signal module always overwrites SIGINT on interpreter shutdown

[yol]

BPO	30654
Nosy	@pitrou, @vstinner, @benjaminp, @ned-deily, @yol
PRs	bpo-30654: Do not reset SIGINT handler to SIG_DFL in finisignal #2162 bpo-30654: Do not reset SIGINT handler to SIG_DFL in finisignal #7146 [3.7] bpo-30654: Do not reset SIGINT handler to SIG_DFL in finisignal (GH-7146) #7306 [3.6] bpo-30654: Do not reset SIGINT handler to SIG_DFL in finisignal (GH-7146) #7307 [2.7] bpo-30654: Do not reset SIGINT handler to SIG_DFL in finisignal (GH-7146) #7347

^{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 2018-06-03.19:11:15.189>
created_at = <Date 2017-06-13.13:06:04.818>
labels = ['interpreter-core', '3.8', 'type-bug', '3.7']
title = 'signal module always overwrites SIGINT on interpreter shutdown'
updated_at = <Date 2020-03-09.23:24:11.602>
user = 'https://github.com/yol'

bugs.python.org fields:

activity = <Date 2020-03-09.23:24:11.602>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2018-06-03.19:11:15.189>
closer = 'pitrou'
components = ['Interpreter Core']
creation = <Date 2017-06-13.13:06:04.818>
creator = 'pkerling'
dependencies = []
files = []
hgrepos = []
issue_num = 30654
keywords = ['patch']
message_count = 21.0
messages = ['295915', '318377', '318378', '318379', '318380', '318381', '318382', '318393', '318394', '318395', '318398', '318402', '318414', '318415', '318484', '318490', '318496', '318513', '318577', '318580', '363788']
nosy_count = 5.0
nosy_names = ['pitrou', 'vstinner', 'benjamin.peterson', 'ned.deily', 'pkerling']
pr_nums = ['2162', '7146', '7306', '7307', '7347']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue30654'
versions = ['Python 2.7', 'Python 3.6', 'Python 3.7', 'Python 3.8']

[yol]

The signal module checks the SIGINT handler on startup. It only registers a new custom handler if the default OS handler is still installed, so that when embedding python in an application the SIGINT handler of that application is not overwritten.

But on shutdown in finisignal, it *always* sets SIGINT to SIG_DFL. The reason is that it saves the old handler in old_siginthandler, but *only* if the signal handler is overwritten in init, which only happens when it was SIG_DFL in the first place! If there was already a handler in place in init (-> no overwriting), old_siginthandler will default to the initialization value, which is also SIG_DFL.

This means that when an application embeds Python and needs a custom SIGINT handler, it will stop to work as soon as it shuts down Python since it will always be reset to SIG_DFL.

[pitrou]

New changeset e905c84 by Antoine Pitrou (pkerling) in branch 'master':
bpo-30654: Do not reset SIGINT handler to SIG_DFL in finisignal (GH-7146)
e905c84

[pitrou]

I'd rather not backport this to 2.7 as it's quite late in the maintenance cycle and I'd like to avoid any regressions there.

[vstinner]

I'd rather not backport this to 2.7 as it's quite late in the maintenance cycle and I'd like to avoid any regressions there.

Since it's a subtle behavior change and the PR doesn't add a flag to opt-in for the old behaviour, I'm not sure about backporting the change (to 3.6 or 3.7).

For 3.7, we are very close to the final release. It doesn't give much time to users to test the new behavior :-( I would suggest to keep the old behavior in Python 3.7 as well.

To be honest, I don't understand well the change. So I'm not confortable with it.

I understand that Py_Initialize() + Py_Finalize() restored the SIGINT handler, but now Python *always* sets SIGINT to SIG_DFL in Py_Finalize(). So if an application has its own signal handler, Python replaces it...

But embedded Python also gives the choice of not setting Python signal handlers in Py_Initialize().

[pitrou]

So if an application has its own signal handler, Python replaces it...

You have it backwards. Please read the bug report.

[pitrou]

For 3.7, we are very close to the final release. It doesn't give much time to users to test the new behavior

That's a fair point. What's the procedure here? If I backport the fix to the 3.7 branch, will it go straight into the 3.7.0 release or will it be deferred to 3.7.1? @ned

[vstinner]

You have it backwards. Please read the bug report.

I'm confused by the NEWS entry:

+Fixed reset of the SIGINT handler to SIG_DFL on interpreter shutdown even
+when there was a custom handler set previously. Patch by Philipp Kerling.

I read it as Python now always reset SIGINT to SIG_DFL. The commit title is more explicit: "Do not reset SIGINT (...)".

I propose to rephrase the NEWS entry as:

"""
Fix signal handlers when Python is embedded. On Python shutdown, do not reset the SIGINT handler to SIG_DFL, when a custom handler has been set before Python initialization. Patch by Philipp Kerling.
"""

--

Ok, now I understood the change. In this case, I'm ok to backport it to 3.6 and 3.7.

Python 2.7 has this behavior since 10 years, it seems like people learnt to live with it. I also dislike touching Python 2.7, to prevent any risk of regression if someone really rely on the current behavior.

[ned-deily]

What's the procedure here? If I backport the fix to the 3.7 branch, will it go straight into the 3.7.0 release or will it be deferred to 3.7.1??

"All fixes that have been merged into the 3.7 branch as of cutoff tomorrow
will be in 3.7.0b5 and fixes merged afterwards will be in 3.7.0rc1 up to
its cutoff point. After 3.7.0rc1 cutoff, 3.7 merges will appear in 3.7.1.
Please continue to exercise diligence when deciding whether a change is
appropriate for 3.7; as a rule of thumb, treat the 3.7 branch as if it were
already released and in maintenance mode."

[pitrou]

Ok, I think it go into 3.7.0 after all. 3.7.1 wouldn't get more testing before it's released anyway.

[pitrou]

New changeset 623b439 by Antoine Pitrou (Miss Islington (bot)) in branch '3.7':
bpo-30654: Do not reset SIGINT handler to SIG_DFL in finisignal (GH-7146) (GH-7306)
623b439

[pitrou]

New changeset 1d5198f by Antoine Pitrou (Miss Islington (bot)) in branch '3.6':
bpo-30654: Do not reset SIGINT handler to SIG_DFL in finisignal (GH-7146) (GH-7307)
1d5198f