GCS_MAVLink: divide time allowed to send messages fairly

libraries/GCS_MAVLink/GCS.h

@@ -978,6 +978,12 @@ private:
 
     // timer called to implement pass-thru
     void passthru_timer();
+
+    // this contains the index of the GCS_MAVLINK backend we will
+    // first call update_send on.  It is incremented each time
+    // GCS::update_send is called so we don't starve later links of
+    // time in which they are permitted to send messages.
+    uint8_t first_backend_to_send;
 };
 
 GCS &gcs();

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -1961,9 +1961,19 @@ void GCS::update_send()
     if (_missionitemprotocol_fence != nullptr) {
         _missionitemprotocol_fence->update();
     }
-    for (uint8_t i=0; i<num_gcs(); i++) {
+    // round-robin the GCS_MAVLINK backend that gets to go first so
+    // one backend doesn't monopolise all of the time allowed for sending
+    // messages
+    for (uint8_t i=first_backend_to_send; i<num_gcs(); i++) {
         chan(i)->update_send();
     }
+    for (uint8_t i=0; i<first_backend_to_send; i++) {
+        chan(i)->update_send();
+    }
+    first_backend_to_send++;
+    if (first_backend_to_send >= num_gcs()) {
+        first_backend_to_send = 0;
+    }
     WITH_SEMAPHORE(_statustext_sem);
     service_statustext();
 }