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 6bc2c1e 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 b86636b by Andrew Kuchling in branch '3.8':
[3.8] bpo-25872: Fix KeyError in linecache when multithreaded (GH-18007) (GH-20092)
b86636b

New changeset 852e8a7 by Miss Islington (bot) in branch '3.7':
[3.8] bpo-25872: Fix KeyError in linecache when multithreaded (GH-18007) (GH-20092)
852e8a7

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

I apologize if this is rude, as I am not familiar with this method.
I created a following PR to add unit tests about this issue.

#25913

I would be happy to receive feedback on the PR.

New changeset 373741a by Irit Katriel in branch '3.10':
[3.10] bpo-25872: Add unit tests for linecache and threading (GH-25913) (GH-26212)
373741a

New changeset c05d8a6 by Irit Katriel in branch '3.9':
bpo-25872: Add unit tests for linecache and threading (GH-25913) (GH-26211)
c05d8a6

@uniocto - thank you for the tests.