Dutch T. Meyer created ZOOKEEPER-1863:
-----------------------------------------
Summary: 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: Critical
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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