CQ: Confirm related optimisations (#7516)

* CQ: Optimise shared store remove when nothing to remove

The message(s) was/were already optimised out during write
when the client acked faster than we could process the
write message.

* Optimise sets:subtract in single-confirm case

This happens due to message store optimisations that tries
to confirm as fast as possible on write or ack, even if that
means processing a single confirm. The ack scenario is common
because clients tend to not be built for multi-ack.

The optimisation avoids calling an expensive sets:subtract/2
when there is a single new confirm and instead does a
sets:del_element/2 of the first set.

(cherry picked from commit 0dce848)

Author: Loïc Hoguin

deps/rabbit/src/rabbit_msg_store.erl

@@ -936,8 +936,11 @@ handle_cast({remove, CRef, MsgIds}, State) ->
                       ignore  -> {Removed, State2}
                   end
           end, {[], State}, MsgIds),
-    noreply(maybe_compact(client_confirm(CRef, sets:from_list(RemovedMsgIds, [{version, 2}]),
-                                         ignored, State1)));
+    case RemovedMsgIds of
+        [] -> noreply(State1);
+        _  -> noreply(maybe_compact(client_confirm(CRef, sets:from_list(RemovedMsgIds, [{version, 2}]),
+                                         ignored, State1)))
+    end;
 
 handle_cast({combine_files, Source, Destination, Reclaimed},
             State = #msstate { sum_file_size    = SumFileSize,
@@ -1372,7 +1375,7 @@ client_confirm(CRef, MsgIds, ActionTaken, State) ->
               case maps:find(CRef, CTM) of
                   {ok, Gs} -> MsgOnDiskFun(sets:intersection(Gs, MsgIds),
                                            ActionTaken),
-                              MsgIds1 = sets:subtract(Gs, MsgIds),
+                              MsgIds1 = sets_subtract(Gs, MsgIds),
                               case sets:is_empty(MsgIds1) of
                                   true  -> maps:remove(CRef, CTM);
                                   false -> maps:put(CRef, MsgIds1, CTM)
@@ -1381,6 +1384,13 @@ client_confirm(CRef, MsgIds, ActionTaken, State) ->
               end
       end, CRef, State).
 
+%% Function defined in both rabbit_msg_store and rabbit_variable_queue.
+sets_subtract(Set1, Set2) ->
+    case sets:size(Set2) of
+        1 -> sets:del_element(hd(sets:to_list(Set2)), Set1);
+        _ -> sets:subtract(Set1, Set2)
+    end.
+
 blind_confirm(CRef, MsgIds, ActionTaken, State) ->
     update_pending_confirms(
       fun (MsgOnDiskFun, CTM) -> MsgOnDiskFun(MsgIds, ActionTaken), CTM end,

deps/rabbit/src/rabbit_variable_queue.erl

@@ -2355,11 +2355,18 @@ record_confirms(MsgIdSet, State = #vqstate { msgs_on_disk        = MOD,
                                              unconfirmed         = UC,
                                              confirmed           = C }) ->
     State #vqstate {
-      msgs_on_disk        = sets:subtract(MOD,  MsgIdSet),
-      msg_indices_on_disk = sets:subtract(MIOD, MsgIdSet),
-      unconfirmed         = sets:subtract(UC,   MsgIdSet),
+      msgs_on_disk        = sets_subtract(MOD,  MsgIdSet),
+      msg_indices_on_disk = sets_subtract(MIOD, MsgIdSet),
+      unconfirmed         = sets_subtract(UC,   MsgIdSet),
       confirmed           = sets:union(C, MsgIdSet) }.
 
+%% Function defined in both rabbit_msg_store and rabbit_variable_queue.
+sets_subtract(Set1, Set2) ->
+    case sets:size(Set2) of
+        1 -> sets:del_element(hd(sets:to_list(Set2)), Set1);
+        _ -> sets:subtract(Set1, Set2)
+    end.
+
 msgs_written_to_disk(Callback, MsgIdSet, ignored) ->
     Callback(?MODULE,
              fun (?MODULE, State) -> record_confirms(MsgIdSet, State) end);