Issue #18048 has been reported by ioquatix (Samuel Williams).

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Bug #18048: Thread#join can break with fiber scheduler unblock fails or blocks.
https://bugs.ruby-lang.org/issues/18048

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: REQUIRED
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In addition to https://bugs.ruby-lang.org/issues/17666 we found several more cases that need to be addressed.

Fix potential hang when joining threads.

If the thread termination invokes user code after `th->status` becomes
`THREAD_KILLED`, and the user unblock function causes that `th->status` to
become something else (e.g. `THREAD_RUNNING`), threads waiting in
`thread_join_sleep` will hang forever. We move the unblock function call
to before the thread status is updated, and allow threads to join as soon
as `th->value` becomes defined.

Wake up join list within thread EC context. (#4471)

If `rb_fiber_scheduler_unblock` raises an exception, it can result in a
segfault if `rb_threadptr_join_list_wakeup` is not within a valid EC. This
change moves `rb_threadptr_join_list_wakeup` into the thread's top level EC
which initially caused an infinite loop because on exception will retry. We
explicitly remove items from the thread's join list to avoid this situation.

These are already fixed on master branch. Here is a PR for backport: https://github.com/ruby/ruby/pull/4686



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