The terminate() method of multiprocessing.Pool hangs sporadically.
I could track this issue down to the fact that _handle_results() hangs in the outqueue-cleanup. poll() returned True but get() actually hangs endlessly never returning any data.

To help get an idea of the racing condition I created a trace with several additional debug outputs in pool.py and connection.py

"======= Creating new pool =======" marks a new start of a pool using the Pool.imap() function. The iteration is terminated at some time and a new pool is created with a new call to imap(). The last log entry shows the location of the last call which currently hangs endlessly.

I am not a multithreading/winapi expert so I could not fix the actual issue with _winapi.WaitForMultipleObjects but the fix in the PR actually ignores preceding issues when polling more data than actually needed for the two sentinels.

Remark: I have also seen this issue in linux with the same application but was not able to debug it so far.

Hi, I got bit by this bug last week, I wrote an example that reproduce the basic idea of our program main loop and it hangs

 - around 20% of the time with a release build of Python 3.7.4
 - around 6% of the time with a debug build of Python 3.7, 3.8 and 3.9

With some of our inputs, it hangs nearly all the time but I cannot post them here.

I tested PR 8009 and it solves the issue. It seems to me that it is an appropriate fix for this.

Removed Python 3.6 as it is in security fixes now.

I'm also experiencing hanging on terminate. I haven't made a debug build or anything but it's happening to me consistently on 3.8, although I haven't managed to create a small example to reproduce. Replacing pool.py with https://raw.githubusercontent.com/python/cpython/5f6a05bf5b3f7e3c1d805b3bbd8c5ad18f26d933/Lib/multiprocessing/pool.py (from the PR) did not help. So maybe what I'm experiencing is unrelated.

It gets stuck on `inqueue._rlock.acquire()` in `Pool._help_stuff_finish`. I've attached debugging info from snoop, maybe that will help.

Sorry, I should have looked around more, I think my problem is https://bugs.python.org/issue22393