fix: Improve message sender / handler robustness in resync scenarios

flow-client/src/main/java/com/vaadin/client/communication/MessageHandler.java

@@ -66,6 +66,8 @@ public class MessageHandler {
      */
     private static final int UNDEFINED_SYNC_ID = -1;
 
+    private boolean resyncInProgress;
+
     /**
      * If responseHandlingLocks contains any objects, response handling is
      * suspended until the collection is empty or a timeout has occurred.
@@ -217,7 +219,17 @@ public void handleMessage(final ValueMap json) {
     protected void handleJSON(final ValueMap valueMap) {
         final int serverId = getServerId(valueMap);
 
-        if (isResynchronize(valueMap) && !isNextExpectedMessage(serverId)) {
+        boolean hasResynchronize = isResynchronize(valueMap);
+
+        if (!hasResynchronize && resyncInProgress) {
+            Console.warn(
+                    "Dropping the response of a request before a resync request.");
+            return;
+        }
+
+        resyncInProgress = false;
+
+        if (hasResynchronize && !isNextExpectedMessage(serverId)) {
             // Resynchronize request. We must remove any old pending
             // messages and ensure this is handled next. Otherwise we
             // would keep waiting for an older message forever (if this
@@ -820,4 +832,7 @@ public void setNextResponseSessionExpiredHandler(
         this.nextResponseSessionExpiredHandler = nextResponseSessionExpiredHandler;
     }
 
+    public void onResynchronize() {
+        resyncInProgress = true;
+    }
 }

flow-client/src/main/java/com/vaadin/client/communication/MessageSender.java

@@ -46,6 +46,8 @@ public class MessageSender {
     private final Registry registry;
     private final PushConnectionFactory pushConnectionFactory;
 
+    private boolean resynchronizeRequested = false;
+
     /**
      * Creates a new instance connected to the given registry.
      *
@@ -88,15 +90,15 @@ public void sendInvocationsToServer() {
     private void doSendInvocationsToServer() {
 
         ServerRpcQueue serverRpcQueue = registry.getServerRpcQueue();
-        if (serverRpcQueue.isEmpty()) {
+        if (serverRpcQueue.isEmpty() && !resynchronizeRequested) {
             return;
         }
 
         boolean showLoadingIndicator = serverRpcQueue.showLoadingIndicator();
         JsonArray reqJson = serverRpcQueue.toJson();
         serverRpcQueue.clear();
 
-        if (reqJson.length() == 0) {
+        if (reqJson.length() == 0 && !resynchronizeRequested) {
             // Nothing to send, all invocations were filtered out (for
             // non-existing connectors)
             Console.warn(
@@ -105,6 +107,12 @@ private void doSendInvocationsToServer() {
         }
 
         JsonObject extraJson = Json.createObject();
+        if (resynchronizeRequested) {
+            registry.getMessageHandler().onResynchronize();
+            Console.log("Resynchronizing from server");
+            extraJson.put(ApplicationConstants.RESYNCHRONIZE_ID, true);
+            resynchronizeRequested = false;
+        }
         if (showLoadingIndicator) {
             ConnectionIndicator.setState(ConnectionIndicator.LOADING);
         }
@@ -218,10 +226,9 @@ public String getCommunicationMethodName() {
      * state from the server
      */
     public void resynchronize() {
-        Console.log("Resynchronizing from server");
-        JsonObject resyncParam = Json.createObject();
-        resyncParam.put(ApplicationConstants.RESYNCHRONIZE_ID, true);
-        send(Json.createArray(), resyncParam);
+        Console.log("Resynchronize from server requested");
+        resynchronizeRequested = true;
+        sendInvocationsToServer();
     }
 
     /**