Keep the state returned by the tracer callback

It was mistakenly discarded.

src/cowboy_tracer_h.erl

@@ -144,19 +144,19 @@ tracer_process(StreamID, Req=#{pid := Parent}, Opts=#{tracer_callback := Fun}) -
 	State = Fun(init, {StreamID, Req, Opts}),
 	tracer_loop(Parent, Fun, State).
 
-tracer_loop(Parent, Fun, State) ->
+tracer_loop(Parent, Fun, State0) ->
 	receive
 		Msg when element(1, Msg) =:= trace_ts ->
-			Fun(Msg, State),
+			State = Fun(Msg, State0),
 			tracer_loop(Parent, Fun, State);
 		{'EXIT', Parent, Reason} ->
-			tracer_terminate(Reason, Fun, State);
+			tracer_terminate(Reason, Fun, State0);
 		{system, From, Request} ->
-			sys:handle_system_msg(Request, From, Parent, ?MODULE, [], {Fun, State});
+			sys:handle_system_msg(Request, From, Parent, ?MODULE, [], {Fun, State0});
 		Msg ->
 			error_logger:error_msg("~p: Tracer process received stray message ~9999p~n",
 				[?MODULE, Msg]),
-			tracer_loop(Parent, Fun, State)
+			tracer_loop(Parent, Fun, State0)
 	end.
 
 tracer_terminate(Reason, Fun, State) ->

test/tracer_SUITE.erl

@@ -98,12 +98,13 @@ do_tracer_callback(Pid) ->
 	fun
 		(Event, _) when Event =:= init; Event =:= terminate ->
 			Pid ! Event,
-			undefined;
+			0;
 		(Event={trace_ts, _, call, {cowboy_req, reply, _}, _}, State) ->
 			Pid ! Event,
-			State;
+			Pid ! {state, State},
+			State + 1;
 		(_, State) ->
-			State
+			State + 1
 	end.
 
 %% Tests.
@@ -140,6 +141,23 @@ terminate(Config) ->
 		error(timeout)
 	end.
 
+state(Config) ->
+	doc("Ensure the returned state is used."),
+	Ref = config(ref, Config),
+	Opts = ranch:get_protocol_options(Ref),
+	ranch:set_protocol_options(Ref, Opts#{
+		tracer_callback => do_tracer_callback(self()),
+		tracer_match_specs => [fun(_,_,_) -> true end]
+	}),
+	do_get("/", Config),
+	receive
+		{state, St} ->
+			true = St > 0,
+			ok
+	after 100 ->
+		error(timeout)
+	end.
+
 empty(Config) ->
 	doc("Empty match specs unconditionally enable tracing."),
 	Ref = config(ref, Config),