select.epoll.unregister(fd) should not ignore EBADF

[vstinner]

BPO	39239
Nosy	@vstinner, @tiran
PRs	bpo-39239: epoll.unregister() no longer ignores EBADF #17882

^{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 2020-01-07.20:41:57.629>
created_at = <Date 2020-01-06.23:34:48.175>
labels = ['library', '3.9']
title = 'select.epoll.unregister(fd) should not ignore EBADF'
updated_at = <Date 2020-01-07.20:41:57.629>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2020-01-07.20:41:57.629>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2020-01-07.20:41:57.629>
closer = 'vstinner'
components = ['Library (Lib)']
creation = <Date 2020-01-06.23:34:48.175>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 39239
keywords = ['patch']
message_count = 2.0
messages = ['359481', '359511']
nosy_count = 2.0
nosy_names = ['vstinner', 'christian.heimes']
pr_nums = ['17882']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue39239'
versions = ['Python 3.9']

[vstinner]

The select.epoll.unregister(fd) method currently ignores EBADF error if the file descriptor fd is invalid.

I'm surprised by this undocumented behavior:
https://docs.python.org/dev/library/select.html#select.epoll.unregister

This behavior may lead to bugs if the file descriptor number has been recycled in the meanwhile. I'm not sure that it's a good idea to silently ignore the error.

See bpo-18748 for a similar issue: "io.IOBase destructor silence I/O error on close() by default".

Note: The method also ignores EBADF error if the epoll file descriptor has been closed.

The behavior is as old as the implementation of select.epoll, bpo-1657:

commit 0e9ab5f
Author: Christian Heimes <christian@cheimes.de>
Date: Fri Mar 21 23:49:44 2008 +0000

Applied patch bpo-1657 epoll and kqueue wrappers for the select module
The patch adds wrappers for the Linux epoll syscalls and the BSD kqueue syscalls. Thanks to Thomas Herve and the Twisted people for their support a

nd help.
TODO: Finish documentation documentation

Thomas Herve wrote a first implementation in bpo-1675118, but it seems like it was Christian Heimes who wrote the unregister() method.

[vstinner]

New changeset 5b23f76 by Victor Stinner in branch 'master':
bpo-39239: epoll.unregister() no longer ignores EBADF (GH-17882)
5b23f76