mavlink: Safely avoid send the same vehicle_command and vehicle_command_ack back

The previous approach was checking system id and component id but it
will not work in 100% of cases as external devices can send MAVLink
message with the right system id but with broadcast component id.

msg/vehicle_command.msg

@@ -104,3 +104,4 @@ uint8 target_component		# Component which should execute the command, 0 for all
 uint8 source_system		# System sending the command
 uint8 source_component		# Component sending the command
 uint8 confirmation		# 0: First transmission of this command. 1-255: Confirmation transmissions (e.g. for kill command)
+uint8 from_external

msg/vehicle_command_ack.msg

@@ -8,3 +8,4 @@ uint32 ORB_QUEUE_LENGTH = 3
 
 uint16 command
 uint8 result
+uint8 from_external

src/modules/mavlink/mavlink_main.cpp

@@ -2203,6 +2203,7 @@ Mavlink::task_main(int argc, char *argv[])
 		struct vehicle_command_ack_s command_ack = {};
 
 		if (ack_sub->update(&ack_time, &command_ack)) {
+			if (!command_ack.from_external) {
 				mavlink_command_ack_t msg;
 				msg.result = command_ack.result;
 				msg.command = command_ack.command;
@@ -2210,6 +2211,7 @@ Mavlink::task_main(int argc, char *argv[])
 
 				mavlink_msg_command_ack_send_struct(get_channel(), &msg);
 			}
+		}
 
 		struct mavlink_log_s mavlink_log;
 

src/modules/mavlink/mavlink_messages.cpp

@@ -462,10 +462,7 @@ protected:
 
 		if (_cmd_sub->update(&_cmd_time, &cmd)) {
 
-			/* only send commands for other systems/components, don't forward broadcast commands */
-			if ((cmd.target_system != mavlink_system.sysid || cmd.target_component != mavlink_system.compid) &&
-			    (cmd.target_system != 0)) {
-
+			if (!cmd.from_external) {
 				if (_mavlink->verbose()) {
 					PX4_INFO("sending command %d to %d/%d", cmd.command, cmd.target_system, cmd.target_component);
 				}

src/modules/mavlink/mavlink_receiver.cpp

@@ -473,6 +473,8 @@ MavlinkReceiver::handle_message_command_long(mavlink_message_t *msg)
 
 		vcmd.confirmation =  cmd_mavlink.confirmation;
 
+		vcmd.from_external = 1;
+
 		if (_cmd_pub == nullptr) {
 			_cmd_pub = orb_advertise_queue(ORB_ID(vehicle_command), &vcmd, vehicle_command_s::ORB_QUEUE_LENGTH);
 
@@ -581,6 +583,8 @@ MavlinkReceiver::handle_message_command_int(mavlink_message_t *msg)
 
 		vcmd.source_component = msg->compid;
 
+		vcmd.from_external = 1;
+
 		if (_cmd_pub == nullptr) {
 			_cmd_pub = orb_advertise_queue(ORB_ID(vehicle_command), &vcmd, vehicle_command_s::ORB_QUEUE_LENGTH);
 
@@ -624,6 +628,7 @@ MavlinkReceiver::handle_message_command_ack(mavlink_message_t *msg)
 	command_ack.command = ack.command;
 	command_ack.result = ack.result;
 	command_ack.timestamp = hrt_absolute_time();
+	command_ack.from_external = 1;
 
 	if (_command_ack_pub == nullptr) {
 		_command_ack_pub = orb_advertise_queue(ORB_ID(vehicle_command_ack), &command_ack,
@@ -779,6 +784,7 @@ MavlinkReceiver::handle_message_set_mode(mavlink_message_t *msg)
 	vcmd.source_component = msg->compid;
 	vcmd.confirmation = 1;
 	vcmd.timestamp = hrt_absolute_time();
+	vcmd.from_external = 1;
 
 	if (_cmd_pub == nullptr) {
 		_cmd_pub = orb_advertise_queue(ORB_ID(vehicle_command), &vcmd, vehicle_command_s::ORB_QUEUE_LENGTH);