This closes #2002

artemis-core-client/src/main/java/org/apache/activemq/artemis/core/protocol/core/impl/RemotingConnectionImpl.java

@@ -78,6 +78,12 @@ public class RemotingConnectionImpl extends AbstractRemotingConnection implement
 
    private String clientID;
 
+
+   @Override
+   public void scheduledFlush() {
+      flush();
+   }
+
    // Constructors
    // ---------------------------------------------------------------------------------
 

artemis-core-client/src/main/java/org/apache/activemq/artemis/spi/core/protocol/AbstractRemotingConnection.java

@@ -52,6 +52,11 @@ public abstract class AbstractRemotingConnection implements RemotingConnection {
       this.creationTime = System.currentTimeMillis();
    }
 
+   @Override
+   public void scheduledFlush() {
+      flush();
+   }
+
    @Override
    public List<FailureListener> getFailureListeners() {
       return new ArrayList<>(failureListeners);

artemis-core-client/src/main/java/org/apache/activemq/artemis/spi/core/protocol/RemotingConnection.java

@@ -57,6 +57,8 @@ public interface RemotingConnection extends BufferHandler {
     */
    String getRemoteAddress();
 
+   void scheduledFlush();
+
    /**
     * add a failure listener.
     * <p>

artemis-protocols/artemis-amqp-protocol/src/main/java/org/apache/activemq/artemis/protocol/amqp/broker/ActiveMQProtonRemotingConnection.java

@@ -60,6 +60,12 @@ public class ActiveMQProtonRemotingConnection extends AbstractRemotingConnection
       return manager;
    }
 
+
+   @Override
+   public void scheduledFlush() {
+      amqpConnection.scheduledFlush();
+   }
+
    /*
     * This can be called concurrently by more than one thread so needs to be locked
     */

artemis-protocols/artemis-amqp-protocol/src/main/java/org/apache/activemq/artemis/protocol/amqp/proton/AMQPConnectionContext.java

@@ -123,6 +123,11 @@ public class AMQPConnectionContext extends ProtonInitializable implements EventH
       }
    }
 
+
+   public void scheduledFlush() {
+      handler.scheduledFlush();
+   }
+
    public boolean isIncomingConnection() {
       return isIncomingConnection;
    }

artemis-protocols/artemis-amqp-protocol/src/main/java/org/apache/activemq/artemis/protocol/amqp/proton/handler/ProtonHandler.java

@@ -131,6 +131,17 @@ public class ProtonHandler extends ProtonInitializable implements SaslListener {
       }
    }
 
+   /**
+    * We cannot flush until the initial handshake was finished.
+    * If this happens before the handshake, the connection response will happen without SASL
+    * and the client will respond and fail with an invalid code.
+    * */
+   public void scheduledFlush() {
+      if (receivedFirstPacket) {
+         flush();
+      }
+   }
+
    public int capacity() {
       lock.lock();
       try {

artemis-protocols/artemis-mqtt-protocol/src/main/java/org/apache/activemq/artemis/core/protocol/mqtt/MQTTConnection.java

@@ -58,6 +58,12 @@ public class MQTTConnection implements RemotingConnection {
       this.destroyed = false;
    }
 
+
+   @Override
+   public void scheduledFlush() {
+      flush();
+   }
+
    @Override
    public boolean isWritable(ReadyListener callback) {
       return transportConnection.isWritable(callback);

artemis-protocols/artemis-stomp-protocol/src/main/java/org/apache/activemq/artemis/core/protocol/stomp/StompConnection.java

@@ -116,6 +116,12 @@ public final class StompConnection implements RemotingConnection {
       return frameHandler;
    }
 
+
+   @Override
+   public void scheduledFlush() {
+      flush();
+   }
+
    public StompFrame decode(ActiveMQBuffer buffer) throws ActiveMQStompException {
       StompFrame frame = null;
       try {

artemis-server/src/main/java/org/apache/activemq/artemis/core/remoting/server/impl/RemotingServiceImpl.java

@@ -716,7 +716,7 @@ public class RemotingServiceImpl implements RemotingService, ServerConnectionLif
                               // this is using a different thread
                               // as if anything wrong happens on flush
                               // failure detection could be affected
-                              conn.flush();
+                              conn.scheduledFlush();
                            } catch (Throwable e) {
                               ActiveMQServerLogger.LOGGER.failedToFlushOutstandingDataFromTheConnection(e);
                            }