The current implementation of thesemaphore_tracker creates a new process for each children.
This PR intends to fix this behavior.

The easy fix would be to pass the_pid to the children but the current mechanism to check if thesemaphore_tracker is alive relies onwaitpid which cannot be used in child processes (thesemaphore_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 onwaitpid, I don't see an efficient mechanism.

I took the approach of adding aPROBE command in the semaphore tracker. When the pipe is closed, the send command fails and this means the semaphore tracker is down.

https://bugs.python.org/issue36668