[FIXED] Race in FetchBatch in legacy API

Signed-off-by: Piotr Piotrowski <piotr@synadia.com>

js.go

@@ -3114,20 +3114,27 @@ type MessageBatch interface {
 }
 
 type messageBatch struct {
+	sync.Mutex
 	msgs chan *Msg
 	err  error
 	done chan struct{}
 }
 
 func (mb *messageBatch) Messages() <-chan *Msg {
+	mb.Lock()
+	defer mb.Unlock()
 	return mb.msgs
 }
 
 func (mb *messageBatch) Error() error {
+	mb.Lock()
+	defer mb.Unlock()
 	return mb.err
 }
 
 func (mb *messageBatch) Done() <-chan struct{} {
+	mb.Lock()
+	defer mb.Unlock()
 	return mb.done
 }
 
@@ -3302,12 +3309,11 @@ func (sub *Subscription) FetchBatch(batch int, opts ...PullOpt) (MessageBatch, e
 	}
 	var hbTimer *time.Timer
 	var hbErr error
-	hbLock := sync.Mutex{}
 	if o.hb > 0 {
 		hbTimer = time.AfterFunc(2*o.hb, func() {
-			hbLock.Lock()
+			result.Lock()
 			hbErr = ErrNoHeartbeat
-			hbLock.Unlock()
+			result.Unlock()
 			cancel()
 		})
 	}
@@ -3338,21 +3344,25 @@ func (sub *Subscription) FetchBatch(batch int, opts ...PullOpt) (MessageBatch, e
 				break
 			}
 			if usrMsg {
+				result.Lock()
 				result.msgs <- msg
+				result.Unlock()
 				requestMsgs++
 			}
 		}
 		if err != nil {
-			hbLock.Lock()
+			result.Lock()
 			if hbErr != nil {
 				result.err = hbErr
 			} else {
 				result.err = o.checkCtxErr(err)
 			}
-			hbLock.Unlock()
+			result.Unlock()
 		}
 		close(result.msgs)
+		result.Lock()
 		result.done <- struct{}{}
+		result.Unlock()
 	}()
 	return result, nil
 }

test/js_test.go

@@ -10788,3 +10788,83 @@ func TestJetStreamTransform(t *testing.T) {
 	}
 
 }
+
+func TestPullConsumerFetchRace(t *testing.T) {
+	srv := RunBasicJetStreamServer()
+	defer shutdownJSServerAndRemoveStorage(t, srv)
+
+	nc, js := jsClient(t, srv)
+	defer nc.Close()
+
+	_, err := js.AddStream(&nats.StreamConfig{Name: "foo", Subjects: []string{"FOO.*"}})
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+
+	for i := 0; i < 3; i++ {
+		if _, err := js.Publish("FOO.123", []byte(fmt.Sprintf("msg-%d", i))); err != nil {
+			t.Fatalf("Unexpected error during publish: %s", err)
+		}
+	}
+	sub, err := js.PullSubscribe("FOO.123", "")
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	cons, err := sub.ConsumerInfo()
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	msgs, err := sub.FetchBatch(5)
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	errCh := make(chan error)
+	go func() {
+		for {
+			err := msgs.Error()
+			if err != nil {
+				errCh <- err
+				return
+			}
+		}
+	}()
+	deleteErrCh := make(chan error, 1)
+	go func() {
+		time.Sleep(100 * time.Millisecond)
+		if err := js.DeleteConsumer("foo", cons.Name); err != nil {
+			deleteErrCh <- err
+		}
+		close(deleteErrCh)
+	}()
+
+	var i int
+	for msg := range msgs.Messages() {
+		if string(msg.Data) != fmt.Sprintf("msg-%d", i) {
+			t.Fatalf("Invalid msg on index %d; expected: %s; got: %s", i, fmt.Sprintf("msg-%d", i), string(msg.Data))
+		}
+		i++
+	}
+	if i != 3 {
+		t.Fatalf("Invalid number of messages received; want: %d; got: %d", 5, i)
+	}
+	select {
+	case err := <-errCh:
+		if !errors.Is(err, nats.ErrConsumerDeleted) {
+			t.Fatalf("Expected error: %v; got: %v", nats.ErrConsumerDeleted, err)
+		}
+	case <-time.After(1 * time.Second):
+		t.Fatalf("Expected error: %v; got: %v", nats.ErrConsumerDeleted, nil)
+	}
+
+	// wait until the consumer is deleted, otherwise we may close the connection
+	// before the consumer delete response is received
+	select {
+	case ert, ok := <-deleteErrCh:
+		if !ok {
+			break
+		}
+		t.Fatalf("Error deleting consumer: %s", ert)
+	case <-time.After(1 * time.Second):
+		t.Fatalf("Expected done to be closed")
+	}
+}