copy: check if writer was closed before setting a pipe

If Close is called externally before a request is attempted, then we
will accidentally attempt to send to a closed channel, causing a panic.

To avoid this, we can check to see if Close has been called, using a
done channel. If this channel is ever done, we drop any incoming errors,
requests or pipes - we don't need them, since we're done.

Signed-off-by: Justin Chadwell <me@jedevc.com>

core/remotes/docker/pusher.go

@@ -331,10 +331,12 @@ type pushWriter struct {
 
 	pipe *io.PipeWriter
 
-	pipeC     chan *io.PipeWriter
-	respC     chan *http.Response
+	done      chan struct{}
 	closeOnce sync.Once
-	errC      chan error
+
+	pipeC chan *io.PipeWriter
+	respC chan *http.Response
+	errC  chan error
 
 	isManifest bool
 
@@ -352,19 +354,30 @@ func newPushWriter(db *dockerBase, ref string, expected digest.Digest, tracker S
 		pipeC:      make(chan *io.PipeWriter, 1),
 		respC:      make(chan *http.Response, 1),
 		errC:       make(chan error, 1),
+		done:       make(chan struct{}),
 		isManifest: isManifest,
 	}
 }
 
 func (pw *pushWriter) setPipe(p *io.PipeWriter) {
-	pw.pipeC <- p
+	select {
+	case <-pw.done:
+	case pw.pipeC <- p:
+	}
 }
 
 func (pw *pushWriter) setError(err error) {
-	pw.errC <- err
+	select {
+	case <-pw.done:
+	case pw.errC <- err:
+	}
 }
+
 func (pw *pushWriter) setResponse(resp *http.Response) {
-	pw.respC <- resp
+	select {
+	case <-pw.done:
+	case pw.respC <- resp:
+	}
 }
 
 func (pw *pushWriter) Write(p []byte) (n int, err error) {
@@ -374,22 +387,26 @@ func (pw *pushWriter) Write(p []byte) (n int, err error) {
 	}
 
 	if pw.pipe == nil {
-		p, ok := <-pw.pipeC
-		if !ok {
+		select {
+		case <-pw.done:
 			return 0, io.ErrClosedPipe
+		case p := <-pw.pipeC:
+			pw.pipe = p
 		}
-		pw.pipe = p
 	} else {
 		select {
-		case p, ok := <-pw.pipeC:
-			if !ok {
-				return 0, io.ErrClosedPipe
-			}
+		case <-pw.done:
+			return 0, io.ErrClosedPipe
+		case p := <-pw.pipeC:
 			pw.pipe.CloseWithError(content.ErrReset)
 			pw.pipe = p
 
 			// If content has already been written, the bytes
-			// cannot be written and the caller must reset
+			// cannot be written again and the caller must reset
+			status, err := pw.tracker.GetStatus(pw.ref)
+			if err != nil {
+				return 0, err
+			}
 			status.Offset = 0
 			status.UpdatedAt = time.Now()
 			pw.tracker.SetStatus(pw.ref, status)
@@ -418,7 +435,7 @@ func (pw *pushWriter) Close() error {
 	// Ensure pipeC is closed but handle `Close()` being
 	// called multiple times without panicking
 	pw.closeOnce.Do(func() {
-		close(pw.pipeC)
+		close(pw.done)
 	})
 	if pw.pipe != nil {
 		status, err := pw.tracker.GetStatus(pw.ref)
@@ -458,17 +475,16 @@ func (pw *pushWriter) Commit(ctx context.Context, size int64, expected digest.Di
 	// TODO: timeout waiting for response
 	var resp *http.Response
 	select {
+	case <-pw.done:
+		return io.ErrClosedPipe
 	case err := <-pw.errC:
 		return err
 	case resp = <-pw.respC:
 		defer resp.Body.Close()
-	case p, ok := <-pw.pipeC:
+	case p := <-pw.pipeC:
 		// check whether the pipe has changed in the commit, because sometimes Write
 		// can complete successfully, but the pipe may have changed. In that case, the
 		// content needs to be reset.
-		if !ok {
-			return io.ErrClosedPipe
-		}
 		pw.pipe.CloseWithError(content.ErrReset)
 		pw.pipe = p