1 files changed, 48 insertions, 15 deletions
diff --git a/libgo/go/context/context.go b/libgo/go/context/context.go
index 6259085..b561968 100644
--- a/libgo/go/context/context.go
+++ b/libgo/go/context/context.go
@@ -51,6 +51,7 @@ import (
 	"errors"
 	"internal/reflectlite"
 	"sync"
+	"sync/atomic"
 	"time"
 )
 
@@ -67,6 +68,8 @@ type Context interface {
 	// Done returns a channel that's closed when work done on behalf of this
 	// context should be canceled. Done may return nil if this context can
 	// never be canceled. Successive calls to Done return the same value.
+	// The close of the Done channel may happen asynchronously,
+	// after the cancel function returns.
 	//
 	// WithCancel arranges for Done to be closed when cancel is called;
 	// WithDeadline arranges for Done to be closed when the deadline
@@ -237,11 +240,24 @@ func newCancelCtx(parent Context) cancelCtx {
 	return cancelCtx{Context: parent}
 }
 
+// goroutines counts the number of goroutines ever created; for testing.
+var goroutines int32
+
 // propagateCancel arranges for child to be canceled when parent is.
 func propagateCancel(parent Context, child canceler) {
-	if parent.Done() == nil {
+	done := parent.Done()
+	if done == nil {
 		return // parent is never canceled
 	}
+
+	select {
+	case <-done:
+		// parent is already canceled
+		child.cancel(false, parent.Err())
+		return
+	default:
+	}
+
 	if p, ok := parentCancelCtx(parent); ok {
 		p.mu.Lock()
 		if p.err != nil {
@@ -255,6 +271,7 @@ func propagateCancel(parent Context, child canceler) {
 		}
 		p.mu.Unlock()
 	} else {
+		atomic.AddInt32(&goroutines, +1)
 		go func() {
 			select {
 			case <-parent.Done():
@@ -265,22 +282,31 @@ func propagateCancel(parent Context, child canceler) {
 	}
 }
 
-// parentCancelCtx follows a chain of parent references until it finds a
-// *cancelCtx. This function understands how each of the concrete types in this
-// package represents its parent.
+// &cancelCtxKey is the key that a cancelCtx returns itself for.
+var cancelCtxKey int
+
+// parentCancelCtx returns the underlying *cancelCtx for parent.
+// It does this by looking up parent.Value(&cancelCtxKey) to find
+// the innermost enclosing *cancelCtx and then checking whether
+// parent.Done() matches that *cancelCtx. (If not, the *cancelCtx
+// has been wrapped in a custom implementation providing a
+// different done channel, in which case we should not bypass it.)
 func parentCancelCtx(parent Context) (*cancelCtx, bool) {
-	for {
-		switch c := parent.(type) {
-		case *cancelCtx:
-			return c, true
-		case *timerCtx:
-			return &c.cancelCtx, true
-		case *valueCtx:
-			parent = c.Context
-		default:
-			return nil, false
-		}
+	done := parent.Done()
+	if done == closedchan || done == nil {
+		return nil, false
+	}
+	p, ok := parent.Value(&cancelCtxKey).(*cancelCtx)
+	if !ok {
+		return nil, false
 	}
+	p.mu.Lock()
+	ok = p.done == done
+	p.mu.Unlock()
+	if !ok {
+		return nil, false
+	}
+	return p, true
 }
 
 // removeChild removes a context from its parent.
@@ -321,6 +347,13 @@ type cancelCtx struct {
 	err      error                 // set to non-nil by the first cancel call
 }
 
+func (c *cancelCtx) Value(key interface{}) interface{} {
+	if key == &cancelCtxKey {
+		return c
+	}
+	return c.Context.Value(key)
+}
+
 func (c *cancelCtx) Done() <-chan struct{} {
 	c.mu.Lock()
 	if c.done == nil {