[GitHub] [zookeeper] cnauroth commented on pull request #1774: ZOOKEEPER-4409 NullPointerException in SendAckRequestProcessor

[GitBox <gi...@apache.org>]

cnauroth commented on PR #1774:
URL: https://github.com/apache/zookeeper/pull/1774#issuecomment-1295509378

   Assuming the cause is as described in https://github.com/apache/zookeeper/pull/1774#issuecomment-993437244 , then I'm not sure this patch would completely prevent NPEs. With this patch, we could have 2 independent threads concurrently calling `Learner#closeSockSync()`:
   
   ```
       void closeSockSync() {
           try {
               long startTime = Time.currentElapsedTime();
               if (sock != null) {
                   sock.close();
                   sock = null;
               }
               ServerMetrics.getMetrics().SOCKET_CLOSING_TIME.add(Time.currentElapsedTime() - startTime);
           } catch (IOException e) {
               LOG.warn("Ignoring error closing connection to leader", e);
           }
       }
   ```
   
    I don't believe there is any locking in effect around this operation, so it would be possible that both thread 1 and thread 2 make it past the `if (sock != null)` check, thread 1 closes and nulls the variable, and then thread 2 gets the NPE when it attempts `sock.close()`.


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