Connecting a socket in timeout mode to a server with full listen backlog raises BlockingIOError

Bug report

Bug description:

I'm working on a Debian bookworm system. Please consider this executable script ms:

#!/usr/bin/python3 -i

from pathlib import Path
import socket

SOCK = 'stream.sock'
s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
Path(SOCK).unlink(missing_ok=True)
s.bind(SOCK)
s.listen(0)

and this one mc:

#!/usr/bin/python3

import socket

SOCK = 'stream.sock'
s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
s.settimeout(3)
s.connect(SOCK)

Let's start ./ms, and once it reaches the Python prompt, run ./mc twice in another terminal:

$ ./mc
$ ./mc
Traceback (most recent call last):
  File "/home/wferi/block/./mc", line 8, in <module>
    s.connect(SOCK)
BlockingIOError: [Errno 11] Resource temporarily unavailable

The first[^1] invocation runs to completion all right, but all succeeding ones immediately fail with BlockingIOError, as if the connecting socket was in non-blocking mode, not in timeout mode. I understand that the documentation[^2] notes

at the operating system level, sockets in timeout mode are internally set in non-blocking mode

so that must be where this comes from, but such an implementation detail shouldn't leak out of the abstraction, as the documentation also states that

A socket object can be in one of three modes: blocking, non-blocking, or timeout.

and

exception BlockingIOError: Raised when an operation would block on an object (e.g. socket) set for non-blocking operation. [^3]

[^1]: In general the first backlog+1, but I set the listen backlog to 0 in the code of ms above.
[^2]: https://docs.python.org/3/library/socket.html#socket-timeouts
[^3]: https://docs.python.org/3/library/exceptions.html

CPython versions tested on:

3.11

Operating systems tested on:

Linux

Linked PRs

• gh-134688

Tái hiện hành vi bằng các script ms và mc trong issue, sử dụng Unix socket với listen(0) và các kết nối có hẹn giờ lặp lại. Đọc tài liệu được liên kết về timeout và exception của socket, sau đó kiểm tra phần triển khai socket liên quan của CPython và PR được liên kết gh-134688. Được xem là hoàn tất khi abstraction timeout được ghi chép vẫn được giữ nguyên trong trường hợp listen backlog đầy, kèm theo coverage hồi quy cho trường hợp đã tái hiện.