Merge pull request #17065 from PX4/pr/fix-mavlink-shell

MAVLink shell: Ensure that nothing is written to the file handles during shell creation.

boards/cuav/x7pro/default.cmake

@@ -37,7 +37,7 @@ px4_add_board(
 		imu/invensense/icm20649
 		imu/invensense/icm20689
 		irlock
-		lights/blinkm
+		#lights/blinkm
 		lights/rgbled
 		lights/rgbled_ncp5623c
 		lights/rgbled_pwm

src/modules/mavlink/mavlink_shell.cpp

@@ -84,8 +84,6 @@ int MavlinkShell::start()
 	 * keeps (duplicates) the first 3 fd's when creating a new task, all others are not inherited.
 	 * This means we need to temporarily change the first 3 fd's of the current task (or at least
 	 * the first 2 if stdout=stderr).
-	 * And we hope :-) that during the temporary phase, no other thread from the same task writes to
-	 * stdout (as it would end up in the pipe).
 	 */
 
 	if (pipe(p1) != 0) {
@@ -105,6 +103,16 @@ int MavlinkShell::start()
 	_shell_fds[0]  = p2[0];
 	_shell_fds[1] = p1[1];
 
+	/*
+	 * Ensure that during the temporary phase no other thread from the same task writes to
+	 * stdout (as it would end up in the pipe).
+	 */
+#ifdef __PX4_NUTTX
+	sched_lock();
+#endif /* __PX4_NUTTX */
+	fflush(stdout);
+	fflush(stderr);
+
 	int fd_backups[2]; //we don't touch stderr, we will redirect it to stdout in the startup of the shell task
 
 	for (int i = 0; i < 2; ++i) {
@@ -140,6 +148,10 @@ int MavlinkShell::start()
 		close(fd_backups[i]);
 	}
 
+#ifdef __PX4_NUTTX
+	sched_unlock();
+#endif /* __PX4_NUTTX */
+
 	//close unused pipe fd's
 	close(_shell_fds[0]);
 	close(_shell_fds[1]);