The current implementation of the semaphore_tracker creates a new process for each children.

The easy fix would be to pass the _pid to the children but the current mechanism to check if the semaphore_tracker is alive relies on waitpid which cannot be used in child processes (the semaphore_tracker is only a sibling of these processes). The main issue is to have a reliable check that either:

    The pipe is open. This is what is done here by sending a message. I don't know if there is a more efficient way to check it.
    Check that a given pid is alive. As we cannot rely on waitpid, I don't see an efficient mechanism.

I propose to add a PROBE command in the semaphore tracker. When the pipe is closed, the send command will fail, meaning that the semaphore tracker is down.

New changeset 004b93ea8947bcbe85b6fa16fe0999bfa712d5c1 by Antoine Pitrou (Thomas Moreau) in branch 'master':
bpo-36668: FIX reuse semaphore tracker for child processes (#5172)
https://github.com/python/cpython/commit/004b93ea8947bcbe85b6fa16fe0999bfa712d5c1