multiprocessing SharedMemory.__del__ can fail, leak file descriptor

[takluyver]

Bug report

Bug description:

When a SharedMemory object goes out of scope, its __del__ method calls self.close() to clean it up. This can break if there is still a buffer exported from its mmap instance. It tries to close the mmap before the fd, so in this case it will leave the file descriptor open:

>>> from multiprocessing.shared_memory import SharedMemory
>>> import os
>>> def shview(size=4096):
...     shmem = SharedMemory(create=True, size=size)
...     print("fd is:", shmem._fd)
...     return shmem.buf.cast('i')
...     
>>> mv = shview()
fd is: 5
Exception ignored while calling deallocator <function SharedMemory.__del__ at 0x7f686264d640>:
Traceback (most recent call last):
  File "/usr/lib64/python3.14/multiprocessing/shared_memory.py", line 189, in __del__
    self.close()
  File "/usr/lib64/python3.14/multiprocessing/shared_memory.py", line 232, in close
    self._mmap.close()
BufferError: cannot close exported pointers exist
>>> os.fstat(5).st_size  # fd still open
4096

I think a better option for __del__ would be to close the fd and leave the mmap & buf objects alone, to be cleaned up as normal by reference counting or GC:

>>> class SharedMemoryFix(SharedMemory):
...     def __del__(self):
...         if self._fd >= 0:
...             os.close(self._fd)
...             self._fd = -1
...             
>>> def shview(size=4096):
...     shmem = SharedMemoryFix(create=True, size=size)
...     print("fd is:", shmem._fd)
...     return shmem.buf.cast('i')
...     
>>> mv = shview()
fd is: 7
>>> os.fstat(7)
Traceback (most recent call last):
  File "<python-input-8>", line 1, in <module>
    os.fstat(7)
    ~~~~~~~~^^^
OSError: [Errno 9] Bad file descriptor

(I'm thinking about the POSIX side here. IDK if the behaviour on Windows needs to be different, though I guess it can rely on the mmap's dealloc too)

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-155003: Don't keep file descriptor in multiprocessing SharedMemory object #155007

[takluyver]

Actually, I think we could simplify it further, and not keep the file descriptor around at all. The man page for the mmap() C function says the mapping doesn't depend on the fd:

After the mmap() call has returned, the file descriptor, fd, can be closed immediately without invalidating the mapping.

The Python implementation also duplicates the fd by default and manages the duplicate as part of the mmap object, to allow resizing, though I think SharedMemory's use would work even without this.