Leader.lead iterates over 'learners' set without proper synchronisation
-----------------------------------------------------------------------
Key: ZOOKEEPER-1361
URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1361
Project: ZooKeeper
Issue Type: Bug
Affects Versions: 3.4.2
Reporter: Henry Robinson
Assignee: Henry Robinson
Fix For: 3.5.0
This block:
{code}
HashSet<Long> followerSet = new HashSet<Long>();
for(LearnerHandler f : learners)
followerSet.add(f.getSid());
{code}
is executed without holding the lock on learners, so if there were ever a
condition where a new learner was added during the initial sync phase, I'm
pretty sure we'd see a concurrent modification exception. Certainly other parts
of the code are very careful to lock on learners when iterating.
It would be nice to use a {{ConcurrentHashMap}} to hold the learners instead,
but I can't convince myself that this wouldn't introduce some correctness bugs.
For example the following:
Learners contains A, B, C, D
Thread 1 iterates over learners, and gets as far as B.
Thread 2 removes A, and adds E.
Thread 1 continues iterating and sees a learner view of A, B, C, D, E
This may be a bug if Thread 1 is counting the number of synced followers for a
quorum count, since at no point was A, B, C, D, E a correct view of the quorum.
In practice, I think this is actually ok, because I don't think ZK makes any
strong ordering guarantees on learners joining or leaving (so we don't need a
strong serialisability guarantee on learners) but I don't think I'll make that
change for this patch. Instead I want to clean up the locking protocols on the
follower / learner sets - to avoid another easy deadlock like the one we saw in
ZOOKEEPER-1294 - and to do less with the lock held; i.e. to copy and then
iterate over the copy rather than iterate over a locked set.
--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators:
https://issues.apache.org/jira/secure/ContactAdministrators!default.jspa
For more information on JIRA, see: http://www.atlassian.com/software/jira
