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Minor Description
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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