[jira] [Created] (ZOOKEEPER-2321) C-client session watcher removal is not thread safe

["Hadriel Kaplan (JIRA)" <ji...@apache.org>]

Hadriel Kaplan created ZOOKEEPER-2321:
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             Summary: C-client session watcher removal is not thread safe
                 Key: ZOOKEEPER-2321
                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2321
             Project: ZooKeeper
          Issue Type: Bug
          Components: c client
    Affects Versions: 3.5.1
            Reporter: Hadriel Kaplan


Invoking the C-client API function {{zoo_set_watcher()}} to remove/change a session event watcher is not a thread-safe operation. The IO thread accesses the session watcher (the one stored in the zhandle_t.watcher member) and copies its value into completion events, which are then later processed by the completion thread. This happens when it's processing session events, such as session connected/connecting/expired events. 

Meanwhile after the value has been copied by the IO thread, but before the completion thread has used it, the main thread could change the watcher to NULL using {{zoo_set_watcher()}} because the calling application may be free'ing it. The call to {{zoo_set_watcher()}} will return even though the IO and completion threads still have the old watcher pointer value, and the main application cannot safely free it. But since the function call returns, the main application thinks it can free it, and boom goes the dynamite.

So... either there needs to be a lockout while the IO/completion threads process session events, or the {{zoo_set_watcher()}} needs to become asynchronous itself by going through the same processing pipeline to the completion thread and having a completion callback to tell the calling application when it succeeded/failed.



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