don't conditionally/auto-delete exchanges that still have durable bindings

src/rabbit_exchange.erl

@@ -319,15 +319,13 @@ exchanges_for_queue(QueueName) ->
       sets:from_list(
         mnesia:select(reverse_route, [{MatchHead, [], ['$1']}]))).
 
-has_bindings(ExchangeName) ->
-    MatchHead = #route{binding = #binding{exchange_name = ExchangeName,
-                                          _ = '_'}},
+contains(Table, MatchHead) ->
     try
-        continue(mnesia:select(route, [{MatchHead, [], ['$_']}], 1, read))
+        continue(mnesia:select(Table, [{MatchHead, [], ['$_']}], 1, read))
     catch exit:{aborted, {badarg, _}} ->
             %% work around OTP-7025, which was fixed in R12B-1, by
             %% falling back on a less efficient method
-            case mnesia:match_object(MatchHead) of
+            case mnesia:match_object(Table, MatchHead, read) of
                 []    -> false;
                 [_|_] -> true
             end
@@ -471,7 +469,11 @@ maybe_auto_delete(Exchange = #exchange{auto_delete = true}) ->
     ok.
 
 conditional_delete(Exchange = #exchange{name = ExchangeName}) ->
-    case has_bindings(ExchangeName) of
+    Match = #route{binding = #binding{exchange_name = ExchangeName, _ = '_'}},
+    %% we need to check for durable routes here too in case a bunch of
+    %% routes to durable queues have been removed temporarily as a
+    %% result of a node failure
+    case contains(route, Match) orelse contains(durable_routes, Match) of
         false  -> unconditional_delete(Exchange);
         true   -> {error, in_use}
     end.