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gh-155003: Don't keep file descriptor in multiprocessing SharedMemory object by takluyver · Pull Request #155007 · python/cpython · GitHub

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

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

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c123d1a
Don't keep file descriptor in SharedMemory object
takluyver Jul 31, 2026
fc454e9
Fix cross-reference in news entry
takluyver Jul 31, 2026
08584a7
Optionally use posix_fallocate() if available
takluyver Aug 1, 2026
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29 changes: 13 additions & 16 deletions Lib/multiprocessing/shared_memory.py
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Original file line number Diff line number Diff line change
Expand Up @@ -65,15 +65,14 @@ class SharedMemory:

# Defaults; enables close() and unlink() to run without errors.
_name = None
_fd = -1
_mmap = None
_buf = None
_flags = os.O_RDWR
_mode = 0o600
_prepend_leading_slash = True if _USE_POSIX else False
_track = True

def __init__(self, name=None, create=False, size=0, *, track=True):
def __init__(self, name=None, create=False, size=0, *, track=True, reserve=False):
if not size >= 0:
raise ValueError("'size' must be a positive integer")
if create:
Expand All @@ -92,7 +91,7 @@ def __init__(self, name=None, create=False, size=0, *, track=True):
while True:
name = _make_filename()
try:
self._fd = _posixshmem.shm_open(
fd = _posixshmem.shm_open(
name,
self._flags,
mode=self._mode
Expand All @@ -103,21 +102,28 @@ def __init__(self, name=None, create=False, size=0, *, track=True):
break
else:
name = "/" + name if self._prepend_leading_slash else name
self._fd = _posixshmem.shm_open(
fd = _posixshmem.shm_open(
name,
self._flags,
mode=self._mode
)
self._name = name
try:
if create and size:
os.ftruncate(self._fd, size)
stats = os.fstat(self._fd)
if hasattr(os, 'posix_fallocate') and reserve:
# Ensures the requested size is available
os.posix_fallocate(fd, 0, size)
else:
os.ftruncate(fd, size)
stats = os.fstat(fd)
size = stats.st_size
self._mmap = mmap.mmap(self._fd, size)
self._mmap = mmap.mmap(fd, size)

gpshead Jul 31, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

should we add trackfd=False here? BUT see my other overall comment.
except OSError:
self.unlink()
raise
finally:
os.close(fd)

if self._track:
resource_tracker.register(self._name, "shared_memory")

Expand Down Expand Up @@ -184,12 +190,6 @@ def __init__(self, name=None, create=False, size=0, *, track=True):
self._size = size
self._buf = memoryview(self._mmap)

def __del__(self):
try:
self.close()
except OSError:
pass

def __reduce__(self):
return (
self.__class__,
Expand Down Expand Up @@ -231,9 +231,6 @@ def close(self):
if self._mmap is not None:
self._mmap.close()
self._mmap = None
if _USE_POSIX and self._fd >= 0:
os.close(self._fd)
self._fd = -1

def unlink(self):
"""Requests that the underlying shared memory block be destroyed.
Expand Down
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2026-07-31-20-54-50.gh-issue-155003.rgWjVC.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
:class:`multiprocessing.shared_memory.SharedMemory` no longer holds on to a
file descriptor on Posix platforms, and does not forcibly close its buffer
when finalized.
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