gh-154291: Fix socket.has_dualstack_ipv6() on DragonFly BSD (GH-154292)

On DragonFly BSD setsockopt(IPPROTO_IPV6, IPV6_V6ONLY, 0) succeeds without
actually clearing the flag, so has_dualstack_ipv6() wrongly returned True.
Verify with getsockopt() that IPV6_V6ONLY was cleared.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Lib/socket.py

@@ -900,7 +900,9 @@ def has_dualstack_ipv6():
     try:
         with socket(AF_INET6, SOCK_STREAM) as sock:
             sock.setsockopt(IPPROTO_IPV6, IPV6_V6ONLY, 0)
-            return True
+            # On some platforms (e.g. DragonFly BSD) setting IPV6_V6ONLY to 0
+            # silently has no effect, so check that it was actually cleared.
+            return sock.getsockopt(IPPROTO_IPV6, IPV6_V6ONLY) == 0
     except error:
         return False
 

Misc/NEWS.d/next/Library/2026-07-20-22-03-44.gh-issue-154291.Mi5HeQ.rst

@@ -0,0 +1,3 @@
+Fix :func:`socket.has_dualstack_ipv6` to return ``False`` on platforms such as
+DragonFly BSD where setting ``IPV6_V6ONLY`` to 0 silently has no
+effect.