Fix bug where server sends a 3rd detach.

If the server initiates the detach due to an error condition, it
destroys and therefore forgets the link.

This should be okay because accroding to section 2.6.5:
"When an error occurs at a link endpoint, the endpoint MUST be detached
with appropriate error information supplied in the error field of the detach
frame. The link endpoint MUST then be destroyed."

It is also valid that the client replies with a detach:
"If any input (other than a detach) related to the endpoint either via
the input handle or delivery-ids be received, the session MUST be
terminated with an errant-link session-error."

In this case, the server must not reply again with (i.e do not sent a 3rd)
detach.

Author: ansd

deps/rabbit/src/rabbit_amqp_session.erl

@@ -1079,17 +1079,14 @@ handle_control(#'v1_0.flow'{handle = Handle} = Flow,
         end,
     {noreply, S};
 
-handle_control(#'v1_0.detach'{handle = Handle = ?UINT(HandleInt),
-                              closed = Closed},
+handle_control(Detach = #'v1_0.detach'{handle = ?UINT(HandleInt)},
                State0 = #state{queue_states = QStates0,
                                incoming_links = IncomingLinks,
                                outgoing_links = OutgoingLinks0,
                                outgoing_unsettled_map = Unsettled0,
                                cfg = #cfg{
-                                        writer_pid = WriterPid,
                                         vhost = Vhost,
-                                        user = #user{username = Username},
-                                        channel_num = Ch}}) ->
+                                        user = #user{username = Username}}}) ->
     Ctag = handle_to_ctag(HandleInt),
     %% TODO delete queue if closed flag is set to true? see 2.6.6
     %% TODO keep the state around depending on the lifetime
@@ -1145,8 +1142,7 @@ handle_control(#'v1_0.detach'{handle = Handle = ?UINT(HandleInt),
                          incoming_links = maps:remove(HandleInt, IncomingLinks),
                          outgoing_links = OutgoingLinks,
                          outgoing_unsettled_map = Unsettled},
-    rabbit_amqp_writer:send_command(WriterPid, Ch, #'v1_0.detach'{handle = Handle,
-                                                                  closed = Closed}),
+    maybe_detach_reply(Detach, State, State0),
     publisher_or_consumer_deleted(State, State0),
     {noreply, State};
 
@@ -2457,6 +2453,22 @@ publisher_or_consumer_deleted(
            ok
     end.
 
+%% If we previously already sent a detach with an error condition, and the Detach we
+%% receive here is therefore the client's reply, do not reply again with a 3rd detach.
+maybe_detach_reply(Detach,
+                   #state{incoming_links = NewIncomingLinks,
+                          outgoing_links = NewOutgoingLinks,
+                          cfg = #cfg{writer_pid = WriterPid,
+                                     channel_num = Ch}},
+                   #state{incoming_links = OldIncomingLinks,
+                          outgoing_links = OldOutgoingLinks})
+  when map_size(NewIncomingLinks) < map_size(OldIncomingLinks) orelse
+       map_size(NewOutgoingLinks) < map_size(OldOutgoingLinks) ->
+    Reply = Detach#'v1_0.detach'{error = undefined},
+    rabbit_amqp_writer:send_command(WriterPid, Ch, Reply);
+maybe_detach_reply(_, _, _) ->
+    ok.
+
 check_internal_exchange(#exchange{internal = true,
                                   name = XName}) ->
     protocol_error(?V_1_0_AMQP_ERROR_UNAUTHORIZED_ACCESS,