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multiprocessing SharedMemory.__del__ can fail, leak file descriptor #155003
Description
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
4096I 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
Activity
takluyver commented on Jul 31, 2026
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:
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.