[jira] [Commented] (HDFS-9409) DataNode shutdown does not guarantee full shutdown of all threads due to race condition.

["Chris Nauroth (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HDFS-9409?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14999257#comment-14999257 ] 

Chris Nauroth commented on HDFS-9409:
-------------------------------------

{{DataNode#shutdown}} calls {{BlockPoolManager#getAllNamenodeThreads}} to get every {{BPOfferService}}.  Then, later in {{shutdown}}, these are passed to {{BlockPoolManager#shutDownAll}}, which eventually stops and joins each {{BPServiceActor}} thread.  There are a few problems:

# {{BlockPoolManager#getAllNamenodeThreads}} returns an unmodifiable wrapper over its underlying list, so callers can't mutate the list, but it's still the same shared backing list.  Later during shutdown, the {{BPServiceActor}} is told that it can exit its main loop.  Part of that is a call on the {{BPServiceActor}} thread to {{BlockPoolManager#remove}}.  This effectively removes it from the backing list returned by {{BlockPoolManager#getAllNamenodeThreads}}, so it will appear to vanish from the list before the call to {{BlockPoolManager#shutDownAll}}.
# Even if point 1 is fixed by changing {{BlockPoolManager#getAllNamenodeThreads}} to return a deep copy, there is a similar problem in that {{BPOfferService#shutdownActor}} will remove the actor from its internal tracking list.

Because of these 2 problems, {{DataNode#shutdown}} might no longer have a reference to the {{BPServiceActor}} threads when it tries to stop and join on them.  Therefore, those threads might still be alive even after completion of {{DataNode#shutdown}}.  I noticed this while trying to write a test that asserts a particular thread has exited after DataNode shutdown.

> DataNode shutdown does not guarantee full shutdown of all threads due to race condition.
> ----------------------------------------------------------------------------------------
>
>                 Key: HDFS-9409
>                 URL: https://issues.apache.org/jira/browse/HDFS-9409
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: datanode
>            Reporter: Chris Nauroth
>
> {{DataNode#shutdown}} is documented to return "only after shutdown is complete".  Even after completion of this method, it's possible that threads started by the DataNode are still running.  Race conditions in the shutdown sequence may cause it to skip stopping and joining the {{BPServiceActor}} threads.
> This is likely not a big problem in normal operations, because these are daemon threads that won't block overall process exit.  It is more of a problem for tests, because it makes it impossible to write reliable assertions that these threads exited cleanly.  For large test suites, it can also cause an accumulation of unneeded threads, which might harm test performance.



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