test_threaded_import: KeyError ignored in _get_module_lock.<locals>.cb

[vstinner]

BPO	31070
Nosy	@brettcannon, @ncoghlan, @vstinner, @serhiy-storchaka
PRs	[WIP] bpo-31070: Fix importlib _get_module_lock() race #3029 bpo-31070: Fix a race condition in importlib _get_module_lock(). #3033 [3.6] bpo-31070: Fix a race condition in importlib _get_module_lock(). (GH-3033). #3038

^{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 = None
closed_at = <Date 2017-08-09.11:57:47.504>
created_at = <Date 2017-07-28.15:03:41.001>
labels = ['interpreter-core', 'type-bug', '3.7']
title = 'test_threaded_import: KeyError ignored in _get_module_lock.<locals>.cb'
updated_at = <Date 2017-10-28.04:46:37.586>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2017-10-28.04:46:37.586>
actor = 'ncoghlan'
assignee = 'none'
closed = True
closed_date = <Date 2017-08-09.11:57:47.504>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2017-07-28.15:03:41.001>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 31070
keywords = []
message_count = 13.0
messages = ['299393', '299951', '299952', '299953', '299971', '299980', '299989', '299992', '299993', '299999', '300007', '300009', '305147']
nosy_count = 4.0
nosy_names = ['brett.cannon', 'ncoghlan', 'vstinner', 'serhiy.storchaka']
pr_nums = ['3029', '3033', '3038']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue31070'
versions = ['Python 3.6', 'Python 3.7']

[vstinner]

See bpo-bpo-30891 and bpo-30876 for recent changes in importlib.

http://buildbot.python.org/all/builders/AMD64%20Windows8.1%20Refleaks%203.x/builds/57/steps/test/logs/stdio

1:33:30 [312/406/2] test_threaded_import passed (30 sec) -- running: test_decimal (826 sec), test_set (84 sec), test_mmap (1151 sec)
beginning 6 repetitions
123456
...Exception ignored in: <function _get_module_lock.<locals>.cb at 0x000000813CA18EB8>
Traceback (most recent call last):
  File "<frozen importlib._bootstrap>", line 176, in cb
KeyError: ('random',)
...

[vstinner]

Oh, it's easy to reproduce the bug on the master branch. Example on Linux:

haypo@selma$ ./python -m test test_threaded_import
Run tests sequentially
0:00:00 load avg: 0.16 [1/1] test_threaded_import
Exception ignored in: <function _get_module_lock.<locals>.cb at 0x7f53201e2f70>
Traceback (most recent call last):
  File "<frozen importlib._bootstrap>", line 176, in cb
KeyError: ('random',)
1 test OK.

Total duration: 2 sec
Tests result: SUCCESS

[vstinner]

The problem is the weakref callback of _module_locks. It seems like the callback is called after a second thread replaced _module_locks[name]

• Thread 1: _get_module_lock('random') sets _module_locks['random'] and registers a weakref callback on the newly created lock

• Thread 1: the lock is destroyed, the weakref callback will be called as soon as possible

• Thread 2: _get_module_lock('random') sees that the weakref is dead and so replaces _module_locks['random'] with a new lock, and registers a new weakref callback on the second lock

• Thread 1: Call the weakref callback removing _module_locks['random'] -- BUG: the callback is called too late, after Thread 2 already sets a new lock

• Thread 2: The lock is destroyed, the second weakref callback will be called as soon as possible

• Thread 2: The second callback is called and fails with KeyError since _module_locks['random'] was already called in the Thread 1 in the meanwhile

[vstinner]

I wrote #3029 which seems to fix the issue.

I don't know importlib well enough to understand why we need a weak reference to a lock.

My PR adds a second lock per module lock (!) to be able to wait until _module_locks[name] is deleted when a thread detects that _module_locks[name] exists and the associated lock was destroyed.

[serhiy-storchaka]

I think the solution can be simpler. PR 3033 uses the global import lock for guarding modification of the _module_locks dictionary.

[vstinner]

Serhiy: "I think the solution can be simpler. PR 3033 uses the global import lock for guarding modification of the _module_locks dictionary."

I tried exactly that, but it wasn't enough. But your PR is correct because it also checks the current value of _module_locks[name] using the callback parameter.

I disliked my own PR, so I'm happy that someone else wrote a simpler fix! (I already abandonned my PR.)

[serhiy-storchaka]

New changeset 9b0d1d6 by Serhiy Storchaka in branch 'master':
bpo-31070: Fix a race condition in importlib _get_module_lock(). (bpo-3033)
9b0d1d6

[serhiy-storchaka]

New changeset f3b8917 by Serhiy Storchaka in branch '3.6':
[3.6] bpo-31070: Fix a race condition in importlib _get_module_lock(). (GH-3033). (bpo-3038)
f3b8917

[serhiy-storchaka]

Thank you for review Victor and Nick.