%%--------------------------------------------------------------------
%% Copyright (c) 2020-2021 EMQ Technologies Co., Ltd. All Rights Reserved.
%%
%% Licensed under the Apache License, Version 2.0 (the "License");
%% you may not use this file except in compliance with the License.
%% You may obtain a copy of the License at
%%
%%     http://www.apache.org/licenses/LICENSE-2.0
%%
%% Unless required by applicable law or agreed to in writing, software
%% distributed under the License is distributed on an "AS IS" BASIS,
%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
%% See the License for the specific language governing permissions and
%% limitations under the License.
%%--------------------------------------------------------------------

-module(emqx_congestion).

-export([ maybe_alarm_conn_congestion/3
        , cancel_alarms/3
        ]).

-define(ALARM_CONN_CONGEST(Channel, Reason),
        list_to_binary(
          io_lib:format("~s/~s/~s",
                        [Reason, emqx_channel:info(clientid, Channel),
                         maps:get(username, emqx_channel:info(clientinfo, Channel),
                                  <<"unknown_user">>)]))).

-define(ALARM_CONN_INFO_KEYS, [socktype, sockname, peername, clientid, username,
                               proto_name, proto_ver, connected_at, conn_state]).
-define(ALARM_SOCK_STATS_KEYS, [send_pend, recv_cnt, recv_oct, send_cnt, send_oct]).
-define(ALARM_SOCK_OPTS_KEYS, [high_watermark, high_msgq_watermark, sndbuf, recbuf, buffer]).
-define(PROC_INFO_KEYS, [message_queue_len, memory, reductions]).
-define(ALARM_SENT(REASON), {alarm_sent, REASON}).
-define(ALL_ALARM_REASONS, [conn_congestion]).
-define(WONT_CLEAR_IN, 60000).

maybe_alarm_conn_congestion(Socket, Transport, Channel) ->
    case is_alarm_enabled(Channel) of
        false -> ok;
        true ->
            case is_tcp_congested(Socket, Transport) of
                true -> alarm_congestion(Socket, Transport, Channel, conn_congestion);
                false -> cancel_alarm_congestion(Socket, Transport, Channel, conn_congestion)
            end
    end.

cancel_alarms(Socket, Transport, Channel) ->
    lists:foreach(fun(Reason) ->
        case has_alarm_sent(Reason) of
            true -> do_cancel_alarm_congestion(Socket, Transport, Channel, Reason);
            false -> ok
        end
    end, ?ALL_ALARM_REASONS).

is_alarm_enabled(Channel) ->
    emqx_zone:get_env(emqx_channel:info(zone, Channel),
        conn_congestion_alarm_enabled, false).

alarm_congestion(Socket, Transport, Channel, Reason) ->
    case has_alarm_sent(Reason) of
        false -> do_alarm_congestion(Socket, Transport, Channel, Reason);
        true ->
            %% pretend we have sent an alarm again
            update_alarm_sent_at(Reason)
    end.

cancel_alarm_congestion(Socket, Transport, Channel, Reason) ->
    Zone = emqx_channel:info(zone, Channel),
    WontClearIn = emqx_zone:get_env(Zone, conn_congestion_min_alarm_sustain_duration,
                    ?WONT_CLEAR_IN),
    case has_alarm_sent(Reason) andalso long_time_since_last_alarm(Reason, WontClearIn) of
        true -> do_cancel_alarm_congestion(Socket, Transport, Channel, Reason);
        false -> ok
    end.

do_alarm_congestion(Socket, Transport, Channel, Reason) ->
    ok = update_alarm_sent_at(Reason),
    AlarmDetails = tcp_congestion_alarm_details(Socket, Transport, Channel),
    emqx_alarm:activate(?ALARM_CONN_CONGEST(Channel, Reason), AlarmDetails),
    ok.

do_cancel_alarm_congestion(Socket, Transport, Channel, Reason) ->
    ok = remove_alarm_sent_at(Reason),
    AlarmDetails = tcp_congestion_alarm_details(Socket, Transport, Channel),
    emqx_alarm:deactivate(?ALARM_CONN_CONGEST(Channel, Reason), AlarmDetails),
    ok.

is_tcp_congested(Socket, Transport) ->
    case Transport:getstat(Socket, [send_pend]) of
        {ok, [{send_pend, N}]} when N > 0 -> true;
        _ -> false
    end.

has_alarm_sent(Reason) ->
    case get_alarm_sent_at(Reason) of
        0 -> false;
        _ -> true
    end.
update_alarm_sent_at(Reason) ->
    erlang:put(?ALARM_SENT(Reason), timenow()),
    ok.
remove_alarm_sent_at(Reason) ->
    erlang:erase(?ALARM_SENT(Reason)),
    ok.
get_alarm_sent_at(Reason) ->
    case erlang:get(?ALARM_SENT(Reason)) of
        undefined -> 0;
        LastSentAt -> LastSentAt
    end.
long_time_since_last_alarm(Reason, WontClearIn) ->
    %% only sent clears when the alarm was not triggered in the last
    %% WontClearIn time
    case timenow() - get_alarm_sent_at(Reason) of
        Elapse when Elapse >= WontClearIn -> true;
        _ -> false
    end.

timenow() ->
    erlang:system_time(millisecond).

%%==============================================================================
%% Alarm message
%%==============================================================================
tcp_congestion_alarm_details(Socket, Transport, Channel) ->
    ProcInfo = process_info(self(), ?PROC_INFO_KEYS),
    BasicInfo = [{pid, list_to_binary(pid_to_list(self()))} | ProcInfo],
    Stat = case Transport:getstat(Socket, ?ALARM_SOCK_STATS_KEYS) of
        {ok, Stat0} -> Stat0;
        {error, _} -> []
    end,
    Opts = case Transport:getopts(Socket, ?ALARM_SOCK_OPTS_KEYS) of
        {ok, Opts0} -> Opts0;
        {error, _} -> []
    end,
    SockInfo = Stat ++ Opts,
    ConnInfo = [conn_info(Key, Channel) || Key <- ?ALARM_CONN_INFO_KEYS],
    maps:from_list(BasicInfo ++ ConnInfo ++ SockInfo).

conn_info(Key, Channel) when Key =:= sockname; Key =:= peername ->
    {IPStr, Port} = emqx_channel:info(Key, Channel),
    {Key, iolist_to_binary([inet:ntoa(IPStr), ":", integer_to_list(Port)])};
conn_info(Key, Channel) ->
    {Key, emqx_channel:info(Key, Channel)}.