Nested multiprocessing leads to AttributeError: is_fork_ctx with forkserver or spawn methods

[bwoodsend]

Bug report

Checklist

• I am confident this is a bug in CPython, not a bug in a third-party project
• I have searched the CPython issue tracker,
  and am confident this bug has not been reported before

CPython versions tested on:

3.11

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.11.5 (main, Aug 26 2023, 00:26:34) [GCC 12.2.1 20220924]

A clear and concise description of the bug:

Using nested multiprocessing (i.e. spawn a child process inside a child process) is broken as of Python 3.11.5, leading to an attribute error.

Process Process-1:
Traceback (most recent call last):
  File "/usr/local/lib/python3.11/multiprocessing/process.py", line 314, in _bootstrap
    self.run()
  File "/usr/local/lib/python3.11/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/io/pyi_multiprocessing_nested_process.py", line 15, in process_function
    process.start()
  File "/usr/local/lib/python3.11/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
                  ^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/multiprocessing/context.py", line 224, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/multiprocessing/context.py", line 288, in _Popen
    return Popen(process_obj)
           ^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/usr/local/lib/python3.11/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/usr/local/lib/python3.11/multiprocessing/popen_spawn_posix.py", line 47, in _launch
    reduction.dump(process_obj, fp)
  File "/usr/local/lib/python3.11/multiprocessing/reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
  File "/usr/local/lib/python3.11/multiprocessing/synchronize.py", line 106, in __getstate__
    if self.is_fork_ctx:
       ^^^^^^^^^^^^^^^^
AttributeError: 'Lock' object has no attribute 'is_fork_ctx'
Results: [1]

Minimal code example below. Invoke with argument fork, forkserver or spawn. fork will work. forkserver and spawn will both raise the above error. All three variants work with Python 3.11.4.

import sys
import multiprocessing


def nested_process_function(queue):
    print("Running nested sub-process!")
    queue.put(2)


def process_function(queue):
    print("Running sub-process!")
    queue.put(1)

    process = multiprocessing.Process(target=nested_process_function, args=(queue,))
    process.start()
    process.join()


def main(start_method):
    multiprocessing.set_start_method(start_method)
    queue = multiprocessing.Queue()

    process = multiprocessing.Process(target=process_function, args=(queue,))
    process.start()
    process.join()

    results = []
    while not queue.empty():
        results.append(queue.get())

    print(f"Results: {results}")
    assert results == [1, 2]


if __name__ == '__main__':
    if len(sys.argv) != 2:
        raise SystemExit(f"Usage: {sys.argv[0]} fork|forkserver|spawn")

    main(sys.argv[1])

I believe that the source of this regression is 34ef75d which adds the attribute is_fork_ctx to multiprocessing.Lock() but doesn't update the pickle methods (__getstate__() and __setstate__()) so after being serialised and deserialised, the Lock() object looses that attribute.

The following patch, adding is_fork_ctx to the pickle methods, makes the above work again.

diff --git a/Lib/multiprocessing/synchronize.py b/Lib/multiprocessing/synchronize.py
index 2328d33212..9c5c2aada6 100644
--- a/Lib/multiprocessing/synchronize.py
+++ b/Lib/multiprocessing/synchronize.py
@@ -109,10 +109,11 @@ def __getstate__(self):
                                    'not supported. Please use the same context to create '
                                    'multiprocessing objects and Process.')
             h = sl.handle
-        return (h, sl.kind, sl.maxvalue, sl.name)
+        return (h, sl.kind, sl.maxvalue, sl.name, self.is_fork_ctx)
 
     def __setstate__(self, state):
-        self._semlock = _multiprocessing.SemLock._rebuild(*state)
+        self._semlock = _multiprocessing.SemLock._rebuild(*state[:4])
+        self.is_fork_ctx = state[4]
         util.debug('recreated blocker with handle %r' % state[0])
         self._make_methods()

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Linked PRs

• gh-108520: Fix bad fork detection in nested multiprocessing use case #108568
• [3.12] gh-108520: Fix bad fork detection in nested multiprocessing use case (GH-108568) #108691
• [3.11] gh-108520: Fix bad fork detection in nested multiprocessing use case (GH-108568) #108692

[terryjreedy]

@pitrou

[albanD]

Sorry about that, proposed a fix at #108568

[carbocation]

Just to add confirmation to what has already been documented here and to document for others using the gcloud cli and finding this in search, I'm on python3.11.5 as well (Python 3.11.5 (main, Aug 24 2023, 15:09:45) [Clang 14.0.3 (clang-1403.0.22.14.1)]) and am encountering this error with the gcloud cli tools.

A search for the following error message brought me here:

$ gcloud storage cp gs://bucket/file ./ --verbosity debug
DEBUG: Running [gcloud.storage.cp] with arguments: [--verbosity: "debug", DESTINATION: "./", SOURCE:1: "['gs://bucket/file']"]
DEBUG: Starting new HTTPS connection (1): storage.googleapis.com:443
DEBUG: https://storage.googleapis.com:443 "GET /storage/v1/b/bucket/o/data%2Ffile?alt=json&projection=noAcl HTTP/1.1" 200 896
Copying gs://bucket/file to file://./file
INFO: Using high CPU count, single-file workload config.
⠛ERROR: gcloud crashed (AttributeError): 'Lock' object has no attribute 'is_fork_ctx'                                                                                                                          

This happens regardless of which gcloud version I upgrade or downgrade to:

$ gcloud version
Google Cloud SDK 444.0.0
beta 2023.08.22
bq 2.0.97
core 2023.08.22
gcloud-crc32c 1.0.0
gsutil 5.25

And I can successfully execute the command if I tell gcloud to use an older python version before 3.11.5, e.g.:

$ CLOUDSDK_PYTHON=python3.10 gcloud storage cp gs://bucket/file ./
# (executes just fine)

[bwoodsend]

The fix appears to be merged and backported. Shouldn't this be closed?

[pitrou]

Hmm, interesting. I thought that the auto-merge feature would also auto-close the issue, but it didn't.

[samos123]

It doesn't seem like a fix has been released for Mac yet. I'm still hitting this issue after running brew upgrade since it's still using python 3.11.5. I did confirm the workaround provided by @carbocation works correctly.

[bwoodsend]

Correct, the fix version will be 3.11.6 which doesn't exist yet.

[carbocation]

looks like python3.11.6 was released today. I haven't tested the fix since I'm waiting for brew to support it (to keep my installations manageable).

[nfonrose]

I can confirm that the problem no longer occurs with python 3.11.6 (tested with Google Cloud SDK 446.0.1 on MacOS 13.4). I had the issue with python 3.11.5