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 :)

Go ahead. Fix also other issues in tests:

* All pickling tests should test with all supported protocols, not just with the default one.

* self.assertTrue(
            isinstance(deserialized_sl, shared_memory.ShareableList)
        )
assertIsinstance() can be used here.

* self.assertTrue(deserialized_sl[-1], 9)
What is this? If it tests that deserialized_sl[-1] is true, 9 should be removed. Should it be assertEqual() instead?

* self.assertFalse(sl is deserialized_sl)
assertIs() can be used here.

* self.assertTrue(len(serialized_sl) == len(serialized_larger_sl))
assertEqual() can be used here.

* All close() should either be called in the finally block, or registered with addCleanup(). Otherwise we will heave resource leaks if tests fail.

There may be other issues in other tests. Seems these tests need a clean up.

New changeset 746d648d47d12d16c2afedaeff626fc6aaaf6a46 by Nikita Sobolev in branch 'main':
bpo-45125: Improves pickling docs and tests for `shared_memory` (GH-28294)
https://github.com/python/cpython/commit/746d648d47d12d16c2afedaeff626fc6aaaf6a46

New changeset fc3511f18e92ea345092aa59a4225218bd19e153 by Miss Islington (bot) in branch '3.10':
bpo-45125: Improves pickling docs and tests for `shared_memory` (GH-28294)
https://github.com/python/cpython/commit/fc3511f18e92ea345092aa59a4225218bd19e153

Thanks everyone. It is now fixed