Introduce x-first-death-{exchange,queue,reason}

They can be used to route dead lettered messages using a headers exchange.

Part of #1332.

Author: michaelklishin

src/rabbit_dead_letter.erl

@@ -113,13 +113,22 @@ group_by_queue_and_reason(Tables) ->
     Grouped.
 
 update_x_death_header(Info, Headers) ->
+    X = x_death_event_key(Info, <<"exchange">>),
     Q = x_death_event_key(Info, <<"queue">>),
     R = x_death_event_key(Info, <<"reason">>),
     case rabbit_basic:header(<<"x-death">>, Headers) of
         undefined ->
+            %% First x-death event gets its own top-level headers.
+            %% See rabbitmq/rabbitmq-server#1332.
+            Headers2 = rabbit_misc:set_table_value(Headers, <<"x-first-death-reason">>,
+                                                   longstr, R),
+            Headers3 = rabbit_misc:set_table_value(Headers2, <<"x-first-death-queue">>,
+                                                   longstr, Q),
+            Headers4 = rabbit_misc:set_table_value(Headers3, <<"x-first-death-exchange">>,
+                                                   longstr, X),
             rabbit_basic:prepend_table_header(
               <<"x-death">>,
-              [{<<"count">>, long, 1} | Info], Headers);
+              [{<<"count">>, long, 1} | Info], Headers4);
         {<<"x-death">>, array, Tables} ->
             %% group existing x-death headers in case we have some from
             %% before rabbitmq-server#78