[jira] [Commented] (TWILL-129) Race condition in leader election if leader quitted before follower set the watch

["Terence Yim (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/TWILL-129?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14555632#comment-14555632 ] 

Terence Yim commented on TWILL-129:
-----------------------------------

This is the unit-test to reproduce the problem.

{code:java}
  @Test
  public void testRace() throws InterruptedException {
    ExecutorService executor = Executors.newFixedThreadPool(2);
    final AtomicInteger leaderCount = new AtomicInteger(0);
    final CountDownLatch completeLatch = new CountDownLatch(2);

    // Starts two threads and try to compete for leader and immediate drop leadership.
    // This is to test the case when a follower tries to watch for leader node, but the leader is already gone
    for (int i = 0; i < 2; i++) {
      final ZKClientService zkClient = ZKClientService.Builder.of(zkServer.getConnectionStr()).build();
      zkClient.startAndWait();
      executor.execute(new Runnable() {
        @Override
        public void run() {
          try {
            for (int i = 0; i < 1000; i++) {
              final CountDownLatch leaderLatch = new CountDownLatch(1);
              LeaderElection election = new LeaderElection(zkClient, "/testRace", new ElectionHandler() {
                @Override
                public void leader() {
                  leaderCount.incrementAndGet();
                  leaderLatch.countDown();
                }

                @Override
                public void follower() {
                  // no-op
                }
              });
              election.startAndWait();
              Uninterruptibles.awaitUninterruptibly(leaderLatch);
              election.stopAndWait();
            }
            completeLatch.countDown();
          } finally {
            zkClient.stopAndWait();
          }
        }
      });
    }

    try {
      Assert.assertTrue(completeLatch.await(2, TimeUnit.MINUTES));
    } finally {
      executor.shutdownNow();
    }
  }
{code}

> Race condition in leader election if leader quitted before follower set the watch
> ---------------------------------------------------------------------------------
>
>                 Key: TWILL-129
>                 URL: https://issues.apache.org/jira/browse/TWILL-129
>             Project: Apache Twill
>          Issue Type: Bug
>          Components: zookeeper
>    Affects Versions: 0.4.0-incubating, 0.5.0-incubating, 0.4.1-incubating
>            Reporter: Terence Yim
>            Assignee: Terence Yim
>            Priority: Blocker
>
> In the LeaderElection implementation, if the leader quit and delete its node before the next-in-line follower try to set the watch through the exists() call, the follower will fail silently without noticing the leader node is gone.



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