fix: dashboard_monitor crash

apps/emqx/src/emqx_metrics.erl

@@ -328,14 +328,14 @@ all() ->
     ].
 
 %% @doc Get metric value
--spec val(metric_name()) -> maybe(non_neg_integer()).
+-spec val(metric_name()) -> non_neg_integer().
 val(Name) ->
     case ets:lookup(?TAB, Name) of
         [#metric{idx = Idx}] ->
             CRef = persistent_term:get(?MODULE),
             counters:get(CRef, Idx);
         [] ->
-            undefined
+            0
     end.
 
 %% @doc Increase counter

apps/emqx/src/emqx_stats.erl

@@ -164,11 +164,11 @@ getstats() ->
     end.
 
 %% @doc Get stats by name.
--spec getstat(atom()) -> maybe(non_neg_integer()).
+-spec getstat(atom()) -> non_neg_integer().
 getstat(Name) ->
     case ets:lookup(?TAB, Name) of
         [{Name, Val}] -> Val;
-        [] -> undefined
+        [] -> 0
     end.
 
 %% @doc Set stats

apps/emqx_dashboard/src/emqx_dashboard_monitor.erl

@@ -242,6 +242,9 @@ format(Data) ->
 format(TimeStamp, Data, All) ->
     [Data#{time_stamp => TimeStamp} | All].
 
+cal_rate(_Now, undefined) ->
+    AllSamples = ?GAUGE_SAMPLER_LIST ++ maps:keys(?DELTA_SAMPLER_RATE_MAP),
+    {ok, lists:foldl(fun(Key, Acc) -> Acc#{Key => 0} end, #{}, AllSamples)};
 cal_rate( #emqx_monit{data = NowData, time = NowTime}
         , #emqx_monit{data = LastData, time = LastTime}) ->
     TimeDelta = NowTime - LastTime,

apps/emqx_dashboard/test/emqx_dashboard_monitor_SUITE.erl

@@ -102,6 +102,15 @@ t_monitor_current_api(_) ->
         || Key <- maps:values(?DELTA_SAMPLER_RATE_MAP) ++ ?GAUGE_SAMPLER_LIST],
     ok.
 
+t_monitor_reset(_) ->
+    restart_monitor(),
+    {ok, Rate} = request(["monitor_current"]),
+    [?assert(maps:is_key(atom_to_binary(Key, utf8), Rate))
+        || Key <- maps:values(?DELTA_SAMPLER_RATE_MAP) ++ ?GAUGE_SAMPLER_LIST],
+    {ok, Samplers} = request(["monitor"], "latest=1"),
+    ?assertEqual(1, erlang:length(Samplers)),
+    ok.
+
 t_monitor_api_error(_) ->
     {error, {400, #{<<"code">> := <<"BAD_RPC">>}}} =
         request(["monitor", "nodes", 'emqx@127.0.0.2']),
@@ -147,3 +156,16 @@ do_request_api(Method, Request)->
 auth_header_() ->
     Basic = binary_to_list(base64:encode(<<"admin:public">>)),
     {"Authorization", "Basic " ++ Basic}.
+
+restart_monitor() ->
+    erlang:exit(erlang:whereis(emqx_dashboard_monitor), killed),
+    ?assertEqual(ok, wait_new_monitor(10)).
+
+wait_new_monitor(Count) when Count =< 0 -> timeout;
+wait_new_monitor(Count) ->
+    case is_pid(erlang:whereis(emqx_dashboard_monitor)) of
+        true -> ok;
+        false ->
+            timer:sleep(100),
+            wait_new_monitor(Count - 1)
+    end.