feat(retainer): add option to stop publish clear message

apps/emqx_retainer/etc/emqx_retainer.conf

@@ -39,3 +39,11 @@ retainer.max_payload_size = 1MB
 ##
 ## Defaut: 0
 retainer.expiry_interval = 0
+
+## When the retained flag of the PUBLISH message is set and Payload is empty,
+## whether to continue to publish the message.
+## see: http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718038
+##
+## Value: Boolean
+## Default: false
+#retainer.stop_publish_clear_msg = false

apps/emqx_retainer/priv/emqx_retainer.schema

@@ -28,3 +28,10 @@
   {default, 0},
   {datatype, [integer, {duration, ms}]}
 ]}.
+
+%% Stop publish clear message
+%% {$configurable}
+{mapping, "retainer.stop_publish_clear_msg", "emqx_retainer.stop_publish_clear_msg", [
+  {default, false},
+  {datatype, {enum, [true, false]}}
+]}.

apps/emqx_retainer/src/emqx_retainer.erl

@@ -83,9 +83,14 @@ dispatch(Pid, Topic) ->
 %% RETAIN flag set to 1 and payload containing zero bytes
 on_message_publish(Msg = #message{flags   = #{retain := true},
                                   topic   = Topic,
-                                  payload = <<>>}, _Env) ->
+                                  payload = <<>>}, Env) ->
     mnesia:dirty_delete(?TAB, topic2tokens(Topic)),
-    {ok, Msg};
+    case stop_publish_clear_msg(Env) of
+        true ->
+            {ok, emqx_message:set_header(allow_publish, false, Msg)};
+        _ ->
+            {ok, Msg}
+    end;
 
 on_message_publish(Msg = #message{flags = #{retain := true}}, Env) ->
     Msg1 = emqx_message:set_header(retained, true, Msg),
@@ -224,6 +229,9 @@ store_retained(Msg = #message{topic = Topic, payload = Payload}, Env) ->
                         "for payload is too big!", [Topic, iolist_size(Payload)])
     end.
 
+stop_publish_clear_msg(Env) ->
+    proplists:get_bool(stop_publish_clear_msg, Env).
+
 is_table_full(Env) ->
     Limit = proplists:get_value(max_retained_messages, Env, 0),
     Limit > 0 andalso (retained_count() > Limit).