[jira] [Created] (CURATOR-495) Race and possible dead locks with RetryPolicies and several Curator Recipes

["Jordan Zimmerman (JIRA)" <ji...@apache.org>]

Jordan Zimmerman created CURATOR-495:
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             Summary: Race and possible dead locks with RetryPolicies and several Curator Recipes
                 Key: CURATOR-495
                 URL: https://issues.apache.org/jira/browse/CURATOR-495
             Project: Apache Curator
          Issue Type: Bug
          Components: Recipes
    Affects Versions: 4.0.1
            Reporter: Jordan Zimmerman
            Assignee: Jordan Zimmerman
             Fix For: 4.1.0


In trying to figure out why {{TestInterProcessSemaphoreMutex}} is so flakey I've come across a fairly serious edge case in how several of our recipes work. You can see the issue in {{InterProcessSemaphoreV2}} (which is what {{InterProcessSemaphoreMutex}} uses internally). Look here:

[https://github.com/apache/curator/blob/master/curator-recipes/src/main/java/org/apache/curator/framework/recipes/locks/InterProcessSemaphoreV2.java#L373|http://example.com]

The code synchronizes and then does {{client.getChildren()...}}. This is where the problem is. If there are connection problems inside of getChildren() the retry policy will do configured sleeping, retries, etc. Importantly, this is all done while the thread doing the retires holds InterProcessSemaphoreV2's monitor. If the ZK connection is repaired past the session timeout, ZK will eventually call InterProcessSemaphoreV2's watcher with an Expired message. InterProcessSemaphoreV2's watcher calls this method:

{code}
private synchronized void notifyFromWatcher()
{
    notifyAll();
}
{code}

You can see that this is a race. The thread doing "getChildren" is holding the monitor and is in a retry loop waiting for the connection to be repaired. However, ZK's event loop is trying to obtain that same monitor as a result of trying to call the synchronized notifyFromWatcher(). This means that the retry policy will always fail because ZK's event loop is tied up until that thread exists. Worse still, if someone were to use a retry policy of "RetryForever" they'd have a deadlock.

This pattern is in about 10 files or so. I'm trying to think of a workaround. One possibility is to use a separate thread for this type of notification. i.e. notifyFromWatcher() would just signal another thread that the notifyAll() needs to be called. This would unblock ZK's event thread so that things can progress. I'll play around with this.



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