avformat/udp: always use IP_ADD_SOURCE_MEMBERSHIP for subscribing to an UDP...

avformat/udp: always use IP_ADD_SOURCE_MEMBERSHIP for subscribing to an UDP multicast source group in IPv4

That alone supports specifying the interface based on its address. Getting the
interface index from the local address seems quite a bit of work in a platform
independent way...

Obviously for IPv6 we still always use MCAST_JOIN_SOURCE_GROUP.

As a side effect this also fixes ticket #7459.
Signed-off-by: Marton Balint <cus@passwd.hu>

libavformat/udp.c

@@ -246,9 +246,12 @@ static int udp_set_multicast_sources(URLContext *h,
                                      struct sockaddr_storage *sources,
                                      int nb_sources, int include)
 {
-#if HAVE_STRUCT_GROUP_SOURCE_REQ && defined(MCAST_BLOCK_SOURCE) && !defined(_WIN32) && (!defined(TARGET_OS_TV) || !TARGET_OS_TV)
-    /* These ones are available in the microsoft SDK, but don't seem to work
-     * as on linux, so just prefer the v4-only approach there for now. */
+    int i;
+    if (addr->sa_family != AF_INET) {
+#if HAVE_STRUCT_GROUP_SOURCE_REQ && defined(MCAST_BLOCK_SOURCE)
+        /* For IPv4 prefer the old approach, as that alone works reliably on
+         * Windows and it also supports supplying the interface based on its
+         * address. */
         int i;
         for (i = 0; i < nb_sources; i++) {
             struct group_source_req mreqs;
@@ -269,13 +272,14 @@ static int udp_set_multicast_sources(URLContext *h,
                 return ff_neterrno();
             }
         }
-#elif HAVE_STRUCT_IP_MREQ_SOURCE && defined(IP_BLOCK_SOURCE)
-    int i;
-    if (addr->sa_family != AF_INET) {
+        return 0;
+#else
         av_log(NULL, AV_LOG_ERROR,
                "Setting multicast sources only supported for IPv4\n");
         return AVERROR(EINVAL);
+#endif
     }
+#if HAVE_STRUCT_IP_MREQ_SOURCE && defined(IP_BLOCK_SOURCE)
     for (i = 0; i < nb_sources; i++) {
         struct ip_mreq_source mreqs;
         if (sources[i].ss_family != AF_INET) {