gh-100374: Fixed a bug in socket.getfqdn() (gh-100375)

(cherry picked from commit 12be23c)

Co-authored-by: Dominic Socular <BBH@awsl.rip>

Author: miss-islington

Lib/socket.py

@@ -784,11 +784,11 @@ def getfqdn(name=''):
 
     First the hostname returned by gethostbyaddr() is checked, then
     possibly existing aliases. In case no FQDN is available and `name`
-    was given, it is returned unchanged. If `name` was empty or '0.0.0.0',
+    was given, it is returned unchanged. If `name` was empty, '0.0.0.0' or '::',
     hostname from gethostname() is returned.
     """
     name = name.strip()
-    if not name or name == '0.0.0.0':
+    if not name or name in ('0.0.0.0', '::'):
         name = gethostname()
     try:
         hostname, aliases, ipaddrs = gethostbyaddr(name)

Lib/test/test_socket.py

@@ -1760,6 +1760,10 @@ def test_getaddrinfo_ipv6_basic(self):
         )
         self.assertEqual(sockaddr, ('ff02::1de:c0:face:8d', 1234, 0, 0))
 
+    def test_getfqdn_filter_localhost(self):
+        self.assertEqual(socket.getfqdn(), socket.getfqdn("0.0.0.0"))
+        self.assertEqual(socket.getfqdn(), socket.getfqdn("::"))
+
     @unittest.skipUnless(socket_helper.IPV6_ENABLED, 'IPv6 required for this test.')
     @unittest.skipIf(sys.platform == 'win32', 'does not work on Windows')
     @unittest.skipIf(AIX, 'Symbolic scope id does not work')

Misc/NEWS.d/next/Core and Builtins/2022-12-20-16-14-19.gh-issue-100374.YRrVHT.rst

@@ -0,0 +1 @@
+Fix incorrect result and delay in :func:`socket.getfqdn`. Patch by Dominic Socular.