fix(shared_sub): failed to clean the emqx_shared_subscription tab

A trick that fixes the issue that we demonitored the shared subscriber
too early if it not unsubscribed all of the topics.

apps/emqx/src/emqx_shared_sub.erl

@@ -336,9 +336,13 @@ handle_info({mnesia_table_event, {write, NewRecord, _}}, State = #state{pmon = P
     #emqx_shared_subscription{subpid = SubPid} = NewRecord,
     {noreply, update_stats(State#state{pmon = emqx_pmon:monitor(SubPid, PMon)})};
 
-handle_info({mnesia_table_event, {delete_object, OldRecord, _}}, State = #state{pmon = PMon}) ->
-    #emqx_shared_subscription{subpid = SubPid} = OldRecord,
-    {noreply, update_stats(State#state{pmon = emqx_pmon:demonitor(SubPid, PMon)})};
+%% The subscriber may have subscribed multiple topics, so we need to keep monitoring the PID until
+%% it `unsubscribed` the last topic.
+%% The trick is we don't demonitor the subscriber here, and (after a long time) it will eventually
+%% be disconnected.
+% handle_info({mnesia_table_event, {delete_object, OldRecord, _}}, State = #state{pmon = PMon}) ->
+%     #emqx_shared_subscription{subpid = SubPid} = OldRecord,
+%     {noreply, update_stats(State#state{pmon = emqx_pmon:demonitor(SubPid, PMon)})};
 
 handle_info({mnesia_table_event, _Event}, State) ->
     {noreply, State};