libgo: update to Go1.16beta1 release

This does not yet include support for the //go:embed directive added
in this release.

	* Makefile.am (check-runtime): Don't create check-runtime-dir.
	(mostlyclean-local): Don't remove check-runtime-dir.
	(check-go-tool, check-vet): Copy in go.mod and modules.txt.
	(check-cgo-test, check-carchive-test): Add go.mod file.
	* Makefile.in: Regenerate.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/280172

17 files changed, 1379 insertions, 93 deletions

diff --git a/libgo/go/testing/benchmark.go b/libgo/go/testing/benchmark.go
index e9687bf..a8f75e9 100644
--- a/libgo/go/testing/benchmark.go
+++ b/libgo/go/testing/benchmark.go
@@ -8,6 +8,7 @@ import (
 	"flag"
 	"fmt"
 	"internal/race"
+	"internal/sysinfo"
 	"io"
 	"math"
 	"os"
@@ -262,6 +263,9 @@ func (b *B) run() {
 		if b.importPath != "" {
 			fmt.Fprintf(b.w, "pkg: %s\n", b.importPath)
 		}
+		if cpu := sysinfo.CPU.Name(); cpu != "" {
+			fmt.Fprintf(b.w, "cpu: %s\n", cpu)
+		}
 	})
 	if b.context != nil {
 		// Running go test --test.bench
@@ -447,23 +451,27 @@ func (r BenchmarkResult) String() string {
 
 func prettyPrint(w io.Writer, x float64, unit string) {
 	// Print all numbers with 10 places before the decimal point
-	// and small numbers with three sig figs.
+	// and small numbers with four sig figs. Field widths are
+	// chosen to fit the whole part in 10 places while aligning
+	// the decimal point of all fractional formats.
 	var format string
 	switch y := math.Abs(x); {
-	case y == 0 || y >= 99.95:
+	case y == 0 || y >= 999.95:
 		format = "%10.0f %s"
-	case y >= 9.995:
+	case y >= 99.995:
 		format = "%12.1f %s"
-	case y >= 0.9995:
+	case y >= 9.9995:
 		format = "%13.2f %s"
-	case y >= 0.09995:
+	case y >= 0.99995:
 		format = "%14.3f %s"
-	case y >= 0.009995:
+	case y >= 0.099995:
 		format = "%15.4f %s"
-	case y >= 0.0009995:
+	case y >= 0.0099995:
 		format = "%16.5f %s"
-	default:
+	case y >= 0.00099995:
 		format = "%17.6f %s"
+	default:
+		format = "%18.7f %s"
 	}
 	fmt.Fprintf(w, format, x, unit)
 }
@@ -648,6 +656,9 @@ func (b *B) Run(name string, f func(b *B)) bool {
 			if b.importPath != "" {
 				fmt.Printf("pkg: %s\n", b.importPath)
 			}
+			if cpu := sysinfo.CPU.Name(); cpu != "" {
+				fmt.Printf("cpu: %s\n", cpu)
+			}
 		})
 
 		fmt.Println(benchName)
diff --git a/libgo/go/testing/benchmark_test.go b/libgo/go/testing/benchmark_test.go
index 1434c26..4c1cbd1 100644
--- a/libgo/go/testing/benchmark_test.go
+++ b/libgo/go/testing/benchmark_test.go
@@ -22,13 +22,14 @@ var prettyPrintTests = []struct {
 	{0, "         0 x"},
 	{1234.1, "      1234 x"},
 	{-1234.1, "     -1234 x"},
-	{99.950001, "       100 x"},
-	{99.949999, "        99.9 x"},
-	{9.9950001, "        10.0 x"},
-	{9.9949999, "         9.99 x"},
-	{-9.9949999, "        -9.99 x"},
-	{0.0099950001, "         0.0100 x"},
-	{0.0099949999, "         0.00999 x"},
+	{999.950001, "      1000 x"},
+	{999.949999, "       999.9 x"},
+	{99.9950001, "       100.0 x"},
+	{99.9949999, "        99.99 x"},
+	{-99.9949999, "       -99.99 x"},
+	{0.000999950001, "         0.001000 x"},
+	{0.000999949999, "         0.0009999 x"}, // smallest case
+	{0.0000999949999, "         0.0001000 x"},
 }
 
 func TestPrettyPrint(t *testing.T) {
@@ -50,13 +51,13 @@ func TestResultString(t *testing.T) {
 	if r.NsPerOp() != 2 {
 		t.Errorf("NsPerOp: expected 2, actual %v", r.NsPerOp())
 	}
-	if want, got := "     100\t         2.40 ns/op", r.String(); want != got {
+	if want, got := "     100\t         2.400 ns/op", r.String(); want != got {
 		t.Errorf("String: expected %q, actual %q", want, got)
 	}
 
 	// Test sub-1 ns/op (issue #31005)
 	r.T = 40 * time.Nanosecond
-	if want, got := "     100\t         0.400 ns/op", r.String(); want != got {
+	if want, got := "     100\t         0.4000 ns/op", r.String(); want != got {
 		t.Errorf("String: expected %q, actual %q", want, got)
 	}
 
@@ -130,7 +131,7 @@ func TestReportMetric(t *testing.T) {
 	}
 	// Test stringing.
 	res.N = 1 // Make the output stable
-	want := "       1\t     12345 ns/op\t         0.200 frobs/op"
+	want := "       1\t     12345 ns/op\t         0.2000 frobs/op"
 	if want != res.String() {
 		t.Errorf("expected %q, actual %q", want, res.String())
 	}
diff --git a/libgo/go/testing/example.go b/libgo/go/testing/example.go
index adc91d5..0217c5d 100644
--- a/libgo/go/testing/example.go
+++ b/libgo/go/testing/example.go
@@ -62,9 +62,10 @@ func sortLines(output string) string {
 // If stdout doesn't match the expected output or if recovered is non-nil, it'll print the cause of failure to stdout.
 // If the test is chatty/verbose, it'll print a success message to stdout.
 // If recovered is non-nil, it'll panic with that value.
-func (eg *InternalExample) processRunResult(stdout string, timeSpent time.Duration, recovered interface{}) (passed bool) {
+// If the test panicked with nil, or invoked runtime.Goexit, it'll be
+// made to fail and panic with errNilPanicOrGoexit
+func (eg *InternalExample) processRunResult(stdout string, timeSpent time.Duration, finished bool, recovered interface{}) (passed bool) {
 	passed = true
-
 	dstr := fmtDuration(timeSpent)
 	var fail string
 	got := strings.TrimSpace(stdout)
@@ -78,16 +79,20 @@ func (eg *InternalExample) processRunResult(stdout string, timeSpent time.Durati
 			fail = fmt.Sprintf("got:\n%s\nwant:\n%s\n", got, want)
 		}
 	}
-	if fail != "" || recovered != nil {
+	if fail != "" || !finished || recovered != nil {
 		fmt.Printf("--- FAIL: %s (%s)\n%s", eg.Name, dstr, fail)
 		passed = false
 	} else if *chatty {
 		fmt.Printf("--- PASS: %s (%s)\n", eg.Name, dstr)
 	}
+
 	if recovered != nil {
 		// Propagate the previously recovered result, by panicking.
 		panic(recovered)
 	}
+	if !finished && recovered == nil {
+		panic(errNilPanicOrGoexit)
+	}
 
 	return
 }
diff --git a/libgo/go/testing/fstest/mapfs.go b/libgo/go/testing/fstest/mapfs.go
new file mode 100644
index 0000000..a5d4a23fa
--- /dev/null
+++ b/libgo/go/testing/fstest/mapfs.go
@@ -0,0 +1,238 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fstest
+
+import (
+	"io"
+	"io/fs"
+	"path"
+	"sort"
+	"strings"
+	"time"
+)
+
+// A MapFS is a simple in-memory file system for use in tests,
+// represented as a map from path names (arguments to Open)
+// to information about the files or directories they represent.
+//
+// The map need not include parent directories for files contained
+// in the map; those will be synthesized if needed.
+// But a directory can still be included by setting the MapFile.Mode's ModeDir bit;
+// this may be necessary for detailed control over the directory's FileInfo
+// or to create an empty directory.
+//
+// File system operations read directly from the map,
+// so that the file system can be changed by editing the map as needed.
+// An implication is that file system operations must not run concurrently
+// with changes to the map, which would be a race.
+// Another implication is that opening or reading a directory requires
+// iterating over the entire map, so a MapFS should typically be used with not more
+// than a few hundred entries or directory reads.
+type MapFS map[string]*MapFile
+
+// A MapFile describes a single file in a MapFS.
+type MapFile struct {
+	Data    []byte      // file content
+	Mode    fs.FileMode // FileInfo.Mode
+	ModTime time.Time   // FileInfo.ModTime
+	Sys     interface{} // FileInfo.Sys
+}
+
+var _ fs.FS = MapFS(nil)
+var _ fs.File = (*openMapFile)(nil)
+
+// Open opens the named file.
+func (fsys MapFS) Open(name string) (fs.File, error) {
+	if !fs.ValidPath(name) {
+		return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrNotExist}
+	}
+	file := fsys[name]
+	if file != nil && file.Mode&fs.ModeDir == 0 {
+		// Ordinary file
+		return &openMapFile{name, mapFileInfo{path.Base(name), file}, 0}, nil
+	}
+
+	// Directory, possibly synthesized.
+	// Note that file can be nil here: the map need not contain explicit parent directories for all its files.
+	// But file can also be non-nil, in case the user wants to set metadata for the directory explicitly.
+	// Either way, we need to construct the list of children of this directory.
+	var list []mapFileInfo
+	var elem string
+	var need = make(map[string]bool)
+	if name == "." {
+		elem = "."
+		for fname, f := range fsys {
+			i := strings.Index(fname, "/")
+			if i < 0 {
+				list = append(list, mapFileInfo{fname, f})
+			} else {
+				need[fname[:i]] = true
+			}
+		}
+	} else {
+		elem = name[strings.LastIndex(name, "/")+1:]
+		prefix := name + "/"
+		for fname, f := range fsys {
+			if strings.HasPrefix(fname, prefix) {
+				felem := fname[len(prefix):]
+				i := strings.Index(felem, "/")
+				if i < 0 {
+					list = append(list, mapFileInfo{felem, f})
+				} else {
+					need[fname[len(prefix):len(prefix)+i]] = true
+				}
+			}
+		}
+		// If the directory name is not in the map,
+		// and there are no children of the name in the map,
+		// then the directory is treated as not existing.
+		if file == nil && list == nil && len(need) == 0 {
+			return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrNotExist}
+		}
+	}
+	for _, fi := range list {
+		delete(need, fi.name)
+	}
+	for name := range need {
+		list = append(list, mapFileInfo{name, &MapFile{Mode: fs.ModeDir}})
+	}
+	sort.Slice(list, func(i, j int) bool {
+		return list[i].name < list[j].name
+	})
+
+	if file == nil {
+		file = &MapFile{Mode: fs.ModeDir}
+	}
+	return &mapDir{name, mapFileInfo{elem, file}, list, 0}, nil
+}
+
+// fsOnly is a wrapper that hides all but the fs.FS methods,
+// to avoid an infinite recursion when implementing special
+// methods in terms of helpers that would use them.
+// (In general, implementing these methods using the package fs helpers
+// is redundant and unnecessary, but having the methods may make
+// MapFS exercise more code paths when used in tests.)
+type fsOnly struct{ fs.FS }
+
+func (fsys MapFS) ReadFile(name string) ([]byte, error) {
+	return fs.ReadFile(fsOnly{fsys}, name)
+}
+
+func (fsys MapFS) Stat(name string) (fs.FileInfo, error) {
+	return fs.Stat(fsOnly{fsys}, name)
+}
+
+func (fsys MapFS) ReadDir(name string) ([]fs.DirEntry, error) {
+	return fs.ReadDir(fsOnly{fsys}, name)
+}
+
+func (fsys MapFS) Glob(pattern string) ([]string, error) {
+	return fs.Glob(fsOnly{fsys}, pattern)
+}
+
+type noSub struct {
+	MapFS
+}
+
+func (noSub) Sub() {} // not the fs.SubFS signature
+
+func (fsys MapFS) Sub(dir string) (fs.FS, error) {
+	return fs.Sub(noSub{fsys}, dir)
+}
+
+// A mapFileInfo implements fs.FileInfo and fs.DirEntry for a given map file.
+type mapFileInfo struct {
+	name string
+	f    *MapFile
+}
+
+func (i *mapFileInfo) Name() string               { return i.name }
+func (i *mapFileInfo) Size() int64                { return int64(len(i.f.Data)) }
+func (i *mapFileInfo) Mode() fs.FileMode          { return i.f.Mode }
+func (i *mapFileInfo) Type() fs.FileMode          { return i.f.Mode.Type() }
+func (i *mapFileInfo) ModTime() time.Time         { return i.f.ModTime }
+func (i *mapFileInfo) IsDir() bool                { return i.f.Mode&fs.ModeDir != 0 }
+func (i *mapFileInfo) Sys() interface{}           { return i.f.Sys }
+func (i *mapFileInfo) Info() (fs.FileInfo, error) { return i, nil }
+
+// An openMapFile is a regular (non-directory) fs.File open for reading.
+type openMapFile struct {
+	path string
+	mapFileInfo
+	offset int64
+}
+
+func (f *openMapFile) Stat() (fs.FileInfo, error) { return &f.mapFileInfo, nil }
+
+func (f *openMapFile) Close() error { return nil }
+
+func (f *openMapFile) Read(b []byte) (int, error) {
+	if f.offset >= int64(len(f.f.Data)) {
+		return 0, io.EOF
+	}
+	if f.offset < 0 {
+		return 0, &fs.PathError{Op: "read", Path: f.path, Err: fs.ErrInvalid}
+	}
+	n := copy(b, f.f.Data[f.offset:])
+	f.offset += int64(n)
+	return n, nil
+}
+
+func (f *openMapFile) Seek(offset int64, whence int) (int64, error) {
+	switch whence {
+	case 0:
+		// offset += 0
+	case 1:
+		offset += f.offset
+	case 2:
+		offset += int64(len(f.f.Data))
+	}
+	if offset < 0 || offset > int64(len(f.f.Data)) {
+		return 0, &fs.PathError{Op: "seek", Path: f.path, Err: fs.ErrInvalid}
+	}
+	f.offset = offset
+	return offset, nil
+}
+
+func (f *openMapFile) ReadAt(b []byte, offset int64) (int, error) {
+	if offset < 0 || offset > int64(len(f.f.Data)) {
+		return 0, &fs.PathError{Op: "read", Path: f.path, Err: fs.ErrInvalid}
+	}
+	n := copy(b, f.f.Data[offset:])
+	if n < len(b) {
+		return n, io.EOF
+	}
+	return n, nil
+}
+
+// A mapDir is a directory fs.File (so also an fs.ReadDirFile) open for reading.
+type mapDir struct {
+	path string
+	mapFileInfo
+	entry  []mapFileInfo
+	offset int
+}
+
+func (d *mapDir) Stat() (fs.FileInfo, error) { return &d.mapFileInfo, nil }
+func (d *mapDir) Close() error               { return nil }
+func (d *mapDir) Read(b []byte) (int, error) {
+	return 0, &fs.PathError{Op: "read", Path: d.path, Err: fs.ErrInvalid}
+}
+
+func (d *mapDir) ReadDir(count int) ([]fs.DirEntry, error) {
+	n := len(d.entry) - d.offset
+	if count > 0 && n > count {
+		n = count
+	}
+	if n == 0 && count > 0 {
+		return nil, io.EOF
+	}
+	list := make([]fs.DirEntry, n)
+	for i := range list {
+		list[i] = &d.entry[d.offset+i]
+	}
+	d.offset += n
+	return list, nil
+}
diff --git a/libgo/go/testing/fstest/mapfs_test.go b/libgo/go/testing/fstest/mapfs_test.go
new file mode 100644
index 0000000..2abedd6
--- /dev/null
+++ b/libgo/go/testing/fstest/mapfs_test.go
@@ -0,0 +1,19 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fstest
+
+import (
+	"testing"
+)
+
+func TestMapFS(t *testing.T) {
+	m := MapFS{
+		"hello":             {Data: []byte("hello, world\n")},
+		"fortune/k/ken.txt": {Data: []byte("If a program is too slow, it must have a loop.\n")},
+	}
+	if err := TestFS(m, "hello", "fortune/k/ken.txt"); err != nil {
+		t.Fatal(err)
+	}
+}
diff --git a/libgo/go/testing/fstest/testfs.go b/libgo/go/testing/fstest/testfs.go
new file mode 100644
index 0000000..2602bdf
--- /dev/null
+++ b/libgo/go/testing/fstest/testfs.go
@@ -0,0 +1,602 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package fstest implements support for testing implementations and users of file systems.
+package fstest
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"io/ioutil"
+	"path"
+	"reflect"
+	"sort"
+	"strings"
+	"testing/iotest"
+)
+
+// TestFS tests a file system implementation.
+// It walks the entire tree of files in fsys,
+// opening and checking that each file behaves correctly.
+// It also checks that the file system contains at least the expected files.
+// As a special case, if no expected files are listed, fsys must be empty.
+// Otherwise, fsys must only contain at least the listed files: it can also contain others.
+//
+// If TestFS finds any misbehaviors, it returns an error reporting all of them.
+// The error text spans multiple lines, one per detected misbehavior.
+//
+// Typical usage inside a test is:
+//
+//	if err := fstest.TestFS(myFS, "file/that/should/be/present"); err != nil {
+//		t.Fatal(err)
+//	}
+//
+func TestFS(fsys fs.FS, expected ...string) error {
+	if err := testFS(fsys, expected...); err != nil {
+		return err
+	}
+	for _, name := range expected {
+		if i := strings.Index(name, "/"); i >= 0 {
+			dir, dirSlash := name[:i], name[:i+1]
+			var subExpected []string
+			for _, name := range expected {
+				if strings.HasPrefix(name, dirSlash) {
+					subExpected = append(subExpected, name[len(dirSlash):])
+				}
+			}
+			sub, err := fs.Sub(fsys, dir)
+			if err != nil {
+				return err
+			}
+			if err := testFS(sub, subExpected...); err != nil {
+				return fmt.Errorf("testing fs.Sub(fsys, %s): %v", dir, err)
+			}
+			break // one sub-test is enough
+		}
+	}
+	return nil
+}
+
+func testFS(fsys fs.FS, expected ...string) error {
+	t := fsTester{fsys: fsys}
+	t.checkDir(".")
+	t.checkOpen(".")
+	found := make(map[string]bool)
+	for _, dir := range t.dirs {
+		found[dir] = true
+	}
+	for _, file := range t.files {
+		found[file] = true
+	}
+	delete(found, ".")
+	if len(expected) == 0 && len(found) > 0 {
+		var list []string
+		for k := range found {
+			if k != "." {
+				list = append(list, k)
+			}
+		}
+		sort.Strings(list)
+		if len(list) > 15 {
+			list = append(list[:10], "...")
+		}
+		t.errorf("expected empty file system but found files:\n%s", strings.Join(list, "\n"))
+	}
+	for _, name := range expected {
+		if !found[name] {
+			t.errorf("expected but not found: %s", name)
+		}
+	}
+	if len(t.errText) == 0 {
+		return nil
+	}
+	return errors.New("TestFS found errors:\n" + string(t.errText))
+}
+
+// An fsTester holds state for running the test.
+type fsTester struct {
+	fsys    fs.FS
+	errText []byte
+	dirs    []string
+	files   []string
+}
+
+// errorf adds an error line to errText.
+func (t *fsTester) errorf(format string, args ...interface{}) {
+	if len(t.errText) > 0 {
+		t.errText = append(t.errText, '\n')
+	}
+	t.errText = append(t.errText, fmt.Sprintf(format, args...)...)
+}
+
+func (t *fsTester) openDir(dir string) fs.ReadDirFile {
+	f, err := t.fsys.Open(dir)
+	if err != nil {
+		t.errorf("%s: Open: %v", dir, err)
+		return nil
+	}
+	d, ok := f.(fs.ReadDirFile)
+	if !ok {
+		f.Close()
+		t.errorf("%s: Open returned File type %T, not a io.ReadDirFile", dir, f)
+		return nil
+	}
+	return d
+}
+
+// checkDir checks the directory dir, which is expected to exist
+// (it is either the root or was found in a directory listing with IsDir true).
+func (t *fsTester) checkDir(dir string) {
+	// Read entire directory.
+	t.dirs = append(t.dirs, dir)
+	d := t.openDir(dir)
+	if d == nil {
+		return
+	}
+	list, err := d.ReadDir(-1)
+	if err != nil {
+		d.Close()
+		t.errorf("%s: ReadDir(-1): %v", dir, err)
+		return
+	}
+
+	// Check all children.
+	var prefix string
+	if dir == "." {
+		prefix = ""
+	} else {
+		prefix = dir + "/"
+	}
+	for _, info := range list {
+		name := info.Name()
+		switch {
+		case name == ".", name == "..", name == "":
+			t.errorf("%s: ReadDir: child has invalid name: %#q", dir, name)
+			continue
+		case strings.Contains(name, "/"):
+			t.errorf("%s: ReadDir: child name contains slash: %#q", dir, name)
+			continue
+		case strings.Contains(name, `\`):
+			t.errorf("%s: ReadDir: child name contains backslash: %#q", dir, name)
+			continue
+		}
+		path := prefix + name
+		t.checkStat(path, info)
+		t.checkOpen(path)
+		if info.IsDir() {
+			t.checkDir(path)
+		} else {
+			t.checkFile(path)
+		}
+	}
+
+	// Check ReadDir(-1) at EOF.
+	list2, err := d.ReadDir(-1)
+	if len(list2) > 0 || err != nil {
+		d.Close()
+		t.errorf("%s: ReadDir(-1) at EOF = %d entries, %v, wanted 0 entries, nil", dir, len(list2), err)
+		return
+	}
+
+	// Check ReadDir(1) at EOF (different results).
+	list2, err = d.ReadDir(1)
+	if len(list2) > 0 || err != io.EOF {
+		d.Close()
+		t.errorf("%s: ReadDir(1) at EOF = %d entries, %v, wanted 0 entries, EOF", dir, len(list2), err)
+		return
+	}
+
+	// Check that close does not report an error.
+	if err := d.Close(); err != nil {
+		t.errorf("%s: Close: %v", dir, err)
+	}
+
+	// Check that closing twice doesn't crash.
+	// The return value doesn't matter.
+	d.Close()
+
+	// Reopen directory, read a second time, make sure contents match.
+	if d = t.openDir(dir); d == nil {
+		return
+	}
+	defer d.Close()
+	list2, err = d.ReadDir(-1)
+	if err != nil {
+		t.errorf("%s: second Open+ReadDir(-1): %v", dir, err)
+		return
+	}
+	t.checkDirList(dir, "first Open+ReadDir(-1) vs second Open+ReadDir(-1)", list, list2)
+
+	// Reopen directory, read a third time in pieces, make sure contents match.
+	if d = t.openDir(dir); d == nil {
+		return
+	}
+	defer d.Close()
+	list2 = nil
+	for {
+		n := 1
+		if len(list2) > 0 {
+			n = 2
+		}
+		frag, err := d.ReadDir(n)
+		if len(frag) > n {
+			t.errorf("%s: third Open: ReadDir(%d) after %d: %d entries (too many)", dir, n, len(list2), len(frag))
+			return
+		}
+		list2 = append(list2, frag...)
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			t.errorf("%s: third Open: ReadDir(%d) after %d: %v", dir, n, len(list2), err)
+			return
+		}
+		if n == 0 {
+			t.errorf("%s: third Open: ReadDir(%d) after %d: 0 entries but nil error", dir, n, len(list2))
+			return
+		}
+	}
+	t.checkDirList(dir, "first Open+ReadDir(-1) vs third Open+ReadDir(1,2) loop", list, list2)
+
+	// If fsys has ReadDir, check that it matches and is sorted.
+	if fsys, ok := t.fsys.(fs.ReadDirFS); ok {
+		list2, err := fsys.ReadDir(dir)
+		if err != nil {
+			t.errorf("%s: fsys.ReadDir: %v", dir, err)
+			return
+		}
+		t.checkDirList(dir, "first Open+ReadDir(-1) vs fsys.ReadDir", list, list2)
+
+		for i := 0; i+1 < len(list2); i++ {
+			if list2[i].Name() >= list2[i+1].Name() {
+				t.errorf("%s: fsys.ReadDir: list not sorted: %s before %s", dir, list2[i].Name(), list2[i+1].Name())
+			}
+		}
+	}
+
+	// Check fs.ReadDir as well.
+	list2, err = fs.ReadDir(t.fsys, dir)
+	if err != nil {
+		t.errorf("%s: fs.ReadDir: %v", dir, err)
+		return
+	}
+	t.checkDirList(dir, "first Open+ReadDir(-1) vs fs.ReadDir", list, list2)
+
+	for i := 0; i+1 < len(list2); i++ {
+		if list2[i].Name() >= list2[i+1].Name() {
+			t.errorf("%s: fs.ReadDir: list not sorted: %s before %s", dir, list2[i].Name(), list2[i+1].Name())
+		}
+	}
+
+	t.checkGlob(dir, list)
+}
+
+// formatEntry formats an fs.DirEntry into a string for error messages and comparison.
+func formatEntry(entry fs.DirEntry) string {
+	return fmt.Sprintf("%s IsDir=%v Type=%v", entry.Name(), entry.IsDir(), entry.Type())
+}
+
+// formatInfoEntry formats an fs.FileInfo into a string like the result of formatEntry, for error messages and comparison.
+func formatInfoEntry(info fs.FileInfo) string {
+	return fmt.Sprintf("%s IsDir=%v Type=%v", info.Name(), info.IsDir(), info.Mode().Type())
+}
+
+// formatInfo formats an fs.FileInfo into a string for error messages and comparison.
+func formatInfo(info fs.FileInfo) string {
+	return fmt.Sprintf("%s IsDir=%v Mode=%v Size=%d ModTime=%v", info.Name(), info.IsDir(), info.Mode(), info.Size(), info.ModTime())
+}
+
+// checkGlob checks that various glob patterns work if the file system implements GlobFS.
+func (t *fsTester) checkGlob(dir string, list []fs.DirEntry) {
+	if _, ok := t.fsys.(fs.GlobFS); !ok {
+		return
+	}
+
+	// Make a complex glob pattern prefix that only matches dir.
+	var glob string
+	if dir != "." {
+		elem := strings.Split(dir, "/")
+		for i, e := range elem {
+			var pattern []rune
+			for j, r := range e {
+				if r == '*' || r == '?' || r == '\\' || r == '[' {
+					pattern = append(pattern, '\\', r)
+					continue
+				}
+				switch (i + j) % 5 {
+				case 0:
+					pattern = append(pattern, r)
+				case 1:
+					pattern = append(pattern, '[', r, ']')
+				case 2:
+					pattern = append(pattern, '[', r, '-', r, ']')
+				case 3:
+					pattern = append(pattern, '[', '\\', r, ']')
+				case 4:
+					pattern = append(pattern, '[', '\\', r, '-', '\\', r, ']')
+				}
+			}
+			elem[i] = string(pattern)
+		}
+		glob = strings.Join(elem, "/") + "/"
+	}
+
+	// Test that malformed patterns are detected.
+	// The error is likely path.ErrBadPattern but need not be.
+	if _, err := t.fsys.(fs.GlobFS).Glob(glob + "nonexist/[]"); err == nil {
+		t.errorf("%s: Glob(%#q): bad pattern not detected", dir, glob+"nonexist/[]")
+	}
+
+	// Try to find a letter that appears in only some of the final names.
+	c := rune('a')
+	for ; c <= 'z'; c++ {
+		have, haveNot := false, false
+		for _, d := range list {
+			if strings.ContainsRune(d.Name(), c) {
+				have = true
+			} else {
+				haveNot = true
+			}
+		}
+		if have && haveNot {
+			break
+		}
+	}
+	if c > 'z' {
+		c = 'a'
+	}
+	glob += "*" + string(c) + "*"
+
+	var want []string
+	for _, d := range list {
+		if strings.ContainsRune(d.Name(), c) {
+			want = append(want, path.Join(dir, d.Name()))
+		}
+	}
+
+	names, err := t.fsys.(fs.GlobFS).Glob(glob)
+	if err != nil {
+		t.errorf("%s: Glob(%#q): %v", dir, glob, err)
+		return
+	}
+	if reflect.DeepEqual(want, names) {
+		return
+	}
+
+	if !sort.StringsAreSorted(names) {
+		t.errorf("%s: Glob(%#q): unsorted output:\n%s", dir, glob, strings.Join(names, "\n"))
+		sort.Strings(names)
+	}
+
+	var problems []string
+	for len(want) > 0 || len(names) > 0 {
+		switch {
+		case len(want) > 0 && len(names) > 0 && want[0] == names[0]:
+			want, names = want[1:], names[1:]
+		case len(want) > 0 && (len(names) == 0 || want[0] < names[0]):
+			problems = append(problems, "missing: "+want[0])
+			want = want[1:]
+		default:
+			problems = append(problems, "extra: "+names[0])
+			names = names[1:]
+		}
+	}
+	t.errorf("%s: Glob(%#q): wrong output:\n%s", dir, glob, strings.Join(problems, "\n"))
+}
+
+// checkStat checks that a direct stat of path matches entry,
+// which was found in the parent's directory listing.
+func (t *fsTester) checkStat(path string, entry fs.DirEntry) {
+	file, err := t.fsys.Open(path)
+	if err != nil {
+		t.errorf("%s: Open: %v", path, err)
+		return
+	}
+	info, err := file.Stat()
+	file.Close()
+	if err != nil {
+		t.errorf("%s: Stat: %v", path, err)
+		return
+	}
+	fentry := formatEntry(entry)
+	finfo := formatInfoEntry(info)
+	if fentry != finfo {
+		t.errorf("%s: mismatch:\n\tentry = %s\n\tfile.Stat() = %s", path, fentry, finfo)
+	}
+
+	einfo, err := entry.Info()
+	if err != nil {
+		t.errorf("%s: entry.Info: %v", path, err)
+		return
+	}
+	fentry = formatInfo(einfo)
+	finfo = formatInfo(info)
+	if fentry != finfo {
+		t.errorf("%s: mismatch:\n\tentry.Info() = %s\n\tfile.Stat() = %s\n", path, fentry, finfo)
+	}
+
+	info2, err := fs.Stat(t.fsys, path)
+	if err != nil {
+		t.errorf("%s: fs.Stat: %v", path, err)
+		return
+	}
+	finfo2 := formatInfo(info2)
+	if finfo2 != finfo {
+		t.errorf("%s: fs.Stat(...) = %s\n\twant %s", path, finfo2, finfo)
+	}
+
+	if fsys, ok := t.fsys.(fs.StatFS); ok {
+		info2, err := fsys.Stat(path)
+		if err != nil {
+			t.errorf("%s: fsys.Stat: %v", path, err)
+			return
+		}
+		finfo2 := formatInfo(info2)
+		if finfo2 != finfo {
+			t.errorf("%s: fsys.Stat(...) = %s\n\twant %s", path, finfo2, finfo)
+		}
+	}
+}
+
+// checkDirList checks that two directory lists contain the same files and file info.
+// The order of the lists need not match.
+func (t *fsTester) checkDirList(dir, desc string, list1, list2 []fs.DirEntry) {
+	old := make(map[string]fs.DirEntry)
+	checkMode := func(entry fs.DirEntry) {
+		if entry.IsDir() != (entry.Type()&fs.ModeDir != 0) {
+			if entry.IsDir() {
+				t.errorf("%s: ReadDir returned %s with IsDir() = true, Type() & ModeDir = 0", dir, entry.Name())
+			} else {
+				t.errorf("%s: ReadDir returned %s with IsDir() = false, Type() & ModeDir = ModeDir", dir, entry.Name())
+			}
+		}
+	}
+
+	for _, entry1 := range list1 {
+		old[entry1.Name()] = entry1
+		checkMode(entry1)
+	}
+
+	var diffs []string
+	for _, entry2 := range list2 {
+		entry1 := old[entry2.Name()]
+		if entry1 == nil {
+			checkMode(entry2)
+			diffs = append(diffs, "+ "+formatEntry(entry2))
+			continue
+		}
+		if formatEntry(entry1) != formatEntry(entry2) {
+			diffs = append(diffs, "- "+formatEntry(entry1), "+ "+formatEntry(entry2))
+		}
+		delete(old, entry2.Name())
+	}
+	for _, entry1 := range old {
+		diffs = append(diffs, "- "+formatEntry(entry1))
+	}
+
+	if len(diffs) == 0 {
+		return
+	}
+
+	sort.Slice(diffs, func(i, j int) bool {
+		fi := strings.Fields(diffs[i])
+		fj := strings.Fields(diffs[j])
+		// sort by name (i < j) and then +/- (j < i, because + < -)
+		return fi[1]+" "+fj[0] < fj[1]+" "+fi[0]
+	})
+
+	t.errorf("%s: diff %s:\n\t%s", dir, desc, strings.Join(diffs, "\n\t"))
+}
+
+// checkFile checks that basic file reading works correctly.
+func (t *fsTester) checkFile(file string) {
+	t.files = append(t.files, file)
+
+	// Read entire file.
+	f, err := t.fsys.Open(file)
+	if err != nil {
+		t.errorf("%s: Open: %v", file, err)
+		return
+	}
+
+	data, err := ioutil.ReadAll(f)
+	if err != nil {
+		f.Close()
+		t.errorf("%s: Open+ReadAll: %v", file, err)
+		return
+	}
+
+	if err := f.Close(); err != nil {
+		t.errorf("%s: Close: %v", file, err)
+	}
+
+	// Check that closing twice doesn't crash.
+	// The return value doesn't matter.
+	f.Close()
+
+	// Check that ReadFile works if present.
+	if fsys, ok := t.fsys.(fs.ReadFileFS); ok {
+		data2, err := fsys.ReadFile(file)
+		if err != nil {
+			t.errorf("%s: fsys.ReadFile: %v", file, err)
+			return
+		}
+		t.checkFileRead(file, "ReadAll vs fsys.ReadFile", data, data2)
+
+		t.checkBadPath(file, "ReadFile",
+			func(name string) error { _, err := fsys.ReadFile(name); return err })
+	}
+
+	// Check that fs.ReadFile works with t.fsys.
+	data2, err := fs.ReadFile(t.fsys, file)
+	if err != nil {
+		t.errorf("%s: fs.ReadFile: %v", file, err)
+		return
+	}
+	t.checkFileRead(file, "ReadAll vs fs.ReadFile", data, data2)
+
+	// Use iotest.TestReader to check small reads, Seek, ReadAt.
+	f, err = t.fsys.Open(file)
+	if err != nil {
+		t.errorf("%s: second Open: %v", file, err)
+		return
+	}
+	defer f.Close()
+	if err := iotest.TestReader(f, data); err != nil {
+		t.errorf("%s: failed TestReader:\n\t%s", file, strings.ReplaceAll(err.Error(), "\n", "\n\t"))
+	}
+}
+
+func (t *fsTester) checkFileRead(file, desc string, data1, data2 []byte) {
+	if string(data1) != string(data2) {
+		t.errorf("%s: %s: different data returned\n\t%q\n\t%q", file, desc, data1, data2)
+		return
+	}
+}
+
+// checkBadPath checks that various invalid forms of file's name cannot be opened using t.fsys.Open.
+func (t *fsTester) checkOpen(file string) {
+	t.checkBadPath(file, "Open", func(file string) error {
+		f, err := t.fsys.Open(file)
+		if err == nil {
+			f.Close()
+		}
+		return err
+	})
+}
+
+// checkBadPath checks that various invalid forms of file's name cannot be opened using open.
+func (t *fsTester) checkBadPath(file string, desc string, open func(string) error) {
+	bad := []string{
+		"/" + file,
+		file + "/.",
+	}
+	if file == "." {
+		bad = append(bad, "/")
+	}
+	if i := strings.Index(file, "/"); i >= 0 {
+		bad = append(bad,
+			file[:i]+"//"+file[i+1:],
+			file[:i]+"/./"+file[i+1:],
+			file[:i]+`\`+file[i+1:],
+			file[:i]+"/../"+file,
+		)
+	}
+	if i := strings.LastIndex(file, "/"); i >= 0 {
+		bad = append(bad,
+			file[:i]+"//"+file[i+1:],
+			file[:i]+"/./"+file[i+1:],
+			file[:i]+`\`+file[i+1:],
+			file+"/../"+file[i+1:],
+		)
+	}
+
+	for _, b := range bad {
+		if err := open(b); err == nil {
+			t.errorf("%s: %s(%s) succeeded, want error", file, desc, b)
+		}
+	}
+}
diff --git a/libgo/go/testing/helper_test.go b/libgo/go/testing/helper_test.go
index 7ce58c6..8858196 100644
--- a/libgo/go/testing/helper_test.go
+++ b/libgo/go/testing/helper_test.go
@@ -70,3 +70,34 @@ func TestTBHelperParallel(t *T) {
 		t.Errorf("got output line %q; want %q", got, want)
 	}
 }
+
+type noopWriter int
+
+func (nw *noopWriter) Write(b []byte) (int, error) { return len(b), nil }
+
+func BenchmarkTBHelper(b *B) {
+	w := noopWriter(0)
+	ctx := newTestContext(1, newMatcher(regexp.MatchString, "", ""))
+	t1 := &T{
+		common: common{
+			signal: make(chan bool),
+			w:      &w,
+		},
+		context: ctx,
+	}
+	f1 := func() {
+		t1.Helper()
+	}
+	f2 := func() {
+		t1.Helper()
+	}
+	b.ResetTimer()
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		if i&1 == 0 {
+			f1()
+		} else {
+			f2()
+		}
+	}
+}
diff --git a/libgo/go/testing/internal/testdeps/deps.go b/libgo/go/testing/internal/testdeps/deps.go
index af08dd7..3608d33 100644
--- a/libgo/go/testing/internal/testdeps/deps.go
+++ b/libgo/go/testing/internal/testdeps/deps.go
@@ -121,3 +121,8 @@ func (TestDeps) StopTestLog() error {
 	log.w = nil
 	return err
 }
+
+// SetPanicOnExit0 tells the os package whether to panic on os.Exit(0).
+func (TestDeps) SetPanicOnExit0(v bool) {
+	testlog.SetPanicOnExit0(v)
+}
diff --git a/libgo/go/testing/iotest/example_test.go b/libgo/go/testing/iotest/example_test.go
new file mode 100644
index 0000000..10f6bd3
--- /dev/null
+++ b/libgo/go/testing/iotest/example_test.go
@@ -0,0 +1,22 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package iotest_test
+
+import (
+	"errors"
+	"fmt"
+	"testing/iotest"
+)
+
+func ExampleErrReader() {
+	// A reader that always returns a custom error.
+	r := iotest.ErrReader(errors.New("custom error"))
+	n, err := r.Read(nil)
+	fmt.Printf("n:   %d\nerr: %q\n", n, err)
+
+	// Output:
+	// n:   0
+	// err: "custom error"
+}
diff --git a/libgo/go/testing/iotest/logger_test.go b/libgo/go/testing/iotest/logger_test.go
index c121bf4..fec4467 100644
--- a/libgo/go/testing/iotest/logger_test.go
+++ b/libgo/go/testing/iotest/logger_test.go
@@ -81,14 +81,6 @@ func TestWriteLogger_errorOnWrite(t *testing.T) {
 	}
 }
 
-type errReader struct {
-	err error
-}
-
-func (r errReader) Read([]byte) (int, error) {
-	return 0, r.err
-}
-
 func TestReadLogger(t *testing.T) {
 	olw := log.Writer()
 	olf := log.Flags()
@@ -146,14 +138,14 @@ func TestReadLogger_errorOnRead(t *testing.T) {
 	data := []byte("Hello, World!")
 	p := make([]byte, len(data))
 
-	lr := errReader{err: errors.New("Read Error!")}
+	lr := ErrReader(errors.New("io failure"))
 	rl := NewReadLogger("read", lr)
 	n, err := rl.Read(p)
 	if err == nil {
 		t.Fatalf("Unexpectedly succeeded to read: %v", err)
 	}
 
-	wantLogWithHex := fmt.Sprintf("lr: read %x: %v\n", p[:n], "Read Error!")
+	wantLogWithHex := fmt.Sprintf("lr: read %x: io failure\n", p[:n])
 	if g, w := lOut.String(), wantLogWithHex; g != w {
 		t.Errorf("ReadLogger mismatch\n\tgot:  %q\n\twant: %q", g, w)
 	}
diff --git a/libgo/go/testing/iotest/reader.go b/libgo/go/testing/iotest/reader.go
index 8d82018..770d87f 100644
--- a/libgo/go/testing/iotest/reader.go
+++ b/libgo/go/testing/iotest/reader.go
@@ -6,7 +6,9 @@
 package iotest
 
 import (
+	"bytes"
 	"errors"
+	"fmt"
 	"io"
 )
 
@@ -68,6 +70,7 @@ func (r *dataErrReader) Read(p []byte) (n int, err error) {
 	return
 }
 
+// ErrTimeout is a fake timeout error.
 var ErrTimeout = errors.New("timeout")
 
 // TimeoutReader returns ErrTimeout on the second read
@@ -86,3 +89,180 @@ func (r *timeoutReader) Read(p []byte) (int, error) {
 	}
 	return r.r.Read(p)
 }
+
+// ErrReader returns an io.Reader that returns 0, err from all Read calls.
+func ErrReader(err error) io.Reader {
+	return &errReader{err: err}
+}
+
+type errReader struct {
+	err error
+}
+
+func (r *errReader) Read(p []byte) (int, error) {
+	return 0, r.err
+}
+
+type smallByteReader struct {
+	r   io.Reader
+	off int
+	n   int
+}
+
+func (r *smallByteReader) Read(p []byte) (int, error) {
+	if len(p) == 0 {
+		return 0, nil
+	}
+	r.n = r.n%3 + 1
+	n := r.n
+	if n > len(p) {
+		n = len(p)
+	}
+	n, err := r.r.Read(p[0:n])
+	if err != nil && err != io.EOF {
+		err = fmt.Errorf("Read(%d bytes at offset %d): %v", n, r.off, err)
+	}
+	r.off += n
+	return n, err
+}
+
+// TestReader tests that reading from r returns the expected file content.
+// It does reads of different sizes, until EOF.
+// If r implements io.ReaderAt or io.Seeker, TestReader also checks
+// that those operations behave as they should.
+//
+// If TestReader finds any misbehaviors, it returns an error reporting them.
+// The error text may span multiple lines.
+func TestReader(r io.Reader, content []byte) error {
+	if len(content) > 0 {
+		n, err := r.Read(nil)
+		if n != 0 || err != nil {
+			return fmt.Errorf("Read(0) = %d, %v, want 0, nil", n, err)
+		}
+	}
+
+	data, err := io.ReadAll(&smallByteReader{r: r})
+	if err != nil {
+		return err
+	}
+	if !bytes.Equal(data, content) {
+		return fmt.Errorf("ReadAll(small amounts) = %q\n\twant %q", data, content)
+	}
+	n, err := r.Read(make([]byte, 10))
+	if n != 0 || err != io.EOF {
+		return fmt.Errorf("Read(10) at EOF = %v, %v, want 0, EOF", n, err)
+	}
+
+	if r, ok := r.(io.ReadSeeker); ok {
+		// Seek(0, 1) should report the current file position (EOF).
+		if off, err := r.Seek(0, 1); off != int64(len(content)) || err != nil {
+			return fmt.Errorf("Seek(0, 1) from EOF = %d, %v, want %d, nil", off, err, len(content))
+		}
+
+		// Seek backward partway through file, in two steps.
+		// If middle == 0, len(content) == 0, can't use the -1 and +1 seeks.
+		middle := len(content) - len(content)/3
+		if middle > 0 {
+			if off, err := r.Seek(-1, 1); off != int64(len(content)-1) || err != nil {
+				return fmt.Errorf("Seek(-1, 1) from EOF = %d, %v, want %d, nil", -off, err, len(content)-1)
+			}
+			if off, err := r.Seek(int64(-len(content)/3), 1); off != int64(middle-1) || err != nil {
+				return fmt.Errorf("Seek(%d, 1) from %d = %d, %v, want %d, nil", -len(content)/3, len(content)-1, off, err, middle-1)
+			}
+			if off, err := r.Seek(+1, 1); off != int64(middle) || err != nil {
+				return fmt.Errorf("Seek(+1, 1) from %d = %d, %v, want %d, nil", middle-1, off, err, middle)
+			}
+		}
+
+		// Seek(0, 1) should report the current file position (middle).
+		if off, err := r.Seek(0, 1); off != int64(middle) || err != nil {
+			return fmt.Errorf("Seek(0, 1) from %d = %d, %v, want %d, nil", middle, off, err, middle)
+		}
+
+		// Reading forward should return the last part of the file.
+		data, err := io.ReadAll(&smallByteReader{r: r})
+		if err != nil {
+			return fmt.Errorf("ReadAll from offset %d: %v", middle, err)
+		}
+		if !bytes.Equal(data, content[middle:]) {
+			return fmt.Errorf("ReadAll from offset %d = %q\n\twant %q", middle, data, content[middle:])
+		}
+
+		// Seek relative to end of file, but start elsewhere.
+		if off, err := r.Seek(int64(middle/2), 0); off != int64(middle/2) || err != nil {
+			return fmt.Errorf("Seek(%d, 0) from EOF = %d, %v, want %d, nil", middle/2, off, err, middle/2)
+		}
+		if off, err := r.Seek(int64(-len(content)/3), 2); off != int64(middle) || err != nil {
+			return fmt.Errorf("Seek(%d, 2) from %d = %d, %v, want %d, nil", -len(content)/3, middle/2, off, err, middle)
+		}
+
+		// Reading forward should return the last part of the file (again).
+		data, err = io.ReadAll(&smallByteReader{r: r})
+		if err != nil {
+			return fmt.Errorf("ReadAll from offset %d: %v", middle, err)
+		}
+		if !bytes.Equal(data, content[middle:]) {
+			return fmt.Errorf("ReadAll from offset %d = %q\n\twant %q", middle, data, content[middle:])
+		}
+
+		// Absolute seek & read forward.
+		if off, err := r.Seek(int64(middle/2), 0); off != int64(middle/2) || err != nil {
+			return fmt.Errorf("Seek(%d, 0) from EOF = %d, %v, want %d, nil", middle/2, off, err, middle/2)
+		}
+		data, err = io.ReadAll(r)
+		if err != nil {
+			return fmt.Errorf("ReadAll from offset %d: %v", middle/2, err)
+		}
+		if !bytes.Equal(data, content[middle/2:]) {
+			return fmt.Errorf("ReadAll from offset %d = %q\n\twant %q", middle/2, data, content[middle/2:])
+		}
+	}
+
+	if r, ok := r.(io.ReaderAt); ok {
+		data := make([]byte, len(content), len(content)+1)
+		for i := range data {
+			data[i] = 0xfe
+		}
+		n, err := r.ReadAt(data, 0)
+		if n != len(data) || err != nil && err != io.EOF {
+			return fmt.Errorf("ReadAt(%d, 0) = %v, %v, want %d, nil or EOF", len(data), n, err, len(data))
+		}
+		if !bytes.Equal(data, content) {
+			return fmt.Errorf("ReadAt(%d, 0) = %q\n\twant %q", len(data), data, content)
+		}
+
+		n, err = r.ReadAt(data[:1], int64(len(data)))
+		if n != 0 || err != io.EOF {
+			return fmt.Errorf("ReadAt(1, %d) = %v, %v, want 0, EOF", len(data), n, err)
+		}
+
+		for i := range data {
+			data[i] = 0xfe
+		}
+		n, err = r.ReadAt(data[:cap(data)], 0)
+		if n != len(data) || err != io.EOF {
+			return fmt.Errorf("ReadAt(%d, 0) = %v, %v, want %d, EOF", cap(data), n, err, len(data))
+		}
+		if !bytes.Equal(data, content) {
+			return fmt.Errorf("ReadAt(%d, 0) = %q\n\twant %q", len(data), data, content)
+		}
+
+		for i := range data {
+			data[i] = 0xfe
+		}
+		for i := range data {
+			n, err = r.ReadAt(data[i:i+1], int64(i))
+			if n != 1 || err != nil && (i != len(data)-1 || err != io.EOF) {
+				want := "nil"
+				if i == len(data)-1 {
+					want = "nil or EOF"
+				}
+				return fmt.Errorf("ReadAt(1, %d) = %v, %v, want 1, %s", i, n, err, want)
+			}
+			if data[i] != content[i] {
+				return fmt.Errorf("ReadAt(1, %d) = %q want %q", i, data[i:i+1], content[i:i+1])
+			}
+		}
+	}
+	return nil
+}
diff --git a/libgo/go/testing/iotest/reader_test.go b/libgo/go/testing/iotest/reader_test.go
index 9397837..f149e74 100644
--- a/libgo/go/testing/iotest/reader_test.go
+++ b/libgo/go/testing/iotest/reader_test.go
@@ -6,7 +6,9 @@ package iotest
 
 import (
 	"bytes"
+	"errors"
 	"io"
+	"strings"
 	"testing"
 )
 
@@ -224,3 +226,36 @@ func TestDataErrReader_emptyReader(t *testing.T) {
 		t.Errorf("Unexpectedly read %d bytes, wanted %d", g, w)
 	}
 }
+
+func TestErrReader(t *testing.T) {
+	cases := []struct {
+		name string
+		err  error
+	}{
+		{"nil error", nil},
+		{"non-nil error", errors.New("io failure")},
+		{"io.EOF", io.EOF},
+	}
+
+	for _, tt := range cases {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			n, err := ErrReader(tt.err).Read(nil)
+			if err != tt.err {
+				t.Fatalf("Error mismatch\nGot:  %v\nWant: %v", err, tt.err)
+			}
+			if n != 0 {
+				t.Fatalf("Byte count mismatch: got %d want 0", n)
+			}
+		})
+	}
+}
+
+func TestStringsReader(t *testing.T) {
+	const msg = "Now is the time for all good gophers."
+
+	r := strings.NewReader(msg)
+	if err := TestReader(r, []byte(msg)); err != nil {
+		t.Fatal(err)
+	}
+}
diff --git a/libgo/go/testing/run_example.go b/libgo/go/testing/run_example.go
index 10bde49..4dc83f7 100644
--- a/libgo/go/testing/run_example.go
+++ b/libgo/go/testing/run_example.go
@@ -43,6 +43,7 @@ func runExample(eg InternalExample) (ok bool) {
 		outC <- buf.String()
 	}()
 
+	finished := false
 	start := time.Now()
 
 	// Clean up in a deferred call so we can recover if the example panics.
@@ -55,10 +56,11 @@ func runExample(eg InternalExample) (ok bool) {
 		out := <-outC
 
 		err := recover()
-		ok = eg.processRunResult(out, timeSpent, err)
+		ok = eg.processRunResult(out, timeSpent, finished, err)
 	}()
 
 	// Run example.
 	eg.F()
+	finished = true
 	return
 }
diff --git a/libgo/go/testing/run_example_js.go b/libgo/go/testing/run_example_js.go
index 472e0c5..1d4164b 100644
--- a/libgo/go/testing/run_example_js.go
+++ b/libgo/go/testing/run_example_js.go
@@ -26,6 +26,7 @@ func runExample(eg InternalExample) (ok bool) {
 	stdout := os.Stdout
 	f := createTempFile(eg.Name)
 	os.Stdout = f
+	finished := false
 	start := time.Now()
 
 	// Clean up in a deferred call so we can recover if the example panics.
@@ -50,11 +51,12 @@ func runExample(eg InternalExample) (ok bool) {
 		}
 
 		err := recover()
-		ok = eg.processRunResult(out, timeSpent, err)
+		ok = eg.processRunResult(out, timeSpent, finished, err)
 	}()
 
 	// Run example.
 	eg.F()
+	finished = true
 	return
 }
 
diff --git a/libgo/go/testing/sub_test.go b/libgo/go/testing/sub_test.go
index 5ed3fc4..5b226f8 100644
--- a/libgo/go/testing/sub_test.go
+++ b/libgo/go/testing/sub_test.go
@@ -937,3 +937,30 @@ func TestCleanupParallelSubtests(t *T) {
 		t.Errorf("unexpected cleanup count; got %d want 1", ranCleanup)
 	}
 }
+
+func TestNestedCleanup(t *T) {
+	ranCleanup := 0
+	t.Run("test", func(t *T) {
+		t.Cleanup(func() {
+			if ranCleanup != 2 {
+				t.Errorf("unexpected cleanup count in first cleanup: got %d want 2", ranCleanup)
+			}
+			ranCleanup++
+		})
+		t.Cleanup(func() {
+			if ranCleanup != 0 {
+				t.Errorf("unexpected cleanup count in second cleanup: got %d want 0", ranCleanup)
+			}
+			ranCleanup++
+			t.Cleanup(func() {
+				if ranCleanup != 1 {
+					t.Errorf("unexpected cleanup count in nested cleanup: got %d want 1", ranCleanup)
+				}
+				ranCleanup++
+			})
+		})
+	})
+	if ranCleanup != 3 {
+		t.Errorf("unexpected cleanup count: got %d want 3", ranCleanup)
+	}
+}
diff --git a/libgo/go/testing/testing.go b/libgo/go/testing/testing.go
index 80282fc..795ee32 100644
--- a/libgo/go/testing/testing.go
+++ b/libgo/go/testing/testing.go
@@ -242,7 +242,6 @@ import (
 	"fmt"
 	"internal/race"
 	"io"
-	"io/ioutil"
 	"os"
 	"runtime"
 	"runtime/debug"
@@ -294,6 +293,7 @@ func Init() {
 	blockProfileRate = flag.Int("test.blockprofilerate", 1, "set blocking profile `rate` (see runtime.SetBlockProfileRate)")
 	mutexProfile = flag.String("test.mutexprofile", "", "write a mutex contention profile to the named file after execution")
 	mutexProfileFraction = flag.Int("test.mutexprofilefraction", 1, "if >= 0, calls runtime.SetMutexProfileFraction()")
+	panicOnExit0 = flag.Bool("test.paniconexit0", false, "panic on call to os.Exit(0)")
 	traceFile = flag.String("test.trace", "", "write an execution trace to `file`")
 	timeout = flag.Duration("test.timeout", 0, "panic test binary after duration `d` (default 0, timeout disabled)")
 	cpuListStr = flag.String("test.cpu", "", "comma-separated `list` of cpu counts to run each test with")
@@ -320,6 +320,7 @@ var (
 	blockProfileRate     *int
 	mutexProfile         *string
 	mutexProfileFraction *int
+	panicOnExit0         *bool
 	traceFile            *string
 	timeout              *time.Duration
 	cpuListStr           *string
@@ -382,17 +383,18 @@ const maxStackLen = 50
 // common holds the elements common between T and B and
 // captures common methods such as Errorf.
 type common struct {
-	mu          sync.RWMutex        // guards this group of fields
-	output      []byte              // Output generated by test or benchmark.
-	w           io.Writer           // For flushToParent.
-	ran         bool                // Test or benchmark (or one of its subtests) was executed.
-	failed      bool                // Test or benchmark has failed.
-	skipped     bool                // Test of benchmark has been skipped.
-	done        bool                // Test is finished and all subtests have completed.
-	helpers     map[string]struct{} // functions to be skipped when writing file/line info
-	cleanup     func()              // optional function to be called at the end of the test
-	cleanupName string              // Name of the cleanup function.
-	cleanupPc   []uintptr           // The stack trace at the point where Cleanup was called.
+	mu          sync.RWMutex         // guards this group of fields
+	output      []byte               // Output generated by test or benchmark.
+	w           io.Writer            // For flushToParent.
+	ran         bool                 // Test or benchmark (or one of its subtests) was executed.
+	failed      bool                 // Test or benchmark has failed.
+	skipped     bool                 // Test of benchmark has been skipped.
+	done        bool                 // Test is finished and all subtests have completed.
+	helperPCs   map[uintptr]struct{} // functions to be skipped when writing file/line info
+	helperNames map[string]struct{}  // helperPCs converted to function names
+	cleanups    []func()             // optional functions to be called at the end of the test
+	cleanupName string               // Name of the cleanup function.
+	cleanupPc   []uintptr            // The stack trace at the point where Cleanup was called.
 
 	chatty     *chattyPrinter // A copy of chattyPrinter, if the chatty flag is set.
 	bench      bool           // Whether the current test is a benchmark.
@@ -411,10 +413,10 @@ type common struct {
 	signal   chan bool // To signal a test is done.
 	sub      []*T      // Queue of subtests to be run in parallel.
 
-	tempDirOnce sync.Once
-	tempDir     string
-	tempDirErr  error
-	tempDirSeq  int32
+	tempDirMu  sync.Mutex
+	tempDir    string
+	tempDirErr error
+	tempDirSeq int32
 }
 
 // Short reports whether the -test.short flag is set.
@@ -507,7 +509,7 @@ func (c *common) frameSkip(skip int) runtime.Frame {
 			}
 			return prevFrame
 		}
-		if _, ok := c.helpers[frame.Function]; !ok {
+		if _, ok := c.helperNames[frame.Function]; !ok {
 			// Found a frame that wasn't inside a helper function.
 			return frame
 		}
@@ -519,6 +521,14 @@ func (c *common) frameSkip(skip int) runtime.Frame {
 // and inserts the final newline if needed and indentation spaces for formatting.
 // This function must be called with c.mu held.
 func (c *common) decorate(s string, skip int) string {
+	// If more helper PCs have been added since we last did the conversion
+	if c.helperNames == nil {
+		c.helperNames = make(map[string]struct{})
+		for pc := range c.helperPCs {
+			c.helperNames[pcToName(pc)] = struct{}{}
+		}
+	}
+
 	frame := c.frameSkip(skip)
 	file := frame.File
 	line := frame.Line
@@ -887,38 +897,50 @@ func (c *common) Skipped() bool {
 func (c *common) Helper() {
 	c.mu.Lock()
 	defer c.mu.Unlock()
-	if c.helpers == nil {
-		c.helpers = make(map[string]struct{})
+	if c.helperPCs == nil {
+		c.helperPCs = make(map[uintptr]struct{})
+	}
+	// repeating code from callerName here to save walking a stack frame
+	var pc [1]uintptr
+	n := runtime.Callers(2, pc[:]) // skip runtime.Callers + Helper
+	if n == 0 {
+		panic("testing: zero callers found")
+	}
+	if _, found := c.helperPCs[pc[0]]; !found {
+		c.helperPCs[pc[0]] = struct{}{}
+		c.helperNames = nil // map will be recreated next time it is needed
 	}
-	c.helpers[callerName(1)] = struct{}{}
 }
 
 // Cleanup registers a function to be called when the test and all its
 // subtests complete. Cleanup functions will be called in last added,
 // first called order.
 func (c *common) Cleanup(f func()) {
-	c.mu.Lock()
-	defer c.mu.Unlock()
-	oldCleanup := c.cleanup
-	oldCleanupPc := c.cleanupPc
-	c.cleanup = func() {
-		if oldCleanup != nil {
-			defer func() {
-				c.mu.Lock()
-				c.cleanupPc = oldCleanupPc
-				c.mu.Unlock()
-				oldCleanup()
-			}()
-		}
+	var pc [maxStackLen]uintptr
+	// Skip two extra frames to account for this function and runtime.Callers itself.
+	n := runtime.Callers(2, pc[:])
+	cleanupPc := pc[:n]
+
+	fn := func() {
+		defer func() {
+			c.mu.Lock()
+			defer c.mu.Unlock()
+			c.cleanupName = ""
+			c.cleanupPc = nil
+		}()
+
+		name := callerName(0)
 		c.mu.Lock()
-		c.cleanupName = callerName(0)
+		c.cleanupName = name
+		c.cleanupPc = cleanupPc
 		c.mu.Unlock()
+
 		f()
 	}
-	var pc [maxStackLen]uintptr
-	// Skip two extra frames to account for this function and runtime.Callers itself.
-	n := runtime.Callers(2, pc[:])
-	c.cleanupPc = pc[:n]
+
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.cleanups = append(c.cleanups, fn)
 }
 
 var tempDirReplacer struct {
@@ -934,17 +956,29 @@ var tempDirReplacer struct {
 func (c *common) TempDir() string {
 	// Use a single parent directory for all the temporary directories
 	// created by a test, each numbered sequentially.
-	c.tempDirOnce.Do(func() {
+	c.tempDirMu.Lock()
+	var nonExistent bool
+	if c.tempDir == "" { // Usually the case with js/wasm
+		nonExistent = true
+	} else {
+		_, err := os.Stat(c.tempDir)
+		nonExistent = os.IsNotExist(err)
+		if err != nil && !nonExistent {
+			c.Fatalf("TempDir: %v", err)
+		}
+	}
+
+	if nonExistent {
 		c.Helper()
 
-		// ioutil.TempDir doesn't like path separators in its pattern,
+		// os.MkdirTemp doesn't like path separators in its pattern,
 		// so mangle the name to accommodate subtests.
 		tempDirReplacer.Do(func() {
 			tempDirReplacer.r = strings.NewReplacer("/", "_", "\\", "_", ":", "_")
 		})
 		pattern := tempDirReplacer.r.Replace(c.Name())
 
-		c.tempDir, c.tempDirErr = ioutil.TempDir("", pattern)
+		c.tempDir, c.tempDirErr = os.MkdirTemp("", pattern)
 		if c.tempDirErr == nil {
 			c.Cleanup(func() {
 				if err := os.RemoveAll(c.tempDir); err != nil {
@@ -952,7 +986,9 @@ func (c *common) TempDir() string {
 				}
 			})
 		}
-	})
+	}
+	c.tempDirMu.Unlock()
+
 	if c.tempDirErr != nil {
 		c.Fatalf("TempDir: %v", c.tempDirErr)
 	}
@@ -976,34 +1012,53 @@ const (
 // If catchPanic is true, this will catch panics, and return the recovered
 // value if any.
 func (c *common) runCleanup(ph panicHandling) (panicVal interface{}) {
-	c.mu.Lock()
-	cleanup := c.cleanup
-	c.cleanup = nil
-	c.mu.Unlock()
-	if cleanup == nil {
-		return nil
-	}
-
 	if ph == recoverAndReturnPanic {
 		defer func() {
 			panicVal = recover()
 		}()
 	}
 
-	cleanup()
-	return nil
+	// Make sure that if a cleanup function panics,
+	// we still run the remaining cleanup functions.
+	defer func() {
+		c.mu.Lock()
+		recur := len(c.cleanups) > 0
+		c.mu.Unlock()
+		if recur {
+			c.runCleanup(normalPanic)
+		}
+	}()
+
+	for {
+		var cleanup func()
+		c.mu.Lock()
+		if len(c.cleanups) > 0 {
+			last := len(c.cleanups) - 1
+			cleanup = c.cleanups[last]
+			c.cleanups = c.cleanups[:last]
+		}
+		c.mu.Unlock()
+		if cleanup == nil {
+			return nil
+		}
+		cleanup()
+	}
 }
 
 // callerName gives the function name (qualified with a package path)
 // for the caller after skip frames (where 0 means the current function).
 func callerName(skip int) string {
-	// Make room for the skip PC.
-	var pc [2]uintptr
+	var pc [1]uintptr
 	n := runtime.Callers(skip+2, pc[:]) // skip + runtime.Callers + callerName
 	if n == 0 {
 		panic("testing: zero callers found")
 	}
-	frames := runtime.CallersFrames(pc[:n])
+	return pcToName(pc[0])
+}
+
+func pcToName(pc uintptr) string {
+	pcs := []uintptr{pc}
+	frames := runtime.CallersFrames(pcs)
 	frame, _ := frames.Next()
 	return frame.Function
 }
@@ -1077,6 +1132,7 @@ func tRunner(t *T, fn func(t *T)) {
 		// If the test panicked, print any test output before dying.
 		err := recover()
 		signal := true
+
 		if !t.finished && err == nil {
 			err = errNilPanicOrGoexit
 			for p := t.parent; p != nil; p = p.parent {
@@ -1088,6 +1144,21 @@ func tRunner(t *T, fn func(t *T)) {
 				}
 			}
 		}
+		// Use a deferred call to ensure that we report that the test is
+		// complete even if a cleanup function calls t.FailNow. See issue 41355.
+		didPanic := false
+		defer func() {
+			if didPanic {
+				return
+			}
+			if err != nil {
+				panic(err)
+			}
+			// Only report that the test is complete if it doesn't panic,
+			// as otherwise the test binary can exit before the panic is
+			// reported to the user. See issue 41479.
+			t.signal <- signal
+		}()
 
 		doPanic := func(err interface{}) {
 			t.Fail()
@@ -1105,6 +1176,7 @@ func tRunner(t *T, fn func(t *T)) {
 					fmt.Fprintf(root.parent.w, "cleanup panicked with %v", r)
 				}
 			}
+			didPanic = true
 			panic(err)
 		}
 		if err != nil {
@@ -1146,7 +1218,6 @@ func tRunner(t *T, fn func(t *T)) {
 		if t.parent != nil && atomic.LoadInt32(&t.hasSub) == 0 {
 			t.setRan()
 		}
-		t.signal <- signal
 	}()
 	defer func() {
 		if len(t.sub) == 0 {
@@ -1287,6 +1358,7 @@ func (f matchStringOnly) WriteProfileTo(string, io.Writer, int) error { return e
 func (f matchStringOnly) ImportPath() string                          { return "" }
 func (f matchStringOnly) StartTestLog(io.Writer)                      {}
 func (f matchStringOnly) StopTestLog() error                          { return errMain }
+func (f matchStringOnly) SetPanicOnExit0(bool)                        {}
 
 // Main is an internal function, part of the implementation of the "go test" command.
 // It was exported because it is cross-package and predates "internal" packages.
@@ -1322,6 +1394,7 @@ type M struct {
 type testDeps interface {
 	ImportPath() string
 	MatchString(pat, str string) (bool, error)
+	SetPanicOnExit0(bool)
 	StartCPUProfile(io.Writer) error
 	StopCPUProfile()
 	StartTestLog(io.Writer)
@@ -1547,6 +1620,9 @@ func (m *M) before() {
 		m.deps.StartTestLog(f)
 		testlogFile = f
 	}
+	if *panicOnExit0 {
+		m.deps.SetPanicOnExit0(true)
+	}
 }
 
 // after runs after all testing.
@@ -1554,6 +1630,13 @@ func (m *M) after() {
 	m.afterOnce.Do(func() {
 		m.writeProfiles()
 	})
+
+	// Restore PanicOnExit0 after every run, because we set it to true before
+	// every run. Otherwise, if m.Run is called multiple times the behavior of
+	// os.Exit(0) will not be restored after the second run.
+	if *panicOnExit0 {
+		m.deps.SetPanicOnExit0(false)
+	}
 }
 
 func (m *M) writeProfiles() {
diff --git a/libgo/go/testing/testing_test.go b/libgo/go/testing/testing_test.go
index dbef706..0f09698 100644
--- a/libgo/go/testing/testing_test.go
+++ b/libgo/go/testing/testing_test.go
@@ -5,7 +5,6 @@
 package testing_test
 
 import (
-	"io/ioutil"
 	"os"
 	"path/filepath"
 	"testing"
@@ -19,6 +18,38 @@ func TestMain(m *testing.M) {
 	os.Exit(m.Run())
 }
 
+func TestTempDirInCleanup(t *testing.T) {
+	var dir string
+
+	t.Run("test", func(t *testing.T) {
+		t.Cleanup(func() {
+			dir = t.TempDir()
+		})
+		_ = t.TempDir()
+	})
+
+	fi, err := os.Stat(dir)
+	if fi != nil {
+		t.Fatalf("Directory %q from user Cleanup still exists", dir)
+	}
+	if !os.IsNotExist(err) {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+}
+
+func TestTempDirInBenchmark(t *testing.T) {
+	testing.Benchmark(func(b *testing.B) {
+		if !b.Run("test", func(b *testing.B) {
+			// Add a loop so that the test won't fail. See issue 38677.
+			for i := 0; i < b.N; i++ {
+				_ = b.TempDir()
+			}
+		}) {
+			t.Fatal("Sub test failure in a benchmark")
+		}
+	})
+}
+
 func TestTempDir(t *testing.T) {
 	testTempDir(t)
 	t.Run("InSubtest", testTempDir)
@@ -70,11 +101,11 @@ func testTempDir(t *testing.T) {
 	if !fi.IsDir() {
 		t.Errorf("dir %q is not a dir", dir)
 	}
-	fis, err := ioutil.ReadDir(dir)
+	files, err := os.ReadDir(dir)
 	if err != nil {
 		t.Fatal(err)
 	}
-	if len(fis) > 0 {
-		t.Errorf("unexpected %d files in TempDir: %v", len(fis), fis)
+	if len(files) > 0 {
+		t.Errorf("unexpected %d files in TempDir: %v", len(files), files)
 	}
 }