[3.7] bpo-33725: multiprocessing uses spawn by default on macOS (GH-13603)

[vstinner]

On macOS, the multiprocessing module now uses the "spawn" start
method by default.

(cherry picked from commit 17a5588)

https://bugs.python.org/issue33725

[vstinner]

Failure on macOS:

1342======================================================================
1343FAIL: test_preload_resources (test.test_multiprocessing_forkserver.TestStartMethod)
1344----------------------------------------------------------------------
1345Traceback (most recent call last):
1346  File "/Users/vsts/agent/2.150.3/work/1/s/Lib/test/_test_multiprocessing.py", line 4469, in test_preload_resources
1347    self.fail("failed spawning forkserver or grandchild")
1348AssertionError: failed spawning forkserver or grandchild
1349
1350Stdout:
1351
1352Traceback (most recent call last):
1353  File "/Users/vsts/agent/2.150.3/work/1/s/Lib/multiprocessing/forkserver.py", line 261, in main
1354    old_handlers)
1355  File "/Users/vsts/agent/2.150.3/work/1/s/Lib/multiprocessing/forkserver.py", line 297, in _serve_one
1356    code = spawn._main(child_r)
1357  File "/Users/vsts/agent/2.150.3/work/1/s/Lib/multiprocessing/spawn.py", line 114, in _main
1358    prepare(preparation_data)
1359  File "/Users/vsts/agent/2.150.3/work/1/s/Lib/multiprocessing/spawn.py", line 225, in prepare
1360    _fixup_main_from_path(data['init_main_from_path'])
1361  File "/Users/vsts/agent/2.150.3/work/1/s/Lib/multiprocessing/spawn.py", line 277, in _fixup_main_from_path
1362    run_name="__mp_main__")
1363  File "/Users/vsts/agent/2.150.3/work/1/s/Lib/runpy.py", line 263, in run_path
1364    pkg_name=pkg_name, script_name=fname)
1365  File "/Users/vsts/agent/2.150.3/work/1/s/Lib/runpy.py", line 96, in _run_module_code
1366    mod_name, mod_spec, pkg_name, script_name)
1367  File "/Users/vsts/agent/2.150.3/work/1/s/Lib/runpy.py", line 85, in _run_code
1368    exec(code, run_globals)
1369  File "/Users/vsts/agent/2.150.3/work/1/s/Lib/test/mp_preload.py", line 3, in <module>
1370    multiprocessing.Lock()
1371  File "/Users/vsts/agent/2.150.3/work/1/s/Lib/multiprocessing/context.py", line 67, in Lock
1372    return Lock(ctx=self.get_context())
1373  File "/Users/vsts/agent/2.150.3/work/1/s/Lib/multiprocessing/synchronize.py", line 162, in __init__
1374    SemLock.__init__(self, SEMAPHORE, 1, 1, ctx=ctx)
1375  File "/Users/vsts/agent/2.150.3/work/1/s/Lib/multiprocessing/synchronize.py", line 80, in __init__
1376    register(self._semlock.name)
1377  File "/Users/vsts/agent/2.150.3/work/1/s/Lib/multiprocessing/semaphore_tracker.py", line 83, in register
1378    self._send('REGISTER', name)
1379  File "/Users/vsts/agent/2.150.3/work/1/s/Lib/multiprocessing/semaphore_tracker.py", line 90, in _send
1380    self.ensure_running()
1381  File "/Users/vsts/agent/2.150.3/work/1/s/Lib/multiprocessing/semaphore_tracker.py", line 46, in ensure_running
1382    pid, status = os.waitpid(self._pid, os.WNOHANG)
1383ChildProcessError: [Errno 10] No child processes

[warsaw]

Same error for me when testing locally.

[ned-deily]

See discussion on the bug tracker. This behavior change should not be backported.

[vstinner]

See discussion on the bug tracker. This behavior change should not be backported.

Ok.