the same checks as performed by
-	// the BSD tar utility.
-	for i, s := range sp {
-		switch {
-		case s.offset < 0 || s.numBytes < 0:
-			return nil, ErrHeader // Negative values are never okay
-		case s.offset > math.MaxInt64-s.numBytes:
-			return nil, ErrHeader // Integer overflow with large length
-		case s.offset+s.numBytes > total:
-			return nil, ErrHeader // Region extends beyond the "real" size
-		case i > 0 && sp[i-1].offset+sp[i-1].numBytes > s.offset:
-			return nil, ErrHeader // Regions can't overlap and must be in order
-		}
-	}
-	return &sparseFileReader{rfr: rfr, sp: sp, total: total}, nil
+func (fr regFileReader) PhysicalRemaining() int64 {
+	return fr.nb
+}
+
+// sparseFileReader is a fileReader for reading data from a sparse file entry.
+type sparseFileReader struct {
+	fr  fileReader  // Underlying fileReader
+	sp  sparseHoles // Normalized list of sparse holes
+	pos int64       // Current position in sparse file
 }
 
-// readHole reads a sparse hole ending at endOffset.
-func (sfr *sparseFileReader) readHole(b []byte, endOffset int64) int {
-	n64 := endOffset - sfr.pos
-	if n64 > int64(len(b)) {
-		n64 = int64(len(b))
+func (sr *sparseFileReader) Read(b []byte) (n int, err error) {
+	finished := int64(len(b)) >= sr.LogicalRemaining()
+	if finished {
+		b = b[:sr.LogicalRemaining()]
+	}
+
+	b0 := b
+	endPos := sr.pos + int64(len(b))
+	for endPos > sr.pos && err == nil {
+		var nf int // Bytes read in fragment
+		holeStart, holeEnd := sr.sp[0].Offset, sr.sp[0].endOffset()
+		if sr.pos < holeStart { // In a data fragment
+			bf := b[:min(int64(len(b)), holeStart-sr.pos)]
+			nf, err = tryReadFull(sr.fr, bf)
+		} else { // In a hole fragment
+			bf := b[:min(int64(len(b)), holeEnd-sr.pos)]
+			nf, err = tryReadFull(zeroReader{}, bf)
+		}
+		b = b[nf:]
+		sr.pos += int64(nf)
+		if sr.pos >= holeEnd && len(sr.sp) > 1 {
+			sr.sp = sr.sp[1:] // Ensure last fragment always remains
+		}
 	}
-	n := int(n64)
-	for i := 0; i < n; i++ {
-		b[i] = 0
+
+	n = len(b0) - len(b)
+	switch {
+	case err == io.EOF:
+		return n, errMissData // Less data in dense file than sparse file
+	case err != nil:
+		return n, err
+	case sr.LogicalRemaining() == 0 && sr.PhysicalRemaining() > 0:
+		return n, errUnrefData // More data in dense file than sparse file
+	case finished:
+		return n, io.EOF
+	default:
+		return n, nil
 	}
-	sfr.pos += n64
-	return n
 }
 
-// Read reads the sparse file data in expanded form.
-func (sfr *sparseFileReader) Read(b []byte) (n int, err error) {
-	// Skip past all empty fragments.
-	for len(sfr.sp) > 0 && sfr.sp[0].numBytes == 0 {
-		sfr.sp = sfr.sp[1:]
+func (sr *sparseFileReader) WriteTo(w io.Writer) (n int64, err error) {
+	ws, ok := w.(io.WriteSeeker)
+	if ok {
+		if _, err := ws.Seek(0, io.SeekCurrent); err != nil {
+			ok = false // Not all io.Seeker can really seek
+		}
 	}
-
-	// If there are no more fragments, then it is possible that there
-	// is one last sparse hole.
-	if len(sfr.sp) == 0 {
-		// This behavior matches the BSD tar utility.
-		// However, GNU tar stops returning data even if sfr.total is unmet.
-		if sfr.pos < sfr.total {
-			return sfr.readHole(b, sfr.total), nil
+	if !ok {
+		return io.Copy(w, struct{ io.Reader }{sr})
+	}
+
+	var writeLastByte bool
+	pos0 := sr.pos
+	for sr.LogicalRemaining() > 0 && !writeLastByte && err == nil {
+		var nf int64 // Size of fragment
+		holeStart, holeEnd := sr.sp[0].Offset, sr.sp[0].endOffset()
+		if sr.pos < holeStart { // In a data fragment
+			nf = holeStart - sr.pos
+			nf, err = io.CopyN(ws, sr.fr, nf)
+		} else { // In a hole fragment
+			nf = holeEnd - sr.pos
+			if sr.PhysicalRemaining() == 0 {
+				writeLastByte = true
+				nf--
+			}
+			_, err = ws.Seek(nf, io.SeekCurrent)
 		}
-		return 0, io.EOF
+		sr.pos += nf
+		if sr.pos >= holeEnd && len(sr.sp) > 1 {
+			sr.sp = sr.sp[1:] // Ensure last fragment always remains
+		}
+	}
+
+	// If the last fragment is a hole, then seek to 1-byte before EOF, and
+	// write a single byte to ensure the file is the right size.
+	if writeLastByte && err == nil {
+		_, err = ws.Write([]byte{0})
+		sr.pos++
 	}
 
-	// In front of a data fragment, so read a hole.
-	if sfr.pos < sfr.sp[0].offset {
-		return sfr.readHole(b, sfr.sp[0].offset), nil
+	n = sr.pos - pos0
+	switch {
+	case err == io.EOF:
+		return n, errMissData // Less data in dense file than sparse file
+	case err != nil:
+		return n, err
+	case sr.LogicalRemaining() == 0 && sr.PhysicalRemaining() > 0:
+		return n, errUnrefData // More data in dense file than sparse file
+	default:
+		return n, nil
 	}
+}
+
+func (sr sparseFileReader) LogicalRemaining() int64 {
+	return sr.sp[len(sr.sp)-1].endOffset() - sr.pos
+}
+func (sr sparseFileReader) PhysicalRemaining() int64 {
+	return sr.fr.PhysicalRemaining()
+}
+
+type zeroReader struct{}
 
-	// In a data fragment, so read from it.
-	// This math is overflow free since we verify that offset and numBytes can
-	// be safely added when creating the sparseFileReader.
-	endPos := sfr.sp[0].offset + sfr.sp[0].numBytes // End offset of fragment
-	bytesLeft := endPos - sfr.pos                   // Bytes left in fragment
-	if int64(len(b)) > bytesLeft {
-		b = b[:bytesLeft]
+func (zeroReader) Read(b []byte) (int, error) {
+	for i := range b {
+		b[i] = 0
 	}
+	return len(b), nil
+}
 
-	n, err = sfr.rfr.Read(b)
-	sfr.pos += int64(n)
+// mustReadFull is like io.ReadFull except it returns
+// io.ErrUnexpectedEOF when io.EOF is hit before len(b) bytes are read.
+func mustReadFull(r io.Reader, b []byte) (int, error) {
+	n, err := tryReadFull(r, b)
 	if err == io.EOF {
-		if sfr.pos < endPos {
-			err = io.ErrUnexpectedEOF // There was supposed to be more data
-		} else if sfr.pos < sfr.total {
-			err = nil // There is still an implicit sparse hole at the end
-		}
+		err = io.ErrUnexpectedEOF
 	}
+	return n, err
+}
 
-	if sfr.pos == endPos {
-		sfr.sp = sfr.sp[1:] // We are done with this fragment, so pop it
+// tryReadFull is like io.ReadFull except it returns
+// io.EOF when it is hit before len(b) bytes are read.
+func tryReadFull(r io.Reader, b []byte) (n int, err error) {
+	for len(b) > n && err == nil {
+		var nn int
+		nn, err = r.Read(b[n:])
+		n += nn
+	}
+	if len(b) == n && err == io.EOF {
+		err = nil
 	}
 	return n, err
 }
 
-// numBytes returns the number of bytes left to read in the sparse file's
-// sparse-encoded data in the tar archive.
-func (sfr *sparseFileReader) numBytes() int64 {
-	return sfr.rfr.numBytes()
+// discard skips n bytes in r, reporting an error if unable to do so.
+func discard(r io.Reader, n int64) error {
+	// If possible, Seek to the last byte before the end of the data section.
+	// Do this because Seek is often lazy about reporting errors; this will mask
+	// the fact that the stream may be truncated. We can rely on the
+	// io.CopyN done shortly afterwards to trigger any IO errors.
+	var seekSkipped int64 // Number of bytes skipped via Seek
+	if sr, ok := r.(io.Seeker); ok && n > 1 {
+		// Not all io.Seeker can actually Seek. For example, os.Stdin implements
+		// io.Seeker, but calling Seek always returns an error and performs
+		// no action. Thus, we try an innocent seek to the current position
+		// to see if Seek is really supported.
+		pos1, err := sr.Seek(0, io.SeekCurrent)
+		if pos1 >= 0 && err == nil {
+			// Seek seems supported, so perform the real Seek.
+			pos2, err := sr.Seek(n-1, io.SeekCurrent)
+			if pos2 < 0 || err != nil {
+				return err
+			}
+			seekSkipped = pos2 - pos1
+		}
+	}
+
+	copySkipped, err := io.CopyN(ioutil.Discard, r, n-seekSkipped)
+	if err == io.EOF && seekSkipped+copySkipped < n {
+		err = io.ErrUnexpectedEOF
+	}
+	return err
 }
diff --git a/libgo/go/archive/tar/reader_test.go b/libgo/go/archive/tar/reader_test.go
index 3386868..a6832d3 100644
--- a/libgo/go/archive/tar/reader_test.go
+++ b/libgo/go/archive/tar/reader_test.go
@@ -7,12 +7,15 @@ package tar
 import (
 	"bytes"
 	"crypto/md5"
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
 	"math"
 	"os"
+	"path"
 	"reflect"
+	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -36,6 +39,7 @@ func TestReader(t *testing.T) {
 			Typeflag: '0',
 			Uname:    "dsymonds",
 			Gname:    "eng",
+			Format:   FormatGNU,
 		}, {
 			Name:     "small2.txt",
 			Mode:     0640,
@@ -46,6 +50,7 @@ func TestReader(t *testing.T) {
 			Typeflag: '0',
 			Uname:    "dsymonds",
 			Gname:    "eng",
+			Format:   FormatGNU,
 		}},
 		chksums: []string{
 			"e38b27eaccb4391bdec553a7f3ae6b2f",
@@ -66,6 +71,7 @@ func TestReader(t *testing.T) {
 			Gname:    "david",
 			Devmajor: 0,
 			Devminor: 0,
+			Format:   FormatGNU,
 		}, {
 			Name:     "sparse-posix-0.0",
 			Mode:     420,
@@ -79,6 +85,12 @@ func TestReader(t *testing.T) {
 			Gname:    "david",
 			Devmajor: 0,
 			Devminor: 0,
+			PAXRecords: map[string]string{
+				"GNU.sparse.size":      "200",
+				"GNU.sparse.numblocks": "95",
+				"GNU.sparse.map":       "1,1,3,1,5,1,7,1,9,1,11,1,13,1,15,1,17,1,19,1,21,1,23,1,25,1,27,1,29,1,31,1,33,1,35,1,37,1,39,1,41,1,43,1,45,1,47,1,49,1,51,1,53,1,55,1,57,1,59,1,61,1,63,1,65,1,67,1,69,1,71,1,73,1,75,1,77,1,79,1,81,1,83,1,85,1,87,1,89,1,91,1,93,1,95,1,97,1,99,1,101,1,103,1,105,1,107,1,109,1,111,1,113,1,115,1,117,1,119,1,121,1,123,1,125,1,127,1,129,1,131,1,133,1,135,1,137,1,139,1,141,1,143,1,145,1,147,1,149,1,151,1,153,1,155,1,157,1,159,1,161,1,163,1,165,1,167,1,169,1,171,1,173,1,175,1,177,1,179,1,181,1,183,1,185,1,187,1,189,1",
+			},
+			Format: FormatPAX,
 		}, {
 			Name:     "sparse-posix-0.1",
 			Mode:     420,
@@ -92,6 +104,13 @@ func TestReader(t *testing.T) {
 			Gname:    "david",
 			Devmajor: 0,
 			Devminor: 0,
+			PAXRecords: map[string]string{
+				"GNU.sparse.size":      "200",
+				"GNU.sparse.numblocks": "95",
+				"GNU.sparse.map":       "1,1,3,1,5,1,7,1,9,1,11,1,13,1,15,1,17,1,19,1,21,1,23,1,25,1,27,1,29,1,31,1,33,1,35,1,37,1,39,1,41,1,43,1,45,1,47,1,49,1,51,1,53,1,55,1,57,1,59,1,61,1,63,1,65,1,67,1,69,1,71,1,73,1,75,1,77,1,79,1,81,1,83,1,85,1,87,1,89,1,91,1,93,1,95,1,97,1,99,1,101,1,103,1,105,1,107,1,109,1,111,1,113,1,115,1,117,1,119,1,121,1,123,1,125,1,127,1,129,1,131,1,133,1,135,1,137,1,139,1,141,1,143,1,145,1,147,1,149,1,151,1,153,1,155,1,157,1,159,1,161,1,163,1,165,1,167,1,169,1,171,1,173,1,175,1,177,1,179,1,181,1,183,1,185,1,187,1,189,1",
+				"GNU.sparse.name":      "sparse-posix-0.1",
+			},
+			Format: FormatPAX,
 		}, {
 			Name:     "sparse-posix-1.0",
 			Mode:     420,
@@ -105,6 +124,13 @@ func TestReader(t *testing.T) {
 			Gname:    "david",
 			Devmajor: 0,
 			Devminor: 0,
+			PAXRecords: map[string]string{
+				"GNU.sparse.major":    "1",
+				"GNU.sparse.minor":    "0",
+				"GNU.sparse.realsize": "200",
+				"GNU.sparse.name":     "sparse-posix-1.0",
+			},
+			Format: FormatPAX,
 		}, {
 			Name:     "end",
 			Mode:     420,
@@ -118,6 +144,7 @@ func TestReader(t *testing.T) {
 			Gname:    "david",
 			Devmajor: 0,
 			Devminor: 0,
+			Format:   FormatGNU,
 		}},
 		chksums: []string{
 			"6f53234398c2449fe67c1812d993012f",
@@ -186,6 +213,13 @@ func TestReader(t *testing.T) {
 			ChangeTime: time.Unix(1350244992, 23960108),
 			AccessTime: time.Unix(1350244992, 23960108),
 			Typeflag:   TypeReg,
+			PAXRecords: map[string]string{
+				"path":  "a/123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100",
+				"mtime": "1350244992.023960108",
+				"atime": "1350244992.023960108",
+				"ctime": "1350244992.023960108",
+			},
+			Format: FormatPAX,
 		}, {
 			Name:       "a/b",
 			Mode:       0777,
@@ -199,6 +233,13 @@ func TestReader(t *testing.T) {
 			AccessTime: time.Unix(1350266320, 910238425),
 			Typeflag:   TypeSymlink,
 			Linkname:   "123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100",
+			PAXRecords: map[string]string{
+				"linkpath": "123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100",
+				"mtime":    "1350266320.910238425",
+				"atime":    "1350266320.910238425",
+				"ctime":    "1350266320.910238425",
+			},
+			Format: FormatPAX,
 		}},
 	}, {
 		file: "testdata/pax-bad-hdr-file.tar",
@@ -218,11 +259,64 @@ func TestReader(t *testing.T) {
 			Typeflag: '0',
 			Uname:    "joetsai",
 			Gname:    "eng",
+			PAXRecords: map[string]string{
+				"size": "000000000000000000000999",
+			},
+			Format: FormatPAX,
 		}},
 		chksums: []string{
 			"0afb597b283fe61b5d4879669a350556",
 		},
 	}, {
+		file: "testdata/pax-records.tar",
+		headers: []*Header{{
+			Typeflag: TypeReg,
+			Name:     "file",
+			Uname:    strings.Repeat("long", 10),
+			ModTime:  time.Unix(0, 0),
+			PAXRecords: map[string]string{
+				"GOLANG.pkg": "tar",
+				"comment":    "Hello, 世界",
+				"uname":      strings.Repeat("long", 10),
+			},
+			Format: FormatPAX,
+		}},
+	}, {
+		file: "testdata/pax-global-records.tar",
+		headers: []*Header{{
+			Typeflag:   TypeXGlobalHeader,
+			Name:       "global1",
+			PAXRecords: map[string]string{"path": "global1", "mtime": "1500000000.0"},
+			Format:     FormatPAX,
+		}, {
+			Typeflag: TypeReg,
+			Name:     "file1",
+			ModTime:  time.Unix(0, 0),
+			Format:   FormatUSTAR,
+		}, {
+			Typeflag:   TypeReg,
+			Name:       "file2",
+			PAXRecords: map[string]string{"path": "file2"},
+			ModTime:    time.Unix(0, 0),
+			Format:     FormatPAX,
+		}, {
+			Typeflag:   TypeXGlobalHeader,
+			Name:       "GlobalHead.0.0",
+			PAXRecords: map[string]string{"path": ""},
+			Format:     FormatPAX,
+		}, {
+			Typeflag: TypeReg,
+			Name:     "file3",
+			ModTime:  time.Unix(0, 0),
+			Format:   FormatUSTAR,
+		}, {
+			Typeflag:   TypeReg,
+			Name:       "file4",
+			ModTime:    time.Unix(1400000000, 0),
+			PAXRecords: map[string]string{"mtime": "1400000000"},
+			Format:     FormatPAX,
+		}},
+	}, {
 		file: "testdata/nil-uid.tar", // golang.org/issue/5290
 		headers: []*Header{{
 			Name:     "P1050238.JPG.log",
@@ -237,6 +331,7 @@ func TestReader(t *testing.T) {
 			Gname:    "eyefi",
 			Devmajor: 0,
 			Devminor: 0,
+			Format:   FormatGNU,
 		}},
 	}, {
 		file: "testdata/xattrs.tar",
@@ -258,6 +353,15 @@ func TestReader(t *testing.T) {
 				// Interestingly, selinux encodes the terminating null inside the xattr
 				"security.selinux": "unconfined_u:object_r:default_t:s0\x00",
 			},
+			PAXRecords: map[string]string{
+				"mtime":                         "1386065770.44825232",
+				"atime":                         "1389782991.41987522",
+				"ctime":                         "1389782956.794414986",
+				"SCHILY.xattr.user.key":         "value",
+				"SCHILY.xattr.user.key2":        "value2",
+				"SCHILY.xattr.security.selinux": "unconfined_u:object_r:default_t:s0\x00",
+			},
+			Format: FormatPAX,
 		}, {
 			Name:       "small2.txt",
 			Mode:       0644,
@@ -273,6 +377,13 @@ func TestReader(t *testing.T) {
 			Xattrs: map[string]string{
 				"security.selinux": "unconfined_u:object_r:default_t:s0\x00",
 			},
+			PAXRecords: map[string]string{
+				"mtime": "1386065770.449252304",
+				"atime": "1389782991.41987522",
+				"ctime": "1386065770.449252304",
+				"SCHILY.xattr.security.selinux": "unconfined_u:object_r:default_t:s0\x00",
+			},
+			Format: FormatPAX,
 		}},
 	}, {
 		// Matches the behavior of GNU, BSD, and STAR tar utilities.
@@ -282,6 +393,7 @@ func TestReader(t *testing.T) {
 			Linkname: "GNU4/GNU4/long-linkpath-name",
 			ModTime:  time.Unix(0, 0),
 			Typeflag: '2',
+			Format:   FormatGNU,
 		}},
 	}, {
 		// GNU tar file with atime and ctime fields set.
@@ -300,6 +412,7 @@ func TestReader(t *testing.T) {
 			Gname:      "dsnet",
 			AccessTime: time.Unix(1441974501, 0),
 			ChangeTime: time.Unix(1441973436, 0),
+			Format:     FormatGNU,
 		}, {
 			Name:       "test2/foo",
 			Mode:       33188,
@@ -312,6 +425,7 @@ func TestReader(t *testing.T) {
 			Gname:      "dsnet",
 			AccessTime: time.Unix(1441974501, 0),
 			ChangeTime: time.Unix(1441973436, 0),
+			Format:     FormatGNU,
 		}, {
 			Name:       "test2/sparse",
 			Mode:       33188,
@@ -324,6 +438,7 @@ func TestReader(t *testing.T) {
 			Gname:      "dsnet",
 			AccessTime: time.Unix(1441991948, 0),
 			ChangeTime: time.Unix(1441973436, 0),
+			Format:     FormatGNU,
 		}},
 	}, {
 		// Matches the behavior of GNU and BSD tar utilities.
@@ -333,8 +448,76 @@ func TestReader(t *testing.T) {
 			Linkname: "PAX4/PAX4/long-linkpath-name",
 			ModTime:  time.Unix(0, 0),
 			Typeflag: '2',
+			PAXRecords: map[string]string{
+				"linkpath": "PAX4/PAX4/long-linkpath-name",
+			},
+			Format: FormatPAX,
+		}},
+	}, {
+		// Both BSD and GNU tar truncate long names at first NUL even
+		// if there is data following that NUL character.
+		// This is reasonable as GNU long names are C-strings.
+		file: "testdata/gnu-long-nul.tar",
+		headers: []*Header{{
+			Name:     "0123456789",
+			Mode:     0644,
+			Uid:      1000,
+			Gid:      1000,
+			ModTime:  time.Unix(1486082191, 0),
+			Typeflag: '0',
+			Uname:    "rawr",
+			Gname:    "dsnet",
+			Format:   FormatGNU,
 		}},
 	}, {
+		// This archive was generated by Writer but is readable by both
+		// GNU and BSD tar utilities.
+		// The archive generated by GNU is nearly byte-for-byte identical
+		// to the Go version except the Go version sets a negative Devminor
+		// just to force the GNU format.
+		file: "testdata/gnu-utf8.tar",
+		headers: []*Header{{
+			Name: "☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹",
+			Mode: 0644,
+			Uid:  1000, Gid: 1000,
+			ModTime:  time.Unix(0, 0),
+			Typeflag: '0',
+			Uname:    "☺",
+			Gname:    "⚹",
+			Format:   FormatGNU,
+		}},
+	}, {
+		// This archive was generated by Writer but is readable by both
+		// GNU and BSD tar utilities.
+		// The archive generated by GNU is nearly byte-for-byte identical
+		// to the Go version except the Go version sets a negative Devminor
+		// just to force the GNU format.
+		file: "testdata/gnu-not-utf8.tar",
+		headers: []*Header{{
+			Name:     "hi\x80\x81\x82\x83bye",
+			Mode:     0644,
+			Uid:      1000,
+			Gid:      1000,
+			ModTime:  time.Unix(0, 0),
+			Typeflag: '0',
+			Uname:    "rawr",
+			Gname:    "dsnet",
+			Format:   FormatGNU,
+		}},
+	}, {
+		// BSD tar v3.1.2 and GNU tar v1.27.1 both rejects PAX records
+		// with NULs in the key.
+		file: "testdata/pax-nul-xattrs.tar",
+		err:  ErrHeader,
+	}, {
+		// BSD tar v3.1.2 rejects a PAX path with NUL in the value, while
+		// GNU tar v1.27.1 simply truncates at first NUL.
+		// We emulate the behavior of BSD since it is strange doing NUL
+		// truncations since PAX records are length-prefix strings instead
+		// of NUL-terminated C-strings.
+		file: "testdata/pax-nul-path.tar",
+		err:  ErrHeader,
+	}, {
 		file: "testdata/neg-size.tar",
 		err:  ErrHeader,
 	}, {
@@ -346,483 +529,214 @@ func TestReader(t *testing.T) {
 	}, {
 		file: "testdata/issue12435.tar",
 		err:  ErrHeader,
-	}}
-
-	for i, v := range vectors {
-		f, err := os.Open(v.file)
-		if err != nil {
-			t.Errorf("file %s, test %d: unexpected error: %v", v.file, i, err)
-			continue
-		}
-		defer f.Close()
-
-		// Capture all headers and checksums.
-		var (
-			tr      = NewReader(f)
-			hdrs    []*Header
-			chksums []string
-			rdbuf   = make([]byte, 8)
-		)
-		for {
-			var hdr *Header
-			hdr, err = tr.Next()
-			if err != nil {
-				if err == io.EOF {
-					err = nil // Expected error
-				}
-				break
-			}
-			hdrs = append(hdrs, hdr)
-
-			if v.chksums == nil {
-				continue
-			}
-			h := md5.New()
-			_, err = io.CopyBuffer(h, tr, rdbuf) // Effectively an incremental read
-			if err != nil {
-				break
-			}
-			chksums = append(chksums, fmt.Sprintf("%x", h.Sum(nil)))
-		}
-
-		for j, hdr := range hdrs {
-			if j >= len(v.headers) {
-				t.Errorf("file %s, test %d, entry %d: unexpected header:\ngot %+v",
-					v.file, i, j, *hdr)
-				continue
-			}
-			if !reflect.DeepEqual(*hdr, *v.headers[j]) {
-				t.Errorf("file %s, test %d, entry %d: incorrect header:\ngot  %+v\nwant %+v",
-					v.file, i, j, *hdr, *v.headers[j])
-			}
-		}
-		if len(hdrs) != len(v.headers) {
-			t.Errorf("file %s, test %d: got %d headers, want %d headers",
-				v.file, i, len(hdrs), len(v.headers))
-		}
-
-		for j, sum := range chksums {
-			if j >= len(v.chksums) {
-				t.Errorf("file %s, test %d, entry %d: unexpected sum: got %s",
-					v.file, i, j, sum)
-				continue
-			}
-			if sum != v.chksums[j] {
-				t.Errorf("file %s, test %d, entry %d: incorrect checksum: got %s, want %s",
-					v.file, i, j, sum, v.chksums[j])
-			}
-		}
-
-		if err != v.err {
-			t.Errorf("file %s, test %d: unexpected error: got %v, want %v",
-				v.file, i, err, v.err)
-		}
-		f.Close()
-	}
-}
-
-func TestPartialRead(t *testing.T) {
-	f, err := os.Open("testdata/gnu.tar")
-	if err != nil {
-		t.Fatalf("Unexpected error: %v", err)
-	}
-	defer f.Close()
-
-	tr := NewReader(f)
-
-	// Read the first four bytes; Next() should skip the last byte.
-	hdr, err := tr.Next()
-	if err != nil || hdr == nil {
-		t.Fatalf("Didn't get first file: %v", err)
-	}
-	buf := make([]byte, 4)
-	if _, err := io.ReadFull(tr, buf); err != nil {
-		t.Fatalf("Unexpected error: %v", err)
-	}
-	if expected := []byte("Kilt"); !bytes.Equal(buf, expected) {
-		t.Errorf("Contents = %v, want %v", buf, expected)
-	}
-
-	// Second file
-	hdr, err = tr.Next()
-	if err != nil || hdr == nil {
-		t.Fatalf("Didn't get second file: %v", err)
-	}
-	buf = make([]byte, 6)
-	if _, err := io.ReadFull(tr, buf); err != nil {
-		t.Fatalf("Unexpected error: %v", err)
-	}
-	if expected := []byte("Google"); !bytes.Equal(buf, expected) {
-		t.Errorf("Contents = %v, want %v", buf, expected)
-	}
-}
-
-func TestSparseFileReader(t *testing.T) {
-	vectors := []struct {
-		realSize   int64         // Real size of the output file
-		sparseMap  []sparseEntry // Input sparse map
-		sparseData string        // Input compact data
-		expected   string        // Expected output data
-		err        error         // Expected error outcome
-	}{{
-		realSize: 8,
-		sparseMap: []sparseEntry{
-			{offset: 0, numBytes: 2},
-			{offset: 5, numBytes: 3},
-		},
-		sparseData: "abcde",
-		expected:   "ab\x00\x00\x00cde",
-	}, {
-		realSize: 10,
-		sparseMap: []sparseEntry{
-			{offset: 0, numBytes: 2},
-			{offset: 5, numBytes: 3},
-		},
-		sparseData: "abcde",
-		expected:   "ab\x00\x00\x00cde\x00\x00",
 	}, {
-		realSize: 8,
-		sparseMap: []sparseEntry{
-			{offset: 1, numBytes: 3},
-			{offset: 6, numBytes: 2},
-		},
-		sparseData: "abcde",
-		expected:   "\x00abc\x00\x00de",
-	}, {
-		realSize: 8,
-		sparseMap: []sparseEntry{
-			{offset: 1, numBytes: 3},
-			{offset: 6, numBytes: 0},
-			{offset: 6, numBytes: 0},
-			{offset: 6, numBytes: 2},
-		},
-		sparseData: "abcde",
-		expected:   "\x00abc\x00\x00de",
+		// Ensure that we can read back the original Header as written with
+		// a buggy pre-Go1.8 tar.Writer.
+		file: "testdata/invalid-go17.tar",
+		headers: []*Header{{
+			Name:    "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/foo",
+			Uid:     010000000,
+			ModTime: time.Unix(0, 0),
+		}},
 	}, {
-		realSize: 10,
-		sparseMap: []sparseEntry{
-			{offset: 1, numBytes: 3},
-			{offset: 6, numBytes: 2},
-		},
-		sparseData: "abcde",
-		expected:   "\x00abc\x00\x00de\x00\x00",
-	}, {
-		realSize: 10,
-		sparseMap: []sparseEntry{
-			{offset: 1, numBytes: 3},
-			{offset: 6, numBytes: 2},
-			{offset: 8, numBytes: 0},
-			{offset: 8, numBytes: 0},
-			{offset: 8, numBytes: 0},
-			{offset: 8, numBytes: 0},
-		},
-		sparseData: "abcde",
-		expected:   "\x00abc\x00\x00de\x00\x00",
-	}, {
-		realSize:   2,
-		sparseMap:  []sparseEntry{},
-		sparseData: "",
-		expected:   "\x00\x00",
-	}, {
-		realSize:  -2,
-		sparseMap: []sparseEntry{},
-		err:       ErrHeader,
-	}, {
-		realSize: -10,
-		sparseMap: []sparseEntry{
-			{offset: 1, numBytes: 3},
-			{offset: 6, numBytes: 2},
-		},
-		sparseData: "abcde",
-		err:        ErrHeader,
+		// USTAR archive with a regular entry with non-zero device numbers.
+		file: "testdata/ustar-file-devs.tar",
+		headers: []*Header{{
+			Name:     "file",
+			Mode:     0644,
+			Typeflag: '0',
+			ModTime:  time.Unix(0, 0),
+			Devmajor: 1,
+			Devminor: 1,
+			Format:   FormatUSTAR,
+		}},
 	}, {
-		realSize: 10,
-		sparseMap: []sparseEntry{
-			{offset: 1, numBytes: 3},
-			{offset: 6, numBytes: 5},
-		},
-		sparseData: "abcde",
-		err:        ErrHeader,
+		// Generated by Go, works on BSD tar v3.1.2 and GNU tar v.1.27.1.
+		file: "testdata/gnu-nil-sparse-data.tar",
+		headers: []*Header{{
+			Name:     "sparse.db",
+			Typeflag: TypeGNUSparse,
+			Size:     1000,
+			ModTime:  time.Unix(0, 0),
+			Format:   FormatGNU,
+		}},
 	}, {
-		realSize: 35,
-		sparseMap: []sparseEntry{
-			{offset: 1, numBytes: 3},
-			{offset: 6, numBytes: 5},
-		},
-		sparseData: "abcde",
-		err:        io.ErrUnexpectedEOF,
+		// Generated by Go, works on BSD tar v3.1.2 and GNU tar v.1.27.1.
+		file: "testdata/gnu-nil-sparse-hole.tar",
+		headers: []*Header{{
+			Name:     "sparse.db",
+			Typeflag: TypeGNUSparse,
+			Size:     1000,
+			ModTime:  time.Unix(0, 0),
+			Format:   FormatGNU,
+		}},
 	}, {
-		realSize: 35,
-		sparseMap: []sparseEntry{
-			{offset: 1, numBytes: 3},
-			{offset: 6, numBytes: -5},
-		},
-		sparseData: "abcde",
-		err:        ErrHeader,
+		// Generated by Go, works on BSD tar v3.1.2 and GNU tar v.1.27.1.
+		file: "testdata/pax-nil-sparse-data.tar",
+		headers: []*Header{{
+			Name:     "sparse.db",
+			Typeflag: TypeReg,
+			Size:     1000,
+			ModTime:  time.Unix(0, 0),
+			PAXRecords: map[string]string{
+				"size":                "1512",
+				"GNU.sparse.major":    "1",
+				"GNU.sparse.minor":    "0",
+				"GNU.sparse.realsize": "1000",
+				"GNU.sparse.name":     "sparse.db",
+			},
+			Format: FormatPAX,
+		}},
 	}, {
-		realSize: 35,
-		sparseMap: []sparseEntry{
-			{offset: math.MaxInt64, numBytes: 3},
-			{offset: 6, numBytes: -5},
-		},
-		sparseData: "abcde",
-		err:        ErrHeader,
+		// Generated by Go, works on BSD tar v3.1.2 and GNU tar v.1.27.1.
+		file: "testdata/pax-nil-sparse-hole.tar",
+		headers: []*Header{{
+			Name:     "sparse.db",
+			Typeflag: TypeReg,
+			Size:     1000,
+			ModTime:  time.Unix(0, 0),
+			PAXRecords: map[string]string{
+				"size":                "512",
+				"GNU.sparse.major":    "1",
+				"GNU.sparse.minor":    "0",
+				"GNU.sparse.realsize": "1000",
+				"GNU.sparse.name":     "sparse.db",
+			},
+			Format: FormatPAX,
+		}},
 	}, {
-		realSize: 10,
-		sparseMap: []sparseEntry{
-			{offset: 1, numBytes: 3},
-			{offset: 2, numBytes: 2},
-		},
-		sparseData: "abcde",
-		err:        ErrHeader,
+		file: "testdata/trailing-slash.tar",
+		headers: []*Header{{
+			Typeflag: TypeDir,
+			Name:     strings.Repeat("123456789/", 30),
+			ModTime:  time.Unix(0, 0),
+			PAXRecords: map[string]string{
+				"path": strings.Repeat("123456789/", 30),
+			},
+			Format: FormatPAX,
+		}},
 	}}
 
-	for i, v := range vectors {
-		r := bytes.NewReader([]byte(v.sparseData))
-		rfr := &regFileReader{r: r, nb: int64(len(v.sparseData))}
-
-		var (
-			sfr *sparseFileReader
-			err error
-			buf []byte
-		)
-
-		sfr, err = newSparseFileReader(rfr, v.sparseMap, v.realSize)
-		if err != nil {
-			goto fail
-		}
-		if sfr.numBytes() != int64(len(v.sparseData)) {
-			t.Errorf("test %d, numBytes() before reading: got %d, want %d", i, sfr.numBytes(), len(v.sparseData))
-		}
-		buf, err = ioutil.ReadAll(sfr)
-		if err != nil {
-			goto fail
-		}
-		if string(buf) != v.expected {
-			t.Errorf("test %d, ReadAll(): got %q, want %q", i, string(buf), v.expected)
-		}
-		if sfr.numBytes() != 0 {
-			t.Errorf("test %d, numBytes() after reading: got %d, want %d", i, sfr.numBytes(), 0)
-		}
+	for _, v := range vectors {
+		t.Run(path.Base(v.file), func(t *testing.T) {
+			f, err := os.Open(v.file)
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			defer f.Close()
 
-	fail:
-		if err != v.err {
-			t.Errorf("test %d, unexpected error: got %v, want %v", i, err, v.err)
-		}
-	}
-}
+			// Capture all headers and checksums.
+			var (
+				tr      = NewReader(f)
+				hdrs    []*Header
+				chksums []string
+				rdbuf   = make([]byte, 8)
+			)
+			for {
+				var hdr *Header
+				hdr, err = tr.Next()
+				if err != nil {
+					if err == io.EOF {
+						err = nil // Expected error
+					}
+					break
+				}
+				hdrs = append(hdrs, hdr)
 
-func TestReadOldGNUSparseMap(t *testing.T) {
-	const (
-		t00 = "00000000000\x0000000000000\x00"
-		t11 = "00000000001\x0000000000001\x00"
-		t12 = "00000000001\x0000000000002\x00"
-		t21 = "00000000002\x0000000000001\x00"
-	)
+				if v.chksums == nil {
+					continue
+				}
+				h := md5.New()
+				_, err = io.CopyBuffer(h, tr, rdbuf) // Effectively an incremental read
+				if err != nil {
+					break
+				}
+				chksums = append(chksums, fmt.Sprintf("%x", h.Sum(nil)))
+			}
 
-	mkBlk := func(size, sp0, sp1, sp2, sp3, ext string, format int) *block {
-		var blk block
-		copy(blk.GNU().RealSize(), size)
-		copy(blk.GNU().Sparse().Entry(0), sp0)
-		copy(blk.GNU().Sparse().Entry(1), sp1)
-		copy(blk.GNU().Sparse().Entry(2), sp2)
-		copy(blk.GNU().Sparse().Entry(3), sp3)
-		copy(blk.GNU().Sparse().IsExtended(), ext)
-		if format != formatUnknown {
-			blk.SetFormat(format)
-		}
-		return &blk
-	}
+			for i, hdr := range hdrs {
+				if i >= len(v.headers) {
+					t.Fatalf("entry %d: unexpected header:\ngot %+v", i, *hdr)
+					continue
+				}
+				if !reflect.DeepEqual(*hdr, *v.headers[i]) {
+					t.Fatalf("entry %d: incorrect header:\ngot  %+v\nwant %+v", i, *hdr, *v.headers[i])
+				}
+			}
+			if len(hdrs) != len(v.headers) {
+				t.Fatalf("got %d headers, want %d headers", len(hdrs), len(v.headers))
+			}
 
-	vectors := []struct {
-		data   string        // Input data
-		rawHdr *block        // Input raw header
-		want   []sparseEntry // Expected sparse entries to be outputted
-		err    error         // Expected error to be returned
-	}{
-		{"", mkBlk("", "", "", "", "", "", formatUnknown), nil, ErrHeader},
-		{"", mkBlk("1234", "fewa", "", "", "", "", formatGNU), nil, ErrHeader},
-		{"", mkBlk("0031", "", "", "", "", "", formatGNU), nil, nil},
-		{"", mkBlk("1234", t00, t11, "", "", "", formatGNU),
-			[]sparseEntry{{0, 0}, {1, 1}}, nil},
-		{"", mkBlk("1234", t11, t12, t21, t11, "", formatGNU),
-			[]sparseEntry{{1, 1}, {1, 2}, {2, 1}, {1, 1}}, nil},
-		{"", mkBlk("1234", t11, t12, t21, t11, "\x80", formatGNU),
-			[]sparseEntry{}, io.ErrUnexpectedEOF},
-		{t11 + t11,
-			mkBlk("1234", t11, t12, t21, t11, "\x80", formatGNU),
-			[]sparseEntry{}, io.ErrUnexpectedEOF},
-		{t11 + t21 + strings.Repeat("\x00", 512),
-			mkBlk("1234", t11, t12, t21, t11, "\x80", formatGNU),
-			[]sparseEntry{{1, 1}, {1, 2}, {2, 1}, {1, 1}, {1, 1}, {2, 1}}, nil},
-	}
+			for i, sum := range chksums {
+				if i >= len(v.chksums) {
+					t.Fatalf("entry %d: unexpected sum: got %s", i, sum)
+					continue
+				}
+				if sum != v.chksums[i] {
+					t.Fatalf("entry %d: incorrect checksum: got %s, want %s", i, sum, v.chksums[i])
+				}
+			}
 
-	for i, v := range vectors {
-		tr := Reader{r: strings.NewReader(v.data)}
-		hdr := new(Header)
-		got, err := tr.readOldGNUSparseMap(hdr, v.rawHdr)
-		if !reflect.DeepEqual(got, v.want) && !(len(got) == 0 && len(v.want) == 0) {
-			t.Errorf("test %d, readOldGNUSparseMap(...): got %v, want %v", i, got, v.want)
-		}
-		if err != v.err {
-			t.Errorf("test %d, unexpected error: got %v, want %v", i, err, v.err)
-		}
+			if err != v.err {
+				t.Fatalf("unexpected error: got %v, want %v", err, v.err)
+			}
+			f.Close()
+		})
 	}
 }
 
-func TestReadGNUSparseMap0x1(t *testing.T) {
-	const (
-		maxUint = ^uint(0)
-		maxInt  = int(maxUint >> 1)
-	)
-	var (
-		big1 = fmt.Sprintf("%d", int64(maxInt))
-		big2 = fmt.Sprintf("%d", (int64(maxInt)/2)+1)
-		big3 = fmt.Sprintf("%d", (int64(maxInt) / 3))
-	)
-
+func TestPartialRead(t *testing.T) {
+	type testCase struct {
+		cnt    int    // Number of bytes to read
+		output string // Expected value of string read
+	}
 	vectors := []struct {
-		extHdrs   map[string]string // Input data
-		sparseMap []sparseEntry     // Expected sparse entries to be outputted
-		err       error             // Expected errors that may be raised
+		file  string
+		cases []testCase
 	}{{
-		extHdrs: map[string]string{paxGNUSparseNumBlocks: "-4"},
-		err:     ErrHeader,
-	}, {
-		extHdrs: map[string]string{paxGNUSparseNumBlocks: "fee "},
-		err:     ErrHeader,
-	}, {
-		extHdrs: map[string]string{
-			paxGNUSparseNumBlocks: big1,
-			paxGNUSparseMap:       "0,5,10,5,20,5,30,5",
-		},
-		err: ErrHeader,
-	}, {
-		extHdrs: map[string]string{
-			paxGNUSparseNumBlocks: big2,
-			paxGNUSparseMap:       "0,5,10,5,20,5,30,5",
-		},
-		err: ErrHeader,
-	}, {
-		extHdrs: map[string]string{
-			paxGNUSparseNumBlocks: big3,
-			paxGNUSparseMap:       "0,5,10,5,20,5,30,5",
-		},
-		err: ErrHeader,
-	}, {
-		extHdrs: map[string]string{
-			paxGNUSparseNumBlocks: "4",
-			paxGNUSparseMap:       "0.5,5,10,5,20,5,30,5",
-		},
-		err: ErrHeader,
-	}, {
-		extHdrs: map[string]string{
-			paxGNUSparseNumBlocks: "4",
-			paxGNUSparseMap:       "0,5.5,10,5,20,5,30,5",
-		},
-		err: ErrHeader,
-	}, {
-		extHdrs: map[string]string{
-			paxGNUSparseNumBlocks: "4",
-			paxGNUSparseMap:       "0,fewafewa.5,fewafw,5,20,5,30,5",
+		file: "testdata/gnu.tar",
+		cases: []testCase{
+			{4, "Kilt"},
+			{6, "Google"},
 		},
-		err: ErrHeader,
 	}, {
-		extHdrs: map[string]string{
-			paxGNUSparseNumBlocks: "4",
-			paxGNUSparseMap:       "0,5,10,5,20,5,30,5",
+		file: "testdata/sparse-formats.tar",
+		cases: []testCase{
+			{2, "\x00G"},
+			{4, "\x00G\x00o"},
+			{6, "\x00G\x00o\x00G"},
+			{8, "\x00G\x00o\x00G\x00o"},
+			{4, "end\n"},
 		},
-		sparseMap: []sparseEntry{{0, 5}, {10, 5}, {20, 5}, {30, 5}},
 	}}
 
-	for i, v := range vectors {
-		sp, err := readGNUSparseMap0x1(v.extHdrs)
-		if !reflect.DeepEqual(sp, v.sparseMap) && !(len(sp) == 0 && len(v.sparseMap) == 0) {
-			t.Errorf("test %d, readGNUSparseMap0x1(...): got %v, want %v", i, sp, v.sparseMap)
-		}
-		if err != v.err {
-			t.Errorf("test %d, unexpected error: got %v, want %v", i, err, v.err)
-		}
-	}
-}
-
-func TestReadGNUSparseMap1x0(t *testing.T) {
-	sp := []sparseEntry{{1, 2}, {3, 4}}
-	for i := 0; i < 98; i++ {
-		sp = append(sp, sparseEntry{54321, 12345})
-	}
+	for _, v := range vectors {
+		t.Run(path.Base(v.file), func(t *testing.T) {
+			f, err := os.Open(v.file)
+			if err != nil {
+				t.Fatalf("Open() error: %v", err)
+			}
+			defer f.Close()
 
-	vectors := []struct {
-		input     string        // Input data
-		sparseMap []sparseEntry // Expected sparse entries to be outputted
-		cnt       int           // Expected number of bytes read
-		err       error         // Expected errors that may be raised
-	}{{
-		input: "",
-		cnt:   0,
-		err:   io.ErrUnexpectedEOF,
-	}, {
-		input: "ab",
-		cnt:   2,
-		err:   io.ErrUnexpectedEOF,
-	}, {
-		input: strings.Repeat("\x00", 512),
-		cnt:   512,
-		err:   io.ErrUnexpectedEOF,
-	}, {
-		input: strings.Repeat("\x00", 511) + "\n",
-		cnt:   512,
-		err:   ErrHeader,
-	}, {
-		input: strings.Repeat("\n", 512),
-		cnt:   512,
-		err:   ErrHeader,
-	}, {
-		input:     "0\n" + strings.Repeat("\x00", 510) + strings.Repeat("a", 512),
-		sparseMap: []sparseEntry{},
-		cnt:       512,
-	}, {
-		input:     strings.Repeat("0", 512) + "0\n" + strings.Repeat("\x00", 510),
-		sparseMap: []sparseEntry{},
-		cnt:       1024,
-	}, {
-		input:     strings.Repeat("0", 1024) + "1\n2\n3\n" + strings.Repeat("\x00", 506),
-		sparseMap: []sparseEntry{{2, 3}},
-		cnt:       1536,
-	}, {
-		input: strings.Repeat("0", 1024) + "1\n2\n\n" + strings.Repeat("\x00", 509),
-		cnt:   1536,
-		err:   ErrHeader,
-	}, {
-		input: strings.Repeat("0", 1024) + "1\n2\n" + strings.Repeat("\x00", 508),
-		cnt:   1536,
-		err:   io.ErrUnexpectedEOF,
-	}, {
-		input: "-1\n2\n\n" + strings.Repeat("\x00", 506),
-		cnt:   512,
-		err:   ErrHeader,
-	}, {
-		input: "1\nk\n2\n" + strings.Repeat("\x00", 506),
-		cnt:   512,
-		err:   ErrHeader,
-	}, {
-		input:     "100\n1\n2\n3\n4\n" + strings.Repeat("54321\n0000000000000012345\n", 98) + strings.Repeat("\x00", 512),
-		cnt:       2560,
-		sparseMap: sp,
-	}}
+			tr := NewReader(f)
+			for i, tc := range v.cases {
+				hdr, err := tr.Next()
+				if err != nil || hdr == nil {
+					t.Fatalf("entry %d, Next(): got %v, want %v", i, err, nil)
+				}
+				buf := make([]byte, tc.cnt)
+				if _, err := io.ReadFull(tr, buf); err != nil {
+					t.Fatalf("entry %d, ReadFull(): got %v, want %v", i, err, nil)
+				}
+				if string(buf) != tc.output {
+					t.Fatalf("entry %d, ReadFull(): got %q, want %q", i, string(buf), tc.output)
+				}
+			}
 
-	for i, v := range vectors {
-		r := strings.NewReader(v.input)
-		sp, err := readGNUSparseMap1x0(r)
-		if !reflect.DeepEqual(sp, v.sparseMap) && !(len(sp) == 0 && len(v.sparseMap) == 0) {
-			t.Errorf("test %d, readGNUSparseMap1x0(...): got %v, want %v", i, sp, v.sparseMap)
-		}
-		if numBytes := len(v.input) - r.Len(); numBytes != v.cnt {
-			t.Errorf("test %d, bytes read: got %v, want %v", i, numBytes, v.cnt)
-		}
-		if err != v.err {
-			t.Errorf("test %d, unexpected error: got %v, want %v", i, err, v.err)
-		}
+			if _, err := tr.Next(); err != io.EOF {
+				t.Fatalf("Next(): got %v, want EOF", err)
+			}
+		})
 	}
 }
 
@@ -950,17 +864,17 @@ func TestReadTruncation(t *testing.T) {
 				}
 				cnt++
 				if s2 == "manual" {
-					if _, err = io.Copy(ioutil.Discard, tr); err != nil {
+					if _, err = tr.writeTo(ioutil.Discard); err != nil {
 						break
 					}
 				}
 			}
 			if err != v.err {
-				t.Errorf("test %d, NewReader(%s(...)) with %s discard: got %v, want %v",
+				t.Errorf("test %d, NewReader(%s) with %s discard: got %v, want %v",
 					i, s1, s2, err, v.err)
 			}
 			if cnt != v.cnt {
-				t.Errorf("test %d, NewReader(%s(...)) with %s discard: got %d headers, want %d headers",
+				t.Errorf("test %d, NewReader(%s) with %s discard: got %d headers, want %d headers",
 					i, s1, s2, cnt, v.cnt)
 			}
 		}
@@ -1025,12 +939,18 @@ func TestMergePAX(t *testing.T) {
 			Name:    "a/b/c",
 			Uid:     1000,
 			ModTime: time.Unix(1350244992, 23960108),
+			PAXRecords: map[string]string{
+				"path":  "a/b/c",
+				"uid":   "1000",
+				"mtime": "1350244992.023960108",
+			},
 		},
 		ok: true,
 	}, {
 		in: map[string]string{
 			"gid": "gtgergergersagersgers",
 		},
+		ok: false,
 	}, {
 		in: map[string]string{
 			"missing":          "missing",
@@ -1038,6 +958,10 @@ func TestMergePAX(t *testing.T) {
 		},
 		want: &Header{
 			Xattrs: map[string]string{"key": "value"},
+			PAXRecords: map[string]string{
+				"missing":          "missing",
+				"SCHILY.xattr.key": "value",
+			},
 		},
 		ok: true,
 	}}
@@ -1070,7 +994,7 @@ func TestParsePAX(t *testing.T) {
 		{"13 key1=haha\n13 key2=nana\n13 key3=kaka\n",
 			map[string]string{"key1": "haha", "key2": "nana", "key3": "kaka"}, true},
 		{"13 key1=val1\n13 key2=val2\n8 key1=\n",
-			map[string]string{"key2": "val2"}, true},
+			map[string]string{"key1": "", "key2": "val2"}, true},
 		{"22 GNU.sparse.size=10\n26 GNU.sparse.numblocks=2\n" +
 			"23 GNU.sparse.offset=1\n25 GNU.sparse.numbytes=2\n" +
 			"23 GNU.sparse.offset=3\n25 GNU.sparse.numbytes=4\n",
@@ -1087,10 +1011,600 @@ func TestParsePAX(t *testing.T) {
 		r := strings.NewReader(v.in)
 		got, err := parsePAX(r)
 		if !reflect.DeepEqual(got, v.want) && !(len(got) == 0 && len(v.want) == 0) {
-			t.Errorf("test %d, parsePAX(...):\ngot  %v\nwant %v", i, got, v.want)
+			t.Errorf("test %d, parsePAX():\ngot  %v\nwant %v", i, got, v.want)
 		}
 		if ok := err == nil; ok != v.ok {
-			t.Errorf("test %d, parsePAX(...): got %v, want %v", i, ok, v.ok)
+			t.Errorf("test %d, parsePAX(): got %v, want %v", i, ok, v.ok)
+		}
+	}
+}
+
+func TestReadOldGNUSparseMap(t *testing.T) {
+	populateSparseMap := func(sa sparseArray, sps []string) []string {
+		for i := 0; len(sps) > 0 && i < sa.MaxEntries(); i++ {
+			copy(sa.Entry(i), sps[0])
+			sps = sps[1:]
+		}
+		if len(sps) > 0 {
+			copy(sa.IsExtended(), "\x80")
+		}
+		return sps
+	}
+
+	makeInput := func(format Format, size string, sps ...string) (out []byte) {
+		// Write the initial GNU header.
+		var blk block
+		gnu := blk.GNU()
+		sparse := gnu.Sparse()
+		copy(gnu.RealSize(), size)
+		sps = populateSparseMap(sparse, sps)
+		if format != FormatUnknown {
+			blk.SetFormat(format)
+		}
+		out = append(out, blk[:]...)
+
+		// Write extended sparse blocks.
+		for len(sps) > 0 {
+			var blk block
+			sps = populateSparseMap(blk.Sparse(), sps)
+			out = append(out, blk[:]...)
+		}
+		return out
+	}
+
+	makeSparseStrings := func(sp []sparseEntry) (out []string) {
+		var f formatter
+		for _, s := range sp {
+			var b [24]byte
+			f.formatNumeric(b[:12], s.Offset)
+			f.formatNumeric(b[12:], s.Length)
+			out = append(out, string(b[:]))
+		}
+		return out
+	}
+
+	vectors := []struct {
+		input    []byte
+		wantMap  sparseDatas
+		wantSize int64
+		wantErr  error
+	}{{
+		input:   makeInput(FormatUnknown, ""),
+		wantErr: ErrHeader,
+	}, {
+		input:    makeInput(FormatGNU, "1234", "fewa"),
+		wantSize: 01234,
+		wantErr:  ErrHeader,
+	}, {
+		input:    makeInput(FormatGNU, "0031"),
+		wantSize: 031,
+	}, {
+		input:   makeInput(FormatGNU, "80"),
+		wantErr: ErrHeader,
+	}, {
+		input: makeInput(FormatGNU, "1234",
+			makeSparseStrings(sparseDatas{{0, 0}, {1, 1}})...),
+		wantMap:  sparseDatas{{0, 0}, {1, 1}},
+		wantSize: 01234,
+	}, {
+		input: makeInput(FormatGNU, "1234",
+			append(makeSparseStrings(sparseDatas{{0, 0}, {1, 1}}), []string{"", "blah"}...)...),
+		wantMap:  sparseDatas{{0, 0}, {1, 1}},
+		wantSize: 01234,
+	}, {
+		input: makeInput(FormatGNU, "3333",
+			makeSparseStrings(sparseDatas{{0, 1}, {2, 1}, {4, 1}, {6, 1}})...),
+		wantMap:  sparseDatas{{0, 1}, {2, 1}, {4, 1}, {6, 1}},
+		wantSize: 03333,
+	}, {
+		input: makeInput(FormatGNU, "",
+			append(append(
+				makeSparseStrings(sparseDatas{{0, 1}, {2, 1}}),
+				[]string{"", ""}...),
+				makeSparseStrings(sparseDatas{{4, 1}, {6, 1}})...)...),
+		wantMap: sparseDatas{{0, 1}, {2, 1}, {4, 1}, {6, 1}},
+	}, {
+		input: makeInput(FormatGNU, "",
+			makeSparseStrings(sparseDatas{{0, 1}, {2, 1}, {4, 1}, {6, 1}, {8, 1}, {10, 1}})...)[:blockSize],
+		wantErr: io.ErrUnexpectedEOF,
+	}, {
+		input: makeInput(FormatGNU, "",
+			makeSparseStrings(sparseDatas{{0, 1}, {2, 1}, {4, 1}, {6, 1}, {8, 1}, {10, 1}})...)[:3*blockSize/2],
+		wantErr: io.ErrUnexpectedEOF,
+	}, {
+		input: makeInput(FormatGNU, "",
+			makeSparseStrings(sparseDatas{{0, 1}, {2, 1}, {4, 1}, {6, 1}, {8, 1}, {10, 1}})...),
+		wantMap: sparseDatas{{0, 1}, {2, 1}, {4, 1}, {6, 1}, {8, 1}, {10, 1}},
+	}, {
+		input: makeInput(FormatGNU, "",
+			makeSparseStrings(sparseDatas{{10 << 30, 512}, {20 << 30, 512}})...),
+		wantMap: sparseDatas{{10 << 30, 512}, {20 << 30, 512}},
+	}}
+
+	for i, v := range vectors {
+		var blk block
+		var hdr Header
+		v.input = v.input[copy(blk[:], v.input):]
+		tr := Reader{r: bytes.NewReader(v.input)}
+		got, err := tr.readOldGNUSparseMap(&hdr, &blk)
+		if !equalSparseEntries(got, v.wantMap) {
+			t.Errorf("test %d, readOldGNUSparseMap(): got %v, want %v", i, got, v.wantMap)
+		}
+		if err != v.wantErr {
+			t.Errorf("test %d, readOldGNUSparseMap() = %v, want %v", i, err, v.wantErr)
+		}
+		if hdr.Size != v.wantSize {
+			t.Errorf("test %d, Header.Size = %d, want %d", i, hdr.Size, v.wantSize)
+		}
+	}
+}
+
+func TestReadGNUSparsePAXHeaders(t *testing.T) {
+	padInput := func(s string) string {
+		return s + string(zeroBlock[:blockPadding(int64(len(s)))])
+	}
+
+	vectors := []struct {
+		inputData string
+		inputHdrs map[string]string
+		wantMap   sparseDatas
+		wantSize  int64
+		wantName  string
+		wantErr   error
+	}{{
+		inputHdrs: nil,
+		wantErr:   nil,
+	}, {
+		inputHdrs: map[string]string{
+			paxGNUSparseNumBlocks: strconv.FormatInt(math.MaxInt64, 10),
+			paxGNUSparseMap:       "0,1,2,3",
+		},
+		wantErr: ErrHeader,
+	}, {
+		inputHdrs: map[string]string{
+			paxGNUSparseNumBlocks: "4\x00",
+			paxGNUSparseMap:       "0,1,2,3",
+		},
+		wantErr: ErrHeader,
+	}, {
+		inputHdrs: map[string]string{
+			paxGNUSparseNumBlocks: "4",
+			paxGNUSparseMap:       "0,1,2,3",
+		},
+		wantErr: ErrHeader,
+	}, {
+		inputHdrs: map[string]string{
+			paxGNUSparseNumBlocks: "2",
+			paxGNUSparseMap:       "0,1,2,3",
+		},
+		wantMap: sparseDatas{{0, 1}, {2, 3}},
+	}, {
+		inputHdrs: map[string]string{
+			paxGNUSparseNumBlocks: "2",
+			paxGNUSparseMap:       "0, 1,2,3",
+		},
+		wantErr: ErrHeader,
+	}, {
+		inputHdrs: map[string]string{
+			paxGNUSparseNumBlocks: "2",
+			paxGNUSparseMap:       "0,1,02,3",
+			paxGNUSparseRealSize:  "4321",
+		},
+		wantMap:  sparseDatas{{0, 1}, {2, 3}},
+		wantSize: 4321,
+	}, {
+		inputHdrs: map[string]string{
+			paxGNUSparseNumBlocks: "2",
+			paxGNUSparseMap:       "0,one1,2,3",
+		},
+		wantErr: ErrHeader,
+	}, {
+		inputHdrs: map[string]string{
+			paxGNUSparseMajor:     "0",
+			paxGNUSparseMinor:     "0",
+			paxGNUSparseNumBlocks: "2",
+			paxGNUSparseMap:       "0,1,2,3",
+			paxGNUSparseSize:      "1234",
+			paxGNUSparseRealSize:  "4321",
+			paxGNUSparseName:      "realname",
+		},
+		wantMap:  sparseDatas{{0, 1}, {2, 3}},
+		wantSize: 1234,
+		wantName: "realname",
+	}, {
+		inputHdrs: map[string]string{
+			paxGNUSparseMajor:     "0",
+			paxGNUSparseMinor:     "0",
+			paxGNUSparseNumBlocks: "1",
+			paxGNUSparseMap:       "10737418240,512",
+			paxGNUSparseSize:      "10737418240",
+			paxGNUSparseName:      "realname",
+		},
+		wantMap:  sparseDatas{{10737418240, 512}},
+		wantSize: 10737418240,
+		wantName: "realname",
+	}, {
+		inputHdrs: map[string]string{
+			paxGNUSparseMajor:     "0",
+			paxGNUSparseMinor:     "0",
+			paxGNUSparseNumBlocks: "0",
+			paxGNUSparseMap:       "",
+		},
+		wantMap: sparseDatas{},
+	}, {
+		inputHdrs: map[string]string{
+			paxGNUSparseMajor:     "0",
+			paxGNUSparseMinor:     "1",
+			paxGNUSparseNumBlocks: "4",
+			paxGNUSparseMap:       "0,5,10,5,20,5,30,5",
+		},
+		wantMap: sparseDatas{{0, 5}, {10, 5}, {20, 5}, {30, 5}},
+	}, {
+		inputHdrs: map[string]string{
+			paxGNUSparseMajor:     "1",
+			paxGNUSparseMinor:     "0",
+			paxGNUSparseNumBlocks: "4",
+			paxGNUSparseMap:       "0,5,10,5,20,5,30,5",
+		},
+		wantErr: io.ErrUnexpectedEOF,
+	}, {
+		inputData: padInput("0\n"),
+		inputHdrs: map[string]string{paxGNUSparseMajor: "1", paxGNUSparseMinor: "0"},
+		wantMap:   sparseDatas{},
+	}, {
+		inputData: padInput("0\n")[:blockSize-1] + "#",
+		inputHdrs: map[string]string{paxGNUSparseMajor: "1", paxGNUSparseMinor: "0"},
+		wantMap:   sparseDatas{},
+	}, {
+		inputData: padInput("0"),
+		inputHdrs: map[string]string{paxGNUSparseMajor: "1", paxGNUSparseMinor: "0"},
+		wantErr:   io.ErrUnexpectedEOF,
+	}, {
+		inputData: padInput("ab\n"),
+		inputHdrs: map[string]string{paxGNUSparseMajor: "1", paxGNUSparseMinor: "0"},
+		wantErr:   ErrHeader,
+	}, {
+		inputData: padInput("1\n2\n3\n"),
+		inputHdrs: map[string]string{paxGNUSparseMajor: "1", paxGNUSparseMinor: "0"},
+		wantMap:   sparseDatas{{2, 3}},
+	}, {
+		inputData: padInput("1\n2\n"),
+		inputHdrs: map[string]string{paxGNUSparseMajor: "1", paxGNUSparseMinor: "0"},
+		wantErr:   io.ErrUnexpectedEOF,
+	}, {
+		inputData: padInput("1\n2\n\n"),
+		inputHdrs: map[string]string{paxGNUSparseMajor: "1", paxGNUSparseMinor: "0"},
+		wantErr:   ErrHeader,
+	}, {
+		inputData: string(zeroBlock[:]) + padInput("0\n"),
+		inputHdrs: map[string]string{paxGNUSparseMajor: "1", paxGNUSparseMinor: "0"},
+		wantErr:   ErrHeader,
+	}, {
+		inputData: strings.Repeat("0", blockSize) + padInput("1\n5\n1\n"),
+		inputHdrs: map[string]string{paxGNUSparseMajor: "1", paxGNUSparseMinor: "0"},
+		wantMap:   sparseDatas{{5, 1}},
+	}, {
+		inputData: padInput(fmt.Sprintf("%d\n", int64(math.MaxInt64))),
+		inputHdrs: map[string]string{paxGNUSparseMajor: "1", paxGNUSparseMinor: "0"},
+		wantErr:   ErrHeader,
+	}, {
+		inputData: padInput(strings.Repeat("0", 300) + "1\n" + strings.Repeat("0", 1000) + "5\n" + strings.Repeat("0", 800) + "2\n"),
+		inputHdrs: map[string]string{paxGNUSparseMajor: "1", paxGNUSparseMinor: "0"},
+		wantMap:   sparseDatas{{5, 2}},
+	}, {
+		inputData: padInput("2\n10737418240\n512\n21474836480\n512\n"),
+		inputHdrs: map[string]string{paxGNUSparseMajor: "1", paxGNUSparseMinor: "0"},
+		wantMap:   sparseDatas{{10737418240, 512}, {21474836480, 512}},
+	}, {
+		inputData: padInput("100\n" + func() string {
+			var ss []string
+			for i := 0; i < 100; i++ {
+				ss = append(ss, fmt.Sprintf("%d\n%d\n", int64(i)<<30, 512))
+			}
+			return strings.Join(ss, "")
+		}()),
+		inputHdrs: map[string]string{paxGNUSparseMajor: "1", paxGNUSparseMinor: "0"},
+		wantMap: func() (spd sparseDatas) {
+			for i := 0; i < 100; i++ {
+				spd = append(spd, sparseEntry{int64(i) << 30, 512})
+			}
+			return spd
+		}(),
+	}}
+
+	for i, v := range vectors {
+		var hdr Header
+		hdr.PAXRecords = v.inputHdrs
+		r := strings.NewReader(v.inputData + "#") // Add canary byte
+		tr := Reader{curr: &regFileReader{r, int64(r.Len())}}
+		got, err := tr.readGNUSparsePAXHeaders(&hdr)
+		if !equalSparseEntries(got, v.wantMap) {
+			t.Errorf("test %d, readGNUSparsePAXHeaders(): got %v, want %v", i, got, v.wantMap)
+		}
+		if err != v.wantErr {
+			t.Errorf("test %d, readGNUSparsePAXHeaders() = %v, want %v", i, err, v.wantErr)
+		}
+		if hdr.Size != v.wantSize {
+			t.Errorf("test %d, Header.Size = %d, want %d", i, hdr.Size, v.wantSize)
+		}
+		if hdr.Name != v.wantName {
+			t.Errorf("test %d, Header.Name = %s, want %s", i, hdr.Name, v.wantName)
+		}
+		if v.wantErr == nil && r.Len() == 0 {
+			t.Errorf("test %d, canary byte unexpectedly consumed", i)
+		}
+	}
+}
+
+// testNonEmptyReader wraps an io.Reader and ensures that
+// Read is never called with an empty buffer.
+type testNonEmptyReader struct{ io.Reader }
+
+func (r testNonEmptyReader) Read(b []byte) (int, error) {
+	if len(b) == 0 {
+		return 0, errors.New("unexpected empty Read call")
+	}
+	return r.Reader.Read(b)
+}
+
+func TestFileReader(t *testing.T) {
+	type (
+		testRead struct { // Read(cnt) == (wantStr, wantErr)
+			cnt     int
+			wantStr string
+			wantErr error
+		}
+		testWriteTo struct { // WriteTo(testFile{ops}) == (wantCnt, wantErr)
+			ops     fileOps
+			wantCnt int64
+			wantErr error
+		}
+		testRemaining struct { // LogicalRemaining() == wantLCnt, PhysicalRemaining() == wantPCnt
+			wantLCnt int64
+			wantPCnt int64
+		}
+		testFnc interface{} // testRead | testWriteTo | testRemaining
+	)
+
+	type (
+		makeReg struct {
+			str  string
+			size int64
+		}
+		makeSparse struct {
+			makeReg makeReg
+			spd     sparseDatas
+			size    int64
+		}
+		fileMaker interface{} // makeReg | makeSparse
+	)
+
+	vectors := []struct {
+		maker fileMaker
+		tests []testFnc
+	}{{
+		maker: makeReg{"", 0},
+		tests: []testFnc{
+			testRemaining{0, 0},
+			testRead{0, "", io.EOF},
+			testRead{1, "", io.EOF},
+			testWriteTo{nil, 0, nil},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeReg{"", 1},
+		tests: []testFnc{
+			testRemaining{1, 1},
+			testRead{5, "", io.ErrUnexpectedEOF},
+			testWriteTo{nil, 0, io.ErrUnexpectedEOF},
+			testRemaining{1, 1},
+		},
+	}, {
+		maker: makeReg{"hello", 5},
+		tests: []testFnc{
+			testRemaining{5, 5},
+			testRead{5, "hello", io.EOF},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeReg{"hello, world", 50},
+		tests: []testFnc{
+			testRemaining{50, 50},
+			testRead{7, "hello, ", nil},
+			testRemaining{43, 43},
+			testRead{5, "world", nil},
+			testRemaining{38, 38},
+			testWriteTo{nil, 0, io.ErrUnexpectedEOF},
+			testRead{1, "", io.ErrUnexpectedEOF},
+			testRemaining{38, 38},
+		},
+	}, {
+		maker: makeReg{"hello, world", 5},
+		tests: []testFnc{
+			testRemaining{5, 5},
+			testRead{0, "", nil},
+			testRead{4, "hell", nil},
+			testRemaining{1, 1},
+			testWriteTo{fileOps{"o"}, 1, nil},
+			testRemaining{0, 0},
+			testWriteTo{nil, 0, nil},
+			testRead{0, "", io.EOF},
+		},
+	}, {
+		maker: makeSparse{makeReg{"abcde", 5}, sparseDatas{{0, 2}, {5, 3}}, 8},
+		tests: []testFnc{
+			testRemaining{8, 5},
+			testRead{3, "ab\x00", nil},
+			testRead{10, "\x00\x00cde", io.EOF},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{"abcde", 5}, sparseDatas{{0, 2}, {5, 3}}, 8},
+		tests: []testFnc{
+			testRemaining{8, 5},
+			testWriteTo{fileOps{"ab", int64(3), "cde"}, 8, nil},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{"abcde", 5}, sparseDatas{{0, 2}, {5, 3}}, 10},
+		tests: []testFnc{
+			testRemaining{10, 5},
+			testRead{100, "ab\x00\x00\x00cde\x00\x00", io.EOF},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{"abc", 5}, sparseDatas{{0, 2}, {5, 3}}, 10},
+		tests: []testFnc{
+			testRemaining{10, 5},
+			testRead{100, "ab\x00\x00\x00c", io.ErrUnexpectedEOF},
+			testRemaining{4, 2},
+		},
+	}, {
+		maker: makeSparse{makeReg{"abcde", 5}, sparseDatas{{1, 3}, {6, 2}}, 8},
+		tests: []testFnc{
+			testRemaining{8, 5},
+			testRead{8, "\x00abc\x00\x00de", io.EOF},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{"abcde", 5}, sparseDatas{{1, 3}, {6, 0}, {6, 0}, {6, 2}}, 8},
+		tests: []testFnc{
+			testRemaining{8, 5},
+			testRead{8, "\x00abc\x00\x00de", io.EOF},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{"abcde", 5}, sparseDatas{{1, 3}, {6, 0}, {6, 0}, {6, 2}}, 8},
+		tests: []testFnc{
+			testRemaining{8, 5},
+			testWriteTo{fileOps{int64(1), "abc", int64(2), "de"}, 8, nil},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{"abcde", 5}, sparseDatas{{1, 3}, {6, 2}}, 10},
+		tests: []testFnc{
+			testRead{100, "\x00abc\x00\x00de\x00\x00", io.EOF},
+		},
+	}, {
+		maker: makeSparse{makeReg{"abcde", 5}, sparseDatas{{1, 3}, {6, 2}}, 10},
+		tests: []testFnc{
+			testWriteTo{fileOps{int64(1), "abc", int64(2), "de", int64(1), "\x00"}, 10, nil},
+		},
+	}, {
+		maker: makeSparse{makeReg{"abcde", 5}, sparseDatas{{1, 3}, {6, 2}, {8, 0}, {8, 0}, {8, 0}, {8, 0}}, 10},
+		tests: []testFnc{
+			testRead{100, "\x00abc\x00\x00de\x00\x00", io.EOF},
+		},
+	}, {
+		maker: makeSparse{makeReg{"", 0}, sparseDatas{}, 2},
+		tests: []testFnc{
+			testRead{100, "\x00\x00", io.EOF},
+		},
+	}, {
+		maker: makeSparse{makeReg{"", 8}, sparseDatas{{1, 3}, {6, 5}}, 15},
+		tests: []testFnc{
+			testRead{100, "\x00", io.ErrUnexpectedEOF},
+		},
+	}, {
+		maker: makeSparse{makeReg{"ab", 2}, sparseDatas{{1, 3}, {6, 5}}, 15},
+		tests: []testFnc{
+			testRead{100, "\x00ab", errMissData},
+		},
+	}, {
+		maker: makeSparse{makeReg{"ab", 8}, sparseDatas{{1, 3}, {6, 5}}, 15},
+		tests: []testFnc{
+			testRead{100, "\x00ab", io.ErrUnexpectedEOF},
+		},
+	}, {
+		maker: makeSparse{makeReg{"abc", 3}, sparseDatas{{1, 3}, {6, 5}}, 15},
+		tests: []testFnc{
+			testRead{100, "\x00abc\x00\x00", errMissData},
+		},
+	}, {
+		maker: makeSparse{makeReg{"abc", 8}, sparseDatas{{1, 3}, {6, 5}}, 15},
+		tests: []testFnc{
+			testRead{100, "\x00abc\x00\x00", io.ErrUnexpectedEOF},
+		},
+	}, {
+		maker: makeSparse{makeReg{"abcde", 5}, sparseDatas{{1, 3}, {6, 5}}, 15},
+		tests: []testFnc{
+			testRead{100, "\x00abc\x00\x00de", errMissData},
+		},
+	}, {
+		maker: makeSparse{makeReg{"abcde", 5}, sparseDatas{{1, 3}, {6, 5}}, 15},
+		tests: []testFnc{
+			testWriteTo{fileOps{int64(1), "abc", int64(2), "de"}, 8, errMissData},
+		},
+	}, {
+		maker: makeSparse{makeReg{"abcde", 8}, sparseDatas{{1, 3}, {6, 5}}, 15},
+		tests: []testFnc{
+			testRead{100, "\x00abc\x00\x00de", io.ErrUnexpectedEOF},
+		},
+	}, {
+		maker: makeSparse{makeReg{"abcdefghEXTRA", 13}, sparseDatas{{1, 3}, {6, 5}}, 15},
+		tests: []testFnc{
+			testRemaining{15, 13},
+			testRead{100, "\x00abc\x00\x00defgh\x00\x00\x00\x00", errUnrefData},
+			testWriteTo{nil, 0, errUnrefData},
+			testRemaining{0, 5},
+		},
+	}, {
+		maker: makeSparse{makeReg{"abcdefghEXTRA", 13}, sparseDatas{{1, 3}, {6, 5}}, 15},
+		tests: []testFnc{
+			testRemaining{15, 13},
+			testWriteTo{fileOps{int64(1), "abc", int64(2), "defgh", int64(4)}, 15, errUnrefData},
+			testRead{100, "", errUnrefData},
+			testRemaining{0, 5},
+		},
+	}}
+
+	for i, v := range vectors {
+		var fr fileReader
+		switch maker := v.maker.(type) {
+		case makeReg:
+			r := testNonEmptyReader{strings.NewReader(maker.str)}
+			fr = &regFileReader{r, maker.size}
+		case makeSparse:
+			if !validateSparseEntries(maker.spd, maker.size) {
+				t.Fatalf("invalid sparse map: %v", maker.spd)
+			}
+			sph := invertSparseEntries(maker.spd, maker.size)
+			r := testNonEmptyReader{strings.NewReader(maker.makeReg.str)}
+			fr = &regFileReader{r, maker.makeReg.size}
+			fr = &sparseFileReader{fr, sph, 0}
+		default:
+			t.Fatalf("test %d, unknown make operation: %T", i, maker)
+		}
+
+		for j, tf := range v.tests {
+			switch tf := tf.(type) {
+			case testRead:
+				b := make([]byte, tf.cnt)
+				n, err := fr.Read(b)
+				if got := string(b[:n]); got != tf.wantStr || err != tf.wantErr {
+					t.Errorf("test %d.%d, Read(%d):\ngot  (%q, %v)\nwant (%q, %v)", i, j, tf.cnt, got, err, tf.wantStr, tf.wantErr)
+				}
+			case testWriteTo:
+				f := &testFile{ops: tf.ops}
+				got, err := fr.WriteTo(f)
+				if _, ok := err.(testError); ok {
+					t.Errorf("test %d.%d, WriteTo(): %v", i, j, err)
+				} else if got != tf.wantCnt || err != tf.wantErr {
+					t.Errorf("test %d.%d, WriteTo() = (%d, %v), want (%d, %v)", i, j, got, err, tf.wantCnt, tf.wantErr)
+				}
+				if len(f.ops) > 0 {
+					t.Errorf("test %d.%d, expected %d more operations", i, j, len(f.ops))
+				}
+			case testRemaining:
+				if got := fr.LogicalRemaining(); got != tf.wantLCnt {
+					t.Errorf("test %d.%d, LogicalRemaining() = %d, want %d", i, j, got, tf.wantLCnt)
+				}
+				if got := fr.PhysicalRemaining(); got != tf.wantPCnt {
+					t.Errorf("test %d.%d, PhysicalRemaining() = %d, want %d", i, j, got, tf.wantPCnt)
+				}
+			default:
+				t.Fatalf("test %d.%d, unknown test operation: %T", i, j, tf)
+			}
 		}
 	}
 }
diff --git a/libgo/go/archive/tar/stat_atim.go b/libgo/go/archive/tar/stat_actime1.go
index cf9cc79..cf9cc79 100644
--- a/libgo/go/archive/tar/stat_atim.go
+++ b/libgo/go/archive/tar/stat_actime1.go
diff --git a/libgo/go/archive/tar/stat_atimespec.go b/libgo/go/archive/tar/stat_actime2.go
index 6f17dbe..6f17dbe 100644
--- a/libgo/go/archive/tar/stat_atimespec.go
+++ b/libgo/go/archive/tar/stat_actime2.go
diff --git a/libgo/go/archive/tar/stat_unix.go b/libgo/go/archive/tar/stat_unix.go
index cb843db..868105f 100644
--- a/libgo/go/archive/tar/stat_unix.go
+++ b/libgo/go/archive/tar/stat_unix.go
@@ -8,6 +8,10 @@ package tar
 
 import (
 	"os"
+	"os/user"
+	"runtime"
+	"strconv"
+	"sync"
 	"syscall"
 )
 
@@ -15,6 +19,10 @@ func init() {
 	sysStat = statUnix
 }
 
+// userMap and groupMap caches UID and GID lookups for performance reasons.
+// The downside is that renaming uname or gname by the OS never takes effect.
+var userMap, groupMap sync.Map // map[int]string
+
 func statUnix(fi os.FileInfo, h *Header) error {
 	sys, ok := fi.Sys().(*syscall.Stat_t)
 	if !ok {
@@ -22,11 +30,67 @@ func statUnix(fi os.FileInfo, h *Header) error {
 	}
 	h.Uid = int(sys.Uid)
 	h.Gid = int(sys.Gid)
-	// TODO(bradfitz): populate username & group.  os/user
-	// doesn't cache LookupId lookups, and lacks group
-	// lookup functions.
+
+	// Best effort at populating Uname and Gname.
+	// The os/user functions may fail for any number of reasons
+	// (not implemented on that platform, cgo not enabled, etc).
+	if u, ok := userMap.Load(h.Uid); ok {
+		h.Uname = u.(string)
+	} else if u, err := user.LookupId(strconv.Itoa(h.Uid)); err == nil {
+		h.Uname = u.Username
+		userMap.Store(h.Uid, h.Uname)
+	}
+	if g, ok := groupMap.Load(h.Gid); ok {
+		h.Gname = g.(string)
+	} else if g, err := user.LookupGroupId(strconv.Itoa(h.Gid)); err == nil {
+		h.Gname = g.Name
+		groupMap.Store(h.Gid, h.Gname)
+	}
+
 	h.AccessTime = statAtime(sys)
 	h.ChangeTime = statCtime(sys)
-	// TODO(bradfitz): major/minor device numbers?
+
+	// Best effort at populating Devmajor and Devminor.
+	if h.Typeflag == TypeChar || h.Typeflag == TypeBlock {
+		dev := uint64(sys.Rdev) // May be int32 or uint32
+		switch runtime.GOOS {
+		case "linux":
+			// Copied from golang.org/x/sys/unix/dev_linux.go.
+			major := uint32((dev & 0x00000000000fff00) >> 8)
+			major |= uint32((dev & 0xfffff00000000000) >> 32)
+			minor := uint32((dev & 0x00000000000000ff) >> 0)
+			minor |= uint32((dev & 0x00000ffffff00000) >> 12)
+			h.Devmajor, h.Devminor = int64(major), int64(minor)
+		case "darwin":
+			// Copied from golang.org/x/sys/unix/dev_darwin.go.
+			major := uint32((dev >> 24) & 0xff)
+			minor := uint32(dev & 0xffffff)
+			h.Devmajor, h.Devminor = int64(major), int64(minor)
+		case "dragonfly":
+			// Copied from golang.org/x/sys/unix/dev_dragonfly.go.
+			major := uint32((dev >> 8) & 0xff)
+			minor := uint32(dev & 0xffff00ff)
+			h.Devmajor, h.Devminor = int64(major), int64(minor)
+		case "freebsd":
+			// Copied from golang.org/x/sys/unix/dev_freebsd.go.
+			major := uint32((dev >> 8) & 0xff)
+			minor := uint32(dev & 0xffff00ff)
+			h.Devmajor, h.Devminor = int64(major), int64(minor)
+		case "netbsd":
+			// Copied from golang.org/x/sys/unix/dev_netbsd.go.
+			major := uint32((dev & 0x000fff00) >> 8)
+			minor := uint32((dev & 0x000000ff) >> 0)
+			minor |= uint32((dev & 0xfff00000) >> 12)
+			h.Devmajor, h.Devminor = int64(major), int64(minor)
+		case "openbsd":
+			// Copied from golang.org/x/sys/unix/dev_openbsd.go.
+			major := uint32((dev & 0x0000ff00) >> 8)
+			minor := uint32((dev & 0x000000ff) >> 0)
+			minor |= uint32((dev & 0xffff0000) >> 8)
+			h.Devmajor, h.Devminor = int64(major), int64(minor)
+		default:
+			// TODO: Implement solaris (see https://golang.org/issue/8106)
+		}
+	}
 	return nil
 }
diff --git a/libgo/go/archive/tar/strconv.go b/libgo/go/archive/tar/strconv.go
index bb5b51c..d144485 100644
--- a/libgo/go/archive/tar/strconv.go
+++ b/libgo/go/archive/tar/strconv.go
@@ -12,26 +12,34 @@ import (
 	"time"
 )
 
+// hasNUL reports whether the NUL character exists within s.
+func hasNUL(s string) bool {
+	return strings.IndexByte(s, 0) >= 0
+}
+
+// isASCII reports whether the input is an ASCII C-style string.
 func isASCII(s string) bool {
 	for _, c := range s {
-		if c >= 0x80 {
+		if c >= 0x80 || c == 0x00 {
 			return false
 		}
 	}
 	return true
 }
 
+// toASCII converts the input to an ASCII C-style string.
+// This a best effort conversion, so invalid characters are dropped.
 func toASCII(s string) string {
 	if isASCII(s) {
 		return s
 	}
-	var buf bytes.Buffer
+	b := make([]byte, 0, len(s))
 	for _, c := range s {
-		if c < 0x80 {
-			buf.WriteByte(byte(c))
+		if c < 0x80 && c != 0x00 {
+			b = append(b, byte(c))
 		}
 	}
-	return buf.String()
+	return string(b)
 }
 
 type parser struct {
@@ -45,23 +53,28 @@ type formatter struct {
 // parseString parses bytes as a NUL-terminated C-style string.
 // If a NUL byte is not found then the whole slice is returned as a string.
 func (*parser) parseString(b []byte) string {
-	n := 0
-	for n < len(b) && b[n] != 0 {
-		n++
+	if i := bytes.IndexByte(b, 0); i >= 0 {
+		return string(b[:i])
 	}
-	return string(b[0:n])
+	return string(b)
 }
 
-// Write s into b, terminating it with a NUL if there is room.
+// formatString copies s into b, NUL-terminating if possible.
 func (f *formatter) formatString(b []byte, s string) {
 	if len(s) > len(b) {
 		f.err = ErrFieldTooLong
-		return
 	}
-	ascii := toASCII(s)
-	copy(b, ascii)
-	if len(ascii) < len(b) {
-		b[len(ascii)] = 0
+	copy(b, s)
+	if len(s) < len(b) {
+		b[len(s)] = 0
+	}
+
+	// Some buggy readers treat regular files with a trailing slash
+	// in the V7 path field as a directory even though the full path
+	// recorded elsewhere (e.g., via PAX record) contains no trailing slash.
+	if len(s) > len(b) && b[len(b)-1] == '/' {
+		n := len(strings.TrimRight(s[:len(b)], "/"))
+		b[n] = 0 // Replace trailing slash with NUL terminator
 	}
 }
 
@@ -73,7 +86,7 @@ func (f *formatter) formatString(b []byte, s string) {
 // that the first byte can only be either 0x80 or 0xff. Thus, the first byte is
 // equivalent to the sign bit in two's complement form.
 func fitsInBase256(n int, x int64) bool {
-	var binBits = uint(n-1) * 8
+	binBits := uint(n-1) * 8
 	return n >= 9 || (x >= -1<<binBits && x < 1<<binBits)
 }
 
@@ -121,8 +134,14 @@ func (p *parser) parseNumeric(b []byte) int64 {
 	return p.parseOctal(b)
 }
 
-// Write x into b, as binary (GNUtar/star extension).
+// formatNumeric encodes x into b using base-8 (octal) encoding if possible.
+// Otherwise it will attempt to use base-256 (binary) encoding.
 func (f *formatter) formatNumeric(b []byte, x int64) {
+	if fitsInOctal(len(b), x) {
+		f.formatOctal(b, x)
+		return
+	}
+
 	if fitsInBase256(len(b), x) {
 		for i := len(b) - 1; i >= 0; i-- {
 			b[i] = byte(x)
@@ -155,6 +174,11 @@ func (p *parser) parseOctal(b []byte) int64 {
 }
 
 func (f *formatter) formatOctal(b []byte, x int64) {
+	if !fitsInOctal(len(b), x) {
+		x = 0 // Last resort, just write zero
+		f.err = ErrFieldTooLong
+	}
+
 	s := strconv.FormatInt(x, 8)
 	// Add leading zeros, but leave room for a NUL.
 	if n := len(b) - len(s) - 1; n > 0 {
@@ -163,6 +187,13 @@ func (f *formatter) formatOctal(b []byte, x int64) {
 	f.formatString(b, s)
 }
 
+// fitsInOctal reports whether the integer x fits in a field n-bytes long
+// using octal encoding with the appropriate NUL terminator.
+func fitsInOctal(n int, x int64) bool {
+	octBits := uint(n-1) * 3
+	return x >= 0 && (n >= 22 || x < 1<<octBits)
+}
+
 // parsePAXTime takes a string of the form %d.%d as described in the PAX
 // specification. Note that this implementation allows for negative timestamps,
 // which is allowed for by the PAX specification, but not always portable.
@@ -195,19 +226,32 @@ func parsePAXTime(s string) (time.Time, error) {
 	}
 	nsecs, _ := strconv.ParseInt(sn, 10, 64) // Must succeed
 	if len(ss) > 0 && ss[0] == '-' {
-		return time.Unix(secs, -1*int64(nsecs)), nil // Negative correction
+		return time.Unix(secs, -1*nsecs), nil // Negative correction
 	}
-	return time.Unix(secs, int64(nsecs)), nil
+	return time.Unix(secs, nsecs), nil
 }
 
-// TODO(dsnet): Implement formatPAXTime.
+// formatPAXTime converts ts into a time of the form %d.%d as described in the
+// PAX specification. This function is capable of negative timestamps.
+func formatPAXTime(ts time.Time) (s string) {
+	secs, nsecs := ts.Unix(), ts.Nanosecond()
+	if nsecs == 0 {
+		return strconv.FormatInt(secs, 10)
+	}
+
+	// If seconds is negative, then perform correction.
+	sign := ""
+	if secs < 0 {
+		sign = "-"             // Remember sign
+		secs = -(secs + 1)     // Add a second to secs
+		nsecs = -(nsecs - 1E9) // Take that second away from nsecs
+	}
+	return strings.TrimRight(fmt.Sprintf("%s%d.%09d", sign, secs, nsecs), "0")
+}
 
 // parsePAXRecord parses the input PAX record string into a key-value pair.
 // If parsing is successful, it will slice off the currently read record and
 // return the remainder as r.
-//
-// A PAX record is of the following form:
-//	"%d %s=%s\n" % (size, key, value)
 func parsePAXRecord(s string) (k, v, r string, err error) {
 	// The size field ends at the first space.
 	sp := strings.IndexByte(s, ' ')
@@ -232,21 +276,51 @@ func parsePAXRecord(s string) (k, v, r string, err error) {
 	if eq == -1 {
 		return "", "", s, ErrHeader
 	}
-	return rec[:eq], rec[eq+1:], rem, nil
+	k, v = rec[:eq], rec[eq+1:]
+
+	if !validPAXRecord(k, v) {
+		return "", "", s, ErrHeader
+	}
+	return k, v, rem, nil
 }
 
 // formatPAXRecord formats a single PAX record, prefixing it with the
 // appropriate length.
-func formatPAXRecord(k, v string) string {
+func formatPAXRecord(k, v string) (string, error) {
+	if !validPAXRecord(k, v) {
+		return "", ErrHeader
+	}
+
 	const padding = 3 // Extra padding for ' ', '=', and '\n'
 	size := len(k) + len(v) + padding
 	size += len(strconv.Itoa(size))
-	record := fmt.Sprintf("%d %s=%s\n", size, k, v)
+	record := strconv.Itoa(size) + " " + k + "=" + v + "\n"
 
 	// Final adjustment if adding size field increased the record size.
 	if len(record) != size {
 		size = len(record)
-		record = fmt.Sprintf("%d %s=%s\n", size, k, v)
+		record = strconv.Itoa(size) + " " + k + "=" + v + "\n"
+	}
+	return record, nil
+}
+
+// validPAXRecord reports whether the key-value pair is valid where each
+// record is formatted as:
+//	"%d %s=%s\n" % (size, key, value)
+//
+// Keys and values should be UTF-8, but the number of bad writers out there
+// forces us to be a more liberal.
+// Thus, we only reject all keys with NUL, and only reject NULs in values
+// for the PAX version of the USTAR string fields.
+// The key must not contain an '=' character.
+func validPAXRecord(k, v string) bool {
+	if k == "" || strings.IndexByte(k, '=') >= 0 {
+		return false
+	}
+	switch k {
+	case paxPath, paxLinkpath, paxUname, paxGname:
+		return !hasNUL(v)
+	default:
+		return !hasNUL(k)
 	}
-	return record
 }
diff --git a/libgo/go/archive/tar/strconv_test.go b/libgo/go/archive/tar/strconv_test.go
index beb7093..4cc388c 100644
--- a/libgo/go/archive/tar/strconv_test.go
+++ b/libgo/go/archive/tar/strconv_test.go
@@ -110,6 +110,25 @@ func TestFormatNumeric(t *testing.T) {
 		want string
 		ok   bool
 	}{
+		// Test base-8 (octal) encoded values.
+		{0, "0\x00", true},
+		{7, "7\x00", true},
+		{8, "\x80\x08", true},
+		{077, "77\x00", true},
+		{0100, "\x80\x00\x40", true},
+		{0, "0000000\x00", true},
+		{0123, "0000123\x00", true},
+		{07654321, "7654321\x00", true},
+		{07777777, "7777777\x00", true},
+		{010000000, "\x80\x00\x00\x00\x00\x20\x00\x00", true},
+		{0, "00000000000\x00", true},
+		{000001234567, "00001234567\x00", true},
+		{076543210321, "76543210321\x00", true},
+		{012345670123, "12345670123\x00", true},
+		{077777777777, "77777777777\x00", true},
+		{0100000000000, "\x80\x00\x00\x00\x00\x00\x00\x02\x00\x00\x00\x00", true},
+		{math.MaxInt64, "777777777777777777777\x00", true},
+
 		// Test base-256 (binary) encoded values.
 		{-1, "\xff", true},
 		{-1, "\xff\xff", true},
@@ -155,6 +174,45 @@ func TestFormatNumeric(t *testing.T) {
 	}
 }
 
+func TestFitsInOctal(t *testing.T) {
+	vectors := []struct {
+		input int64
+		width int
+		ok    bool
+	}{
+		{-1, 1, false},
+		{-1, 2, false},
+		{-1, 3, false},
+		{0, 1, true},
+		{0 + 1, 1, false},
+		{0, 2, true},
+		{07, 2, true},
+		{07 + 1, 2, false},
+		{0, 4, true},
+		{0777, 4, true},
+		{0777 + 1, 4, false},
+		{0, 8, true},
+		{07777777, 8, true},
+		{07777777 + 1, 8, false},
+		{0, 12, true},
+		{077777777777, 12, true},
+		{077777777777 + 1, 12, false},
+		{math.MaxInt64, 22, true},
+		{012345670123, 12, true},
+		{01564164, 12, true},
+		{-012345670123, 12, false},
+		{-01564164, 12, false},
+		{-1564164, 30, false},
+	}
+
+	for _, v := range vectors {
+		ok := fitsInOctal(v.width, v.input)
+		if ok != v.ok {
+			t.Errorf("checkOctal(%d, %d): got %v, want %v", v.input, v.width, ok, v.ok)
+		}
+	}
+}
+
 func TestParsePAXTime(t *testing.T) {
 	vectors := []struct {
 		in   string
@@ -236,6 +294,51 @@ func TestParsePAXTime(t *testing.T) {
 	}
 }
 
+func TestFormatPAXTime(t *testing.T) {
+	vectors := []struct {
+		sec, nsec int64
+		want      string
+	}{
+		{1350244992, 0, "1350244992"},
+		{1350244992, 300000000, "1350244992.3"},
+		{1350244992, 23960100, "1350244992.0239601"},
+		{1350244992, 23960108, "1350244992.023960108"},
+		{+1, +1E9 - 1E0, "1.999999999"},
+		{+1, +1E9 - 1E3, "1.999999"},
+		{+1, +1E9 - 1E6, "1.999"},
+		{+1, +0E0 - 0E0, "1"},
+		{+1, +1E6 - 0E0, "1.001"},
+		{+1, +1E3 - 0E0, "1.000001"},
+		{+1, +1E0 - 0E0, "1.000000001"},
+		{0, 1E9 - 1E0, "0.999999999"},
+		{0, 1E9 - 1E3, "0.999999"},
+		{0, 1E9 - 1E6, "0.999"},
+		{0, 0E0, "0"},
+		{0, 1E6 + 0E0, "0.001"},
+		{0, 1E3 + 0E0, "0.000001"},
+		{0, 1E0 + 0E0, "0.000000001"},
+		{-1, -1E9 + 1E0, "-1.999999999"},
+		{-1, -1E9 + 1E3, "-1.999999"},
+		{-1, -1E9 + 1E6, "-1.999"},
+		{-1, -0E0 + 0E0, "-1"},
+		{-1, -1E6 + 0E0, "-1.001"},
+		{-1, -1E3 + 0E0, "-1.000001"},
+		{-1, -1E0 + 0E0, "-1.000000001"},
+		{-1350244992, 0, "-1350244992"},
+		{-1350244992, -300000000, "-1350244992.3"},
+		{-1350244992, -23960100, "-1350244992.0239601"},
+		{-1350244992, -23960108, "-1350244992.023960108"},
+	}
+
+	for _, v := range vectors {
+		got := formatPAXTime(time.Unix(v.sec, v.nsec))
+		if got != v.want {
+			t.Errorf("formatPAXTime(%ds, %dns): got %q, want %q",
+				v.sec, v.nsec, got, v.want)
+		}
+	}
+}
+
 func TestParsePAXRecord(t *testing.T) {
 	medName := strings.Repeat("CD", 50)
 	longName := strings.Repeat("AB", 100)
@@ -256,7 +359,7 @@ func TestParsePAXRecord(t *testing.T) {
 		{"18 foo=b=\nar=\n==\x00\n", "", "foo", "b=\nar=\n==\x00", true},
 		{"27 foo=hello9 foo=ba\nworld\n", "", "foo", "hello9 foo=ba\nworld", true},
 		{"27 ☺☻☹=日a本b語ç\nmeow mix", "meow mix", "☺☻☹", "日a本b語ç", true},
-		{"17 \x00hello=\x00world\n", "", "\x00hello", "\x00world", true},
+		{"17 \x00hello=\x00world\n", "17 \x00hello=\x00world\n", "", "", false},
 		{"1 k=1\n", "1 k=1\n", "", "", false},
 		{"6 k~1\n", "6 k~1\n", "", "", false},
 		{"6_k=1\n", "6_k=1\n", "", "", false},
@@ -296,21 +399,33 @@ func TestFormatPAXRecord(t *testing.T) {
 		inKey string
 		inVal string
 		want  string
+		ok    bool
 	}{
-		{"k", "v", "6 k=v\n"},
-		{"path", "/etc/hosts", "19 path=/etc/hosts\n"},
-		{"path", longName, "210 path=" + longName + "\n"},
-		{"path", medName, "110 path=" + medName + "\n"},
-		{"foo", "ba", "9 foo=ba\n"},
-		{"foo", "bar", "11 foo=bar\n"},
-		{"foo", "b=\nar=\n==\x00", "18 foo=b=\nar=\n==\x00\n"},
-		{"foo", "hello9 foo=ba\nworld", "27 foo=hello9 foo=ba\nworld\n"},
-		{"☺☻☹", "日a本b語ç", "27 ☺☻☹=日a本b語ç\n"},
-		{"\x00hello", "\x00world", "17 \x00hello=\x00world\n"},
+		{"k", "v", "6 k=v\n", true},
+		{"path", "/etc/hosts", "19 path=/etc/hosts\n", true},
+		{"path", longName, "210 path=" + longName + "\n", true},
+		{"path", medName, "110 path=" + medName + "\n", true},
+		{"foo", "ba", "9 foo=ba\n", true},
+		{"foo", "bar", "11 foo=bar\n", true},
+		{"foo", "b=\nar=\n==\x00", "18 foo=b=\nar=\n==\x00\n", true},
+		{"foo", "hello9 foo=ba\nworld", "27 foo=hello9 foo=ba\nworld\n", true},
+		{"☺☻☹", "日a本b語ç", "27 ☺☻☹=日a本b語ç\n", true},
+		{"xhello", "\x00world", "17 xhello=\x00world\n", true},
+		{"path", "null\x00", "", false},
+		{"null\x00", "value", "", false},
+		{paxSchilyXattr + "key", "null\x00", "26 SCHILY.xattr.key=null\x00\n", true},
 	}
 
 	for _, v := range vectors {
-		got := formatPAXRecord(v.inKey, v.inVal)
+		got, err := formatPAXRecord(v.inKey, v.inVal)
+		ok := (err == nil)
+		if ok != v.ok {
+			if v.ok {
+				t.Errorf("formatPAXRecord(%q, %q): got format failure, want success", v.inKey, v.inVal)
+			} else {
+				t.Errorf("formatPAXRecord(%q, %q): got format success, want failure", v.inKey, v.inVal)
+			}
+		}
 		if got != v.want {
 			t.Errorf("formatPAXRecord(%q, %q): got %q, want %q",
 				v.inKey, v.inVal, got, v.want)
diff --git a/libgo/go/archive/tar/tar_test.go b/libgo/go/archive/tar/tar_test.go
index fb7a9dc..af80d6e 100644
--- a/libgo/go/archive/tar/tar_test.go
+++ b/libgo/go/archive/tar/tar_test.go
@@ -6,8 +6,12 @@ package tar
 
 import (
 	"bytes"
+	"errors"
+	"fmt"
 	"internal/testenv"
+	"io"
 	"io/ioutil"
+	"math"
 	"os"
 	"path"
 	"path/filepath"
@@ -17,6 +21,193 @@ import (
 	"time"
 )
 
+type testError struct{ error }
+
+type fileOps []interface{} // []T where T is (string | int64)
+
+// testFile is an io.ReadWriteSeeker where the IO operations performed
+// on it must match the list of operations in ops.
+type testFile struct {
+	ops fileOps
+	pos int64
+}
+
+func (f *testFile) Read(b []byte) (int, error) {
+	if len(b) == 0 {
+		return 0, nil
+	}
+	if len(f.ops) == 0 {
+		return 0, io.EOF
+	}
+	s, ok := f.ops[0].(string)
+	if !ok {
+		return 0, errors.New("unexpected Read operation")
+	}
+
+	n := copy(b, s)
+	if len(s) > n {
+		f.ops[0] = s[n:]
+	} else {
+		f.ops = f.ops[1:]
+	}
+	f.pos += int64(len(b))
+	return n, nil
+}
+
+func (f *testFile) Write(b []byte) (int, error) {
+	if len(b) == 0 {
+		return 0, nil
+	}
+	if len(f.ops) == 0 {
+		return 0, errors.New("unexpected Write operation")
+	}
+	s, ok := f.ops[0].(string)
+	if !ok {
+		return 0, errors.New("unexpected Write operation")
+	}
+
+	if !strings.HasPrefix(s, string(b)) {
+		return 0, testError{fmt.Errorf("got Write(%q), want Write(%q)", b, s)}
+	}
+	if len(s) > len(b) {
+		f.ops[0] = s[len(b):]
+	} else {
+		f.ops = f.ops[1:]
+	}
+	f.pos += int64(len(b))
+	return len(b), nil
+}
+
+func (f *testFile) Seek(pos int64, whence int) (int64, error) {
+	if pos == 0 && whence == io.SeekCurrent {
+		return f.pos, nil
+	}
+	if len(f.ops) == 0 {
+		return 0, errors.New("unexpected Seek operation")
+	}
+	s, ok := f.ops[0].(int64)
+	if !ok {
+		return 0, errors.New("unexpected Seek operation")
+	}
+
+	if s != pos || whence != io.SeekCurrent {
+		return 0, testError{fmt.Errorf("got Seek(%d, %d), want Seek(%d, %d)", pos, whence, s, io.SeekCurrent)}
+	}
+	f.pos += s
+	f.ops = f.ops[1:]
+	return f.pos, nil
+}
+
+func equalSparseEntries(x, y []sparseEntry) bool {
+	return (len(x) == 0 && len(y) == 0) || reflect.DeepEqual(x, y)
+}
+
+func TestSparseEntries(t *testing.T) {
+	vectors := []struct {
+		in   []sparseEntry
+		size int64
+
+		wantValid    bool          // Result of validateSparseEntries
+		wantAligned  []sparseEntry // Result of alignSparseEntries
+		wantInverted []sparseEntry // Result of invertSparseEntries
+	}{{
+		in: []sparseEntry{}, size: 0,
+		wantValid:    true,
+		wantInverted: []sparseEntry{{0, 0}},
+	}, {
+		in: []sparseEntry{}, size: 5000,
+		wantValid:    true,
+		wantInverted: []sparseEntry{{0, 5000}},
+	}, {
+		in: []sparseEntry{{0, 5000}}, size: 5000,
+		wantValid:    true,
+		wantAligned:  []sparseEntry{{0, 5000}},
+		wantInverted: []sparseEntry{{5000, 0}},
+	}, {
+		in: []sparseEntry{{1000, 4000}}, size: 5000,
+		wantValid:    true,
+		wantAligned:  []sparseEntry{{1024, 3976}},
+		wantInverted: []sparseEntry{{0, 1000}, {5000, 0}},
+	}, {
+		in: []sparseEntry{{0, 3000}}, size: 5000,
+		wantValid:    true,
+		wantAligned:  []sparseEntry{{0, 2560}},
+		wantInverted: []sparseEntry{{3000, 2000}},
+	}, {
+		in: []sparseEntry{{3000, 2000}}, size: 5000,
+		wantValid:    true,
+		wantAligned:  []sparseEntry{{3072, 1928}},
+		wantInverted: []sparseEntry{{0, 3000}, {5000, 0}},
+	}, {
+		in: []sparseEntry{{2000, 2000}}, size: 5000,
+		wantValid:    true,
+		wantAligned:  []sparseEntry{{2048, 1536}},
+		wantInverted: []sparseEntry{{0, 2000}, {4000, 1000}},
+	}, {
+		in: []sparseEntry{{0, 2000}, {8000, 2000}}, size: 10000,
+		wantValid:    true,
+		wantAligned:  []sparseEntry{{0, 1536}, {8192, 1808}},
+		wantInverted: []sparseEntry{{2000, 6000}, {10000, 0}},
+	}, {
+		in: []sparseEntry{{0, 2000}, {2000, 2000}, {4000, 0}, {4000, 3000}, {7000, 1000}, {8000, 0}, {8000, 2000}}, size: 10000,
+		wantValid:    true,
+		wantAligned:  []sparseEntry{{0, 1536}, {2048, 1536}, {4096, 2560}, {7168, 512}, {8192, 1808}},
+		wantInverted: []sparseEntry{{10000, 0}},
+	}, {
+		in: []sparseEntry{{0, 0}, {1000, 0}, {2000, 0}, {3000, 0}, {4000, 0}, {5000, 0}}, size: 5000,
+		wantValid:    true,
+		wantInverted: []sparseEntry{{0, 5000}},
+	}, {
+		in: []sparseEntry{{1, 0}}, size: 0,
+		wantValid: false,
+	}, {
+		in: []sparseEntry{{-1, 0}}, size: 100,
+		wantValid: false,
+	}, {
+		in: []sparseEntry{{0, -1}}, size: 100,
+		wantValid: false,
+	}, {
+		in: []sparseEntry{{0, 0}}, size: -100,
+		wantValid: false,
+	}, {
+		in: []sparseEntry{{math.MaxInt64, 3}, {6, -5}}, size: 35,
+		wantValid: false,
+	}, {
+		in: []sparseEntry{{1, 3}, {6, -5}}, size: 35,
+		wantValid: false,
+	}, {
+		in: []sparseEntry{{math.MaxInt64, math.MaxInt64}}, size: math.MaxInt64,
+		wantValid: false,
+	}, {
+		in: []sparseEntry{{3, 3}}, size: 5,
+		wantValid: false,
+	}, {
+		in: []sparseEntry{{2, 0}, {1, 0}, {0, 0}}, size: 3,
+		wantValid: false,
+	}, {
+		in: []sparseEntry{{1, 3}, {2, 2}}, size: 10,
+		wantValid: false,
+	}}
+
+	for i, v := range vectors {
+		gotValid := validateSparseEntries(v.in, v.size)
+		if gotValid != v.wantValid {
+			t.Errorf("test %d, validateSparseEntries() = %v, want %v", i, gotValid, v.wantValid)
+		}
+		if !v.wantValid {
+			continue
+		}
+		gotAligned := alignSparseEntries(append([]sparseEntry{}, v.in...), v.size)
+		if !equalSparseEntries(gotAligned, v.wantAligned) {
+			t.Errorf("test %d, alignSparseEntries():\ngot  %v\nwant %v", i, gotAligned, v.wantAligned)
+		}
+		gotInverted := invertSparseEntries(append([]sparseEntry{}, v.in...), v.size)
+		if !equalSparseEntries(gotInverted, v.wantInverted) {
+			t.Errorf("test %d, inverseSparseEntries():\ngot  %v\nwant %v", i, gotInverted, v.wantInverted)
+		}
+	}
+}
+
 func TestFileInfoHeader(t *testing.T) {
 	fi, err := os.Stat("testdata/small.txt")
 	if err != nil {
@@ -109,15 +300,12 @@ func TestRoundTrip(t *testing.T) {
 	var b bytes.Buffer
 	tw := NewWriter(&b)
 	hdr := &Header{
-		Name: "file.txt",
-		Uid:  1 << 21, // too big for 8 octal digits
-		Size: int64(len(data)),
-		// AddDate to strip monotonic clock reading,
-		// and Round to discard sub-second precision,
-		// both of which are not included in the tar header
-		// and would otherwise break the round-trip check
-		// below.
-		ModTime: time.Now().AddDate(0, 0, 0).Round(1 * time.Second),
+		Name:       "file.txt",
+		Uid:        1 << 21, // Too big for 8 octal digits
+		Size:       int64(len(data)),
+		ModTime:    time.Now().Round(time.Second),
+		PAXRecords: map[string]string{"uid": "2097152"},
+		Format:     FormatPAX,
 	}
 	if err := tw.WriteHeader(hdr); err != nil {
 		t.Fatalf("tw.WriteHeader: %v", err)
@@ -329,3 +517,338 @@ func TestHeaderRoundTrip(t *testing.T) {
 		}
 	}
 }
+
+func TestHeaderAllowedFormats(t *testing.T) {
+	vectors := []struct {
+		header  *Header           // Input header
+		paxHdrs map[string]string // Expected PAX headers that may be needed
+		formats Format            // Expected formats that can encode the header
+	}{{
+		header:  &Header{},
+		formats: FormatUSTAR | FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{Size: 077777777777},
+		formats: FormatUSTAR | FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{Size: 077777777777, Format: FormatUSTAR},
+		formats: FormatUSTAR,
+	}, {
+		header:  &Header{Size: 077777777777, Format: FormatPAX},
+		formats: FormatUSTAR | FormatPAX,
+	}, {
+		header:  &Header{Size: 077777777777, Format: FormatGNU},
+		formats: FormatGNU,
+	}, {
+		header:  &Header{Size: 077777777777 + 1},
+		paxHdrs: map[string]string{paxSize: "8589934592"},
+		formats: FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{Size: 077777777777 + 1, Format: FormatPAX},
+		paxHdrs: map[string]string{paxSize: "8589934592"},
+		formats: FormatPAX,
+	}, {
+		header:  &Header{Size: 077777777777 + 1, Format: FormatGNU},
+		paxHdrs: map[string]string{paxSize: "8589934592"},
+		formats: FormatGNU,
+	}, {
+		header:  &Header{Mode: 07777777},
+		formats: FormatUSTAR | FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{Mode: 07777777 + 1},
+		formats: FormatGNU,
+	}, {
+		header:  &Header{Devmajor: -123},
+		formats: FormatGNU,
+	}, {
+		header:  &Header{Devmajor: 1<<56 - 1},
+		formats: FormatGNU,
+	}, {
+		header:  &Header{Devmajor: 1 << 56},
+		formats: FormatUnknown,
+	}, {
+		header:  &Header{Devmajor: -1 << 56},
+		formats: FormatGNU,
+	}, {
+		header:  &Header{Devmajor: -1<<56 - 1},
+		formats: FormatUnknown,
+	}, {
+		header:  &Header{Name: "用戶名", Devmajor: -1 << 56},
+		formats: FormatGNU,
+	}, {
+		header:  &Header{Size: math.MaxInt64},
+		paxHdrs: map[string]string{paxSize: "9223372036854775807"},
+		formats: FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{Size: math.MinInt64},
+		paxHdrs: map[string]string{paxSize: "-9223372036854775808"},
+		formats: FormatUnknown,
+	}, {
+		header:  &Header{Uname: "0123456789abcdef0123456789abcdef"},
+		formats: FormatUSTAR | FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{Uname: "0123456789abcdef0123456789abcdefx"},
+		paxHdrs: map[string]string{paxUname: "0123456789abcdef0123456789abcdefx"},
+		formats: FormatPAX,
+	}, {
+		header:  &Header{Name: "foobar"},
+		formats: FormatUSTAR | FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{Name: strings.Repeat("a", nameSize)},
+		formats: FormatUSTAR | FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{Name: strings.Repeat("a", nameSize+1)},
+		paxHdrs: map[string]string{paxPath: strings.Repeat("a", nameSize+1)},
+		formats: FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{Linkname: "用戶名"},
+		paxHdrs: map[string]string{paxLinkpath: "用戶名"},
+		formats: FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{Linkname: strings.Repeat("用戶名\x00", nameSize)},
+		paxHdrs: map[string]string{paxLinkpath: strings.Repeat("用戶名\x00", nameSize)},
+		formats: FormatUnknown,
+	}, {
+		header:  &Header{Linkname: "\x00hello"},
+		paxHdrs: map[string]string{paxLinkpath: "\x00hello"},
+		formats: FormatUnknown,
+	}, {
+		header:  &Header{Uid: 07777777},
+		formats: FormatUSTAR | FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{Uid: 07777777 + 1},
+		paxHdrs: map[string]string{paxUid: "2097152"},
+		formats: FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{Xattrs: nil},
+		formats: FormatUSTAR | FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{Xattrs: map[string]string{"foo": "bar"}},
+		paxHdrs: map[string]string{paxSchilyXattr + "foo": "bar"},
+		formats: FormatPAX,
+	}, {
+		header:  &Header{Xattrs: map[string]string{"foo": "bar"}, Format: FormatGNU},
+		paxHdrs: map[string]string{paxSchilyXattr + "foo": "bar"},
+		formats: FormatUnknown,
+	}, {
+		header:  &Header{Xattrs: map[string]string{"用戶名": "\x00hello"}},
+		paxHdrs: map[string]string{paxSchilyXattr + "用戶名": "\x00hello"},
+		formats: FormatPAX,
+	}, {
+		header:  &Header{Xattrs: map[string]string{"foo=bar": "baz"}},
+		formats: FormatUnknown,
+	}, {
+		header:  &Header{Xattrs: map[string]string{"foo": ""}},
+		paxHdrs: map[string]string{paxSchilyXattr + "foo": ""},
+		formats: FormatPAX,
+	}, {
+		header:  &Header{ModTime: time.Unix(0, 0)},
+		formats: FormatUSTAR | FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{ModTime: time.Unix(077777777777, 0)},
+		formats: FormatUSTAR | FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{ModTime: time.Unix(077777777777+1, 0)},
+		paxHdrs: map[string]string{paxMtime: "8589934592"},
+		formats: FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{ModTime: time.Unix(math.MaxInt64, 0)},
+		paxHdrs: map[string]string{paxMtime: "9223372036854775807"},
+		formats: FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{ModTime: time.Unix(math.MaxInt64, 0), Format: FormatUSTAR},
+		paxHdrs: map[string]string{paxMtime: "9223372036854775807"},
+		formats: FormatUnknown,
+	}, {
+		header:  &Header{ModTime: time.Unix(-1, 0)},
+		paxHdrs: map[string]string{paxMtime: "-1"},
+		formats: FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{ModTime: time.Unix(1, 500)},
+		paxHdrs: map[string]string{paxMtime: "1.0000005"},
+		formats: FormatUSTAR | FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{ModTime: time.Unix(1, 0)},
+		formats: FormatUSTAR | FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{ModTime: time.Unix(1, 0), Format: FormatPAX},
+		formats: FormatUSTAR | FormatPAX,
+	}, {
+		header:  &Header{ModTime: time.Unix(1, 500), Format: FormatUSTAR},
+		paxHdrs: map[string]string{paxMtime: "1.0000005"},
+		formats: FormatUSTAR,
+	}, {
+		header:  &Header{ModTime: time.Unix(1, 500), Format: FormatPAX},
+		paxHdrs: map[string]string{paxMtime: "1.0000005"},
+		formats: FormatPAX,
+	}, {
+		header:  &Header{ModTime: time.Unix(1, 500), Format: FormatGNU},
+		paxHdrs: map[string]string{paxMtime: "1.0000005"},
+		formats: FormatGNU,
+	}, {
+		header:  &Header{ModTime: time.Unix(-1, 500)},
+		paxHdrs: map[string]string{paxMtime: "-0.9999995"},
+		formats: FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{ModTime: time.Unix(-1, 500), Format: FormatGNU},
+		paxHdrs: map[string]string{paxMtime: "-0.9999995"},
+		formats: FormatGNU,
+	}, {
+		header:  &Header{AccessTime: time.Unix(0, 0)},
+		paxHdrs: map[string]string{paxAtime: "0"},
+		formats: FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{AccessTime: time.Unix(0, 0), Format: FormatUSTAR},
+		paxHdrs: map[string]string{paxAtime: "0"},
+		formats: FormatUnknown,
+	}, {
+		header:  &Header{AccessTime: time.Unix(0, 0), Format: FormatPAX},
+		paxHdrs: map[string]string{paxAtime: "0"},
+		formats: FormatPAX,
+	}, {
+		header:  &Header{AccessTime: time.Unix(0, 0), Format: FormatGNU},
+		paxHdrs: map[string]string{paxAtime: "0"},
+		formats: FormatGNU,
+	}, {
+		header:  &Header{AccessTime: time.Unix(-123, 0)},
+		paxHdrs: map[string]string{paxAtime: "-123"},
+		formats: FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{AccessTime: time.Unix(-123, 0), Format: FormatPAX},
+		paxHdrs: map[string]string{paxAtime: "-123"},
+		formats: FormatPAX,
+	}, {
+		header:  &Header{ChangeTime: time.Unix(123, 456)},
+		paxHdrs: map[string]string{paxCtime: "123.000000456"},
+		formats: FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{ChangeTime: time.Unix(123, 456), Format: FormatUSTAR},
+		paxHdrs: map[string]string{paxCtime: "123.000000456"},
+		formats: FormatUnknown,
+	}, {
+		header:  &Header{ChangeTime: time.Unix(123, 456), Format: FormatGNU},
+		paxHdrs: map[string]string{paxCtime: "123.000000456"},
+		formats: FormatGNU,
+	}, {
+		header:  &Header{ChangeTime: time.Unix(123, 456), Format: FormatPAX},
+		paxHdrs: map[string]string{paxCtime: "123.000000456"},
+		formats: FormatPAX,
+	}, {
+		header:  &Header{Name: "foo/", Typeflag: TypeDir},
+		formats: FormatUSTAR | FormatPAX | FormatGNU,
+	}, {
+		header:  &Header{Name: "foo/", Typeflag: TypeReg},
+		formats: FormatUnknown,
+	}, {
+		header:  &Header{Name: "foo/", Typeflag: TypeSymlink},
+		formats: FormatUSTAR | FormatPAX | FormatGNU,
+	}}
+
+	for i, v := range vectors {
+		formats, paxHdrs, err := v.header.allowedFormats()
+		if formats != v.formats {
+			t.Errorf("test %d, allowedFormats(): got %v, want %v", i, formats, v.formats)
+		}
+		if formats&FormatPAX > 0 && !reflect.DeepEqual(paxHdrs, v.paxHdrs) && !(len(paxHdrs) == 0 && len(v.paxHdrs) == 0) {
+			t.Errorf("test %d, allowedFormats():\ngot  %v\nwant %s", i, paxHdrs, v.paxHdrs)
+		}
+		if (formats != FormatUnknown) && (err != nil) {
+			t.Errorf("test %d, unexpected error: %v", i, err)
+		}
+		if (formats == FormatUnknown) && (err == nil) {
+			t.Errorf("test %d, got nil-error, want non-nil error", i)
+		}
+	}
+}
+
+func Benchmark(b *testing.B) {
+	type file struct {
+		hdr  *Header
+		body []byte
+	}
+
+	vectors := []struct {
+		label string
+		files []file
+	}{{
+		"USTAR",
+		[]file{{
+			&Header{Name: "bar", Mode: 0640, Size: int64(3)},
+			[]byte("foo"),
+		}, {
+			&Header{Name: "world", Mode: 0640, Size: int64(5)},
+			[]byte("hello"),
+		}},
+	}, {
+		"GNU",
+		[]file{{
+			&Header{Name: "bar", Mode: 0640, Size: int64(3), Devmajor: -1},
+			[]byte("foo"),
+		}, {
+			&Header{Name: "world", Mode: 0640, Size: int64(5), Devmajor: -1},
+			[]byte("hello"),
+		}},
+	}, {
+		"PAX",
+		[]file{{
+			&Header{Name: "bar", Mode: 0640, Size: int64(3), Xattrs: map[string]string{"foo": "bar"}},
+			[]byte("foo"),
+		}, {
+			&Header{Name: "world", Mode: 0640, Size: int64(5), Xattrs: map[string]string{"foo": "bar"}},
+			[]byte("hello"),
+		}},
+	}}
+
+	b.Run("Writer", func(b *testing.B) {
+		for _, v := range vectors {
+			b.Run(v.label, func(b *testing.B) {
+				b.ReportAllocs()
+				for i := 0; i < b.N; i++ {
+					// Writing to ioutil.Discard because we want to
+					// test purely the writer code and not bring in disk performance into this.
+					tw := NewWriter(ioutil.Discard)
+					for _, file := range v.files {
+						if err := tw.WriteHeader(file.hdr); err != nil {
+							b.Errorf("unexpected WriteHeader error: %v", err)
+						}
+						if _, err := tw.Write(file.body); err != nil {
+							b.Errorf("unexpected Write error: %v", err)
+						}
+					}
+					if err := tw.Close(); err != nil {
+						b.Errorf("unexpected Close error: %v", err)
+					}
+				}
+			})
+		}
+	})
+
+	b.Run("Reader", func(b *testing.B) {
+		for _, v := range vectors {
+			var buf bytes.Buffer
+			var r bytes.Reader
+
+			// Write the archive to a byte buffer.
+			tw := NewWriter(&buf)
+			for _, file := range v.files {
+				tw.WriteHeader(file.hdr)
+				tw.Write(file.body)
+			}
+			tw.Close()
+			b.Run(v.label, func(b *testing.B) {
+				b.ReportAllocs()
+				// Read from the byte buffer.
+				for i := 0; i < b.N; i++ {
+					r.Reset(buf.Bytes())
+					tr := NewReader(&r)
+					if _, err := tr.Next(); err != nil {
+						b.Errorf("unexpected Next error: %v", err)
+					}
+					if _, err := io.Copy(ioutil.Discard, tr); err != nil {
+						b.Errorf("unexpected Copy error : %v", err)
+					}
+				}
+			})
+		}
+	})
+
+}
diff --git a/libgo/go/archive/tar/testdata/gnu-long-nul.tar b/libgo/go/archive/tar/testdata/gnu-long-nul.tar
new file mode 100644
index 0000000..28bc812
--- /dev/null
+++ b/libgo/go/archive/tar/testdata/gnu-long-nul.tar
diff --git a/libgo/go/archive/tar/testdata/gnu-nil-sparse-data.tar b/libgo/go/archive/tar/testdata/gnu-nil-sparse-data.tar
new file mode 100644
index 0000000..df1aa83
--- /dev/null
+++ b/libgo/go/archive/tar/testdata/gnu-nil-sparse-data.tar
diff --git a/libgo/go/archive/tar/testdata/gnu-nil-sparse-hole.tar b/libgo/go/archive/tar/testdata/gnu-nil-sparse-hole.tar
new file mode 100644
index 0000000..496abfe
--- /dev/null
+++ b/libgo/go/archive/tar/testdata/gnu-nil-sparse-hole.tar
diff --git a/libgo/go/archive/tar/testdata/gnu-not-utf8.tar b/libgo/go/archive/tar/testdata/gnu-not-utf8.tar
new file mode 100644
index 0000000..81cec67d
--- /dev/null
+++ b/libgo/go/archive/tar/testdata/gnu-not-utf8.tar
diff --git a/libgo/go/archive/tar/testdata/gnu-sparse-big.tar b/libgo/go/archive/tar/testdata/gnu-sparse-big.tar
new file mode 100644
index 0000000..1a5cfc9
--- /dev/null
+++ b/libgo/go/archive/tar/testdata/gnu-sparse-big.tar
diff --git a/libgo/go/archive/tar/testdata/gnu-utf8.tar b/libgo/go/archive/tar/testdata/gnu-utf8.tar
new file mode 100644
index 0000000..2c9c807
--- /dev/null
+++ b/libgo/go/archive/tar/testdata/gnu-utf8.tar
diff --git a/libgo/go/archive/tar/testdata/invalid-go17.tar b/libgo/go/archive/tar/testdata/invalid-go17.tar
new file mode 100644
index 0000000..58f2488
--- /dev/null
+++ b/libgo/go/archive/tar/testdata/invalid-go17.tar
diff --git a/libgo/go/archive/tar/testdata/pax-global-records.tar b/libgo/go/archive/tar/testdata/pax-global-records.tar
new file mode 100644
index 0000000..3d3d241
--- /dev/null
+++ b/libgo/go/archive/tar/testdata/pax-global-records.tar
diff --git a/libgo/go/archive/tar/testdata/pax-nil-sparse-data.tar b/libgo/go/archive/tar/testdata/pax-nil-sparse-data.tar
new file mode 100644
index 0000000..e59bd94
--- /dev/null
+++ b/libgo/go/archive/tar/testdata/pax-nil-sparse-data.tar
diff --git a/libgo/go/archive/tar/testdata/ustar.issue12594.tar b/libgo/go/archive/tar/testdata/pax-nil-sparse-hole.tar
index c7910ae..b44327b 100644
--- a/libgo/go/archive/tar/testdata/ustar.issue12594.tar
+++ b/libgo/go/archive/tar/testdata/pax-nil-sparse-hole.tar
diff --git a/libgo/go/archive/tar/testdata/pax-nul-path.tar b/libgo/go/archive/tar/testdata/pax-nul-path.tar
new file mode 100644
index 0000000..c78f82b
--- /dev/null
+++ b/libgo/go/archive/tar/testdata/pax-nul-path.tar
diff --git a/libgo/go/archive/tar/testdata/pax-nul-xattrs.tar b/libgo/go/archive/tar/testdata/pax-nul-xattrs.tar
new file mode 100644
index 0000000..881f517
--- /dev/null
+++ b/libgo/go/archive/tar/testdata/pax-nul-xattrs.tar
diff --git a/libgo/go/archive/tar/testdata/pax-pos-size-file.tar b/libgo/go/archive/tar/testdata/pax-pos-size-file.tar
index aed9a8a..ea5ccf9 100644
--- a/libgo/go/archive/tar/testdata/pax-pos-size-file.tar
+++ b/libgo/go/archive/tar/testdata/pax-pos-size-file.tar
diff --git a/libgo/go/archive/tar/testdata/pax-records.tar b/libgo/go/archive/tar/testdata/pax-records.tar
new file mode 100644
index 0000000..276c211
--- /dev/null
+++ b/libgo/go/archive/tar/testdata/pax-records.tar
diff --git a/libgo/go/archive/tar/testdata/pax-sparse-big.tar b/libgo/go/archive/tar/testdata/pax-sparse-big.tar
new file mode 100644
index 0000000..65d1f8e
--- /dev/null
+++ b/libgo/go/archive/tar/testdata/pax-sparse-big.tar
diff --git a/libgo/go/archive/tar/testdata/trailing-slash.tar b/libgo/go/archive/tar/testdata/trailing-slash.tar
new file mode 100644
index 0000000..bf1b2ec
--- /dev/null
+++ b/libgo/go/archive/tar/testdata/trailing-slash.tar
diff --git a/libgo/go/archive/tar/testdata/ustar-file-devs.tar b/libgo/go/archive/tar/testdata/ustar-file-devs.tar
new file mode 100644
index 0000000..146e25b
--- /dev/null
+++ b/libgo/go/archive/tar/testdata/ustar-file-devs.tar
diff --git a/libgo/go/archive/tar/testdata/writer-big-long.tar b/libgo/go/archive/tar/testdata/writer-big-long.tar
index 52bd748..09fc5dd 100644
--- a/libgo/go/archive/tar/testdata/writer-big-long.tar
+++ b/libgo/go/archive/tar/testdata/writer-big-long.tar
diff --git a/libgo/go/archive/tar/testdata/writer-big.tar b/libgo/go/archive/tar/testdata/writer-big.tar
index 753e883..435dcbc 100644
--- a/libgo/go/archive/tar/testdata/writer-big.tar
+++ b/libgo/go/archive/tar/testdata/writer-big.tar
diff --git a/libgo/go/archive/tar/writer.go b/libgo/go/archive/tar/writer.go
index c51c243..97d23f8 100644
--- a/libgo/go/archive/tar/writer.go
+++ b/libgo/go/archive/tar/writer.go
@@ -4,255 +4,391 @@
 
 package tar
 
-// TODO(dsymonds):
-// - catch more errors (no first header, etc.)
-
 import (
 	"bytes"
-	"errors"
 	"fmt"
 	"io"
 	"path"
 	"sort"
-	"strconv"
 	"strings"
 	"time"
 )
 
-var (
-	ErrWriteTooLong    = errors.New("archive/tar: write too long")
-	ErrFieldTooLong    = errors.New("archive/tar: header field too long")
-	ErrWriteAfterClose = errors.New("archive/tar: write after close")
-	errInvalidHeader   = errors.New("archive/tar: header field too long or contains invalid values")
-)
-
-// A Writer provides sequential writing of a tar archive in POSIX.1 format.
-// A tar archive consists of a sequence of files.
-// Call WriteHeader to begin a new file, and then call Write to supply that file's data,
-// writing at most hdr.Size bytes in total.
+// Writer provides sequential writing of a tar archive.
+// Write.WriteHeader begins a new file with the provided Header,
+// and then Writer can be treated as an io.Writer to supply that file's data.
 type Writer struct {
-	w          io.Writer
-	err        error
-	nb         int64 // number of unwritten bytes for current file entry
-	pad        int64 // amount of padding to write after current file entry
-	closed     bool
-	usedBinary bool  // whether the binary numeric field extension was used
-	preferPax  bool  // use PAX header instead of binary numeric header
-	hdrBuff    block // buffer to use in writeHeader when writing a regular header
-	paxHdrBuff block // buffer to use in writeHeader when writing a PAX header
+	w    io.Writer
+	pad  int64      // Amount of padding to write after current file entry
+	curr fileWriter // Writer for current file entry
+	hdr  Header     // Shallow copy of Header that is safe for mutations
+	blk  block      // Buffer to use as temporary local storage
+
+	// err is a persistent error.
+	// It is only the responsibility of every exported method of Writer to
+	// ensure that this error is sticky.
+	err error
 }
 
 // NewWriter creates a new Writer writing to w.
-func NewWriter(w io.Writer) *Writer { return &Writer{w: w} }
+func NewWriter(w io.Writer) *Writer {
+	return &Writer{w: w, curr: &regFileWriter{w, 0}}
+}
+
+type fileWriter interface {
+	io.Writer
+	fileState
 
-// Flush finishes writing the current file (optional).
+	ReadFrom(io.Reader) (int64, error)
+}
+
+// Flush finishes writing the current file's block padding.
+// The current file must be fully written before Flush can be called.
+//
+// This is unnecessary as the next call to WriteHeader or Close
+// will implicitly flush out the file's padding.
 func (tw *Writer) Flush() error {
-	if tw.nb > 0 {
-		tw.err = fmt.Errorf("archive/tar: missed writing %d bytes", tw.nb)
+	if tw.err != nil {
 		return tw.err
 	}
-
-	n := tw.nb + tw.pad
-	for n > 0 && tw.err == nil {
-		nr := n
-		if nr > blockSize {
-			nr = blockSize
-		}
-		var nw int
-		nw, tw.err = tw.w.Write(zeroBlock[0:nr])
-		n -= int64(nw)
+	if nb := tw.curr.LogicalRemaining(); nb > 0 {
+		return fmt.Errorf("archive/tar: missed writing %d bytes", nb)
+	}
+	if _, tw.err = tw.w.Write(zeroBlock[:tw.pad]); tw.err != nil {
+		return tw.err
 	}
-	tw.nb = 0
 	tw.pad = 0
-	return tw.err
+	return nil
 }
 
-var (
-	minTime = time.Unix(0, 0)
-	// There is room for 11 octal digits (33 bits) of mtime.
-	maxTime = minTime.Add((1<<33 - 1) * time.Second)
-)
-
 // WriteHeader writes hdr and prepares to accept the file's contents.
-// WriteHeader calls Flush if it is not the first header.
-// Calling after a Close will return ErrWriteAfterClose.
+// The Header.Size determines how many bytes can be written for the next file.
+// If the current file is not fully written, then this returns an error.
+// This implicitly flushes any padding necessary before writing the header.
 func (tw *Writer) WriteHeader(hdr *Header) error {
-	return tw.writeHeader(hdr, true)
-}
-
-// WriteHeader writes hdr and prepares to accept the file's contents.
-// WriteHeader calls Flush if it is not the first header.
-// Calling after a Close will return ErrWriteAfterClose.
-// As this method is called internally by writePax header to allow it to
-// suppress writing the pax header.
-func (tw *Writer) writeHeader(hdr *Header, allowPax bool) error {
-	if tw.closed {
-		return ErrWriteAfterClose
+	if err := tw.Flush(); err != nil {
+		return err
 	}
-	if tw.err == nil {
-		tw.Flush()
+	tw.hdr = *hdr // Shallow copy of Header
+
+	// Round ModTime and ignore AccessTime and ChangeTime unless
+	// the format is explicitly chosen.
+	// This ensures nominal usage of WriteHeader (without specifying the format)
+	// does not always result in the PAX format being chosen, which
+	// causes a 1KiB increase to every header.
+	if tw.hdr.Format == FormatUnknown {
+		tw.hdr.ModTime = tw.hdr.ModTime.Round(time.Second)
+		tw.hdr.AccessTime = time.Time{}
+		tw.hdr.ChangeTime = time.Time{}
 	}
-	if tw.err != nil {
+
+	allowedFormats, paxHdrs, err := tw.hdr.allowedFormats()
+	switch {
+	case allowedFormats.has(FormatUSTAR):
+		tw.err = tw.writeUSTARHeader(&tw.hdr)
 		return tw.err
+	case allowedFormats.has(FormatPAX):
+		tw.err = tw.writePAXHeader(&tw.hdr, paxHdrs)
+		return tw.err
+	case allowedFormats.has(FormatGNU):
+		tw.err = tw.writeGNUHeader(&tw.hdr)
+		return tw.err
+	default:
+		return err // Non-fatal error
 	}
+}
 
-	// a map to hold pax header records, if any are needed
-	paxHeaders := make(map[string]string)
-
-	// TODO(dsnet): we might want to use PAX headers for
-	// subsecond time resolution, but for now let's just capture
-	// too long fields or non ascii characters
-
-	// We need to select which scratch buffer to use carefully,
-	// since this method is called recursively to write PAX headers.
-	// If allowPax is true, this is the non-recursive call, and we will use hdrBuff.
-	// If allowPax is false, we are being called by writePAXHeader, and hdrBuff is
-	// already being used by the non-recursive call, so we must use paxHdrBuff.
-	header := &tw.hdrBuff
-	if !allowPax {
-		header = &tw.paxHdrBuff
+func (tw *Writer) writeUSTARHeader(hdr *Header) error {
+	// Check if we can use USTAR prefix/suffix splitting.
+	var namePrefix string
+	if prefix, suffix, ok := splitUSTARPath(hdr.Name); ok {
+		namePrefix, hdr.Name = prefix, suffix
 	}
-	copy(header[:], zeroBlock[:])
 
-	// Wrappers around formatter that automatically sets paxHeaders if the
-	// argument extends beyond the capacity of the input byte slice.
+	// Pack the main header.
 	var f formatter
-	var formatString = func(b []byte, s string, paxKeyword string) {
-		needsPaxHeader := paxKeyword != paxNone && len(s) > len(b) || !isASCII(s)
-		if needsPaxHeader {
-			paxHeaders[paxKeyword] = s
+	blk := tw.templateV7Plus(hdr, f.formatString, f.formatOctal)
+	f.formatString(blk.USTAR().Prefix(), namePrefix)
+	blk.SetFormat(FormatUSTAR)
+	if f.err != nil {
+		return f.err // Should never happen since header is validated
+	}
+	return tw.writeRawHeader(blk, hdr.Size, hdr.Typeflag)
+}
+
+func (tw *Writer) writePAXHeader(hdr *Header, paxHdrs map[string]string) error {
+	realName, realSize := hdr.Name, hdr.Size
+
+	// TODO(dsnet): Re-enable this when adding sparse support.
+	// See https://golang.org/issue/22735
+	/*
+		// Handle sparse files.
+		var spd sparseDatas
+		var spb []byte
+		if len(hdr.SparseHoles) > 0 {
+			sph := append([]sparseEntry{}, hdr.SparseHoles...) // Copy sparse map
+			sph = alignSparseEntries(sph, hdr.Size)
+			spd = invertSparseEntries(sph, hdr.Size)
+
+			// Format the sparse map.
+			hdr.Size = 0 // Replace with encoded size
+			spb = append(strconv.AppendInt(spb, int64(len(spd)), 10), '\n')
+			for _, s := range spd {
+				hdr.Size += s.Length
+				spb = append(strconv.AppendInt(spb, s.Offset, 10), '\n')
+				spb = append(strconv.AppendInt(spb, s.Length, 10), '\n')
+			}
+			pad := blockPadding(int64(len(spb)))
+			spb = append(spb, zeroBlock[:pad]...)
+			hdr.Size += int64(len(spb)) // Accounts for encoded sparse map
+
+			// Add and modify appropriate PAX records.
+			dir, file := path.Split(realName)
+			hdr.Name = path.Join(dir, "GNUSparseFile.0", file)
+			paxHdrs[paxGNUSparseMajor] = "1"
+			paxHdrs[paxGNUSparseMinor] = "0"
+			paxHdrs[paxGNUSparseName] = realName
+			paxHdrs[paxGNUSparseRealSize] = strconv.FormatInt(realSize, 10)
+			paxHdrs[paxSize] = strconv.FormatInt(hdr.Size, 10)
+			delete(paxHdrs, paxPath) // Recorded by paxGNUSparseName
+		}
+	*/
+	_ = realSize
+
+	// Write PAX records to the output.
+	isGlobal := hdr.Typeflag == TypeXGlobalHeader
+	if len(paxHdrs) > 0 || isGlobal {
+		// Sort keys for deterministic ordering.
+		var keys []string
+		for k := range paxHdrs {
+			keys = append(keys, k)
+		}
+		sort.Strings(keys)
+
+		// Write each record to a buffer.
+		var buf bytes.Buffer
+		for _, k := range keys {
+			rec, err := formatPAXRecord(k, paxHdrs[k])
+			if err != nil {
+				return err
+			}
+			buf.WriteString(rec)
 		}
 
-		// Write string in a best-effort manner to satisfy readers that expect
-		// the field to be non-empty.
-		s = toASCII(s)
-		if len(s) > len(b) {
-			s = s[:len(b)]
+		// Write the extended header file.
+		var name string
+		var flag byte
+		if isGlobal {
+			name = realName
+			if name == "" {
+				name = "GlobalHead.0.0"
+			}
+			flag = TypeXGlobalHeader
+		} else {
+			dir, file := path.Split(realName)
+			name = path.Join(dir, "PaxHeaders.0", file)
+			flag = TypeXHeader
 		}
-		f.formatString(b, s) // Should never error
-	}
-	var formatNumeric = func(b []byte, x int64, paxKeyword string) {
-		// Try octal first.
-		s := strconv.FormatInt(x, 8)
-		if len(s) < len(b) {
-			f.formatOctal(b, x)
-			return
+		data := buf.String()
+		if err := tw.writeRawFile(name, data, flag, FormatPAX); err != nil || isGlobal {
+			return err // Global headers return here
 		}
+	}
 
-		// If it is too long for octal, and PAX is preferred, use a PAX header.
-		if paxKeyword != paxNone && tw.preferPax {
-			f.formatOctal(b, 0)
-			s := strconv.FormatInt(x, 10)
-			paxHeaders[paxKeyword] = s
-			return
+	// Pack the main header.
+	var f formatter // Ignore errors since they are expected
+	fmtStr := func(b []byte, s string) { f.formatString(b, toASCII(s)) }
+	blk := tw.templateV7Plus(hdr, fmtStr, f.formatOctal)
+	blk.SetFormat(FormatPAX)
+	if err := tw.writeRawHeader(blk, hdr.Size, hdr.Typeflag); err != nil {
+		return err
+	}
+
+	// TODO(dsnet): Re-enable this when adding sparse support.
+	// See https://golang.org/issue/22735
+	/*
+		// Write the sparse map and setup the sparse writer if necessary.
+		if len(spd) > 0 {
+			// Use tw.curr since the sparse map is accounted for in hdr.Size.
+			if _, err := tw.curr.Write(spb); err != nil {
+				return err
+			}
+			tw.curr = &sparseFileWriter{tw.curr, spd, 0}
 		}
+	*/
+	return nil
+}
 
-		tw.usedBinary = true
-		f.formatNumeric(b, x)
+func (tw *Writer) writeGNUHeader(hdr *Header) error {
+	// Use long-link files if Name or Linkname exceeds the field size.
+	const longName = "././@LongLink"
+	if len(hdr.Name) > nameSize {
+		data := hdr.Name + "\x00"
+		if err := tw.writeRawFile(longName, data, TypeGNULongName, FormatGNU); err != nil {
+			return err
+		}
+	}
+	if len(hdr.Linkname) > nameSize {
+		data := hdr.Linkname + "\x00"
+		if err := tw.writeRawFile(longName, data, TypeGNULongLink, FormatGNU); err != nil {
+			return err
+		}
 	}
 
-	// Handle out of range ModTime carefully.
-	var modTime int64
-	if !hdr.ModTime.Before(minTime) && !hdr.ModTime.After(maxTime) {
-		modTime = hdr.ModTime.Unix()
+	// Pack the main header.
+	var f formatter // Ignore errors since they are expected
+	var spd sparseDatas
+	var spb []byte
+	blk := tw.templateV7Plus(hdr, f.formatString, f.formatNumeric)
+	if !hdr.AccessTime.IsZero() {
+		f.formatNumeric(blk.GNU().AccessTime(), hdr.AccessTime.Unix())
+	}
+	if !hdr.ChangeTime.IsZero() {
+		f.formatNumeric(blk.GNU().ChangeTime(), hdr.ChangeTime.Unix())
+	}
+	// TODO(dsnet): Re-enable this when adding sparse support.
+	// See https://golang.org/issue/22735
+	/*
+		if hdr.Typeflag == TypeGNUSparse {
+			sph := append([]sparseEntry{}, hdr.SparseHoles...) // Copy sparse map
+			sph = alignSparseEntries(sph, hdr.Size)
+			spd = invertSparseEntries(sph, hdr.Size)
+
+			// Format the sparse map.
+			formatSPD := func(sp sparseDatas, sa sparseArray) sparseDatas {
+				for i := 0; len(sp) > 0 && i < sa.MaxEntries(); i++ {
+					f.formatNumeric(sa.Entry(i).Offset(), sp[0].Offset)
+					f.formatNumeric(sa.Entry(i).Length(), sp[0].Length)
+					sp = sp[1:]
+				}
+				if len(sp) > 0 {
+					sa.IsExtended()[0] = 1
+				}
+				return sp
+			}
+			sp2 := formatSPD(spd, blk.GNU().Sparse())
+			for len(sp2) > 0 {
+				var spHdr block
+				sp2 = formatSPD(sp2, spHdr.Sparse())
+				spb = append(spb, spHdr[:]...)
+			}
+
+			// Update size fields in the header block.
+			realSize := hdr.Size
+			hdr.Size = 0 // Encoded size; does not account for encoded sparse map
+			for _, s := range spd {
+				hdr.Size += s.Length
+			}
+			copy(blk.V7().Size(), zeroBlock[:]) // Reset field
+			f.formatNumeric(blk.V7().Size(), hdr.Size)
+			f.formatNumeric(blk.GNU().RealSize(), realSize)
+		}
+	*/
+	blk.SetFormat(FormatGNU)
+	if err := tw.writeRawHeader(blk, hdr.Size, hdr.Typeflag); err != nil {
+		return err
 	}
 
-	v7 := header.V7()
-	formatString(v7.Name(), hdr.Name, paxPath)
-	// TODO(dsnet): The GNU format permits the mode field to be encoded in
-	// base-256 format. Thus, we can use formatNumeric instead of formatOctal.
-	f.formatOctal(v7.Mode(), hdr.Mode)
-	formatNumeric(v7.UID(), int64(hdr.Uid), paxUid)
-	formatNumeric(v7.GID(), int64(hdr.Gid), paxGid)
-	formatNumeric(v7.Size(), hdr.Size, paxSize)
-	// TODO(dsnet): Consider using PAX for finer time granularity.
-	formatNumeric(v7.ModTime(), modTime, paxNone)
-	v7.TypeFlag()[0] = hdr.Typeflag
-	formatString(v7.LinkName(), hdr.Linkname, paxLinkpath)
-
-	ustar := header.USTAR()
-	formatString(ustar.UserName(), hdr.Uname, paxUname)
-	formatString(ustar.GroupName(), hdr.Gname, paxGname)
-	formatNumeric(ustar.DevMajor(), hdr.Devmajor, paxNone)
-	formatNumeric(ustar.DevMinor(), hdr.Devminor, paxNone)
-
-	// TODO(dsnet): The logic surrounding the prefix field is broken when trying
-	// to encode the header as GNU format. The challenge with the current logic
-	// is that we are unsure what format we are using at any given moment until
-	// we have processed *all* of the fields. The problem is that by the time
-	// all fields have been processed, some work has already been done to handle
-	// each field under the assumption that it is for one given format or
-	// another. In some situations, this causes the Writer to be confused and
-	// encode a prefix field when the format being used is GNU. Thus, producing
-	// an invalid tar file.
-	//
-	// As a short-term fix, we disable the logic to use the prefix field, which
-	// will force the badly generated GNU files to become encoded as being
-	// the PAX format.
-	//
-	// As an alternative fix, we could hard-code preferPax to be true. However,
-	// this is problematic for the following reasons:
-	//	* The preferPax functionality is not tested at all.
-	//	* This can result in headers that try to use both the GNU and PAX
-	//	features at the same time, which is also wrong.
-	//
-	// The proper fix for this is to use a two-pass method:
-	//	* The first pass simply determines what set of formats can possibly
-	//	encode the given header.
-	//	* The second pass actually encodes the header as that given format
-	//	without worrying about violating the format.
-	//
-	// See the following:
-	//	https://golang.org/issue/12594
-	//	https://golang.org/issue/17630
-	//	https://golang.org/issue/9683
-	const usePrefix = false
-
-	// try to use a ustar header when only the name is too long
-	_, paxPathUsed := paxHeaders[paxPath]
-	if usePrefix && !tw.preferPax && len(paxHeaders) == 1 && paxPathUsed {
-		prefix, suffix, ok := splitUSTARPath(hdr.Name)
-		if ok {
-			// Since we can encode in USTAR format, disable PAX header.
-			delete(paxHeaders, paxPath)
-
-			// Update the path fields
-			formatString(v7.Name(), suffix, paxNone)
-			formatString(ustar.Prefix(), prefix, paxNone)
+	// Write the extended sparse map and setup the sparse writer if necessary.
+	if len(spd) > 0 {
+		// Use tw.w since the sparse map is not accounted for in hdr.Size.
+		if _, err := tw.w.Write(spb); err != nil {
+			return err
 		}
+		tw.curr = &sparseFileWriter{tw.curr, spd, 0}
 	}
+	return nil
+}
+
+type (
+	stringFormatter func([]byte, string)
+	numberFormatter func([]byte, int64)
+)
 
-	if tw.usedBinary {
-		header.SetFormat(formatGNU)
-	} else {
-		header.SetFormat(formatUSTAR)
+// templateV7Plus fills out the V7 fields of a block using values from hdr.
+// It also fills out fields (uname, gname, devmajor, devminor) that are
+// shared in the USTAR, PAX, and GNU formats using the provided formatters.
+//
+// The block returned is only valid until the next call to
+// templateV7Plus or writeRawFile.
+func (tw *Writer) templateV7Plus(hdr *Header, fmtStr stringFormatter, fmtNum numberFormatter) *block {
+	tw.blk.Reset()
+
+	modTime := hdr.ModTime
+	if modTime.IsZero() {
+		modTime = time.Unix(0, 0)
 	}
 
-	// Check if there were any formatting errors.
-	if f.err != nil {
-		tw.err = f.err
-		return tw.err
+	v7 := tw.blk.V7()
+	v7.TypeFlag()[0] = hdr.Typeflag
+	fmtStr(v7.Name(), hdr.Name)
+	fmtStr(v7.LinkName(), hdr.Linkname)
+	fmtNum(v7.Mode(), hdr.Mode)
+	fmtNum(v7.UID(), int64(hdr.Uid))
+	fmtNum(v7.GID(), int64(hdr.Gid))
+	fmtNum(v7.Size(), hdr.Size)
+	fmtNum(v7.ModTime(), modTime.Unix())
+
+	ustar := tw.blk.USTAR()
+	fmtStr(ustar.UserName(), hdr.Uname)
+	fmtStr(ustar.GroupName(), hdr.Gname)
+	fmtNum(ustar.DevMajor(), hdr.Devmajor)
+	fmtNum(ustar.DevMinor(), hdr.Devminor)
+
+	return &tw.blk
+}
+
+// writeRawFile writes a minimal file with the given name and flag type.
+// It uses format to encode the header format and will write data as the body.
+// It uses default values for all of the other fields (as BSD and GNU tar does).
+func (tw *Writer) writeRawFile(name, data string, flag byte, format Format) error {
+	tw.blk.Reset()
+
+	// Best effort for the filename.
+	name = toASCII(name)
+	if len(name) > nameSize {
+		name = name[:nameSize]
 	}
+	name = strings.TrimRight(name, "/")
 
-	if allowPax {
-		for k, v := range hdr.Xattrs {
-			paxHeaders[paxXattr+k] = v
-		}
+	var f formatter
+	v7 := tw.blk.V7()
+	v7.TypeFlag()[0] = flag
+	f.formatString(v7.Name(), name)
+	f.formatOctal(v7.Mode(), 0)
+	f.formatOctal(v7.UID(), 0)
+	f.formatOctal(v7.GID(), 0)
+	f.formatOctal(v7.Size(), int64(len(data))) // Must be < 8GiB
+	f.formatOctal(v7.ModTime(), 0)
+	tw.blk.SetFormat(format)
+	if f.err != nil {
+		return f.err // Only occurs if size condition is violated
 	}
 
-	if len(paxHeaders) > 0 {
-		if !allowPax {
-			return errInvalidHeader
-		}
-		if err := tw.writePAXHeader(hdr, paxHeaders); err != nil {
-			return err
-		}
+	// Write the header and data.
+	if err := tw.writeRawHeader(&tw.blk, int64(len(data)), flag); err != nil {
+		return err
 	}
-	tw.nb = hdr.Size
-	tw.pad = (blockSize - (tw.nb % blockSize)) % blockSize
+	_, err := io.WriteString(tw, data)
+	return err
+}
 
-	_, tw.err = tw.w.Write(header[:])
-	return tw.err
+// writeRawHeader writes the value of blk, regardless of its value.
+// It sets up the Writer such that it can accept a file of the given size.
+// If the flag is a special header-only flag, then the size is treated as zero.
+func (tw *Writer) writeRawHeader(blk *block, size int64, flag byte) error {
+	if err := tw.Flush(); err != nil {
+		return err
+	}
+	if _, err := tw.w.Write(blk[:]); err != nil {
+		return err
+	}
+	if isHeaderOnlyType(flag) {
+		size = 0
+	}
+	tw.curr = &regFileWriter{tw.w, size}
+	tw.pad = blockPadding(size)
+	return nil
 }
 
 // splitUSTARPath splits a path according to USTAR prefix and suffix rules.
@@ -276,95 +412,233 @@ func splitUSTARPath(name string) (prefix, suffix string, ok bool) {
 	return name[:i], name[i+1:], true
 }
 
-// writePaxHeader writes an extended pax header to the
-// archive.
-func (tw *Writer) writePAXHeader(hdr *Header, paxHeaders map[string]string) error {
-	// Prepare extended header
-	ext := new(Header)
-	ext.Typeflag = TypeXHeader
-	// Setting ModTime is required for reader parsing to
-	// succeed, and seems harmless enough.
-	ext.ModTime = hdr.ModTime
-	// The spec asks that we namespace our pseudo files
-	// with the current pid. However, this results in differing outputs
-	// for identical inputs. As such, the constant 0 is now used instead.
-	// golang.org/issue/12358
-	dir, file := path.Split(hdr.Name)
-	fullName := path.Join(dir, "PaxHeaders.0", file)
-
-	ascii := toASCII(fullName)
-	if len(ascii) > nameSize {
-		ascii = ascii[:nameSize]
-	}
-	ext.Name = ascii
-	// Construct the body
-	var buf bytes.Buffer
-
-	// Keys are sorted before writing to body to allow deterministic output.
-	keys := make([]string, 0, len(paxHeaders))
-	for k := range paxHeaders {
-		keys = append(keys, k)
-	}
-	sort.Strings(keys)
-
-	for _, k := range keys {
-		fmt.Fprint(&buf, formatPAXRecord(k, paxHeaders[k]))
-	}
-
-	ext.Size = int64(len(buf.Bytes()))
-	if err := tw.writeHeader(ext, false); err != nil {
-		return err
-	}
-	if _, err := tw.Write(buf.Bytes()); err != nil {
-		return err
+// Write writes to the current file in the tar archive.
+// Write returns the error ErrWriteTooLong if more than
+// Header.Size bytes are written after WriteHeader.
+//
+// Calling Write on special types like TypeLink, TypeSymlink, TypeChar,
+// TypeBlock, TypeDir, and TypeFifo returns (0, ErrWriteTooLong) regardless
+// of what the Header.Size claims.
+func (tw *Writer) Write(b []byte) (int, error) {
+	if tw.err != nil {
+		return 0, tw.err
 	}
-	if err := tw.Flush(); err != nil {
-		return err
+	n, err := tw.curr.Write(b)
+	if err != nil && err != ErrWriteTooLong {
+		tw.err = err
 	}
-	return nil
+	return n, err
 }
 
-// Write writes to the current entry in the tar archive.
-// Write returns the error ErrWriteTooLong if more than
-// hdr.Size bytes are written after WriteHeader.
-func (tw *Writer) Write(b []byte) (n int, err error) {
-	if tw.closed {
-		err = ErrWriteAfterClose
-		return
-	}
-	overwrite := false
-	if int64(len(b)) > tw.nb {
-		b = b[0:tw.nb]
-		overwrite = true
-	}
-	n, err = tw.w.Write(b)
-	tw.nb -= int64(n)
-	if err == nil && overwrite {
-		err = ErrWriteTooLong
-		return
-	}
-	tw.err = err
-	return
+// readFrom populates the content of the current file by reading from r.
+// The bytes read must match the number of remaining bytes in the current file.
+//
+// If the current file is sparse and r is an io.ReadSeeker,
+// then readFrom uses Seek to skip past holes defined in Header.SparseHoles,
+// assuming that skipped regions are all NULs.
+// This always reads the last byte to ensure r is the right size.
+//
+// TODO(dsnet): Re-export this when adding sparse file support.
+// See https://golang.org/issue/22735
+func (tw *Writer) readFrom(r io.Reader) (int64, error) {
+	if tw.err != nil {
+		return 0, tw.err
+	}
+	n, err := tw.curr.ReadFrom(r)
+	if err != nil && err != ErrWriteTooLong {
+		tw.err = err
+	}
+	return n, err
 }
 
-// Close closes the tar archive, flushing any unwritten
-// data to the underlying writer.
+// Close closes the tar archive by flushing the padding, and writing the footer.
+// If the current file (from a prior call to WriteHeader) is not fully written,
+// then this returns an error.
 func (tw *Writer) Close() error {
-	if tw.err != nil || tw.closed {
-		return tw.err
+	if tw.err == ErrWriteAfterClose {
+		return nil
 	}
-	tw.Flush()
-	tw.closed = true
 	if tw.err != nil {
 		return tw.err
 	}
 
-	// trailer: two zero blocks
-	for i := 0; i < 2; i++ {
-		_, tw.err = tw.w.Write(zeroBlock[:])
-		if tw.err != nil {
-			break
+	// Trailer: two zero blocks.
+	err := tw.Flush()
+	for i := 0; i < 2 && err == nil; i++ {
+		_, err = tw.w.Write(zeroBlock[:])
+	}
+
+	// Ensure all future actions are invalid.
+	tw.err = ErrWriteAfterClose
+	return err // Report IO errors
+}
+
+// regFileWriter is a fileWriter for writing data to a regular file entry.
+type regFileWriter struct {
+	w  io.Writer // Underlying Writer
+	nb int64     // Number of remaining bytes to write
+}
+
+func (fw *regFileWriter) Write(b []byte) (n int, err error) {
+	overwrite := int64(len(b)) > fw.nb
+	if overwrite {
+		b = b[:fw.nb]
+	}
+	if len(b) > 0 {
+		n, err = fw.w.Write(b)
+		fw.nb -= int64(n)
+	}
+	switch {
+	case err != nil:
+		return n, err
+	case overwrite:
+		return n, ErrWriteTooLong
+	default:
+		return n, nil
+	}
+}
+
+func (fw *regFileWriter) ReadFrom(r io.Reader) (int64, error) {
+	return io.Copy(struct{ io.Writer }{fw}, r)
+}
+
+func (fw regFileWriter) LogicalRemaining() int64 {
+	return fw.nb
+}
+func (fw regFileWriter) PhysicalRemaining() int64 {
+	return fw.nb
+}
+
+// sparseFileWriter is a fileWriter for writing data to a sparse file entry.
+type sparseFileWriter struct {
+	fw  fileWriter  // Underlying fileWriter
+	sp  sparseDatas // Normalized list of data fragments
+	pos int64       // Current position in sparse file
+}
+
+func (sw *sparseFileWriter) Write(b []byte) (n int, err error) {
+	overwrite := int64(len(b)) > sw.LogicalRemaining()
+	if overwrite {
+		b = b[:sw.LogicalRemaining()]
+	}
+
+	b0 := b
+	endPos := sw.pos + int64(len(b))
+	for endPos > sw.pos && err == nil {
+		var nf int // Bytes written in fragment
+		dataStart, dataEnd := sw.sp[0].Offset, sw.sp[0].endOffset()
+		if sw.pos < dataStart { // In a hole fragment
+			bf := b[:min(int64(len(b)), dataStart-sw.pos)]
+			nf, err = zeroWriter{}.Write(bf)
+		} else { // In a data fragment
+			bf := b[:min(int64(len(b)), dataEnd-sw.pos)]
+			nf, err = sw.fw.Write(bf)
+		}
+		b = b[nf:]
+		sw.pos += int64(nf)
+		if sw.pos >= dataEnd && len(sw.sp) > 1 {
+			sw.sp = sw.sp[1:] // Ensure last fragment always remains
+		}
+	}
+
+	n = len(b0) - len(b)
+	switch {
+	case err == ErrWriteTooLong:
+		return n, errMissData // Not possible; implies bug in validation logic
+	case err != nil:
+		return n, err
+	case sw.LogicalRemaining() == 0 && sw.PhysicalRemaining() > 0:
+		return n, errUnrefData // Not possible; implies bug in validation logic
+	case overwrite:
+		return n, ErrWriteTooLong
+	default:
+		return n, nil
+	}
+}
+
+func (sw *sparseFileWriter) ReadFrom(r io.Reader) (n int64, err error) {
+	rs, ok := r.(io.ReadSeeker)
+	if ok {
+		if _, err := rs.Seek(0, io.SeekCurrent); err != nil {
+			ok = false // Not all io.Seeker can really seek
+		}
+	}
+	if !ok {
+		return io.Copy(struct{ io.Writer }{sw}, r)
+	}
+
+	var readLastByte bool
+	pos0 := sw.pos
+	for sw.LogicalRemaining() > 0 && !readLastByte && err == nil {
+		var nf int64 // Size of fragment
+		dataStart, dataEnd := sw.sp[0].Offset, sw.sp[0].endOffset()
+		if sw.pos < dataStart { // In a hole fragment
+			nf = dataStart - sw.pos
+			if sw.PhysicalRemaining() == 0 {
+				readLastByte = true
+				nf--
+			}
+			_, err = rs.Seek(nf, io.SeekCurrent)
+		} else { // In a data fragment
+			nf = dataEnd - sw.pos
+			nf, err = io.CopyN(sw.fw, rs, nf)
 		}
+		sw.pos += nf
+		if sw.pos >= dataEnd && len(sw.sp) > 1 {
+			sw.sp = sw.sp[1:] // Ensure last fragment always remains
+		}
+	}
+
+	// If the last fragment is a hole, then seek to 1-byte before EOF, and
+	// read a single byte to ensure the file is the right size.
+	if readLastByte && err == nil {
+		_, err = mustReadFull(rs, []byte{0})
+		sw.pos++
+	}
+
+	n = sw.pos - pos0
+	switch {
+	case err == io.EOF:
+		return n, io.ErrUnexpectedEOF
+	case err == ErrWriteTooLong:
+		return n, errMissData // Not possible; implies bug in validation logic
+	case err != nil:
+		return n, err
+	case sw.LogicalRemaining() == 0 && sw.PhysicalRemaining() > 0:
+		return n, errUnrefData // Not possible; implies bug in validation logic
+	default:
+		return n, ensureEOF(rs)
+	}
+}
+
+func (sw sparseFileWriter) LogicalRemaining() int64 {
+	return sw.sp[len(sw.sp)-1].endOffset() - sw.pos
+}
+func (sw sparseFileWriter) PhysicalRemaining() int64 {
+	return sw.fw.PhysicalRemaining()
+}
+
+// zeroWriter may only be written with NULs, otherwise it returns errWriteHole.
+type zeroWriter struct{}
+
+func (zeroWriter) Write(b []byte) (int, error) {
+	for i, c := range b {
+		if c != 0 {
+			return i, errWriteHole
+		}
+	}
+	return len(b), nil
+}
+
+// ensureEOF checks whether r is at EOF, reporting ErrWriteTooLong if not so.
+func ensureEOF(r io.Reader) error {
+	n, err := tryReadFull(r, []byte{0})
+	switch {
+	case n > 0:
+		return ErrWriteTooLong
+	case err == io.EOF:
+		return nil
+	default:
+		return err
 	}
-	return tw.err
 }
diff --git a/libgo/go/archive/tar/writer_test.go b/libgo/go/archive/tar/writer_test.go
index d88b8f4..24e8da2 100644
--- a/libgo/go/archive/tar/writer_test.go
+++ b/libgo/go/archive/tar/writer_test.go
@@ -6,10 +6,12 @@ package tar
 
 import (
 	"bytes"
-	"fmt"
+	"encoding/hex"
+	"errors"
 	"io"
 	"io/ioutil"
 	"os"
+	"path"
 	"reflect"
 	"sort"
 	"strings"
@@ -18,120 +20,127 @@ import (
 	"time"
 )
 
-// Render byte array in a two-character hexadecimal string, spaced for easy visual inspection.
-func bytestr(offset int, b []byte) string {
-	const rowLen = 32
-	s := fmt.Sprintf("%04x ", offset)
-	for _, ch := range b {
-		switch {
-		case '0' <= ch && ch <= '9', 'A' <= ch && ch <= 'Z', 'a' <= ch && ch <= 'z':
-			s += fmt.Sprintf("  %c", ch)
-		default:
-			s += fmt.Sprintf(" %02x", ch)
+func bytediff(a, b []byte) string {
+	const (
+		uniqueA  = "-  "
+		uniqueB  = "+  "
+		identity = "   "
+	)
+	var ss []string
+	sa := strings.Split(strings.TrimSpace(hex.Dump(a)), "\n")
+	sb := strings.Split(strings.TrimSpace(hex.Dump(b)), "\n")
+	for len(sa) > 0 && len(sb) > 0 {
+		if sa[0] == sb[0] {
+			ss = append(ss, identity+sa[0])
+		} else {
+			ss = append(ss, uniqueA+sa[0])
+			ss = append(ss, uniqueB+sb[0])
 		}
+		sa, sb = sa[1:], sb[1:]
+	}
+	for len(sa) > 0 {
+		ss = append(ss, uniqueA+sa[0])
+		sa = sa[1:]
+	}
+	for len(sb) > 0 {
+		ss = append(ss, uniqueB+sb[0])
+		sb = sb[1:]
 	}
-	return s
+	return strings.Join(ss, "\n")
 }
 
-// Render a pseudo-diff between two blocks of bytes.
-func bytediff(a []byte, b []byte) string {
-	const rowLen = 32
-	s := fmt.Sprintf("(%d bytes vs. %d bytes)\n", len(a), len(b))
-	for offset := 0; len(a)+len(b) > 0; offset += rowLen {
-		na, nb := rowLen, rowLen
-		if na > len(a) {
-			na = len(a)
+func TestWriter(t *testing.T) {
+	type (
+		testHeader struct { // WriteHeader(hdr) == wantErr
+			hdr     Header
+			wantErr error
 		}
-		if nb > len(b) {
-			nb = len(b)
+		testWrite struct { // Write(str) == (wantCnt, wantErr)
+			str     string
+			wantCnt int
+			wantErr error
 		}
-		sa := bytestr(offset, a[0:na])
-		sb := bytestr(offset, b[0:nb])
-		if sa != sb {
-			s += fmt.Sprintf("-%v\n+%v\n", sa, sb)
+		testReadFrom struct { // ReadFrom(testFile{ops}) == (wantCnt, wantErr)
+			ops     fileOps
+			wantCnt int64
+			wantErr error
 		}
-		a = a[na:]
-		b = b[nb:]
-	}
-	return s
-}
-
-func TestWriter(t *testing.T) {
-	type entry struct {
-		header   *Header
-		contents string
-	}
+		testClose struct { // Close() == wantErr
+			wantErr error
+		}
+		testFnc interface{} // testHeader | testWrite | testReadFrom | testClose
+	)
 
 	vectors := []struct {
-		file    string // filename of expected output
-		entries []*entry
+		file  string // Optional filename of expected output
+		tests []testFnc
 	}{{
 		// The writer test file was produced with this command:
 		// tar (GNU tar) 1.26
 		//   ln -s small.txt link.txt
 		//   tar -b 1 --format=ustar -c -f writer.tar small.txt small2.txt link.txt
 		file: "testdata/writer.tar",
-		entries: []*entry{{
-			header: &Header{
+		tests: []testFnc{
+			testHeader{Header{
+				Typeflag: TypeReg,
 				Name:     "small.txt",
+				Size:     5,
 				Mode:     0640,
 				Uid:      73025,
 				Gid:      5000,
-				Size:     5,
-				ModTime:  time.Unix(1246508266, 0),
-				Typeflag: '0',
 				Uname:    "dsymonds",
 				Gname:    "eng",
-			},
-			contents: "Kilts",
-		}, {
-			header: &Header{
+				ModTime:  time.Unix(1246508266, 0),
+			}, nil},
+			testWrite{"Kilts", 5, nil},
+
+			testHeader{Header{
+				Typeflag: TypeReg,
 				Name:     "small2.txt",
+				Size:     11,
 				Mode:     0640,
 				Uid:      73025,
-				Gid:      5000,
-				Size:     11,
-				ModTime:  time.Unix(1245217492, 0),
-				Typeflag: '0',
 				Uname:    "dsymonds",
 				Gname:    "eng",
-			},
-			contents: "Google.com\n",
-		}, {
-			header: &Header{
+				Gid:      5000,
+				ModTime:  time.Unix(1245217492, 0),
+			}, nil},
+			testWrite{"Google.com\n", 11, nil},
+
+			testHeader{Header{
+				Typeflag: TypeSymlink,
 				Name:     "link.txt",
+				Linkname: "small.txt",
 				Mode:     0777,
 				Uid:      1000,
 				Gid:      1000,
-				Size:     0,
-				ModTime:  time.Unix(1314603082, 0),
-				Typeflag: '2',
-				Linkname: "small.txt",
 				Uname:    "strings",
 				Gname:    "strings",
-			},
-			// no contents
-		}},
+				ModTime:  time.Unix(1314603082, 0),
+			}, nil},
+			testWrite{"", 0, nil},
+
+			testClose{nil},
+		},
 	}, {
 		// The truncated test file was produced using these commands:
 		//   dd if=/dev/zero bs=1048576 count=16384 > /tmp/16gig.txt
 		//   tar -b 1 -c -f- /tmp/16gig.txt | dd bs=512 count=8 > writer-big.tar
 		file: "testdata/writer-big.tar",
-		entries: []*entry{{
-			header: &Header{
+		tests: []testFnc{
+			testHeader{Header{
+				Typeflag: TypeReg,
 				Name:     "tmp/16gig.txt",
+				Size:     16 << 30,
 				Mode:     0640,
 				Uid:      73025,
 				Gid:      5000,
-				Size:     16 << 30,
-				ModTime:  time.Unix(1254699560, 0),
-				Typeflag: '0',
 				Uname:    "dsymonds",
 				Gname:    "eng",
-			},
-			// fake contents
-			contents: strings.Repeat("\x00", 4<<10),
-		}},
+				ModTime:  time.Unix(1254699560, 0),
+				Format:   FormatGNU,
+			}, nil},
+		},
 	}, {
 		// This truncated file was produced using this library.
 		// It was verified to work with GNU tar 1.27.1 and BSD tar 3.1.2.
@@ -141,117 +150,377 @@ func TestWriter(t *testing.T) {
 		//
 		// This file is in PAX format.
 		file: "testdata/writer-big-long.tar",
-		entries: []*entry{{
-			header: &Header{
+		tests: []testFnc{
+			testHeader{Header{
+				Typeflag: TypeReg,
 				Name:     strings.Repeat("longname/", 15) + "16gig.txt",
+				Size:     16 << 30,
 				Mode:     0644,
 				Uid:      1000,
 				Gid:      1000,
-				Size:     16 << 30,
-				ModTime:  time.Unix(1399583047, 0),
-				Typeflag: '0',
 				Uname:    "guillaume",
 				Gname:    "guillaume",
-			},
-			// fake contents
-			contents: strings.Repeat("\x00", 4<<10),
-		}},
-	}, {
-		// TODO(dsnet): The Writer output should match the following file.
-		// To fix an issue (see https://golang.org/issue/12594), we disabled
-		// prefix support, which alters the generated output.
-		/*
-			// This file was produced using gnu tar 1.17
-			// gnutar  -b 4 --format=ustar (longname/)*15 + file.txt
-			file: "testdata/ustar.tar"
-		*/
-		file: "testdata/ustar.issue12594.tar", // This is a valid tar file, but not expected
-		entries: []*entry{{
-			header: &Header{
+				ModTime:  time.Unix(1399583047, 0),
+			}, nil},
+		},
+	}, {
+		// This file was produced using GNU tar v1.17.
+		//	gnutar -b 4 --format=ustar (longname/)*15 + file.txt
+		file: "testdata/ustar.tar",
+		tests: []testFnc{
+			testHeader{Header{
+				Typeflag: TypeReg,
 				Name:     strings.Repeat("longname/", 15) + "file.txt",
+				Size:     6,
 				Mode:     0644,
-				Uid:      0765,
-				Gid:      024,
-				Size:     06,
-				ModTime:  time.Unix(1360135598, 0),
-				Typeflag: '0',
+				Uid:      501,
+				Gid:      20,
 				Uname:    "shane",
 				Gname:    "staff",
-			},
-			contents: "hello\n",
-		}},
-	}, {
-		// This file was produced using gnu tar 1.26
-		// echo "Slartibartfast" > file.txt
-		// ln file.txt hard.txt
-		// tar -b 1 --format=ustar -c -f hardlink.tar file.txt hard.txt
+				ModTime:  time.Unix(1360135598, 0),
+			}, nil},
+			testWrite{"hello\n", 6, nil},
+			testClose{nil},
+		},
+	}, {
+		// This file was produced using GNU tar v1.26:
+		//	echo "Slartibartfast" > file.txt
+		//	ln file.txt hard.txt
+		//	tar -b 1 --format=ustar -c -f hardlink.tar file.txt hard.txt
 		file: "testdata/hardlink.tar",
-		entries: []*entry{{
-			header: &Header{
+		tests: []testFnc{
+			testHeader{Header{
+				Typeflag: TypeReg,
 				Name:     "file.txt",
+				Size:     15,
 				Mode:     0644,
 				Uid:      1000,
 				Gid:      100,
-				Size:     15,
-				ModTime:  time.Unix(1425484303, 0),
-				Typeflag: '0',
 				Uname:    "vbatts",
 				Gname:    "users",
-			},
-			contents: "Slartibartfast\n",
-		}, {
-			header: &Header{
+				ModTime:  time.Unix(1425484303, 0),
+			}, nil},
+			testWrite{"Slartibartfast\n", 15, nil},
+
+			testHeader{Header{
+				Typeflag: TypeLink,
 				Name:     "hard.txt",
+				Linkname: "file.txt",
 				Mode:     0644,
 				Uid:      1000,
 				Gid:      100,
-				Size:     0,
-				ModTime:  time.Unix(1425484303, 0),
-				Typeflag: '1',
-				Linkname: "file.txt",
 				Uname:    "vbatts",
 				Gname:    "users",
-			},
-			// no contents
-		}},
+				ModTime:  time.Unix(1425484303, 0),
+			}, nil},
+			testWrite{"", 0, nil},
+
+			testClose{nil},
+		},
+	}, {
+		tests: []testFnc{
+			testHeader{Header{
+				Typeflag: TypeReg,
+				Name:     "bad-null.txt",
+				Xattrs:   map[string]string{"null\x00null\x00": "fizzbuzz"},
+			}, headerError{}},
+		},
+	}, {
+		tests: []testFnc{
+			testHeader{Header{
+				Typeflag: TypeReg,
+				Name:     "null\x00.txt",
+			}, headerError{}},
+		},
+	}, {
+		file: "testdata/pax-records.tar",
+		tests: []testFnc{
+			testHeader{Header{
+				Typeflag: TypeReg,
+				Name:     "file",
+				Uname:    strings.Repeat("long", 10),
+				PAXRecords: map[string]string{
+					"path":           "FILE", // Should be ignored
+					"GNU.sparse.map": "0,0",  // Should be ignored
+					"comment":        "Hello, 世界",
+					"GOLANG.pkg":     "tar",
+				},
+			}, nil},
+			testClose{nil},
+		},
+	}, {
+		// Craft a theoretically valid PAX archive with global headers.
+		// The GNU and BSD tar tools do not parse these the same way.
+		//
+		// BSD tar v3.1.2 parses and ignores all global headers;
+		// the behavior is verified by researching the source code.
+		//
+		//	$ bsdtar -tvf pax-global-records.tar
+		//	----------  0 0      0           0 Dec 31  1969 file1
+		//	----------  0 0      0           0 Dec 31  1969 file2
+		//	----------  0 0      0           0 Dec 31  1969 file3
+		//	----------  0 0      0           0 May 13  2014 file4
+		//
+		// GNU tar v1.27.1 applies global headers to subsequent records,
+		// but does not do the following properly:
+		//	* It does not treat an empty record as deletion.
+		//	* It does not use subsequent global headers to update previous ones.
+		//
+		//	$ gnutar -tvf pax-global-records.tar
+		//	---------- 0/0               0 2017-07-13 19:40 global1
+		//	---------- 0/0               0 2017-07-13 19:40 file2
+		//	gnutar: Substituting `.' for empty member name
+		//	---------- 0/0               0 1969-12-31 16:00
+		//	gnutar: Substituting `.' for empty member name
+		//	---------- 0/0               0 2014-05-13 09:53
+		//
+		// According to the PAX specification, this should have been the result:
+		//	---------- 0/0               0 2017-07-13 19:40 global1
+		//	---------- 0/0               0 2017-07-13 19:40 file2
+		//	---------- 0/0               0 2017-07-13 19:40 file3
+		//	---------- 0/0               0 2014-05-13 09:53 file4
+		file: "testdata/pax-global-records.tar",
+		tests: []testFnc{
+			testHeader{Header{
+				Typeflag:   TypeXGlobalHeader,
+				PAXRecords: map[string]string{"path": "global1", "mtime": "1500000000.0"},
+			}, nil},
+			testHeader{Header{
+				Typeflag: TypeReg, Name: "file1",
+			}, nil},
+			testHeader{Header{
+				Typeflag:   TypeReg,
+				Name:       "file2",
+				PAXRecords: map[string]string{"path": "file2"},
+			}, nil},
+			testHeader{Header{
+				Typeflag:   TypeXGlobalHeader,
+				PAXRecords: map[string]string{"path": ""}, // Should delete "path", but keep "mtime"
+			}, nil},
+			testHeader{Header{
+				Typeflag: TypeReg, Name: "file3",
+			}, nil},
+			testHeader{Header{
+				Typeflag:   TypeReg,
+				Name:       "file4",
+				ModTime:    time.Unix(1400000000, 0),
+				PAXRecords: map[string]string{"mtime": "1400000000"},
+			}, nil},
+			testClose{nil},
+		},
+	}, {
+		file: "testdata/gnu-utf8.tar",
+		tests: []testFnc{
+			testHeader{Header{
+				Typeflag: TypeReg,
+				Name:     "☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹",
+				Mode:     0644,
+				Uid:      1000, Gid: 1000,
+				Uname:   "☺",
+				Gname:   "⚹",
+				ModTime: time.Unix(0, 0),
+				Format:  FormatGNU,
+			}, nil},
+			testClose{nil},
+		},
+	}, {
+		file: "testdata/gnu-not-utf8.tar",
+		tests: []testFnc{
+			testHeader{Header{
+				Typeflag: TypeReg,
+				Name:     "hi\x80\x81\x82\x83bye",
+				Mode:     0644,
+				Uid:      1000,
+				Gid:      1000,
+				Uname:    "rawr",
+				Gname:    "dsnet",
+				ModTime:  time.Unix(0, 0),
+				Format:   FormatGNU,
+			}, nil},
+			testClose{nil},
+		},
+		// TODO(dsnet): Re-enable this test when adding sparse support.
+		// See https://golang.org/issue/22735
+		/*
+			}, {
+				file: "testdata/gnu-nil-sparse-data.tar",
+				tests: []testFnc{
+					testHeader{Header{
+						Typeflag:    TypeGNUSparse,
+						Name:        "sparse.db",
+						Size:        1000,
+						SparseHoles: []sparseEntry{{Offset: 1000, Length: 0}},
+					}, nil},
+					testWrite{strings.Repeat("0123456789", 100), 1000, nil},
+					testClose{},
+				},
+			}, {
+				file: "testdata/gnu-nil-sparse-hole.tar",
+				tests: []testFnc{
+					testHeader{Header{
+						Typeflag:    TypeGNUSparse,
+						Name:        "sparse.db",
+						Size:        1000,
+						SparseHoles: []sparseEntry{{Offset: 0, Length: 1000}},
+					}, nil},
+					testWrite{strings.Repeat("\x00", 1000), 1000, nil},
+					testClose{},
+				},
+			}, {
+				file: "testdata/pax-nil-sparse-data.tar",
+				tests: []testFnc{
+					testHeader{Header{
+						Typeflag:    TypeReg,
+						Name:        "sparse.db",
+						Size:        1000,
+						SparseHoles: []sparseEntry{{Offset: 1000, Length: 0}},
+					}, nil},
+					testWrite{strings.Repeat("0123456789", 100), 1000, nil},
+					testClose{},
+				},
+			}, {
+				file: "testdata/pax-nil-sparse-hole.tar",
+				tests: []testFnc{
+					testHeader{Header{
+						Typeflag:    TypeReg,
+						Name:        "sparse.db",
+						Size:        1000,
+						SparseHoles: []sparseEntry{{Offset: 0, Length: 1000}},
+					}, nil},
+					testWrite{strings.Repeat("\x00", 1000), 1000, nil},
+					testClose{},
+				},
+			}, {
+				file: "testdata/gnu-sparse-big.tar",
+				tests: []testFnc{
+					testHeader{Header{
+						Typeflag: TypeGNUSparse,
+						Name:     "gnu-sparse",
+						Size:     6e10,
+						SparseHoles: []sparseEntry{
+							{Offset: 0e10, Length: 1e10 - 100},
+							{Offset: 1e10, Length: 1e10 - 100},
+							{Offset: 2e10, Length: 1e10 - 100},
+							{Offset: 3e10, Length: 1e10 - 100},
+							{Offset: 4e10, Length: 1e10 - 100},
+							{Offset: 5e10, Length: 1e10 - 100},
+						},
+					}, nil},
+					testReadFrom{fileOps{
+						int64(1e10 - blockSize),
+						strings.Repeat("\x00", blockSize-100) + strings.Repeat("0123456789", 10),
+						int64(1e10 - blockSize),
+						strings.Repeat("\x00", blockSize-100) + strings.Repeat("0123456789", 10),
+						int64(1e10 - blockSize),
+						strings.Repeat("\x00", blockSize-100) + strings.Repeat("0123456789", 10),
+						int64(1e10 - blockSize),
+						strings.Repeat("\x00", blockSize-100) + strings.Repeat("0123456789", 10),
+						int64(1e10 - blockSize),
+						strings.Repeat("\x00", blockSize-100) + strings.Repeat("0123456789", 10),
+						int64(1e10 - blockSize),
+						strings.Repeat("\x00", blockSize-100) + strings.Repeat("0123456789", 10),
+					}, 6e10, nil},
+					testClose{nil},
+				},
+			}, {
+				file: "testdata/pax-sparse-big.tar",
+				tests: []testFnc{
+					testHeader{Header{
+						Typeflag: TypeReg,
+						Name:     "pax-sparse",
+						Size:     6e10,
+						SparseHoles: []sparseEntry{
+							{Offset: 0e10, Length: 1e10 - 100},
+							{Offset: 1e10, Length: 1e10 - 100},
+							{Offset: 2e10, Length: 1e10 - 100},
+							{Offset: 3e10, Length: 1e10 - 100},
+							{Offset: 4e10, Length: 1e10 - 100},
+							{Offset: 5e10, Length: 1e10 - 100},
+						},
+					}, nil},
+					testReadFrom{fileOps{
+						int64(1e10 - blockSize),
+						strings.Repeat("\x00", blockSize-100) + strings.Repeat("0123456789", 10),
+						int64(1e10 - blockSize),
+						strings.Repeat("\x00", blockSize-100) + strings.Repeat("0123456789", 10),
+						int64(1e10 - blockSize),
+						strings.Repeat("\x00", blockSize-100) + strings.Repeat("0123456789", 10),
+						int64(1e10 - blockSize),
+						strings.Repeat("\x00", blockSize-100) + strings.Repeat("0123456789", 10),
+						int64(1e10 - blockSize),
+						strings.Repeat("\x00", blockSize-100) + strings.Repeat("0123456789", 10),
+						int64(1e10 - blockSize),
+						strings.Repeat("\x00", blockSize-100) + strings.Repeat("0123456789", 10),
+					}, 6e10, nil},
+					testClose{nil},
+				},
+		*/
+	}, {
+		file: "testdata/trailing-slash.tar",
+		tests: []testFnc{
+			testHeader{Header{Name: strings.Repeat("123456789/", 30)}, nil},
+			testClose{nil},
+		},
 	}}
 
-testLoop:
-	for i, v := range vectors {
-		expected, err := ioutil.ReadFile(v.file)
-		if err != nil {
-			t.Errorf("test %d: Unexpected error: %v", i, err)
-			continue
-		}
-
-		buf := new(bytes.Buffer)
-		tw := NewWriter(iotest.TruncateWriter(buf, 4<<10)) // only catch the first 4 KB
-		big := false
-		for j, entry := range v.entries {
-			big = big || entry.header.Size > 1<<10
-			if err := tw.WriteHeader(entry.header); err != nil {
-				t.Errorf("test %d, entry %d: Failed writing header: %v", i, j, err)
-				continue testLoop
-			}
-			if _, err := io.WriteString(tw, entry.contents); err != nil {
-				t.Errorf("test %d, entry %d: Failed writing contents: %v", i, j, err)
-				continue testLoop
-			}
-		}
-		// Only interested in Close failures for the small tests.
-		if err := tw.Close(); err != nil && !big {
-			t.Errorf("test %d: Failed closing archive: %v", i, err)
-			continue testLoop
+	equalError := func(x, y error) bool {
+		_, ok1 := x.(headerError)
+		_, ok2 := y.(headerError)
+		if ok1 || ok2 {
+			return ok1 && ok2
 		}
+		return x == y
+	}
+	for _, v := range vectors {
+		t.Run(path.Base(v.file), func(t *testing.T) {
+			const maxSize = 10 << 10 // 10KiB
+			buf := new(bytes.Buffer)
+			tw := NewWriter(iotest.TruncateWriter(buf, maxSize))
 
-		actual := buf.Bytes()
-		if !bytes.Equal(expected, actual) {
-			t.Errorf("test %d: Incorrect result: (-=expected, +=actual)\n%v",
-				i, bytediff(expected, actual))
-		}
-		if testing.Short() { // The second test is expensive.
-			break
-		}
+			for i, tf := range v.tests {
+				switch tf := tf.(type) {
+				case testHeader:
+					err := tw.WriteHeader(&tf.hdr)
+					if !equalError(err, tf.wantErr) {
+						t.Fatalf("test %d, WriteHeader() = %v, want %v", i, err, tf.wantErr)
+					}
+				case testWrite:
+					got, err := tw.Write([]byte(tf.str))
+					if got != tf.wantCnt || !equalError(err, tf.wantErr) {
+						t.Fatalf("test %d, Write() = (%d, %v), want (%d, %v)", i, got, err, tf.wantCnt, tf.wantErr)
+					}
+				case testReadFrom:
+					f := &testFile{ops: tf.ops}
+					got, err := tw.readFrom(f)
+					if _, ok := err.(testError); ok {
+						t.Errorf("test %d, ReadFrom(): %v", i, err)
+					} else if got != tf.wantCnt || !equalError(err, tf.wantErr) {
+						t.Errorf("test %d, ReadFrom() = (%d, %v), want (%d, %v)", i, got, err, tf.wantCnt, tf.wantErr)
+					}
+					if len(f.ops) > 0 {
+						t.Errorf("test %d, expected %d more operations", i, len(f.ops))
+					}
+				case testClose:
+					err := tw.Close()
+					if !equalError(err, tf.wantErr) {
+						t.Fatalf("test %d, Close() = %v, want %v", i, err, tf.wantErr)
+					}
+				default:
+					t.Fatalf("test %d, unknown test operation: %T", i, tf)
+				}
+			}
+
+			if v.file != "" {
+				want, err := ioutil.ReadFile(v.file)
+				if err != nil {
+					t.Fatalf("ReadFile() = %v, want nil", err)
+				}
+				got := buf.Bytes()
+				if !bytes.Equal(want, got) {
+					t.Fatalf("incorrect result: (-got +want)\n%v", bytediff(got, want))
+				}
+			}
+		})
 	}
 }
 
@@ -546,21 +815,104 @@ func TestValidTypeflagWithPAXHeader(t *testing.T) {
 	}
 }
 
-func TestWriteAfterClose(t *testing.T) {
-	var buffer bytes.Buffer
-	tw := NewWriter(&buffer)
+// failOnceWriter fails exactly once and then always reports success.
+type failOnceWriter bool
 
-	hdr := &Header{
-		Name: "small.txt",
-		Size: 5,
-	}
-	if err := tw.WriteHeader(hdr); err != nil {
-		t.Fatalf("Failed to write header: %s", err)
-	}
-	tw.Close()
-	if _, err := tw.Write([]byte("Kilts")); err != ErrWriteAfterClose {
-		t.Fatalf("Write: got %v; want ErrWriteAfterClose", err)
+func (w *failOnceWriter) Write(b []byte) (int, error) {
+	if !*w {
+		return 0, io.ErrShortWrite
 	}
+	*w = true
+	return len(b), nil
+}
+
+func TestWriterErrors(t *testing.T) {
+	t.Run("HeaderOnly", func(t *testing.T) {
+		tw := NewWriter(new(bytes.Buffer))
+		hdr := &Header{Name: "dir/", Typeflag: TypeDir}
+		if err := tw.WriteHeader(hdr); err != nil {
+			t.Fatalf("WriteHeader() = %v, want nil", err)
+		}
+		if _, err := tw.Write([]byte{0x00}); err != ErrWriteTooLong {
+			t.Fatalf("Write() = %v, want %v", err, ErrWriteTooLong)
+		}
+	})
+
+	t.Run("NegativeSize", func(t *testing.T) {
+		tw := NewWriter(new(bytes.Buffer))
+		hdr := &Header{Name: "small.txt", Size: -1}
+		if err := tw.WriteHeader(hdr); err == nil {
+			t.Fatalf("WriteHeader() = nil, want non-nil error")
+		}
+	})
+
+	t.Run("BeforeHeader", func(t *testing.T) {
+		tw := NewWriter(new(bytes.Buffer))
+		if _, err := tw.Write([]byte("Kilts")); err != ErrWriteTooLong {
+			t.Fatalf("Write() = %v, want %v", err, ErrWriteTooLong)
+		}
+	})
+
+	t.Run("AfterClose", func(t *testing.T) {
+		tw := NewWriter(new(bytes.Buffer))
+		hdr := &Header{Name: "small.txt"}
+		if err := tw.WriteHeader(hdr); err != nil {
+			t.Fatalf("WriteHeader() = %v, want nil", err)
+		}
+		if err := tw.Close(); err != nil {
+			t.Fatalf("Close() = %v, want nil", err)
+		}
+		if _, err := tw.Write([]byte("Kilts")); err != ErrWriteAfterClose {
+			t.Fatalf("Write() = %v, want %v", err, ErrWriteAfterClose)
+		}
+		if err := tw.Flush(); err != ErrWriteAfterClose {
+			t.Fatalf("Flush() = %v, want %v", err, ErrWriteAfterClose)
+		}
+		if err := tw.Close(); err != nil {
+			t.Fatalf("Close() = %v, want nil", err)
+		}
+	})
+
+	t.Run("PrematureFlush", func(t *testing.T) {
+		tw := NewWriter(new(bytes.Buffer))
+		hdr := &Header{Name: "small.txt", Size: 5}
+		if err := tw.WriteHeader(hdr); err != nil {
+			t.Fatalf("WriteHeader() = %v, want nil", err)
+		}
+		if err := tw.Flush(); err == nil {
+			t.Fatalf("Flush() = %v, want non-nil error", err)
+		}
+	})
+
+	t.Run("PrematureClose", func(t *testing.T) {
+		tw := NewWriter(new(bytes.Buffer))
+		hdr := &Header{Name: "small.txt", Size: 5}
+		if err := tw.WriteHeader(hdr); err != nil {
+			t.Fatalf("WriteHeader() = %v, want nil", err)
+		}
+		if err := tw.Close(); err == nil {
+			t.Fatalf("Close() = %v, want non-nil error", err)
+		}
+	})
+
+	t.Run("Persistence", func(t *testing.T) {
+		tw := NewWriter(new(failOnceWriter))
+		if err := tw.WriteHeader(&Header{}); err != io.ErrShortWrite {
+			t.Fatalf("WriteHeader() = %v, want %v", err, io.ErrShortWrite)
+		}
+		if err := tw.WriteHeader(&Header{Name: "small.txt"}); err == nil {
+			t.Errorf("WriteHeader() = got %v, want non-nil error", err)
+		}
+		if _, err := tw.Write(nil); err == nil {
+			t.Errorf("Write() = %v, want non-nil error", err)
+		}
+		if err := tw.Flush(); err == nil {
+			t.Errorf("Flush() = %v, want non-nil error", err)
+		}
+		if err := tw.Close(); err == nil {
+			t.Errorf("Close() = %v, want non-nil error", err)
+		}
+	})
 }
 
 func TestSplitUSTARPath(t *testing.T) {
@@ -631,7 +983,7 @@ func TestIssue12594(t *testing.T) {
 		if i := strings.IndexByte(prefix, 0); i >= 0 {
 			prefix = prefix[:i] // Truncate at the NUL terminator
 		}
-		if blk.GetFormat() == formatGNU && len(prefix) > 0 && strings.HasPrefix(name, prefix) {
+		if blk.GetFormat() == FormatGNU && len(prefix) > 0 && strings.HasPrefix(name, prefix) {
 			t.Errorf("test %d, found prefix in GNU format: %s", i, prefix)
 		}
 
@@ -645,3 +997,306 @@ func TestIssue12594(t *testing.T) {
 		}
 	}
 }
+
+// testNonEmptyWriter wraps an io.Writer and ensures that
+// Write is never called with an empty buffer.
+type testNonEmptyWriter struct{ io.Writer }
+
+func (w testNonEmptyWriter) Write(b []byte) (int, error) {
+	if len(b) == 0 {
+		return 0, errors.New("unexpected empty Write call")
+	}
+	return w.Writer.Write(b)
+}
+
+func TestFileWriter(t *testing.T) {
+	type (
+		testWrite struct { // Write(str) == (wantCnt, wantErr)
+			str     string
+			wantCnt int
+			wantErr error
+		}
+		testReadFrom struct { // ReadFrom(testFile{ops}) == (wantCnt, wantErr)
+			ops     fileOps
+			wantCnt int64
+			wantErr error
+		}
+		testRemaining struct { // LogicalRemaining() == wantLCnt, PhysicalRemaining() == wantPCnt
+			wantLCnt int64
+			wantPCnt int64
+		}
+		testFnc interface{} // testWrite | testReadFrom | testRemaining
+	)
+
+	type (
+		makeReg struct {
+			size    int64
+			wantStr string
+		}
+		makeSparse struct {
+			makeReg makeReg
+			sph     sparseHoles
+			size    int64
+		}
+		fileMaker interface{} // makeReg | makeSparse
+	)
+
+	vectors := []struct {
+		maker fileMaker
+		tests []testFnc
+	}{{
+		maker: makeReg{0, ""},
+		tests: []testFnc{
+			testRemaining{0, 0},
+			testWrite{"", 0, nil},
+			testWrite{"a", 0, ErrWriteTooLong},
+			testReadFrom{fileOps{""}, 0, nil},
+			testReadFrom{fileOps{"a"}, 0, ErrWriteTooLong},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeReg{1, "a"},
+		tests: []testFnc{
+			testRemaining{1, 1},
+			testWrite{"", 0, nil},
+			testWrite{"a", 1, nil},
+			testWrite{"bcde", 0, ErrWriteTooLong},
+			testWrite{"", 0, nil},
+			testReadFrom{fileOps{""}, 0, nil},
+			testReadFrom{fileOps{"a"}, 0, ErrWriteTooLong},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeReg{5, "hello"},
+		tests: []testFnc{
+			testRemaining{5, 5},
+			testWrite{"hello", 5, nil},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeReg{5, "\x00\x00\x00\x00\x00"},
+		tests: []testFnc{
+			testRemaining{5, 5},
+			testReadFrom{fileOps{"\x00\x00\x00\x00\x00"}, 5, nil},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeReg{5, "\x00\x00\x00\x00\x00"},
+		tests: []testFnc{
+			testRemaining{5, 5},
+			testReadFrom{fileOps{"\x00\x00\x00\x00\x00extra"}, 5, ErrWriteTooLong},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeReg{5, "abc\x00\x00"},
+		tests: []testFnc{
+			testRemaining{5, 5},
+			testWrite{"abc", 3, nil},
+			testRemaining{2, 2},
+			testReadFrom{fileOps{"\x00\x00"}, 2, nil},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeReg{5, "\x00\x00abc"},
+		tests: []testFnc{
+			testRemaining{5, 5},
+			testWrite{"\x00\x00", 2, nil},
+			testRemaining{3, 3},
+			testWrite{"abc", 3, nil},
+			testReadFrom{fileOps{"z"}, 0, ErrWriteTooLong},
+			testWrite{"z", 0, ErrWriteTooLong},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{5, "abcde"}, sparseHoles{{2, 3}}, 8},
+		tests: []testFnc{
+			testRemaining{8, 5},
+			testWrite{"ab\x00\x00\x00cde", 8, nil},
+			testWrite{"a", 0, ErrWriteTooLong},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{5, "abcde"}, sparseHoles{{2, 3}}, 8},
+		tests: []testFnc{
+			testWrite{"ab\x00\x00\x00cdez", 8, ErrWriteTooLong},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{5, "abcde"}, sparseHoles{{2, 3}}, 8},
+		tests: []testFnc{
+			testWrite{"ab\x00", 3, nil},
+			testRemaining{5, 3},
+			testWrite{"\x00\x00cde", 5, nil},
+			testWrite{"a", 0, ErrWriteTooLong},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{5, "abcde"}, sparseHoles{{2, 3}}, 8},
+		tests: []testFnc{
+			testWrite{"ab", 2, nil},
+			testRemaining{6, 3},
+			testReadFrom{fileOps{int64(3), "cde"}, 6, nil},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{5, "abcde"}, sparseHoles{{2, 3}}, 8},
+		tests: []testFnc{
+			testReadFrom{fileOps{"ab", int64(3), "cde"}, 8, nil},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{5, "abcde"}, sparseHoles{{2, 3}}, 8},
+		tests: []testFnc{
+			testReadFrom{fileOps{"ab", int64(3), "cdeX"}, 8, ErrWriteTooLong},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{4, "abcd"}, sparseHoles{{2, 3}}, 8},
+		tests: []testFnc{
+			testReadFrom{fileOps{"ab", int64(3), "cd"}, 7, io.ErrUnexpectedEOF},
+			testRemaining{1, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{4, "abcd"}, sparseHoles{{2, 3}}, 8},
+		tests: []testFnc{
+			testReadFrom{fileOps{"ab", int64(3), "cde"}, 7, errMissData},
+			testRemaining{1, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{6, "abcde"}, sparseHoles{{2, 3}}, 8},
+		tests: []testFnc{
+			testReadFrom{fileOps{"ab", int64(3), "cde"}, 8, errUnrefData},
+			testRemaining{0, 1},
+		},
+	}, {
+		maker: makeSparse{makeReg{4, "abcd"}, sparseHoles{{2, 3}}, 8},
+		tests: []testFnc{
+			testWrite{"ab", 2, nil},
+			testRemaining{6, 2},
+			testWrite{"\x00\x00\x00", 3, nil},
+			testRemaining{3, 2},
+			testWrite{"cde", 2, errMissData},
+			testRemaining{1, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{6, "abcde"}, sparseHoles{{2, 3}}, 8},
+		tests: []testFnc{
+			testWrite{"ab", 2, nil},
+			testRemaining{6, 4},
+			testWrite{"\x00\x00\x00", 3, nil},
+			testRemaining{3, 4},
+			testWrite{"cde", 3, errUnrefData},
+			testRemaining{0, 1},
+		},
+	}, {
+		maker: makeSparse{makeReg{3, "abc"}, sparseHoles{{0, 2}, {5, 2}}, 7},
+		tests: []testFnc{
+			testRemaining{7, 3},
+			testWrite{"\x00\x00abc\x00\x00", 7, nil},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{3, "abc"}, sparseHoles{{0, 2}, {5, 2}}, 7},
+		tests: []testFnc{
+			testRemaining{7, 3},
+			testReadFrom{fileOps{int64(2), "abc", int64(1), "\x00"}, 7, nil},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{3, ""}, sparseHoles{{0, 2}, {5, 2}}, 7},
+		tests: []testFnc{
+			testWrite{"abcdefg", 0, errWriteHole},
+		},
+	}, {
+		maker: makeSparse{makeReg{3, "abc"}, sparseHoles{{0, 2}, {5, 2}}, 7},
+		tests: []testFnc{
+			testWrite{"\x00\x00abcde", 5, errWriteHole},
+		},
+	}, {
+		maker: makeSparse{makeReg{3, "abc"}, sparseHoles{{0, 2}, {5, 2}}, 7},
+		tests: []testFnc{
+			testWrite{"\x00\x00abc\x00\x00z", 7, ErrWriteTooLong},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{3, "abc"}, sparseHoles{{0, 2}, {5, 2}}, 7},
+		tests: []testFnc{
+			testWrite{"\x00\x00", 2, nil},
+			testRemaining{5, 3},
+			testWrite{"abc", 3, nil},
+			testRemaining{2, 0},
+			testWrite{"\x00\x00", 2, nil},
+			testRemaining{0, 0},
+		},
+	}, {
+		maker: makeSparse{makeReg{2, "ab"}, sparseHoles{{0, 2}, {5, 2}}, 7},
+		tests: []testFnc{
+			testWrite{"\x00\x00", 2, nil},
+			testWrite{"abc", 2, errMissData},
+			testWrite{"\x00\x00", 0, errMissData},
+		},
+	}, {
+		maker: makeSparse{makeReg{4, "abc"}, sparseHoles{{0, 2}, {5, 2}}, 7},
+		tests: []testFnc{
+			testWrite{"\x00\x00", 2, nil},
+			testWrite{"abc", 3, nil},
+			testWrite{"\x00\x00", 2, errUnrefData},
+		},
+	}}
+
+	for i, v := range vectors {
+		var wantStr string
+		bb := new(bytes.Buffer)
+		w := testNonEmptyWriter{bb}
+		var fw fileWriter
+		switch maker := v.maker.(type) {
+		case makeReg:
+			fw = &regFileWriter{w, maker.size}
+			wantStr = maker.wantStr
+		case makeSparse:
+			if !validateSparseEntries(maker.sph, maker.size) {
+				t.Fatalf("invalid sparse map: %v", maker.sph)
+			}
+			spd := invertSparseEntries(maker.sph, maker.size)
+			fw = &regFileWriter{w, maker.makeReg.size}
+			fw = &sparseFileWriter{fw, spd, 0}
+			wantStr = maker.makeReg.wantStr
+		default:
+			t.Fatalf("test %d, unknown make operation: %T", i, maker)
+		}
+
+		for j, tf := range v.tests {
+			switch tf := tf.(type) {
+			case testWrite:
+				got, err := fw.Write([]byte(tf.str))
+				if got != tf.wantCnt || err != tf.wantErr {
+					t.Errorf("test %d.%d, Write(%s):\ngot  (%d, %v)\nwant (%d, %v)", i, j, tf.str, got, err, tf.wantCnt, tf.wantErr)
+				}
+			case testReadFrom:
+				f := &testFile{ops: tf.ops}
+				got, err := fw.ReadFrom(f)
+				if _, ok := err.(testError); ok {
+					t.Errorf("test %d.%d, ReadFrom(): %v", i, j, err)
+				} else if got != tf.wantCnt || err != tf.wantErr {
+					t.Errorf("test %d.%d, ReadFrom() = (%d, %v), want (%d, %v)", i, j, got, err, tf.wantCnt, tf.wantErr)
+				}
+				if len(f.ops) > 0 {
+					t.Errorf("test %d.%d, expected %d more operations", i, j, len(f.ops))
+				}
+			case testRemaining:
+				if got := fw.LogicalRemaining(); got != tf.wantLCnt {
+					t.Errorf("test %d.%d, LogicalRemaining() = %d, want %d", i, j, got, tf.wantLCnt)
+				}
+				if got := fw.PhysicalRemaining(); got != tf.wantPCnt {
+					t.Errorf("test %d.%d, PhysicalRemaining() = %d, want %d", i, j, got, tf.wantPCnt)
+				}
+			default:
+				t.Fatalf("test %d.%d, unknown test operation: %T", i, j, tf)
+			}
+		}
+
+		if got := bb.String(); got != wantStr {
+			t.Fatalf("test %d, String() = %q, want %q", i, got, wantStr)
+		}
+	}
+}