BUG: avoid deadlocks with C++ shared mutex in dispatch cache #28583
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Backport of #28577.
@pitrou correctly pointed out here that the C++
shared_mutexwe use in the dispatch might deadlock with the GIL (if it's re-enabled) or other global synchronization events in the interpreter, and we need to explicitly callPy_BEGIN_ALLOW_THREADS/Py_END_ALLOW_THREADSaround a possibly blocking call.I don't have a case where a deadlock happens right now, so no new test. Happy to add one if someone can come up with a way to trigger the deadlock.
I ran the test script from #27786 and don't see any performance hit.