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6d6099f7fe89 introduced a regression.
It causes the following code to hang fairly reliably for me:
"""
import concurrent.futures
import math
def is_prime(n):
 print(sqt(81)) <---- Note the type error!
def main():
 with concurrent.futures.ProcessPoolExecutor(2) as executor:
 executor.map(is_prime, [1, 2])
if __name__ == '__main__':
 main()
"""
From my limited knowledge of multiprocessing and concurrent futures, it seems that in shutdown_worker(), call_queue.put(None) hangs because the queue is full and there are no more workers consuming items in the queue (they exited early).
Increasing EXTRA_QUEUED_CALLS fixes the problem, though its probably not the correct solution.