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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CURATOR-495) Race and possible dead locks with RetryPolicies and several Curator Recipes
Date Fri, 14 Dec 2018 19:13:00 GMT

    [ https://issues.apache.org/jira/browse/CURATOR-495?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16721712#comment-16721712
] 

ASF GitHub Bot commented on CURATOR-495:
----------------------------------------

Github user Randgalt commented on a diff in the pull request:

    https://github.com/apache/curator/pull/297#discussion_r241859977
  
    --- Diff: curator-framework/src/main/java/org/apache/curator/framework/CuratorFramework.java
---
    @@ -331,4 +329,29 @@
          * @return true/false
          */
         boolean isZk34CompatibilityMode();
    +
    +    /**
    +     * Calls {@link #notifyAll()} on the given object after first synchronizing on it.
This is
    +     * done from the {@link #runSafe(Runnable)} thread.
    +     *
    +     * @param monitorHolder object to sync on and notify
    +     * @since 4.1.0
    +     */
    +    default void postSafeNotify(Object monitorHolder)
    --- End diff --
    
    Good idea - I'll add that


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