AP_HAL_SITL: provide method to get amount of data still pending in outbound system queues

libraries/AP_HAL_SITL/UARTDriver.cpp

@@ -770,5 +770,25 @@ uint64_t UARTDriver::receive_time_constraint_us(uint16_t nbytes)
     return last_receive_us;
 }
 
+ssize_t UARTDriver::get_system_outqueue_length() const
+{
+    if (!_connected) {
+        return 0;
+    }
+
+#if defined(__CYGWIN__) || defined(__CYGWIN64__) || defined(CYGWIN_BUILD)
+    return 0;
+#elif defined(__APPLE__) && defined(__MACH__)
+    return 0;
+#else
+    int size;
+    if (ioctl(_fd, TIOCOUTQ, &size) == -1) {
+        // ::fprintf(stderr, "ioctl TIOCOUTQ failed: %m\n");
+        return 0;
+    }
+    return size;
+#endif
+}
+
 #endif // CONFIG_HAL_BOARD
 

libraries/AP_HAL_SITL/UARTDriver.h

@@ -33,6 +33,8 @@ public:
         return true;
     }
 
+    ssize_t get_system_outqueue_length() const;
+
     void set_blocking_writes(bool blocking) override
     {
         _nonblocking_writes = !blocking;