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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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bpo-42178: Fix issue causing cmd to hang 
Try writing subprocess data before creating subprocess
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@John-Ted
John-Ted committed Nov 14, 2020
commit c9a059cac00a8e1393fa6118df5947a70f327d0b
14 changes: 14 additions & 0 deletions Lib/multiprocessing/popen_spawn_win32.py
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Original file line number Diff line number Diff line change
@@ -1,4 +1,5 @@
import os
import io
import msvcrt
import signal
import sys
Expand Down Expand Up @@ -44,6 +45,19 @@ class Popen(object):
def __init__(self, process_obj):
prep_data = spawn.get_preparation_data(process_obj._name)

# bpo-42178: Check for pickle error before creating subprocess
self.sentinel = None
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)
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.

# read end of pipe will be duplicated by the child process
# -- see spawn_main() in spawn.py.
#
Expand Down
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Windows/2020-11-14-23-45-11.bpo-42178.w04kL7.rst
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@@ -0,0 +1 @@
Fix issue causing cmd to hang in Windows