asyncio.create_unix_server has an off-by-one error concerning the backlog parameter

[jnsnow]

BPO	46715
Nosy	@asvetlov, @1st1, @jnsnow
Files	issue.patch

^{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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assignee = None
closed_at = None
created_at = <Date 2022-02-10.21:24:35.963>
labels = ['type-bug', '3.8', '3.9', '3.10', '3.11', '3.7', 'expert-asyncio']
title = 'asyncio.create_unix_server has an off-by-one error concerning the backlog parameter'
updated_at = <Date 2022-02-10.21:24:35.963>
user = 'https://github.com/jnsnow'

bugs.python.org fields:

activity = <Date 2022-02-10.21:24:35.963>
actor = 'jnsnow'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['asyncio']
creation = <Date 2022-02-10.21:24:35.963>
creator = 'jnsnow'
dependencies = []
files = ['50618']
hgrepos = []
issue_num = 46715
keywords = ['patch']
message_count = 1.0
messages = ['413025']
nosy_count = 3.0
nosy_names = ['asvetlov', 'yselivanov', 'jnsnow']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46715'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10', 'Python 3.11']

[jnsnow]

Hi, asyncio.create_unix_server appears to treat the "backlog" parameter as where 0 means that *no connection will ever possibly be pending*, which (at the very least for UNIX sockets on my machine) is untrue.

Consider a (non-asyncio) server:

import os, socket, sys, time

sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
sock.bind('test.sock')

sock.listen(backlog=0)

while True:
    print('.', end='', file=sys.stderr)
    time.sleep(1)

This server never calls accept(), and uses a backlog of zero. However, a client can actually still successfully call connect against such a server:

import os, socket, time

sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
sock.setblocking(False)

sock.connect('test.sock')
print("Connected!")

When run against the server example, the first invocation of this client will actually connect successfully (Surprising, but that's how the C syscalls work too, so... alright) but the second invocation of this client will raise BlockingIOError (EAGAIN).

Further, if we amend the first server example to actually call accept(), it will succeed when the first client connects -- demonstrating that the actual total queue length here was actually effectively 1, not 0.

(i.e. there's always room for at least one connection to be considered, and the backlog counts everybody else.)

However, in asyncio.BaseSelectorEventLoop._accept_connection(...), the code uses for _ in range(backlog) to determine the maximum number of accept calls to make. When backlog is set to zero, this means we will never call accept, even when there are pending connections.

Note that when backlog=1, this actually allows for *two* pending connections before clients are rejected, but this loop will only fire once. This behavior is surprising, because backlog==0 means we'll accept no clients, but backlog==1 means we will allow for two to enqueue before accepting both. There is seemingly no way with asyncio to actually specify "Exactly one pending connection".

I think this loop should be amended to reflect the actual truth of the backlog parameter, and it should iterate over backlog + 1. This does necessitate a change to [Lib/test/test_asyncio/test_selector_events.py](https://github.com/python/cpython/blob/main/Lib/test/test_asyncio/test_selector_events.py) which believes that backlog=100 means that accept() should be called 100 times (instead of 101.)

A (very) simple fix is attached here; if it seems sound, I can spin a real PR on GitHub.

[gvanrossum]

Sure. Could you send a PR? Ideally it would include a test for the new behavior and a docs update.

Does this apply only to UNIX sockets, or also to INET sockets?

[jnsnow]

Sure. Could you send a PR? Ideally it would include a test for the new behavior and a docs update.

I'll give it a shot. I haven't submitted anything to the project before so it might take me a handful of minutes to figure out how to tie and/or untie my shoelaces.

Does this apply only to UNIX sockets, or also to INET sockets?

I'll be honest, I don't know. I'll have to experiment. I don't know if there are cross-platform gotchas at play here between Windows/macOS/BSD/Linux. socket programming is, famously, very easy and nobody ever gets it wrong. :)

[gvanrossum]

I would start with a test -- such a test should be easily reused for INET sockets, so then you can test that case, and we will find out in CI.

While it's not authoritative, the FreeBSD developers handbook has this clear sentence: "[t]he backlog variable tells sockets how many incoming requests to accept while you are busy processing the last request."