gh-94821: Fix autobind of empty unix domain address (GH-94826) (GH-94875)

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 f6b3a07b7df6 (bpo-44493: Add missing terminated NUL in sockaddr_un's length (GH-26866)
(cherry picked from commit c22f134211743cd5ad14cec1dd4f527bee542b4c)

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

1 files changed, 2 insertions, 0 deletions

diff --git a/Misc/NEWS.d/next/Library/2022-07-14-00-43-52.gh-issue-94821.e17ghU.rst b/Misc/NEWS.d/next/Library/2022-07-14-00-43-52.gh-issue-94821.e17ghU.rst
new file mode 100644
index 0000000..bf7885a
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2022-07-14-00-43-52.gh-issue-94821.e17ghU.rst
@@ -0,0 +1,2 @@
+Fix binding of unix socket to empty address on Linux to use an available
+address from the abstract namespace, instead of "\0".