maybe_trace.erl

% works with https://gist.github.com/davisp/626acba57bb0f6da84a1d0de4d7237c3

maybe_trace(MochiReq) ->
    case MochiReq:get_header_value("x-couchdb-trace") of
        "true" ->
            case is_tracer_running() of
                true ->
                    couch_log:info("Tracer updating scope with request pid:~p", [self()]),
                    % https://github.com/rabbitmq/looking_glass/blob/master/src/lg.erl#L125
                    Children = supervisor:which_children(looking_glass_sup),
                    {default, PoolPid, supervisor, _} = lists:keyfind(default, 1, Children),
                    Tracers = lg_tracer_pool:tracers(PoolPid),
                    TracersMap = maps:from_list(lists:zip(lists:seq(0, length(Tracers) - 1), Tracers)),
                    TracerState = #{mode => profile, tracers => TracersMap},
                    TraceFlags = [call, procs, timestamp, arity, return_to, set_on_spawn],
                    erlang:trace(self(), true, [{tracer, lg_tracer, TracerState} | TraceFlags]);
                false ->
                    couch_log:info("Trace initializing...", []),
                    lg:trace(['_', {scope, [self()]}], lg_file_tracer, "traces.lz4", #{mode => profile})
            end;
        "false" ->
            case is_tracer_running() of false -> ok; true ->
                couch_log:info("Trace stopping...", []),
                lg:stop()
            end;
        _ ->
            ok
    end.


is_tracer_running() ->
   case whereis(looking_glass_sup) of
        undefined ->
            false;
        Pid when is_pid(Pid) ->
            Children = supervisor:which_children(Pid),
            lists:keymember(default, 1, Children)
   end.