New submission from Antoine Pitrou:
The following script:
import multiprocessing
import os
def f():
pass
multiprocessing.Lock()
if __name__ == "__main__":
ctx = multiprocessing.get_context('forkserver')
# modname is the script's importable name (not "__main__")
modname = os.path.basename(__file__).split(".")[0]
ctx.set_forkserver_preload([modname])
proc = ctx.Process(target=f)
proc.start()
proc.join()
Fails with the following error:
Traceback (most recent call last):
File
"/home/antoine/miniconda3/envs/dask35/lib/python3.5/multiprocessing/forkserver.py",
line 178, in main
_serve_one(s, listener, alive_r, handler)
File
"/home/antoine/miniconda3/envs/dask35/lib/python3.5/multiprocessing/forkserver.py",
line 212, in _serve_one
code = spawn._main(child_r)
File
"/home/antoine/miniconda3/envs/dask35/lib/python3.5/multiprocessing/spawn.py",
line 115, in _main
prepare(preparation_data)
File
"/home/antoine/miniconda3/envs/dask35/lib/python3.5/multiprocessing/spawn.py",
line 221, in prepare
set_start_method(data['start_method'])
File
"/home/antoine/miniconda3/envs/dask35/lib/python3.5/multiprocessing/context.py",
line 231, in set_start_method
raise RuntimeError('context has already been set')
RuntimeError: context has already been set
This makes set_forkserver_preload() quite fragile if you preload any library
that may create multiprocessing resources (such as locks) at the top level.
----------
components: Library (Lib)
messages: 281565
nosy: davin, pitrou, sbt
priority: normal
severity: normal
stage: needs patch
status: open
title: set_forkserver_preload() can crash the forkserver if preloaded module
instantiate multiprocessing classes
type: behavior
versions: Python 3.5, Python 3.6, Python 3.7
_______________________________________
Python tracker <[email protected]>
<http://bugs.python.org/issue28779>
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