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https://issues.apache.org/jira/browse/ZOOKEEPER-1863?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13882403#comment-13882403
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Camille Fournier commented on ZOOKEEPER-1863:
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So, it seems like if this happens it must happen due to the fact that we have
two paths in CommitProcessor, one on line 203, where we set the nextPending to
null, and one where we never set it to null, the else case below on lines
205-210.
Thawan, has your background thread ever done that in prod?
I am curious as to why we have that case where we get a commit with no
corresponding pending request waiting for it. The comment " // this request
came from someone else so just
// send the commit packet"
Doesn't really make much sense to me, does anyone have an explanation? If this
is the only way we think this could happen, maybe we check the queuedRequest
queue at the time we get the unmatched request, and try to match it then.
> 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
> Attachments: stack.17512
>
>
> 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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