fix(emqx_retainer): Dialyzer warnings

apps/emqx_retainer/src/emqx_retainer.erl

@@ -56,8 +56,9 @@
 %%--------------------------------------------------------------------
 
 load(Env) ->
-    emqx:hook('session.subscribed', fun ?MODULE:on_session_subscribed/3, []),
-    emqx:hook('message.publish', fun ?MODULE:on_message_publish/2, [Env]).
+    _ = emqx:hook('session.subscribed', fun ?MODULE:on_session_subscribed/3, []),
+    _ = emqx:hook('message.publish', fun ?MODULE:on_message_publish/2, [Env]),
+    ok.
 
 unload() ->
     emqx:unhook('message.publish', fun ?MODULE:on_message_publish/2),
@@ -169,15 +170,17 @@ handle_info(stats, State = #state{stats_fun = StatsFun}) ->
     {noreply, State, hibernate};
 
 handle_info(expire, State) ->
-    expire_messages(),
+    ok = expire_messages(),
     {noreply, State, hibernate};
 
 handle_info(Info, State) ->
     ?LOG(error, "Unexpected info: ~p", [Info]),
     {noreply, State}.
 
-terminate(_Reason, _State = #state{stats_timer = TRef1, expiry_timer = TRef2}) ->
-    timer:cancel(TRef1), timer:cancel(TRef2).
+terminate(_Reason, #state{stats_timer = TRef1, expiry_timer = TRef2} = State) ->
+    _ = timer:cancel(TRef1),
+    _ = timer:cancel(TRef2),
+    ok.
 
 code_change(_OldVsn, State, _Extra) ->
     {ok, State}.
@@ -248,11 +251,12 @@ expire_messages() ->
     NowMs = erlang:system_time(millisecond),
     MsHd = #retained{topic = '$1', msg = '_', expiry_time = '$3'},
     Ms = [{MsHd, [{'=/=','$3',0}, {'<','$3',NowMs}], ['$1']}],
-    mnesia:transaction(
+    {atomic, _} = mnesia:transaction(
         fun() ->
             Keys = mnesia:select(?TAB, Ms, write),
             lists:foreach(fun(Key) -> mnesia:delete({?TAB, Key}) end, Keys)
-        end).
+        end),
+    ok.
 
 -spec(read_messages(emqx_types:topic())
       -> [emqx_types:message()]).