sock_accept() does not remove server socket reader on cancellation

[agronholm]

BPO	41317
Nosy	@asvetlov, @agronholm, @1st1, @miss-islington
PRs	bpo-41317: Remove reader on cancellation in asyncio.loop.sock_accept() #21595 [3.9] bpo-41317: Remove reader on cancellation in asyncio.loop.sock_accept() (GH-21595) #21603
Files	bug.py: Script to reproduce the error

^{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 = None
created_at = <Date 2020-07-16.22:12:05.498>
labels = ['3.8', 'type-bug', '3.7', '3.9', 'expert-asyncio']
title = 'sock_accept() does not remove server socket reader on cancellation'
updated_at = <Date 2020-07-23.20:02:57.073>
user = 'https://github.com/agronholm'

bugs.python.org fields:

activity = <Date 2020-07-23.20:02:57.073>
actor = 'miss-islington'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['asyncio']
creation = <Date 2020-07-16.22:12:05.498>
creator = 'alex.gronholm'
dependencies = []
files = ['49319']
hgrepos = []
issue_num = 41317
keywords = ['patch']
message_count = 7.0
messages = ['373777', '373840', '374110', '374111', '374112', '374145', '374146']
nosy_count = 4.0
nosy_names = ['asvetlov', 'alex.gronholm', 'yselivanov', 'miss-islington']
pr_nums = ['21595', '21603']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41317'
versions = ['Python 3.5', 'Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9']

[agronholm]

Unlike with all the other sock_* functions, sock_accept() only removes the reader on the server socket when the socket becomes available for reading (ie. when there's an incoming connection). If the operation is cancelled instead, the reader is not removed.

If then the server socket is closed and a new socket is created which has the same file number and it is used for a socket operation, it will cause a FileNotFoundError because the event loop thinks it has this fd registered but the epoll object does not agree since all closed sockets are automatically removed from it.

The attached script reproduces the problem on Fedora Linux 32 (all relevant Python versions), but not on Windows (on any tested Python versions from 3.6 to 3.8).

[agronholm]

This bug is the same as https://bugs.python.org/issue30064 except for sock_accept().

[1st1]

Alex, do you want to submit a PR?

[agronholm]

Sure, it should be a rather straightforward fix. I have to check if the tests for the related methods test for proper cancellation semantics. It should be even faster if they do.

[agronholm]

Looks like they do – fantastic!

[1st1]

New changeset 0dd98c2 by Alex Grönholm in branch 'master':
bpo-41317: Remove reader on cancellation in asyncio.loop.sock_accept() (bpo-21595)
0dd98c2

[miss-islington]

New changeset 4ff8e5b by Miss Islington (bot) in branch '3.9':
bpo-41317: Remove reader on cancellation in asyncio.loop.sock_accept() (GH-21595)
4ff8e5b