use crate::common_state::{CommonState, Context, IoState, State};
use crate::enums::{AlertDescription, ContentType};
use crate::error::{Error, PeerMisbehaved};
#[cfg(feature = "logging")]
use crate::log::trace;
use crate::msgs::deframer::{Deframed, MessageDeframer};
use crate::msgs::handshake::Random;
use crate::msgs::message::{Message, MessagePayload, PlainMessage};
#[cfg(feature = "secret_extraction")]
use crate::suites::{ExtractedSecrets, PartiallyExtractedSecrets};
use crate::vecbuf::ChunkVecBuffer;

use std::fmt::Debug;
use std::io;
use std::mem;
use std::ops::{Deref, DerefMut};

/// A client or server connection.
#[derive(Debug)]
pub enum Connection {
    /// A client connection
    Client(crate::client::ClientConnection),
    /// A server connection
    Server(crate::server::ServerConnection),
}

impl Connection {
    /// Read TLS content from `rd`.
    ///
    /// See [`ConnectionCommon::read_tls()`] for more information.
    pub fn read_tls(&mut self, rd: &mut dyn io::Read) -> Result<usize, io::Error> {
        match self {
            Self::Client(conn) => conn.read_tls(rd),
            Self::Server(conn) => conn.read_tls(rd),
        }
    }

    /// Writes TLS messages to `wr`.
    ///
    /// See [`ConnectionCommon::write_tls()`] for more information.
    pub fn write_tls(&mut self, wr: &mut dyn io::Write) -> Result<usize, io::Error> {
        self.sendable_tls.write_to(wr)
    }

    /// Returns an object that allows reading plaintext.
    pub fn reader(&mut self) -> Reader {
        match self {
            Self::Client(conn) => conn.reader(),
            Self::Server(conn) => conn.reader(),
        }
    }

    /// Returns an object that allows writing plaintext.
    pub fn writer(&mut self) -> Writer {
        match self {
            Self::Client(conn) => Writer::new(&mut **conn),
            Self::Server(conn) => Writer::new(&mut **conn),
        }
    }

    /// Processes any new packets read by a previous call to [`Connection::read_tls`].
    ///
    /// See [`ConnectionCommon::process_new_packets()`] for more information.
    pub fn process_new_packets(&mut self) -> Result<IoState, Error> {
        match self {
            Self::Client(conn) => conn.process_new_packets(),
            Self::Server(conn) => conn.process_new_packets(),
        }
    }

    /// Derives key material from the agreed connection secrets.
    ///
    /// See [`ConnectionCommon::export_keying_material()`] for more information.
    pub fn export_keying_material<T: AsMut<[u8]>>(
        &self,
        output: T,
        label: &[u8],
        context: Option<&[u8]>,
    ) -> Result<T, Error> {
        match self {
            Self::Client(conn) => conn.export_keying_material(output, label, context),
            Self::Server(conn) => conn.export_keying_material(output, label, context),
        }
    }

    /// Extract secrets, to set up kTLS for example
    #[cfg(feature = "secret_extraction")]
    #[cfg_attr(docsrs, doc(cfg(feature = "secret_extraction")))]
    pub fn extract_secrets(self) -> Result<ExtractedSecrets, Error> {
        match self {
            Self::Client(conn) => conn.extract_secrets(),
            Self::Server(conn) => conn.extract_secrets(),
        }
    }

    /// This function uses `io` to complete any outstanding IO for this connection.
    ///
    /// See [`ConnectionCommon::complete_io()`] for more information.
    pub fn complete_io<T>(&mut self, io: &mut T) -> Result<(usize, usize), io::Error>
    where
        Self: Sized,
        T: io::Read + io::Write,
    {
        match self {
            Self::Client(conn) => conn.complete_io(io),
            Self::Server(conn) => conn.complete_io(io),
        }
    }
}

impl Deref for Connection {
    type Target = CommonState;

    fn deref(&self) -> &Self::Target {
        match self {
            Self::Client(conn) => &conn.core.common_state,
            Self::Server(conn) => &conn.core.common_state,
        }
    }
}

impl DerefMut for Connection {
    fn deref_mut(&mut self) -> &mut Self::Target {
        match self {
            Self::Client(conn) => &mut conn.core.common_state,
            Self::Server(conn) => &mut conn.core.common_state,
        }
    }
}

/// A structure that implements [`std::io::Read`] for reading plaintext.
pub struct Reader<'a> {
    received_plaintext: &'a mut ChunkVecBuffer,
    peer_cleanly_closed: bool,
    has_seen_eof: bool,
}

impl<'a> io::Read for Reader<'a> {
    /// Obtain plaintext data received from the peer over this TLS connection.
    ///
    /// If the peer closes the TLS session cleanly, this returns `Ok(0)`  once all
    /// the pending data has been read. No further data can be received on that
    /// connection, so the underlying TCP connection should be half-closed too.
    ///
    /// If the peer closes the TLS session uncleanly (a TCP EOF without sending a
    /// `close_notify` alert) this function returns `Err(ErrorKind::UnexpectedEof.into())`
    /// once any pending data has been read.
    ///
    /// Note that support for `close_notify` varies in peer TLS libraries: many do not
    /// support it and uncleanly close the TCP connection (this might be
    /// vulnerable to truncation attacks depending on the application protocol).
    /// This means applications using rustls must both handle EOF
    /// from this function, *and* unexpected EOF of the underlying TCP connection.
    ///
    /// If there are no bytes to read, this returns `Err(ErrorKind::WouldBlock.into())`.
    ///
    /// You may learn the number of bytes available at any time by inspecting
    /// the return of [`Connection::process_new_packets`].
    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        let len = self.received_plaintext.read(buf)?;

        if len == 0 && !buf.is_empty() {
            // No bytes available:
            match (self.peer_cleanly_closed, self.has_seen_eof) {
                // cleanly closed; don't care about TCP EOF: express this as Ok(0)
                (true, _) => {}
                // unclean closure
                (false, true) => return Err(io::ErrorKind::UnexpectedEof.into()),
                // connection still going, but need more data: signal `WouldBlock` so that
                // the caller knows this
                (false, false) => return Err(io::ErrorKind::WouldBlock.into()),
            }
        }

        Ok(len)
    }

    /// Obtain plaintext data received from the peer over this TLS connection.
    ///
    /// If the peer closes the TLS session, this returns `Ok(())` without filling
    /// any more of the buffer once all the pending data has been read. No further
    /// data can be received on that connection, so the underlying TCP connection
    /// should be half-closed too.
    ///
    /// If the peer closes the TLS session uncleanly (a TCP EOF without sending a
    /// `close_notify` alert) this function returns `Err(ErrorKind::UnexpectedEof.into())`
    /// once any pending data has been read.
    ///
    /// Note that support for `close_notify` varies in peer TLS libraries: many do not
    /// support it and uncleanly close the TCP connection (this might be
    /// vulnerable to truncation attacks depending on the application protocol).
    /// This means applications using rustls must both handle EOF
    /// from this function, *and* unexpected EOF of the underlying TCP connection.
    ///
    /// If there are no bytes to read, this returns `Err(ErrorKind::WouldBlock.into())`.
    ///
    /// You may learn the number of bytes available at any time by inspecting
    /// the return of [`Connection::process_new_packets`].
    #[cfg(read_buf)]
    fn read_buf(&mut self, mut cursor: core::io::BorrowedCursor<'_>) -> io::Result<()> {
        let before = cursor.written();
        self.received_plaintext
            .read_buf(cursor.reborrow())?;
        let len = cursor.written() - before;

        if len == 0 && cursor.capacity() > 0 {
            // No bytes available:
            match (self.peer_cleanly_closed, self.has_seen_eof) {
                // cleanly closed; don't care about TCP EOF: express this as Ok(0)
                (true, _) => {}
                // unclean closure
                (false, true) => return Err(io::ErrorKind::UnexpectedEof.into()),
                // connection still going, but need more data: signal `WouldBlock` so that
                // the caller knows this
                (false, false) => return Err(io::ErrorKind::WouldBlock.into()),
            }
        }

        Ok(())
    }
}

/// Internal trait implemented by the [`ServerConnection`]/[`ClientConnection`]
/// allowing them to be the subject of a [`Writer`].
pub(crate) trait PlaintextSink {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize>;
    fn write_vectored(&mut self, bufs: &[io::IoSlice<'_>]) -> io::Result<usize>;
    fn flush(&mut self) -> io::Result<()>;
}

impl<T> PlaintextSink for ConnectionCommon<T> {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        Ok(self.send_some_plaintext(buf))
    }

    fn write_vectored(&mut self, bufs: &[io::IoSlice<'_>]) -> io::Result<usize> {
        let mut sz = 0;
        for buf in bufs {
            sz += self.send_some_plaintext(buf);
        }
        Ok(sz)
    }

    fn flush(&mut self) -> io::Result<()> {
        Ok(())
    }
}

/// A structure that implements [`std::io::Write`] for writing plaintext.
pub struct Writer<'a> {
    sink: &'a mut dyn PlaintextSink,
}

impl<'a> Writer<'a> {
    /// Create a new Writer.
    ///
    /// This is not an external interface.  Get one of these objects
    /// from [`Connection::writer`].
    pub(crate) fn new(sink: &'a mut dyn PlaintextSink) -> Writer<'a> {
        Writer { sink }
    }
}

impl<'a> io::Write for Writer<'a> {
    /// Send the plaintext `buf` to the peer, encrypting
    /// and authenticating it.  Once this function succeeds
    /// you should call [`Connection::write_tls`] which will output the
    /// corresponding TLS records.
    ///
    /// This function buffers plaintext sent before the
    /// TLS handshake completes, and sends it as soon
    /// as it can.  See [`CommonState::set_buffer_limit`] to control
    /// the size of this buffer.
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        self.sink.write(buf)
    }

    fn write_vectored(&mut self, bufs: &[io::IoSlice<'_>]) -> io::Result<usize> {
        self.sink.write_vectored(bufs)
    }

    fn flush(&mut self) -> io::Result<()> {
        self.sink.flush()
    }
}

#[derive(Debug)]
pub(crate) struct ConnectionRandoms {
    pub(crate) client: [u8; 32],
    pub(crate) server: [u8; 32],
}

/// How many ChangeCipherSpec messages we accept and drop in TLS1.3 handshakes.
/// The spec says 1, but implementations (namely the boringssl test suite) get
/// this wrong.  BoringSSL itself accepts up to 32.
static TLS13_MAX_DROPPED_CCS: u8 = 2u8;

impl ConnectionRandoms {
    pub(crate) fn new(client: Random, server: Random) -> Self {
        Self {
            client: client.0,
            server: server.0,
        }
    }
}

// --- Common (to client and server) connection functions ---

fn is_valid_ccs(msg: &PlainMessage) -> bool {
    // We passthrough ChangeCipherSpec messages in the deframer without decrypting them.
    // nb. this is prior to the record layer, so is unencrypted. see
    // third paragraph of section 5 in RFC8446.
    msg.typ == ContentType::ChangeCipherSpec && msg.payload.0 == [0x01]
}

/// Interface shared by client and server connections.
pub struct ConnectionCommon<Data> {
    pub(crate) core: ConnectionCore<Data>,
}

impl<Data> ConnectionCommon<Data> {
    /// Returns an object that allows reading plaintext.
    pub fn reader(&mut self) -> Reader {
        let common = &mut self.core.common_state;
        Reader {
            received_plaintext: &mut common.received_plaintext,
            // Are we done? i.e., have we processed all received messages, and received a
            // close_notify to indicate that no new messages will arrive?
            peer_cleanly_closed: common.has_received_close_notify
                && !self.core.message_deframer.has_pending(),
            has_seen_eof: common.has_seen_eof,
        }
    }

    /// Returns an object that allows writing plaintext.
    pub fn writer(&mut self) -> Writer {
        Writer::new(self)
    }

    /// This function uses `io` to complete any outstanding IO for
    /// this connection.
    ///
    /// This is a convenience function which solely uses other parts
    /// of the public API.
    ///
    /// What this means depends on the connection  state:
    ///
    /// - If the connection [`is_handshaking`], then IO is performed until
    ///   the handshake is complete.
    /// - Otherwise, if [`wants_write`] is true, [`write_tls`] is invoked
    ///   until it is all written.
    /// - Otherwise, if [`wants_read`] is true, [`read_tls`] is invoked
    ///   once.
    ///
    /// The return value is the number of bytes read from and written
    /// to `io`, respectively.
    ///
    /// This function will block if `io` blocks.
    ///
    /// Errors from TLS record handling (i.e., from [`process_new_packets`])
    /// are wrapped in an `io::ErrorKind::InvalidData`-kind error.
    ///
    /// [`is_handshaking`]: CommonState::is_handshaking
    /// [`wants_read`]: CommonState::wants_read
    /// [`wants_write`]: CommonState::wants_write
    /// [`write_tls`]: ConnectionCommon::write_tls
    /// [`read_tls`]: ConnectionCommon::read_tls
    /// [`process_new_packets`]: ConnectionCommon::process_new_packets
    pub fn complete_io<T>(&mut self, io: &mut T) -> Result<(usize, usize), io::Error>
    where
        Self: Sized,
        T: io::Read + io::Write,
    {
        let mut eof = false;
        let mut wrlen = 0;
        let mut rdlen = 0;

        loop {
            let until_handshaked = self.is_handshaking();

            while self.wants_write() {
                wrlen += self.write_tls(io)?;
            }
            io.flush()?;

            if !until_handshaked && wrlen > 0 {
                return Ok((rdlen, wrlen));
            }

            while !eof && self.wants_read() {
                let read_size = match self.read_tls(io) {
                    Ok(0) => {
                        eof = true;
                        Some(0)
                    }
                    Ok(n) => {
                        rdlen += n;
                        Some(n)
                    }
                    Err(ref err) if err.kind() == io::ErrorKind::Interrupted => None, // nothing to do
                    Err(err) => return Err(err),
                };
                if read_size.is_some() {
                    break;
                }
            }

            match self.process_new_packets() {
                Ok(_) => {}
                Err(e) => {
                    // In case we have an alert to send describing this error,
                    // try a last-gasp write -- but don't predate the primary
                    // error.
                    let _ignored = self.write_tls(io);
                    let _ignored = io.flush();

                    return Err(io::Error::new(io::ErrorKind::InvalidData, e));
                }
            };

            // if we're doing IO until handshaked, and we believe we've finished handshaking,
            // but process_new_packets() has queued TLS data to send, loop around again to write
            // the queued messages.
            if until_handshaked && !self.is_handshaking() && self.wants_write() {
                continue;
            }

            match (eof, until_handshaked, self.is_handshaking()) {
                (_, true, false) => return Ok((rdlen, wrlen)),
                (_, false, _) => return Ok((rdlen, wrlen)),
                (true, true, true) => return Err(io::Error::from(io::ErrorKind::UnexpectedEof)),
                (..) => {}
            }
        }
    }

    /// Extract the first handshake message.
    ///
    /// This is a shortcut to the `process_new_packets()` -> `process_msg()` ->
    /// `process_handshake_messages()` path, specialized for the first handshake message.
    pub(crate) fn first_handshake_message(&mut self) -> Result<Option<Message>, Error> {
        match self
            .core
            .deframe(None)?
            .map(Message::try_from)
        {
            Some(Ok(msg)) => Ok(Some(msg)),
            Some(Err(err)) => Err(self.send_fatal_alert(AlertDescription::DecodeError, err)),
            None => Ok(None),
        }
    }

    pub(crate) fn replace_state(&mut self, new: Box<dyn State<Data>>) {
        self.core.state = Ok(new);
    }

    /// Processes any new packets read by a previous call to
    /// [`Connection::read_tls`].
    ///
    /// Errors from this function relate to TLS protocol errors, and
    /// are fatal to the connection.  Future calls after an error will do
    /// no new work and will return the same error. After an error is
    /// received from [`process_new_packets`], you should not call [`read_tls`]
    /// any more (it will fill up buffers to no purpose). However, you
    /// may call the other methods on the connection, including `write`,
    /// `send_close_notify`, and `write_tls`. Most likely you will want to
    /// call `write_tls` to send any alerts queued by the error and then
    /// close the underlying connection.
    ///
    /// Success from this function comes with some sundry state data
    /// about the connection.
    ///
    /// [`read_tls`]: Connection::read_tls
    /// [`process_new_packets`]: Connection::process_new_packets
    #[inline]
    pub fn process_new_packets(&mut self) -> Result<IoState, Error> {
        self.core.process_new_packets()
    }

    /// Read TLS content from `rd` into the internal buffer.
    ///
    /// Due to the internal buffering, `rd` can supply TLS messages in arbitrary-sized chunks (like
    /// a socket or pipe might).
    ///
    /// You should call [`process_new_packets()`] each time a call to this function succeeds in order
    /// to empty the incoming TLS data buffer.
    ///
    /// This function returns `Ok(0)` when the underlying `rd` does so. This typically happens when
    /// a socket is cleanly closed, or a file is at EOF. Errors may result from the IO done through
    /// `rd`; additionally, errors of `ErrorKind::Other` are emitted to signal backpressure:
    ///
    /// * In order to empty the incoming TLS data buffer, you should call [`process_new_packets()`]
    ///   each time a call to this function succeeds.
    /// * In order to empty the incoming plaintext data buffer, you should empty it through
    ///   the [`reader()`] after the call to [`process_new_packets()`].
    ///
    /// [`process_new_packets()`]: ConnectionCommon::process_new_packets
    /// [`reader()`]: ConnectionCommon::reader
    pub fn read_tls(&mut self, rd: &mut dyn io::Read) -> Result<usize, io::Error> {
        if self.received_plaintext.is_full() {
            return Err(io::Error::new(
                io::ErrorKind::Other,
                "received plaintext buffer full",
            ));
        }

        let res = self.core.message_deframer.read(rd);
        if let Ok(0) = res {
            self.has_seen_eof = true;
        }
        res
    }

    /// Writes TLS messages to `wr`.
    ///
    /// On success, this function returns `Ok(n)` where `n` is a number of bytes written to `wr`
    /// (after encoding and encryption).
    ///
    /// After this function returns, the connection buffer may not yet be fully flushed. The
    /// [`CommonState::wants_write`] function can be used to check if the output buffer is empty.
    pub fn write_tls(&mut self, wr: &mut dyn io::Write) -> Result<usize, io::Error> {
        self.sendable_tls.write_to(wr)
    }

    /// Derives key material from the agreed connection secrets.
    ///
    /// This function fills in `output` with `output.len()` bytes of key
    /// material derived from the master session secret using `label`
    /// and `context` for diversification. Ownership of the buffer is taken
    /// by the function and returned via the Ok result to ensure no key
    /// material leaks if the function fails.
    ///
    /// See RFC5705 for more details on what this does and is for.
    ///
    /// For TLS1.3 connections, this function does not use the
    /// "early" exporter at any point.
    ///
    /// This function fails if called prior to the handshake completing;
    /// check with [`CommonState::is_handshaking`] first.
    #[inline]
    pub fn export_keying_material<T: AsMut<[u8]>>(
        &self,
        output: T,
        label: &[u8],
        context: Option<&[u8]>,
    ) -> Result<T, Error> {
        self.core
            .export_keying_material(output, label, context)
    }

    /// Extract secrets, so they can be used when configuring kTLS, for example.
    #[cfg(feature = "secret_extraction")]
    #[cfg_attr(docsrs, doc(cfg(feature = "secret_extraction")))]
    pub fn extract_secrets(self) -> Result<ExtractedSecrets, Error> {
        if !self.enable_secret_extraction {
            return Err(Error::General("Secret extraction is disabled".into()));
        }

        let st = self.core.state?;

        let record_layer = self.core.common_state.record_layer;
        let PartiallyExtractedSecrets { tx, rx } = st.extract_secrets()?;
        Ok(ExtractedSecrets {
            tx: (record_layer.write_seq(), tx),
            rx: (record_layer.read_seq(), rx),
        })
    }
}

impl<'a, Data> From<&'a mut ConnectionCommon<Data>> for Context<'a, Data> {
    fn from(conn: &'a mut ConnectionCommon<Data>) -> Self {
        Self {
            common: &mut conn.core.common_state,
            data: &mut conn.core.data,
        }
    }
}

impl<T> Deref for ConnectionCommon<T> {
    type Target = CommonState;

    fn deref(&self) -> &Self::Target {
        &self.core.common_state
    }
}

impl<T> DerefMut for ConnectionCommon<T> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.core.common_state
    }
}

impl<Data> From<ConnectionCore<Data>> for ConnectionCommon<Data> {
    fn from(core: ConnectionCore<Data>) -> Self {
        Self { core }
    }
}

pub(crate) struct ConnectionCore<Data> {
    pub(crate) state: Result<Box<dyn State<Data>>, Error>,
    pub(crate) data: Data,
    pub(crate) common_state: CommonState,
    pub(crate) message_deframer: MessageDeframer,
}

impl<Data> ConnectionCore<Data> {
    pub(crate) fn new(state: Box<dyn State<Data>>, data: Data, common_state: CommonState) -> Self {
        Self {
            state: Ok(state),
            data,
            common_state,
            message_deframer: MessageDeframer::default(),
        }
    }

    pub(crate) fn process_new_packets(&mut self) -> Result<IoState, Error> {
        let mut state = match mem::replace(&mut self.state, Err(Error::HandshakeNotComplete)) {
            Ok(state) => state,
            Err(e) => {
                self.state = Err(e.clone());
                return Err(e);
            }
        };

        while let Some(msg) = self.deframe(Some(&*state))? {
            match self.process_msg(msg, state) {
                Ok(new) => state = new,
                Err(e) => {
                    self.state = Err(e.clone());
                    return Err(e);
                }
            }
        }

        self.state = Ok(state);
        Ok(self.common_state.current_io_state())
    }

    /// Pull a message out of the deframer and send any messages that need to be sent as a result.
    fn deframe(&mut self, state: Option<&dyn State<Data>>) -> Result<Option<PlainMessage>, Error> {
        match self
            .message_deframer
            .pop(&mut self.common_state.record_layer)
        {
            Ok(Some(Deframed {
                want_close_before_decrypt,
                aligned,
                trial_decryption_finished,
                message,
            })) => {
                if want_close_before_decrypt {
                    self.common_state.send_close_notify();
                }

                if trial_decryption_finished {
                    self.common_state
                        .record_layer
                        .finish_trial_decryption();
                }

                self.common_state.aligned_handshake = aligned;
                Ok(Some(message))
            }
            Ok(None) => Ok(None),
            Err(err @ Error::InvalidMessage(_)) => {
                #[cfg(feature = "quic")]
                if self.common_state.is_quic() {
                    self.common_state.quic.alert = Some(AlertDescription::DecodeError);
                }

                Err(if !self.common_state.is_quic() {
                    self.common_state
                        .send_fatal_alert(AlertDescription::DecodeError, err)
                } else {
                    err
                })
            }
            Err(err @ Error::PeerSentOversizedRecord) => Err(self
                .common_state
                .send_fatal_alert(AlertDescription::RecordOverflow, err)),
            Err(err @ Error::DecryptError) => {
                if let Some(state) = state {
                    state.handle_decrypt_error();
                }
                Err(self
                    .common_state
                    .send_fatal_alert(AlertDescription::BadRecordMac, err))
            }
            Err(e) => Err(e),
        }
    }

    fn process_msg(
        &mut self,
        msg: PlainMessage,
        state: Box<dyn State<Data>>,
    ) -> Result<Box<dyn State<Data>>, Error> {
        // Drop CCS messages during handshake in TLS1.3
        if msg.typ == ContentType::ChangeCipherSpec
            && !self
                .common_state
                .may_receive_application_data
            && self.common_state.is_tls13()
        {
            if !is_valid_ccs(&msg)
                || self.common_state.received_middlebox_ccs > TLS13_MAX_DROPPED_CCS
            {
                // "An implementation which receives any other change_cipher_spec value or
                //  which receives a protected change_cipher_spec record MUST abort the
                //  handshake with an "unexpected_message" alert."
                return Err(self.common_state.send_fatal_alert(
                    AlertDescription::UnexpectedMessage,
                    PeerMisbehaved::IllegalMiddleboxChangeCipherSpec,
                ));
            } else {
                self.common_state.received_middlebox_ccs += 1;
                trace!("Dropping CCS");
                return Ok(state);
            }
        }

        // Now we can fully parse the message payload.
        let msg = match Message::try_from(msg) {
            Ok(msg) => msg,
            Err(err) => {
                return Err(self
                    .common_state
                    .send_fatal_alert(AlertDescription::DecodeError, err));
            }
        };

        // For alerts, we have separate logic.
        if let MessagePayload::Alert(alert) = &msg.payload {
            self.common_state.process_alert(alert)?;
            return Ok(state);
        }

        self.common_state
            .process_main_protocol(msg, state, &mut self.data)
    }

    pub(crate) fn export_keying_material<T: AsMut<[u8]>>(
        &self,
        mut output: T,
        label: &[u8],
        context: Option<&[u8]>,
    ) -> Result<T, Error> {
        match self.state.as_ref() {
            Ok(st) => st
                .export_keying_material(output.as_mut(), label, context)
                .map(|_| output),
            Err(e) => Err(e.clone()),
        }
    }
}

/// Data specific to the peer's side (client or server).
pub trait SideData {}