Multiprocessing.Queue._feed deadlocks on import

[ffinkernagel]

BPO	22853
Nosy	@serhiy-storchaka, @applio
Files	show_queue_import_bug.py issue_22853_fix_and_test_import_lock_in_queue_py27.patch: Florian's fix and a matching test for 2.7 issue_22853_only_test_import_lock_in_queue_py34_and_py35.patch: Ignore -- replaced by newer patch file. issue_22853_revised_only_test_import_lock_in_queue_py34_and_py35.patch: Regression test-only for 3.4 and 3.5 (both), replaces prior patch for 3.[4-5].

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/serhiy-storchaka'
closed_at = <Date 2015-03-06.21:36:21.616>
created_at = <Date 2014-11-12.12:29:31.069>
labels = ['type-bug', 'library']
title = 'Multiprocessing.Queue._feed deadlocks on import'
updated_at = <Date 2015-03-06.21:36:21.615>
user = 'https://bugs.python.org/ffinkernagel'

bugs.python.org fields:

activity = <Date 2015-03-06.21:36:21.615>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2015-03-06.21:36:21.616>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2014-11-12.12:29:31.069>
creator = 'ffinkernagel'
dependencies = []
files = ['37185', '38272', '38273', '38361']
hgrepos = []
issue_num = 22853
keywords = ['patch']
message_count = 12.0
messages = ['231073', '236790', '236854', '236865', '237375', '237376', '237377', '237378', '237379', '237382', '237384', '237385']
nosy_count = 6.0
nosy_names = ['BreamoreBoy', 'python-dev', 'sbt', 'serhiy.storchaka', 'ffinkernagel', 'davin']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'commit review'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue22853'
versions = ['Python 2.7']

[ffinkernagel]

If you import a module that creates a multiprocessing.Queue, puts a value, and then waits for to be received again from the queue, you run into a deadlock.

The issue is that Queue._feed does 'from .util import is_existing' - which needs the import lock, but is still being held by the main thread.

Attached a script that illustrates this.

Patch is a two line change, import is_exiting in line 49, remove the import inside the thread:

49c49
< from multiprocessing.util import debug, info, Finalize, register_after_fork
---

from multiprocessing.util import debug, info, Finalize, register_after_fork, is_exiting
232d231
< from .util import is_exiting

[BreamoreBoy]

@davin I believe that you're interested in multiprocessing issues.

[applio]

Confirmed that the issue can be reproduced under 2.7.9 on OS X 10.10.

It is not possible to reproduce the issue with default (3.5) -- taking a look at what's different there, notably the import of is_exiting has moved to the top of the queues module and is no longer only imported at the time Queue._feed is invoked, which is just as Florian advocates doing for 2.7.

I should take a closer look to understand what else has changed.

@mark: Cool -- thanks.

[applio]

Attaching a patch for 2.7 that applies Florian's fix and provides a test for it as well.

Although the issue is not triggered on 3.4 or default (3.5), there is the potential for regression there -- attaching a single patch that works for both 3.4 and 3.5 to provide a regression test (only a test, nothing to "fix").

These patches have been tested on OS X 10.10 and Ubuntu 12.04.5 64-bit for each of 2.7, 3.4, and default (3.5).

[serhiy-storchaka]

I'm not sure there is a need to fix this issue. Using multiprocessing Queue at import time looks as yet one way to shoot yourself in the foot. But the patch looks harmful, I'll commit it.

[BreamoreBoy]

That's all we need, harmful patches being committed :) Harmless possibly?

[serhiy-storchaka]

Oh, right. Harmless. Thanks for all the fish Mark. I meant the bug.

[python-dev]

New changeset 069c13ca7a70 by Serhiy Storchaka in branch '2.7':
Issue bpo-22853: Fixed a deadlock when use multiprocessing.Queue at import time.
https://hg.python.org/cpython/rev/069c13ca7a70

[serhiy-storchaka]

As for 3.x, underscored test does not run, and when remove the underscore it runs, but produce a warning (you should run regrtests with -vv to see detailed warnings):

$ ./python -m test.regrtest -vv -m '*no_import_lock_contention*' test_multiprocessing_spawn
...
Warning -- threading._dangling was modified by test_multiprocessing_spawn
  Before: <_weakrefset.WeakSet object at 0xb6a960ac>
  After:  <_weakrefset.WeakSet object at 0xb6c4cc0c> 
1 test altered the execution environment:
    test_multiprocessing_spawn

[applio]

Corrected patch for 3.4 and default/3.5 -- newly introduced test is now turned on this time and the dangling weak references are properly addressed as well as the reference to Empty. Nastiness.

Good save, Serhiy.

[python-dev]

New changeset cf12856bde17 by Serhiy Storchaka in branch '3.4':
Issue bpo-22853: Added regression test for using multiprocessing.Queue at import
https://hg.python.org/cpython/rev/cf12856bde17

New changeset dcd6d41f2c9a by Serhiy Storchaka in branch 'default':
Issue bpo-22853: Added regression test for using multiprocessing.Queue at import
https://hg.python.org/cpython/rev/dcd6d41f2c9a

[serhiy-storchaka]

Thank you Florian and Davin for your contribution.