gh-125828 : Fix 'get_value' missing for [Bounded]Semaphore on multiprocessing MacOSX - C workaround

[YvesDup]

Following #125829 discussions and after a long deeping in the _multiprocessing.semaphore.c C file, I suggest a workaround based on a shared memory that contains an array to store all the internal semaphore counters.

At the moment, all the tests are passing, but I'm having 2 side problems, the second of which is really embarrassing:

1. How to calculate the size of the array stored in the shared memory, knowing that:
   1. I sized the array from the value of sysconf(SC_SEM_NSEMS_MAX) which is set to 87381 on a mac OSX 15. This is very high.
   2. But I can't create more than about 4900 semaphores regardless of the number of python instances. I get an OSError(28, 'No space left on device'). What is the right value ?
2. When and how to call the system function shm_unlink to properly close the shared memory (First I called shm_open)?

Regarding this last point, I wonder if only the process that opened the shared memory should call this function? Or should all processes that called shm_open call shm_unlink? Unfortunately, the Posix documentation isn't precise.

I worked on the count of opened/closed processes, with the aim of calling shm_unlink when the last process has closed, without success. I also had a look to the posixshmem module, without success too.
Could the resource_tracker and/or the use of the Finalize class of the multiprocessing.util module help me to fix this problem ? Is it possible to use these 2 Python libs from C API ?

This memory management difficulty make me think that this choice may not be appropriate. I am a bit stuck.
Thank you for your feedback and ideas.

• Issue: Multiprocessing Semaphore and BoundedSemaphore - incomplete features ('get_value' missing) on MacOSX. #125828