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From "Timothy Bish (JIRA)" <j...@apache.org>
Subject [jira] [Assigned] (AMQ-4022) Receiving event while disconnecting -> AMQ deadlock -> "Already connected" error
Date Wed, 19 Sep 2012 08:39:09 GMT

     [ https://issues.apache.org/jira/browse/AMQ-4022?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Timothy Bish reassigned AMQ-4022:
---------------------------------

    Assignee: Timothy Bish
    
> Receiving event while disconnecting -> AMQ deadlock -> "Already connected" error
> --------------------------------------------------------------------------------
>
>                 Key: AMQ-4022
>                 URL: https://issues.apache.org/jira/browse/AMQ-4022
>             Project: ActiveMQ
>          Issue Type: Bug
>          Components: stomp
>    Affects Versions: 5.5.1
>         Environment: SLES11-SP1 x86_64
>            Reporter: Gabriel Gerhardsson
>            Assignee: Timothy Bish
>             Fix For: NEEDS_REVIEWED
>
>         Attachments: AMQ_4022_possible_fix.diff, amqtest.pl, Net_Stomp.pm.diff
>
>
> Executive summary
> ---
> When an event is about to be delivered to a client while that client simultaneously sends
a DISCONNECT message, there's a risk for a deadlock between two threads in ActiveMQ, causing
no DISCONNECT receipt as well as "Already connected" error if another client connects with
the same client ID.
> Details
> ---
> Here's steps to reproduce, using STOMP, tho it's possible that this issue exists for
other protocols also, since the deadlock happens in broker/TransportConnection.java. I could
reproduce the error easily with these steps, tho it's entirely possible that some of the steps
aren't strictly required but rather provides certain needed timing.
> 1) Connect to ActiveMQ using STOMP
> 2) Send a SUBSCRIBE message with destination=/topic/foo, without asking for a receipt.
No other options sent in message. 
> 3) Send a message to /topic/foo, without asking for a receipt. Note: It's important that
the message is auto-acked by the server.
> 4) Wait 10ms for message
> 5) Go right ahead and send DISCONNECT message, asking for a receipt.
> 5.1) About 20% of the time, the message is never received, nor is the DISCONNECT receipt.
> 5.2) Run 'jstack -p <activemq pid>', watch how the JVM prints info on stdout about
a deadlock between two threads.
> Test script
> ---
> Apply the small patch to the perl library Net/Stomp.pm
> It's a naive minimal patch to make it wait for a receipt for the DISCONNECT message.
It will actually just wait for any frame to arrive and won't check what it is, but it's enough
for this test.
> Run amqtest.pl a few times. A non-buggy run completes almost immediately. A buggy run
hangs the script during disconnect.
> Workaround
> ---
> -When sending the SUBSCRIBE with receipt requested, and waiting for the receipt before
sending the SEND, I was unable to reproduce the issue, even after running the script 100000
times.-
> No, it turns out that the workaround suggested above does not completely eliminate the
problem. It probably just introduces a large enough delay to almost always avoid the race
condition. After 30000 additional test runs with the script the error occurred again.

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