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