Return ErrPipelineOverflow from PipelineClient.Do if the pending requests' queue is overflown. This should prevent from caller's goroutines leak on stalled pipeline client

client.go

@@ -1547,7 +1547,15 @@ func (c *PipelineClient) Do(req *Request, resp *Response) error {
 		w.resp = &w.respCopy
 	}
 
-	c.chW <- w
+	// Put the request to outgoing queue
+	select {
+	case c.chW <- w:
+	default:
+		releasePipelineWork(&c.workPool, w)
+		return ErrPipelineOverflow
+	}
+
+	// Wait for the response
 	<-w.done
 	err := w.err
 
@@ -1556,6 +1564,10 @@ func (c *PipelineClient) Do(req *Request, resp *Response) error {
 	return err
 }
 
+// ErrPipelineOverflow may be returned from PipelineClient.Do
+// if the requests' queue is overflown.
+var ErrPipelineOverflow = errors.New("pipelined requests' queue has been overflown. Increase MaxPendingRequests")
+
 // DefaultMaxPendingRequests is the default value
 // for PipelineClient.MaxPendingRequests.
 const DefaultMaxPendingRequests = 1024
@@ -1756,7 +1768,10 @@ func (c *PipelineClient) logger() Logger {
 func (c *PipelineClient) PendingRequests() int {
 	c.init()
 
-	return len(c.chR)
+	c.chLock.Lock()
+	n := len(c.chR)
+	c.chLock.Unlock()
+	return n
 }
 
 var errPipelineClientStopped = errors.New("pipeline client has been stopped")

client_test.go

@@ -64,6 +64,10 @@ func testPipelineClientDoConcurrent(t *testing.T, concurrency int) {
 		}
 	}
 
+	if c.PendingRequests() != 0 {
+		t.Fatalf("unexpected number of pending requests: %d. Expecting zero", c.PendingRequests())
+	}
+
 	if err := ln.Close(); err != nil {
 		t.Fatalf("unexpected error: %s", err)
 	}
@@ -86,6 +90,10 @@ func testPipelineClientDo(t *testing.T, c *PipelineClient) {
 			err = c.Do(req, resp)
 		}
 		if err != nil {
+			if err == ErrPipelineOverflow {
+				time.Sleep(10 * time.Millisecond)
+				continue
+			}
 			t.Fatalf("unexpected error on iteration %d: %s", i, err)
 		}
 		if resp.StatusCode() != StatusOK {