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GeezerVison is showing github.com — gh-77377: Ensure multiprocessing SemLock is valid for spawn-based Process before serializing it by albanD · Pull Request #107275 · python/cpython

gh-77377: Ensure multiprocessing SemLock is valid for spawn-based Process before serializing it #107275

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Merged
pitrou merged 5 commits into python:main from albanD:fix-issue-77377
Aug 23, 2023

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Merged

gh-77377: Ensure multiprocessing SemLock is valid for spawn-based Process before serializing it #107275

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d9aeedf
Ensure multiprocessing lock is valid for spawn Process before seriali…
albanD Jul 25, 2023
df69347
📜🤖 Added by blurb_it.
blurb-it[bot] Jul 25, 2023
152800c
Improve test to run on macos and cover forkserver
albanD Aug 23, 2023
29d37e1
Fix typos and wording
pitrou Aug 23, 2023
e5f4e32
lint
albanD Aug 23, 2023
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9 changes: 7 additions & 2 deletions 9 Lib/multiprocessing/synchronize.py
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Original file line number Diff line number Diff line change
Expand Up @@ -50,8 +50,8 @@ class SemLock(object):
def __init__(self, kind, value, maxvalue, *, ctx):
if ctx is None:
ctx = context._default_context.get_context()
name = ctx.get_start_method()
unlink_now = sys.platform == 'win32' or name == 'fork'
self.is_fork_ctx = ctx.get_start_method() == 'fork'
unlink_now = sys.platform == 'win32' or self.is_fork_ctx
for i in range(100):
try:
sl = self._semlock = _multiprocessing.SemLock(
Expand Down Expand Up @@ -103,6 +103,11 @@ def __getstate__(self):
if sys.platform == 'win32':
h = context.get_spawning_popen().duplicate_for_child(sl.handle)
else:
if self.is_fork_ctx:
raise RuntimeError('A SemLock created in a fork context is being '
'shared with a process in a spawn context. This is '
'not supported. Please use the same context to create '
'multiprocessing objects and Process.')
h = sl.handle
return (h, sl.kind, sl.maxvalue, sl.name)

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22 changes: 22 additions & 0 deletions 22 Lib/test/_test_multiprocessing.py
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Expand Up @@ -5421,6 +5421,28 @@ def test_preload_resources(self):
print(err)
self.fail("failed spawning forkserver or grandchild")

@unittest.skipIf(sys.platform == "win32",
"Only Spawn on windows so no risk of mixing")
@only_run_in_spawn_testsuite("avoids redundant testing.")
def test_mixed_startmethod(self):
# Fork-based locks cannot be used with spawned process
for process_method in ["spawn", "forkserver"]:
queue = multiprocessing.get_context("fork").Queue()
process_ctx = multiprocessing.get_context(process_method)
p = process_ctx.Process(target=close_queue, args=(queue,))
err_msg = "A SemLock created in a fork"
with self.assertRaisesRegex(RuntimeError, err_msg):
p.start()

# non-fork-based locks can be used with all other start methods
for queue_method in ["spawn", "forkserver"]:
for process_method in multiprocessing.get_all_start_methods():
queue = multiprocessing.get_context(queue_method).Queue()
process_ctx = multiprocessing.get_context(process_method)
p = process_ctx.Process(target=close_queue, args=(queue,))
p.start()
p.join()


@unittest.skipIf(sys.platform == "win32",
"test semantics don't make sense on Windows")
Expand Down
1 change: 1 addition & 0 deletions 1 Misc/NEWS.d/next/Core and Builtins/2023-07-25-22-35-35.gh-issue-77377.EHAbXx.rst
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@@ -0,0 +1 @@
Ensure that multiprocessing synchronization objects created in a fork context are not sent to a different process created in a spawn context. This changes a segfault into an actionable RuntimeError in the parent process.
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