%%% @doc Per-tunnel server session for HTTP/1.1 MASQUE (CONNECT-UDP).
%%%
%%% Spawned by the h1 request-handler after a `GET' with
%%% `Upgrade: connect-udp' passes validation. The session itself calls
%%% `h1:accept_upgrade/3' so that socket ownership lands on this
%%% process (the h1 connection transfers controlling_process to the
%%% caller of accept_upgrade).
%%%
%%% After the 101 response the raw TLS socket becomes the tunnel.
%%% Datagrams flow as RFC 9297 DATAGRAM capsules; the capsule wire
%%% format is identical to the h2 / h3 paths so `masque_datagram' and
%%% `masque_capsule' are reused unchanged.
-module(masque_h1_server_session).
-behaviour(gen_server).

-export([start_link/1]).

-export([init/1, handle_call/3, handle_cast/2, handle_info/2,
         terminate/2, code_change/3]).

-include("masque.hrl").

-record(state, {
    transport  :: gen_tcp | ssl,
    socket     :: ssl:sslsocket() | gen_tcp:socket(),
    handler    :: module(),
    h_state    :: term(),
    req        :: map(),
    cap_buf = <<>> :: binary(),
    max_cap    :: pos_integer(),
    idle_ms    :: non_neg_integer() | infinity,
    idle_ref   :: reference() | undefined
}).

%%====================================================================
%% API
%%====================================================================

-spec start_link(map()) -> {ok, pid()} | ignore | {error, term()}.
start_link(Args) ->
    gen_server:start_link(?MODULE, Args, []).

%%====================================================================
%% gen_server
%%====================================================================

init(#{conn := Conn, stream_id := StreamId,
       handler := Handler, handler_opts := HOpts, req := Req}) ->
    process_flag(trap_exit, true),
    MaxCap = maps:get(max_capsule_size, HOpts,
                      ?MASQUE_DEFAULT_MAX_CAPSULE_SIZE),
    IdleMs = maps:get(idle_timeout_ms, HOpts, 300000),
    %% Run the handler's init/2 first so a rejection surfaces as a
    %% clean 502 on the as-yet-unupgraded h1 connection. Only then
    %% call accept_upgrade, which writes 101 and transfers socket
    %% ownership to this process.
    case init_handler(Handler, Req, HOpts) of
        {ok, HState, Actions} ->
            case h1:accept_upgrade(Conn, StreamId,
                                    [{<<"capsule-protocol">>, <<"?1">>}]) of
                {ok, Socket, Buffer} ->
                    Transport = socket_transport(Socket),
                    State0 = arm_idle(#state{
                        transport = Transport,
                        socket    = Socket,
                        handler   = Handler,
                        h_state   = HState,
                        req       = Req,
                        cap_buf   = Buffer,
                        max_cap   = MaxCap,
                        idle_ms   = IdleMs
                    }),
                    %% Drain anything already past the 101 CRLF before
                    %% arming the socket.
                    case drain_and_arm(State0) of
                        {ok, State1} ->
                            apply_actions(Actions, State1);
                        {stop, Reason, _State} ->
                            _ = close_socket(State0),
                            {stop, Reason}
                    end;
                {error, Reason} ->
                    try_callback(Handler, terminate,
                                  [{accept_upgrade, Reason}, HState]),
                    {stop, {accept_upgrade, Reason}}
            end;
        {stop, Reason} ->
            {stop, Reason}
    end.

handle_call(_Req, _From, S) ->
    {reply, {error, unknown_call}, S}.

handle_cast(_Msg, S) ->
    {noreply, S}.

handle_info({ssl, Sock, Bytes}, #state{socket = Sock, cap_buf = Buf,
                                        max_cap = Max} = S) ->
    S1 = arm_idle(S),
    New = <<Buf/binary, Bytes/binary>>,
    case byte_size(New) > Max of
        true  -> {stop, capsule_buffer_overflow, S1};
        false -> step(S1#state{cap_buf = New})
    end;
handle_info({tcp, Sock, Bytes}, #state{socket = Sock, cap_buf = Buf,
                                        max_cap = Max} = S) ->
    S1 = arm_idle(S),
    New = <<Buf/binary, Bytes/binary>>,
    case byte_size(New) > Max of
        true  -> {stop, capsule_buffer_overflow, S1};
        false -> step(S1#state{cap_buf = New})
    end;
handle_info({timeout, Ref, idle},
            #state{idle_ref = Ref} = S) ->
    {stop, idle_timeout, S};
handle_info({ssl_closed, Sock}, #state{socket = Sock} = S) ->
    {stop, peer_closed, S};
handle_info({tcp_closed, Sock}, #state{socket = Sock} = S) ->
    {stop, peer_closed, S};
handle_info({ssl_error, Sock, Reason}, #state{socket = Sock} = S) ->
    {stop, {ssl_error, Reason}, S};
handle_info({tcp_error, Sock, Reason}, #state{socket = Sock} = S) ->
    {stop, {tcp_error, Reason}, S};
handle_info({'EXIT', _Pid, _Reason}, S) ->
    {noreply, S};
handle_info(Msg, S) ->
    dispatch(handle_info, [Msg], S).

terminate(_Reason, #state{handler = Handler, h_state = HState} = S) ->
    _ = cancel_idle(S),
    _ = close_socket(S),
    try_callback(Handler, terminate, [_Reason, HState]),
    ok.

code_change(_OldVsn, S, _Extra) ->
    {ok, S}.

%%====================================================================
%% Capsule decode loop
%%====================================================================

%% Drain everything buffered, then re-arm active-once. Used once at
%% startup (to consume bytes that arrived past the 101 CRLF) and after
%% every inbound {tcp|ssl, _, _} message.
step(#state{cap_buf = Buf} = S) ->
    case h1_capsule:decode(Buf) of
        {ok, {Type, Inner}, Rest} ->
            case dispatch_capsule(Type, Inner, S#state{cap_buf = Rest}) of
                {noreply, S2} -> step(S2);
                Stop          -> Stop
            end;
        {more, _} ->
            _ = arm_once(S),
            {noreply, S}
    end.

drain_and_arm(S) ->
    case step(S) of
        {noreply, S2}        -> {ok, S2};
        {stop, Reason, S2}   -> {stop, Reason, S2}
    end.

dispatch_capsule(datagram, Inner, S) ->
    case masque_datagram:decode(Inner) of
        {ok, {?MASQUE_CONTEXT_ID_UDP, UdpBytes}}
          when byte_size(UdpBytes) =< ?MASQUE_MAX_UDP_PAYLOAD ->
            dispatch(handle_packet, [UdpBytes], S);
        _ ->
            %% RFC 9298 §5: unknown context-id or oversize -> drop.
            {noreply, S}
    end;
dispatch_capsule(Type, Inner, S) when is_integer(Type) ->
    %% RFC 9297 §3.3 mandates silent-drop for unknown capsule types.
    %% `dispatch/3' preserves that: if the handler does not export
    %% `handle_capsule/3' it returns `{noreply, S}' unchanged. Handlers
    %% that do export it are treated as an extension hook.
    dispatch(handle_capsule, [Type, Inner], S).

%%====================================================================
%% Handler dispatch (mirrors masque_h2_server_session)
%%====================================================================

init_handler(Handler, Req, HOpts) ->
    case exported(Handler, init, 2) of
        true ->
            case safe_apply(Handler, init, [Req, HOpts]) of
                {ok, HState}          -> {ok, HState, []};
                {ok, HState, Actions} -> {ok, HState, Actions};
                {stop, Reason}        -> {stop, Reason};
                Other                 -> {stop, {bad_init, Other}}
            end;
        false ->
            {ok, undefined, []}
    end.

dispatch(CB, Extra, #state{handler = Handler, h_state = HS} = S) ->
    case exported(Handler, CB, length(Extra) + 1) of
        true ->
            case safe_apply(Handler, CB, Extra ++ [HS]) of
                {ok, HS2}           -> {noreply, S#state{h_state = HS2}};
                {ok, HS2, Actions}  -> apply_actions_noreply(
                                         Actions, S#state{h_state = HS2});
                {stop, Reason, HS2} -> {stop, Reason,
                                              S#state{h_state = HS2}};
                _                   -> {noreply, S}
            end;
        false ->
            {noreply, S}
    end.

exported(Mod, Fun, Arity) ->
    _ = code:ensure_loaded(Mod),
    erlang:function_exported(Mod, Fun, Arity).

apply_actions(Actions, State) ->
    case do_actions(Actions, State) of
        {ok, S2}          -> {ok, S2};
        {stop, Reason, _} -> {stop, Reason}
    end.

apply_actions_noreply(Actions, State) ->
    case do_actions(Actions, State) of
        {ok, S2}           -> {noreply, S2};
        {stop, Reason, S2} -> {stop, Reason, S2}
    end.

do_actions([], S) -> {ok, S};
do_actions([{send, Data} | Rest], S) ->
    do_actions([{send, ?MASQUE_CONTEXT_ID_UDP, Data} | Rest], S);
do_actions([{send, Ctx, Data} | Rest], S) ->
    PayloadSize = iolist_size(Data),
    case Ctx =:= ?MASQUE_CONTEXT_ID_UDP
         andalso PayloadSize > ?MASQUE_MAX_UDP_PAYLOAD of
        true ->
            do_actions(Rest, S);
        false ->
            Inner = iolist_to_binary(masque_datagram:encode(Ctx, Data)),
            _ = h1_upgrade:send_capsule(S#state.transport, S#state.socket,
                                         datagram, Inner),
            do_actions(Rest, S)
    end;
do_actions([{send_capsule, Type, Value} | Rest], S) ->
    _ = h1_upgrade:send_capsule(S#state.transport, S#state.socket,
                                 Type, Value),
    do_actions(Rest, S);
do_actions([close_session | _Rest], S) ->
    {stop, normal, S};
do_actions([{close_session, _Code, _Msg} | _Rest], S) ->
    {stop, normal, S};
do_actions([_Unknown | Rest], S) ->
    do_actions(Rest, S).

safe_apply(M, F, A) ->
    try apply(M, F, A)
    catch
        Class:Reason:Stack ->
            error_logger:error_msg(
                "masque h1 handler ~p:~p/~p failed: ~p:~p~n~p~n",
                [M, F, length(A), Class, Reason, Stack]),
            {stop, {handler_crash, Reason}}
    end.

try_callback(Mod, Fun, Args) ->
    Arity = length(Args),
    case erlang:function_exported(Mod, Fun, Arity) of
        true  -> (try apply(Mod, Fun, Args) catch _:_ -> ok end);
        false -> ok
    end.

%%====================================================================
%% Socket helpers
%%====================================================================

arm_once(#state{transport = ssl, socket = Sock}) ->
    ssl:setopts(Sock, [{active, once}, {mode, binary}]);
arm_once(#state{transport = gen_tcp, socket = Sock}) ->
    inet:setopts(Sock, [{active, once}, {mode, binary}]).

close_socket(#state{transport = T, socket = S}) ->
    close_transport(T, S).

close_transport(ssl, S)     -> try ssl:close(S) catch _:_ -> ok end;
close_transport(gen_tcp, S) -> try gen_tcp:close(S) catch _:_ -> ok end.

%% `h1:accept_upgrade/3' returns the raw socket without identifying the
%% transport. Infer it from the shape: ssl sockets are `#sslsocket{}'
%% records; gen_tcp sockets are ports (or nif socket records).
socket_transport(Socket) when is_tuple(Socket),
                               element(1, Socket) =:= sslsocket ->
    ssl;
socket_transport(_) ->
    gen_tcp.

%%====================================================================
%% Idle timer
%%====================================================================

arm_idle(#state{idle_ms = infinity} = S) -> S;
arm_idle(#state{idle_ms = 0}        = S) -> S;
arm_idle(#state{idle_ref = OldRef, idle_ms = Ms} = S) ->
    case OldRef of
        undefined -> ok;
        _         -> _ = erlang:cancel_timer(OldRef), ok
    end,
    Ref = erlang:start_timer(Ms, self(), idle),
    S#state{idle_ref = Ref}.

cancel_idle(#state{idle_ref = undefined}) -> ok;
cancel_idle(#state{idle_ref = Ref}) ->
    _ = erlang:cancel_timer(Ref),
    ok.