Remove teardown hook to allow hard shutdown #6395
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This is a cherry-pick of the crucial commit from jruby#6213 that should also fix jruby#6379, where users reported that at_exit and other "clean shutdown" hooks were improperly running during a "hard shutdown" using `exit!`. The original commit message for c941c41 follows: Remove JVM shutdown hook for teardown There's not a lot of context on the commit from 2013 that added this, but I suspect it's not really correct behavior anymore. In the process of reordering teardown (jruby#6213) this logic began to cause a spec hang for Kernel.exit! specs due to the teardown deadlocking with the main thread: * Main thread starts up and joins a thread that calls exit!. The java.lang.Thread.join call locks the thread object. * Thread calls exit!, which calls System.exit (as opposed to the non-bang exit, which terminates the main thread and lets it tear things down safely). * System exit calls this teardown hook, which attempts to shut terminate and join remaining threads. * Because the teardown is not called from the main thread, the system deadlocks trying to shut down the child thread, which is locked in the main thread doing join. The chain of deadlock is main => thread 1 join => system exit => thread 2 for teardown => try to kill and join thread 1. The ultimate deadlock is due to a circular sequence of calls that can't be interrupted (System.exit's wait on the teardown thread) combined with the teardown happening from a non-main thread. It's not clear why this started to hang after the reordering. Moving the ThreadService teardown back to its former relative position does not fix the hang, and logically I don't know why it would... there should still be a deadlock if main is joining a thread and the System exit hook tries to tear that thread down.
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This is a cherry-pick of the crucial commit from #6213 that
should also fix #6379, where users reported that at_exit and
other "clean shutdown" hooks were improperly running during a
"hard shutdown" using
exit!.See c941c41 for the original commit and #6379 (comment) for a discussion of this fix.