[New-bugs-announce] [issue45125] Improve tests and docs of how `pickle` works with `SharedMemory` obejcts

[Nikita Sobolev]

New submission from Nikita Sobolev <mail at sobolevn.me>:

As we've discussed in https://bugs.python.org/msg401146 we need to improve how `SharedMemory` is documented and tested. Right now docs do not cover `pickle` at all (https://github.com/python/cpython/blob/main/Doc/library/multiprocessing.shared_memory.rst). And this is an important aspect in multiprocessing.

`SharedMemory` and `pickle` in `test_shared_memory_basics` (https://github.com/python/cpython/blob/a5c6bcf24479934fe9c5b859dd1cf72685a0003a/Lib/test/_test_multiprocessing.py#L3789-L3794):

```
sms.buf[0:6] = b'pickle'
pickled_sms = pickle.dumps(sms)
sms2 = pickle.loads(pickled_sms)
self.assertEqual(sms.name, sms2.name)
self.assertEqual(bytes(sms.buf[0:6]), bytes(sms2.buf[0:6]), b'pickle')
```

`ShareableList` has better coverage in this regard (https://github.com/python/cpython/blob/a5c6bcf24479934fe9c5b859dd1cf72685a0003a/Lib/test/_test_multiprocessing.py#L4121-L4144):

```
    def test_shared_memory_ShareableList_pickling(self):
        sl = shared_memory.ShareableList(range(10))
        self.addCleanup(sl.shm.unlink)

        serialized_sl = pickle.dumps(sl)
        deserialized_sl = pickle.loads(serialized_sl)
        self.assertTrue(
            isinstance(deserialized_sl, shared_memory.ShareableList)
        )
        self.assertTrue(deserialized_sl[-1], 9)
        self.assertFalse(sl is deserialized_sl)
        deserialized_sl[4] = "changed"
        self.assertEqual(sl[4], "changed")

        # Verify data is not being put into the pickled representation.
        name = 'a' * len(sl.shm.name)
        larger_sl = shared_memory.ShareableList(range(400))
        self.addCleanup(larger_sl.shm.unlink)
        serialized_larger_sl = pickle.dumps(larger_sl)
        self.assertTrue(len(serialized_sl) == len(serialized_larger_sl))
        larger_sl.shm.close()

        deserialized_sl.shm.close()
        sl.shm.close()
```

So, my plan is:
1. Improve testing of `SharedMemory` after pickling/unpickling. I will create a separate test with all the `pickle`-related stuff there
2. Improve docs: user must understand what will have when `SharedMemory`  / `SharableList` is pickled and unpickled. For example, the fact that it will still be shared can be a surprise to many.

I am going to send a PR with both thing somewhere this week.

I will glad to head any feedback before / after :)

----------
assignee: docs at python
components: Documentation, Tests
messages: 401219
nosy: docs at python, sobolevn
priority: normal
severity: normal
status: open
title: Improve tests and docs of how `pickle` works with `SharedMemory` obejcts
type: behavior
versions: Python 3.10, Python 3.11, Python 3.8, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue45125>
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