Ensure that the mavlink start call only returns once the new instance is fully initialized. This avoids race conditions in getopt() and it ensures that the mavlink debug fd is ready when other processes start

src/modules/mavlink/mavlink_main.cpp

@@ -254,7 +254,6 @@ Mavlink::Mavlink() :
 		break;
 	}
 
-	LL_APPEND(_mavlink_instances, this);
 }
 
 Mavlink::~Mavlink()
@@ -1778,6 +1777,9 @@ Mavlink::task_main(int argc, char *argv[])
 	/* set main loop delay depending on data rate to minimize CPU overhead */
 	_main_loop_delay = MAIN_LOOP_DELAY / rate_mult;
 
+	/* now the instance is fully initialized and we can bump the instance count */
+	LL_APPEND(_mavlink_instances, this);
+
 	while (!_task_should_exit) {
 		/* main loop */
 		usleep(_main_loop_delay);
@@ -1924,10 +1926,18 @@ int Mavlink::start_helper(int argc, char *argv[])
 int
 Mavlink::start(int argc, char *argv[])
 {
+	// Wait for the instance count to go up one
+	// before returning to the shell
+	int ic = Mavlink::instance_count();
+
 	// Instantiate thread
 	char buf[24];
-	sprintf(buf, "mavlink_if%d", Mavlink::instance_count());
+	sprintf(buf, "mavlink_if%d", ic);
 
+	// This is where the control flow splits
+	// between the starting task and the spawned
+	// task - start_helper() only returns
+	// when the started task exits.
 	task_spawn_cmd(buf,
 		       SCHED_DEFAULT,
 		       SCHED_PRIORITY_DEFAULT,
@@ -1935,6 +1945,16 @@ Mavlink::start(int argc, char *argv[])
 		       (main_t)&Mavlink::start_helper,
 		       (const char **)argv);
 
+	// Ensure that this shell command
+	// does not return before the instance
+	// is fully initialized. As this is also
+	// the only path to create a new instance,
+	// this is effectively a lock on concurrent
+	// instance starting. XXX do a real lock.
+	while (ic == Mavlink::instance_count()) {
+		::usleep(500);
+	}
+
 	return OK;
 }