fix(mavlink): add send timeout to UDP socket to avoid infinite waiting on UDP IOB on NuttX (#26988)

Co-authored-by: Ivan Zivotic <ivan@sunflower-labs.com>

src/modules/mavlink/mavlink_main.cpp

@@ -61,6 +61,10 @@
 #include "mavlink_receiver.h"
 #include "mavlink_main.h"
 
+#ifdef MAVLINK_UDP
+#include <sys/time.h>
+#endif
+
 // Guard against MAVLink misconfiguration
 #ifndef MAVLINK_CRC_EXTRA
 #error MAVLINK_CRC_EXTRA has to be defined on PX4 systems
@@ -1022,6 +1026,19 @@ void Mavlink::init_udp()
 		return;
 	}
 
+	// Cap UDP send time to avoid blocking indefinitely if the NuttX net
+	// stack hasn't finished initializing the IOB write-buffer semaphore.
+	// 10ms is ~4x the worst-case send time measured on STM32H7.
+	constexpr long UDP_SEND_TIMEOUT_US = 10000;
+	struct timeval tv {
+		.tv_sec = 0,
+		.tv_usec = UDP_SEND_TIMEOUT_US
+	};
+
+	if (setsockopt(_socket_fd, SOL_SOCKET, SO_SNDTIMEO, &tv, sizeof(tv)) < 0) {
+		PX4_WARN("setting socket timeout failed: %s", strerror(errno));
+	}
+
 	if (bind(_socket_fd, (struct sockaddr *)&_myaddr, sizeof(_myaddr)) < 0) {
 		PX4_WARN("bind failed: %s", strerror(errno));
 		return;