cf-socket: drop feature check for `IPV6_V6ONLY` on Windows

The macro is present in all supported Windows toolchains.

It's present in mingw-w64 v3+, and in MS SDK 6.0A+ (maybe earlier).

Also:
- restrict this logic to `USE_WINSOCK` (was: `_WIN32`), to exclude
  alternate socket libraries (i.e. lwIP). lwIP supports `IPV6_V6ONLY`
  since its 2.0.0 (2016-11-10) release and it's disabled by default,
  unlike in Winsock.
  Ref: https://github.com/lwip-tcpip/lwip/commit/e65202f8257e55b09e6309b1aa405b5e6a017f0d
- delete interim setter function/dummy macro `set_ipv6_v6only()`.

Follow-up to a28f5f68b965119d9dd1ab6c2a2ccc66c6ed5d1f #18010
Follow-up to ca3f6decb927a4c3eb4c10fba09848b626a526d6 #10975

Closes #19769

diff --git a/lib/cf-socket.c b/lib/cf-socket.c
index a12995941610a4c960478e8069dc2954b75cb172..0e431976c915908efb502c2ae3ef95ccccd9b70d 100644 (file)
--- a/lib/cf-socket.c
+++ b/lib/cf-socket.c
@@ -83,22 +83,6 @@
 #include "curlx/strparse.h"
 
 
-#if defined(USE_IPV6) && defined(IPV6_V6ONLY) && defined(_WIN32)
-/* It makes support for IPv4-mapped IPv6 addresses.
- * Linux kernel, NetBSD, FreeBSD and Darwin: default is off;
- * Windows Vista and later: default is on;
- * DragonFly BSD: acts like off, and dummy setting;
- * OpenBSD and earlier Windows: unsupported.
- * Linux: controlled by /proc/sys/net/ipv6/bindv6only.
- */
-static void set_ipv6_v6only(curl_socket_t sockfd, int on)
-{
-  (void)setsockopt(sockfd, IPPROTO_IPV6, IPV6_V6ONLY, (void *)&on, sizeof(on));
-}
-#else
-#define set_ipv6_v6only(x, y)
-#endif
-
 static void tcpnodelay(struct Curl_cfilter *cf,
                        struct Curl_easy *data,
                        curl_socket_t sockfd)
@@ -1114,7 +1098,18 @@ static CURLcode cf_socket_open(struct Curl_cfilter *cf,
 
 #ifdef USE_IPV6
   if(ctx->addr.family == AF_INET6) {
-    set_ipv6_v6only(ctx->sock, 0);
+#ifdef USE_WINSOCK
+    /* Turn on support for IPv4-mapped IPv6 addresses.
+     * Linux kernel, NetBSD, FreeBSD, Darwin, lwIP: default is off;
+     * Windows Vista and later: default is on;
+     * DragonFly BSD: acts like off, and dummy setting;
+     * OpenBSD and earlier Windows: unsupported.
+     * Linux: controlled by /proc/sys/net/ipv6/bindv6only.
+     */
+    int on = 0;
+    (void)setsockopt(ctx->sock, IPPROTO_IPV6, IPV6_V6ONLY,
+                     (void *)&on, sizeof(on));
+#endif
     infof(data, "  Trying [%s]:%d...", ctx->ip.remote_ip, ctx->ip.remote_port);
   }
   else