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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