# Source code for asyncssh.channel

    # Copyright (c) 2013-2015 by Ron Frederick <ronf@timeheart.net>.
    # All rights reserved.
    #
    # This program and the accompanying materials are made available under
    # the terms of the Eclipse Public License v1.0 which accompanies this
    # distribution and is available at:
    #
    #     http://www.eclipse.org/legal/epl-v10.html
    #
    # Contributors:
    #     Ron Frederick - initial implementation, API, and documentation

    """SSH channel and session handlers"""

    import asyncio

    from .constants import DEFAULT_LANG, DISC_PROTOCOL_ERROR, 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_PTY_FAILED, OPEN_REQUEST_SESSION_FAILED
    from .misc import ChannelOpenError, DisconnectError
    from .packet import Boolean, Byte, String, UInt32, SSHPacketHandler
    from .sftp import SFTPServerSession

    _EOF = object()

    class SSHChannel(SSHPacketHandler):
        """Parent class for SSH channels"""

        _read_datatypes = set()
        _write_datatypes = set()

        def __init__(self, conn, loop, encoding, window, max_pktsize):
            """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 = conn
            self._loop = loop
            self._session = None
            self._encoding = encoding
            self._extra = {'connection': conn}

            self._send_state = 'closed'
            self._send_chan = None
            self._send_window = None
            self._send_pktsize = None
            self._send_paused = False
            self._send_buf = []
            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 = True
            self._recv_buf = []
            self._recv_partial = {}

            self._open_waiter = None
            self._request_waiters = []
            self._close_waiters = []

            self.set_write_buffer_limits()

            self._recv_chan = conn.add_channel(self)

        def get_loop(self):
            """Return the event loop used by this channel"""

            return self._loop

        def get_encoding(self):
            """Return the encoding used by this channel"""

            return self._encoding

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

            return self._init_recv_window

        def get_read_datatypes(self):
            """Return the legal read data types for this channel"""

            return self._read_datatypes

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

            if self._open_waiter:
                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:
                    waiter.set_exception(exc)

                self._request_waiters = []

            if self._close_waiters:
                for waiter in self._close_waiters:
                    if not waiter.cancelled():
                        waiter.set_result(None)

                self._close_waiters = []

            if self._session:
                self._session.connection_lost(exc)
                self._session = None

            if self._conn:
                if self._recv_chan:
                    self._conn.remove_channel(self._recv_chan)
                    self._recv_chan = None

                self._conn = None

            self._send_state = 'closed'
            self._recv_state = 'closed'

        def _pause_resume_writing(self):
            """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._send_paused = False
                    self._session.resume_writing()
            else:
                if self._send_buf_len > self._send_high_water:
                    self._send_paused = True
                    self._session.pause_writing()

        def _flush_send_buf(self):
            """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_sent'
                elif self._send_state == 'close_pending':
                    self._send_packet(MSG_CHANNEL_CLOSE)
                    self._send_state = 'close_sent'

        def _deliver_data(self, data, datatype):
            """Deliver incoming data to the session"""

            if data == _EOF:
                if datatype in self._recv_partial:
                    raise DisconnectError(DISC_PROTOCOL_ERROR,
                                          'Unicode decode error')

                if not self._session.eof_received() and self._send_state == 'open':
                    self.write_eof()
            else:
                self._recv_window -= len(data)

                if self._recv_window < self._init_recv_window / 2:
                    self._send_packet(MSG_CHANNEL_WINDOW_ADJUST,
                                      UInt32(self._init_recv_window -
                                             self._recv_window))
                    self._recv_window = self._init_recv_window

                if self._encoding:
                    if datatype in self._recv_partial:
                        encdata = self._recv_partial.pop(datatype) + data
                    else:
                        encdata = data

                    while encdata:
                        try:
                            data = encdata.decode(self._encoding)
                            encdata = b''
                        except UnicodeDecodeError as exc:
                            if exc.start > 0:
                                # Avoid pylint false positive
                                # pylint: disable=invalid-slice-index
                                data = encdata[:exc.start].decode()
                                encdata = encdata[exc.start:]
                            elif exc.reason == 'unexpected end of data':
                                break
                            else:
                                raise DisconnectError(DISC_PROTOCOL_ERROR,
                                                      'Unicode decode error')

                        self._session.data_received(data, datatype)

                    if encdata:
                        self._recv_partial[datatype] = encdata
                else:
                    self._session.data_received(data, datatype)

        def _accept_data(self, data, datatype=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', 'close_sent', 'closed'}:
                return

            if data != _EOF and len(data) > self._recv_window:
                raise DisconnectError(DISC_PROTOCOL_ERROR, 'Window exceeded')

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

        def process_connection_close(self, exc):
            """Process the SSH connection closing"""

            self._cleanup(exc)

        def process_open(self, send_chan, send_window, send_pktsize, session):
            """Process a channel open request"""

            if self._recv_state != 'closed':
                raise DisconnectError(DISC_PROTOCOL_ERROR, 'Channel already open')

            self._send_state = 'open_received'
            self._send_chan = send_chan
            self._send_window = send_window
            self._send_pktsize = send_pktsize

            asyncio.async(self._finish_open_request(session), loop=self._loop)

        @asyncio.coroutine
        def _finish_open_request(self, session):
            """Finish processing a channel open request"""

            try:
                if asyncio.iscoroutine(session):
                    session = yield from session

                self._session = session

                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(self)
            except ChannelOpenError as exc:
                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, send_window,
                                      send_pktsize, packet):
            """Process a channel open confirmation"""

            if not self._open_waiter:
                raise DisconnectError(DISC_PROTOCOL_ERROR,
                                      'Channel not being opened')

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

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

            if not self._open_waiter.cancelled():
                self._open_waiter.set_result(packet)
            self._open_waiter = None

        def process_open_failure(self, code, reason, lang):
            """Process a channel open failure"""

            if not self._open_waiter:
                raise DisconnectError(DISC_PROTOCOL_ERROR,
                                      'Channel not being opened')

            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, packet):
            """Process a send window adjustment"""

            # pylint: disable=unused-argument

            if self._recv_state not in {'open', 'eof_received'}:
                raise DisconnectError(DISC_PROTOCOL_ERROR, 'Channel not open')

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

            self._send_window += adjust
            self._flush_send_buf()

        def _process_data(self, pkttype, packet):
            """Process incoming data"""

            # pylint: disable=unused-argument

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

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

            self._accept_data(data)

        def _process_extended_data(self, pkttype, packet):
            """Process incoming extended data"""

            # pylint: disable=unused-argument

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

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

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

            self._accept_data(data, datatype)

        def _process_eof(self, pkttype, packet):
            """Process an incoming end of file"""

            # pylint: disable=unused-argument

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

            packet.check_end()

            self._recv_state = 'eof_received'
            self._accept_data(_EOF)

        def _process_close(self, pkttype, packet):
            """Process an incoming channel close"""

            # pylint: disable=unused-argument

            if self._recv_state not in {'open', 'eof_received'}:
                raise DisconnectError(DISC_PROTOCOL_ERROR, 'Channel not open')

            packet.check_end()

            # Flush any unsent data
            self._send_buf = []
            self._send_buf_len = 0

            # If we haven't yet sent a close, send one now
            if self._send_state not in {'close_sent', 'closed'}:
                self._send_packet(MSG_CHANNEL_CLOSE)
                self._send_state = 'close_sent'

            self._loop.call_soon(self._cleanup)

        def _process_request(self, pkttype, packet):
            """Process an incoming channel request"""

            # pylint: disable=unused-argument

            if self._recv_state not in {'open', 'eof_received'}:
                raise DisconnectError(DISC_PROTOCOL_ERROR, 'Channel not open')

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

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

            try:
                request = request.decode('ascii')
            except UnicodeDecodeError:
                raise DisconnectError(DISC_PROTOCOL_ERROR,
                                      'Invalid channel request') from None

            name = '_process_' + request.replace('-', '_') + '_request'
            handler = getattr(self, name, None)
            result = handler(packet) if callable(handler) else False

            if want_reply:
                if result:
                    self._send_packet(MSG_CHANNEL_SUCCESS)
                else:
                    self._send_packet(MSG_CHANNEL_FAILURE)

            if result and request in ('shell', 'exec', 'subsystem'):
                self._session.session_started()
                self.resume_reading()

        def _process_response(self, pkttype, packet):
            """Process a success or failure response"""

            # pylint: disable=unused-argument

            if self._send_state not in {'open', 'eof_pending', 'eof_sent',
                                        'close_pending', 'close_sent'}:
                raise DisconnectError(DISC_PROTOCOL_ERROR, 'Channel not open')

            packet.check_end()

            if self._request_waiters:
                waiter = self._request_waiters.pop(0)
                if not waiter.cancelled():
                    waiter.set_result(pkttype == MSG_CHANNEL_SUCCESS)
            else:
                raise DisconnectError(DISC_PROTOCOL_ERROR,
                                      'Unexpected channel response')

        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
        }

        @asyncio.coroutine
        def _open(self, chantype, *args):
            """Make a request to open the channel"""

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

            self._open_waiter = asyncio.Future(loop=self._loop)

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

            self._send_state = 'open_sent'
            return (yield from self._open_waiter)

        def _send_packet(self, pkttype, *args):
            """Send a packet on the channel"""

            if self._send_chan is None:
                raise OSError('Channel not open')

            self._conn.send_packet(Byte(pkttype), UInt32(self._send_chan), *args)

        def _send_request(self, request, *args, want_reply=False):
            """Send a channel request"""

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

        @asyncio.coroutine
        def _make_request(self, request, *args):
            """Make a channel request and wait for the response"""

            waiter = asyncio.Future(loop=self._loop)
            self._request_waiters.append(waiter)
            self._send_request(request, *args, want_reply=True)
            return (yield from waiter)

        def abort(self):
            """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.

            """

            if self._send_state not in {'close_sent', 'closed'}:
                self._send_packet(MSG_CHANNEL_CLOSE)
                self._send_state = 'close_sent'

        def close(self):
            """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.

            """

            if self._send_state not in {'close_pending', 'close_sent', 'closed'}:
                self._send_state = 'close_pending'
                self._flush_send_buf()

        @asyncio.coroutine
        def wait_closed(self):
            """Wait for this channel to close

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

            """

            if self._session:
                waiter = asyncio.Future(loop=self._loop)
                self._close_waiters.append(waiter)
                yield from waiter

        def get_extra_info(self, name, default=None):
            """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.

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

            """

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

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

               This method sets the limits used when deciding when to call
               the ``pause_writing()`` and ``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._send_high_water = high
            self._send_low_water = low
            self._pause_resume_writing()

        def write(self, data, datatype=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 integer datatype: (optional)
                   The extended data type of the data, from :ref:`extended
                   data types <ExtendedDataTypes>`
               :type data: string or bytes

               :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 len(data) == 0:
                return

            if self._encoding:
                data = data.encode(self._encoding)

            self._send_buf.append((bytearray(data), datatype))
            self._send_buf_len += len(data)
            self._flush_send_buf()

        def writelines(self, list_of_data, datatype=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 integer datatype: (optional)
                   The extended data type of the data, from :ref:`extended
                   data types <ExtendedDataTypes>`
               :type list_of_data: iterable of ``string`` or ``bytes`` objects

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

            """

            sep = '' if self._encoding else b''
            return self.write(sep.join(list_of_data), datatype)

        def write_eof(self):
            """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

            """

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

            self._send_state = 'eof_pending'
            self._flush_send_buf()

        def pause_reading(self):
            """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._recv_paused = True

        def resume_reading(self):
            """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._recv_paused = False

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

    class SSHClientChannel(SSHChannel):
        """SSH client channel"""

        _read_datatypes = {EXTENDED_DATA_STDERR}

        def __init__(self, conn, loop, encoding, window, max_pktsize):
            super().__init__(conn, loop, encoding, window, max_pktsize)

            self._exit_status = None
            self._exit_signal = None

        @asyncio.coroutine
        def create(self, session_factory, command, subsystem, env,
                   term_type, term_size, term_modes):
            """Create an SSH client session"""

            packet = yield from 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)

            for name, value in env.items():
                name = str(name).encode('utf-8')
                value = str(value).encode('utf-8')
                self._send_request(b'env', String(name), String(value))

            if term_type:
                term_type = term_type.encode('ascii')

                if len(term_size) == 4:
                    width, height, pixwidth, pixheight = term_size
                elif len(term_size) == 2:
                    width, height = term_size
                    pixwidth = pixheight = 0
                elif not term_size:
                    width = height = pixwidth = pixheight = 0
                else:
                    raise ValueError('If set, terminal size must be a tuple of '
                                     '2 or 4 integers')

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

                    modes += Byte(mode) + UInt32(value)

                modes += Byte(PTY_OP_END)

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

            if command:
                result = yield from self._make_request(b'exec', String(command))
            elif subsystem:
                result = yield from self._make_request(b'subsystem',
                                                       String(subsystem))
            else:
                result = yield from self._make_request(b'shell')

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

            if not self._session:
                raise ChannelOpenError(OPEN_REQUEST_SESSION_FAILED,
                                       'Channel closed during session startup')

            self._session.session_started()
            self.resume_reading()

            return self, self._session

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

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

            self._session.xon_xoff_requested(client_can_do)
            return True

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

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

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

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

            signal = packet.get_string()
            core_dumped = packet.get_boolean()
            msg = packet.get_string()
            lang = packet.get_string()
            packet.check_end()

            try:
                signal = signal.decode('ascii')
                msg = msg.decode('utf-8')
                lang = lang.decode('ascii')
            except UnicodeDecodeError:
                raise DisconnectError(DISC_PROTOCOL_ERROR,
                                      'Invalid exit signal request') from None

            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):
            """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 received, 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):
            """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. If no exit signal was sent, ``None`` is
               returned.

            """

            return self._exit_signal

        def change_terminal_size(self, width, height, pixwidth=0, pixheight=0):
            """Change the terminal window size for this session

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

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

            """

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

        def send_break(self, msec):
            """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 integer msec:
                   The duration of the break in milliseconds

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

            """

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

        def send_signal(self, signal):
            """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:`4254#section-6.10`.

               :param string signal:
                   The signal to deliver

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

            """

            signal = signal.encode('ascii')

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

        def terminate(self):
            """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

            """

            self.send_signal('TERM')

        def kill(self):
            """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

            """

            self.send_signal('KILL')

    class SSHServerChannel(SSHChannel):
        """SSH server channel"""

        _write_datatypes = {EXTENDED_DATA_STDERR}

        def __init__(self, conn, loop, encoding, window, max_pktsize):
            """Initialize an SSH server channel"""

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

            self._env = self._conn.get_key_option('environment', {})
            self._command = None
            self._subsystem = None
            self._term_type = None
            self._term_size = (0, 0, 0, 0)
            self._term_modes = {}

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

            term_type = 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()

            try:
                self._term_type = term_type.decode('ascii')
            except UnicodeDecodeError:
                raise DisconnectError(DISC_PROTOCOL_ERROR,
                                      'Invalid pty request') from None

            if not self._conn.check_key_permission('pty') or \
               not self._conn.check_certificate_permission('pty'):
                return False

            self._term_size = (width, height, pixwidth, pixheight)

            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):
                    self._term_modes[mode] = int.from_bytes(modes[idx:idx+4],
                                                            'big')
                    idx += 4
                else:
                    raise DisconnectError(DISC_PROTOCOL_ERROR,
                                          'Invalid pty modes string')

            return self._session.pty_requested(self._term_type, self._term_size,
                                               self._term_modes)

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

            name = packet.get_string()
            value = packet.get_string()
            packet.check_end()

            try:
                name = name.decode('utf-8')
                value = value.decode('utf-8')
            except UnicodeDecodeError:
                return False

            self._env[name] = value
            return True

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

            forced_command = self._conn.get_certificate_option('force-command')

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

            if forced_command is not None:
                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):
            """Process a request to open a shell"""

            packet.check_end()

            return self._start_session()

        def _process_exec_request(self, packet):
            """Process a request to execute a command"""

            command = packet.get_string()
            packet.check_end()

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

            return self._start_session(command=command)

        def _process_subsystem_request(self, packet):
            """Process a request to open a subsystem"""

            subsystem = packet.get_string()
            packet.check_end()

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

            return self._start_session(subsystem=subsystem)

        def _process_window_change_request(self, packet):
            """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()

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

        def _process_signal_request(self, packet):
            """Process a request to send a signal"""

            signal = packet.get_string()
            packet.check_end()

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

            self._session.signal_received(signal)
            return True

        def _process_break_request(self, packet):
            """Process a request to send a break"""

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

            return self._session.break_received(msec)

        def start_sftp_server(self, sftp_factory):
            """Start an SFTP server for this session

               This method can be used by an existing :class:`SSHServerSession`
               to replace itself with an SFTP server session. Calls to this
               method should be made from  :meth:`session_started
               <SSHServerSession.session_started>` before any data is read
               or written.  Once called, no further calls will be made on the
               original session.

                 .. note:: The :meth:`connection_lost
                           <SSHServerSession.connection_lost>` method will not
                           be called on the original server session when this
                           is used.

               :param callable sftp_server:
                   A callable which returns an :class:`SFTPServer` object
                   that will be created to handle SFTP requests on this
                   channel.

            """

            # Reset the encoding to allow the transfer of binary data
            self._encoding = None

            # Replace the session with an SFTPServerSession
            self._session = SFTPServerSession(sftp_factory(self._conn))
            self._session.connection_made(self)
            self._session.session_started()

        def get_environment(self):
            """Return the environment for this session

               This method returns the environment set by the client
               when the session was opened. Calls to this method should
               only be made after :meth:`session_started
               <SSHServerSession.session_started>` has been called on
               the :class:`SSHServerSession`.

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

            """

            return self._env

        def get_command(self):
            """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``. 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._command

        def get_subsystem(self):
            """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``. 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

        def get_terminal_type(self):
            """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 string containing the terminal type or ``None`` if
                         no pseudo-terminal was requested

            """

            return self._term_type

        def get_terminal_size(self):
            """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 integers 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):
            """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 integer mode:
                   POSIX terminal mode taken from :ref:`POSIX terminal modes
                   <PTYModes>` to look up

               :returns: An integer 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 set_xon_xoff(self, client_can_do):
            """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 boolean client_can_do:
                   Whether or not the client should enable XON/XOFF flow control

            """

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

        def write_stderr(self, data):
            """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: string 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):
            """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):
            """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 integer status:
                   The exit status to report to the client

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

            """

            if self._send_state not in {'open', 'eof_pending', 'eof_sent'}:
                raise OSError('Channel not open')

            self._send_request(b'exit-status', UInt32(status & 0xff))
            self.close()

        def exit_with_signal(self, signal, core_dumped=False,
                             msg='', lang=DEFAULT_LANG):
            """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 string signal:
                   The signal which caused the process to exit
               :param boolean 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

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

            """

            if self._send_state not in {'open', 'eof_pending', 'eof_sent'}:
                raise OSError('Channel not open')

            signal = signal.encode('ascii')
            msg = msg.encode('utf-8')
            lang = lang.encode('ascii')

            self._send_request(b'exit-signal', String(signal),
                               Boolean(core_dumped), String(msg), String(lang))
            self.close()

    class SSHTCPChannel(SSHChannel):
        """SSH TCP channel"""

        @asyncio.coroutine
        def _finish_open_request(self, session):
            """Finish processing a TCP channel open request"""

            yield from super()._finish_open_request(session)

            if self._session:
                self._session.session_started()
                self.resume_reading()

        @asyncio.coroutine
        def _open_tcp(self, session_factory, chantype, host, port,
                      orig_host, orig_port):
            """Open a TCP channel"""

            self._extra['local_peername'] = (orig_host, orig_port)
            self._extra['remote_peername'] = (host, port)

            host = host.encode('utf-8')
            orig_host = orig_host.encode('utf-8')

            packet = yield from super()._open(chantype, String(host), UInt32(port),
                                              String(orig_host), UInt32(orig_port))

            # TCP sessions 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()
            self.resume_reading()

            return self, self._session

        @asyncio.coroutine
        def connect(self, session_factory, host, port, orig_host, orig_port):
            """Create a new outbound TCP session"""

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

        @asyncio.coroutine
        def accept(self, session_factory, host, port, orig_host, orig_port):
            """Create a new forwarded TCP session"""

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

        def set_inbound_peer_names(self, dest_host, dest_port,
                                   orig_host, orig_port):
            """Set local and remote peer names for inbound connections"""

            self._extra['local_peername'] = (dest_host, dest_port)
            self._extra['remote_peername'] = (orig_host, orig_port)