fix mavlink: fixes for mavlink on USB instance

Open the UART after adding the instance: mavlink_open_uart() might block,
and in that case the parent task waiting for mavlink to be started times
out.

And while waiting for the USB UART device to come up:
- check for _task_should_exit
- check for check_requested_subscriptions()

Side-effects when USB is not plugged in during boot:
- reduces boot duration by 100ms
- fixes duplicate instance name in 'top':
 201 mavlink_if0                     1  0.000  1328/ 2572 100 (100)  w:sig  3
 204 mavlink_if0                   572  4.221  1632/ 2540 100 (100)  w:sig  4
- 'mavlink stop-all' now stops the usb instance as well

src/modules/mavlink/mavlink_main.cpp

@@ -578,8 +578,7 @@ Mavlink::mavlink_open_uart(const int baud, const char *uart_name, const bool for
 	}
 
 	/* back off 1800 ms to avoid running into the USB setup timing */
-	while (_is_usb_uart &&
-	       hrt_absolute_time() < 1800U * 1000U) {
+	while (_is_usb_uart && hrt_absolute_time() < 1800U * 1000U) {
 		px4_usleep(50000);
 	}
 
@@ -592,7 +591,10 @@ Mavlink::mavlink_open_uart(const int baud, const char *uart_name, const bool for
 		uORB::SubscriptionData<actuator_armed_s> armed_sub{ORB_ID(actuator_armed)};
 
 		/* get the system arming state and abort on arming */
-		while (_uart_fd < 0) {
+		while (_uart_fd < 0 && !_task_should_exit) {
+
+			/* another task might have requested subscriptions: make sure we handle it */
+			check_requested_subscriptions();
 
 			/* abort if an arming topic is published and system is armed */
 			armed_sub.update();
@@ -1881,6 +1883,12 @@ Mavlink::task_main(int argc, char *argv[])
 		case 'd':
 			_device_name = myoptarg;
 			set_protocol(Protocol::SERIAL);
+
+			if (access(_device_name, F_OK) == -1) {
+				PX4_ERR("Device %s does not exist", _device_name);
+				err_flag = true;
+			}
+
 			break;
 
 		case 'n':
@@ -2048,7 +2056,7 @@ Mavlink::task_main(int argc, char *argv[])
 		return PX4_ERROR;
 	}
 
-	/* USB serial is indicated by /dev/ttyACM0*/
+	/* USB serial is indicated by /dev/ttyACMx */
 	if (strcmp(_device_name, "/dev/ttyACM0") == OK || strcmp(_device_name, "/dev/ttyACM1") == OK) {
 		if (_datarate == 0) {
 			_datarate = 800000;
@@ -2086,19 +2094,6 @@ Mavlink::task_main(int argc, char *argv[])
 
 		/* flush stdout in case MAVLink is about to take it over */
 		fflush(stdout);
-
-		/* default values for arguments */
-		_uart_fd = mavlink_open_uart(_baudrate, _device_name, _force_flow_control);
-
-		if (_uart_fd < 0 && !_is_usb_uart) {
-			PX4_ERR("could not open %s", _device_name);
-			return PX4_ERROR;
-
-		} else if (_uart_fd < 0 && _is_usb_uart) {
-			/* the config link is optional */
-			return OK;
-		}
-
 	}
 
 #if defined(MAVLINK_UDP)
@@ -2198,6 +2193,20 @@ Mavlink::task_main(int argc, char *argv[])
 	/* now the instance is fully initialized and we can bump the instance count */
 	LL_APPEND(_mavlink_instances, this);
 
+	/* open the UART device after setting the instance, as it might block */
+	if (get_protocol() == Protocol::SERIAL) {
+		_uart_fd = mavlink_open_uart(_baudrate, _device_name, _force_flow_control);
+
+		if (_uart_fd < 0 && !_is_usb_uart) {
+			PX4_ERR("could not open %s", _device_name);
+			return PX4_ERROR;
+
+		} else if (_uart_fd < 0 && _is_usb_uart) {
+			/* the config link is optional */
+			return PX4_OK;
+		}
+	}
+
 #if defined(MAVLINK_UDP)
 
 	/* init socket if necessary */