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https://issues.apache.org/jira/browse/HADOOP-18217?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Steve Loughran resolved HADOOP-18217.
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Fix Version/s: 3.4.0
3.3.9
Resolution: Fixed
committed to branch-3.3 and above
> shutdownhookmanager should not be multithreaded (deadlock possible)
> -------------------------------------------------------------------
>
> Key: HADOOP-18217
> URL: https://issues.apache.org/jira/browse/HADOOP-18217
> Project: Hadoop Common
> Issue Type: Bug
> Components: util
> Affects Versions: 2.10.1
> Environment: linux, windows, any version
> Reporter: Catherinot Remi
> Priority: Minor
> Labels: pull-request-available
> Fix For: 3.4.0, 3.3.9
>
> Attachments: wtf.java
>
> Time Spent: 4h 40m
> Remaining Estimate: 0h
>
> the ShutdownHookManager class uses an executor to run hooks to have a
> "timeout" notion around them. It does this using a single threaded executor.
> It can leads to deadlock leaving a never-shutting-down JVM with this
> execution flow:
> * JVM need to exit (only daemon threads remaining or someone called
> System.exit)
> * ShutdowHookManager kicks in
> * SHMngr executor start running some hooks
> * SHMngr executor thread kicks in and, as a side effect, run some code from
> one of the hook that calls System.exit (as a side effect from an external lib
> for example)
> * the executor thread is waiting for a lock because another thread already
> entered System.exit and has its internal lock, so the executor never returns.
> * SHMngr never returns
> * 1st call to System.exit never returns
> * JVM stuck
>
> using an executor with a single thread does "fake" timeouts (the task keeps
> running, you can interrupt it but until it stumble upon some piece of code
> that is interruptible (like an IO) it will keep running) especially since the
> executor is a single threaded one. So it has this bug for example :
> * caller submit 1st hook (bad one that would need 1 hour of runtime and that
> cannot be interrupted)
> * executor start 1st hook
> * caller of the future 1st hook result timeout
> * caller submit 2nd hook
> * bug : 1 hook still running, 2nd hook triggers a timeout but never got the
> chance to run anyway, so 1st faulty hook makes it impossible for any other
> hook to have a chance to run, so running hooks in a single separate thread
> does not allow to run other hooks in parallel to long ones.
>
> If we really really want to timeout the JVM shutdown, even accepting maybe
> dirty shutdown, it should rather handle the hooks inside the initial thread
> (not spawning new one(s) so not triggering the deadlock described on the 1st
> place) and if a timeout was configured, only spawn a single parallel daemon
> thread that sleeps the timeout delay, and then use Runtime.halt (which bypass
> the hook system so should not trigger the deadlock). If the normal
> System.exit ends before the timeout delay everything is fine. If the
> System.exit took to much time, the JVM is killed and so the reason why this
> multithreaded shutdown hook implementation was created is satisfied (avoding
> having hanging JVMs)
>
> Had the bug with both oracle and open jdk builds, all in 1.8 major version.
> hadoop 2.6 and 2.7 did not have the issue because they do not run hooks in
> another thread
>
> Another solution is of course to configure the timeout AND to have as many
> threads as needed to run the hooks so to have at least some gain to offset
> the pain of the dealock scenario
>
> EDIT: added some logs and reproduced the problem. in fact it is located after
> triggering all the hook entries and before shutting down the executor.
> Current code, after running the hooks, creates a new Configuration object and
> reads the configured timeout from it, applies this timeout to shutdown the
> executor. I sometimes run with a classloader doing remote classloading,
> Configuration loads its content using this classloader, so when shutting down
> the JVM and some network error occurs the classloader fails to load the
> ressources needed by Configuration. So the code crash before shutting down
> the executor and ends up inside the thread's default uncaught throwable
> handler, which was calling System.exit, so got stuck, so shutting down the
> executor never returned, so does the JVM.
> So, forget about the halt stuff (even if it is a last ressort very robust
> safety net). Still I'll do a small adjustement to the final executor shutdown
> code to be slightly more robust to even the strangest exceptions/errors it
> encounters.
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