proxy: Make proxy requests separate responses

As an incoming proxy request may delay its response (e.g. initiate an
ongoing connection), send back empty ACK immediately so that the
client does not timeout and re-transmit the request.

Do not process duplicated (i.e. re-transmitted) proxy requests.

Correct a couple of typos in example proxy code.

examples/coap-server.c

@@ -1110,7 +1110,7 @@ hnd_proxy_uri(coap_resource_t *resource COAP_UNUSED,
      */
     pdu = coap_pdu_init(req_type, req_code,
                         coap_new_message_id(ongoing),
-                        coap_session_max_pdu_size(session));
+                        coap_session_max_pdu_size(ongoing));
     if (!pdu) {
       coap_pdu_set_code(response, COAP_RESPONSE_CODE_INTERNAL_ERROR);
       goto cleanup;
@@ -1208,7 +1208,7 @@ add_in:
     coap_delete_optlist(optlist);
 
     if (size) {
-      if (!coap_add_data_large_request(session, pdu, size, data,
+      if (!coap_add_data_large_request(ongoing, pdu, size, data,
                                        release_proxy_body_data, body_data)) {
         coap_log(LOG_DEBUG, "cannot add data to proxy request\n");
       }

src/net.c

@@ -2600,6 +2600,7 @@ handle_request(coap_context_t *context, coap_session_t *session, coap_pdu_t *pdu
   int is_proxy_scheme = 0;
   int skip_hop_limit_check = 0;
   int resp;
+  int send_early_empty_ack = 0;
   coap_binary_t token = { pdu->token_length, pdu->token };
   coap_string_t *query = NULL;
   coap_opt_t *observe = NULL;
@@ -2900,6 +2901,24 @@ handle_request(coap_context_t *context, coap_session_t *session, coap_pdu_t *pdu
         }
       }
 
+      /* TODO for non-proxy requests */
+      if (resource == context->proxy_uri_resource &&
+          COAP_PROTO_NOT_RELIABLE(session->proto) &&
+          pdu->type == COAP_MESSAGE_CON) {
+        /* Make the proxy response separate and fix response later */
+        send_early_empty_ack = 1;
+      }
+      if (send_early_empty_ack) {
+        coap_send_ack(session, pdu);
+        if (pdu->mid == session->last_con_mid) {
+          /* request has already been processed - do not process it again */
+          coap_log(LOG_DEBUG,
+                   "Duplicate request with mid=0x%04x - not processed\n",
+                   pdu->mid);
+          goto drop_it_no_debug;
+        }
+        session->last_con_mid = pdu->mid;
+      }
       if (session->block_mode & COAP_BLOCK_USE_LIBCOAP) {
         if (coap_handle_request_put_block(context, session, pdu, response,
                                           resource, uri_path, observe,
@@ -2925,6 +2944,15 @@ handle_request(coap_context_t *context, coap_session_t *session, coap_pdu_t *pdu
       coap_check_code_lg_xmit(session, response, resource, query, pdu->code);
 
 skip_handler:
+      if (send_early_empty_ack &&
+          response->type == COAP_MESSAGE_ACK) {
+        /* Response is now separate - convert to CON as needed */
+        response->type = COAP_MESSAGE_CON;
+        /* Check for empty ACK - need to drop as already sent */
+        if (response->code == 0) {
+          goto drop_it_no_debug;
+        }
+      }
       respond = no_response(pdu, response, session, resource);
       if (respond != RESPONSE_DROP) {
         coap_mid_t mid = pdu->mid;
@@ -2969,6 +2997,7 @@ skip_handler:
             goto clean_up;
           }
           if (!coap_pdu_encode_header(response, session->proto)) {
+            coap_delete_node(node);
             goto clean_up;
           }
 
@@ -2995,6 +3024,7 @@ skip_handler:
                  coap_session_str(session),
                  response->mid);
         coap_show_pdu(LOG_DEBUG, response);
+drop_it_no_debug:
         coap_delete_pdu(response);
       }
 clean_up: