%%
%% %CopyrightBegin%
%%
%% Copyright Ericsson AB 2010-2024. 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.
%%
%% %CopyrightEnd%
%%

-module(diameter_sctp).
-moduledoc """
Diameter transport over SCTP.

This module implements diameter transport over SCTP using `m:gen_sctp`. It can
be specified as the value of a transport_module option to
`diameter:add_transport/2` and implements the behaviour documented in
`m:diameter_transport`.

[](){: #start }

## SEE ALSO

`m:diameter`, `m:diameter_transport`, `m:gen_sctp`, `m:inet`
""".
-moduledoc(#{since => "OTP R14B03"}).
-behaviour(gen_server).

%% interface
-export([start/3]).

%% child start from supervisor
-export([start_link/1]).

%% child start from here
-export([init/1]).

%% gen_server callbacks
-export([handle_call/3,
         handle_cast/2,
         handle_info/2,
         code_change/3,
         terminate/2]).

-export([listener/1,%% diameter_sync callback
         info/1]).  %% service_info callback

-export([ports/0,
         ports/1]).

-export_type([listen_option/0,
              connect_option/0]).

-include_lib("kernel/include/inet_sctp.hrl").
-include_lib("diameter/include/diameter.hrl").

%% Keys into process dictionary.
-define(INFO_KEY, info).
-define(REF_KEY,  ref).
-define(TRANSPORT_KEY, transport).

-define(ERROR(T), erlang:error({T, ?MODULE, ?LINE})).

%% The default port for a listener.
-define(DEFAULT_PORT, 3868).  %% RFC 3588, ch 2.1

%% How long to wait for a transport process to attach after
%% association establishment.
-define(ACCEPT_TIMEOUT, 5000).

-type connect_option() :: {raddr, inet:ip_address()}
                        | {rport, inet:port_number()}
                        | option()
                        | term(). %% gen_sctp:open_option().

-type match() :: inet:ip_address()
               | string()
               | [match()].

-type listen_option() :: {accept, match()}
                       | option()
                       | term().  %% gen_sctp:open_option().

-type option() :: {sender, boolean()}
                | sender
                | {packet, boolean() | raw}
                | {message_cb, false | diameter:eval()}.

-type uint() :: non_neg_integer().

%% Accepting/connecting transport process state.
-record(transport,
        {parent  :: pid() | undefined,
         mode :: {accept, pid()}
               | accept
               | {connect, {[inet:ip_address()], uint(), list()}}
                        %% {RAs, RP, Errors}
               | connect,
         socket   :: gen_sctp:sctp_socket() | undefined,
         active = false :: boolean(),      %% is socket active?
         recv = true    :: boolean(),      %% should it be active?
         assoc_id :: gen_sctp:assoc_id()   %% association identifier
                   | undefined
                   | true,
         peer     :: {[inet:ip_address()], uint()} %% {RAs, RP}
                   | undefined,
         streams  :: {uint(), uint()}      %% {InStream, OutStream} counts
                   | undefined,
         os = 0   :: uint(),               %% next output stream
         rotate = 1 :: boolean() | 0 | 1,  %% rotate os?
         unordered = false :: boolean()    %% always send unordered?
                            | pos_integer(),% or if =< N outbound streams?
         packet = true :: boolean()        %% legacy transport_data?
                        | raw,
         message_cb = false :: false | diameter:eval(),
         send = false :: pid() | boolean()}).      %% sending process

%% Monitor process state.
-record(monitor,
        {transport :: pid(),
         ack = false :: boolean(),
         socket :: gen_sctp:sctp_socket(),
         assoc_id :: gen_sctp:assoc_id()}).

%% Listener process state.
-record(listener,
        {ref       :: reference(),
         socket    :: gen_sctp:sctp_socket(),
         service   :: pid(), %% service process
         pending = {0, queue:new()},
         opts      :: [[match()] | boolean() | diameter:eval()]}).
%% Field pending implements two queues: the first of transport-to-be
%% processes to which an association has been assigned but for which
%% diameter hasn't yet spawned a transport process, a short-lived
%% state of affairs as a new transport is spawned as a consequence of
%% a peer being taken up, transport processes being spawned by the
%% listener on demand; the second of started transport processes that
%% have not yet been assigned an association.
%%
%% When diameter calls start/3, the transport process is either taken
%% from the first queue or spawned and placed in the second queue
%% until an association is established. When an association is
%% established, a controlling process is either taken from the second
%% queue or spawned and placed in the first queue. Thus, there are
%% only elements in one queue at a time, so share an ets table queue
%% and tag it with a positive length if it contains the first queue, a
%% negative length if it contains the second queue.

%% ---------------------------------------------------------------------------
%% # start/3
%% ---------------------------------------------------------------------------

-doc """
The start function required by `m:diameter_transport`.

Options `raddr` and `rport` specify the remote address and port for a connecting
transport and not valid for a listening transport: the former is required while
latter defaults to 3868 if unspecified. Multiple `raddr` options can be
specified, in which case the connecting transport in question attempts each in
sequence until an association is established.

Option `accept` specifies remote addresses for a listening transport and is not
valid for a connecting transport. If specified, a remote address that does not
match one of the specified addresses causes the association to be aborted.
Multiple `accept` options can be specified. A string-valued `Match` that does
not parse as an address is interpreted as a regular expression.

Option `unordered` specifies whether or not to use unordered delivery, integer
`N` being equivalent to `N =< OS`, where `OS` is the number of outbound streams
negotiated on the association in question. Regardless of configuration, sending
is ordered on stream 0 until reception of a second incoming message, to ensure
that a peer receives capabilities exchange messages before any other. Defaults
to `false`.

Option `packet` determines how/if an incoming message is packaged into a
diameter_packet record. If `false` then messages are received as binary(). If
`true` then as a record with the binary() message in the `bin` field and a
`{stream, Id}` tuple in the `transport_data` field, where `Id` is the identifier
of the inbound stream the message was received on. If `raw` then as a record
with the received ancillary sctp_sndrcvinfo record in the `transport_data`
field. Defaults to `true`.

Options `message_cb` and `sender` have semantics identical to those documented
in [diameter_tcp(3)](`m:diameter_tcp#sender`), but with the message argument to
a `recv` callback being as directed by the `packet` option.

An `{outstream, Id}` tuple in the `transport_data` field of a outgoing
diameter_packet record sets the outbound stream on which the message is sent,
modulo the negotiated number of outbound streams. Any other value causes
successive such sends to cycle though all outbound streams.

Remaining options are any accepted by `gen_sctp:open/1`, with the exception of
options `mode`, `binary`, `list`, `active` and `sctp_events`. Note that options
`ip` and `port` specify the local address and port respectively.

Multiple `ip` options can be specified for a multihomed peer. If none are
specified then the values of `Host-IP-Address` in the `diameter_service` record
are used. Option `port` defaults to 3868 for a listening transport and 0 for a
connecting transport.

> #### Warning {: .warning }
>
> An small receive buffer may result in a peer having to resend incoming
> messages: set the `m:inet` option `recbuf` to increase the buffer size.
>
> An small send buffer may result in outgoing messages being discarded: set the
> `m:inet` option `sndbuf` to increase the buffer size.
""".
-doc(#{since => <<"OTP R14B03">>}).
-spec start({accept, Ref}, #diameter_service{}, [listen_option()])
   -> {ok, pid(), [inet:ip_address()]}
 when Ref :: diameter:transport_ref();
           ({connect, Ref}, #diameter_service{}, [connect_option()])
   -> {ok, pid(), [inet:ip_address()]}
 when Ref :: diameter:transport_ref().

start(TypeRef, Svc, Options)
  when is_list(Options) ->
    #diameter_service{capabilities = Caps,
                      pid = Pid}
        = Svc,
    diameter_sctp_sup:start(),  %% start supervisors on demand
    Addrs = Caps#diameter_caps.host_ip_address,
    s(TypeRef, Addrs, Pid, Options).

%% A listener spawns transports either as a consequence of this call
%% when there is not yet an association to assign it, or at comm_up on
%% a new association in which case the call retrieves a transport from
%% the pending queue.
s({accept, Ref} = A, Addrs, SvcPid, Opts) ->
    {ok, LPid, LAs} = listener(Ref, {Opts, SvcPid, Addrs}),
    try gen_server:call(LPid, {A, self()}, infinity) of
        {ok, TPid} ->
            {ok, TPid, LAs};
        No ->
            {error, No}
    catch
        exit: Reason ->
            {error, Reason}
    end;
%% This implementation is due to there being no accept call in
%% gen_sctp in order to be able to accept a new association only
%% *after* an accepting transport has been spawned.

s({connect = C, Ref}, Addrs, _SvcPid, Opts) ->
    diameter_sctp_sup:start_child({C, self(), Opts, Addrs, Ref}).

%% start_link/1

-doc false.
start_link(T) ->
    proc_lib:start_link(?MODULE,
                        init,
                        [T],
                        infinity,
                        diameter_lib:spawn_opts(server, [])).

%% ---------------------------------------------------------------------------
%% # info/1
%% ---------------------------------------------------------------------------

-doc false.
info({gen_sctp, Sock}) ->
    lists:flatmap(fun(K) -> info(K, Sock) end,
                  [{socket, socknames},
                   {peer, peernames},
                   {statistics, getstat}]).

info({K,F}, Sock) ->
    case inet:F(Sock) of
        {ok, V} ->
            [{K, map(F,V)}];
        _ ->
            []
    end.

%% inet:{sock,peer}names/1 returns [{Addr, Port}] but the port number
%% should be the same in each tuple. Map to a {[Addr], Port} tuple if
%% so.
map(K, [{_, Port} | _] = APs)
  when K == socknames;
       K == peernames ->
    try [A || {A,P} <- APs, P == Port orelse throw(?MODULE)] of
        As -> {As, Port}
    catch
        ?MODULE -> APs
    end;

map(_, V) ->
    V.

%% ---------------------------------------------------------------------------
%% # init/1
%% ---------------------------------------------------------------------------

-doc false.
init(T) ->
    gen_server:enter_loop(?MODULE, [], i(T)).

%% i/1

i(#monitor{transport = TPid} = S) ->
    monitor(process, TPid),
    putr(?TRANSPORT_KEY, TPid),
    proc_lib:init_ack({ok, self()}),
    S;

%% A process owning a listening socket.
i({listen, Ref, {Opts, SvcPid, Addrs}}) ->
    monitor(process, SvcPid),
    [_] = diameter_config:subscribe(Ref, transport), %% assert existence
    {Split, Rest} = proplists:split(Opts, [accept,
                                           packet,
                                           sender,
                                           message_cb,
                                           unordered]),
    OwnOpts = lists:append(Split),
    {LAs, Sock} = AS = open(Addrs, Rest, ?DEFAULT_PORT),
    ok = gen_sctp:listen(Sock, true),
    true = diameter_reg:add_new({?MODULE, listener, {Ref, AS}}),
    proc_lib:init_ack({ok, self(), LAs}),
    #listener{ref = Ref,
              service = SvcPid,
              socket = Sock,
              opts = [[[M] || {accept, M} <- OwnOpts],
                      proplists:get_value(packet, OwnOpts, true)
                      | [proplists:get_value(K, OwnOpts, false)
                         || K <- [sender, message_cb, unordered]]]};

%% A connecting transport.
i({connect, Pid, Opts, Addrs, Ref}) ->
    {[Ps | Split], Rest} = proplists:split(Opts, [rport,
                                                  raddr,
                                                  packet,
                                                  sender,
                                                  message_cb,
                                                  unordered]),
    OwnOpts = lists:append(Split),
    CB = proplists:get_value(message_cb, OwnOpts, false),
    false == CB orelse (Pid ! {diameter, ack}),
    RAs  = [diameter_lib:ipaddr(A) || {raddr, A} <- OwnOpts],
    [RP] = [P || {rport, P} <- Ps] ++ [P || P <- [?DEFAULT_PORT], [] == Ps],
    {LAs, Sock} = open(Addrs, Rest, 0),
    putr(?REF_KEY, Ref),
    proc_lib:init_ack({ok, self(), LAs}),
    monitor(process, Pid),
    #transport{parent = Pid,
               mode = {connect, connect(Sock, RAs, RP, [])},
               socket = Sock,
               message_cb = CB,
               unordered = proplists:get_value(unordered, OwnOpts, false),
               packet = proplists:get_value(packet, OwnOpts, true),
               send = proplists:get_value(sender, OwnOpts, false)};

%% An accepting transport spawned by diameter, not yet owning an
%% association.
i({accept, Ref, LPid, Pid})
  when is_pid(Pid) ->
    putr(?REF_KEY, Ref),
    proc_lib:init_ack({ok, self()}),
    monitor(process, Pid),
    MRef = monitor(process, LPid),
    wait([{peeloff, MRef}], #transport{parent = Pid,
                                       mode = {accept, LPid}});

%% An accepting transport spawned at association establishment, whose
%% parent is not yet known.
i({accept, Ref, LPid}) ->
    putr(?REF_KEY, Ref),
    proc_lib:init_ack({ok, self()}),
    erlang:send_after(?ACCEPT_TIMEOUT, self(), accept_timeout),
    MRef = monitor(process, LPid),
    wait([{parent, Ref}, {peeloff, MRef}], #transport{mode = {accept, LPid}}).

%% wait/2
%%
%% Wait for diameter to start the transport process and for the
%% association to be peeled off before processing other messages.

wait(Keys, S) ->
    lists:foldl(fun i/2, S, Keys).

i({K, Ref}, #transport{mode = {accept, _}} = S) ->
    receive
        {Ref, Pid} when K == parent ->  %% transport process started
            S#transport{parent = Pid};
        {K, T, Opts} when K == peeloff ->  %% association
            {sctp, Sock, _RA, _RP, _Data} = T,
            [Matches, Packet, Sender, CB, Unordered] = Opts,
            ok = accept_peer(Sock, Matches),
            demonitor(Ref, [flush]),
            false == CB orelse (S#transport.parent ! {diameter, ack}),
            t(T, S#transport{socket = Sock,
                             message_cb = CB,
                             unordered = Unordered,
                             packet = Packet,
                             send = Sender});
        accept_timeout = T ->
            x(T);
        {'DOWN', _, process, _, _} = T ->
            x(T)
    end.

%% listener/2

%% Accepting processes can be started concurrently: ensure only one
%% listener is started.
listener(Ref, T) ->
    diameter_sync:call({?MODULE, listener, Ref},
                       {?MODULE, listener, [{Ref, T}]},
                       infinity,
                       infinity).

-doc false.
listener({Ref, T}) ->
    l(diameter_reg:match({?MODULE, listener, {Ref, '_'}}), Ref, T).

%% Existing listening process ...
l([{{?MODULE, listener, {_, AS}}, LPid}], _, _) ->
    {LAs, _Sock} = AS,
    {ok, LPid, LAs};

%% ... or not.
l([], Ref, T) ->
    diameter_sctp_sup:start_child({listen, Ref, T}).

%% open/3

open(Addrs, Opts, PortNr) ->
    case gen_sctp:open(gen_opts(portnr(addrs(Addrs, Opts), PortNr))) of
        {ok, Sock} ->
            {addrs(Sock), Sock};
        {error, Reason} ->
            x({open, Reason})
    end.

addrs(Addrs, Opts) ->
    case lists:mapfoldl(fun ipaddr/2, false, Opts) of
        {Os, true} ->
            Os;
        {_, false} ->
            Opts ++ [{ip, A} || A <- Addrs]
    end.

ipaddr({K,A}, _)
  when K == ifaddr;
       K == ip ->
    {{ip, ipaddr(A)}, true};
ipaddr(T, B) ->
    {T, B}.

ipaddr(A)
  when A == loopback;
       A == any ->
    A;
ipaddr(A) ->
    diameter_lib:ipaddr(A).

portnr(Opts, PortNr) ->
    case proplists:get_value(port, Opts) of
        undefined ->
            [{port, PortNr} | Opts];
        _ ->
            Opts
    end.

addrs(Sock) ->
    case inet:socknames(Sock) of
        {ok, As} ->
            [A || {A,_} <- As];
        {error, Reason} ->
            x({socknames, Reason})
    end.

%% x/1

x(Reason) ->
    exit({shutdown, Reason}).

%% gen_opts/1

gen_opts(Opts) ->
    {L,_} = proplists:split(Opts, [binary, list, mode, active, sctp_events]),
    [[],[],[],[],[]] == L orelse ?ERROR({reserved_options, Opts}),
    [binary, {active, once} | Opts].

%% ---------------------------------------------------------------------------
%% # ports/0-1
%% ---------------------------------------------------------------------------

-doc false.
ports() ->
    Ts = diameter_reg:match({?MODULE, '_', '_'}),
    [{type(T), N, Pid} || {{?MODULE, T, {_, {_, S}}}, Pid} <- Ts,
                          {ok, N} <- [inet:port(S)]].

-doc false.
ports(Ref) ->
    Ts = diameter_reg:match({?MODULE, '_', {Ref, '_'}}),
    [{type(T), N, Pid} || {{?MODULE, T, {R, {_, S}}}, Pid} <- Ts,
                          R == Ref,
                          {ok, N} <- [inet:port(S)]].

type(listener) ->
    listen;
type(T) ->
    T.

%% ---------------------------------------------------------------------------
%% # handle_call/3
%% ---------------------------------------------------------------------------

-doc false.
handle_call({{accept, Ref}, Pid}, _, #listener{ref = Ref} = S) ->
    {TPid, NewS} = accept(Ref, Pid, S),
    {reply, {ok, TPid}, NewS};

%% Transport is telling us of parent death.
handle_call({stop, _Pid} = Reason, _From, #monitor{} = S) ->
    {stop, {shutdown, Reason}, ok, S};

handle_call(_, _, State) ->
    {reply, nok, State}.

%% ---------------------------------------------------------------------------
%% # handle_cast/2
%% ---------------------------------------------------------------------------

-doc false.
handle_cast(_, State) ->
    {noreply, State}.

%% ---------------------------------------------------------------------------
%% # handle_info/2
%% ---------------------------------------------------------------------------

-doc false.
handle_info(T, #transport{} = S) ->
    {noreply, #transport{} = t(T,S)};

handle_info(T, #listener{} = S) ->
    {noreply, #listener{} = l(T,S)};

handle_info(T, #monitor{} = S) ->
    m(T,S),
    {noreply, S}.

%% Prior to the possibility of setting pool_size on in transport
%% configuration, a new accepting transport was only started following
%% the death of a predecessor, so that there was only at most one
%% previously started transport process waiting for an association.
%% This assumption no longer holds with pool_size > 1, in which case
%% several accepting transports are started concurrently. Deal with
%% this by placing the started transports in a new queue of transport
%% processes waiting for an association.

%% ---------------------------------------------------------------------------
%% # code_change/3
%% ---------------------------------------------------------------------------

-doc false.
code_change(_, State, _) ->
    {ok, State}.

%% ---------------------------------------------------------------------------
%% # terminate/2
%% ---------------------------------------------------------------------------

-doc false.
terminate(_, #monitor{}) ->
    ok;

terminate(_, #transport{assoc_id = undefined}) ->
    ok;

terminate(_, #transport{socket = Sock}) ->
    gen_sctp:close(Sock);

terminate(_, #listener{socket = Sock}) ->
    gen_sctp:close(Sock).

%% ---------------------------------------------------------------------------

putr(Key, Val) ->
    put({?MODULE, Key}, Val).

getr(Key) ->
    get({?MODULE, Key}).

%% l/2
%%
%% Transition listener state.

%% Incoming message from SCTP.
l({sctp, Sock, _RA, _RP, Data} = T, #listener{socket = Sock,
                                              opts = Opts}
                                    = S) ->
    Id = assoc_id(Data),
    {TPid, NewS} = accept(S),
    TPid ! {peeloff, setelement(2, T, peeloff(Sock, Id, TPid)), Opts},
    setopts(Sock),
    NewS;

%% Service process has died.
l({'DOWN', _, process, Pid, _} = T, #listener{service = Pid,
                                              socket = Sock}) ->
    gen_sctp:close(Sock),
    x(T);

%% Accepting process has died.
l({'DOWN', _MRef, process, TPid, _}, #listener{pending = {_,Q}} = S) ->
    down(queue:member(TPid, Q), TPid, S);

%% Transport has been removed.
l({transport, remove, _} = T, #listener{socket = Sock}) ->
    gen_sctp:close(Sock),
    x(T).

%% down/3
%%
%% Accepting transport has died.

%% One that's waiting for transport start in the pending queue ...
down(true, TPid, #listener{pending = {N,Q}} = S) ->
    NQ = queue:filter(fun(P) -> P /= TPid end, Q),
    if N < 0 ->  %% awaiting an association ...
            S#listener{pending = {N+1, NQ}};
       true ->   %% ... or one has been assigned
            S#listener{pending = {N-1, NQ}}
    end;

%% ... or one that's already attached.
down(false, _TPid, S) ->
    S.

%% t/2
%%
%% Transition transport state.

t(T,S) ->
    case transition(T,S) of
        ok ->
            S;
        #transport{} = NS ->
            NS;
        stop ->
            x(T)
    end.

%% transition/2

%% Incoming message.
transition({sctp, Sock, _RA, _RP, Data}, #transport{socket = Sock} = S) ->
    setopts(S, recv(Data, S#transport{active = false}));

%% Outgoing message.
transition({diameter, {send, Msg}}, S) ->
    message(send, Msg, S);

%% Monitor has sent an outgoing message.
transition(Msg, S)
  when is_record(Msg, diameter_packet);
       is_binary(Msg) ->
    message(ack, Msg, S);

%% Deferred actions from a message_cb.
transition({actions, Dir, Acts}, S) ->
    setopts(ok, actions(Acts, Dir, S));

%% Request to close the transport connection.
transition({diameter, {close, Pid}}, #transport{parent = Pid}) ->
    stop;

%% TLS over SCTP is described in RFC 3436 but has limitations as
%% described in RFC 6083. The latter describes DTLS over SCTP, which
%% addresses these limitations, DTLS itself being described in RFC
%% 4347. TLS is primarily used over TCP, which RFC 6733 acknowledges
%% by equating TLS with TLS/TCP and DTLS/SCTP.
transition({diameter, {tls, _Ref, _Type, _Bool}}, _) ->
    stop;

%% Parent process has died: call the monitor to not close the socket
%% during an ongoing send, but don't let it take forever.
transition({'DOWN', _, process, Pid, _}, #transport{parent = Pid,
                                                    send = MPid}) ->
    is_boolean(MPid)
        orelse ok == (catch gen_server:call(MPid, {stop, Pid}))
        orelse exit(MPid, kill),
    stop;

%% Monitor process has died.
transition({'DOWN', _, process, MPid, _}, #transport{send = MPid})
  when is_pid(MPid) ->
    stop;

%% Timeout after transport process has been started.
transition(accept_timeout, _) ->
    ok;

%% Request for the local port number.
transition({resolve_port, Pid}, #transport{socket = Sock})
  when is_pid(Pid) ->
    Pid ! inet:port(Sock),
    ok.

%% m/2

m({Msg, StreamId}, #monitor{socket = Sock,
                            transport = TPid,
                            assoc_id = AId,
                            ack = B}) ->
    send(Sock, AId, StreamId, Msg),
    B andalso (TPid ! Msg);

m({'DOWN', _, process, TPid, _} = T, #monitor{transport = TPid}) ->
    x(T).

%% Crash on anything unexpected.

ok({ok, T}) ->
    T;
ok(T) ->
    x(T).

%% accept_peer/2

accept_peer(_, []) ->
    ok;

accept_peer(Sock, Matches) ->
    RAddrs = [A || {A,_} <- ok(inet:peernames(Sock))],
    diameter_peer:match(RAddrs, Matches)
        orelse x({accept, RAddrs, Matches}),
    ok.

%% accept/3
%%
%% Start a new transport process or use one that's already been
%% started as a consequence of diameter requesting a transport
%% process.

accept(Ref, Pid, #listener{pending = {N,_}} = S) ->
    {TPid, NQ} = q(Ref, Pid, S),
    {TPid, S#listener{pending = {N-1, NQ}}}.

%% Pending associations: attach to the first in the queue.
q(_, Pid, #listener{ref = Ref,
                    pending = {N,Q}})
  when 0 < N ->
    {TPid, _} = T = dq(Q),
    TPid ! {Ref, Pid},
    T;

%% No pending associations: spawn a new transport.
q(Ref, Pid, #listener{pending = {_,Q}}) ->
    nq({accept, Ref, self(), Pid}, Q).

%% send/2

%% Start monitor process on first send.
send(Msg, #transport{send = true,
                     socket = Sock,
                     assoc_id = AId,
                     message_cb = CB}
          = S) ->
    {ok, MPid} = diameter_sctp_sup:start_child(#monitor{transport = self(),
                                                        socket = Sock,
                                                        assoc_id = AId,
                                                        ack = false /= CB}),
    monitor(process, MPid),
    send(Msg, S#transport{send = MPid});

%% Outbound Diameter message on a specified stream ...
send(#diameter_packet{transport_data = {outstream, SId}}
     = Msg,
     #transport{streams = {_, OS}}
     = S) ->
    send(SId rem OS, Msg, S);

%% ... or not: rotate when sending on multiple streams ...
send(Msg, #transport{rotate = true,
                     streams = {_, OS},
                     os = N}
          = S) ->
    send(N, Msg, S#transport{os = (N + 1) rem OS});

%% ... or send on the only stream available.
send(Msg, S) ->
    send(0, Msg, S).

%% send/3

send(StreamId, Msg, #transport{send = false,
                               socket = Sock,
                               assoc_id = AId}
                    = S) ->
    send(Sock, AId, StreamId, Msg),
    message(ack, Msg, S);

send(StreamId, Msg, #transport{send = MPid} = S) ->
    MPid ! {Msg, StreamId},
    S.

%% send/4

send(Sock, AssocId, StreamId, #diameter_packet{bin = Bin}) ->
    send(Sock, AssocId, StreamId, Bin);

send(Sock, AssocId, StreamId, Bin) ->
    case gen_sctp:send(Sock, AssocId, StreamId, Bin) of
        ok ->
            ok;
        {error, Reason} ->
            x({send, Reason})
    end.

%% recv/2

%% Association established ...
recv({_, #sctp_assoc_change{state = comm_up,
                            outbound_streams = OS,
                            inbound_streams = IS,
                            assoc_id = Id}},
     #transport{assoc_id = undefined,
                mode = {T, _},
                socket = Sock}
     = S) ->
    Ref = getr(?REF_KEY),
    publish(T, Ref, Id, Sock),
    %% Deal with different association id after peeloff on Solaris by
    %% taking the id from the first reception.
    up(S#transport{assoc_id = T == accept orelse Id,
                   streams = {IS, OS}});

%% ... or not: try the next address.
recv({_, #sctp_assoc_change{} = E},
     #transport{assoc_id = undefined,
                socket = Sock,
                mode = {connect = C, {[RA|RAs], RP, Es}}}
     = S) ->
    S#transport{mode = {C, connect(Sock, RAs, RP, [{RA,E} | Es])}};

%% Association failure.
recv({_, #sctp_assoc_change{}}, _) ->
    stop;

%% First inbound on an accepting transport.
recv({[#sctp_sndrcvinfo{assoc_id = Id}], _Bin}
     = T,
     #transport{assoc_id = true}
     = S) ->
    recv(T, S#transport{assoc_id = Id});

%% Inbound Diameter message.
recv({[#sctp_sndrcvinfo{}], Bin} = Msg, S)
  when is_binary(Bin) ->
    message(recv, Msg, recv(S));

recv({_, #sctp_shutdown_event{}}, _) ->
    stop;

%% Note that diameter_sctp(3) documents that sctp_events cannot be
%% specified in the list of options passed to gen_sctp and that
%% gen_opts/1 guards against this. This is to ensure that we know what
%% events to expect and also to ensure that we receive
%% #sctp_sndrcvinfo{} with each incoming message (data_io_event =
%% true). Adaptation layer events (ie. #sctp_adaptation_event{}) are
%% disabled by default so don't handle it. We could simply disable
%% events we don't react to but don't.

recv({_, #sctp_paddr_change{}}, _) ->
    ok;

recv({_, #sctp_pdapi_event{}}, _) ->
    ok.

%% recv/1
%%
%% Start sending unordered after the second reception, so that an
%% outgoing CER/CEA will arrive at the peer before another request.

recv(#transport{rotate = B} = S)
  when is_boolean(B) ->
    S;

recv(#transport{rotate = 0,
                streams = {_,OS},
                socket = Sock,
                assoc_id = Id,
                unordered = B}
     = S) ->
    ok = unordered(Sock, Id, OS, B),
    S#transport{rotate = 1 < OS};

recv(#transport{rotate = N} = S) ->
    S#transport{rotate = N-1}.

%% unordered/4

unordered(Sock, Id, OS, B)
  when B;
       is_integer(B), OS =< B ->
    inet:setopts(Sock, [{sctp_default_send_param,
                         #sctp_sndrcvinfo{flags = [unordered],
                                          assoc_id = Id}}]);

unordered(_, _, OS, B)
  when not B;
       is_integer(B), B < OS ->
    ok.

%% publish/4

publish(T, Ref, Id, Sock) ->
    true = diameter_reg:add_new({?MODULE, T, {Ref, {Id, Sock}}}),
    putr(?INFO_KEY, {gen_sctp, Sock}).  %% for info/1

%% up/1

up(#transport{parent = Pid,
              mode = {connect = C, {[RA | _], RP, _}}}
   = S) ->
    diameter_peer:up(Pid, {RA,RP}),
    S#transport{mode = C};

up(#transport{parent = Pid,
              mode = {accept = A, _}}
   = S) ->
    diameter_peer:up(Pid),
    S#transport{mode = A}.

%% accept/1
%%
%% Start a new transport process or use one that's already been
%% started as a consequence of an event to a listener process.

accept(#listener{pending = {N,_}} = S) ->
    {TPid, NQ} = q(S),
    {TPid, S#listener{pending = {N+1, NQ}}}.

%% Transport waiting for an association: use it.
q(#listener{pending = {N,Q}})
  when N < 0 ->
    dq(Q);

%% No transport start yet: spawn one and queue.
q(#listener{ref = Ref,
            pending = {_,Q}}) ->
    nq({accept, Ref, self()}, Q).

%% nq/2
%%
%% Place a transport process in the second pending queue to make it
%% available to the next association.

nq(Arg, Q) ->
    {ok, TPid} = diameter_sctp_sup:start_child(Arg),
    monitor(process, TPid),
    {TPid, queue:in(TPid, Q)}.

%% dq/1
%%
%% Remove a transport process from the first pending queue to assign
%% it to an existing association.

dq(Q) ->
    {{value, TPid}, NQ} = queue:out(Q),
    {TPid, NQ}.

%% assoc_id/1
%%
%% It's unclear if this is needed, or if the first message on an
%% association is always sctp_assoc_change, but don't assume since
%% SCTP behaviour differs between operating systems.

assoc_id({[#sctp_sndrcvinfo{assoc_id = Id}], _}) ->
    Id;
assoc_id({_, Rec}) ->
    id(Rec).

id(#sctp_shutdown_event{assoc_id = Id}) ->
    Id;
id(#sctp_assoc_change{assoc_id = Id}) ->
    Id;
id(#sctp_sndrcvinfo{assoc_id = Id}) ->
    Id;
id(#sctp_paddr_change{assoc_id = Id}) ->
    Id;
id(#sctp_adaptation_event{assoc_id = Id}) ->
    Id.

%% peeloff/3

peeloff(LSock, Id, TPid) ->
    {ok, Sock} = gen_sctp:peeloff(LSock, Id),
    ok = gen_sctp:controlling_process(Sock, TPid),
    Sock.

%% connect/4

connect(_, [], _, Reasons) ->
    x({connect, lists:reverse(Reasons)});

connect(Sock, [Addr | AT] = As, Port, Reasons) ->
    case gen_sctp:connect_init(Sock, Addr, Port, []) of
        ok ->
            {As, Port, Reasons};
        {error, _} = E ->
            connect(Sock, AT, Port, [{Addr, E} | Reasons])
    end.

%% setopts/2

setopts(_, #transport{socket = Sock,
                      active = A,
                      recv = B}
        = S)
  when B, not A ->
    setopts(Sock),
    S#transport{active = true};

setopts(_, #transport{} = S) ->
    S;

setopts(#transport{socket = Sock}, T) ->
    setopts(Sock),
    T.

%% setopts/1

setopts(Sock) ->
    case inet:setopts(Sock, [{active, once}]) of
        ok -> ok;
        X  -> x({setopts, Sock, X})  %% possibly on peer disconnect
    end.

%% A message_cb is invoked whenever a message is sent or received, or
%% to provide acknowledgement of a completed send or discarded
%% request. See diameter_tcp for semantics, the only difference being
%% that a recv callback can get a diameter_packet record as Msg
%% depending on how/if option packet has been specified.

%% message/3

message(send, false = M, S) ->
    message(ack, M, S);

message(ack, _, #transport{message_cb = false} = S) ->
    S;

message(Dir, Msg, S) ->
    setopts(S, actions(cb(S, Dir, Msg), Dir, S)).

%% actions/3

actions([], _, S) ->
    S;

actions([B | As], Dir, S)
  when is_boolean(B) ->
    actions(As, Dir, S#transport{recv = B});

actions([Dir | As], _, S)
  when Dir == send;
       Dir == recv ->
    actions(As, Dir, S);

actions([Msg | As], send = Dir, S)
  when is_record(Msg, diameter_packet);
       is_binary(Msg) ->
    actions(As, Dir, send(Msg, S));

actions([Msg | As], recv = Dir, #transport{parent = Pid} = S)
  when is_record(Msg, diameter_packet);
       is_binary(Msg) ->
    diameter_peer:recv(Pid, Msg),
    actions(As, Dir, S);

actions([{defer, Tmo, Acts} | As], Dir, S) ->
    erlang:send_after(Tmo, self(), {actions, Dir, Acts}),
    actions(As, Dir, S);

actions(CB, _, S) ->
    S#transport{message_cb = CB}.

%% cb/3

cb(#transport{message_cb = false, packet = P}, recv, Msg) ->
    [pkt(P, true, Msg)];

cb(#transport{message_cb = CB, packet = P}, recv = D, Msg) ->
    cb(CB, D, pkt(P, false, Msg));

cb(#transport{message_cb = CB}, Dir, Msg) ->
    cb(CB, Dir, Msg);

cb(false, send, Msg) ->
    [Msg];

cb(CB, Dir, Msg) ->
    diameter_lib:eval([CB, Dir, Msg]).

%% pkt/3

pkt(false, _, {_Info, Bin}) ->
    Bin;

pkt(true, _, {[#sctp_sndrcvinfo{stream = Id}], Bin}) ->
    #diameter_packet{bin = Bin, transport_data = {stream, Id}};

pkt(raw, true, {[Info], Bin}) ->
    #diameter_packet{bin = Bin, transport_data = Info};

pkt(raw, false, {[_], _} = Msg) ->
    Msg.
