select which was interrupted by EINTR isn't re-run if the timeout has passed

[BrianMaissy]

BPO	35310
Nosy	@vstinner, @miss-islington, @oranav
PRs	bpo-35310: Retry select() calls on EINTR even if deadline has passed #10700 bpo-35310: Clear select() lists before returning upon EINTR #10877 [3.6] bpo-35310: Clear select() lists before returning upon EINTR (GH-10877) #10948 [3.7] bpo-35310: Clear select() lists before returning upon EINTR (GH-10877) #10949
Files	select_eintr.py

^{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 2018-12-05.23:17:30.083>
created_at = <Date 2018-11-25.13:19:06.435>
labels = ['3.8', 'type-bug', '3.7']
title = "select which was interrupted by EINTR isn't re-run if the timeout has passed"
updated_at = <Date 2018-12-05.23:17:30.082>
user = 'https://bugs.python.org/BrianMaissy'

bugs.python.org fields:

activity = <Date 2018-12-05.23:17:30.082>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2018-12-05.23:17:30.083>
closer = 'vstinner'
components = []
creation = <Date 2018-11-25.13:19:06.435>
creator = 'Brian Maissy'
dependencies = []
files = ['47945']
hgrepos = []
issue_num = 35310
keywords = ['patch']
message_count = 10.0
messages = ['330388', '330389', '330464', '330493', '330504', '331158', '331161', '331172', '331173', '331191']
nosy_count = 4.0
nosy_names = ['vstinner', 'miss-islington', 'Brian Maissy', 'oranav']
pr_nums = ['10700', '10877', '10948', '10949']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue35310'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[BrianMaissy]

If a call to select.select() was interrupted by a signal and the select syscall set an errno of EINTR, then under PEP-475 the select call should be re-executed.

If, however, there is a signal handler which takes enough time that the select's timeout expires, select() is not retried, and the rlist is returned as-is (with fds in it which are not ready for reading).

Under this condition, either select() should be re-run with a timeout of 0, or the fd lists should be emptied before returning.

Example code which reproduces the problem attached.

[BrianMaissy]

I believe the offending line in the source is this break statement:
https://github.com/python/cpython/blob/master/Modules/selectmodule.c#L339

[vstinner]

Under this condition, either select() should be re-run with a timeout of 0, or the fd lists should be emptied before returning.

What is the current behavior?

Are you trying to fix a bug?

[BrianMaissy]

The current behavior is:

select() is not retried, and the rlist is returned as-is (with fds in it which are not ready for reading)

Yes, this is a bug. It results in select() indicating that fd are ready for reading when they, in fact, are not.

[vstinner]

Ah! I missed attached select_eintr.py script. Ok, I reproduced the bug and I agree that it must be fixed.

[vstinner]

New changeset 7f52415 by Victor Stinner (Oran Avraham) in branch 'master':
bpo-35310: Clear select() lists before returning upon EINTR (GH-10877)
7f52415

[vstinner]

Python 2.7 is not affected: select() isn't retried if it fails with EINTR.

[miss-islington]

New changeset b2e0649 by Miss Islington (bot) in branch '3.6':
bpo-35310: Clear select() lists before returning upon EINTR (GH-10877)
b2e0649

[miss-islington]

New changeset 3451078 by Miss Islington (bot) in branch '3.7':
bpo-35310: Clear select() lists before returning upon EINTR (GH-10877)
3451078

[vstinner]

Ok, the bug should now be fixed in 3.6, 3.7 and master branches.

Thanks Brian Maissy for the bug report and thanks Oran Avraham for the fix!

I hope that we will understand why I didn't want to add a new functional test ;-)