On running this: shm = SharedMemory(create=True, size=0)
I get the following error:



Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.8/multiprocessing/shared_memory.py", line 111, in __init__
    self._mmap = mmap.mmap(self._fd, size)
ValueError: cannot mmap an empty file
Error in sys.excepthook:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 63, in apport_excepthook
    from apport.fileutils import likely_packaged, get_recent_crashes
  File "/usr/lib/python3/dist-packages/apport/__init__.py", line 5, in <module>
    from apport.report import Report
  File "/usr/lib/python3/dist-packages/apport/report.py", line 30, in <module>
    import apport.fileutils
  File "/usr/lib/python3/dist-packages/apport/fileutils.py", line 23, in <module>
    from apport.packaging_impl import impl as packaging
  File "/usr/lib/python3/dist-packages/apport/packaging_impl.py", line 24, in <module>
    import apt
  File "/usr/lib/python3/dist-packages/apt/__init__.py", line 23, in <module>
    import apt_pkg
ModuleNotFoundError: No module named 'apt_pkg'

Original exception was:
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.8/multiprocessing/shared_memory.py", line 111, in __init__
    self._mmap = mmap.mmap(self._fd, size)
ValueError: cannot mmap an empty file
>>> shm = SharedMemory(create=True, size=0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.8/multiprocessing/shared_memory.py", line 111, in __init__
    self._mmap = mmap.mmap(self._fd, size)
ValueError: cannot mmap an empty file
Error in sys.excepthook:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 63, in apport_excepthook
    from apport.fileutils import likely_packaged, get_recent_crashes
  File "/usr/lib/python3/dist-packages/apport/__init__.py", line 5, in <module>
    from apport.report import Report
  File "/usr/lib/python3/dist-packages/apport/report.py", line 30, in <module>
    import apport.fileutils
  File "/usr/lib/python3/dist-packages/apport/fileutils.py", line 23, in <module>
    from apport.packaging_impl import impl as packaging
  File "/usr/lib/python3/dist-packages/apport/packaging_impl.py", line 24, in <module>
    import apt
  File "/usr/lib/python3/dist-packages/apt/__init__.py", line 23, in <module>
    import apt_pkg
ModuleNotFoundError: No module named 'apt_pkg'

Original exception was:
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.8/multiprocessing/shared_memory.py", line 111, in __init__
    self._mmap = mmap.mmap(self._fd, size)
ValueError: cannot mmap an empty file


This can be simply resolved by adding a check when size passed is 0, so that a shared memory segment is never created.
Currently, the following is coded:

if not size >= 0:
    raise ValueError("'size' must be a positive integer")

I believe this should be changed to:
if not size > 0:
    raise ValueError("'size' must be a positive integer")

As zero is not a positive and integer and is causing problems.

Python 3.10.0a0  Jul 23 2020, win32 (without patch)

>>> from multiprocessing import shared_memory
>>> shm_a = shared_memory.SharedMemory(create=True, size=0)
Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
    shm_a = shared_memory.SharedMemory(create=True, size=0)
  File "f:\dev\3x\lib\multiprocessing\shared_memory.py", line 129, in __init__
    h_map = _winapi.CreateFileMapping(
OSError: [WinError 87] The parameter is incorrect: 'wnsm_4ab39616'

After the patch, I get the value error.

Hi, The patch aims to raise a value error, because before the patch also, ValueError was being raised in case size was negative. That's why I thought it would be correct behaviour if I raise ValueError in case size is 0.

Do you mean to say, that OSError should be raise in the case size=0.
Also, I think handling of Windows was fine even before the patch, it was MacOS and Linux which were causing problems.

Also, Linux created the shared memory with the passed name if size = 0, but threw error when it was being mapped to the process's virtual memory. Therefore, this scenario should be prevented before actually creating the shared memory segment in cases of Linux.

No, I think ValueError on all systems is better.  I have no idea what 'wnsm_4ab39616' is about other than something internal to Windows.  It does not say what to do to correct the call.

New changeset 475a5fbb5644ea200c990d85d8c264e78ab6c7ea by Vinay Sharma in branch 'master':
bpo-41344: Raise ValueError when creating shared memory of size 0 (GH-21556)
https://github.com/python/cpython/commit/475a5fbb5644ea200c990d85d8c264e78ab6c7ea

New changeset ca55ecbf9aab305fa301ec69410ca3d3d18ec848 by Miss Islington (bot) in branch '3.9':
bpo-41344: Raise ValueError when creating shared memory of size 0 (GH-21556) (GH-22018)
https://github.com/python/cpython/commit/ca55ecbf9aab305fa301ec69410ca3d3d18ec848

New changeset 38e32872eb3cf0dc9dd8cef9b05e47ba03638d34 by Miss Islington (bot) in branch '3.8':
bpo-41344: Raise ValueError when creating shared memory of size 0 (GH-21556) (GH-22019)
https://github.com/python/cpython/commit/38e32872eb3cf0dc9dd8cef9b05e47ba03638d34

Thanks for the report and the PR, vinay0410!