Autobind of empty unix socket on Linux broken since python 3.9

[nirs]

Bug report

When using empty address for unix socket on Linux, the socket is bound to a random
address in the abstract namespace.

from unix(7):

   Autobind feature
       If  a  bind(2)  call  specifies  addrlen as sizeof(sa_family_t), or the
       SO_PASSCRED socket option was specified for a socket that was  not  ex‐
       plicitly  bound  to  an address, then the socket is autobound to an ab‐
       stract address.  The address consists of a  null  byte  followed  by  5
       bytes  in  the  character set [0-9a-f].  Thus, there is a limit of 2^20
       autobind addresses.  (From Linux 2.1.15, when the autobind feature  was
       added,  8  bytes  were  used,  and the limit was thus 2^32 autobind ad‐
       dresses.  The change to 5 bytes came in Linux 2.3.15.)

Since python 3.9, when using empty socket address (""), bind() is called with
with path.len + offsetof(struct sockaddr_un, sun_path) + 1, so the socket is
bound to the specific address "\0" instead of a random address (e.g. "\075499").
This breaks code assuming that multiple sockets can be bound to a random address
at the same time.

In python 3.8 (not including this change):

$ python3.8
Python 3.8.13 (default, Jun 10 2022, 00:00:00) 
[GCC 12.1.1 20220507 (Red Hat 12.1.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import socket
>>> s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
>>> s.bind("")
>>> s.getsockname()
b'\x0075499'

With python 3.9 (including this change):

$ python3.9
Python 3.9.13 (main, Jun  9 2022, 00:00:00) 
[GCC 12.1.1 20220507 (Red Hat 12.1.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import socket
>>> s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
>>> s.bind("")
>>> s.getsockname()
b'\x00'

Your environment

• CPython versions tested on: python 3.9, python 3.10
• Operating system and architecture: Fedora 36, Ubuntu latest (GitHub actions)

The breaking change is f6b3a07, backported
to python 3.9. I suspect that change was not needed since it did not include
any test showing a real issue, but it is possible to fix handling of empty
unix socket address without reverting this change.