Mitigate deadlock on DLL unload

[felixweilbach]

Summary:
ThreadPool gets stored in a static variable here extension/threadpool/threadpool.cpp:146

This means the destructor of ThreadPool will be run when the process exits or a DLL containing this code unloads.

While working with ExecuTorch I experienced a deadlock during unloading our DLL (which contained ExecuTorch) at runtime. This was caused by the pthreadpool_destroy function pthreadpool/src/windows.c:366 waiting forever on the worker threads.

Why this is happening exactly is unclear to me. It is likely a race condition inside Windows Parallel Loader (https://blogs.blackberry.com/en/2017/10/windows-10-parallel-loading-breakdown) as I could see its functions in the stack trace of the stuck worker threads after they returned from their main function.

The issue was mitigated on my side by calling executorch::extension::threadpool::get_threadpool()->_unsafe_reset_threadpool(0); before unloading the DLL.

This is just a workaround. I think a proper fix would be to rework the ThreadPool singleton and allow for explicit termination of it.

Differential Revision: D89889628

[pytorch-bot]

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❌ 2 New Failures, 1 Unrelated Failure

As of commit 2d43467 with merge base daf93a1 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner / linux-job (gh)
  >>> Lint for extension/threadpool/threadpool.h:
• pull / unittest-editable / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 2

UNSTABLE - The following job is marked as unstable, possibly due to flakiness on trunk:

• pull / android / run-emulator (gh) (#16137)
  Timeout waiting for emulator to boot.

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[meta-codesync]

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[kimishpatel]

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