cPickle may raise AttributeError when loading concurrently in threads

[sangongs]

BPO	34110
Nosy	@pitrou, @csabella, @pablogsal, @sangongs
PRs	[2.7] bpo-34110: Fix module load problem of cPickle in case of multithreading #8276

^{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 2020-01-10.12:37:17.418>
created_at = <Date 2018-07-13.18:40:08.000>
labels = ['library', 'type-crash']
title = 'cPickle may raise AttributeError when loading concurrently in threads'
updated_at = <Date 2020-01-10.12:37:17.409>
user = 'https://github.com/sangongs'

bugs.python.org fields:

activity = <Date 2020-01-10.12:37:17.409>
actor = 'cheryl.sabella'
assignee = 'none'
closed = True
closed_date = <Date 2020-01-10.12:37:17.418>
closer = 'cheryl.sabella'
components = ['Library (Lib)']
creation = <Date 2018-07-13.18:40:08.000>
creator = 'sangongs'
dependencies = []
files = []
hgrepos = []
issue_num = 34110
keywords = ['patch']
message_count = 4.0
messages = ['321627', '321639', '339750', '359723']
nosy_count = 4.0
nosy_names = ['pitrou', 'cheryl.sabella', 'pablogsal', 'sangongs']
pr_nums = ['8276']
priority = 'normal'
resolution = 'wont fix'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue34110'
versions = ['Python 2.7']

[sangongs]

If two threads use cPickle to load objects simultaneously, one thread may raise an AttributeError. This problem is caused by the partially loaded module.

To reproduce, create a file 'foo.py':
import time
time.sleep(0.1)
class foo():
pass

Then in main.py:
import threading
import cPickle

threads = [threading.Thread(target=cPickle.loads, args=('cfoo\nfoo\np0\n.',)) for _ in range(2)]
[thread.start() for thread in threads]
[thread.join() for thread in threads]

Run main.py, there should be a crash:
Exception in thread Thread-2:
Traceback (most recent call last):
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/threading.py", line 810, in __bootstrap_inner
    self.run()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/threading.py", line 763, in run
    self.__target(*self.__args, **self.__kwargs)
AttributeError: 'module' object has no attribute 'foo'

[pablogsal]

This behaviour also happens on the current master.

[csabella]

Adding @pitrou to the nosy list as he last modified the section of code changed in the PR.

[csabella]

Fixed in Python 3 with bpo-34572.