[jira] Commented: (QPID-1871) During Rollback Client Rejects Message after sending TxRollback

["Martin Ritchie (JIRA)" <qp...@incubator.apache.org>]

    [ https://issues.apache.org/jira/browse/QPID-1871?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12757073#action_12757073 ] 

Martin Ritchie commented on QPID-1871:
--------------------------------------

=== QPID-1971 Spec ===

During Rollback Client Rejects Message after sending TxRollback

Problem:

Broker does not obey Flow=false when the client calls TxRollback. In the TxRollbackHandler we rollback 
any TxOps then resend any unacknowledged messages.

The question is who is wrong here.
 - Is the client wrong in requsting TxRollback whilst in a Flowed state.
 - Is the broker wrong for sending messages when the client has requested flow.false.
 
 
Either way the client is erroneously requesting TxRollback before it has fully verified
that it no longer has any messages in the session.

The problem occurs here as the Dispatcher has the ability to hold on to a message so when the rollback 
process is beleived to have completed the Dispatcher then rejects the final message AFTER the TxRollback
so that one message gets sent ahead of the other messages. The reject is dropped as the message has been 
resent.

Analysis of current process:

session.rollback()

0-8 : Client Side
 - Suspend the Channel
 
  - for ClientAck && Tx
    - Reject all the delivered messages, Reject (requeue=true)

  - if we have a dispatcher (.rollback)
   - verify connection is stopped  
   - loops all active consumers  
  	 - rolling them back, if they are Consumers 
	  *** Consumer Rollback
 	 - clearing their recieve Queue, if they are Browsers
   - If we are in a Transacted Session we hae recorded all the consumers that have been closed.
    - we then rollback and remove each of these recorded consumers from the list.
     *** Consumer Rollback
   - restore the connection state(i.e. if we were stopped stay stopped)
   
  - Send Rollback 
  - Mark Session Clean
 - Unsuspend the channel
---

Consumer Rollback
- Not sure why the recorded sessions would need to rollback as the first check in this method is
 to check if there are any messages pending for this consumer.
  - All deregistered consumers will have an empty queue... I would have thought.

- The contents of the synchronouseQueue are then processed.
 - Any AbstractJMSMessages are rejected (requeue=true)
 - Any thing else is dropped from the queue.:)

----
Java Broker Side	
 - all TxOps are rollback
 - TxRollbackOk is sent then
 - channel.resend is called which resends all currently unacked messages

	
Other Issues:
Java doc wrong: rollback does NOT commit all messages!

---- 
Solution Approach:

I believe there are two possibilities.

1) If we do not reject the delivered messages then the channel.resend process will resend all the messages
back to the client. The order of receipt will be identical to the first time.

2) We can correct the dispatcher flaw so that all messages are correctly rejected so the resend will have 
nothing to do. This means that the order of received messages may change but will be in line with the JMS API.
- Expired messages will expire
- Priority messages will be expodited.
- Also those messages will be made available to other consumers of that queue (Ordering on that client is not defined by JMS)

Whilst 1 is simpist I beleive performing 2 is the correct thing to do.


> During Rollback Client Rejects Message after sending TxRollback
> ---------------------------------------------------------------
>
>                 Key: QPID-1871
>                 URL: https://issues.apache.org/jira/browse/QPID-1871
>             Project: Qpid
>          Issue Type: Bug
>          Components: Java Broker, Java Client
>    Affects Versions: M4, 0.5
>            Reporter: Martin Ritchie
>            Assignee: Martin Ritchie
>
> Summary:
> See QPID-1864 for annotated log output.
> The log output is from a run with the Java broker, but highlights that the client dispatcher thread is not synchronized with the main thread during rollback.
> As a result the main thread sends the TxRollback before the Dispatcher has sent its Reject message. This results, on the java broker, in the unrejected message being redelivered, which may be out of order depending on what other messages have been released on the message queue.
> If we are to continue to rely on the dispatcher thread rejecting/releasing the message it is currently processing (i.e. the message that is neither in the _queue preDispatchQueue nor the _synchronousQueue for receiver delivery) then we will need to synchronize with the main thread's rollback/recover calls so that the dispatcher can finish processing its message before the rollback/recover completes.
> The message that the Dispatcher thread has can be seen in AMQSession L:2866:dispatchMessage(). 
> On Rollback we stop the dispatcher (L2763) which can result in the dispatcher thread stopping on L2877 and holding on the the message it is in the middle of delivery. More likely during recover the dispatcher will block on the lock L2870.
> When the dispatcher is restarted (L2792) it is then free to reject its message. However, the thread that restarted the dispatcher's next call is to send the rollback command(L1553) Which is where the race condition occurs.
> Potential Fix:
> Message Rejection should be performed BEFORE we stop the dispatcher.
> On L:2825 we remove the message from the _queue (preDispatchQueue) and then potentailly sit on the message L:2877 when we get stopped.
> If the reject call in L:2888 was before the wait then we could reject the message rather than sit on it.
> Note: Now that I look at this a bit more the rollback (L2754) code looks to be over synchronized. I'm not sure the dispatcher will actually ever stop on the wait L2877 during rollback as the dispatcher is stopped and started again inside the one syncronisation which would prevent the dispatcher getting to the wait. So will more likely block on the sync L2870
> Moving the setConnectionStopped calls out of the sync block along and ensuring that the _rollbackMark is updated before the connection is stopped then we should ok.

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