[issue42340] KeyboardInterrupt should come with a warning

[Jelle Zijlstra]

Jelle Zijlstra  added the comment:

Thanks for the patch!

--
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue42340] KeyboardInterrupt should come with a warning

[miss-islington]

miss-islington  added the comment:


New changeset c26af2bc531eb114c378cdad81935f6e838a7ee0 by Miss Islington (bot) 
in branch '3.9':
bpo-42340: Document issues around KeyboardInterrupt (GH-23255)
https://github.com/python/cpython/commit/c26af2bc531eb114c378cdad81935f6e838a7ee0


--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue42340] KeyboardInterrupt should come with a warning

[miss-islington]

miss-islington  added the comment:


New changeset 66cde7c51a871a86cf8667adf4bd1d03e9b98eb1 by Miss Islington (bot) 
in branch '3.10':
bpo-42340: Document issues around KeyboardInterrupt (GH-23255)
https://github.com/python/cpython/commit/66cde7c51a871a86cf8667adf4bd1d03e9b98eb1


--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue42340] KeyboardInterrupt should come with a warning

[miss-islington]

Change by miss-islington :


--
pull_requests: +30262
pull_request: https://github.com/python/cpython/pull/32185

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue42340] KeyboardInterrupt should come with a warning

[Jelle Zijlstra]

Jelle Zijlstra  added the comment:


New changeset d0906c90fcfbc4cfb9bb963eaa6bb152dd543b56 by benfogle in branch 
'main':
bpo-42340: Document issues around KeyboardInterrupt (GH-23255)
https://github.com/python/cpython/commit/d0906c90fcfbc4cfb9bb963eaa6bb152dd543b56


--
nosy: +JelleZijlstra

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue42340] KeyboardInterrupt should come with a warning

[miss-islington]

Change by miss-islington :


--
nosy: +miss-islington
nosy_count: 2.0 -> 3.0
pull_requests: +30261
pull_request: https://github.com/python/cpython/pull/32184

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue42340] KeyboardInterrupt should come with a warning

[Benjamin Fogle]

Change by Benjamin Fogle :


--
keywords: +patch
pull_requests: +22151
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/23255

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue42340] KeyboardInterrupt should come with a warning

[Benjamin Fogle]

Change by Benjamin Fogle :


Added file: https://bugs.python.org/file49595/sigint_condition_1.py

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue42340] KeyboardInterrupt should come with a warning

[Benjamin Fogle]

Change by Benjamin Fogle :


Added file: https://bugs.python.org/file49597/sigint_tempfile.py

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue42340] KeyboardInterrupt should come with a warning

[Benjamin Fogle]

Change by Benjamin Fogle :


Added file: https://bugs.python.org/file49598/sigint_zipfile.py

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue42340] KeyboardInterrupt should come with a warning

[Benjamin Fogle]

New submission from Benjamin Fogle :

This is related to bpo-29988, and I'm happy to move this to there. I made this 
a separate issue because this is a workaround, not a fix as was being discussed 
there. Also unlike bpo-29988, this is not restricted to context managers or 
finally blocks.

TL;DR: Raising exceptions from interrupt handlers (most notably 
KeyboardInterrupt) can wreak havoc in ways that are impossible to fix. This 
should be noted in the documentation, with a workaround.

I've attached a few example scripts that cause various strange behavior on 
Linux when a KeyboardInterrupt is raised at just the right time. There are 
likely many, many more possible examples:
  - sigint_condition_1.py: Cause a deadlock with threading.Condition
  - sigint_condition_2.py: Cause a double-release and/or notify on unacquired 
threading.Condition
  - sigint_tempfile.py: Cause NamedTemporaryFiles to not be deleted
  - sigint_zipfile.py: Cause ZipExtFile to corrupt its state

When a user presses Ctrl+C, a KeyboardInterrupt will be raised on the main 
thread at some later time. This exception may be raised after any bytecode, and 
most Python code, including the standard library, is not designed to handle 
exceptions that spring up from nowhere.

As a simple example, consider threading.Condition:

def __enter__(self):
return self._lock.__enter__()

The KeyboardInterrupt could be raised just prior to return. In this case, 
__exit__ will never be called, and the underlying lock will remain acquired. A 
similar problem occurs if KeyboardInterrupt occurs at the start of __exit__.

This can be mitigated by attempting to catch a KeyboardInterrupt *absolutely 
everywhere*, but even then, it can't be fixed completely.

def __enter__(self):
try:
# it could happen here, in which case we should not unlock
ret = self._lock.__enter__()
# it could happen here, in which case we must unlock
except KeyboardInterrupt:
# it could, in theory, happen again right here
...
raise
return ret
# it could happen here, which is the same problem we had before

This is not restricted to context handlers or try/finally blocks. The zipfile 
module is a good example of code that is almost certain to enter an 
inconsistent state if a KeyboardInterrupt is raised while it's doing work:

class ZipExtFile:
...
def read1(self, n):
...
self._readbuffer = b''
# what happens if KeyboardInterrupt happens here?
self._offset = 0
...

Due to how widespread this is, it's not worth "fixing". (And honestly, it seems 
to be a rare problem in practice.) I believe that it would be better to clearly 
document that KeyboardInterrupt (or any exception propagated from a signal 
handler) may leave the system
in an inconsistent state. Complex or high reliability applications should avoid 
catching KeyboardInterrupt as a way of gracefully shutting down, and should 
prefer registering their own SIGINT handler. They should also avoid raising 
exceptions from signal handlers at all.

--
assignee: docs@python
components: Documentation
messages: 380868
nosy: benfogle, docs@python
priority: normal
severity: normal
status: open
title: KeyboardInterrupt should come with a warning
type: enhancement
versions: Python 3.10

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue42340] KeyboardInterrupt should come with a warning

[Benjamin Fogle]

Change by Benjamin Fogle :


Added file: https://bugs.python.org/file49596/sigint_condition_2.py

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com