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bpo-42178: Fix issue causing cmd to hang #23290

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bpo-42178: Fix issue causing cmd to hang #23290

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c9a059c
bpo-42178: Fix issue causing cmd to hang
John-Ted Nov 14, 2020
fa00b0c
Fix whitespace issues
John-Ted Nov 14, 2020
29ccd55
Implement reviewer feedback
John-Ted Nov 16, 2020
659f57e
Implement reviewer feedback
John-Ted Nov 16, 2020
5ad8ead
Implement reviewer feedback
John-Ted Nov 16, 2020
1cfdcbe
Fix error handling
John-Ted Nov 16, 2020
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Fix whitespace issues
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@John-Ted
John-Ted committed Nov 14, 2020
commit fa00b0c0a05aca115caaa016d0527261854ecaf6
11 changes: 6 additions & 5 deletions Lib/multiprocessing/popen_spawn_win32.py
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Expand Up @@ -23,6 +23,7 @@
def _path_eq(p1, p2):
return p1 == p2 or os.path.normcase(p1) == os.path.normcase(p2)


WINENV = not _path_eq(sys.executable, sys._base_executable)


Expand Down Expand Up @@ -50,11 +51,11 @@ def __init__(self, process_obj):
set_spawning_popen(self)
try:
with io.BytesIO() as f:
reduction.dump(prep_data, f)
try:
reduction.pickle.dump(process_obj, f)
except:
reduction.dump(process_obj, f)
reduction.dump(prep_data, f)
try:
reduction.pickle.dump(process_obj, f)
except:
reduction.dump(process_obj, f)
finally:
set_spawning_popen(None)

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Pickling Semaphore or SemLock objects entails duplicating an OS handle to the child process via duplicate_for_child. So we need the child process in order to do the reduction only once. What we can do to improve the situation is to spawn the child process with a suspended thread. This will require adding the CREATE_SUSPENDED flag and ResumeThread function to the _winapi module. After creating the child process with a suspended thread, set up the self attributes (e.g. self.sentinel) and dump the reduction to a BytesIO instance. If the latter fails, kill the child process via _winapi.TerminateProcess. Otherwise resume the thread in the child, and write the reduction from the BytesIO instance to the pipe.

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