refactor(mqtt-worker): avoid unnecessary abstraction

So the code is easier to follow.

apps/emqx_connector/src/emqx_connector_mqtt.erl

@@ -198,8 +198,9 @@ on_query_async(_InstId, {send_message, Msg}, Callback, #{name := InstanceId}) ->
     ?TRACE("QUERY", "async_send_msg_to_remote_node", #{message => Msg, connector => InstanceId}),
     case emqx_connector_mqtt_worker:send_to_remote_async(InstanceId, Msg, Callback) of
         ok ->
-            % TODO this is racy
-            {ok, emqx_connector_mqtt_worker:pid(InstanceId)};
+            ok;
+        {ok, Pid} ->
+            {ok, Pid};
         {error, Reason} ->
             classify_error(Reason)
     end.

apps/emqx_connector/src/mqtt/emqx_connector_mqtt_worker.erl

@@ -67,8 +67,7 @@
 %% APIs
 -export([
     start_link/2,
-    stop/1,
-    pid/1
+    stop/1
 ]).
 
 %% management APIs
@@ -175,7 +174,7 @@ mk_client_event_handler(undefined, _Opts) ->
 
 connect(Name) ->
     #{subscriptions := Subscriptions} = get_config(Name),
-    case emqtt:connect(pid(Name)) of
+    case emqtt:connect(get_pid(Name)) of
         {ok, Properties} ->
             case subscribe_remote_topics(Name, Subscriptions) of
                 ok ->
@@ -206,37 +205,28 @@ subscribe_remote_topics(_Ref, undefined) ->
 stop(Ref) ->
     emqtt:stop(ref(Ref)).
 
-pid(Name) ->
-    gproc:lookup_pid(?NAME(Name)).
-
 status(Ref) ->
-    trycall(
-        fun() ->
-            Info = emqtt:info(ref(Ref)),
-            case proplists:get_value(socket, Info) of
-                Socket when Socket /= undefined ->
-                    connected;
-                undefined ->
-                    connecting
-            end
-        end,
-        #{noproc => disconnected}
-    ).
+    try
+        Info = emqtt:info(ref(Ref)),
+        case proplists:get_value(socket, Info) of
+            Socket when Socket /= undefined ->
+                connected;
+            undefined ->
+                connecting
+        end
+    catch
+        exit:{noproc, _} ->
+            disconnected
+    end.
 
 ping(Ref) ->
     emqtt:ping(ref(Ref)).
 
 send_to_remote(Name, MsgIn) ->
-    trycall(
-        fun() -> do_send(Name, export_msg(Name, MsgIn)) end,
-        #{
-            badarg => {error, disconnected},
-            noproc => {error, disconnected}
-        }
-    ).
+    trycall(fun() -> do_send(Name, export_msg(Name, MsgIn)) end).
 
 do_send(Name, {true, Msg}) ->
-    case emqtt:publish(pid(Name), Msg) of
+    case emqtt:publish(get_pid(Name), Msg) of
         ok ->
             ok;
         {ok, #{reason_code := RC}} when
@@ -263,13 +253,16 @@ do_send(_Name, false) ->
     ok.
 
 send_to_remote_async(Name, MsgIn, Callback) ->
-    trycall(
-        fun() -> do_send_async(Name, export_msg(Name, MsgIn), Callback) end,
-        #{badarg => {error, disconnected}}
-    ).
+    trycall(fun() -> do_send_async(Name, export_msg(Name, MsgIn), Callback) end).
 
 do_send_async(Name, {true, Msg}, Callback) ->
-    emqtt:publish_async(pid(Name), Msg, _Timeout = infinity, Callback);
+    Pid = get_pid(Name),
+    case emqtt:publish_async(Pid, Msg, _Timeout = infinity, Callback) of
+        ok ->
+            {ok, Pid};
+        {error, _} = Error ->
+            Error
+    end;
 do_send_async(_Name, false, _Callback) ->
     ok.
 
@@ -278,14 +271,14 @@ ref(Pid) when is_pid(Pid) ->
 ref(Term) ->
     ?REF(Term).
 
-trycall(Fun, Else) ->
+trycall(Fun) ->
     try
         Fun()
     catch
-        error:badarg ->
-            maps:get(badarg, Else);
+        throw:noproc ->
+            {error, disconnected};
         exit:{noproc, _} ->
-            maps:get(noproc, Else)
+            {error, disconnected}
     end.
 
 format_mountpoint(undefined) ->
@@ -325,8 +318,21 @@ pre_process_conf(Key, Conf) ->
             Conf#{Key => Val}
     end.
 
+get_pid(Name) ->
+    case gproc:where(?NAME(Name)) of
+        Pid when is_pid(Pid) ->
+            Pid;
+        undefined ->
+            throw(noproc)
+    end.
+
 get_config(Name) ->
-    gproc:lookup_value(?NAME(Name)).
+    try
+        gproc:lookup_value(?NAME(Name))
+    catch
+        error:badarg ->
+            throw(noproc)
+    end.
 
 export_msg(Name, Msg) ->
     case get_config(Name) of