refactor(shared_sub): improve sticky strategy performance

* avoid calling ets:select twice for sticky strategy
* when the calling process is terminating, no loop-back dispatch

apps/emqx/src/emqx_shared_sub.erl

@@ -97,6 +97,7 @@
 -define(NACK(Reason), {shared_sub_nack, Reason}).
 -define(NO_ACK, no_ack).
 -define(REDISPATCH_TO(GROUP, TOPIC), {GROUP, TOPIC}).
+-define(SUBSCRIBER_DOWN, noproc).
 
 -type redispatch_to() :: ?REDISPATCH_TO(emqx_topic:group(), emqx_topic:topic()).
 
@@ -262,7 +263,8 @@ redispatch_shared_message(#message{} = Msg) ->
     %% Note that dispatch is called with self() in failed subs
     %% This is done to avoid dispatching back to caller
     Delivery = #delivery{sender = self(), message = Msg},
-    FailedSubs = #{self() => sender},
+    %% Self is terminating, it makes no sense to loop-back the dispatch
+    FailedSubs = #{self() => ?SUBSCRIBER_DOWN},
     dispatch(Group, Topic, Delivery, FailedSubs).
 
 %% @hidden Return the `redispatch_to` group-topic in the message header.
@@ -308,7 +310,7 @@ maybe_ack(Msg) ->
 
 pick(sticky, ClientId, SourceTopic, Group, Topic, FailedSubs) ->
     Sub0 = erlang:get({shared_sub_sticky, Group, Topic}),
-    All = subscribers(Group, Topic),
+    All = subscribers(Group, Topic, FailedSubs),
     case is_active_sub(Sub0, FailedSubs, All) of
         true ->
             %% the old subscriber is still alive
@@ -316,27 +318,25 @@ pick(sticky, ClientId, SourceTopic, Group, Topic, FailedSubs) ->
             {fresh, Sub0};
         false ->
             %% randomly pick one for the first message
-            {Type, Sub} = do_pick(
-                random,
-                ClientId,
-                SourceTopic,
-                Group,
-                Topic,
-                FailedSubs#{Sub0 => noproc}
-            ),
-            %% stick to whatever pick result
-            erlang:put({shared_sub_sticky, Group, Topic}, Sub),
-            {Type, Sub}
+            FailedSubs1 = FailedSubs#{Sub0 => ?SUBSCRIBER_DOWN},
+            Res = do_pick(All, random, ClientId, SourceTopic, Group, Topic, FailedSubs1),
+            case Res of
+                {_, Sub} ->
+                    %% stick to whatever pick result
+                    erlang:put({shared_sub_sticky, Group, Topic}, Sub);
+                _ ->
+                    ok
+            end,
+            Res
     end;
 pick(Strategy, ClientId, SourceTopic, Group, Topic, FailedSubs) ->
-    do_pick(Strategy, ClientId, SourceTopic, Group, Topic, FailedSubs).
+    All = subscribers(Group, Topic, FailedSubs),
+    do_pick(All, Strategy, ClientId, SourceTopic, Group, Topic, FailedSubs).
 
-do_pick(Strategy, ClientId, SourceTopic, Group, Topic, FailedSubs) ->
-    All = subscribers(Group, Topic),
+do_pick([], _Strategy, _ClientId, _SourceTopic, _Group, _Topic, _FailedSubs) ->
+    false;
+do_pick(All, Strategy, ClientId, SourceTopic, Group, Topic, FailedSubs) ->
     case lists:filter(fun(Sub) -> not maps:is_key(Sub, FailedSubs) end, All) of
-        [] when All =:= [] ->
-            %% Genuinely no subscriber
-            false;
         [] ->
             %% All offline? pick one anyway
             {retry, pick_subscriber(Group, Topic, Strategy, ClientId, SourceTopic, All)};
@@ -383,6 +383,16 @@ do_pick_subscriber(Group, Topic, round_robin_per_group, _ClientId, _SourceTopic,
         {Group, Topic}, 0
     }).
 
+%% Select ETS table to get all subscriber pids which are not down.
+subscribers(Group, Topic, FailedSubs) ->
+    lists:filter(
+        fun(P) ->
+            ?SUBSCRIBER_DOWN =/= maps:get(P, FailedSubs, false)
+        end,
+        subscribers(Group, Topic)
+    ).
+
+%% Select ETS table to get all subscriber pids.
 subscribers(Group, Topic) ->
     ets:select(?TAB, [{{emqx_shared_subscription, Group, Topic, '$1'}, [], ['$1']}]).
 
@@ -523,8 +533,8 @@ update_stats(State) ->
 %% Return 'true' if the subscriber process is alive AND not in the failed list
 is_active_sub(Pid, FailedSubs, All) ->
     lists:member(Pid, All) andalso
-        is_alive_sub(Pid) andalso
-        (not maps:is_key(Pid, FailedSubs)).
+        (not maps:is_key(Pid, FailedSubs)) andalso
+        is_alive_sub(Pid).
 
 %% erlang:is_process_alive/1 does not work with remote pid.
 is_alive_sub(Pid) when ?IS_LOCAL_PID(Pid) ->