Avoid deadlock in unpacker.

Signed-off-by: Lantao Liu <lantaol@google.com>

pull.go

@@ -70,6 +70,11 @@ func (c *Client) Pull(ctx context.Context, ref string, opts ...RemoteOpt) (_ Ima
 		}
 		unpackWrapper, eg := u.handlerWrapper(ctx, &unpacks)
 		defer func() {
+			if retErr != nil {
+				// Forcibly stop the unpacker if there is
+				// an error.
+				eg.Cancel()
+			}
 			if err := eg.Wait(); err != nil {
 				if retErr == nil {
 					retErr = errors.Wrap(err, "unpack")

unpacker.go

@@ -186,8 +186,32 @@ func (u *unpacker) unpack(ctx context.Context, config ocispec.Descriptor, layers
 	return nil
 }
 
-func (u *unpacker) handlerWrapper(uctx context.Context, unpacks *int32) (func(images.Handler) images.Handler, *errgroup.Group) {
-	eg, uctx := errgroup.WithContext(uctx)
+type errGroup struct {
+	*errgroup.Group
+	cancel context.CancelFunc
+}
+
+func newErrGroup(ctx context.Context) (*errGroup, context.Context) {
+	ctx, cancel := context.WithCancel(ctx)
+	eg, ctx := errgroup.WithContext(ctx)
+	return &errGroup{
+		Group:  eg,
+		cancel: cancel,
+	}, ctx
+}
+
+func (e *errGroup) Cancel() {
+	e.cancel()
+}
+
+func (e *errGroup) Wait() error {
+	err := e.Group.Wait()
+	e.cancel()
+	return err
+}
+
+func (u *unpacker) handlerWrapper(uctx context.Context, unpacks *int32) (func(images.Handler) images.Handler, *errGroup) {
+	eg, uctx := newErrGroup(uctx)
 	return func(f images.Handler) images.Handler {
 		var (
 			lock    sync.Mutex
@@ -234,7 +258,19 @@ func (u *unpacker) handlerWrapper(uctx context.Context, unpacks *int32) (func(im
 				update := !schema1
 				lock.Unlock()
 				if update {
-					u.updateCh <- desc
+					select {
+					case <-uctx.Done():
+						// Do not send update if unpacker is not running.
+					default:
+						select {
+						case u.updateCh <- desc:
+						case <-uctx.Done():
+							// Do not send update if unpacker is not running.
+						}
+					}
+					// Checking ctx.Done() prevents the case that unpacker
+					// exits unexpectedly, but update continues to be generated,
+					// and eventually fills up updateCh and blocks forever.
 				}
 			}
 			return children, nil