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

[Davin Potts]

New submission from Davin Potts <python at discontinuity.net>:

The resource tracker currently destroys (via _posixshmem.shm_unlink) shared memory segments on posix systems when any independently created Python process with a handle on a shared memory segment exits (gracefully or otherwise).  This breaks the expected cross-platform behavior that a shared memory segment persists at least as long as any running process has a handle on that segment.

As described with an example scenario in issue37754:
Let's say a three processes P1, P2 and P3 are trying to communicate using shared memory.
 --> P1 creates the shared memory block, and waits for P2 and P3 to access it.
 --> P2 starts and attaches this shared memory segment, writes some data to it and exits.
 --> Now in case of Unix, shm_unlink is called as soon as P2 exits. (This is by action of the resource tracker.)
 --> Now, P3 starts and tries to attach the shared memory segment.
 --> P3 will not be able to attach the shared memory segment in Unix, because shm_unlink has been called on that segment.
 --> Whereas, P3 will be able to attach to the shared memory segment in Windows.

Another key scenario we expect to work but does not currently:
1. A multiprocessing.managers.SharedMemoryManager is instantiated and started in process A.
2. A shared memory segment is created using that manager in process A.
3. A serialized representation of that shared memory segment is deserialized in process B.
4. Process B does work with the shared memory segment that is also still visible to process A.
5. Process B exits cleanly.
6. Process A reads data from the shared memory segment after process B is gone.  (This currently fails.)

The SharedMemoryManager provides a flexible means for ensuring cleanup of shared memory segments.  The current resource tracker attempts to treat shared memory segments as equivalent to semaphore references, which is too narrow of an interpretation.  As such, the current resource tracker should not be attempting to enforce cleanup of shared memory segments because it breaks expected behavior and significantly limits functionality.

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assignee: davin
components: Library (Lib)
messages: 351960
nosy: davin, pablogsal, pitrou, vinay0410, vstinner
priority: normal
severity: normal
status: open
title: resource tracker destroys shared memory segments when other processes should still have valid access
type: behavior
versions: Python 3.8, Python 3.9

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