make sure after resume we just deliver one message at one moment

src/main/java/io/vertx/core/eventbus/impl/MessageConsumerImpl.java

@@ -133,36 +133,24 @@ public synchronized Future<Void> unregister() {
   }
 
   protected boolean doReceive(Message<T> message) {
-    Handler<Message<T>> theHandler;
     synchronized (this) {
       if (handler == null) {
         return false;
       }
-      if (demand == 0L) {
-        if (pending.size() < maxBufferedMessages) {
-          pending.add(message);
-          return true;
-        } else {
-          discard(message);
-          if (discardHandler != null) {
-            discardHandler.handle(message);
-          } else {
-            log.warn("Discarding message as more than " + maxBufferedMessages + " buffered in paused consumer. address: " + address);
-          }
-        }
+      if (pending.size() < maxBufferedMessages) {
+        pending.add(message);
+        checkNextTick();
         return true;
       } else {
-        if (pending.size() > 0) {
-          pending.add(message);
-          message = pending.poll();
-        }
-        if (demand != Long.MAX_VALUE) {
-          demand--;
+        discard(message);
+        if (discardHandler != null) {
+          discardHandler.handle(message);
+        } else {
+          String pause = demand == 0 ? "paused" : "NOT paused";
+          log.warn("Discarding message as more than " + maxBufferedMessages + " buffered in " + pause + " consumer. address: " + address);
         }
-        theHandler = handler;
       }
     }
-    deliver(theHandler, message);
     return true;
   }
 
@@ -171,7 +159,7 @@ protected void dispatch(Message<T> msg, ContextInternal context, Handler<Message
     if (handler == null) {
       throw new NullPointerException();
     }
-    context.emit(msg, handler);
+    context.dispatch(msg, handler);
   }
 
   private void deliver(Handler<Message<T>> theHandler, Message<T> message) {
@@ -183,8 +171,8 @@ private void deliver(Handler<Message<T>> theHandler, Message<T> message) {
 
   private synchronized void checkNextTick() {
     // Check if there are more pending messages in the queue that can be processed next time around
-    if (!pending.isEmpty() && demand > 0L) {
-      context.nettyEventLoop().execute(() -> {
+    if (demand > 0L && !pending.isEmpty()) {
+      context.emit(__ -> {
         Message<T> message;
         Handler<Message<T>> theHandler;
         synchronized (MessageConsumerImpl.this) {

src/test/java/io/vertx/core/eventbus/LocalEventBusTest.java

@@ -1401,7 +1401,6 @@ public void testUnregisterConsumerDiscardPendingMessages() {
       eb.send(ADDRESS1, "val1");
       Context ctx = Vertx.currentContext();
       ctx.runOnContext(v -> {
-        consumer.resume();
         ((MessageConsumerImpl<?>) consumer).discardHandler(discarded -> {
           assertEquals("val1", discarded.body());
           testComplete();

src/test/java/io/vertx/core/eventbus/MessageConsumerTest.java

@@ -7,33 +7,41 @@
 import java.util.HashMap;
 import java.util.Map;
 import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.atomic.AtomicBoolean;
 
 public class MessageConsumerTest extends VertxTestBase {
 
 
   @Test
-  public void testMessageConsumptionStayOnWorkerThreadAfterResume() throws Exception {
-    TestVerticle verticle = new TestVerticle(2);
+  public void testMessageConsumptionStayOnWorkerThreadAfterResumeAndOnlyDispatchOneMessageAtOneMoment() throws Exception {
+    int numberOfExpectedMessages = 10;
+    TestVerticle verticle = new TestVerticle(numberOfExpectedMessages);
+    EchoVerticle echoVerticle = new EchoVerticle();
     Future<String> deployVerticle = vertx.deployVerticle(verticle, new DeploymentOptions().setThreadingModel(ThreadingModel.WORKER));
+    Future<String> deployEchoVerticle = vertx.deployVerticle(echoVerticle, new DeploymentOptions().setThreadingModel(ThreadingModel.WORKER));
 
     CountDownLatch startLatch = new CountDownLatch(1);
-    deployVerticle.onComplete(onSuccess(cf -> startLatch.countDown()));
+    Future.all(deployVerticle, deployEchoVerticle)
+      .onComplete(onSuccess(cf -> startLatch.countDown()));
     awaitLatch(startLatch);
 
-    vertx.eventBus().send("testAddress", "message1");
-    vertx.eventBus().send("testAddress", "message2");
+    for (int i = 1; i <= numberOfExpectedMessages; i++) {
+      vertx.eventBus().send("testAddress", "message" + i);
+    }
 
     awaitLatch(verticle.msgLatch);
 
-    assertEquals(2, verticle.messageArrivedOnWorkerThread.size());
-    assertTrue("message1 should be processed on worker thread", verticle.messageArrivedOnWorkerThread.get("message1"));
-    assertTrue("message2 should be processed on worker thread", verticle.messageArrivedOnWorkerThread.get("message2"));
+    assertEquals(numberOfExpectedMessages, verticle.messageArrivedOnWorkerThread.size());
+    for (int i = 1; i <= numberOfExpectedMessages; i++) {
+      assertTrue("message" + i + " should be processed on worker thread", verticle.messageArrivedOnWorkerThread.get("message" + i));
+    }
   }
 
 
   private static class TestVerticle extends AbstractVerticle {
 
     private final CountDownLatch msgLatch;
+    private final AtomicBoolean messageProcessingOngoing = new AtomicBoolean();
 
     private final Map<String, Boolean> messageArrivedOnWorkerThread = new HashMap<>();
 
@@ -51,11 +59,34 @@ private void handleMessages(MessageConsumer<String> consumer) {
       consumer.handler(msg -> {
         consumer.pause();
         messageArrivedOnWorkerThread.putIfAbsent(msg.body(), Context.isOnWorkerThread());
-        msgLatch.countDown();
-        vertx.setTimer(20, id -> {
-          consumer.resume();
+        if (messageProcessingOngoing.compareAndSet(false, true)) {
+          msgLatch.countDown();
+        } else {
+          System.err.println("Received message while processing another message");
+        }
+        vertx.eventBus().request("echoAddress", 20)
+          .onComplete(ar -> {
+            messageProcessingOngoing.set(false);
+            consumer.resume();
+          });
+      });
+    }
+  }
+
+  private static class EchoVerticle extends AbstractVerticle {
+    @Override
+    public void start() {
+      MessageConsumer<Integer> consumer = vertx.eventBus().localConsumer("echoAddress");
+      handleMessages(consumer);
+    }
+
+    private void handleMessages(MessageConsumer<Integer> consumer) {
+      consumer.handler(msg -> {
+        vertx.setTimer(msg.body(), id -> {
+          msg.reply(msg.body());
         });
       });
     }
   }
+
 }