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https://issues.apache.org/jira/browse/ZOOKEEPER-1863?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13872397#comment-13872397
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Dutch T. Meyer commented on ZOOKEEPER-1863:
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I'd be happy to, but it may take me quite some time to get to it - I have many
commitments right now.
I think I'd first look at whether that second block can be protected with a
short circuit check on isWaitingForCommit(). If that's not true then I don't
think we can grab a sync off the committedRequests before it has been pulled
off queuedRequests, which seems to be the core problem. We should probably
think about whether there are any other orderings that would get you to a
similar state though.
> Race condition in commit processor leading to out of order request
> completion, xid mismatch on client.
> ------------------------------------------------------------------------------------------------------
>
> Key: ZOOKEEPER-1863
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1863
> Project: ZooKeeper
> Issue Type: Bug
> Components: server
> Affects Versions: 3.5.0
> Reporter: Dutch T. Meyer
> Priority: Blocker
>
> In CommitProcessor.java processor, if we are at the primary request handler
> on line 167:
> {noformat}
> while (!stopped && !isWaitingForCommit() &&
> !isProcessingCommit() &&
> (request = queuedRequests.poll()) != null) {
> if (needCommit(request)) {
> nextPending.set(request);
> } else {
> sendToNextProcessor(request);
> }
> }
> {noformat}
> A request can be handled in this block and be quickly processed and completed
> on another thread. If queuedRequests is empty, we then exit the block. Next,
> before this thread makes any more progress, we can get 2 more requests, one
> get_children(say), and a sync placed on queuedRequests for the processor.
> Then, if we are very unlucky, the sync request can complete and this object's
> commit() routine is called (from FollowerZookeeperServer), which places the
> sync request on the previously empty committedRequests queue. At that point,
> this thread continues.
> We reach line 182, which is a check on sync requests.
> {noformat}
> if (!stopped && !isProcessingRequest() &&
> (request = committedRequests.poll()) != null) {
> {noformat}
> Here we are not processing any requests, because the original request has
> completed. We haven't dequeued either the read or the sync request in this
> processor. Next, the poll above will pull the sync request off the queue, and
> in the following block, the sync will get forwarded to the next processor.
> This is a problem because the read request hasn't been forwarded yet, so
> requests are now out of order.
> I've been able to reproduce this bug reliably by injecting a
> Thread.sleep(5000) between the two blocks above to make the race condition
> far more likely, then in a client program.
> {noformat}
> zoo_aget_children(zh, "/", 0, getchildren_cb, NULL);
> //Wait long enough for queuedRequests to drain
> sleep(1);
> zoo_aget_children(zh, "/", 0, getchildren_cb, &th_ctx[0]);
> zoo_async(zh, "/", sync_cb, &th_ctx[0]);
> {noformat}
> When this bug is triggered, 3 things can happen:
> 1) Clients will see requests complete out of order and fail on xid mismatches.
> 2) Kazoo in particular doesn't handle this runtime exception well, and can
> orphan outstanding requests.
> 3) I've seen zookeeper servers deadlock, likely because the commit cannot be
> completed, which can wedge the commit processor.
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