From 7fbc80c6f600bfb068250dd21ee81fc7e2e135fe Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Mon, 20 Jul 2026 22:59:56 +0300
Subject: [PATCH] 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.
(cherry picked from commit c7bbf040e721781be8f11a3b0716220835d4de2f)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 Lib/socket.py                                                 | 4 +++-
 .../Library/2026-07-20-22-03-44.gh-issue-154291.Mi5HeQ.rst    | 3 +++
 2 files changed, 6 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-07-20-22-03-44.gh-issue-154291.Mi5HeQ.rst

diff --git a/Lib/socket.py b/Lib/socket.py
index bbb476f2fc010e2..9ac0badc6606c19 100644
--- a/Lib/socket.py
+++ b/Lib/socket.py
@@ -891,7 +891,9 @@ 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
 
diff --git a/Misc/NEWS.d/next/Library/2026-07-20-22-03-44.gh-issue-154291.Mi5HeQ.rst b/Misc/NEWS.d/next/Library/2026-07-20-22-03-44.gh-issue-154291.Mi5HeQ.rst
new file mode 100644
index 000000000000000..d962f8987ad491e
--- /dev/null
+++ b/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.