[issue38119] resource tracker destroys shared memory segments when other processes should still have valid access

[Damian Barabonkov]

Damian Barabonkov <damianb at mit.edu> added the comment:

As per Guido's comment (https://github.com/python/cpython/pull/21516#issuecomment-668110711), I'm going to use this space to discuss ways to go forward with resource tracking and SharedMemory.

Taking inspiration from Vinay (https://bugs.python.org/issue37754#msg351445), I think the simplest and best way forward is to use a small section of the shared memory at the start as a reference counter. 

Every time a process latches onto a shared memory block, it does and atomic increment to the reference counter. And if it detaches, it does an atomic decrement. This atomic operations are available in C via hardware specific instructions. This would require modifying the Python C code posixshmem.c. It should not be a difficult change.

This would then change the SharedMemory API such that a call to `close()` could check the reference count at the end, and aromatically unlink if it reaches 0. Basically, the purpose of the explicit `unlink()` call is dissolved.

I think this would also play nice with the current implementation of the `resource_tracker`. A small change would need to take place such that it calls `close()` instead of `unlink()` as the clean up function. Nonetheless, it would keep track if all attachments of shared memory call `close()` at the end, which they should, and issue a warning if they do not. It would do this with the current code, no need to change anything.

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nosy: +damian.barabonkov

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38119>
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