refactor(hook): Add emqx_hooks:put to hide already_exists error

This also fixes dialyzer warnings in emqx_mod_presence

src/emqx_hooks.erl

@@ -31,6 +31,7 @@
 -export([ add/2
         , add/3
         , add/4
+        , put/2
         , del/2
         , run/2
         , run_fold/3
@@ -111,6 +112,14 @@ add(HookPoint, Action, Priority) when is_integer(Priority) ->
 add(HookPoint, Action, Filter, Priority) when is_integer(Priority) ->
     add(HookPoint, #callback{action = Action, filter = Filter, priority = Priority}).
 
+%% @doc Like add/2, it register a callback, discard 'already_exists' error.
+-spec(put(hookpoint(), action() | #callback{}) -> ok).
+put(HookPoint, Callback) ->
+    case add(HookPoint, Callback) of
+        ok -> ok;
+        {error, already_exists} -> ok
+    end.
+
 %% @doc Unregister a callback.
 -spec(del(hookpoint(), function() | {module(), atom()}) -> ok).
 del(HookPoint, Action) ->

src/emqx_mod_presence.erl

@@ -38,8 +38,8 @@
 -endif.
 
 load(Env) ->
-    emqx_hooks:add('client.connected',    {?MODULE, on_client_connected, [Env]}),
-    emqx_hooks:add('client.disconnected', {?MODULE, on_client_disconnected, [Env]}).
+    emqx_hooks:put('client.connected',    {?MODULE, on_client_connected, [Env]}),
+    emqx_hooks:put('client.disconnected', {?MODULE, on_client_disconnected, [Env]}).
 
 unload(_Env) ->
     emqx_hooks:del('client.connected',    {?MODULE, on_client_connected}),