# Copyright (C) 2007 Giampaolo Rodola' <g.rodola@gmail.com>.
# Use of this source code is governed by MIT license that can be
# found in the LICENSE file.

import asynchat
import errno
import glob
import logging
import os
import socket
import sys
import time
import traceback
from datetime import datetime

try:
    import grp
    import pwd
except ImportError:
    pwd = grp = None

from pyftpdlib import __ver__
from pyftpdlib.authorizers import DummyAuthorizer
from pyftpdlib.exceptions import AuthenticationFailed
from pyftpdlib.exceptions import AuthorizerError
from pyftpdlib.exceptions import FilesystemError
from pyftpdlib.filesystems import AbstractedFS
from pyftpdlib.ioloop import AsyncChat
from pyftpdlib.log import debug
from pyftpdlib.log import logger
from pyftpdlib.utils import has_dualstack_ipv6
from pyftpdlib.utils import is_ssl_sock
from pyftpdlib.utils import strerror

from .data import DTPHandler
from .dispatchers import ActiveDTP
from .dispatchers import PassiveDTP
from .producers import BufferedIteratorProducer
from .producers import FileProducer

__all__ = ["FTPHandler"]

proto_cmds = {
    "ABOR": dict(
        perm=None, auth=True, arg=False, help="Syntax: ABOR (abort transfer)."
    ),
    "ALLO": dict(
        perm=None,
        auth=True,
        arg=True,
        help="Syntax: ALLO <SP> bytes (noop; allocate storage).",
    ),
    "APPE": dict(
        perm="a",
        auth=True,
        arg=True,
        help="Syntax: APPE <SP> file-name (append data to file).",
    ),
    "CDUP": dict(
        perm="e",
        auth=True,
        arg=False,
        help="Syntax: CDUP (go to parent directory).",
    ),
    "CWD": dict(
        perm="e",
        auth=True,
        arg=None,
        help="Syntax: CWD [<SP> dir-name] (change working directory).",
    ),
    "DELE": dict(
        perm="d",
        auth=True,
        arg=True,
        help="Syntax: DELE <SP> file-name (delete file).",
    ),
    "EPRT": dict(
        perm=None,
        auth=True,
        arg=True,
        help="Syntax: EPRT <SP> |proto|ip|port| (extended active mode).",
    ),
    "EPSV": dict(
        perm=None,
        auth=True,
        arg=None,
        help='Syntax: EPSV [<SP> proto/"ALL"] (extended passive mode).',
    ),
    "FEAT": dict(
        perm=None,
        auth=False,
        arg=False,
        help="Syntax: FEAT (list all new features supported).",
    ),
    "HELP": dict(
        perm=None,
        auth=False,
        arg=None,
        help="Syntax: HELP [<SP> cmd] (show help).",
    ),
    "LIST": dict(
        perm="l",
        auth=True,
        arg=None,
        help="Syntax: LIST [<SP> path] (list files).",
    ),
    "MDTM": dict(
        perm="l",
        auth=True,
        arg=True,
        help="Syntax: MDTM [<SP> path] (file last modification time).",
    ),
    "MFMT": dict(
        perm="T",
        auth=True,
        arg=True,
        help=(
            "Syntax: MFMT <SP> timeval <SP> path (file update last "
            "modification time)."
        ),
    ),
    "MLSD": dict(
        perm="l",
        auth=True,
        arg=None,
        help="Syntax: MLSD [<SP> path] (list directory).",
    ),
    "MLST": dict(
        perm="l",
        auth=True,
        arg=None,
        help="Syntax: MLST [<SP> path] (show information about path).",
    ),
    "MODE": dict(
        perm=None,
        auth=True,
        arg=True,
        help="Syntax: MODE <SP> mode (noop; set data transfer mode).",
    ),
    "MKD": dict(
        perm="m",
        auth=True,
        arg=True,
        help="Syntax: MKD <SP> path (create directory).",
    ),
    "NLST": dict(
        perm="l",
        auth=True,
        arg=None,
        help="Syntax: NLST [<SP> path] (list path in a compact form).",
    ),
    "NOOP": dict(
        perm=None,
        auth=False,
        arg=False,
        help="Syntax: NOOP (just do nothing).",
    ),
    "OPTS": dict(
        perm=None,
        auth=True,
        arg=True,
        help="Syntax: OPTS <SP> cmd [<SP> option] (set option for command).",
    ),
    "PASS": dict(
        perm=None,
        auth=False,
        arg=None,
        help="Syntax: PASS [<SP> password] (set user password).",
    ),
    "PASV": dict(
        perm=None,
        auth=True,
        arg=False,
        help="Syntax: PASV (open passive data connection).",
    ),
    "PORT": dict(
        perm=None,
        auth=True,
        arg=True,
        help="Syntax: PORT <sp> h,h,h,h,p,p (open active data connection).",
    ),
    "PWD": dict(
        perm=None,
        auth=True,
        arg=False,
        help="Syntax: PWD (get current working directory).",
    ),
    "QUIT": dict(
        perm=None,
        auth=False,
        arg=False,
        help="Syntax: QUIT (quit current session).",
    ),
    "REIN": dict(
        perm=None, auth=True, arg=False, help="Syntax: REIN (flush account)."
    ),
    "REST": dict(
        perm=None,
        auth=True,
        arg=True,
        help="Syntax: REST <SP> offset (set file offset).",
    ),
    "RETR": dict(
        perm="r",
        auth=True,
        arg=True,
        help="Syntax: RETR <SP> file-name (retrieve a file).",
    ),
    "RMD": dict(
        perm="d",
        auth=True,
        arg=True,
        help="Syntax: RMD <SP> dir-name (remove directory).",
    ),
    "RNFR": dict(
        perm="f",
        auth=True,
        arg=True,
        help="Syntax: RNFR <SP> file-name (rename (source name)).",
    ),
    "RNTO": dict(
        perm="f",
        auth=True,
        arg=True,
        help="Syntax: RNTO <SP> file-name (rename (destination name)).",
    ),
    "SITE": dict(
        perm=None,
        auth=False,
        arg=True,
        help="Syntax: SITE <SP> site-command (execute SITE command).",
    ),
    "SITE HELP": dict(
        perm=None,
        auth=False,
        arg=None,
        help="Syntax: SITE HELP [<SP> cmd] (show SITE command help).",
    ),
    "SITE CHMOD": dict(
        perm="M",
        auth=True,
        arg=True,
        help="Syntax: SITE CHMOD <SP> mode path (change file mode).",
    ),
    "SIZE": dict(
        perm="l",
        auth=True,
        arg=True,
        help="Syntax: SIZE <SP> file-name (get file size).",
    ),
    "STAT": dict(
        perm="l",
        auth=False,
        arg=None,
        help="Syntax: STAT [<SP> path name] (server stats [list files]).",
    ),
    "STOR": dict(
        perm="w",
        auth=True,
        arg=True,
        help="Syntax: STOR <SP> file-name (store a file).",
    ),
    "STOU": dict(
        perm="w",
        auth=True,
        arg=None,
        help="Syntax: STOU [<SP> name] (store a file with a unique name).",
    ),
    "STRU": dict(
        perm=None,
        auth=True,
        arg=True,
        help="Syntax: STRU <SP> type (noop; set file structure).",
    ),
    "SYST": dict(
        perm=None,
        auth=False,
        arg=False,
        help="Syntax: SYST (get operating system type).",
    ),
    "TYPE": dict(
        perm=None,
        auth=True,
        arg=True,
        help="Syntax: TYPE <SP> [A | I] (set transfer type).",
    ),
    "USER": dict(
        perm=None,
        auth=False,
        arg=True,
        help="Syntax: USER <SP> user-name (set username).",
    ),
    "XCUP": dict(
        perm="e",
        auth=True,
        arg=False,
        help="Syntax: XCUP (obsolete; go to parent directory).",
    ),
    "XCWD": dict(
        perm="e",
        auth=True,
        arg=None,
        help="Syntax: XCWD [<SP> dir-name] (obsolete; change directory).",
    ),
    "XMKD": dict(
        perm="m",
        auth=True,
        arg=True,
        help="Syntax: XMKD <SP> dir-name (obsolete; create directory).",
    ),
    "XPWD": dict(
        perm=None,
        auth=True,
        arg=False,
        help="Syntax: XPWD (obsolete; get current dir).",
    ),
    "XRMD": dict(
        perm="d",
        auth=True,
        arg=True,
        help="Syntax: XRMD <SP> dir-name (obsolete; remove directory).",
    ),
}

if not hasattr(os, "chmod"):
    del proto_cmds["SITE CHMOD"]


class FTPHandler(AsyncChat):
    """Implements the FTP server Protocol Interpreter (see RFC-959),
    handling commands received from the client on the control channel.

    All relevant session information is stored in class attributes
    reproduced below and can be modified before instantiating this
    class.

     - (int) timeout:
       The timeout which is the maximum time a remote client may spend
       between FTP commands. If the timeout triggers, the remote client
       will be kicked off.  Defaults to 300 seconds.

     - (str) banner: the message sent when client connects.

     - (int) max_login_attempts:
        the maximum number of wrong authentications before disconnecting
        the client (default 3).

     - (bool)permit_foreign_addresses:
        FTP site-to-site transfer feature: also referenced as "FXP" it
        permits for transferring a file between two remote FTP servers
        without the transfer going through the client's host (not
        recommended for security reasons as described in RFC-2577).
        Having this attribute set to False means that all data
        connections from/to remote IP addresses which do not match the
        client's IP address will be dropped (default False).

     - (bool) permit_privileged_ports:
        set to True if you want to permit active data connections (PORT)
        over privileged TCP ports (not recommended, defaulting to False).

     - (str) masquerade_address:
        the "masqueraded" IP address to provide along PASV reply when
        pyftpdlib is running behind a NAT or other types of gateways.
        When configured pyftpdlib will hide its local address and
        instead use the public address of your NAT (default None).

     - (dict) masquerade_address_map:
        in case the server has multiple IP addresses which are all
        behind a NAT router, you may wish to specify individual
        masquerade_addresses for each of them. The map expects a
        dictionary containing private IP addresses as keys, and their
        corresponding public (masquerade) addresses as values.

     - (list) passive_ports:
        what ports the ftpd will use for its passive data transfers.
        Value expected is a list of integers (e.g. range(60000, 65535)).
        When configured pyftpdlib will no longer use kernel-assigned
        random ports (default None).

     - (bool) use_gmt_times:
        when True causes the server to report all ls and MDTM times in
        GMT and not local time (default True).

     - (bool) use_sendfile: when True uses sendfile() system call to
        send a file resulting in faster uploads (from server to client).
        Linux only.

     - (bool) tcp_no_delay: controls the use of the TCP_NODELAY socket
        option which disables the Nagle algorithm resulting in
        significantly better performances (default True on all systems
        where it is supported).

     - (str) encoding: the encoding used for client / server
       communication. Defaults to 'utf-8'.

     - (str) unicode_errors:
       the error handler passed to ''.encode() and ''.decode():
       https://docs.python.org/library/stdtypes.html#str.decode
       (detaults to 'replace').

     - (str) log_prefix:
       the prefix string preceding any log line; all instance
       attributes can be used as arguments.


    All relevant instance attributes initialized when client connects
    are reproduced below.  You may be interested in them in case you
    want to subclass the original FTPHandler.

     - (bool) authenticated: True if client authenticated himself.
     - (str) username: the name of the connected user (if any).
     - (int) attempted_logins: number of currently attempted logins.
     - (str) current_type: the current transfer type (default "a")
     - (int) af: the connection's address family (IPv4/IPv6)
     - (instance) server: the FTPServer class instance.
     - (instance) data_channel: the data channel instance (if any).
    """

    # these are overridable defaults

    # default classes
    authorizer = DummyAuthorizer()
    active_dtp = ActiveDTP
    passive_dtp = PassiveDTP
    dtp_handler = DTPHandler
    abstracted_fs = AbstractedFS
    proto_cmds = proto_cmds

    # session attributes (explained in the docstring)
    timeout = 300
    banner = f"pyftpdlib {__ver__} ready."
    max_login_attempts = 3
    permit_foreign_addresses = False
    permit_privileged_ports = False
    masquerade_address = None
    masquerade_address_map = {}
    passive_ports = None
    use_gmt_times = True
    use_sendfile = hasattr(os, "sendfile")  # added in python 3.3
    tcp_no_delay = hasattr(socket, "TCP_NODELAY")
    encoding = "utf8"
    unicode_errors = "replace"
    log_prefix = "%(remote_ip)s:%(remote_port)s-[%(username)s]"
    auth_failed_timeout = 3

    def __init__(self, conn, server, ioloop=None):
        """Initialize the command channel.

        - (instance) conn: the socket object instance of the newly
           established connection.
        - (instance) server: the ftp server class instance.
        """
        # public session attributes
        self.server = server
        self.fs = None
        self.authenticated = False
        self.username = ""
        self.password = ""
        self.attempted_logins = 0
        self.data_channel = None
        self.remote_ip = ""
        self.remote_port = ""
        self.started = time.time()

        # private session attributes
        self._last_response = ""
        self._current_type = "a"
        self._restart_position = 0
        self._quit_pending = False
        self._in_buffer = []
        self._in_buffer_len = 0
        self._epsvall = False
        self._dtp_acceptor = None
        self._dtp_connector = None
        self._in_dtp_queue = None
        self._out_dtp_queue = None
        self._extra_feats = []
        self._current_facts = ["type", "perm", "size", "modify"]
        self._rnfr = None
        self._idler = None
        self._log_debug = (
            logging.getLogger("pyftpdlib").getEffectiveLevel() <= logging.DEBUG
        )

        if os.name == "posix":
            self._current_facts.append("unique")
        self._available_facts = self._current_facts[:]
        if pwd and grp:
            self._available_facts += ["unix.mode", "unix.uid", "unix.gid"]
        if os.name == "nt":
            self._available_facts.append("create")

        try:
            AsyncChat.__init__(self, conn, ioloop=ioloop)
        except OSError as err:
            # if we get an exception here we want the dispatcher
            # instance to set socket attribute before closing, see:
            # https://github.com/giampaolo/pyftpdlib/issues/188
            AsyncChat.__init__(self, socket.socket(), ioloop=ioloop)
            self.close()
            debug(f"call: FTPHandler.__init__, err {err!r}", self)
            if err.errno == errno.EINVAL:
                # https://github.com/giampaolo/pyftpdlib/issues/143
                return
            self.handle_error()
            return
        self.set_terminator(b"\r\n")

        # connection properties
        try:
            self.remote_ip, self.remote_port = self.socket.getpeername()[:2]
        except OSError as err:
            debug(
                f"call: FTPHandler.__init__, err on getpeername() {err!r}",
                self,
            )
            # A race condition  may occur if the other end is closing
            # before we can get the peername, hence ENOTCONN (see issue
            # #100) while EINVAL can occur on OSX (see issue #143).
            self.connected = False
            if err.errno in (errno.ENOTCONN, errno.EINVAL):
                self.close()
            else:
                self.handle_error()
            return
        else:
            self.log("FTP session opened (connect)")

        # try to handle urgent data inline
        try:
            self.socket.setsockopt(socket.SOL_SOCKET, socket.SO_OOBINLINE, 1)
        except OSError as err:
            debug(
                f"call: FTPHandler.__init__, err on SO_OOBINLINE {err!r}", self
            )

        # disable Nagle algorithm for the control socket only, resulting
        # in significantly better performances
        if self.tcp_no_delay:
            try:
                self.socket.setsockopt(socket.SOL_TCP, socket.TCP_NODELAY, 1)
            except OSError as err:
                debug(
                    f"call: FTPHandler.__init__, err on TCP_NODELAY {err!r}",
                    self,
                )

        # remove this instance from IOLoop's socket_map
        if not self.connected:
            self.close()
            return

        if self.timeout:
            self._idler = self.ioloop.call_later(
                self.timeout, self.handle_timeout, _errback=self.handle_error
            )

    def get_repr_info(self, as_str=False, extra_info=None):
        if extra_info is None:
            extra_info = {}
        info = {}
        info["id"] = id(self)
        info["addr"] = f"{self.remote_ip}:{self.remote_port}"
        if is_ssl_sock(self.socket):
            info["ssl"] = True
        if self.username:
            info["user"] = self.username
        # If threads are involved sometimes "self" may be None (?!?).
        dc = getattr(self, "data_channel", None)
        if dc is not None:
            if is_ssl_sock(dc.socket):
                info["ssl-data"] = True
            if dc.file_obj:
                if self.data_channel.receive:
                    info["sending-file"] = dc.file_obj
                    if dc.use_sendfile():
                        info["use-sendfile(2)"] = True
                else:
                    info["receiving-file"] = dc.file_obj
                info["bytes-trans"] = dc.get_transmitted_bytes()
        info.update(extra_info)
        if as_str:
            return ", ".join([f"{k}={v!r}" for (k, v) in info.items()])
        return info

    def __repr__(self):
        return f"<{self.__class__.__name__}({self.get_repr_info(True)})>"

    __str__ = __repr__

    def handle(self):
        """Return a 220 'ready' response to the client over the command
        channel.
        """
        self.on_connect()
        if not self._closed and not self._closing:
            if len(self.banner) <= 75:
                self.respond(f"220 {self.banner!s}")
            else:
                self.push(f"220-{self.banner!s}\r\n")
                self.respond("220 ")

    def handle_max_cons(self):
        """Called when limit for maximum number of connections is reached."""
        msg = "421 Too many connections. Service temporarily unavailable."
        self.respond_w_warning(msg)
        # If self.push is used, data could not be sent immediately in
        # which case a new "loop" will occur exposing us to the risk of
        # accepting new connections.  Since this could cause asyncore to
        # run out of fds in case we're using select() on Windows  we
        # immediately close the channel by using close() instead of
        # close_when_done(). If data has not been sent yet client will
        # be silently disconnected.
        self.close()

    def handle_max_cons_per_ip(self):
        """Called when too many clients are connected from the same IP."""
        msg = "421 Too many connections from the same IP address."
        self.respond_w_warning(msg)
        self.close_when_done()

    def handle_timeout(self):
        """Called when client does not send any command within the time
        specified in <timeout> attribute."""
        msg = "Control connection timed out."
        self.respond("421 " + msg, logfun=logger.info)
        self.close_when_done()

    # --- asyncore / asynchat overridden methods

    def readable(self):
        # Checking for self.connected seems to be necessary as per:
        # https://github.com/giampaolo/pyftpdlib/issues/188#c18
        # In contrast to DTPHandler, here we are not interested in
        # attempting to receive any further data from a closed socket.
        return self.connected and AsyncChat.readable(self)

    def writable(self):
        return self.connected and AsyncChat.writable(self)

    def collect_incoming_data(self, data):
        """Read incoming data and append to the input buffer."""
        self._in_buffer.append(data)
        self._in_buffer_len += len(data)
        # Flush buffer if it gets too long (possible DoS attacks).
        # RFC-959 specifies that a 500 response could be given in
        # such cases
        buflimit = 2048
        if self._in_buffer_len > buflimit:
            self.respond_w_warning("500 Command too long.")
            self._in_buffer = []
            self._in_buffer_len = 0

    def decode(self, bytes):
        return bytes.decode(self.encoding, self.unicode_errors)

    def found_terminator(self):
        r"""Called when the incoming data stream matches the \r\n
        terminator.
        """
        if self._idler is not None and not self._idler.cancelled:
            self._idler.reset()

        line = b"".join(self._in_buffer)
        try:
            line = self.decode(line)
        except UnicodeDecodeError:
            # By default we'll never get here as we replace errors
            # but user might want to override this behavior.
            # RFC-2640 doesn't mention what to do in this case so
            # we'll just return 501 (bad arg).
            return self.respond("501 Can't decode command.")

        self._in_buffer = []
        self._in_buffer_len = 0

        cmd = line.split(" ")[0].upper()
        arg = line[len(cmd) + 1 :]
        try:
            self.pre_process_command(line, cmd, arg)
        except UnicodeEncodeError:
            self.respond(
                "501 can't decode path (server filesystem encoding is"
                f" {sys.getfilesystemencoding()})"
            )

    def pre_process_command(self, line, cmd, arg):
        kwargs = {}
        if cmd == "SITE" and arg:
            cmd = f"SITE {arg.split(' ')[0].upper()}"
            arg = line[len(cmd) + 1 :]

        if cmd != "PASS":
            self.logline(f"<- {line}")
        else:
            self.logline(f"<- {line.split(' ')[0]} {'*' * 6}")

        # Recognize those commands having a "special semantic". They
        # should be sent by following the RFC-959 procedure of sending
        # Telnet IP/Synch sequence (chr 242 and 255) as OOB data but
        # since many ftp clients don't do it correctly we check the
        # last 4 characters only.
        if cmd not in self.proto_cmds:
            if cmd[-4:] in ("ABOR", "STAT", "QUIT"):
                cmd = cmd[-4:]
            else:
                msg = f'Command "{cmd}" not understood.'
                self.respond("500 " + msg)
                if cmd:
                    self.log_cmd(cmd, arg, 500, msg)
                return

        if not arg and self.proto_cmds[cmd]["arg"] is True:
            msg = "Syntax error: command needs an argument."
            self.respond("501 " + msg)
            self.log_cmd(cmd, "", 501, msg)
            return
        if arg and self.proto_cmds[cmd]["arg"] is False:
            msg = "Syntax error: command does not accept arguments."
            self.respond("501 " + msg)
            self.log_cmd(cmd, arg, 501, msg)
            return

        if not self.authenticated:
            if self.proto_cmds[cmd]["auth"] or (cmd == "STAT" and arg):
                msg = "Log in with USER and PASS first."
                self.respond("530 " + msg)
                self.log_cmd(cmd, arg, 530, msg)
            else:
                # call the proper ftp_* method
                self.process_command(cmd, arg)
                return
        else:
            if (cmd == "STAT") and not arg:
                self.ftp_STAT("")
                return

            # for file-system related commands check whether real path
            # destination is valid
            if self.proto_cmds[cmd]["perm"] and (cmd != "STOU"):
                if cmd in ("CWD", "XCWD"):
                    arg = self.fs.ftp2fs(arg or "/")
                elif cmd in ("CDUP", "XCUP"):
                    arg = self.fs.ftp2fs("..")
                elif cmd == "LIST":
                    if arg.lower() in ("-a", "-l", "-al", "-la"):
                        arg = self.fs.ftp2fs(self.fs.cwd)
                    else:
                        arg = self.fs.ftp2fs(arg or self.fs.cwd)
                elif cmd == "STAT":
                    if glob.has_magic(arg):
                        msg = "Globbing not supported."
                        self.respond("550 " + msg)
                        self.log_cmd(cmd, arg, 550, msg)
                        return
                    arg = self.fs.ftp2fs(arg or self.fs.cwd)
                elif cmd == "SITE CHMOD":
                    if " " not in arg:
                        msg = "Syntax error: command needs two arguments."
                        self.respond("501 " + msg)
                        self.log_cmd(cmd, "", 501, msg)
                        return
                    else:
                        mode, arg = arg.split(" ", 1)
                        arg = self.fs.ftp2fs(arg)
                        kwargs = dict(mode=mode)
                elif cmd == "MFMT":
                    if " " not in arg:
                        msg = "Syntax error: command needs two arguments."
                        self.respond("501 " + msg)
                        self.log_cmd(cmd, "", 501, msg)
                        return
                    else:
                        timeval, arg = arg.split(" ", 1)
                        arg = self.fs.ftp2fs(arg)
                        kwargs = dict(timeval=timeval)

                else:  # LIST, NLST, MLSD, MLST
                    arg = self.fs.ftp2fs(arg or self.fs.cwd)

                if not self.fs.validpath(arg):
                    line = self.fs.fs2ftp(arg)
                    msg = f"{line!r} points to a path which is outside "
                    msg += "the user's root directory"
                    self.respond(f"550 {msg}.")
                    self.log_cmd(cmd, arg, 550, msg)
                    return

            # check permission
            perm = self.proto_cmds[cmd]["perm"]
            if perm is not None and cmd != "STOU":
                if not self.authorizer.has_perm(self.username, perm, arg):
                    msg = "Not enough privileges."
                    self.respond("550 " + msg)
                    self.log_cmd(cmd, arg, 550, msg)
                    return

            # call the proper ftp_* method
            self.process_command(cmd, arg, **kwargs)

    def process_command(self, cmd, *args, **kwargs):
        """Process command by calling the corresponding ftp_* class
        method (e.g. for received command "MKD pathname", ftp_MKD()
        method is called with "pathname" as the argument).
        """
        if self._closed:
            return
        self._last_response = ""
        method = getattr(self, "ftp_" + cmd.replace(" ", "_"))
        method(*args, **kwargs)
        if self._last_response:
            code = int(self._last_response[:3])
            resp = self._last_response[4:]
            self.log_cmd(cmd, args[0], code, resp)

    def handle_error(self):
        try:
            self.log_exception(self)
            self.close()
        except Exception:
            logger.critical(traceback.format_exc())

    def handle_close(self):
        self.close()

    def close(self):
        """Close the current channel disconnecting the client."""
        debug("call: close()", inst=self)
        if not self._closed:
            AsyncChat.close(self)

            self._shutdown_connecting_dtp()

            if self.data_channel is not None:
                self.data_channel.close()
                del self.data_channel

            if self._out_dtp_queue is not None:
                file = self._out_dtp_queue[2]
                if file is not None:
                    file.close()
            if self._in_dtp_queue is not None:
                file = self._in_dtp_queue[0]
                if file is not None:
                    file.close()

            del self._out_dtp_queue
            del self._in_dtp_queue

            if self._idler is not None and not self._idler.cancelled:
                self._idler.cancel()

            # remove client IP address from ip map
            if self.remote_ip in self.server.ip_map:
                self.server.ip_map.remove(self.remote_ip)

            if self.fs is not None:
                self.fs.cmd_channel = None
                self.fs = None
            self.log("FTP session closed (disconnect).")
            # Having self.remote_ip not set means that no connection
            # actually took place, hence we're not interested in
            # invoking the callback.
            if self.remote_ip:
                self.ioloop.call_later(
                    0, self.on_disconnect, _errback=self.handle_error
                )

    def _shutdown_connecting_dtp(self):
        """Close any ActiveDTP or PassiveDTP instance waiting to
        establish a connection (passive or active).
        """
        if self._dtp_acceptor is not None:
            self._dtp_acceptor.close()
            self._dtp_acceptor = None
        if self._dtp_connector is not None:
            self._dtp_connector.close()
            self._dtp_connector = None

    # --- public callbacks
    # Note: to run a time consuming task make sure to use a separate
    # process or thread (see FAQs).

    def on_connect(self):
        """Called when client connects, *before* sending the initial
        220 reply.
        """

    def on_disconnect(self):
        """Called when connection is closed."""

    def on_login(self, username):
        """Called on user login."""

    def on_login_failed(self, username, password):
        """Called on failed login attempt.
        At this point client might have already been disconnected if it
        failed too many times.
        """

    def on_logout(self, username):
        """Called when user "cleanly" logs out due to QUIT or USER
        issued twice (re-login). This is not called if the connection
        is simply closed by client.
        """

    def on_file_sent(self, file):
        """Called every time a file has been successfully sent.
        "file" is the absolute name of the file just being sent.
        """

    def on_file_received(self, file):
        """Called every time a file has been successfully received.
        "file" is the absolute name of the file just being received.
        """

    def on_incomplete_file_sent(self, file):
        """Called every time a file has not been entirely sent.
        (e.g. ABOR during transfer or client disconnected).
        "file" is the absolute name of that file.
        """

    def on_incomplete_file_received(self, file):
        """Called every time a file has not been entirely received
        (e.g. ABOR during transfer or client disconnected).
        "file" is the absolute name of that file.
        """

    # --- internal callbacks

    def _on_dtp_connection(self):
        """Called every time data channel connects, either active or
        passive.

        Incoming and outgoing queues are checked for pending data.
        If outbound data is pending, it is pushed into the data channel.
        If awaiting inbound data, the data channel is enabled for
        receiving.
        """
        # Close accepting DTP only. By closing ActiveDTP DTPHandler
        # would receive a closed socket object.
        # self._shutdown_connecting_dtp()
        if self._dtp_acceptor is not None:
            self._dtp_acceptor.close()
            self._dtp_acceptor = None

        # stop the idle timer as long as the data transfer is not finished
        if self._idler is not None and not self._idler.cancelled:
            self._idler.cancel()

        # check for data to send
        if self._out_dtp_queue is not None:
            data, isproducer, file, cmd = self._out_dtp_queue
            self._out_dtp_queue = None
            self.data_channel.cmd = cmd
            if file:
                self.data_channel.file_obj = file
            try:
                if not isproducer:
                    self.data_channel.push(data)
                else:
                    self.data_channel.push_with_producer(data)
                if self.data_channel is not None:
                    self.data_channel.close_when_done()
            except Exception:
                # dealing with this exception is up to DTP (see bug #84)
                self.data_channel.handle_error()

        # check for data to receive
        elif self._in_dtp_queue is not None:
            file, cmd = self._in_dtp_queue
            self.data_channel.file_obj = file
            self._in_dtp_queue = None
            self.data_channel.enable_receiving(self._current_type, cmd)

    def _on_dtp_close(self):
        """Called every time the data channel is closed."""
        self.data_channel = None
        if self._quit_pending:
            self.close()
        elif self.timeout:
            # data transfer finished, restart the idle timer
            if self._idler is not None and not self._idler.cancelled:
                self._idler.cancel()
            self._idler = self.ioloop.call_later(
                self.timeout, self.handle_timeout, _errback=self.handle_error
            )

    # --- utility

    def push(self, data):
        asynchat.async_chat.push(self, data.encode(self.encoding))

    def respond(self, resp, logfun=logger.debug):
        """Send a response to the client using the command channel."""
        self._last_response = resp
        self.push(resp + "\r\n")
        if self._log_debug:
            self.logline(f"-> {resp}", logfun=logfun)
        else:
            self.log(resp[4:], logfun=logfun)

    def respond_w_warning(self, resp):
        self.respond(resp, logfun=logger.warning)

    def push_dtp_data(self, data, isproducer=False, file=None, cmd=None):
        """Pushes data into the data channel.

        It is usually called for those commands requiring some data to
        be sent over the data channel (e.g. RETR).
        If data channel does not exist yet, it queues the data to send
        later; data will then be pushed into data channel when
        _on_dtp_connection() will be called.

         - (str/classobj) data: the data to send which may be a string
            or a producer object).
         - (bool) isproducer: whether treat data as a producer.
         - (file) file: the file[-like] object to send (if any).
        """
        if self.data_channel is not None:
            self.respond(
                "125 Data connection already open. Transfer starting."
            )
            if file:
                self.data_channel.file_obj = file
            try:
                if not isproducer:
                    self.data_channel.push(data)
                else:
                    self.data_channel.push_with_producer(data)
                if self.data_channel is not None:
                    self.data_channel.cmd = cmd
                    self.data_channel.close_when_done()
            except Exception:
                # dealing with this exception is up to DTP (see bug #84)
                self.data_channel.handle_error()
        else:
            self.respond(
                "150 File status okay. About to open data connection."
            )
            self._out_dtp_queue = (data, isproducer, file, cmd)

    def flush_account(self):
        """Flush account information by clearing attributes that need
        to be reset on a REIN or new USER command.
        """
        self._shutdown_connecting_dtp()
        # if there's a transfer in progress RFC-959 states we are
        # supposed to let it finish
        if self.data_channel is not None:
            if not self.data_channel.transfer_in_progress():
                self.data_channel.close()
                self.data_channel = None

        username = self.username
        if self.authenticated and username:
            self.on_logout(username)
        self.authenticated = False
        self.username = ""
        self.password = ""
        self.attempted_logins = 0
        self._current_type = "a"
        self._restart_position = 0
        self._quit_pending = False
        self._in_dtp_queue = None
        self._rnfr = None
        self._out_dtp_queue = None

    def run_as_current_user(self, function, *args, **kwargs):
        """Execute a function impersonating the current logged-in user."""
        self.authorizer.impersonate_user(self.username, self.password)
        try:
            return function(*args, **kwargs)
        finally:
            self.authorizer.terminate_impersonation(self.username)

    # --- logging wrappers

    # this is defined earlier
    # log_prefix = '%(remote_ip)s:%(remote_port)s-[%(username)s]'

    def log(self, msg, logfun=logger.info):
        """Log a message, including additional identifying session data."""
        prefix = self.log_prefix % self.__dict__
        logfun(f"{prefix} {msg}")

    def logline(self, msg, logfun=logger.debug):
        """Log a line including additional identifying session data.
        By default this is disabled unless logging level == DEBUG.
        """
        if self._log_debug:
            prefix = self.log_prefix % self.__dict__
            logfun(f"{prefix} {msg}")

    def logerror(self, msg):
        """Log an error including additional identifying session data."""
        prefix = self.log_prefix % self.__dict__
        logger.error(f"{prefix} {msg}")

    def log_exception(self, instance):
        """Log an unhandled exception. 'instance' is the instance
        where the exception was generated.
        """
        logger.exception("unhandled exception in instance %r", instance)

    # the list of commands which gets logged when logging level
    # is >= logging.INFO
    log_cmds_list = [
        "DELE",
        "RNFR",
        "RNTO",
        "MKD",
        "RMD",
        "CWD",
        "XMKD",
        "XRMD",
        "XCWD",
        "REIN",
        "SITE CHMOD",
        "MFMT",
    ]

    def log_cmd(self, cmd, arg, respcode, respstr):
        """Log commands and responses in a standardized format.
        This is disabled in case the logging level is set to DEBUG.

         - (str) cmd:
            the command sent by client

         - (str) arg:
            the command argument sent by client.
            For filesystem commands such as DELE, MKD, etc. this is
            already represented as an absolute real filesystem path
            like "/home/user/file.ext".

         - (int) respcode:
            the response code as being sent by server. Response codes
            starting with 4xx or 5xx are returned if the command has
            been rejected for some reason.

         - (str) respstr:
            the response string as being sent by server.

        By default only DELE, RMD, RNTO, MKD, CWD, ABOR, REIN, SITE CHMOD
        commands are logged and the output is redirected to self.log
        method.

        Can be overridden to provide alternate formats or to log
        further commands.
        """
        if not self._log_debug and cmd in self.log_cmds_list:
            line = f"{cmd.strip()} {arg.strip()} {respcode}"
            if str(respcode)[0] in ("4", "5"):
                line += f" {respstr!r}"
            self.log(line)

    def log_transfer(self, cmd, filename, receive, completed, elapsed, bytes):
        """Log all file transfers in a standardized format.

        - (str) cmd:
           the original command who caused the transfer.

        - (str) filename:
           the absolutized name of the file on disk.

        - (bool) receive:
           True if the transfer was used for client uploading (STOR,
           STOU, APPE), False otherwise (RETR).

        - (bool) completed:
           True if the file has been entirely sent, else False.

        - (float) elapsed:
           transfer elapsed time in seconds.

        - (int) bytes:
           number of bytes transmitted.
        """
        line = "%s %s completed=%s bytes=%s seconds=%s" % (
            cmd,
            filename,
            (completed and 1) or 0,
            bytes,
            elapsed,
        )
        self.log(line)

    # --- connection
    def _make_eport(self, ip, port):
        """Establish an active data channel with remote client which
        issued a PORT or EPRT command.
        """
        # FTP bounce attacks protection: according to RFC-2577 it's
        # recommended to reject PORT if IP address specified in it
        # does not match client IP address.
        remote_ip = self.remote_ip
        if remote_ip.startswith("::ffff:"):
            # In this scenario, the server has an IPv6 socket, but
            # the remote client is using IPv4 and its address is
            # represented as an IPv4-mapped IPv6 address which
            # looks like this ::ffff:151.12.5.65, see:
            # https://en.wikipedia.org/wiki/IPv6#IPv4-mapped_addresses
            # https://datatracker.ietf.org/doc/html/rfc3493.html#section-3.7
            # We truncate the first bytes to make it look like a
            # common IPv4 address.
            remote_ip = remote_ip[7:]
        if not self.permit_foreign_addresses and ip != remote_ip:
            msg = (
                f"501 Rejected data connection to foreign address {ip}:{port}."
            )
            self.respond_w_warning(msg)
            return

        # ...another RFC-2577 recommendation is rejecting connections
        # to privileged ports (< 1024) for security reasons.
        if not self.permit_privileged_ports and port < 1024:
            msg = f'501 PORT against the privileged port "{port}" refused.'
            self.respond_w_warning(msg)
            return

        # close establishing DTP instances, if any
        self._shutdown_connecting_dtp()

        if self.data_channel is not None:
            self.data_channel.close()
            self.data_channel = None

        # make sure we are not hitting the max connections limit
        if not self.server._accept_new_cons():
            msg = "425 Too many connections. Can't open data channel."
            self.respond_w_warning(msg)
            return

        # open data channel
        self._dtp_connector = self.active_dtp(ip, port, self)

    def _make_epasv(self, extmode=False):
        """Initialize a passive data channel with remote client which
        issued a PASV or EPSV command.
        If extmode argument is True we assume that client issued EPSV in
        which case extended passive mode will be used (see RFC-2428).
        """
        # close establishing DTP instances, if any
        self._shutdown_connecting_dtp()

        # close established data connections, if any
        if self.data_channel is not None:
            self.data_channel.close()
            self.data_channel = None

        # make sure we are not hitting the max connections limit
        if not self.server._accept_new_cons():
            msg = "425 Too many connections. Can't open data channel."
            self.respond_w_warning(msg)
            return

        # open data channel
        self._dtp_acceptor = self.passive_dtp(self, extmode)

    def ftp_PORT(self, line):
        """Start an active data channel by using IPv4."""
        if self._epsvall:
            self.respond("501 PORT not allowed after EPSV ALL.")
            return
        # Parse PORT request for getting IP and PORT.
        # Request comes in as:
        # > h1,h2,h3,h4,p1,p2
        # ...where the client's IP address is h1.h2.h3.h4 and the TCP
        # port number is (p1 * 256) + p2.
        try:
            addr = list(map(int, line.split(",")))
            if len(addr) != 6:
                raise ValueError
            for x in addr[:4]:
                if not 0 <= x <= 255:
                    raise ValueError
            ip = "%d.%d.%d.%d" % tuple(addr[:4])
            port = (addr[4] * 256) + addr[5]
            if not 0 <= port <= 65535:
                raise ValueError
        except (ValueError, OverflowError):
            self.respond("501 Invalid PORT format.")
            return
        self._make_eport(ip, port)

    def ftp_EPRT(self, line):
        """Start an active data channel by choosing the network protocol
        to use (IPv4/IPv6) as defined in RFC-2428.
        """
        if self._epsvall:
            self.respond("501 EPRT not allowed after EPSV ALL.")
            return
        # Parse EPRT request for getting protocol, IP and PORT.
        # Request comes in as:
        # <d>proto<d>ip<d>port<d>
        # ...where <d> is an arbitrary delimiter character (usually "|") and
        # <proto> is the network protocol to use (1 for IPv4, 2 for IPv6).
        try:
            af, ip, port = line.split(line[0])[1:-1]
            port = int(port)
            if not 0 <= port <= 65535:
                raise ValueError
        except (ValueError, IndexError, OverflowError):
            self.respond("501 Invalid EPRT format.")
            return

        if af == "1":
            # test if AF_INET6 and IPV6_V6ONLY
            if (
                self.socket.family == socket.AF_INET6
                and not has_dualstack_ipv6()
            ):
                self.respond("522 Network protocol not supported (use 2).")
            else:
                try:
                    octs = list(map(int, ip.split(".")))
                    if len(octs) != 4:
                        raise ValueError
                    for x in octs:
                        if not 0 <= x <= 255:
                            raise ValueError
                except (ValueError, OverflowError):
                    self.respond("501 Invalid EPRT format.")
                else:
                    self._make_eport(ip, port)
        elif af == "2":
            if self.socket.family == socket.AF_INET:
                self.respond("522 Network protocol not supported (use 1).")
            else:
                self._make_eport(ip, port)
        elif self.socket.family == socket.AF_INET:
            self.respond("501 Unknown network protocol (use 1).")
        else:
            self.respond("501 Unknown network protocol (use 2).")

    def ftp_PASV(self, line):
        """Start a passive data channel by using IPv4."""
        if self._epsvall:
            self.respond("501 PASV not allowed after EPSV ALL.")
            return
        self._make_epasv(extmode=False)

    def ftp_EPSV(self, line):
        """Start a passive data channel by using IPv4 or IPv6 as defined
        in RFC-2428.
        """
        # RFC-2428 specifies that if an optional parameter is given,
        # we have to determine the address family from that otherwise
        # use the same address family used on the control connection.
        # In such a scenario a client may use IPv4 on the control channel
        # and choose to use IPv6 for the data channel.
        # But how could we use IPv6 on the data channel without knowing
        # which IPv6 address to use for binding the socket?
        # Unfortunately RFC-2428 does not provide satisfying information
        # on how to do that.  The assumption is that we don't have any way
        # to know wich address to use, hence we just use the same address
        # family used on the control connection.
        if not line:
            self._make_epasv(extmode=True)
        # IPv4
        elif line == "1":
            if self.socket.family != socket.AF_INET:
                self.respond("522 Network protocol not supported (use 2).")
            else:
                self._make_epasv(extmode=True)
        # IPv6
        elif line == "2":
            if self.socket.family == socket.AF_INET:
                self.respond("522 Network protocol not supported (use 1).")
            else:
                self._make_epasv(extmode=True)
        elif line.lower() == "all":
            self._epsvall = True
            self.respond(
                "220 Other commands other than EPSV are now disabled."
            )
        elif self.socket.family == socket.AF_INET:
            self.respond("501 Unknown network protocol (use 1).")
        else:
            self.respond("501 Unknown network protocol (use 2).")

    def ftp_QUIT(self, line):
        """Quit the current session disconnecting the client."""
        if self.authenticated:
            msg_quit = self.authorizer.get_msg_quit(self.username)
        else:
            msg_quit = "Goodbye."
        if len(msg_quit) <= 75:
            self.respond(f"221 {msg_quit}")
        else:
            self.push(f"221-{msg_quit}\r\n")
            self.respond("221 ")

        # From RFC-959:
        # If file transfer is in progress, the connection must remain
        # open for result response and the server will then close it.
        # We also stop responding to any further command.
        if self.data_channel:
            self._quit_pending = True
            self.del_channel()
        else:
            self._shutdown_connecting_dtp()
            self.close_when_done()
        if self.authenticated and self.username:
            self.on_logout(self.username)

        # --- data transferring

    def ftp_LIST(self, path):
        """Return a list of files in the specified directory to the
        client.
        On success return the directory path, else None.
        """
        # - If no argument, fall back on cwd as default.
        # - Some older FTP clients erroneously issue /bin/ls-like LIST
        #   formats in which case we fall back on cwd as default.
        try:
            isdir = self.fs.isdir(path)
            if isdir:
                listing = self.run_as_current_user(self.fs.listdir, path)
                # RFC 959 recommends the listing to be sorted.
                listing.sort()
                iterator = self.fs.format_list(path, listing)
            else:
                basedir, filename = os.path.split(path)
                self.fs.lstat(path)  # raise exc in case of problems
                iterator = self.fs.format_list(basedir, [filename])
        except (OSError, FilesystemError) as err:
            why = strerror(err)
            self.respond(f"550 {why}.")
        else:
            producer = BufferedIteratorProducer(iterator)
            self.push_dtp_data(producer, isproducer=True, cmd="LIST")
            return path

    def ftp_NLST(self, path):
        """Return a list of files in the specified directory in a
        compact form to the client.
        On success return the directory path, else None.
        """
        try:
            if self.fs.isdir(path):
                listing = list(self.run_as_current_user(self.fs.listdir, path))
            else:
                # if path is a file we just list its name
                self.fs.lstat(path)  # raise exc in case of problems
                listing = [os.path.basename(path)]
        except (OSError, FilesystemError) as err:
            self.respond(f"550 {strerror(err)}.")
        else:
            data = ""
            if listing:
                # RFC 959 recommends the listing to be sorted.
                listing.sort()
                data = "\r\n".join(listing) + "\r\n"
            data = data.encode(self.encoding, self.unicode_errors)
            self.push_dtp_data(data, cmd="NLST")
            return path

        # --- MLST and MLSD commands

    # The MLST and MLSD commands are intended to standardize the file and
    # directory information returned by the server-FTP process.  These
    # commands differ from the LIST command in that the format of the
    # replies is strictly defined although extensible.

    def ftp_MLST(self, path):
        """Return information about a pathname in a machine-processable
        form as defined in RFC-3659.
        On success return the path just listed, else None.
        """
        line = self.fs.fs2ftp(path)
        basedir, basename = os.path.split(path)
        perms = self.authorizer.get_perms(self.username)
        try:
            iterator = self.run_as_current_user(
                self.fs.format_mlsx,
                basedir,
                [basename],
                perms,
                self._current_facts,
                ignore_err=False,
            )
            data = b"".join(iterator)
        except (OSError, FilesystemError) as err:
            self.respond(f"550 {strerror(err)}.")
        else:
            data = data.decode(self.encoding, self.unicode_errors)
            # since TVFS is supported (see RFC-3659 chapter 6), a fully
            # qualified pathname should be returned
            data = data.split(" ")[0] + f" {line}\r\n"
            # response is expected on the command channel
            self.push(f'250-Listing "{line}":\r\n')
            # the fact set must be preceded by a space
            self.push(" " + data)
            self.respond("250 End MLST.")
            return path

    def ftp_MLSD(self, path):
        """Return contents of a directory in a machine-processable form
        as defined in RFC-3659.
        On success return the path just listed, else None.
        """
        # RFC-3659 requires 501 response code if path is not a directory
        if not self.fs.isdir(path):
            self.respond("501 No such directory.")
            return
        try:
            listing = self.run_as_current_user(self.fs.listdir, path)
        except (OSError, FilesystemError) as err:
            why = strerror(err)
            self.respond(f"550 {why}.")
        else:
            perms = self.authorizer.get_perms(self.username)
            iterator = self.fs.format_mlsx(
                path, listing, perms, self._current_facts
            )
            producer = BufferedIteratorProducer(iterator)
            self.push_dtp_data(producer, isproducer=True, cmd="MLSD")
            return path

    def ftp_RETR(self, file):
        """Retrieve the specified file (transfer from the server to the
        client).  On success return the file path else None.
        """
        rest_pos = self._restart_position
        self._restart_position = 0
        try:
            fd = self.run_as_current_user(self.fs.open, file, "rb")
        except (OSError, FilesystemError) as err:
            why = strerror(err)
            self.respond(f"550 {why}.")
            return

        try:
            if rest_pos:
                # Make sure that the requested offset is valid (within the
                # size of the file being resumed).
                # According to RFC-1123 a 554 reply may result in case that
                # the existing file cannot be repositioned as specified in
                # the REST.
                ok = 0
                try:
                    fsize = self.fs.getsize(file)
                    if rest_pos > fsize:
                        raise ValueError
                    fd.seek(rest_pos)
                    ok = 1
                except ValueError:
                    why = f"REST position ({rest_pos}) > file size ({fsize})"
                except (OSError, FilesystemError) as err:
                    why = strerror(err)
                if not ok:
                    fd.close()
                    self.respond(f"554 {why}")
                    return
            producer = FileProducer(fd, self._current_type)
            self.push_dtp_data(producer, isproducer=True, file=fd, cmd="RETR")
            return file
        except Exception:
            fd.close()
            raise

    def ftp_STOR(self, file, mode="w"):
        """Store a file (transfer from the client to the server).
        On success return the file path, else None.
        """
        # A resume could occur in case of APPE or REST commands.
        # In that case we have to open file object in different ways:
        # STOR: mode = 'w'
        # APPE: mode = 'a'
        # REST: mode = 'r+' (to permit seeking on file object)
        cmd = "APPE" if "a" in mode else "STOR"
        rest_pos = self._restart_position
        self._restart_position = 0
        if rest_pos:
            mode = "r+"
        try:
            fd = self.run_as_current_user(self.fs.open, file, mode + "b")
        except (OSError, FilesystemError) as err:
            why = strerror(err)
            self.respond(f"550 {why}.")
            return

        try:
            if rest_pos:
                # Make sure that the requested offset is valid (within the
                # size of the file being resumed).
                # According to RFC-1123 a 554 reply may result in case
                # that the existing file cannot be repositioned as
                # specified in the REST.
                ok = 0
                try:
                    fsize = self.fs.getsize(file)
                    if rest_pos > fsize:
                        raise ValueError
                    fd.seek(rest_pos)
                    ok = 1
                except ValueError:
                    why = f"REST position ({rest_pos}) > file size ({fsize})"
                except (OSError, FilesystemError) as err:
                    why = strerror(err)
                if not ok:
                    fd.close()
                    self.respond(f"554 {why}")
                    return

            if self.data_channel is not None:
                resp = "Data connection already open. Transfer starting."
                self.respond("125 " + resp)
                self.data_channel.file_obj = fd
                self.data_channel.enable_receiving(self._current_type, cmd)
            else:
                resp = "File status okay. About to open data connection."
                self.respond("150 " + resp)
                self._in_dtp_queue = (fd, cmd)
            return file
        except Exception:
            fd.close()
            raise

    def ftp_STOU(self, line):
        """Store a file on the server with a unique name.
        On success return the file path, else None.
        """
        # Note 1: RFC-959 prohibited STOU parameters, but this
        # prohibition is obsolete.
        # Note 2: 250 response wanted by RFC-959 has been declared
        # incorrect in RFC-1123 that wants 125/150 instead.
        # Note 3: RFC-1123 also provided an exact output format
        # defined to be as follow:
        # > 125 FILE: pppp
        # ...where pppp represents the unique path name of the
        # file that will be written.

        # watch for STOU preceded by REST, which makes no sense.
        if self._restart_position:
            self.respond("450 Can't STOU while REST request is pending.")
            return

        if line:
            basedir, prefix = os.path.split(self.fs.ftp2fs(line))
            prefix += "."
        else:
            basedir = self.fs.ftp2fs(self.fs.cwd)
            prefix = "ftpd."
        try:
            fd = self.run_as_current_user(
                self.fs.mkstemp, prefix=prefix, dir=basedir
            )
        except (OSError, FilesystemError) as err:
            # likely, we hit the max number of retries to find out a
            # file with a unique name
            if getattr(err, "errno", -1) == errno.EEXIST:
                why = "No usable unique file name found"
            # something else happened
            else:
                why = strerror(err)
            self.respond(f"450 {why}.")
            return

        try:
            if not self.authorizer.has_perm(self.username, "w", fd.name):
                try:
                    fd.close()
                    self.run_as_current_user(self.fs.remove, fd.name)
                except (OSError, FilesystemError):
                    pass
                self.respond("550 Not enough privileges.")
                return

            # now just acts like STOR except that restarting isn't allowed
            filename = os.path.basename(fd.name)
            if self.data_channel is not None:
                self.respond(f"125 FILE: {filename}")
                self.data_channel.file_obj = fd
                self.data_channel.enable_receiving(self._current_type, "STOU")
            else:
                self.respond(f"150 FILE: {filename}")
                self._in_dtp_queue = (fd, "STOU")
            return filename
        except Exception:
            fd.close()
            raise

    def ftp_APPE(self, file):
        """Append data to an existing file on the server.
        On success return the file path, else None.
        """
        # watch for APPE preceded by REST, which makes no sense.
        if self._restart_position:
            self.respond("450 Can't APPE while REST request is pending.")
        else:
            return self.ftp_STOR(file, mode="a")

    def ftp_REST(self, line):
        """Restart a file transfer from a previous mark."""
        if self._current_type == "a":
            self.respond("501 Resuming transfers not allowed in ASCII mode.")
            return
        try:
            marker = int(line)
            if marker < 0:
                raise ValueError
        except (ValueError, OverflowError):
            self.respond("501 Invalid parameter.")
        else:
            self.respond(f"350 Restarting at position {marker}.")
            self._restart_position = marker

    def ftp_ABOR(self, line):
        """Abort the current data transfer."""
        # ABOR received while no data channel exists
        if (
            self._dtp_acceptor is None
            and self._dtp_connector is None
            and self.data_channel is None
        ):
            self.respond("225 No transfer to abort.")
            return
        else:
            # a PASV or PORT was received but connection wasn't made yet
            if (
                self._dtp_acceptor is not None
                or self._dtp_connector is not None
            ):
                self._shutdown_connecting_dtp()
                resp = "225 ABOR command successful; data channel closed."

            # If a data transfer is in progress the server must first
            # close the data connection, returning a 426 reply to
            # indicate that the transfer terminated abnormally, then it
            # must send a 226 reply, indicating that the abort command
            # was successfully processed.
            # If no data has been transmitted we just respond with 225
            # indicating that no transfer was in progress.
            if self.data_channel is not None:
                if self.data_channel.transfer_in_progress():
                    self.data_channel.close()
                    self.data_channel = None
                    self.respond(
                        "426 Transfer aborted via ABOR.", logfun=logger.info
                    )
                    resp = "226 ABOR command successful."
                else:
                    self.data_channel.close()
                    self.data_channel = None
                    resp = "225 ABOR command successful; data channel closed."
        self.respond(resp)

        # --- authentication

    def ftp_USER(self, line):
        """Set the username for the current session."""
        # RFC-959 specifies a 530 response to the USER command if the
        # username is not valid.  If the username is valid is required
        # ftpd returns a 331 response instead.  In order to prevent a
        # malicious client from determining valid usernames on a server,
        # it is suggested by RFC-2577 that a server always return 331 to
        # the USER command and then reject the combination of username
        # and password for an invalid username when PASS is provided later.
        if not self.authenticated:
            self.respond("331 Username ok, send password.")
        else:
            # a new USER command could be entered at any point in order
            # to change the access control flushing any user, password,
            # and account information already supplied and beginning the
            # login sequence again.
            self.flush_account()
            msg = "Previous account information was flushed"
            self.respond(f"331 {msg}, send password.", logfun=logger.info)
        self.username = line

    def handle_auth_failed(self, msg, password):
        def callback(username, password, msg):
            self.add_channel()
            if hasattr(self, "_closed") and not self._closed:
                self.attempted_logins += 1
                if self.attempted_logins >= self.max_login_attempts:
                    msg += " Disconnecting."
                    self.respond("530 " + msg)
                    self.close_when_done()
                else:
                    self.respond("530 " + msg)
                self.log(f"USER '{username}' failed login.")
            self.on_login_failed(username, password)

        self.del_channel()
        if not msg:
            if self.username == "anonymous":
                msg = "Anonymous access not allowed."
            else:
                msg = "Authentication failed."
        else:
            # response string should be capitalized as per RFC-959
            msg = msg.capitalize()
        self.ioloop.call_later(
            self.auth_failed_timeout,
            callback,
            self.username,
            password,
            msg,
            _errback=self.handle_error,
        )
        self.username = ""

    def handle_auth_success(self, home, password, msg_login):
        if len(msg_login) <= 75:
            self.respond(f"230 {msg_login}")
        else:
            self.push(f"230-{msg_login}\r\n")
            self.respond("230 ")
        self.log(f"USER '{self.username}' logged in.")
        self.authenticated = True
        self.password = password
        self.attempted_logins = 0

        self.fs = self.abstracted_fs(home, self)
        self.on_login(self.username)

    def ftp_PASS(self, line):
        """Check username's password against the authorizer."""
        if self.authenticated:
            self.respond("503 User already authenticated.")
            return
        if not self.username:
            self.respond("503 Login with USER first.")
            return

        try:
            self.authorizer.validate_authentication(self.username, line, self)
            home = self.authorizer.get_home_dir(self.username)
            msg_login = self.authorizer.get_msg_login(self.username)
        except (AuthenticationFailed, AuthorizerError) as err:
            self.handle_auth_failed(str(err), line)
        else:
            self.handle_auth_success(home, line, msg_login)

    def ftp_REIN(self, line):
        """Reinitialize user's current session."""
        # From RFC-959:
        # REIN command terminates a USER, flushing all I/O and account
        # information, except to allow any transfer in progress to be
        # completed.  All parameters are reset to the default settings
        # and the control connection is left open.  This is identical
        # to the state in which a user finds himself immediately after
        # the control connection is opened.
        self.flush_account()
        # Note: RFC-959 erroneously mention "220" as the correct response
        # code to be given in this case, but this is wrong...
        self.respond("230 Ready for new user.")

        # --- filesystem operations

    def ftp_PWD(self, line):
        """Return the name of the current working directory to the client."""
        # The 257 response is supposed to include the directory
        # name and in case it contains embedded double-quotes
        # they must be doubled (see RFC-959, chapter 7, appendix 2).
        cwd = self.fs.cwd
        self.respond(
            '257 "%s" is the current directory.' % cwd.replace('"', '""')
        )

    def ftp_CWD(self, path):
        """Change the current working directory.
        On success return the new directory path, else None.
        """
        # Temporarily join the specified directory to see if we have
        # permissions to do so, then get back to original process's
        # current working directory.
        # Note that if for some reason os.getcwd() gets removed after
        # the process is started we'll get into troubles (os.getcwd()
        # will fail with ENOENT) but we can't do anything about that
        # except logging an error.
        init_cwd = os.getcwd()
        try:
            self.run_as_current_user(self.fs.chdir, path)
        except (OSError, FilesystemError) as err:
            why = strerror(err)
            self.respond(f"550 {why}.")
        else:
            cwd = self.fs.cwd
            self.respond(f'250 "{cwd}" is the current directory.')
            if os.getcwd() != init_cwd:
                os.chdir(init_cwd)
            return path

    def ftp_CDUP(self, path):
        """Change into the parent directory.
        On success return the new directory, else None.
        """
        # Note: RFC-959 says that code 200 is required but it also says
        # that CDUP uses the same codes as CWD.
        return self.ftp_CWD(path)

    def ftp_SIZE(self, path):
        """Return size of file in a format suitable for using with
        RESTart as defined in RFC-3659."""

        # Implementation note: properly handling the SIZE command when
        # TYPE ASCII is used would require to scan the entire file to
        # perform the ASCII translation logic
        # (file.read().replace(os.linesep, '\r\n')) and then calculating
        # the len of such data which may be different than the actual
        # size of the file on the server.  Considering that calculating
        # such result could be very resource-intensive and also dangerous
        # (DoS) we reject SIZE when the current TYPE is ASCII.
        # However, clients in general should not be resuming downloads
        # in ASCII mode.  Resuming downloads in binary mode is the
        # recommended way as specified in RFC-3659.

        line = self.fs.fs2ftp(path)
        if self._current_type == "a":
            why = "SIZE not allowed in ASCII mode"
            self.respond(f"550 {why}.")
            return
        if not self.fs.isfile(self.fs.realpath(path)):
            why = f"{line} is not retrievable"
            self.respond(f"550 {why}.")
            return
        try:
            size = self.run_as_current_user(self.fs.getsize, path)
        except (OSError, FilesystemError) as err:
            why = strerror(err)
            self.respond(f"550 {why}.")
        else:
            self.respond(f"213 {size}")

    def ftp_MDTM(self, path):
        """Return last modification time of file to the client as an ISO
        3307 style timestamp (YYYYMMDDHHMMSS) as defined in RFC-3659.
        On success return the file path, else None.
        """
        line = self.fs.fs2ftp(path)
        if not self.fs.isfile(self.fs.realpath(path)):
            self.respond(f"550 {line} is not retrievable")
            return
        timefunc = time.gmtime if self.use_gmt_times else time.localtime
        try:
            secs = self.run_as_current_user(self.fs.getmtime, path)
            lmt = time.strftime("%Y%m%d%H%M%S", timefunc(secs))
        except (ValueError, OSError, FilesystemError) as err:
            if isinstance(err, ValueError):
                # It could happen if file's last modification time
                # happens to be too old (prior to year 1900)
                why = "Can't determine file's last modification time"
            else:
                why = strerror(err)
            self.respond(f"550 {why}.")
        else:
            self.respond(f"213 {lmt}")
            return path

    def ftp_MFMT(self, path, timeval):
        """Sets the last modification time of file to timeval
        3307 style timestamp (YYYYMMDDHHMMSS) as defined in RFC-3659.
        On success return the modified time and file path, else None.
        """
        # Note: the MFMT command is not a formal RFC command
        # but stated in the following MEMO:
        # https://tools.ietf.org/html/draft-somers-ftp-mfxx-04
        # this is implemented to assist with file synchronization

        line = self.fs.fs2ftp(path)

        if len(timeval) != len("YYYYMMDDHHMMSS"):
            why = "Invalid time format; expected: YYYYMMDDHHMMSS"
            self.respond(f"550 {why}.")
            return
        if not self.fs.isfile(self.fs.realpath(path)):
            self.respond(f"550 {line} is not retrievable")
            return
        timefunc = time.gmtime if self.use_gmt_times else time.localtime
        try:
            # convert timeval string to epoch seconds
            epoch = datetime.utcfromtimestamp(0)
            timeval_datetime_obj = datetime.strptime(timeval, "%Y%m%d%H%M%S")
            timeval_secs = (timeval_datetime_obj - epoch).total_seconds()
        except ValueError:
            why = "Invalid time format; expected: YYYYMMDDHHMMSS"
            self.respond(f"550 {why}.")
            return
        try:
            # Modify Time
            self.run_as_current_user(self.fs.utime, path, timeval_secs)
            # Fetch Time
            secs = self.run_as_current_user(self.fs.getmtime, path)
            lmt = time.strftime("%Y%m%d%H%M%S", timefunc(secs))
        except (ValueError, OSError, FilesystemError) as err:
            if isinstance(err, ValueError):
                # It could happen if file's last modification time
                # happens to be too old (prior to year 1900)
                why = "Can't determine file's last modification time"
            else:
                why = strerror(err)
            self.respond(f"550 {why}.")
        else:
            self.respond(f"213 Modify={lmt}; {line}.")
            return (lmt, path)

    def ftp_MKD(self, path):
        """Create the specified directory.
        On success return the directory path, else None.
        """
        line = self.fs.fs2ftp(path)
        try:
            self.run_as_current_user(self.fs.mkdir, path)
        except (OSError, FilesystemError) as err:
            why = strerror(err)
            self.respond(f"550 {why}.")
        else:
            # The 257 response is supposed to include the directory
            # name and in case it contains embedded double-quotes
            # they must be doubled (see RFC-959, chapter 7, appendix 2).
            self.respond(
                '257 "%s" directory created.' % line.replace('"', '""')
            )
            return path

    def ftp_RMD(self, path):
        """Remove the specified directory.
        On success return the directory path, else None.
        """
        if self.fs.realpath(path) == self.fs.realpath(self.fs.root):
            msg = "Can't remove root directory."
            self.respond(f"550 {msg}")
            return
        try:
            self.run_as_current_user(self.fs.rmdir, path)
        except (OSError, FilesystemError) as err:
            why = strerror(err)
            self.respond(f"550 {why}.")
        else:
            self.respond("250 Directory removed.")

    def ftp_DELE(self, path):
        """Delete the specified file.
        On success return the file path, else None.
        """
        try:
            self.run_as_current_user(self.fs.remove, path)
        except (OSError, FilesystemError) as err:
            why = strerror(err)
            self.respond(f"550 {why}.")
        else:
            self.respond("250 File removed.")
            return path

    def ftp_RNFR(self, path):
        """Rename the specified (only the source name is specified
        here, see RNTO command)."""
        if not self.fs.lexists(path):
            self.respond("550 No such file or directory.")
        elif self.fs.realpath(path) == self.fs.realpath(self.fs.root):
            self.respond("550 Can't rename home directory.")
        else:
            self._rnfr = path
            self.respond("350 Ready for destination name.")

    def ftp_RNTO(self, path):
        """Rename file (destination name only, source is specified with
        RNFR).
        On success return a (source_path, destination_path) tuple.
        """
        if not self._rnfr:
            self.respond("503 Bad sequence of commands: use RNFR first.")
            return
        src = self._rnfr
        self._rnfr = None
        try:
            self.run_as_current_user(self.fs.rename, src, path)
        except (OSError, FilesystemError) as err:
            why = strerror(err)
            self.respond(f"550 {why}.")
        else:
            self.respond("250 Renaming ok.")
            return (src, path)

        # --- others

    def ftp_TYPE(self, line):
        """Set current type data type to binary/ascii."""
        type = line.upper().replace(" ", "")
        if type in ("A", "L7"):
            self.respond("200 Type set to: ASCII.")
            self._current_type = "a"
        elif type in ("I", "L8"):
            self.respond("200 Type set to: Binary.")
            self._current_type = "i"
        else:
            self.respond(f'504 Unsupported type "{line}".')

    def ftp_STRU(self, line):
        """Set file structure ("F" is the only one supported (noop))."""
        stru = line.upper()
        if stru == "F":
            self.respond("200 File transfer structure set to: F.")
        elif stru in ("P", "R"):
            # R is required in minimum implementations by RFC-959, 5.1.
            # RFC-1123, 4.1.2.13, amends this to only apply to servers
            # whose file systems support record structures, but also
            # suggests that such a server "may still accept files with
            # STRU R, recording the byte stream literally".
            # Should we accept R but with no operational difference from
            # F? proftpd and wu-ftpd don't accept STRU R. We just do
            # the same.
            #
            # RFC-1123 recommends against implementing P.
            self.respond("504 Unimplemented STRU type.")
        else:
            self.respond("501 Unrecognized STRU type.")

    def ftp_MODE(self, line):
        """Set data transfer mode ("S" is the only one supported (noop))."""
        mode = line.upper()
        if mode == "S":
            self.respond("200 Transfer mode set to: S")
        elif mode in ("B", "C"):
            self.respond("504 Unimplemented MODE type.")
        else:
            self.respond("501 Unrecognized MODE type.")

    def ftp_STAT(self, path):
        """Return statistics about current ftp session. If an argument
        is provided return directory listing over command channel.

        Implementation note:

        RFC-959 does not explicitly mention globbing but many FTP
        servers do support it as a measure of convenience for FTP
        clients and users.

        In order to search for and match the given globbing expression,
        the code has to search (possibly) many directories, examine
        each contained filename, and build a list of matching files in
        memory.  Since this operation can be quite intensive, both CPU-
        and memory-wise, we do not support globbing.
        """
        # return STATus information about ftpd
        if not path:
            s = []
            s.append("Connected to: %s:%s" % self.socket.getsockname()[:2])
            if self.authenticated:
                s.append(f"Logged in as: {self.username}")
            elif not self.username:
                s.append("Waiting for username.")
            else:
                s.append("Waiting for password.")
            type = "ASCII" if self._current_type == "a" else "Binary"
            s.append(f"TYPE: {type}; STRUcture: File; MODE: Stream")
            if self._dtp_acceptor is not None:
                s.append("Passive data channel waiting for connection.")
            elif self.data_channel is not None:
                bytes_sent = self.data_channel.tot_bytes_sent
                bytes_recv = self.data_channel.tot_bytes_received
                elapsed_time = self.data_channel.get_elapsed_time()
                s.extend((
                    "Data connection open:",
                    f"Total bytes sent: {bytes_sent}",
                    f"Total bytes received: {bytes_recv}",
                    f"Transfer elapsed time: {elapsed_time} secs",
                ))
            else:
                s.append("Data connection closed.")

            self.push("211-FTP server status:\r\n")
            self.push("".join([f" {item}\r\n" for item in s]))
            self.respond("211 End of status.")
        # return directory LISTing over the command channel
        else:
            line = self.fs.fs2ftp(path)
            try:
                isdir = self.fs.isdir(path)
                if isdir:
                    listing = self.run_as_current_user(self.fs.listdir, path)
                    if isinstance(listing, list):
                        # RFC 959 recommends the listing to be sorted.
                        listing.sort()
                    iterator = self.fs.format_list(path, listing)
                else:
                    basedir, filename = os.path.split(path)
                    self.fs.lstat(path)  # raise exc in case of problems
                    iterator = self.fs.format_list(basedir, [filename])
            except (OSError, FilesystemError) as err:
                why = strerror(err)
                self.respond(f"550 {why}.")
            else:
                self.push(f'213-Status of "{line}":\r\n')
                self.push_with_producer(BufferedIteratorProducer(iterator))
                self.respond("213 End of status.")
                return path

    def ftp_FEAT(self, line):
        """List all new features supported as defined in RFC-2398."""
        features = {"TVFS"}
        if self.encoding.lower() in ("utf8", "utf-8"):
            features.add("UTF8")
        features.update([
            feat
            for feat in ("EPRT", "EPSV", "MDTM", "MFMT", "SIZE")
            if feat in self.proto_cmds
        ])
        features.update(self._extra_feats)
        if "MLST" in self.proto_cmds or "MLSD" in self.proto_cmds:
            facts = ""
            for fact in self._available_facts:
                if fact in self._current_facts:
                    facts += fact + "*;"
                else:
                    facts += fact + ";"
            features.add("MLST " + facts)
        if "REST" in self.proto_cmds:
            features.add("REST STREAM")
        features = sorted(features)
        self.push("211-Features supported:\r\n")
        self.push("".join([f" {x}\r\n" for x in features]))
        self.respond("211 End FEAT.")

    def ftp_OPTS(self, line):
        """Specify options for FTP commands as specified in RFC-2389."""
        try:
            if line.count(" ") > 1:
                raise ValueError("Invalid number of arguments")
            if " " in line:
                cmd, arg = line.split(" ")
            else:
                cmd, arg = line, ""
            # the only commands able to accept options are MLST and UTF8
            if cmd.upper() in ("UTF8", "UTF-8"):
                self.respond("200 Always in UTF8 mode.")
            elif cmd.upper() == "MLST" and "MLST" in self.proto_cmds:
                if arg and ";" not in arg:
                    raise ValueError("Invalid argument")
                facts = [x.lower() for x in arg.split(";")]
                self._current_facts = [
                    x for x in facts if x in self._available_facts
                ]
                f = "".join([x + ";" for x in self._current_facts])
                self.respond("200 MLST OPTS " + f)
            else:
                raise ValueError(f'Unsupported command "{cmd}"')
        except ValueError as err:
            self.respond(f"501 {err}.")

    def ftp_NOOP(self, line):
        """Do nothing."""
        self.respond("200 I successfully did nothing'.")

    def ftp_SYST(self, line):
        """Return system type (always returns UNIX type: L8)."""
        # This command is used to find out the type of operating system
        # at the server.  The reply shall have as its first word one of
        # the system names listed in RFC-943.
        # Since that we always return a "/bin/ls -lA"-like output on
        # LIST we  prefer to respond as if we would on Unix in any case.
        self.respond("215 UNIX Type: L8")

    def ftp_ALLO(self, line):
        """Allocate bytes for storage (noop)."""
        # not necessary (always respond with 202)
        self.respond("202 No storage allocation necessary.")

    def ftp_HELP(self, line):
        """Return help text to the client."""
        if line:
            line = line.upper()
            if line in self.proto_cmds:
                self.respond(f"214 {self.proto_cmds[line]['help']}")
            else:
                self.respond("501 Unrecognized command.")
        else:
            # provide a compact list of recognized commands
            def formatted_help():
                cmds = []
                keys = sorted(
                    [x for x in self.proto_cmds if not x.startswith("SITE ")]
                )
                while keys:
                    elems = tuple(keys[0:8])
                    cmds.append(" %-6s" * len(elems) % elems + "\r\n")
                    del keys[0:8]
                return "".join(cmds)

            self.push("214-The following commands are recognized:\r\n")
            self.push(formatted_help())
            self.respond("214 Help command successful.")

        # --- site commands

    # The user willing to add support for a specific SITE command must
    # update self.proto_cmds dictionary and define a new ftp_SITE_%CMD%
    # method in the subclass.

    def ftp_SITE_CHMOD(self, path, mode):
        """Change file mode.
        On success return a (file_path, mode) tuple.
        """
        # Note: although most UNIX servers implement it, SITE CHMOD is not
        # defined in any official RFC.
        try:
            assert len(mode) in (3, 4)
            for x in mode:
                assert 0 <= int(x) <= 7
            mode = int(mode, 8)
        except (AssertionError, ValueError):
            self.respond("501 Invalid SITE CHMOD format.")
        else:
            try:
                self.run_as_current_user(self.fs.chmod, path, mode)
            except (OSError, FilesystemError) as err:
                why = strerror(err)
                self.respond(f"550 {why}.")
            else:
                self.respond("200 SITE CHMOD successful.")
                return (path, mode)

    def ftp_SITE_HELP(self, line):
        """Return help text to the client for a given SITE command."""
        if line:
            line = line.upper()
            if line in self.proto_cmds:
                self.respond(f"214 {self.proto_cmds[line]['help']}")
            else:
                self.respond("501 Unrecognized SITE command.")
        else:
            self.push("214-The following SITE commands are recognized:\r\n")
            site_cmds = []
            for cmd in sorted(self.proto_cmds.keys()):
                if cmd.startswith("SITE "):
                    site_cmds.append(f" {cmd[5:]}\r\n")
            self.push("".join(site_cmds))
            self.respond("214 Help SITE command successful.")

        # --- support for deprecated cmds

    # RFC-1123 requires that the server treat XCUP, XCWD, XMKD, XPWD
    # and XRMD commands as synonyms for CDUP, CWD, MKD, LIST and RMD.
    # Such commands are obsoleted but some ftp clients (e.g. Windows
    # ftp.exe) still use them.

    def ftp_XCUP(self, line):
        """Change to the parent directory. Synonym for CDUP. Deprecated."""
        return self.ftp_CDUP(line)

    def ftp_XCWD(self, line):
        """Change the current working directory. Synonym for CWD.
        Deprecated."""
        return self.ftp_CWD(line)

    def ftp_XMKD(self, line):
        """Create the specified directory. Synonym for MKD. Deprecated."""
        return self.ftp_MKD(line)

    def ftp_XPWD(self, line):
        """Return the current working directory. Synonym for PWD.
        Deprecated."""
        return self.ftp_PWD(line)

    def ftp_XRMD(self, line):
        """Remove the specified directory. Synonym for RMD. Deprecated."""
        return self.ftp_RMD(line)
