Add support for NOP.

src/guacd/client.c

@@ -74,17 +74,17 @@ void* __guacd_client_output_thread(void* data) {
     /* Guacamole client output loop */
     while (client->state == GUAC_CLIENT_RUNNING) {
 
-        /* Occasionally ping client with repeat of last sync */
+        /* Occasionally send client NOP keep-alive */
         guac_timestamp timestamp = guac_timestamp_current();
         if (timestamp - last_ping_timestamp > GUACD_SYNC_FREQUENCY) {
 
             /* Record time of last synnc */
             last_ping_timestamp = timestamp;
 
-            /* Send sync */
-            if (guac_protocol_send_sync(socket, client->last_sent_timestamp)) {
+            /* Send NOP */
+            if (guac_protocol_send_nop(socket)) {
                 guacd_client_log_guac_error(client,
-                        "Error sending \"sync\" instruction");
+                        "Error sending \"nop\" keep-alive");
                 guac_client_stop(client);
                 return NULL;
             }

src/libguac/guacamole/protocol.h

@@ -232,6 +232,18 @@ int guac_protocol_send_error(guac_socket* socket, const char* error);
 int guac_protocol_send_nest(guac_socket* socket, int index,
         const char* data);
 
+/**
+ * Sends a nop instruction (null-operation) over the given guac_socket
+ * connection.
+ *
+ * If an error occurs sending the instruction, a non-zero value is
+ * returned, and guac_error is set appropriately.
+ *
+ * @param socket The guac_socket connection to use.
+ * @return Zero on success, non-zero on error.
+ */
+int guac_protocol_send_nop(guac_socket* socket);
+
 /**
  * Sends a set instruction over the given guac_socket connection.
  *

src/libguac/protocol.c

@@ -951,6 +951,18 @@ int guac_protocol_send_nest(guac_socket* socket, int index,
 
 }
 
+int guac_protocol_send_nop(guac_socket* socket) {
+
+    int ret_val;
+
+    guac_socket_instruction_begin(socket);
+    ret_val = guac_socket_write_string(socket, "3.nop;");
+    guac_socket_instruction_end(socket);
+
+    return ret_val;
+
+}
+
 int guac_protocol_send_png(guac_socket* socket, guac_composite_mode mode,
         const guac_layer* layer, int x, int y, cairo_surface_t* surface) {