Preventing port from release in asynchronous operations.

Router reconfiguration may lead to starting new application processes, opening
listen ports etc.  These actions are asynchronous and require message
exchange with master process.  Router stores reference for controller
(response) port in structure associated with reconfiguration process (tmcf).
The port used to provide reconfiguration response (either success or failed)
to controller.

This commit increases response port use counter to make sure port structure
not freed while router waits for result of asynchronous operations.

src/nxt_router.c

@@ -874,6 +874,14 @@ nxt_router_conf_data_handler(nxt_task_t *task, nxt_port_recv_msg_t *msg)
                                        msg->port_msg.pid,
                                        msg->port_msg.reply_port);
 
+    if (nxt_slow_path(tmcf->port == NULL)) {
+        nxt_alert(task, "reply port not found");
+
+        return;
+    }
+
+    nxt_port_use(task, tmcf->port, 1);
+
     b = nxt_buf_chk_make_plain(tmcf->router_conf->mem_pool,
                                msg->buf, msg->size);
     if (nxt_slow_path(b == NULL)) {
@@ -1204,6 +1212,10 @@ nxt_router_conf_send(nxt_task_t *task, nxt_router_temp_conf_t *tmcf,
     nxt_port_msg_type_t type)
 {
     nxt_port_socket_write(task, tmcf->port, type, -1, tmcf->stream, 0, NULL);
+
+    nxt_port_use(task, tmcf->port, -1);
+
+    tmcf->port = NULL;
 }