bpo-38119: Fix shmem resource tracking

[applio]

Eliminates the use of the resource tracker on shared memory segments but preserves its use on semaphores, etc. where it is appropriately applied.

Adds a test that fails with the current code (an independent Python process started via subprocess accesses shared memory created by a different Python process, then exits normally, expecting the original creator process to continue to be able to access that shared memory) but passes with this patch to defend against future regressions.

https://bugs.python.org/issue38119

[vinay0410]

Updated PR fixing merge conflicts.

[vinay0410]

@lihaoxiang1989 , this PR has merge conflicts, I have created a PR fixing those, containing the same changes. Can you approve that instead.
Updated PR.

[lthakur007]

Hello Team,
As am facing the same issue for which this PR has a solution.

May I know, once the PR is merged, how can I get the fix on Ubuntu 18.04?

[vinay0410]

@lthakur007 , these changes are now being tracked by #21516, once those changes are merged you can either wait for an alpha or beta tag release, or you can directly build python from source (master).

[lthakur007]

@lthakur007 , these changes are now being tracked by #21516, once those changes are merged you can either wait for an alpha or beta tag release, or you can directly build python from source (master).

Thank you for the response.

[zhuowater]

good idea,it works,nice job!

[atarashansky]

Astonishing that this is still not merged... I've been manually unregistering shared memory from child processes for the last year. What's the point of shared memory if it doesn't persist between processes by default...

[jcarlitooliveira]

any news on this?

[buzmeg]

@jcarlitooliveira Does #82300 help you in any way?

[gpshead]

there's an updated PR mentioned above: #21516 - it has a lot more discussion on it so i'm closing this earlier version of the proposed change.

[jcarlitooliveira]

@jcarlitooliveira Does #82300 help you in any way?

im using this monkey patch at the beginning of the application:

TODO: CHANGE FIX REGISTER/UNREGISTER TO INCLUDE SEMAPHORES

def remove_shm_from_resource_tracker():
"""Monkey-patch multiprocessing.resource_tracker so SharedMemory won't be tracked

More details at: https://bugs.python.org/issue38119
"""

def fix_register(name, rtype):
    if rtype == "shared_memory":
        return
    return resource_tracker._resource_tracker.register(self, name, rtype)
resource_tracker.register = fix_register

def fix_unregister(name, rtype):
    if rtype == "shared_memory":
        return
    return resource_tracker._resource_tracker.unregister(self, name, rtype)
resource_tracker.unregister = fix_unregister

if "shared_memory" in resource_tracker._CLEANUP_FUNCS:
    del resource_tracker._CLEANUP_FUNCS["shared_memory"]