endpoint: fix incorrect early release of maxConcurrentZFSSendSemaphore

fixes #274

endpoint/endpoint.go

@@ -198,7 +198,11 @@ func (s *Sender) Send(ctx context.Context, r *pdu.SendReq) (*pdu.SendRes, zfs.St
 	if err != nil {
 		return nil, nil, err
 	}
-	defer guard.Release()
+	defer func(guardp **semaphore.AcquireGuard) {
+		if *guardp != nil {
+			(*guardp).Release()
+		}
+	}(&guard)
 
 	si, err := zfs.ZFSSendDry(ctx, sendArgs)
 	if err != nil {
@@ -255,6 +259,13 @@ func (s *Sender) Send(ctx context.Context, r *pdu.SendReq) (*pdu.SendRes, zfs.St
 	if err != nil {
 		return nil, nil, errors.Wrap(err, "zfs send failed")
 	}
+
+	// defer releasing guard until streamCopier is closed
+	streamCopier.SetPostCloseCallback(func(_ error) {
+		guard.Release()
+	})
+	guard = nil
+
 	return res, streamCopier, nil
 }
 

zfs/zfs.go

@@ -351,6 +351,7 @@ func (a ZFSSendArgs) buildCommonSendArgs() ([]string, error) {
 
 type ReadCloserCopier struct {
 	recorder          readErrRecorder
+	postCloseCallback func(closeErr error)
 }
 
 type readErrRecorder struct {
@@ -402,8 +403,17 @@ func (c *ReadCloserCopier) Read(p []byte) (n int, err error) {
 	return c.recorder.Read(p)
 }
 
+// caller must ensure that this function is not executing concurrently to Close
+func (c *ReadCloserCopier) SetPostCloseCallback(callback func(closeErr error)) {
+	c.postCloseCallback = callback
+}
+
 func (c *ReadCloserCopier) Close() error {
-	return c.recorder.ReadCloser.Close()
+	err := c.recorder.Close()
+	if c.postCloseCallback != nil {
+		c.postCloseCallback(err)
+	}
+	return err
 }
 
 func pipeWithCapacityHint(capacity int) (r, w *os.File, err error) {