# Source code for asyncssh.channel

    # Copyright (c) 2013-2022 by Ron Frederick <ronf@timeheart.net> and others.
    #
    # This program and the accompanying materials are made available under
    # the terms of the Eclipse Public License v2.0 which accompanies this
    # distribution and is available at:
    #
    #     http://www.eclipse.org/legal/epl-2.0/
    #
    # This program may also be made available under the following secondary
    # licenses when the conditions for such availability set forth in the
    # Eclipse Public License v2.0 are satisfied:
    #
    #    GNU General Public License, Version 2.0, or any later versions of
    #    that license
    #
    # SPDX-License-Identifier: EPL-2.0 OR GPL-2.0-or-later
    #
    # Contributors:
    #     Ron Frederick - initial implementation, API, and documentation

    """SSH channel and session handlers"""

    import asyncio
    import binascii
    import codecs
    import inspect
    import re
    import signal as _signal
    from types import MappingProxyType
    from typing import TYPE_CHECKING, Any, AnyStr, Awaitable, Callable
    from typing import Dict, Generic, Iterable, List, Mapping, Optional
    from typing import Set, Tuple, Union, cast

    from . import constants
    from .constants import DEFAULT_LANG, EXTENDED_DATA_STDERR
    from .constants import MSG_CHANNEL_OPEN, MSG_CHANNEL_WINDOW_ADJUST
    from .constants import MSG_CHANNEL_DATA, MSG_CHANNEL_EXTENDED_DATA
    from .constants import MSG_CHANNEL_EOF, MSG_CHANNEL_CLOSE, MSG_CHANNEL_REQUEST
    from .constants import MSG_CHANNEL_SUCCESS, MSG_CHANNEL_FAILURE
    from .constants import OPEN_CONNECT_FAILED, PTY_OP_RESERVED, PTY_OP_END
    from .constants import OPEN_REQUEST_X11_FORWARDING_FAILED
    from .constants import OPEN_REQUEST_PTY_FAILED, OPEN_REQUEST_SESSION_FAILED

    from .editor import SSHLineEditorChannel, SSHLineEditorSession

    from .logging import SSHLogger

    from .misc import ChannelOpenError, MaybeAwait, ProtocolError
    from .misc import get_symbol_names, map_handler_name

    from .packet import Boolean, Byte, String, UInt32, SSHPacket, SSHPacketHandler

    from .session import TermModes, TermSize, TermSizeArg
    from .session import SSHSession, SSHClientSession, SSHServerSession
    from .session import SSHTCPSession, SSHUNIXSession
    from .session import SSHSessionFactory, SSHClientSessionFactory
    from .session import SSHTCPSessionFactory, SSHUNIXSessionFactory

    from .stream import DataType

    if TYPE_CHECKING:
        # pylint: disable=cyclic-import
        from .connection import SSHConnection, SSHClientConnection
        from .connection import SSHServerConnection

    _const_dict: Mapping[str, int] = constants.__dict__
    _pty_mode_names = get_symbol_names(_const_dict, 'PTY_', 4)
    _data_type_names = get_symbol_names(_const_dict, 'EXTENDED_DATA_', 14)

    _signal_regex = re.compile(r'SIG[^_]')
    _signal_numbers = {k[3:]: int(v) for (k, v) in vars(_signal).items()
                       if _signal_regex.match(k)}
    _signal_names = {v: k for (k, v) in _signal_numbers.items()}

    _ExitSignal = Tuple[str, bool, str, str]
    _RequestHandler = Callable[[SSHPacket], Optional[bool]]

    class SSHChannel(Generic[AnyStr], SSHPacketHandler):
        """Parent class for SSH channels"""

        _handler_names = get_symbol_names(globals(), 'MSG_CHANNEL_')

        _read_datatypes: Set[int] = set()
        _write_datatypes: Set[int] = set()

        def __init__(self, conn: 'SSHConnection',
                     loop: asyncio.AbstractEventLoop, encoding: Optional[str],
                     errors: str, window: int, max_pktsize: int):
            """Initialize an SSH channel

               If encoding is set, data sent and received will be in the form
               of strings, converted on the wire to bytes using the specified
               encoding. If encoding is None, data sent and received must be
               provided as bytes.

               Window specifies the initial receive window size.

               Max_pktsize specifies the maximum length of a single data packet.

            """

            self._conn: Optional['SSHConnection'] = conn
            self._loop = loop
            self._session: Optional[SSHSession[AnyStr]] = None
            self._extra: Dict[str, object] = {'connection': conn}
            self._encoding: Optional[str]
            self._errors: str
            self._send_high_water: int
            self._send_low_water: int

            self._env: Dict[str, str] = {}
            self._command: Optional[str] = None
            self._subsystem: Optional[str] = None

            self._send_state = 'closed'
            self._send_chan: Optional[int] = None
            self._send_window: int = 0
            self._send_pktsize: int = 0
            self._send_paused = False
            self._send_buf: List[Tuple[bytearray, DataType]] = []
            self._send_buf_len = 0

            self._recv_state = 'closed'
            self._init_recv_window = window
            self._recv_window = window
            self._recv_pktsize = max_pktsize
            self._recv_paused: Union[bool, str] = 'starting'
            self._recv_buf: List[Tuple[bytes, DataType]] = []

            self._request_queue: List[Tuple[str, SSHPacket, bool]] = []

            self._open_waiter: Optional[asyncio.Future[SSHPacket]] = None
            self._request_waiters: List[asyncio.Future[bool]] = []

            self._close_event = asyncio.Event()

            self._recv_chan: Optional[int] = conn.add_channel(self)

            self._logger = conn.logger.get_child(context='chan=%d' %
                                                 self._recv_chan)

            self.set_encoding(encoding, errors)
            self.set_write_buffer_limits()

        @property
        def logger(self) -> SSHLogger:
            """A logger associated with this channel"""

            return self._logger

        def get_connection(self) -> 'SSHConnection':
            """Return the connection used by this channel"""

            assert self._conn is not None
            return self._conn

        def get_loop(self) -> asyncio.AbstractEventLoop:
            """Return the event loop used by this channel"""

            return self._loop

        def get_encoding(self) -> Tuple[Optional[str], str]:
            """Return the encoding used by this channel"""

            return self._encoding, self._errors

        def set_encoding(self, encoding: Optional[str],
                         errors: str = 'strict') -> None:
            """Set the encoding on this channel"""

            self._encoding = encoding
            self._errors = errors

            if encoding:
                self._encoder: Optional[codecs.IncrementalEncoder] = \
                    codecs.getincrementalencoder(encoding)(errors)
                self._decoder: Optional[codecs.IncrementalDecoder] = \
                    codecs.getincrementaldecoder(encoding)(errors)
            else:
                self._encoder = None
                self._decoder = None

        def get_recv_window(self) -> int:
            """Return the configured receive window for this channel"""

            return self._init_recv_window

        def get_read_datatypes(self) -> Set[int]:
            """Return the legal read data types for this channel"""

            return self._read_datatypes

        def get_write_datatypes(self) -> Set[int]:
            """Return the legal write data types for this channel"""

            return self._write_datatypes

        def _cleanup(self, exc: Exception = None) -> None:
            """Clean up this channel"""

            if self._open_waiter:
                if not self._open_waiter.cancelled(): # pragma: no branch
                    self._open_waiter.set_exception(
                        ChannelOpenError(OPEN_CONNECT_FAILED,
                                         'SSH connection closed'))

                self._open_waiter = None

            if self._request_waiters:
                for waiter in self._request_waiters:
                    if not waiter.cancelled(): # pragma: no cover
                        if exc:
                            waiter.set_exception(exc)
                        else:
                            waiter.set_result(False)

                self._request_waiters = []

            if self._session is not None:
                self._session.connection_lost(exc)
                self._session = None

            self._close_event.set()

            if self._conn: # pragma: no branch
                self.logger.info('Channel closed%s',
                                 ': ' + str(exc) if exc else '')

                self._conn.detach_x11_listener(self)

                assert self._recv_chan is not None
                self._conn.remove_channel(self._recv_chan)
                self._send_chan = None
                self._recv_chan = None
                self._conn = None

        def _close_send(self) -> None:
            """Discard unsent data and close the channel for sending"""

            # Discard unsent data
            self._send_buf = []
            self._send_buf_len = 0

            if self._send_state != 'closed':
                self.send_packet(MSG_CHANNEL_CLOSE)
                self._send_chan = None
                self._send_state = 'closed'

        def _discard_recv(self) -> None:
            """Discard unreceived data and clean up if close received"""

            # Discard unreceived data
            self._recv_buf = []
            self._recv_paused = False

            # If recv is close_pending, we know send is already closed
            if self._recv_state == 'close_pending':
                self._recv_state = 'closed'
                self._loop.call_soon(self._cleanup)

        async def _start_reading(self) -> None:
            """Start processing data on a new connection"""

            # If owner of the channel  didn't explicitly pause it at
            # startup, begin processing incoming data.

            if self._recv_paused == 'starting':
                self.logger.debug2('Reading from channel started')
                self._recv_paused = False
                self._flush_recv_buf()

        def _pause_resume_writing(self) -> None:
            """Pause or resume writing based on send buffer low/high water marks"""

            if self._send_paused:
                if self._send_buf_len <= self._send_low_water:
                    self.logger.debug2('Writing from session resumed')

                    self._send_paused = False
                    assert self._session is not None
                    self._session.resume_writing()
            else:
                if self._send_buf_len > self._send_high_water:
                    self.logger.debug2('Writing from session paused')

                    self._send_paused = True
                    assert self._session is not None
                    self._session.pause_writing()

        def _flush_send_buf(self) -> None:
            """Flush as much data in send buffer as the send window allows"""

            while self._send_buf and self._send_window:
                pktsize = min(self._send_window, self._send_pktsize)
                buf, datatype = self._send_buf[0]

                if len(buf) > pktsize:
                    data = buf[:pktsize]
                    del buf[:pktsize]
                else:
                    data = buf
                    del self._send_buf[0]

                self._send_buf_len -= len(data)
                self._send_window -= len(data)

                if datatype is None:
                    self.send_packet(MSG_CHANNEL_DATA, String(data))
                else:
                    self.send_packet(MSG_CHANNEL_EXTENDED_DATA,
                                     UInt32(datatype), String(data))

            self._pause_resume_writing()

            if not self._send_buf:
                if self._send_state == 'eof_pending':
                    self.send_packet(MSG_CHANNEL_EOF)
                    self._send_state = 'eof'
                elif self._send_state == 'close_pending':
                    self._close_send()

        def _flush_recv_buf(self, exc: Exception = None) -> None:
            """Flush as much data in the recv buffer as the application allows"""

            while self._recv_buf and not self._recv_paused:
                self._deliver_data(*self._recv_buf.pop(0))

            if not self._recv_buf and self._recv_paused != 'starting':
                if self._encoding and not exc and \
                        self._recv_state in ('eof_pending', 'close_pending'):
                    try:
                        assert self._decoder is not None
                        self._decoder.decode(b'', True)
                    except UnicodeDecodeError as unicode_exc:
                        raise ProtocolError(str(unicode_exc)) from None

                if self._recv_state == 'eof_pending':
                    self._recv_state = 'eof'

                    assert self._session is not None

                    if (not self._session.eof_received() and
                            self._send_state == 'open'):
                        self.write_eof()

            if not self._recv_buf and self._recv_state == 'close_pending':
                self._recv_state = 'closed'
                self._loop.call_soon(self._cleanup, exc)

        def _deliver_data(self, data: bytes, datatype: DataType) -> None:
            """Deliver incoming data to the session"""

            self._recv_window -= len(data)

            if self._recv_window < self._init_recv_window / 2:
                adjust = self._init_recv_window - self._recv_window

                self.logger.debug2('Sending window adjust of %d bytes, '
                                   'new window %d', adjust, self._init_recv_window)

                self.send_packet(MSG_CHANNEL_WINDOW_ADJUST, UInt32(adjust))
                self._recv_window = self._init_recv_window

            if self._encoding:
                try:
                    assert self._decoder is not None
                    decoded_data = cast(AnyStr, self._decoder.decode(data))
                except UnicodeDecodeError as unicode_exc:
                    raise ProtocolError(str(unicode_exc)) from None
            else:
                decoded_data = cast(AnyStr, data)

            if self._session is not None:
                self._session.data_received(decoded_data, datatype)

        def _accept_data(self, data: bytes, datatype: DataType = None) -> None:
            """Accept new data on the channel

               This method accepts new data on the channel, immediately
               delivering it to the session if it hasn't paused reading.
               If it has paused, data is buffered until reading is resumed.

               Data sent after the channel has been closed by the session
               is dropped.

            """

            if not data:
                return

            if self._send_state in {'close_pending', 'closed'}:
                return

            datalen = len(data)

            if datalen > self._recv_window:
                raise ProtocolError('Window exceeded')

            if datatype:
                typename = ' from %s' % _data_type_names[datatype]
            else:
                typename = ''

            self.logger.debug2('Received %d data byte%s%s', datalen,
                               's' if datalen > 1 else '', typename)

            if self._recv_paused:
                self._recv_buf.append((data, datatype))
            else:
                self._deliver_data(data, datatype)

        def _service_next_request(self) -> None:
            """Process next item on channel request queue"""

            request, packet, _ = self._request_queue[0]

            name = '_process_' + map_handler_name(request) + '_request'
            handler = cast(_RequestHandler, getattr(self, name, None))

            if handler:
                result = cast(Optional[bool], handler(packet))
            else:
                self.logger.debug1('Received unknown channel request: %s', request)
                result = False

            if result is not None:
                self._report_response(result)

        def _report_response(self, result: bool) -> None:
            """Report back the response to a previously issued channel request"""

            request, _, want_reply = self._request_queue.pop(0)

            if want_reply and self._send_state not in {'close_pending', 'closed'}:
                if result:
                    self.send_packet(MSG_CHANNEL_SUCCESS)
                else:
                    self.send_packet(MSG_CHANNEL_FAILURE)

            if result and request in {'shell', 'exec', 'subsystem'}:
                assert self._session is not None
                self._session.session_started()
                self.resume_reading()

            if self._request_queue:
                self._service_next_request()

        def process_connection_close(self, exc: Optional[Exception]) -> None:
            """Process the SSH connection closing"""

            self.logger.info('Closing channel due to connection close')

            self._cleanup(exc)

        def process_open(self, send_chan: int, send_window: int, send_pktsize: int,
                         session: MaybeAwait[SSHSession[AnyStr]]) -> None:
            """Process a channel open request"""

            self._send_chan = send_chan
            self._send_window = send_window
            self._send_pktsize = send_pktsize

            self.logger.debug2('  Initial send window %d, packet size %d',
                               send_window, send_pktsize)

            assert self._conn is not None
            self._conn.create_task(self._finish_open_request(session), self.logger)

        def _wrap_session(self, session: SSHSession[AnyStr]) -> \
                Tuple['SSHChannel[AnyStr]', SSHSession[AnyStr]]:
            """Hook to optionally wrap channel and session objects"""

            # By default, return the original channel and session objects
            return self, session

        async def _finish_open_request(
                self, result: MaybeAwait[SSHSession[AnyStr]]) -> None:
            """Finish processing a channel open request"""

            try:
                if inspect.isawaitable(result):
                    session = await cast(Awaitable[SSHSession[AnyStr]], result)
                else:
                    session = cast(SSHSession[AnyStr], result)

                if not self._conn:
                    raise ChannelOpenError(OPEN_CONNECT_FAILED,
                                           'SSH connection closed')

                chan, self._session = self._wrap_session(session)

                self.logger.debug2('  Initial recv window %d, packet size %d',
                                   self._recv_window, self._recv_pktsize)

                assert self._send_chan is not None
                assert self._recv_chan is not None

                self._conn.send_channel_open_confirmation(self._send_chan,
                                                          self._recv_chan,
                                                          self._recv_window,
                                                          self._recv_pktsize)

                self._send_state = 'open'
                self._recv_state = 'open'

                self._session.connection_made(chan)
            except ChannelOpenError as exc:
                if self._conn:
                    assert self._send_chan is not None
                    self._conn.send_channel_open_failure(self._send_chan, exc.code,
                                                         exc.reason, exc.lang)

                self._loop.call_soon(self._cleanup)

        def process_open_confirmation(self, send_chan: int, send_window: int,
                                      send_pktsize: int, packet: SSHPacket) -> None:
            """Process a channel open confirmation"""

            if not self._open_waiter:
                raise ProtocolError('Channel not being opened')

            self._send_chan = send_chan
            self._send_window = send_window
            self._send_pktsize = send_pktsize

            self.logger.debug2('  Initial send window %d, packet size %d',
                               send_window, send_pktsize)

            self._send_state = 'open'
            self._recv_state = 'open'

            if not self._open_waiter.cancelled(): # pragma: no branch
                self._open_waiter.set_result(packet)

            self._open_waiter = None

        def process_open_failure(self, code: int, reason: str, lang: str) -> None:
            """Process a channel open failure"""

            if not self._open_waiter:
                raise ProtocolError('Channel not being opened')

            if not self._open_waiter.cancelled(): # pragma: no branch
                self._open_waiter.set_exception(
                    ChannelOpenError(code, reason, lang))

            self._open_waiter = None
            self._loop.call_soon(self._cleanup)

        def _process_window_adjust(self, _pkttype: int, _pktid: int,
                                   packet: SSHPacket) -> None:
            """Process a send window adjustment"""

            if self._recv_state not in {'open', 'eof_pending', 'eof'}:
                raise ProtocolError('Channel not open')

            adjust = packet.get_uint32()
            packet.check_end()

            self._send_window += adjust

            self.logger.debug2('Received window adjust of %d bytes, '
                               'new window %d', adjust, self._send_window)

            self._flush_send_buf()

        def _process_data(self, _pkttype: int, _pktid: int,
                          packet: SSHPacket) -> None:
            """Process incoming data"""

            if self._recv_state != 'open':
                raise ProtocolError('Channel not open for sending')

            data = packet.get_string()
            packet.check_end()

            self._accept_data(data)

        def _process_extended_data(self, _pkttype: int, _pktid: int,
                                   packet: SSHPacket) -> None:
            """Process incoming extended data"""

            if self._recv_state != 'open':
                raise ProtocolError('Channel not open for sending')

            datatype = packet.get_uint32()
            data = packet.get_string()
            packet.check_end()

            if datatype not in self._read_datatypes:
                raise ProtocolError('Invalid extended data type')

            self._accept_data(data, datatype)

        def _process_eof(self, _pkttype: int, _pktid: int,
                         packet: SSHPacket) -> None:
            """Process an incoming end of file"""

            if self._recv_state != 'open':
                raise ProtocolError('Channel not open for sending')

            packet.check_end()

            self.logger.debug2('Received EOF')

            self._recv_state = 'eof_pending'
            self._flush_recv_buf()

        def _process_close(self, _pkttype: int, _pktid: int,
                           packet: SSHPacket) -> None:
            """Process an incoming channel close"""

            if self._recv_state not in {'open', 'eof_pending', 'eof'}:
                raise ProtocolError('Channel not open')

            packet.check_end()

            self.logger.info('Received channel close')

            self._close_send()

            self._recv_state = 'close_pending'
            self._flush_recv_buf()

        def _process_request(self, _pkttype: int, _pktid: int,
                             packet: SSHPacket) -> None:
            """Process an incoming channel request"""

            if self._recv_state not in {'open', 'eof_pending', 'eof'}:
                raise ProtocolError('Channel not open')

            request_bytes = packet.get_string()
            want_reply = packet.get_boolean()

            try:
                request = request_bytes.decode('ascii')
            except UnicodeDecodeError:
                raise ProtocolError('Invalid channel request') from None

            self._request_queue.append((request, packet, want_reply))
            if len(self._request_queue) == 1:
                self._service_next_request()

        def _process_response(self, pkttype: int, _pktid: int,
                              packet: SSHPacket) -> None:
            """Process a success or failure response"""

            packet.check_end()

            if self._request_waiters:
                waiter = self._request_waiters.pop(0)
                if not waiter.cancelled(): # pragma: no branch
                    waiter.set_result(pkttype == MSG_CHANNEL_SUCCESS)
            else:
                raise ProtocolError('Unexpected channel response')

        def _process_keepalive_at_openssh_dot_com_request(
                self, packet: SSHPacket) -> bool:
            """Process an incoming OpenSSH keepalive request"""

            packet.check_end()

            self.logger.debug2('Received OpenSSH keepalive channel request')
            return False

        _packet_handlers = {
            MSG_CHANNEL_WINDOW_ADJUST:      _process_window_adjust,
            MSG_CHANNEL_DATA:               _process_data,
            MSG_CHANNEL_EXTENDED_DATA:      _process_extended_data,
            MSG_CHANNEL_EOF:                _process_eof,
            MSG_CHANNEL_CLOSE:              _process_close,
            MSG_CHANNEL_REQUEST:            _process_request,
            MSG_CHANNEL_SUCCESS:            _process_response,
            MSG_CHANNEL_FAILURE:            _process_response
        }

        async def _open(self, chantype: bytes, *args: bytes) -> SSHPacket:
            """Make a request to open the channel"""

            if self._send_state != 'closed':
                raise OSError('Channel already open')

            self._open_waiter = self._loop.create_future()

            self.logger.debug2('  Initial recv window %d, packet size %d',
                               self._recv_window, self._recv_pktsize)

            assert self._conn is not None
            assert self._recv_chan is not None

            self._conn.send_packet(MSG_CHANNEL_OPEN, String(chantype),
                                   UInt32(self._recv_chan),
                                   UInt32(self._recv_window),
                                   UInt32(self._recv_pktsize), *args, handler=self)

            return await self._open_waiter

        def send_packet(self, pkttype: int, *args: bytes) -> None:
            """Send a packet on the channel"""

            if self._send_chan is None: # pragma: no cover
                return

            payload = UInt32(self._send_chan) + b''.join(args)

            assert self._conn is not None
            self._conn.send_packet(pkttype, payload, handler=self)

        def _send_request(self, request: bytes, *args: bytes,
                          want_reply: bool = False) -> None:
            """Send a channel request"""

            self.send_packet(MSG_CHANNEL_REQUEST, String(request),
                             Boolean(want_reply), *args)

        async def _make_request(self, request: bytes,
                                *args: bytes) -> Optional[bool]:
            """Make a channel request and wait for the response"""

            if self._send_chan is None:
                return False

            waiter = self._loop.create_future()
            self._request_waiters.append(waiter)
            self._send_request(request, *args, want_reply=True)
            return await waiter

        def abort(self) -> None:
            """Forcibly close the channel

               This method can be called to forcibly close the channel, after
               which no more data can be sent or received. Any unsent buffered
               data and any incoming data in flight will be discarded.

            """

            self.logger.info('Aborting channel')

            if self._send_state not in {'close_pending', 'closed'}:
                # Send an immediate close, discarding unsent data
                self._close_send()

            if self._recv_state != 'closed':
                # Discard unreceived data
                self._discard_recv()

        def close(self) -> None:
            """Cleanly close the channel

               This method can be called to cleanly close the channel, after
               which no more data can be sent or received. Any unsent buffered
               data will be flushed asynchronously before the channel is
               closed.

            """

            self.logger.info('Closing channel')

            if self._send_state not in {'close_pending', 'closed'}:
                # Send a close only after sending unsent data
                self._send_state = 'close_pending'
                self._flush_send_buf()

            if self._recv_state != 'closed':
                # Discard unreceived data
                self._discard_recv()

        def is_closing(self) -> bool:
            """Return if the channel is closing or is closed"""

            return self._send_state != 'open'

        async def wait_closed(self) -> None:
            """Wait for this channel to close

               This method is a coroutine which can be called to block until
               this channel has finished closing.

            """

            await self._close_event.wait()

        def get_extra_info(self, name: str, default: Any = None) -> Any:
            """Get additional information about the channel

               This method returns extra information about the channel once
               it is established. Supported values include `'connection'`
               to return the SSH connection this channel is running over plus
               all of the values supported on that connection.

               For TCP channels, the values `'local_peername'` and
               `'remote_peername'` are added to return the local and remote
               host and port information for the tunneled TCP connection.

               For UNIX channels, the values `'local_peername'` and
               `'remote_peername'` are added to return the local and remote
               path information for the tunneled UNIX domain socket connection.
               Since UNIX domain sockets provide no "source" address, only
               one of these will be filled in.

               See :meth:`get_extra_info() <SSHClientConnection.get_extra_info>`
               on :class:`SSHClientConnection` for more information.

               Additional information stored on the channel by calling
               :meth:`set_extra_info` can also be returned here.

            """

            return self._extra.get(name, self._conn.get_extra_info(name, default)
                                   if self._conn else default)

        def set_extra_info(self, **kwargs: Any) -> None:
            """Store additional information associated with the channel

               This method allows extra information to be associated with the
               channel. The information to store should be passed in as
               keyword parameters and can later be returned by calling
               :meth:`get_extra_info` with one of the keywords as the name
               to retrieve.

            """

            self._extra.update(**kwargs)

        def can_write_eof(self) -> bool:
            """Return whether the channel supports :meth:`write_eof`

               This method always returns `True`.

            """

            # pylint: disable=no-self-use
            return True

        def get_write_buffer_size(self) -> int:
            """Return the current size of the channel's output buffer

               This method returns how many bytes are currently in the
               channel's output buffer waiting to be written.

            """

            return self._send_buf_len

        def set_write_buffer_limits(self, high: int = None,
                                    low: int = None) -> None:
            """Set the high- and low-water limits for write flow control

               This method sets the limits used when deciding when to call
               the :meth:`pause_writing() <SSHClientSession.pause_writing>`
               and :meth:`resume_writing() <SSHClientSession.resume_writing>`
               methods on SSH sessions. Writing will be paused when the write
               buffer size exceeds the high-water mark, and resumed when the
               write buffer size equals or drops below the low-water mark.

            """

            if high is None:
                high = 4*low if low is not None else 65536

            if low is None:
                low = high // 4

            if not 0 <= low <= high:
                raise ValueError('high (%r) must be >= low (%r) must be >= 0' %
                                 (high, low))

            self.logger.debug1('Set write buffer limits: low-water=%d, '
                               'high-water=%d', low, high)

            self._send_high_water = high
            self._send_low_water = low
            self._pause_resume_writing()

        def write(self, data: AnyStr, datatype: DataType = None) -> None:
            """Write data on the channel

               This method can be called to send data on the channel. If
               an encoding was specified when the channel was created, the
               data should be provided as a string and will be converted
               using that encoding. Otherwise, the data should be provided
               as bytes.

               An extended data type can optionally be provided. For
               instance, this is used from a :class:`SSHServerSession`
               to write data to `stderr`.

               :param data:
                   The data to send on the channel
               :param datatype: (optional)
                   The extended data type of the data, from :ref:`extended
                   data types <ExtendedDataTypes>`
               :type data: `str` or `bytes`
               :type datatype: `int`

               :raises: :exc:`OSError` if the channel isn't open for sending
                        or the extended data type is not valid for this type
                        of channel

            """

            if self._send_state != 'open':
                raise BrokenPipeError('Channel not open for sending')

            if datatype is not None and datatype not in self._write_datatypes:
                raise OSError('Invalid extended data type')

            if not data:
                return

            if self._encoding:
                assert self._encoder is not None
                encoded_data = self._encoder.encode(cast(str, data))
            else:
                encoded_data = cast(bytes, data)

            datalen = len(encoded_data)

            if datatype:
                typename = ' to %s' % _data_type_names[datatype]
            else:
                typename = ''

            self.logger.debug2('Sending %d data byte%s%s', datalen,
                               's' if datalen > 1 else '', typename)

            self._send_buf.append((bytearray(encoded_data), datatype))
            self._send_buf_len += datalen
            self._flush_send_buf()

        def writelines(self, list_of_data: Iterable[AnyStr],
                       datatype: DataType = None) -> None:
            """Write a list of data bytes on the channel

               This method can be called to write a list (or any iterable) of
               data bytes to the channel. It is functionality equivalent to
               calling :meth:`write` on each element in the list.

               :param list_of_data:
                   The data to send on the channel
               :param datatype: (optional)
                   The extended data type of the data, from :ref:`extended
                   data types <ExtendedDataTypes>`
               :type list_of_data: iterable of `str` or `bytes`
               :type datatype: `int`

               :raises: :exc:`OSError` if the channel isn't open for sending
                        or the extended data type is not valid for this type
                        of channel

            """

            if self._encoding:
                data = cast(AnyStr, ''.join(cast(Iterable[str], list_of_data)))
            else:
                data = cast(AnyStr, b''.join(cast(Iterable[bytes], list_of_data)))

            return self.write(data, datatype)

        def write_eof(self) -> None:
            """Write EOF on the channel

               This method sends an end-of-file indication on the
               channel, after which no more data can be sent. The
               channel remains open, though, and data may still be
               sent in the other direction.

               :raises: :exc:`OSError` if the channel isn't open for sending

            """

            self.logger.debug2('Sending EOF')

            if self._send_state == 'open':
                self._send_state = 'eof_pending'
                self._flush_send_buf()

        def pause_reading(self) -> None:
            """Pause delivery of incoming data

               This method is used to temporarily suspend delivery of incoming
               channel data. After this call, incoming data will no longer
               be delivered until :meth:`resume_reading` is called. Data will be
               buffered locally up to the configured SSH channel window size,
               but window updates will no longer be sent, eventually causing
               back pressure on the remote system.

               .. note:: Channel close notifications are not suspended by this
                         call. If the remote system closes the channel while
                         delivery is suspended, the channel will be closed even
                         though some buffered data may not have been delivered.

            """

            self.logger.debug2('Reading from channel paused')

            self._recv_paused = True

        def resume_reading(self) -> None:
            """Resume delivery of incoming data

               This method can be called to resume delivery of incoming data
               which was suspended by a call to :meth:`pause_reading`. As soon
               as this method is called, any buffered data will be delivered
               immediately. A pending end-of-file notication may also be
               delivered if one was queued while reading was paused.

            """

            if self._recv_paused:
                self.logger.debug2('Reading from channel resumed')

                self._recv_paused = False
                self._flush_recv_buf()

        def get_environment(self) -> Mapping[str, str]:
            """Return the environment for this session

               This method returns the environment set by the client when
               the session was opened. On the server, calls to this method
               should only be made after :meth:`session_started
               <SSHServerSession.session_started>` has been called on the
               :class:`SSHServerSession`. When using the stream-based API,
               calls to this can be made at any time after the handler
               function has started up.

               :returns: A dictionary containing the environment variables
                         set by the client

            """

            return MappingProxyType(self._env)

        def get_command(self) -> Optional[str]:
            """Return the command the client requested to execute, if any

               This method returns the command the client requested to
               execute when the session was opened, if any. If the client
               did not request that a command be executed, this method
               will return `None`. On the server, alls to this method
               should only be made after :meth:`session_started
               <SSHServerSession.session_started>` has been called on the
               :class:`SSHServerSession`. When using the stream-based API,
               calls to this can be made at any time after the handler
               function has started up.

            """

            return self._command

        def get_subsystem(self) -> Optional[str]:
            """Return the subsystem the client requested to open, if any

               This method returns the subsystem the client requested to
               open when the session was opened, if any. If the client
               did not request that a subsystem be opened, this method will
               return `None`. On the server, calls to this method should
               only be made after :meth:`session_started
               <SSHServerSession.session_started>` has been called on the
               :class:`SSHServerSession`. When using the stream-based API,
               calls to this can be made at any time after the handler
               function has started up.

            """

            return self._subsystem

    class SSHClientChannel(SSHChannel, Generic[AnyStr]):
        """SSH client channel"""

        _conn: 'SSHClientConnection'
        _session: SSHClientSession[AnyStr]

        _read_datatypes = {EXTENDED_DATA_STDERR}

        def __init__(self, conn: 'SSHClientConnection',
                     loop: asyncio.AbstractEventLoop, encoding: Optional[str],
                     errors: str, window: int, max_pktsize: int):
            super().__init__(conn, loop, encoding, errors, window, max_pktsize)

            self._exit_status: Optional[int] = None
            self._exit_signal: Optional[_ExitSignal] = None

        async def create(self, session_factory: SSHClientSessionFactory[AnyStr],
                         command: Optional[str], subsystem: Optional[str],
                         env: Dict[str, str], request_pty: bool,
                         term_type: Optional[str], term_size: TermSizeArg,
                         term_modes: TermModes, x11_forwarding: Union[bool, str],
                         x11_display: Optional[str], x11_auth_path: Optional[str],
                         x11_single_connection: bool,
                         agent_forwarding: bool) -> SSHClientSession[AnyStr]:
            """Create an SSH client session"""

            self.logger.info('Requesting new SSH session')

            packet = await self._open(b'session')

            # Client sessions should have no extra data in the open confirmation
            packet.check_end()

            self._session = session_factory()
            self._session.connection_made(self)

            self._env = env
            self._command = command
            self._subsystem = subsystem

            for name, env_value in env.items():
                self.logger.debug1('  Env: %s=%s', name, env_value)
                self._send_request(b'env', String(str(name)),
                                   String(str(env_value)))

            if request_pty:
                self.logger.debug1('  Terminal type: %s', term_type or 'None')

                if not term_size:
                    width = height = pixwidth = pixheight = 0
                elif len(term_size) == 2:
                    width, height = cast(Tuple[int, int], term_size)
                    pixwidth = pixheight = 0
                    self.logger.debug1('  Terminal size: %sx%s', width, height)
                elif len(term_size) == 4:
                    width, height, pixwidth, pixheight = cast(TermSize, term_size)
                    self.logger.debug1('  Terminal size: %sx%s (%sx%s pixels)',
                                       width, height, pixwidth, pixheight)
                else:
                    raise ValueError('If set, terminal size must be a tuple of '
                                     '2 or 4 integers')

                modes = b''
                for mode, mode_value in term_modes.items():
                    if mode <= PTY_OP_END or mode >= PTY_OP_RESERVED:
                        raise ValueError('Invalid pty mode: %s' % mode)

                    name = _pty_mode_names.get(mode, str(mode))
                    self.logger.debug2('  Mode %s: %d', name, mode_value)
                    modes += Byte(mode) + UInt32(mode_value)

                modes += Byte(PTY_OP_END)

                if not (await self._make_request(b'pty-req',
                                                 String(term_type or ''),
                                                 UInt32(width), UInt32(height),
                                                 UInt32(pixwidth),
                                                 UInt32(pixheight),
                                                 String(modes))):
                    self.close()
                    raise ChannelOpenError(OPEN_REQUEST_PTY_FAILED,
                                           'PTY request failed')

            if x11_forwarding:
                self.logger.debug1('  X11 forwarding enabled')

                try:
                    attach_result: Optional[Tuple[bytes, bytes, int]] = \
                        await self._conn.attach_x11_listener(
                            self, x11_display, x11_auth_path, x11_single_connection)
                except ValueError as exc:
                    if x11_forwarding != 'ignore_failure':
                        raise ChannelOpenError(OPEN_REQUEST_X11_FORWARDING_FAILED,
                                               str(exc)) from None
                    else:
                        attach_result = None
                        self.logger.info('  X11 forwarding attach failure ignored')

                if attach_result:
                    auth_proto, remote_auth, screen = attach_result

                    result = await self._make_request(
                        b'x11-req', Boolean(x11_single_connection),
                        String(auth_proto), String(binascii.b2a_hex(remote_auth)),
                        UInt32(screen))

                    if not result:
                        if self._conn: # pragma: no branch
                            self._conn.detach_x11_listener(self)

                        if x11_forwarding != 'ignore_failure':
                            raise ChannelOpenError(
                                OPEN_REQUEST_X11_FORWARDING_FAILED,
                                'X11 forwarding request failed')
                        else:
                            self.logger.info(
                                '  X11 forwarding request failure ignored')

            if agent_forwarding:
                self.logger.debug1('  Agent forwarding enabled')
                self._send_request(b'auth-agent-req@openssh.com')

            if command:
                self.logger.info('  Command: %s', command)
                result = await self._make_request(b'exec', String(command))
            elif subsystem:
                self.logger.info('  Subsystem: %s', subsystem)
                result = await self._make_request(b'subsystem', String(subsystem))
            else:
                self.logger.info('  Interactive shell requested')
                result = await self._make_request(b'shell')

            if not result:
                self.close()
                raise ChannelOpenError(OPEN_REQUEST_SESSION_FAILED,
                                       'Session request failed')

            self._session.session_started()
            self._conn.create_task(self._start_reading(), self.logger)

            return self._session

        def _process_xon_xoff_request(self, packet: SSHPacket) -> bool:
            """Process a request to set up XON/XOFF processing"""

            client_can_do = packet.get_boolean()
            packet.check_end()

            self.logger.info('Received XON/XOFF flow control %s request',
                             'enable' if client_can_do else 'disable')

            self._session.xon_xoff_requested(client_can_do)
            return True

        def _process_exit_status_request(self, packet: SSHPacket) -> bool:
            """Process a request to deliver exit status"""

            status = packet.get_uint32() & 0xff
            packet.check_end()

            self.logger.info('Received exit status %d', status)

            self._exit_status = status
            self._session.exit_status_received(status)
            return True

        def _process_exit_signal_request(self, packet: SSHPacket) -> bool:
            """Process a request to deliver an exit signal"""

            signal_bytes = packet.get_string()
            core_dumped = packet.get_boolean()
            msg_bytes = packet.get_string()
            lang_bytes = packet.get_string()
            packet.check_end()

            try:
                signal = signal_bytes.decode('ascii')
                msg = msg_bytes.decode('utf-8')
                lang = lang_bytes.decode('ascii')
            except UnicodeDecodeError:
                raise ProtocolError('Invalid exit signal request') from None

            self.logger.info('Received exit signal %s', signal)
            self.logger.debug1('  Core dumped: %s', core_dumped)
            self.logger.debug1('  Message: %s', msg)

            self._exit_signal = (signal, core_dumped, msg, lang)
            self._session.exit_signal_received(signal, core_dumped, msg, lang)
            return True

        def get_exit_status(self) -> Optional[int]:
            """Return the session's exit status

               This method returns the exit status of the session if one has
               been sent. If an exit signal was sent, this method returns -1
               and the exit signal information can be collected by calling
               :meth:`get_exit_signal`. If neither has been sent, this method
               returns `None`.

            """

            if self._exit_status is not None:
                return self._exit_status
            elif self._exit_signal:
                return -1
            else:
                return None

        def get_exit_signal(self) -> Optional[_ExitSignal]:
            """Return the session's exit signal, if one was sent

               This method returns information about the exit signal sent on
               this session. If an exit signal was sent, a tuple is returned
               containing the signal name, a boolean for whether a core dump
               occurred, a message associated with the signal, and the language
               the message was in. Otherwise, this method returns `None`.

            """

            return self._exit_signal

        def get_returncode(self) -> Optional[int]:
            """Return the session's exit status or signal

               This method returns the exit status of the session if one has
               been sent. If an exit signal was sent, this method returns
               the negative of the numeric value of that signal, matching
               the behavior of :meth:`asyncio.SubprocessTransport.get_returncode`.
               If neither has been sent, this method returns `None`.

               :returns: `int` or `None`

            """

            if self._exit_status is not None:
                return self._exit_status
            elif self._exit_signal:
                return -_signal_numbers.get(self._exit_signal[0], 99)
            else:
                return None

        def change_terminal_size(self, width: int, height: int,
                                 pixwidth: int = 0, pixheight: int = 0) -> None:
            """Change the terminal window size for this session

               This method changes the width and height of the terminal
               associated with this session.

               :param width:
                   The width of the terminal in characters
               :param height:
                   The height of the terminal in characters
               :param pixwidth: (optional)
                   The width of the terminal in pixels
               :param pixheight: (optional)
                   The height of the terminal in pixels
               :type width: `int`
               :type height: `int`
               :type pixwidth: `int`
               :type pixheight: `int`

            """

            if pixwidth or pixheight:
                self.logger.info('Sending window size change: %sx%s (%sx%s pixels)',
                                 width, height, pixwidth, pixheight)
            else:
                self.logger.info('Sending window size change: %sx%s', width, height)

            self._send_request(b'window-change', UInt32(width), UInt32(height),
                               UInt32(pixwidth), UInt32(pixheight))

        def send_break(self, msec: int) -> None:
            """Send a break to the remote process

               This method requests that the server perform a break
               operation on the remote process or service as described in
               :rfc:`4335`.

               :param msec:
                   The duration of the break in milliseconds
               :type msec: `int`

               :raises: :exc:`OSError` if the channel is not open

            """

            self.logger.info('Sending %d msec break', msec)

            self._send_request(b'break', UInt32(msec))

        def send_signal(self, signal: Union[str, int]) -> None:
            """Send a signal to the remote process

               This method can be called to deliver a signal to the remote
               process or service. Signal names should be as described in
               section 6.10 of :rfc:`RFC 4254 <4254#section-6.10>`, or
               can be integer values as defined in the :mod:`signal`
               module, in which case they will be translated to their
               corresponding signal name before being sent.

               .. note:: OpenSSH's SSH server implementation prior to version
                         7.9 does not support this message, so attempts to
                         use :meth:`send_signal`, :meth:`terminate`, or
                         :meth:`kill` with an older OpenSSH SSH server will
                         end up being ignored. This was tracked in OpenSSH
                         `bug 1424`__.

                         __ https://bugzilla.mindrot.org/show_bug.cgi?id=1424

               :param signal:
                   The signal to deliver
               :type signal: `str` or `int`

               :raises: | :exc:`OSError` if the channel is not open
                        | :exc:`ValueError` if the signal number is unknown

            """

            if isinstance(signal, int):
                try:
                    signal = _signal_names[signal]
                except KeyError:
                    raise ValueError('Unknown signal: %s' % int(signal)) from None

            self.logger.info('Sending %s signal', signal)

            self._send_request(b'signal', String(signal))

        def terminate(self) -> None:
            """Terminate the remote process

               This method can be called to terminate the remote process or
               service by sending it a `TERM` signal.

               :raises: :exc:`OSError` if the channel is not open

               .. note:: If your server-side runs on OpenSSH,
                         this might be ineffective;
                         for more details, see the note in
                         :meth:`send_signal`

            """

            self.send_signal('TERM')

        def kill(self) -> None:
            """Forcibly kill the remote process

               This method can be called to forcibly stop the remote process
               or service by sending it a `KILL` signal.

               :raises: :exc:`OSError` if the channel is not open

               .. note:: If your server-side runs on OpenSSH,
                         this might be ineffective;
                         for more details, see the note in
                         :meth:`send_signal`

            """

            self.send_signal('KILL')

    class SSHServerChannel(SSHChannel, Generic[AnyStr]):
        """SSH server channel"""

        _conn: 'SSHServerConnection'
        _session: SSHServerSession[AnyStr]

        _write_datatypes = {EXTENDED_DATA_STDERR}

        def __init__(self, conn: 'SSHServerConnection',
                     loop: asyncio.AbstractEventLoop, allow_pty: bool,
                     line_editor: bool, line_echo: bool, line_history: int,
                     max_line_length: int, encoding: Optional[str], errors: str,
                     window: int, max_pktsize: int):
            """Initialize an SSH server channel"""

            super().__init__(conn, loop, encoding, errors, window, max_pktsize)

            self._env = cast(Dict[str, str],
                             conn.get_key_option('environment', {}))

            self._allow_pty = allow_pty
            self._line_editor = line_editor
            self._line_echo = line_echo
            self._line_history = line_history
            self._max_line_length = max_line_length
            self._term_type: Optional[str] = None
            self._term_size = (0, 0, 0, 0)
            self._term_modes: TermModes = {}
            self._x11_display: Optional[str] = None

            self.logger.info('New SSH session requested')

        def _wrap_session(self, session: SSHSession[AnyStr]) -> \
                Tuple[SSHChannel[AnyStr], SSHSession[AnyStr]]:
            """Wrap a line editor around the session if enabled"""

            if self._line_editor:
                server_chan = cast(SSHServerChannel[str], self)
                server_session = cast(SSHServerSession[str], session)

                editor_chan = SSHLineEditorChannel(server_chan, server_session,
                                                   self._line_echo,
                                                   self._line_history,
                                                   self._max_line_length)
                editor_session = SSHLineEditorSession(editor_chan, server_session)

                chan = cast(SSHChannel[AnyStr], editor_chan)
                session = cast(SSHSession[AnyStr], editor_session)
            else:
                chan = self

            return chan, session

        def _process_pty_req_request(self, packet: SSHPacket) -> bool:
            """Process a request to open a pseudo-terminal"""

            term_type_bytes = packet.get_string()
            width = packet.get_uint32()
            height = packet.get_uint32()
            pixwidth = packet.get_uint32()
            pixheight = packet.get_uint32()
            modes = packet.get_string()
            packet.check_end()

            if not self._allow_pty or \
               not self._conn.check_key_permission('pty') or \
               not self._conn.check_certificate_permission('pty'):
                self.logger.info('PTY request denied: PTY not permitted')
                return False

            try:
                term_type = term_type_bytes.decode('ascii')
            except UnicodeDecodeError:
                raise ProtocolError('Invalid pty request') from None

            term_size = (width, height, pixwidth, pixheight)
            term_modes = {}

            self.logger.debug1('  Terminal type: %s', term_type)

            if pixwidth or pixheight:
                self.logger.debug1('  Terminal size: %sx%s (%sx%s pixels)',
                                   width, height, pixwidth, pixheight)
            else:
                self.logger.debug1('  Terminal size: %sx%s', width, height)

            idx = 0
            while idx < len(modes):
                mode = modes[idx]
                idx += 1
                if mode == PTY_OP_END or mode >= PTY_OP_RESERVED:
                    break

                if idx+4 <= len(modes):
                    name = _pty_mode_names.get(mode, str(mode))
                    value = int.from_bytes(modes[idx:idx+4], 'big')
                    self.logger.debug2('  Mode %s: %s', name, value)
                    term_modes[mode] = value
                    idx += 4
                else:
                    raise ProtocolError('Invalid pty modes string')

            result = self._session.pty_requested(term_type, term_size, term_modes)

            if result:
                self.logger.info('  PTY created')

                if self._line_editor:
                    self.logger.info('  Line editor enabled')

                self._term_type = term_type
                self._term_size = term_size
                self._term_modes = term_modes
            else:
                self.logger.info('  PTY creation failed')

            return result

        def _process_x11_req_request(self, packet: SSHPacket) -> Optional[bool]:
            """Process request to enable X11 forwarding"""

            _ = packet.get_boolean()                        # single_connection
            auth_proto = packet.get_string()
            auth_data = packet.get_string()
            screen = packet.get_uint32()
            packet.check_end()

            try:
                auth_data = binascii.a2b_hex(auth_data)
            except binascii.Error:
                self.logger.debug1('  Invalid X11 auth data')
                return False

            self._conn.create_task(self._finish_x11_req_request(auth_proto,
                                                                auth_data, screen),
                                   self.logger)
            return None

        async def _finish_x11_req_request(self, auth_proto: bytes,
                                          auth_data: bytes, screen: int) -> None:
            """Finish processing request to enable X11 forwarding"""

            self._x11_display = await self._conn.attach_x11_listener(
                self, auth_proto, auth_data, screen)

            if self._x11_display:
                self.logger.debug1('  X11 forwarding enabled')
                self._report_response(True)
            else:
                self.logger.debug1('  X11 forwarding failed')
                self._report_response(False)

        def _process_auth_agent_req_at_openssh_dot_com_request(
                self, packet: SSHPacket) -> None:
            """Process a request to enable ssh-agent forwarding"""

            packet.check_end()

            self._conn.create_task(self._finish_agent_req_request(), self.logger)

        async def _finish_agent_req_request(self) -> None:
            """Finish processing request to enable agent forwarding"""

            if await self._conn.create_agent_listener():
                self.logger.debug1('  Agent forwarding enabled')
                self._report_response(True)
            else:
                self.logger.debug1('  Agent forwarding failed')
                self._report_response(False)

        def _process_env_request(self, packet: SSHPacket) -> bool:
            """Process a request to set an environment variable"""

            name_bytes = packet.get_string()
            value_bytes = packet.get_string()
            packet.check_end()

            try:
                name = name_bytes.decode('utf-8')
                value = value_bytes.decode('utf-8')
            except UnicodeDecodeError:
                self.logger.debug1('Invalid environment data')
                return False

            self.logger.debug1('  Env: %s=%s', name, value)
            self._env[name] = value
            return True

        def _start_session(self, command: str = None,
                           subsystem: str = None) -> bool:
            """Tell the session what type of channel is being requested"""

            forced_command = \
                cast(str, self._conn.get_certificate_option('force-command'))

            if forced_command is None:
                forced_command = cast(str, self._conn.get_key_option('command'))

            if forced_command is not None:
                self.logger.info('  Forced command override: %s', forced_command)

                command = forced_command

            if command is not None:
                self._command = command
                result = self._session.exec_requested(command)
            elif subsystem is not None:
                self._subsystem = subsystem
                result = self._session.subsystem_requested(subsystem)
            else:
                result = self._session.shell_requested()

            return result

        def _process_shell_request(self, packet: SSHPacket) -> bool:
            """Process a request to open a shell"""

            packet.check_end()

            self.logger.info('  Interactive shell requested')
            return self._start_session()

        def _process_exec_request(self, packet: SSHPacket) -> bool:
            """Process a request to execute a command"""

            command_bytes = packet.get_string()
            packet.check_end()

            try:
                command = command_bytes.decode('utf-8')
            except UnicodeDecodeError:
                return False

            self.logger.info('  Command: %s', command)
            return self._start_session(command=command)

        def _process_subsystem_request(self, packet: SSHPacket) -> bool:
            """Process a request to open a subsystem"""

            subsystem_bytes = packet.get_string()
            packet.check_end()

            try:
                subsystem = subsystem_bytes.decode('ascii')
            except UnicodeDecodeError:
                return False

            self.logger.info('  Subsystem: %s', subsystem)
            return self._start_session(subsystem=subsystem)

        def _process_window_change_request(self, packet: SSHPacket) -> bool:
            """Process a request to change the window size"""

            width = packet.get_uint32()
            height = packet.get_uint32()
            pixwidth = packet.get_uint32()
            pixheight = packet.get_uint32()
            packet.check_end()

            if pixwidth or pixheight:
                self.logger.info('Received window change: %sx%s (%sx%s pixels)',
                                 width, height, pixwidth, pixheight)
            else:
                self.logger.info('Recceived window change: %sx%s', width, height)

            self._term_size = (width, height, pixwidth, pixheight)
            self._session.terminal_size_changed(width, height, pixwidth, pixheight)
            return True

        def _process_signal_request(self, packet: SSHPacket) -> bool:
            """Process a request to send a signal"""

            signal_bytes = packet.get_string()
            packet.check_end()

            try:
                signal = signal_bytes.decode('ascii')
            except UnicodeDecodeError:
                return False

            self.logger.info('Received %s signal', signal)

            self._session.signal_received(signal)
            return True

        def _process_break_request(self, packet: SSHPacket) -> bool:
            """Process a request to send a break"""

            msec = packet.get_uint32()
            packet.check_end()

            self.logger.info('Received %d msec break', msec)

            return self._session.break_received(msec)

        def get_terminal_type(self) -> Optional[str]:
            """Return the terminal type for this session

               This method returns the terminal type set by the client
               when the session was opened. If the client didn't request
               a pseudo-terminal, this method will return `None`. Calls
               to this method should only be made after :meth:`session_started
               <SSHServerSession.session_started>` has been called on the
               :class:`SSHServerSession`. When using the stream-based API,
               calls to this can be made at any time after the handler
               function has started up.

               :returns: A `str` containing the terminal type or `None` if
                         no pseudo-terminal was requested

            """

            return self._term_type

        def get_terminal_size(self) -> TermSize:
            """Return terminal size information for this session

               This method returns the latest terminal size information set
               by the client. If the client didn't set any terminal size
               information, all values returned will be zero. Calls to
               this method should only be made after :meth:`session_started
               <SSHServerSession.session_started>` has been called on the
               :class:`SSHServerSession`. When using the stream-based API,
               calls to this can be made at any time after the handler
               function has started up.

               Also see :meth:`terminal_size_changed()
               <SSHServerSession.terminal_size_changed>` or the
               :exc:`TerminalSizeChanged` exception for how to get notified
               when the terminal size changes.

               :returns: A tuple of four `int` values containing the width and
                         height of the terminal in characters and the width
                         and height of the terminal in pixels

            """

            return self._term_size

        def get_terminal_mode(self, mode: int) -> Optional[int]:
            """Return the requested TTY mode for this session

               This method looks up the value of a POSIX terminal mode
               set by the client when the session was opened. If the client
               didn't request a pseudo-terminal or didn't set the requested
               TTY mode opcode, this method will return `None`. Calls to
               this method should only be made after :meth:`session_started
               <SSHServerSession.session_started>` has been called on the
               :class:`SSHServerSession`. When using the stream-based API,
               calls to this can be made at any time after the handler
               function has started up.

               :param mode:
                   POSIX terminal mode taken from :ref:`POSIX terminal modes
                   <PTYModes>` to look up
               :type mode: `int`

               :returns: An `int` containing the value of the requested
                         POSIX terminal mode or `None` if the requested
                         mode was not set

            """

            return self._term_modes.get(mode)

        def get_terminal_modes(self) -> TermModes:
            """Return the TTY modes for this session

               This method returns a mapping of all the POSIX terminal modes
               set by the client when the session was opened. If the client
               didn't request a pseudo-terminal, this method will return an
               empty mapping. Calls to this method should only be made after
               :meth:`session_started <SSHServerSession.session_started>`
               has been called on the :class:`SSHServerSession`. When using
               the stream-based API, calls to this can be made at any time
               after the handler function has started up.

                :returns: A mapping containing all the POSIX terminal modes
                          set by the client or an empty mapping if no
                          pseudo-terminal was requested

            """

            return MappingProxyType(self._term_modes)

        def get_x11_display(self) -> Optional[str]:
            """Return the display to use for X11 forwarding

               When X11 forwarding has been requested by the client, this
               method returns the X11 display which should be used to open
               a forwarded connection. If the client did not request X11
               forwarding, this method returns `None`.

               :returns: A `str` containing the X11 display or `None` if
                         X11 fowarding was not requested

            """

            return self._x11_display

        def get_agent_path(self) -> Optional[str]:
            """Return the path of the ssh-agent listening socket

               When agent forwarding has been requested by the client,
               this method returns the path of the listening socket which
               should be used to open a forwarded agent connection. If the
               client did not request agent forwarding, this method returns
               `None`.

               :returns: A `str` containing the ssh-agent socket path or
                         `None` if agent fowarding was not requested

            """

            return self._conn.get_agent_path()

        def set_xon_xoff(self, client_can_do: bool) -> None:
            """Set whether the client should enable XON/XOFF flow control

               This method can be called to tell the client whether or not
               to enable XON/XOFF flow control, indicating that it should
               intercept Control-S and Control-Q coming from its local
               terminal to pause and resume output, respectively.
               Applications should set client_can_do to `True` to
               enable this functionality or to `False` to tell the client
               to forward Control-S and Control-Q through as normal input.

               :param client_can_do:
                   Whether or not the client should enable XON/XOFF flow control
               :type client_can_do: `bool`

            """

            self.logger.info('Sending XON/XOFF flow control %s',
                             'enable' if client_can_do else 'disable')

            self._send_request(b'xon-xoff', Boolean(client_can_do))

        def write_stderr(self, data: AnyStr) -> None:
            """Write output to stderr

               This method can be called to send output to the client which
               is intended to be displayed on stderr. If an encoding was
               specified when the channel was created, the data should be
               provided as a string and will be converted using that
               encoding. Otherwise, the data should be provided as bytes.

               :param data:
                   The data to send to stderr
               :type data: `str` or `bytes`

               :raises: :exc:`OSError` if the channel isn't open for sending

            """

            self.write(data, EXTENDED_DATA_STDERR)

        def writelines_stderr(self, list_of_data: Iterable[AnyStr]) -> None:
            """Write a list of data bytes to stderr

               This method can be called to write a list (or any iterable) of
               data bytes to the channel. It is functionality equivalent to
               calling :meth:`write_stderr` on each element in the list.

            """

            self.writelines(list_of_data, EXTENDED_DATA_STDERR)

        def exit(self, status: int) -> None:
            """Send exit status and close the channel

               This method can be called to report an exit status for the
               process back to the client and close the channel. A zero
               exit status is generally returned when the operation was
               successful. After reporting the status, the channel is
               closed.

               :param status:
                   The exit status to report to the client
               :type status: `int`

               :raises: :exc:`OSError` if the channel isn't open

            """

            status &= 0xff

            if self._send_state not in {'close_pending', 'closed'}:
                self.logger.info('Sending exit status %d', status)

                self._send_request(b'exit-status', UInt32(status))
                self.close()

        def exit_with_signal(self, signal: str, core_dumped: bool = False,
                             msg: str = '', lang: str = DEFAULT_LANG) -> None:
            """Send exit signal and close the channel

               This method can be called to report that the process
               terminated abnormslly with a signal. A more detailed
               error message may also provided, along with an indication
               of whether or not the process dumped core. After
               reporting the signal, the channel is closed.

               :param signal:
                   The signal which caused the process to exit
               :param core_dumped: (optional)
                   Whether or not the process dumped core
               :param msg: (optional)
                   Details about what error occurred
               :param lang: (optional)
                   The language the error message is in
               :type signal: `str`
               :type core_dumped: `bool`
               :type msg: `str`
               :type lang: `str`

               :raises: :exc:`OSError` if the channel isn't open

            """

            self.logger.info('Sending exit signal %s', signal)
            self.logger.debug1('  Core dumped: %s', core_dumped)
            self.logger.debug1('  Message: %s', msg)

            if self._send_state not in {'close_pending', 'closed'}:
                self._send_request(b'exit-signal', String(signal),
                                   Boolean(core_dumped), String(msg), String(lang))
                self.close()

    class SSHForwardChannel(SSHChannel, Generic[AnyStr]):
        """SSH channel for forwarding TCP and UNIX domain connections"""

        async def _finish_open_request(
                self, result: MaybeAwait[SSHSession[AnyStr]]) -> None:
            """Finish processing a forward channel open request"""

            await super()._finish_open_request(result)

            if self._session is not None:
                self._session.session_started()
                self.resume_reading()

        async def _open_forward(self, session_factory: SSHSessionFactory[AnyStr],
                                chantype: bytes, *args: bytes) -> \
                SSHSession[AnyStr]:
            """Open a forward channel"""

            packet = await super()._open(chantype, *args)

            # Forward channels should have no extra data in the open confirmation
            packet.check_end()

            self._session = session_factory()
            self._session.connection_made(self)
            self._session.session_started()

            assert self._conn is not None
            self._conn.create_task(self._start_reading(), self.logger)

            return self._session

    class SSHTCPChannel(SSHForwardChannel, Generic[AnyStr]):
        """SSH TCP channel"""

        async def _open_tcp(self, session_factory: SSHTCPSessionFactory[AnyStr],
                            chantype: bytes, host: str, port: int, orig_host: str,
                            orig_port: int) -> SSHTCPSession[AnyStr]:
            """Open a TCP channel"""

            self.set_extra_info(peername=('', 0),
                                local_peername=(orig_host, orig_port),
                                remote_peername=(host, port))

            return cast(SSHTCPSession[AnyStr],
                        await self._open_forward(session_factory, chantype,
                                                 String(host), UInt32(port),
                                                 String(orig_host),
                                                 UInt32(orig_port)))

        async def connect(self, session_factory: SSHTCPSessionFactory[AnyStr],
                         host: str, port: int, orig_host: str, orig_port: int) -> \
                SSHTCPSession[AnyStr]:
            """Create a new outbound TCP session"""

            return (await self._open_tcp(session_factory, b'direct-tcpip',
                                         host, port, orig_host, orig_port))

        async def accept(self, session_factory: SSHTCPSessionFactory[AnyStr],
                         host: str, port: int, orig_host: str,
                         orig_port: int) -> SSHTCPSession[AnyStr]:
            """Create a new forwarded TCP session"""

            return (await self._open_tcp(session_factory, b'forwarded-tcpip',
                                         host, port, orig_host, orig_port))

        def set_inbound_peer_names(self, dest_host: str, dest_port: int,
                                   orig_host: str, orig_port: int) -> None:
            """Set local and remote peer names for inbound connections"""

            self.set_extra_info(peername=('', 0),
                                local_peername=(dest_host, dest_port),
                                remote_peername=(orig_host, orig_port))

    class SSHUNIXChannel(SSHForwardChannel, Generic[AnyStr]):
        """SSH UNIX channel"""

        async def _open_unix(self, session_factory: SSHUNIXSessionFactory[AnyStr],
                             chantype: bytes, path: str,
                              *args: bytes) -> SSHUNIXSession[AnyStr]:
            """Open a UNIX channel"""

            self.set_extra_info(local_peername='', remote_peername=path)

            return cast(SSHUNIXSession[AnyStr],
                        await self._open_forward(session_factory, chantype,
                                                 String(path), *args))

        async def connect(self, session_factory: SSHUNIXSessionFactory[AnyStr],
                          path: str) -> SSHUNIXSession[AnyStr]:
            """Create a new outbound UNIX session"""

            # OpenSSH appears to have a bug which requires an originator
            # host and port to be sent after the path name to connect to
            # when opening a direct streamlocal channel.
            return await self._open_unix(session_factory,
                                         b'direct-streamlocal@openssh.com',
                                         path, String(''), UInt32(0))

        async def accept(self, session_factory: SSHUNIXSessionFactory[AnyStr],
                         path: str) -> SSHUNIXSession[AnyStr]:
            """Create a new forwarded UNIX session"""

            return await self._open_unix(session_factory,
                                         b'forwarded-streamlocal@openssh.com',
                                         path, String(''))

        def set_inbound_peer_names(self, dest_path: str) -> None:
            """Set local and remote peer names for inbound connections"""

            self.set_extra_info(local_peername=dest_path, remote_peername='')

    class SSHX11Channel(SSHForwardChannel[bytes]):
        """SSH X11 channel"""

        async def open(self, session_factory: SSHTCPSessionFactory[bytes],
                       orig_host: str, orig_port: int) -> SSHTCPSession[bytes]:
            """Open an SSH X11 channel"""

            self.set_extra_info(local_peername=(orig_host, orig_port),
                                remote_peername=('', 0))

            return cast(SSHTCPSession[bytes],
                        await self._open_forward(session_factory, b'x11',
                                                 String(orig_host),
                                                 UInt32(orig_port)))

        def set_inbound_peer_names(self, orig_host: str, orig_port: int) -> None:
            """Set local and remote peer name for inbound connections"""

            self.set_extra_info(local_peername=('', 0),
                                remote_peername=(orig_host, orig_port))

    class SSHAgentChannel(SSHForwardChannel[bytes]):
        """SSH agent channel"""

        async def open(self, session_factory: SSHUNIXSessionFactory[bytes]) -> \
                SSHUNIXSession[bytes]:
            """Open an SSH agent channel"""

            return cast(SSHUNIXSession[bytes],
                        await self._open_forward(session_factory,
                                                 b'auth-agent@openssh.com'))