[3.10] gh-94821: Fix autobind of empty unix domain address (GH-94826)

[miss-islington]

When binding a unix socket to an empty address on Linux, the socket is
automatically bound to an available address in the abstract namespace.

>>> s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
>>> s.bind("")
>>> s.getsockname()
b'\x0075499'

Since python 3.9, the socket is bound to the one address:

>>> s.getsockname()
b'\x00'

And trying to bind multiple sockets will fail with:

Traceback (most recent call last):
File "/home/nsoffer/src/cpython/Lib/test/test_socket.py", line 5553, in testAutobind
s2.bind("")
OSError: [Errno 98] Address already in use

Added 2 tests:

• Auto binding empty address on Linux
• Failing to bind an empty address on other platforms

Fixes f6b3a07 (bpo-44493: Add missing terminated NUL in sockaddr_un's length (GH-26866)
(cherry picked from commit c22f134)

Co-authored-by: Nir Soffer nsoffer@redhat.com

• Issue: Autobind of empty unix socket on Linux broken since python 3.9 #94821

[miss-islington]

Status check is done, and it's a success ✅ .

[miss-islington]

Status check is done, and it's a success ✅ .

[miss-islington]

Status check is done, and it's a success ✅ .