sockets: allow port to be NULL when listening on IP address

If the port in the SocketAddress struct is NULL, it can allow
the kernel to automatically select a free port. This is useful
in particular in unit tests to avoid a race trying to find a
free port to run a test case on.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

qapi-schema.json

@@ -2614,7 +2614,9 @@
 #
 # @host: host part of the address
 #
-# @port: port part of the address, or lowest port if @to is present
+# @port: port part of the address, or lowest port if @to is present.
+#        Kernel selects a free port if omitted for listener addresses.
+#        #optional
 #
 # @to: highest port to try
 #
@@ -2629,7 +2631,7 @@
 { 'struct': 'InetSocketAddress',
   'data': {
     'host': 'str',
-    'port': 'str',
+    '*port': 'str',
     '*to': 'uint16',
     '*ipv4': 'bool',
     '*ipv6': 'bool' } }

util/qemu-sockets.c

@@ -128,12 +128,15 @@ int inet_listen_opts(QemuOpts *opts, int port_offset, Error **errp)
     ai.ai_family = PF_UNSPEC;
     ai.ai_socktype = SOCK_STREAM;
 
-    if ((qemu_opt_get(opts, "host") == NULL) ||
-        (qemu_opt_get(opts, "port") == NULL)) {
-        error_setg(errp, "host and/or port not specified");
+    if ((qemu_opt_get(opts, "host") == NULL)) {
+        error_setg(errp, "host not specified");
         return -1;
     }
-    pstrcpy(port, sizeof(port), qemu_opt_get(opts, "port"));
+    if (qemu_opt_get(opts, "port") != NULL) {
+        pstrcpy(port, sizeof(port), qemu_opt_get(opts, "port"));
+    } else {
+        port[0] = '\0';
+    }
     addr = qemu_opt_get(opts, "host");
 
     to = qemu_opt_get_number(opts, "to", 0);
@@ -145,6 +148,10 @@ int inet_listen_opts(QemuOpts *opts, int port_offset, Error **errp)
     /* lookup */
     if (port_offset) {
         unsigned long long baseport;
+        if (strlen(port) == 0) {
+            error_setg(errp, "port not specified");
+            return -1;
+        }
         if (parse_uint_full(port, &baseport, 10) < 0) {
             error_setg(errp, "can't convert to a number: %s", port);
             return -1;
@@ -156,7 +163,8 @@ int inet_listen_opts(QemuOpts *opts, int port_offset, Error **errp)
         }
         snprintf(port, sizeof(port), "%d", (int)baseport + port_offset);
     }
-    rc = getaddrinfo(strlen(addr) ? addr : NULL, port, &ai, &res);
+    rc = getaddrinfo(strlen(addr) ? addr : NULL,
+                     strlen(port) ? port : NULL, &ai, &res);
     if (rc != 0) {
         error_setg(errp, "address resolution failed for %s:%s: %s", addr, port,
                    gai_strerror(rc));