Fix panic on network timeout during push

`Upload` already closes the reader returned by `compress` and the
progressreader passed into it, before returning. But even so, the
io.Copy inside compress' goroutine needs to attempt a read from the
progressreader to notice that it's closed, and this read has a side
effect of outputting a progress message. If this happens after `Upload`
returns, it can result in a write to a closed channel. Change `compress`
to return a channel that allows the caller to wait for its goroutine to
finish before freeing any resources connected to the reader that was
passed to it.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

distribution/push.go

@@ -170,7 +170,14 @@ func Push(ctx context.Context, ref reference.Named, imagePushConfig *ImagePushCo
 // argument so that it can be used with httpBlobWriter's ReadFrom method.
 // Using httpBlobWriter's Write method would send a PATCH request for every
 // Write call.
-func compress(in io.Reader) io.ReadCloser {
+//
+// The second return value is a channel that gets closed when the goroutine
+// is finished. This allows the caller to make sure the goroutine finishes
+// before it releases any resources connected with the reader that was
+// passed in.
+func compress(in io.Reader) (io.ReadCloser, chan struct{}) {
+	compressionDone := make(chan struct{})
+
 	pipeReader, pipeWriter := io.Pipe()
 	// Use a bufio.Writer to avoid excessive chunking in HTTP request.
 	bufWriter := bufio.NewWriterSize(pipeWriter, compressionBufSize)
@@ -189,7 +196,8 @@ func compress(in io.Reader) io.ReadCloser {
 		} else {
 			pipeWriter.Close()
 		}
+		close(compressionDone)
 	}()
 
-	return pipeReader
+	return pipeReader, compressionDone
 }

distribution/push_v2.go

@@ -345,8 +345,11 @@ func (pd *v2PushDescriptor) Upload(ctx context.Context, progressOutput progress.
 	size, _ := pd.layer.DiffSize()
 
 	reader := progress.NewProgressReader(ioutils.NewCancelReadCloser(ctx, arch), progressOutput, size, pd.ID(), "Pushing")
-	defer reader.Close()
+	compressedReader, compressionDone := compress(reader)
-	compressedReader := compress(reader)
+	defer func() {
+		reader.Close()
+		<-compressionDone
+	}()
 
 	digester := digest.Canonical.New()
 	tee := io.TeeReader(compressedReader, digester.Hash())