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

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 f6b3a07b7d (bpo-44493: Add missing terminated NUL in sockaddr_un's length (GH-26866)
(cherry picked from commit c22f134211)

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

Lib/test/test_socket.py

@@ -5480,6 +5480,20 @@ class TestLinuxAbstractNamespace(unittest.TestCase):
             s.bind(bytearray(b"\x00python\x00test\x00"))
             self.assertEqual(s.getsockname(), b"\x00python\x00test\x00")
 
+    def testAutobind(self):
+        # Check that binding to an empty string binds to an available address
+        # in the abstract namespace as specified in unix(7) "Autobind feature".
+        abstract_address = b"^\0[0-9a-f]{5}"
+        with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as s1:
+            s1.bind("")
+            self.assertRegex(s1.getsockname(), abstract_address)
+            # Each socket is bound to a different abstract address.
+            with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as s2:
+                s2.bind("")
+                self.assertRegex(s2.getsockname(), abstract_address)
+                self.assertNotEqual(s1.getsockname(), s2.getsockname())
+
+
 @unittest.skipUnless(hasattr(socket, 'AF_UNIX'), 'test needs socket.AF_UNIX')
 class TestUnixDomain(unittest.TestCase):
 
@@ -5549,6 +5563,11 @@ class TestUnixDomain(unittest.TestCase):
         self.addCleanup(os_helper.unlink, path)
         self.assertEqual(self.sock.getsockname(), path)
 
+    @unittest.skipIf(sys.platform == 'linux', 'Linux specific test')
+    def testEmptyAddress(self):
+        # Test that binding empty address fails.
+        self.assertRaises(OSError, self.sock.bind, "")
+
 
 class BufferIOTest(SocketConnectedTest):
     """