Persistent term optimisations (#148)

In `telemetry:attach_many/4`, we check if the function given to the
config is an external function and log noisy errors if it is not,
because the Erlang efficiency guide says these are more efficient.
Enforce the same for the `telemetry_handler_table` module at least for
the very hot path that looks up the handlers for an event. Also remove a
case-of from the execution path for another picosecond shave off.

Author: NelsonVides

src/telemetry_handler_table.erl

@@ -46,60 +46,49 @@ delete(HandlerId) ->
     gen_server:call(?MODULE, {delete, HandlerId}).
 
 persist() ->
-    {Mod, State} = impl_get(),
+    {Mod, _, State} = impl_get(),
     case Mod:persist(State) of
         {ok, NewState} ->
-            persistent_term:put(telemetry, {telemetry_pt, NewState}),
-            ok;
+            ListForEventFun = fun telemetry_pt:list_for_event/2,
+            persistent_term:put(telemetry, {telemetry_pt, ListForEventFun, NewState});
         _ ->
             ok
     end.
 
-impl_get() -> persistent_term:get(telemetry, undefined).
+impl_get() ->
+    persistent_term:get(telemetry, default_impl()).
 
 -spec list_for_event(telemetry:event_name()) -> [#handler{}].
 list_for_event(EventName) ->
-    case impl_get() of
-        {Mod, State} ->
-            Mod:list_for_event(State, EventName);
-        undefined ->
-            ?LOG_WARNING("Failed to lookup telemetry handlers. "
-                         "Ensure the telemetry application has been started. ", []),
-            []
-    end.
+    {_Mod, ListForEventFun, State} = impl_get(),
+    ListForEventFun(State, EventName).
 
 -spec list_by_prefix(telemetry:event_prefix()) -> [#handler{}].
 list_by_prefix(EventPrefix) ->
-    case impl_get() of
-        {Mod, State} ->
-            Mod:list_by_prefix(State, EventPrefix);
-        undefined ->
-            ?LOG_WARNING("Failed to lookup telemetry handlers. "
-                         "Ensure the telemetry application has been started. ", []),
-            []
-    end.
+    {Mod, _ListForEventFun, State} = impl_get(),
+    Mod:list_by_prefix(State, EventPrefix).
 
 init([]) ->
+    process_flag(trap_exit, true),
     TID = create_table(),
-
-    persistent_term:put(telemetry, {telemetry_ets, TID}),
-
+    ListForEventFun = fun telemetry_ets:list_for_event/2,
+    persistent_term:put(telemetry, {telemetry_ets, ListForEventFun, TID}),
     {ok, []}.
 
 handle_call({insert, HandlerId, EventNames, Function, Config}, _From, State) ->
-    {Mod, MState} = impl_get(),
+    {Mod, ListForEventFun, MState} = impl_get(),
     case Mod:insert(MState, HandlerId, EventNames, Function, Config) of
         {ok, NewState} ->
-            persistent_term:put(telemetry, {Mod, NewState}),
+            persistent_term:put(telemetry, {Mod, ListForEventFun, NewState}),
             {reply, ok, State};
         {error, _} = Error ->
             {reply, Error, State}
     end;
 handle_call({delete, HandlerId}, _From, State) ->
-    {Mod, MState} = impl_get(),
+    {Mod, ListForEventFun, MState} = impl_get(),
     case Mod:delete(MState, HandlerId) of
         {ok, NewState} ->
-            persistent_term:put(telemetry, {Mod, NewState}),
+            persistent_term:put(telemetry, {Mod, ListForEventFun, NewState}),
             {reply, ok, State};
         {error, _} = Error ->
             {reply, Error, State}
@@ -120,6 +109,10 @@ terminate(_Reason, _State) ->
 
 %%
 
+default_impl() ->
+    ListForEventFun = fun(_, _) -> [] end,
+    {telemetry_ets, ListForEventFun, #{}}.
+
 create_table() ->
     ets:new(?MODULE, [duplicate_bag, protected,
                       {keypos, #handler.event_name}, {read_concurrency, true}]).

src/telemetry_pt.erl

@@ -29,14 +29,15 @@ starts_with(_Haystack, _Needle) -> false.
 insert(Map, _HandlerId, [], _Function, _Config) ->
     {ok, Map};
 insert(Map, HandlerId, [EventName | Rest], Function, Config) ->
-    Handler = #handler{id=HandlerId,
-                       event_name=EventName,
-                       function=Function,
-                       config=Config},
     OldHandlers = maps:get(EventName, Map, []),
     case OldHandlers of
-        #{HandlerId := _} -> {error, already_exists};
+        #{HandlerId := _} ->
+            {error, already_exists};
         _ ->
+            Handler = #handler{id=HandlerId,
+                               event_name=EventName,
+                               function=Function,
+                               config=Config},
             case put_new(Handler, OldHandlers) of
                 {ok, NewHandlers} ->
                     NewMap = Map#{EventName => NewHandlers},