[FIXED] Avoid stale KV reads on server restart for replicated KV stores.

server/jetstream_cluster.go

@@ -2284,27 +2284,19 @@ func (js *jetStream) monitorStream(mset *stream, sa *streamAssignment, sendSnaps
 			// Here we are checking if we are not the leader but we have been asked to allow
 			// direct access. We now allow non-leaders to participate in the queue group.
 			if !isLeader && mset != nil {
-				mset.mu.Lock()
-				// Check direct gets first.
-				if mset.cfg.AllowDirect {
-					if mset.directSub == nil && mset.isCurrent() {
-						mset.subscribeToDirect()
-					} else {
-						startDirectAccessMonitoring()
-					}
-				}
-				// Now check for mirror directs as well.
-				if mset.cfg.MirrorDirect {
-					if mset.mirror != nil && mset.mirror.dsub == nil && mset.isCurrent() {
-						mset.subscribeToMirrorDirect()
-					} else {
-						startDirectAccessMonitoring()
-					}
-				}
-				mset.mu.Unlock()
+				startDirectAccessMonitoring()
 			}
 
 		case <-datc:
+			if mset == nil || isRecovering {
+				return
+			}
+			// If we are leader we can stop, we know this is setup now.
+			if isLeader {
+				stopDirectMonitoring()
+				return
+			}
+
 			mset.mu.Lock()
 			ad, md, current := mset.cfg.AllowDirect, mset.cfg.MirrorDirect, mset.isCurrent()
 			if !current {
@@ -2328,7 +2320,7 @@ func (js *jetStream) monitorStream(mset *stream, sa *streamAssignment, sendSnaps
 				mset.subscribeToMirrorDirect()
 			}
 			mset.mu.Unlock()
-			// Stop monitoring.
+			// Stop direct monitoring.
 			stopDirectMonitoring()
 
 		case <-t.C:

server/jetstream_cluster_3_test.go

@@ -4046,3 +4046,79 @@ func TestJetStreamClusterStreamScaleUpNoGroupCluster(t *testing.T) {
 	})
 	require_NoError(t, err)
 }
+
+// https://github.com/nats-io/nats-server/issues/4162
+func TestJetStreamClusterStaleDirectGetOnRestart(t *testing.T) {
+	c := createJetStreamClusterExplicit(t, "NATS", 3)
+	defer c.shutdown()
+
+	nc, js := jsClientConnect(t, c.randomServer())
+	defer nc.Close()
+
+	kv, err := js.CreateKeyValue(&nats.KeyValueConfig{
+		Bucket:   "TEST",
+		Replicas: 3,
+	})
+	require_NoError(t, err)
+
+	_, err = kv.PutString("foo", "bar")
+	require_NoError(t, err)
+
+	// Close client in case we were connected to server below.
+	// We will recreate.
+	nc.Close()
+
+	// Shutdown a non-leader.
+	s := c.randomNonStreamLeader(globalAccountName, "KV_TEST")
+	s.Shutdown()
+
+	nc, js = jsClientConnect(t, c.randomServer())
+	defer nc.Close()
+
+	kv, err = js.KeyValue("TEST")
+	require_NoError(t, err)
+
+	_, err = kv.PutString("foo", "baz")
+	require_NoError(t, err)
+
+	errCh := make(chan error, 100)
+	done := make(chan struct{})
+
+	go func() {
+		nc, js := jsClientConnect(t, c.randomServer())
+		defer nc.Close()
+
+		kv, err := js.KeyValue("TEST")
+		if err != nil {
+			errCh <- err
+			return
+		}
+
+		for {
+			select {
+			case <-done:
+				return
+			default:
+				entry, err := kv.Get("foo")
+				if err != nil {
+					errCh <- err
+					return
+				}
+				if v := string(entry.Value()); v != "baz" {
+					errCh <- fmt.Errorf("Got wrong value: %q", v)
+				}
+			}
+		}
+	}()
+
+	// Restart
+	c.restartServer(s)
+	// Wait for a bit to make sure as this server participates in direct gets
+	// it does not server stale reads.
+	time.Sleep(2 * time.Second)
+	close(done)
+
+	if len(errCh) > 0 {
+		t.Fatalf("Expected no errors but got %v", <-errCh)
+	}
+}