[jira] [Created] (HTTPASYNC-56) Deadlock in DefaultClientExchangeHandlerImpl.start()

["Dmitry Potapov (JIRA)" <ji...@apache.org>]

Dmitry Potapov created HTTPASYNC-56:
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             Summary: Deadlock in DefaultClientExchangeHandlerImpl.start()
                 Key: HTTPASYNC-56
                 URL: https://issues.apache.org/jira/browse/HTTPASYNC-56
             Project: HttpComponents HttpAsyncClient
          Issue Type: Bug
    Affects Versions: 4.0-beta4
            Reporter: Dmitry Potapov


How it looks like:
1. Theads A and B calls CloseableHttpAsyncClient.execute()
2. Each CloseableHttpAsyncClient in turn calls InternalHttpAsyncClient.execute() which creates DefaultClientExchangeHandlerImpl instance and calls DefaultClientExchangeHandlerImpl.start(), which is synchronized
3. At this point, we have two DefaultClientExchangeHandlerImpl with locked monitors, let these instances have names AH and BH.
4. DefaultClientExchangeHandlerImpl.start() calls requestConnection(), which in turn calls PoolingNHttpClientConnectionManager.requestConnection()
5. At thread A: AbstractNIOConnPool.lease() adds completed request to the completedRequests queue (line 271). This request callback has reference to the AH
6. At thread B: AbstractNIOConnPool.lease() adds completed request to the completedRequests queue. This request callback has reference to BH
7. At thread B: AbstractNIOConnPool.fireCallbacks() is called. It polls AH from completedRequests and calls AH callback, which tries to enter AH monitor and locks, because this monitor is already locked.
8. At thread A: AbstractNIOConnPool.fireCallbacks() is called. It polls BH from completedRequests (AH was polled at step 7) and calls BH callback, which tries to enter BH monitor and locks, because this monitor is already locked.

At this point we have threads A and B deadlocked.

I have obvious solution for this particular dead-lock: make DefaultClientExchangeHandlerImpl.start() not synchronized, because this object created only at single point, and .start() is called immediately after construction.

I'm not sure that there is no problems in other scenarious where .fireCallbacks() involved, because DefaultClientExchangeHandlerImpl.requestConnection() may be called from other synchronized methods.



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