GCS_MAVLink: reserve some space when param fetch fails

this ensures we give some buffer space for parameter fetch when we are
low on buffer space

we reserve 100 bytes for 2 seconds after a param fetch fails due to
low buffer space

libraries/GCS_MAVLink/GCS.h

@@ -306,6 +306,9 @@ private:
     uint8_t next_deferred_message;
     uint8_t num_deferred_messages;
 
+    // time when we missed sending a parameter for GCS
+    static uint32_t reserve_param_space_start_ms;
+    
     // bitmask of what mavlink channels are active
     static uint8_t mavlink_active;
 

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -29,6 +29,7 @@ extern const AP_HAL::HAL& hal;
 uint32_t GCS_MAVLINK::last_radio_status_remrssi_ms;
 uint8_t GCS_MAVLINK::mavlink_active = 0;
 ObjectArray<GCS_MAVLINK::statustext_t> GCS_MAVLINK::_statustext_queue(GCS_MAVLINK_PAYLOAD_STATUS_CAPACITY);
+uint32_t GCS_MAVLINK::reserve_param_space_start_ms;
 
 GCS_MAVLINK::GCS_MAVLINK()
 {
@@ -579,9 +580,17 @@ void GCS_MAVLINK::handle_param_request_read(mavlink_message_t *msg)
     mavlink_param_request_read_t packet;
     mavlink_msg_param_request_read_decode(msg, &packet);
 
+    /*
+      we reserve some space for sending parameters if the client ever
+      fails to get a parameter due to lack of space
+     */
+    uint32_t saved_reserve_param_space_start_ms = reserve_param_space_start_ms;
+    reserve_param_space_start_ms = 0;
     if (!HAVE_PAYLOAD_SPACE(chan, PARAM_VALUE)) {
+        reserve_param_space_start_ms = AP_HAL::millis();
         return;
     }
+    reserve_param_space_start_ms = saved_reserve_param_space_start_ms;
     
     enum ap_var_type p_type;
     AP_Param *vp;

libraries/GCS_MAVLink/GCS_Signing.cpp

@@ -234,10 +234,23 @@ bool GCS_MAVLINK::signing_enabled(void) const
  */
 uint8_t GCS_MAVLINK::packet_overhead_chan(mavlink_channel_t chan)
 {
+    /*
+      reserve 100 bytes for parameters when a GCS fails to fetch a
+      parameter due to lack of buffer space. The reservation lasts 2
+      seconds
+     */
+    uint8_t reserved_space = 0;
+    if (reserve_param_space_start_ms != 0 &&
+        AP_HAL::millis() - reserve_param_space_start_ms < 2000) {
+        reserved_space = 100;
+    } else {
+        reserve_param_space_start_ms = 0;
+    }
+    
     const mavlink_status_t *status = mavlink_get_channel_status(chan);
     if (status->signing && (status->signing->flags & MAVLINK_SIGNING_FLAG_SIGN_OUTGOING)) {
-        return MAVLINK_NUM_NON_PAYLOAD_BYTES + MAVLINK_SIGNATURE_BLOCK_LEN;
+        return MAVLINK_NUM_NON_PAYLOAD_BYTES + MAVLINK_SIGNATURE_BLOCK_LEN + reserved_space;
     }
-    return MAVLINK_NUM_NON_PAYLOAD_BYTES;
+    return MAVLINK_NUM_NON_PAYLOAD_BYTES + reserved_space;
 }