Merge pull request #13206 from emqx/0607-patch-qlc-leak-for-5.6

fix(retainer): fix qlc cursor cleanup

apps/emqx_retainer/src/emqx_retainer_dispatcher.erl

@@ -255,6 +255,8 @@ deliver([], Context, Pid, Topic, Cursor, Limiter) ->
 deliver(Result, Context, Pid, Topic, Cursor, Limiter) ->
     case erlang:is_process_alive(Pid) of
         false ->
+            ok = close_cursor(Cursor),
+            ?tp(debug, retainer_dispatcher_no_receiver, #{topic => Topic}),
             {ok, Limiter};
         _ ->
             DeliverNum = emqx_conf:get([retainer, flow_control, batch_deliver_number], undefined),
@@ -272,6 +274,11 @@ deliver(Result, Context, Pid, Topic, Cursor, Limiter) ->
             end
     end.
 
+close_cursor({{qlc_cursor, _} = Cursor, _}) ->
+    qlc:delete_cursor(Cursor);
+close_cursor(_Cursor) ->
+    ok.
+
 do_deliver([], _DeliverNum, _Pid, _Topic, Limiter) ->
     {ok, Limiter};
 do_deliver(Msgs, DeliverNum, Pid, Topic, Limiter) ->

apps/emqx_retainer/test/emqx_retainer_SUITE.erl

@@ -21,6 +21,7 @@
 
 -include("emqx_retainer.hrl").
 
+-include_lib("emqx/include/asserts.hrl").
 -include_lib("eunit/include/eunit.hrl").
 -include_lib("common_test/include/ct.hrl").
 -include_lib("snabbkaffe/include/snabbkaffe.hrl").
@@ -461,6 +462,66 @@ t_flow_control(_) ->
     }),
     ok.
 
+t_cursor_cleanup(_) ->
+    Rate = emqx_ratelimiter_SUITE:to_rate("1/1s"),
+    LimiterCfg = make_limiter_cfg(Rate),
+    JsonCfg = make_limiter_json(<<"1/1s">>),
+    emqx_limiter_server:add_bucket(emqx_retainer, internal, LimiterCfg),
+    emqx_retainer:update_config(#{
+        <<"delivery_rate">> => <<"1/1s">>,
+        <<"flow_control">> =>
+            #{
+                <<"batch_read_number">> => 1,
+                <<"batch_deliver_number">> => 1,
+                <<"batch_deliver_limiter">> => JsonCfg
+            }
+    }),
+    {ok, C1} = emqtt:start_link([{clean_start, true}, {proto_ver, v5}]),
+    {ok, _} = emqtt:connect(C1),
+    lists:foreach(
+        fun(I) ->
+            emqtt:publish(
+                C1,
+                <<"retained/", (integer_to_binary(I))/binary>>,
+                <<"this is a retained message">>,
+                [{qos, 0}, {retain, true}]
+            )
+        end,
+        lists:seq(1, 5)
+    ),
+    {ok, #{}, [0]} = emqtt:subscribe(C1, <<"retained/#">>, [{qos, 0}, {rh, 0}]),
+
+    snabbkaffe:start_trace(),
+
+    ?assertWaitEvent(
+        emqtt:disconnect(C1),
+        #{?snk_kind := retainer_dispatcher_no_receiver, topic := <<"retained/#">>},
+        2000
+    ),
+
+    QLCProcesses = lists:filter(
+        fun(Pid) ->
+            {current_function, {qlc, wait_for_request, 3}} =:=
+                erlang:process_info(Pid, current_function)
+        end,
+        erlang:processes()
+    ),
+
+    ?assertEqual(0, length(QLCProcesses)),
+
+    snabbkaffe:stop(),
+
+    emqx_limiter_server:del_bucket(emqx_retainer, internal),
+    emqx_retainer:update_config(#{
+        <<"flow_control">> =>
+            #{
+                <<"batch_read_number">> => 1,
+                <<"batch_deliver_number">> => 1
+            }
+    }),
+
+    ok.
+
 t_clear_expired(_) ->
     ConfMod = fun(Conf) ->
         Conf#{