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Henry Robinson commented on ZOOKEEPER-1361: ------------------------------------------- It applies to trunk for me (I don't know that this has to into 3.4, since we've not seen bug reports on this issue). My trunk is at ZOOKEEPER-1386, and I can apply the patch cleanly with patch -p0 < ZOOKEEPER-1361-no-whitespace.patch Am I out of date? > 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 > > Attachments: ZOOKEEPER-1361-no-whitespace.patch, ZOOKEEPER-1361.patch > > > 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