coap_event: Add COAP_EVENT_MSG_RETRANSMITTED

Apart from debug log messages, there is no way for a user to be
notified when a message retransmission happens.

This event can be useful for diagnosing issues related to
lossy networks or unresponsive CoAP servers (e.g. user can
track total retransmissions as an indicator for general link
quality).

Assigned event ID to 0x5002 to avoid creating a new 0x6000
event group, and because it's similar to COAP_EVENT_BAD_PACKET (0x5001).

examples/coap-client.c

@@ -358,6 +358,7 @@ event_handler(coap_session_t *session COAP_UNUSED,
   case COAP_EVENT_SERVER_SESSION_NEW:
   case COAP_EVENT_SERVER_SESSION_DEL:
   case COAP_EVENT_BAD_PACKET:
+  case COAP_EVENT_MSG_RETRANSMITTED:
   default:
     break;
   }

examples/coap-server.c

@@ -1582,6 +1582,7 @@ proxy_event_handler(coap_session_t *session,
   case COAP_EVENT_SERVER_SESSION_NEW:
   case COAP_EVENT_SERVER_SESSION_DEL:
   case COAP_EVENT_BAD_PACKET:
+  case COAP_EVENT_MSG_RETRANSMITTED:
   default:
     break;
   }

include/coap3/coap_event.h

@@ -94,10 +94,12 @@ typedef enum {
   COAP_EVENT_SERVER_SESSION_DEL = 0x4002,
 
 /*
- * Received packet formatting errors
+ * Message receive and transmit events
  */
   /** Triggered when badly formatted packet received */
-  COAP_EVENT_BAD_PACKET         = 0x5001
+  COAP_EVENT_BAD_PACKET         = 0x5001,
+  /** Triggered when a message is retransmitted */
+  COAP_EVENT_MSG_RETRANSMITTED  = 0x5002
 
 } coap_event_t;
 

man/coap_handler.txt.in

@@ -219,9 +219,10 @@ COAP_EVENT_XMIT_BLOCK_FAIL    0x3002
 COAP_EVENT_SERVER_SESSION_NEW 0x4001
 COAP_EVENT_SERVER_SESSION_DEL 0x4002
 /**
- * Unrecognizable received packet events
+ * Message receive and transmit events
  */
 COAP_EVENT_BAD_PACKET         0x5001
+COAP_EVENT_MSG_RETRANSMITTED  0x5002
 ----
 
 EXAMPLES

src/net.c

@@ -1439,6 +1439,8 @@ coap_retransmit(coap_context_t *context, coap_queue_t *node) {
     coap_tick_t now;
 
     node->retransmit_cnt++;
+    coap_handle_event(context, COAP_EVENT_MSG_RETRANSMITTED, node->session);
+
     coap_ticks(&now);
     if (context->sendqueue == NULL) {
       node->t = node->timeout << node->retransmit_cnt;