Modifying a file while getting a stacktrace across multiple threads causes linecache's cache to bust and del to be called on the global cache variable. This is not thread safe and raises a KeyError.

Reproducible with,

import threading
import traceback

def main():
    with open(__file__, 'a') as fp:
        fp.write(' ')
        traceback.format_stack()

threads = [
    threading.Thread(target=main)
    for i in range(100)
]
map(lambda t: t.start(), threads)
map(lambda t: t.join(), threads)

I see the following error,

Exception in thread Thread-56:
Traceback (most recent call last):
  File "/Users/me/.pyenv/versions/2.7.10/lib/python2.7/threading.py", line 810, in __bootstrap_inner
    self.run()
  File "/Users/me/.pyenv/versions/2.7.10/lib/python2.7/threading.py", line 763, in run
    self.__target(*self.__args, **self.__kwargs)
  File "test.py", line 7, in main
    traceback.format_stack()
  File "/Users/me/.pyenv/versions/2.7.10/lib/python2.7/traceback.py", line 279, in format_stack
    return format_list(extract_stack(f, limit))
  File "/Users/me/.pyenv/versions/2.7.10/lib/python2.7/traceback.py", line 305, in extract_stack
    linecache.checkcache(filename)
  File "/Users/me/.pyenv/versions/2.7.10/lib/python2.7/linecache.py", line 69, in checkcache
    del cache[filename]
KeyError: 'test.py'

Possible solution is to ignore KeyError on del cache[filename].

It seems there was a major refactor in traceback module with 6bc2c1e7ebf359224e5e547f58ffc2c42cb36a39 where this was fixed in Python 3. Ignoring the KeyError seems reasonable to me.

Thanks

As @xtreak said, this looks like it was fixed for Python 3 and was only an issue for 2.7, so I'm closing the issue.

This issue still exists in Python 3. The repro just needs to be changed so that the threads are actually started.

- map(lambda t: t.start(), threads)
- map(lambda t: t.join(), threads)
+ [t.start() for t in threads]
+ [t.join() for t in threads]

My fix is linked.

Thanks Michael, reopening. I was wrong while trying the reproducer since map is lazy in Python 3 and threads were not executed.

New changeset b86636bff4b29ce23c886df079715dd951f13a07 by Andrew Kuchling in branch '3.8':
[3.8] bpo-25872: Fix KeyError in linecache when multithreaded (GH-18007) (GH-20092)
https://github.com/python/cpython/commit/b86636bff4b29ce23c886df079715dd951f13a07

New changeset 852e8a7ed4d3d48e5c1c8120cfc932eb6a84bb8e by Miss Islington (bot) in branch '3.7':
[3.8] bpo-25872: Fix KeyError in linecache when multithreaded (GH-18007) (GH-20092)
https://github.com/python/cpython/commit/852e8a7ed4d3d48e5c1c8120cfc932eb6a84bb8e

The issue was fixed but a unit test for this still needs to be added.