add config directive forward_in_turn to support forward streams to one of the server

in forward_in_turn list in turn

@@ -9,6 +9,8 @@ daemon              off;
 srs_log_tank        console;
 vhost __defaultVhost__ {
     forward         127.0.0.1:19350;
+#select on of the listed servers in turn to forward,not all of them
+    forward_in_turn 127.0.0.1:19351 127.0.0.1:19352;
 #the forward destination server type,default true. if the server is not srs server,connect app maybe fail,try config as false 
     forward_server_srs  true;
 }

@@ -1823,7 +1823,7 @@ int SrsConfig::check_config()
                 && n != "dvr" && n != "ingest" && n != "hls" && n != "http_hooks"
                 && n != "gop_cache" && n != "queue_length"
                 && n != "refer" && n != "refer_publish" && n != "refer_play"
-                && n != "forward" && n != "forward_server_srs" && n != "transcode" && n != "bandcheck"
+                && n != "forward" && n != "forward_server_srs" && n != "forward_in_turn" && n != "transcode" && n != "bandcheck"
                 && n != "time_jitter" && n != "mix_correct"
                 && n != "atc" && n != "atc_auto"
                 && n != "debug_srs_upnode"
@@ -2772,6 +2772,17 @@ bool SrsConfig::get_forward_server_srs(string vhost)
     return conf->arg0()=="true";
 }
 
+SrsConfDirective* SrsConfig::get_forward_in_turn(string vhost)
+{
+    SrsConfDirective* conf = get_vhost(vhost);
+
+    if (!conf) {
+        return NULL;
+    }
+
+    return conf->get("forward_in_turn");
+}
+
 SrsConfDirective* SrsConfig::get_vhost_http_hooks(string vhost)
 {
     SrsConfDirective* conf = get_vhost(vhost);

@@ -573,6 +573,10 @@ public:
     * get the forward server type  of vhost,if srs return true,else false.
     */
     virtual bool                get_forward_server_srs(std::string vhost);
+    /**
+    * get the forward_in_turn directive of vhost.
+    */
+    virtual SrsConfDirective*   get_forward_in_turn(std::string vhost);
 // http_hooks section
 private:
     /**

@@ -2448,6 +2448,44 @@ int SrsSource::create_forwarders()
         }
     }
 
+    //forward a server in forward_in_turn
+    static int server_index = 0;
+    conf = _srs_config->get_forward_in_turn(_req->vhost);
+    if(!conf){
+    	return ret;
+    }
+     
+    int servers_size = (int)conf->args.size();
+    if(servers_size <= 0){
+    	return ret;
+    }
+
+    if(server_index >= servers_size){
+    	server_index = 0;
+    }
+
+    std::string forward_server = conf->args.at(server_index);
+    server_index ++;
+
+    SrsForwarder* forwarder = new SrsForwarder(this);
+    forwarders.push_back(forwarder);
+
+    // initialize the forwarder with request.
+    if ((ret = forwarder->initialize(_req, forward_server)) != ERROR_SUCCESS) {
+        return ret;
+    }
+
+    double queue_size = _srs_config->get_queue_length(_req->vhost);
+    forwarder->set_queue_size(queue_size);
+
+    if ((ret = forwarder->on_publish()) != ERROR_SUCCESS) {
+            srs_error("start forwarder failed. "
+                "vhost=%s, app=%s, stream=%s, forward-to=%s",
+                _req->vhost.c_str(), _req->app.c_str(), _req->stream.c_str(),
+                forward_server.c_str());
+        return ret;
+    }
+
     return ret;
 }