sockets: Support multipath TCP

Multipath TCP allows combining multiple interfaces/routes into a single
socket, with very little work for the user/admin.

It's enabled by 'mptcp' on most socket addresses:

   ./qemu-system-x86_64 -nographic -incoming tcp:0:4444,mptcp

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20210421112834.107651-6-dgilbert@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

io/dns-resolver.c

@@ -122,6 +122,10 @@ static int qio_dns_resolver_lookup_sync_inet(QIODNSResolver *resolver,
             .ipv4 = iaddr->ipv4,
             .has_ipv6 = iaddr->has_ipv6,
             .ipv6 = iaddr->ipv6,
+#ifdef IPPROTO_MPTCP
+            .has_mptcp = iaddr->has_mptcp,
+            .mptcp = iaddr->mptcp,
+#endif
         };
 
         (*addrs)[i] = newaddr;

qapi/sockets.json

@@ -57,6 +57,8 @@
 # @keep-alive: enable keep-alive when connecting to this socket. Not supported
 #              for passive sockets. (Since 4.2)
 #
+# @mptcp: enable multi-path TCP. (Since 6.1)
+#
 # Since: 1.3
 ##
 { 'struct': 'InetSocketAddress',
@@ -66,7 +68,8 @@
     '*to': 'uint16',
     '*ipv4': 'bool',
     '*ipv6': 'bool',
-    '*keep-alive': 'bool' } }
+    '*keep-alive': 'bool',
+    '*mptcp': { 'type': 'bool', 'if': 'defined(IPPROTO_MPTCP)' } } }
 
 ##
 # @UnixSocketAddress:

util/qemu-sockets.c

@@ -278,6 +278,11 @@ static int inet_listen_saddr(InetSocketAddress *saddr,
 
     /* create socket + bind/listen */
     for (e = res; e != NULL; e = e->ai_next) {
+#ifdef IPPROTO_MPTCP
+        if (saddr->has_mptcp && saddr->mptcp) {
+            e->ai_protocol = IPPROTO_MPTCP;
+        }
+#endif
         getnameinfo((struct sockaddr*)e->ai_addr,e->ai_addrlen,
                         uaddr,INET6_ADDRSTRLEN,uport,32,
                         NI_NUMERICHOST | NI_NUMERICSERV);
@@ -456,6 +461,13 @@ int inet_connect_saddr(InetSocketAddress *saddr, Error **errp)
     for (e = res; e != NULL; e = e->ai_next) {
         error_free(local_err);
         local_err = NULL;
+
+#ifdef IPPROTO_MPTCP
+        if (saddr->has_mptcp && saddr->mptcp) {
+            e->ai_protocol = IPPROTO_MPTCP;
+        }
+#endif
+
         sock = inet_connect_addr(saddr, e, &local_err);
         if (sock >= 0) {
             break;
@@ -687,6 +699,17 @@ int inet_parse(InetSocketAddress *addr, const char *str, Error **errp)
         }
         addr->has_keep_alive = true;
     }
+#ifdef IPPROTO_MPTCP
+    begin = strstr(optstr, ",mptcp");
+    if (begin) {
+        if (inet_parse_flag("mptcp", begin + strlen(",mptcp"),
+                            &addr->mptcp, errp) < 0)
+        {
+            return -1;
+        }
+        addr->has_mptcp = true;
+    }
+#endif
     return 0;
 }