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https://issues.apache.org/jira/browse/HTTPCLIENT-633?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12473237
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Michael Becke commented on HTTPCLIENT-633:
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Hi John,
I would agree, using object notification is probably a cleaner way to do this.
I'm not sure why I did it this way originally, simplicity perhaps. My question
is if interrupting a waiting thread outside of the connection manager is
something we want to support. If we did, would we just throw a
ConnectionPoolTimeoutException when it occurs? This would seem to break the
contract of doGetConnection. I'm not sure how we will handle HttpConnection
getConnection(HostConfiguration hostConfiguration) without an API change.
Assuming we did want to change this, we would need to wait on the
HostConnectionPool instead of the ConnectionPool. Doing this brings up a
couple of potential issues, but I think they're all fixable. In particular:
- We would have two layers of synchronization to go through, ConnectionPool
and HostConnectionPool, and would need to handle this carefully to avoid
potential dead-locks, race conditions, etc.
- We lose the FIFO nature currently guaranteed to waiting threads.
Object.notify() makes no guarantees about which thread will be woken first.
To get this into 3.1 we will definitely want to have an RC release with this
change first. Given the complexity of synchronization I would want to give it
some time for testing.
Any other thoughts?
Mike
> MultiThreadedHttpConnectionManager does not properly respond to thread
> interrupts
> ---------------------------------------------------------------------------------
>
> Key: HTTPCLIENT-633
> URL: https://issues.apache.org/jira/browse/HTTPCLIENT-633
> Project: HttpComponents HttpClient
> Issue Type: Bug
> Components: HttpConn
> Affects Versions: 3.1 Beta 1
> Reporter: John Goodwin
>
> MultiThreadedHttpConnectionManager uses interrupts to notify waiting threads
> when a connection is ready for them. Issues arise if the threads are
> interrupted by someone else while they are still waiting on a thread, because
> doGetConnection does not remove the threads from the queue of waiting threads
> when they are interrupted:
> connectionPool.wait(timeToWait);
> // we have not been interrupted so we need to remove
> ourselves from the
> // wait queue
> hostPool.waitingThreads.remove(waitingThread);
> connectionPool.waitingThreads.remove(waitingThread);
> } catch (InterruptedException e) {
> // do nothing } finally {
> if (useTimeout) {
> endWait = System.currentTimeMillis();
> timeToWait -= (endWait - startWait);
> } }
> Under ordinary circumstances, the queue maintenance is done by the
> notifyWaitingThread method. However, if the thread is interrupted by any
> other part of the system, it will (1) not actually be released, since the
> loop in doGetConnection will force it back to the wait, and (2) will be added
> the waiting thread to the queue repeatedly, which basically means that the
> thread will eventually receive the interrupt from notifyWaitingThread at some
> later point, when it is no longer actually waiting for a connection.
> This code could probably be re-architected to make it less error-prone, but
> the fundamental issue seems to be the use of interrupts to signal waiting
> threads, as opposed to something like a notify.
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