# -*- test-case-name: twisted.internet.test.test_endpoints -*-
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.

"""
Implementations of L{IStreamServerEndpoint} and L{IStreamClientEndpoint} that
wrap the L{IReactorTCP}, L{IReactorSSL}, and L{IReactorUNIX} interfaces.

This also implements an extensible mini-language for describing endpoints,
parsed by the L{clientFromString} and L{serverFromString} functions.

@since: 10.1
"""

from __future__ import division, absolute_import

import os
import re
import socket
import warnings

from socket import AF_INET6, AF_INET

from zope.interface import implementer, directlyProvides

from twisted.python.compat import _PY3
from twisted.internet import interfaces, defer, error, fdesc, threads
from twisted.internet.protocol import ClientFactory, Factory
from twisted.internet.protocol import ProcessProtocol, Protocol
from twisted.internet.interfaces import (
    IStreamServerEndpointStringParser, IStreamClientEndpointStringParser,
    IStreamClientEndpointStringParserWithReactor)
from twisted.python.filepath import FilePath
from twisted.python.systemd import ListenFDs
from twisted.internet.abstract import isIPv6Address
from twisted.python.failure import Failure
from twisted.python import log
from twisted.internet.address import _ProcessAddress, HostnameAddress
from twisted.python.components import proxyForInterface
from twisted.internet.task import LoopingCall

if not _PY3:
    from twisted.plugin import IPlugin, getPlugins
    from twisted.internet import stdio
    from twisted.internet.stdio import PipeAddress
    from twisted.python.constants import NamedConstant, Names
else:
    from zope.interface import Interface
    class IPlugin(Interface):
        pass
    NamedConstant = object
    Names = object

__all__ = ["clientFromString", "serverFromString",
           "TCP4ServerEndpoint", "TCP6ServerEndpoint",
           "TCP4ClientEndpoint", "TCP6ClientEndpoint",
           "UNIXServerEndpoint", "UNIXClientEndpoint",
           "SSL4ServerEndpoint", "SSL4ClientEndpoint",
           "AdoptedStreamServerEndpoint", "StandardIOEndpoint",
           "ProcessEndpoint", "HostnameEndpoint",
           "StandardErrorBehavior", "connectProtocol"]

__all3__ = ["TCP4ServerEndpoint", "TCP6ServerEndpoint",
            "TCP4ClientEndpoint", "TCP6ClientEndpoint",
            "SSL4ServerEndpoint", "SSL4ClientEndpoint",
            "connectProtocol", "HostnameEndpoint"]


class _WrappingProtocol(Protocol):
    """
    Wrap another protocol in order to notify my user when a connection has
    been made.
    """

    def __init__(self, connectedDeferred, wrappedProtocol):
        """
        @param connectedDeferred: The L{Deferred} that will callback
            with the C{wrappedProtocol} when it is connected.

        @param wrappedProtocol: An L{IProtocol} provider that will be
            connected.
        """
        self._connectedDeferred = connectedDeferred
        self._wrappedProtocol = wrappedProtocol

        for iface in [interfaces.IHalfCloseableProtocol,
                      interfaces.IFileDescriptorReceiver]:
            if iface.providedBy(self._wrappedProtocol):
                directlyProvides(self, iface)


    def logPrefix(self):
        """
        Transparently pass through the wrapped protocol's log prefix.
        """
        if interfaces.ILoggingContext.providedBy(self._wrappedProtocol):
            return self._wrappedProtocol.logPrefix()
        return self._wrappedProtocol.__class__.__name__


    def connectionMade(self):
        """
        Connect the C{self._wrappedProtocol} to our C{self.transport} and
        callback C{self._connectedDeferred} with the C{self._wrappedProtocol}
        """
        self._wrappedProtocol.makeConnection(self.transport)
        self._connectedDeferred.callback(self._wrappedProtocol)


    def dataReceived(self, data):
        """
        Proxy C{dataReceived} calls to our C{self._wrappedProtocol}
        """
        return self._wrappedProtocol.dataReceived(data)


    def fileDescriptorReceived(self, descriptor):
        """
        Proxy C{fileDescriptorReceived} calls to our C{self._wrappedProtocol}
        """
        return self._wrappedProtocol.fileDescriptorReceived(descriptor)


    def connectionLost(self, reason):
        """
        Proxy C{connectionLost} calls to our C{self._wrappedProtocol}
        """
        return self._wrappedProtocol.connectionLost(reason)


    def readConnectionLost(self):
        """
        Proxy L{IHalfCloseableProtocol.readConnectionLost} to our
        C{self._wrappedProtocol}
        """
        self._wrappedProtocol.readConnectionLost()


    def writeConnectionLost(self):
        """
        Proxy L{IHalfCloseableProtocol.writeConnectionLost} to our
        C{self._wrappedProtocol}
        """
        self._wrappedProtocol.writeConnectionLost()



class _WrappingFactory(ClientFactory):
    """
    Wrap a factory in order to wrap the protocols it builds.

    @ivar _wrappedFactory: A provider of I{IProtocolFactory} whose buildProtocol
        method will be called and whose resulting protocol will be wrapped.

    @ivar _onConnection: A L{Deferred} that fires when the protocol is
        connected

    @ivar _connector: A L{connector <twisted.internet.interfaces.IConnector>}
        that is managing the current or previous connection attempt.
    """
    protocol = _WrappingProtocol

    def __init__(self, wrappedFactory):
        """
        @param wrappedFactory: A provider of I{IProtocolFactory} whose
            buildProtocol method will be called and whose resulting protocol
            will be wrapped.
        """
        self._wrappedFactory = wrappedFactory
        self._onConnection = defer.Deferred(canceller=self._canceller)


    def startedConnecting(self, connector):
        """
        A connection attempt was started.  Remember the connector which started
        said attempt, for use later.
        """
        self._connector = connector


    def _canceller(self, deferred):
        """
        The outgoing connection attempt was cancelled.  Fail that L{Deferred}
        with an L{error.ConnectingCancelledError}.

        @param deferred: The L{Deferred <defer.Deferred>} that was cancelled;
            should be the same as C{self._onConnection}.
        @type deferred: L{Deferred <defer.Deferred>}

        @note: This relies on startedConnecting having been called, so it may
            seem as though there's a race condition where C{_connector} may not
            have been set.  However, using public APIs, this condition is
            impossible to catch, because a connection API
            (C{connectTCP}/C{SSL}/C{UNIX}) is always invoked before a
            L{_WrappingFactory}'s L{Deferred <defer.Deferred>} is returned to
            C{connect()}'s caller.

        @return: C{None}
        """
        deferred.errback(
            error.ConnectingCancelledError(
                self._connector.getDestination()))
        self._connector.stopConnecting()


    def doStart(self):
        """
        Start notifications are passed straight through to the wrapped factory.
        """
        self._wrappedFactory.doStart()


    def doStop(self):
        """
        Stop notifications are passed straight through to the wrapped factory.
        """
        self._wrappedFactory.doStop()


    def buildProtocol(self, addr):
        """
        Proxy C{buildProtocol} to our C{self._wrappedFactory} or errback
        the C{self._onConnection} L{Deferred}.

        @return: An instance of L{_WrappingProtocol} or C{None}
        """
        try:
            proto = self._wrappedFactory.buildProtocol(addr)
        except:
            self._onConnection.errback()
        else:
            return self.protocol(self._onConnection, proto)


    def clientConnectionFailed(self, connector, reason):
        """
        Errback the C{self._onConnection} L{Deferred} when the
        client connection fails.
        """
        if not self._onConnection.called:
            self._onConnection.errback(reason)



@implementer(interfaces.IStreamServerEndpoint)
class StandardIOEndpoint(object):
    """
    A Standard Input/Output endpoint
    """

    def __init__(self, reactor):
        """
        @param reactor: The reactor for the endpoint
        """
        self._reactor = reactor


    def listen(self, stdioProtocolFactory):
        """
        Implement L{IStreamServerEndpoint.listen} to listen on stdin/stdout
        """
        return defer.execute(stdio.StandardIO,
                             stdioProtocolFactory.buildProtocol(PipeAddress()),
                             reactor=self._reactor)



@implementer(interfaces.ITransport)
class _ProcessEndpointTransport(proxyForInterface(
                                interfaces.IProcessTransport, '_process')):
    """
    An L{ITransport} provider for the L{IProtocol} instance passed to the
    process endpoint.

    @ivar _process: An active process transport which will be used by write
        methods on this object to write data to a child process.
    @type _process: L{interfaces.IProcessTransport} provider
    """

    def write(self, data):
        """
        Write to the child process's standard input.

        @param data: The data to write on stdin.
        """
        self._process.writeToChild(0, data)


    def writeSequence(self, data):
        """
        Write a list of strings to child process's stdin.

        @param data: The list of chunks to write on stdin.
        """
        for chunk in data:
            self._process.writeToChild(0, chunk)



class _WrapIProtocol(ProcessProtocol):
    """
    An L{IProcessProtocol} provider that wraps an L{IProtocol}.

    @ivar transport: A L{_ProcessEndpointTransport} provider that is hooked to
        the wrapped L{IProtocol} provider.

    @see: L{protocol.ProcessProtocol}
    """

    def __init__(self, proto, executable, errFlag):
        """
        @param proto: An L{IProtocol} provider.
        @param errFlag: A constant belonging to L{StandardErrorBehavior}
            that determines if stderr is logged or dropped.
        @param executable: The file name (full path) to spawn.
        """
        self.protocol = proto
        self.errFlag = errFlag
        self.executable = executable


    def makeConnection(self, process):
        """
        Call L{IProtocol} provider's makeConnection method with an
        L{ITransport} provider.

        @param process: An L{IProcessTransport} provider.
        """
        self.transport = _ProcessEndpointTransport(process)
        return self.protocol.makeConnection(self.transport)


    def childDataReceived(self, childFD, data):
        """
        This is called with data from the process's stdout or stderr pipes. It
        checks the status of the errFlag to setermine if stderr should be
        logged (default) or dropped.
        """
        if childFD == 1:
            return self.protocol.dataReceived(data)
        elif childFD == 2 and self.errFlag == StandardErrorBehavior.LOG:
            log.msg(
                format="Process %(executable)r wrote stderr unhandled by "
                       "%(protocol)s: %(data)s",
                executable=self.executable, protocol=self.protocol,
                data=data)


    def processEnded(self, reason):
        """
        If the process ends with L{error.ProcessDone}, this method calls the
        L{IProtocol} provider's L{connectionLost} with a
        L{error.ConnectionDone}

        @see: L{ProcessProtocol.processEnded}
        """
        if (reason.check(error.ProcessDone) == error.ProcessDone) and (
                reason.value.status == 0):
            return self.protocol.connectionLost(
                Failure(error.ConnectionDone()))
        else:
            return self.protocol.connectionLost(reason)



class StandardErrorBehavior(Names):
    """
    Constants used in ProcessEndpoint to decide what to do with stderr.

    @cvar LOG: Indicates that stderr is to be logged.
    @cvar DROP: Indicates that stderr is to be dropped (and not logged).

    @since: 13.1
    """
    LOG = NamedConstant()
    DROP = NamedConstant()



@implementer(interfaces.IStreamClientEndpoint)
class ProcessEndpoint(object):
    """
    An endpoint for child processes

    @ivar _spawnProcess: A hook used for testing the spawning of child process.

    @since: 13.1
    """
    def __init__(self, reactor, executable, args=(), env={}, path=None,
                 uid=None, gid=None, usePTY=0, childFDs=None,
                 errFlag=StandardErrorBehavior.LOG):
        """
        See L{IReactorProcess.spawnProcess}.

        @param errFlag: Determines if stderr should be logged.
        @type errFlag: L{endpoints.StandardErrorBehavior}
        """
        self._reactor = reactor
        self._executable = executable
        self._args = args
        self._env = env
        self._path = path
        self._uid = uid
        self._gid = gid
        self._usePTY = usePTY
        self._childFDs = childFDs
        self._errFlag = errFlag
        self._spawnProcess = self._reactor.spawnProcess


    def connect(self, protocolFactory):
        """
        Implement L{IStreamClientEndpoint.connect} to launch a child process
        and connect it to a protocol created by C{protocolFactory}.

        @param protocolFactory: A factory for an L{IProtocol} provider which
            will be notified of all events related to the created process.
        """
        proto = protocolFactory.buildProtocol(_ProcessAddress())
        try:
            self._spawnProcess(
                _WrapIProtocol(proto, self._executable, self._errFlag),
                self._executable, self._args, self._env, self._path, self._uid,
                self._gid, self._usePTY, self._childFDs)
        except:
            return defer.fail()
        else:
            return defer.succeed(proto)



@implementer(interfaces.IStreamServerEndpoint)
class _TCPServerEndpoint(object):
    """
    A TCP server endpoint interface
    """

    def __init__(self, reactor, port, backlog, interface):
        """
        @param reactor: An L{IReactorTCP} provider.

        @param port: The port number used for listening
        @type port: int

        @param backlog: Size of the listen queue
        @type backlog: int

        @param interface: The hostname to bind to
        @type interface: str
        """
        self._reactor = reactor
        self._port = port
        self._backlog = backlog
        self._interface = interface


    def listen(self, protocolFactory):
        """
        Implement L{IStreamServerEndpoint.listen} to listen on a TCP
        socket
        """
        return defer.execute(self._reactor.listenTCP,
                             self._port,
                             protocolFactory,
                             backlog=self._backlog,
                             interface=self._interface)



class TCP4ServerEndpoint(_TCPServerEndpoint):
    """
    Implements TCP server endpoint with an IPv4 configuration
    """
    def __init__(self, reactor, port, backlog=50, interface=''):
        """
        @param reactor: An L{IReactorTCP} provider.

        @param port: The port number used for listening
        @type port: int

        @param backlog: Size of the listen queue
        @type backlog: int

        @param interface: The hostname to bind to, defaults to '' (all)
        @type interface: str
        """
        _TCPServerEndpoint.__init__(self, reactor, port, backlog, interface)



class TCP6ServerEndpoint(_TCPServerEndpoint):
    """
    Implements TCP server endpoint with an IPv6 configuration
    """
    def __init__(self, reactor, port, backlog=50, interface='::'):
        """
        @param reactor: An L{IReactorTCP} provider.

        @param port: The port number used for listening
        @type port: int

        @param backlog: Size of the listen queue
        @type backlog: int

        @param interface: The hostname to bind to, defaults to '' (all)
        @type interface: str
        """
        _TCPServerEndpoint.__init__(self, reactor, port, backlog, interface)



@implementer(interfaces.IStreamClientEndpoint)
class TCP4ClientEndpoint(object):
    """
    TCP client endpoint with an IPv4 configuration.
    """

    def __init__(self, reactor, host, port, timeout=30, bindAddress=None):
        """
        @param reactor: An L{IReactorTCP} provider

        @param host: A hostname, used when connecting
        @type host: str

        @param port: The port number, used when connecting
        @type port: int

        @param timeout: The number of seconds to wait before assuming the
            connection has failed.
        @type timeout: int

        @param bindAddress: A (host, port) tuple of local address to bind to,
            or None.
        @type bindAddress: tuple
        """
        self._reactor = reactor
        self._host = host
        self._port = port
        self._timeout = timeout
        self._bindAddress = bindAddress


    def connect(self, protocolFactory):
        """
        Implement L{IStreamClientEndpoint.connect} to connect via TCP.
        """
        try:
            wf = _WrappingFactory(protocolFactory)
            self._reactor.connectTCP(
                self._host, self._port, wf,
                timeout=self._timeout, bindAddress=self._bindAddress)
            return wf._onConnection
        except:
            return defer.fail()



@implementer(interfaces.IStreamClientEndpoint)
class TCP6ClientEndpoint(object):
    """
    TCP client endpoint with an IPv6 configuration.

    @ivar _getaddrinfo: A hook used for testing name resolution.

    @ivar _deferToThread: A hook used for testing deferToThread.

    @ivar _GAI_ADDRESS: Index of the address portion in result of
        getaddrinfo to be used.

    @ivar _GAI_ADDRESS_HOST: Index of the actual host-address in the
        5-tuple L{_GAI_ADDRESS}.
    """

    _getaddrinfo = socket.getaddrinfo
    _deferToThread = staticmethod(threads.deferToThread)
    _GAI_ADDRESS = 4
    _GAI_ADDRESS_HOST = 0

    def __init__(self, reactor, host, port, timeout=30, bindAddress=None):
        """
        @param host: An IPv6 address literal or a hostname with an
            IPv6 address

        @see: L{twisted.internet.interfaces.IReactorTCP.connectTCP}
        """
        self._reactor = reactor
        self._host = host
        self._port = port
        self._timeout = timeout
        self._bindAddress = bindAddress


    def connect(self, protocolFactory):
        """
        Implement L{IStreamClientEndpoint.connect} to connect via TCP,
        once the hostname resolution is done.
        """
        if isIPv6Address(self._host):
            d = self._resolvedHostConnect(self._host, protocolFactory)
        else:
            d = self._nameResolution(self._host)
            d.addCallback(lambda result: result[0][self._GAI_ADDRESS]
                          [self._GAI_ADDRESS_HOST])
            d.addCallback(self._resolvedHostConnect, protocolFactory)
        return d


    def _nameResolution(self, host):
        """
        Resolve the hostname string into a tuple containing the host
        IPv6 address.
        """
        return self._deferToThread(
            self._getaddrinfo, host, 0, socket.AF_INET6)


    def _resolvedHostConnect(self, resolvedHost, protocolFactory):
        """
        Connect to the server using the resolved hostname.
        """
        try:
            wf = _WrappingFactory(protocolFactory)
            self._reactor.connectTCP(resolvedHost, self._port, wf,
                timeout=self._timeout, bindAddress=self._bindAddress)
            return wf._onConnection
        except:
            return defer.fail()



@implementer(interfaces.IStreamClientEndpoint)
class HostnameEndpoint(object):
    """
    A name-based endpoint that connects to the fastest amongst the
    resolved host addresses.

    @ivar _getaddrinfo: A hook used for testing name resolution.

    @ivar _deferToThread: A hook used for testing deferToThread.
    """
    _getaddrinfo = socket.getaddrinfo
    _deferToThread = staticmethod(threads.deferToThread)

    def __init__(self, reactor, host, port, timeout=30, bindAddress=None):
        """
        @param host: A hostname to connect to.
        @type host: L{bytes}

        @param timeout: For each individual connection attempt, the number of
            seconds to wait before assuming the connection has failed.
        @type timeout: L{int}

        @see: L{twisted.internet.interfaces.IReactorTCP.connectTCP}
        """
        self._reactor = reactor
        self._host = host
        self._port = port
        self._timeout = timeout
        self._bindAddress = bindAddress


    def connect(self, protocolFactory):
        """
        Attempts a connection to each address returned by gai, and returns a
        connection which is established first.
        """
        wf = protocolFactory
        pending = []

        def _canceller(d):
            """
            The outgoing connection attempt was cancelled.  Fail that L{Deferred}
            with an L{error.ConnectingCancelledError}.

            @param d: The L{Deferred <defer.Deferred>} that was cancelled
            @type d: L{Deferred <defer.Deferred>}

            @return: C{None}
            """
            d.errback(error.ConnectingCancelledError(
                HostnameAddress(self._host, self._port)))
            for p in pending[:]:
                p.cancel()

        def errbackForGai(failure):
            """
            Errback for when L{_nameResolution} returns a Deferred that fires
            with failure.
            """
            return defer.fail(error.DNSLookupError(
                "Couldn't find the hostname '%s'" % (self._host,)))

        def _endpoints(gaiResult):
            """
            This method matches the host address family with an endpoint for
            every address returned by GAI.

            @param gaiResult: A list of 5-tuples as returned by GAI.
            @type gaiResult: list
            """
            for family, socktype, proto, canonname, sockaddr in gaiResult:
                if family in [AF_INET6]:
                    yield TCP6ClientEndpoint(self._reactor, sockaddr[0],
                            sockaddr[1], self._timeout, self._bindAddress)
                elif family in [AF_INET]:
                    yield TCP4ClientEndpoint(self._reactor, sockaddr[0],
                            sockaddr[1], self._timeout, self._bindAddress)
                        # Yields an endpoint for every address returned by GAI

        def attemptConnection(endpoints):
            """
            When L{endpoints} yields an endpoint, this method attempts to connect it.
            """
            # The trial attempts for each endpoints, the recording of
            # successful and failed attempts, and the algorithm to pick the
            # winner endpoint goes here.
            # Return a Deferred that fires with the endpoint that wins,
            # or `failures` if none succeed.

            endpointsListExhausted = []
            successful = []
            failures = []
            winner = defer.Deferred(canceller=_canceller)

            def usedEndpointRemoval(connResult, connAttempt):
                pending.remove(connAttempt)
                return connResult

            def afterConnectionAttempt(connResult):
                if lc.running:
                    lc.stop()

                successful.append(True)
                for p in pending[:]:
                    p.cancel()
                winner.callback(connResult)
                return None

            def checkDone():
                if endpointsListExhausted and not pending and not successful:
                    winner.errback(failures.pop())

            def connectFailed(reason):
                failures.append(reason)
                checkDone()
                return None

            def iterateEndpoint():
                try:
                    endpoint = next(endpoints)
                except StopIteration:
                    # The list of endpoints ends.
                    endpointsListExhausted.append(True)
                    lc.stop()
                    checkDone()
                else:
                    dconn = endpoint.connect(wf)
                    pending.append(dconn)
                    dconn.addBoth(usedEndpointRemoval, dconn)
                    dconn.addCallback(afterConnectionAttempt)
                    dconn.addErrback(connectFailed)

            lc = LoopingCall(iterateEndpoint)
            lc.clock = self._reactor
            lc.start(0.3)
            return winner

        d = self._nameResolution(self._host, self._port)
        d.addErrback(errbackForGai)
        d.addCallback(_endpoints)
        d.addCallback(attemptConnection)
        return d

    def _nameResolution(self, host, port):
        """
        Resolve the hostname string into a tuple containig the host
        address.
        """
        return self._deferToThread(self._getaddrinfo, host, port, 0,
                socket.SOCK_STREAM)



@implementer(interfaces.IStreamServerEndpoint)
class SSL4ServerEndpoint(object):
    """
    SSL secured TCP server endpoint with an IPv4 configuration.
    """

    def __init__(self, reactor, port, sslContextFactory,
                 backlog=50, interface=''):
        """
        @param reactor: An L{IReactorSSL} provider.

        @param port: The port number used for listening
        @type port: int

        @param sslContextFactory: An instance of
            L{twisted.internet.ssl.ContextFactory}.

        @param backlog: Size of the listen queue
        @type backlog: int

        @param interface: The hostname to bind to, defaults to '' (all)
        @type interface: str
        """
        self._reactor = reactor
        self._port = port
        self._sslContextFactory = sslContextFactory
        self._backlog = backlog
        self._interface = interface


    def listen(self, protocolFactory):
        """
        Implement L{IStreamServerEndpoint.listen} to listen for SSL on a
        TCP socket.
        """
        return defer.execute(self._reactor.listenSSL, self._port,
                             protocolFactory,
                             contextFactory=self._sslContextFactory,
                             backlog=self._backlog,
                             interface=self._interface)



@implementer(interfaces.IStreamClientEndpoint)
class SSL4ClientEndpoint(object):
    """
    SSL secured TCP client endpoint with an IPv4 configuration
    """

    def __init__(self, reactor, host, port, sslContextFactory,
                 timeout=30, bindAddress=None):
        """
        @param reactor: An L{IReactorSSL} provider.

        @param host: A hostname, used when connecting
        @type host: str

        @param port: The port number, used when connecting
        @type port: int

        @param sslContextFactory: SSL Configuration information as an instance
            of L{twisted.internet.ssl.ContextFactory}.

        @param timeout: Number of seconds to wait before assuming the
            connection has failed.
        @type timeout: int

        @param bindAddress: A (host, port) tuple of local address to bind to,
            or None.
        @type bindAddress: tuple
        """
        self._reactor = reactor
        self._host = host
        self._port = port
        self._sslContextFactory = sslContextFactory
        self._timeout = timeout
        self._bindAddress = bindAddress


    def connect(self, protocolFactory):
        """
        Implement L{IStreamClientEndpoint.connect} to connect with SSL over
        TCP.
        """
        try:
            wf = _WrappingFactory(protocolFactory)
            self._reactor.connectSSL(
                self._host, self._port, wf, self._sslContextFactory,
                timeout=self._timeout, bindAddress=self._bindAddress)
            return wf._onConnection
        except:
            return defer.fail()



@implementer(interfaces.IStreamServerEndpoint)
class UNIXServerEndpoint(object):
    """
    UnixSocket server endpoint.
    """
    def __init__(self, reactor, address, backlog=50, mode=0o666, wantPID=0):
        """
        @param reactor: An L{IReactorUNIX} provider.
        @param address: The path to the Unix socket file, used when listening
        @param backlog: number of connections to allow in backlog.
        @param mode: mode to set on the unix socket.  This parameter is
            deprecated.  Permissions should be set on the directory which
            contains the UNIX socket.
        @param wantPID: If True, create a pidfile for the socket.
        """
        self._reactor = reactor
        self._address = address
        self._backlog = backlog
        self._mode = mode
        self._wantPID = wantPID


    def listen(self, protocolFactory):
        """
        Implement L{IStreamServerEndpoint.listen} to listen on a UNIX socket.
        """
        return defer.execute(self._reactor.listenUNIX, self._address,
                             protocolFactory,
                             backlog=self._backlog,
                             mode=self._mode,
                             wantPID=self._wantPID)



@implementer(interfaces.IStreamClientEndpoint)
class UNIXClientEndpoint(object):
    """
    UnixSocket client endpoint.
    """
    def __init__(self, reactor, path, timeout=30, checkPID=0):
        """
        @param reactor: An L{IReactorUNIX} provider.

        @param path: The path to the Unix socket file, used when connecting
        @type path: str

        @param timeout: Number of seconds to wait before assuming the
            connection has failed.
        @type timeout: int

        @param checkPID: If True, check for a pid file to verify that a server
            is listening.
        @type checkPID: bool
        """
        self._reactor = reactor
        self._path = path
        self._timeout = timeout
        self._checkPID = checkPID


    def connect(self, protocolFactory):
        """
        Implement L{IStreamClientEndpoint.connect} to connect via a
        UNIX Socket
        """
        try:
            wf = _WrappingFactory(protocolFactory)
            self._reactor.connectUNIX(
                self._path, wf,
                timeout=self._timeout,
                checkPID=self._checkPID)
            return wf._onConnection
        except:
            return defer.fail()



@implementer(interfaces.IStreamServerEndpoint)
class AdoptedStreamServerEndpoint(object):
    """
    An endpoint for listening on a file descriptor initialized outside of
    Twisted.

    @ivar _used: A C{bool} indicating whether this endpoint has been used to
        listen with a factory yet.  C{True} if so.
    """
    _close = os.close
    _setNonBlocking = staticmethod(fdesc.setNonBlocking)

    def __init__(self, reactor, fileno, addressFamily):
        """
        @param reactor: An L{IReactorSocket} provider.

        @param fileno: An integer file descriptor corresponding to a listening
            I{SOCK_STREAM} socket.

        @param addressFamily: The address family of the socket given by
            C{fileno}.
        """
        self.reactor = reactor
        self.fileno = fileno
        self.addressFamily = addressFamily
        self._used = False


    def listen(self, factory):
        """
        Implement L{IStreamServerEndpoint.listen} to start listening on, and
        then close, C{self._fileno}.
        """
        if self._used:
            return defer.fail(error.AlreadyListened())
        self._used = True

        try:
            self._setNonBlocking(self.fileno)
            port = self.reactor.adoptStreamPort(
                self.fileno, self.addressFamily, factory)
            self._close(self.fileno)
        except:
            return defer.fail()
        return defer.succeed(port)



def _parseTCP(factory, port, interface="", backlog=50):
    """
    Internal parser function for L{_parseServer} to convert the string
    arguments for a TCP(IPv4) stream endpoint into the structured arguments.

    @param factory: the protocol factory being parsed, or C{None}.  (This was a
        leftover argument from when this code was in C{strports}, and is now
        mostly None and unused.)

    @type factory: L{IProtocolFactory} or C{NoneType}

    @param port: the integer port number to bind
    @type port: C{str}

    @param interface: the interface IP to listen on
    @param backlog: the length of the listen queue
    @type backlog: C{str}

    @return: a 2-tuple of (args, kwargs), describing  the parameters to
        L{IReactorTCP.listenTCP} (or, modulo argument 2, the factory, arguments
        to L{TCP4ServerEndpoint}.
    """
    return (int(port), factory), {'interface': interface,
                                  'backlog': int(backlog)}



def _parseUNIX(factory, address, mode='666', backlog=50, lockfile=True):
    """
    Internal parser function for L{_parseServer} to convert the string
    arguments for a UNIX (AF_UNIX/SOCK_STREAM) stream endpoint into the
    structured arguments.

    @param factory: the protocol factory being parsed, or C{None}.  (This was a
        leftover argument from when this code was in C{strports}, and is now
        mostly None and unused.)

    @type factory: L{IProtocolFactory} or C{NoneType}

    @param address: the pathname of the unix socket
    @type address: C{str}

    @param backlog: the length of the listen queue
    @type backlog: C{str}

    @param lockfile: A string '0' or '1', mapping to True and False
        respectively.  See the C{wantPID} argument to C{listenUNIX}

    @return: a 2-tuple of (args, kwargs), describing  the parameters to
        L{IReactorTCP.listenUNIX} (or, modulo argument 2, the factory,
        arguments to L{UNIXServerEndpoint}.
    """
    return (
        (address, factory),
        {'mode': int(mode, 8), 'backlog': int(backlog),
         'wantPID': bool(int(lockfile))})



def _parseSSL(factory, port, privateKey="server.pem", certKey=None,
              sslmethod=None, interface='', backlog=50, extraCertChain=None,
              dhParameters=None):
    """
    Internal parser function for L{_parseServer} to convert the string
    arguments for an SSL (over TCP/IPv4) stream endpoint into the structured
    arguments.

    @param factory: the protocol factory being parsed, or C{None}.  (This was a
        leftover argument from when this code was in C{strports}, and is now
        mostly None and unused.)
    @type factory: L{IProtocolFactory} or C{NoneType}

    @param port: the integer port number to bind
    @type port: C{str}

    @param interface: the interface IP to listen on
    @param backlog: the length of the listen queue
    @type backlog: C{str}

    @param privateKey: The file name of a PEM format private key file.
    @type privateKey: C{str}

    @param certKey: The file name of a PEM format certificate file.
    @type certKey: C{str}

    @param sslmethod: The string name of an SSL method, based on the name of a
        constant in C{OpenSSL.SSL}.  Must be one of: "SSLv23_METHOD",
        "SSLv2_METHOD", "SSLv3_METHOD", "TLSv1_METHOD".
    @type sslmethod: C{str}

    @param extraCertChain: The path of a file containing one or more
        certificates in PEM format that establish the chain from a root CA to
        the CA that signed your C{certKey}.
    @type extraCertChain: L{bytes}

    @param dhParameters: The file name of a file containing parameters that are
        required for Diffie-Hellman key exchange.  If this is not specified,
        the forward secret C{DHE} ciphers aren't available for servers.
    @type dhParameters: L{bytes}

    @return: a 2-tuple of (args, kwargs), describing  the parameters to
        L{IReactorSSL.listenSSL} (or, modulo argument 2, the factory, arguments
        to L{SSL4ServerEndpoint}.
    """
    from twisted.internet import ssl
    if certKey is None:
        certKey = privateKey
    kw = {}
    if sslmethod is not None:
        kw['method'] = getattr(ssl.SSL, sslmethod)
    else:
        kw['method'] = ssl.SSL.SSLv23_METHOD
    certPEM = FilePath(certKey).getContent()
    keyPEM = FilePath(privateKey).getContent()
    privateCertificate = ssl.PrivateCertificate.loadPEM(certPEM + keyPEM)
    if extraCertChain is not None:
        matches = re.findall(
            r'(-----BEGIN CERTIFICATE-----\n.+?\n-----END CERTIFICATE-----)',
            FilePath(extraCertChain).getContent(),
            flags=re.DOTALL
        )
        chainCertificates = [ssl.Certificate.loadPEM(chainCertPEM).original
                             for chainCertPEM in matches]
        if not chainCertificates:
            raise ValueError(
                "Specified chain file '%s' doesn't contain any valid "
                "certificates in PEM format." % (extraCertChain,)
            )
    else:
        chainCertificates = None
    if dhParameters is not None:
        dhParameters = ssl.DiffieHellmanParameters.fromFile(
            FilePath(dhParameters),
        )

    cf = ssl.CertificateOptions(
        privateKey=privateCertificate.privateKey.original,
        certificate=privateCertificate.original,
        extraCertChain=chainCertificates,
        dhParameters=dhParameters,
        **kw
    )
    return ((int(port), factory, cf),
            {'interface': interface, 'backlog': int(backlog)})



@implementer(IPlugin, IStreamServerEndpointStringParser)
class _StandardIOParser(object):
    """
    Stream server endpoint string parser for the Standard I/O type.

    @ivar prefix: See L{IStreamClientEndpointStringParser.prefix}.
    """
    prefix = "stdio"

    def _parseServer(self, reactor):
        """
        Internal parser function for L{_parseServer} to convert the string
        arguments into structured arguments for the L{StandardIOEndpoint}

        @param reactor: Reactor for the endpoint
        """
        return StandardIOEndpoint(reactor)


    def parseStreamServer(self, reactor, *args, **kwargs):
        # Redirects to another function (self._parseServer), tricks zope.interface
        # into believing the interface is correctly implemented.
        return self._parseServer(reactor)



@implementer(IPlugin, IStreamServerEndpointStringParser)
class _SystemdParser(object):
    """
    Stream server endpoint string parser for the I{systemd} endpoint type.

    @ivar prefix: See L{IStreamClientEndpointStringParser.prefix}.

    @ivar _sddaemon: A L{ListenFDs} instance used to translate an index into an
        actual file descriptor.
    """
    _sddaemon = ListenFDs.fromEnvironment()

    prefix = "systemd"

    def _parseServer(self, reactor, domain, index):
        """
        Internal parser function for L{_parseServer} to convert the string
        arguments for a systemd server endpoint into structured arguments for
        L{AdoptedStreamServerEndpoint}.

        @param reactor: An L{IReactorSocket} provider.

        @param domain: The domain (or address family) of the socket inherited
            from systemd.  This is a string like C{"INET"} or C{"UNIX"}, ie the
            name of an address family from the L{socket} module, without the
            C{"AF_"} prefix.
        @type domain: C{str}

        @param index: An offset into the list of file descriptors inherited from
            systemd.
        @type index: C{str}

        @return: A two-tuple of parsed positional arguments and parsed keyword
            arguments (a tuple and a dictionary).  These can be used to
            construct an L{AdoptedStreamServerEndpoint}.
        """
        index = int(index)
        fileno = self._sddaemon.inheritedDescriptors()[index]
        addressFamily = getattr(socket, 'AF_' + domain)
        return AdoptedStreamServerEndpoint(reactor, fileno, addressFamily)


    def parseStreamServer(self, reactor, *args, **kwargs):
        # Delegate to another function with a sane signature.  This function has
        # an insane signature to trick zope.interface into believing the
        # interface is correctly implemented.
        return self._parseServer(reactor, *args, **kwargs)



@implementer(IPlugin, IStreamServerEndpointStringParser)
class _TCP6ServerParser(object):
    """
    Stream server endpoint string parser for the TCP6ServerEndpoint type.

    @ivar prefix: See L{IStreamClientEndpointStringParser.prefix}.
    """
    prefix = "tcp6"     # Used in _parseServer to identify the plugin with the endpoint type

    def _parseServer(self, reactor, port, backlog=50, interface='::'):
        """
        Internal parser function for L{_parseServer} to convert the string
        arguments into structured arguments for the L{TCP6ServerEndpoint}

        @param reactor: An L{IReactorTCP} provider.

        @param port: The port number used for listening
        @type port: int

        @param backlog: Size of the listen queue
        @type backlog: int

        @param interface: The hostname to bind to
        @type interface: str
        """
        port = int(port)
        backlog = int(backlog)
        return TCP6ServerEndpoint(reactor, port, backlog, interface)


    def parseStreamServer(self, reactor, *args, **kwargs):
        # Redirects to another function (self._parseServer), tricks zope.interface
        # into believing the interface is correctly implemented.
        return self._parseServer(reactor, *args, **kwargs)



_serverParsers = {"tcp": _parseTCP,
                  "unix": _parseUNIX,
                  "ssl": _parseSSL,
                  }

_OP, _STRING = range(2)

def _tokenize(description):
    """
    Tokenize a strports string and yield each token.

    @param description: a string as described by L{serverFromString} or
        L{clientFromString}.

    @return: an iterable of 2-tuples of (L{_OP} or L{_STRING}, string).  Tuples
        starting with L{_OP} will contain a second element of either ':' (i.e.
        'next parameter') or '=' (i.e. 'assign parameter value').  For example,
        the string 'hello:greet\=ing=world' would result in a generator
        yielding these values::

            _STRING, 'hello'
            _OP, ':'
            _STRING, 'greet=ing'
            _OP, '='
            _STRING, 'world'
    """
    current = ''
    ops = ':='
    nextOps = {':': ':=', '=': ':'}
    description = iter(description)
    for n in description:
        if n in ops:
            yield _STRING, current
            yield _OP, n
            current = ''
            ops = nextOps[n]
        elif n == '\\':
            current += description.next()
        else:
            current += n
    yield _STRING, current



def _parse(description):
    """
    Convert a description string into a list of positional and keyword
    parameters, using logic vaguely like what Python does.

    @param description: a string as described by L{serverFromString} or
        L{clientFromString}.

    @return: a 2-tuple of C{(args, kwargs)}, where 'args' is a list of all
        ':'-separated C{str}s not containing an '=' and 'kwargs' is a map of
        all C{str}s which do contain an '='.  For example, the result of
        C{_parse('a:b:d=1:c')} would be C{(['a', 'b', 'c'], {'d': '1'})}.
    """
    args, kw = [], {}
    def add(sofar):
        if len(sofar) == 1:
            args.append(sofar[0])
        else:
            kw[sofar[0]] = sofar[1]
    sofar = ()
    for (type, value) in _tokenize(description):
        if type is _STRING:
            sofar += (value,)
        elif value == ':':
            add(sofar)
            sofar = ()
    add(sofar)
    return args, kw


# Mappings from description "names" to endpoint constructors.
_endpointServerFactories = {
    'TCP': TCP4ServerEndpoint,
    'SSL': SSL4ServerEndpoint,
    'UNIX': UNIXServerEndpoint,
    }

_endpointClientFactories = {
    'TCP': TCP4ClientEndpoint,
    'SSL': SSL4ClientEndpoint,
    'UNIX': UNIXClientEndpoint,
    }


_NO_DEFAULT = object()

def _parseServer(description, factory, default=None):
    """
    Parse a strports description into a 2-tuple of arguments and keyword
    values.

    @param description: A description in the format explained by
        L{serverFromString}.
    @type description: C{str}

    @param factory: A 'factory' argument; this is left-over from
        twisted.application.strports, it's not really used.
    @type factory: L{IProtocolFactory} or L{None}

    @param default: Deprecated argument, specifying the default parser mode to
        use for unqualified description strings (those which do not have a ':'
        and prefix).
    @type default: C{str} or C{NoneType}

    @return: a 3-tuple of (plugin or name, arguments, keyword arguments)
    """
    args, kw = _parse(description)
    if not args or (len(args) == 1 and not kw):
        deprecationMessage = (
            "Unqualified strport description passed to 'service'."
            "Use qualified endpoint descriptions; for example, 'tcp:%s'."
            % (description,))
        if default is None:
            default = 'tcp'
            warnings.warn(
                deprecationMessage, category=DeprecationWarning, stacklevel=4)
        elif default is _NO_DEFAULT:
            raise ValueError(deprecationMessage)
        # If the default has been otherwise specified, the user has already
        # been warned.
        args[0:0] = [default]
    endpointType = args[0]
    parser = _serverParsers.get(endpointType)
    if parser is None:
        # If the required parser is not found in _server, check if
        # a plugin exists for the endpointType
        for plugin in getPlugins(IStreamServerEndpointStringParser):
            if plugin.prefix == endpointType:
                return (plugin, args[1:], kw)
        raise ValueError("Unknown endpoint type: '%s'" % (endpointType,))
    return (endpointType.upper(),) + parser(factory, *args[1:], **kw)



def _serverFromStringLegacy(reactor, description, default):
    """
    Underlying implementation of L{serverFromString} which avoids exposing the
    deprecated 'default' argument to anything but L{strports.service}.
    """
    nameOrPlugin, args, kw = _parseServer(description, None, default)
    if type(nameOrPlugin) is not str:
        plugin = nameOrPlugin
        return plugin.parseStreamServer(reactor, *args, **kw)
    else:
        name = nameOrPlugin
    # Chop out the factory.
    args = args[:1] + args[2:]
    return _endpointServerFactories[name](reactor, *args, **kw)



def serverFromString(reactor, description):
    """
    Construct a stream server endpoint from an endpoint description string.

    The format for server endpoint descriptions is a simple byte string.  It is
    a prefix naming the type of endpoint, then a colon, then the arguments for
    that endpoint.

    For example, you can call it like this to create an endpoint that will
    listen on TCP port 80::

        serverFromString(reactor, b"tcp:80")

    Additional arguments may be specified as keywords, separated with colons.
    For example, you can specify the interface for a TCP server endpoint to
    bind to like this::

        serverFromString(reactor, b"tcp:80:interface=127.0.0.1")

    SSL server endpoints may be specified with the 'ssl' prefix, and the
    private key and certificate files may be specified by the C{privateKey} and
    C{certKey} arguments::

        serverFromString(
            reactor, b"ssl:443:privateKey=key.pem:certKey=crt.pem")

    If a private key file name (C{privateKey}) isn't provided, a "server.pem"
    file is assumed to exist which contains the private key. If the certificate
    file name (C{certKey}) isn't provided, the private key file is assumed to
    contain the certificate as well.

    You may escape colons in arguments with a backslash, which you will need to
    use if you want to specify a full pathname argument on Windows::

        serverFromString(reactor,
            b"ssl:443:privateKey=C\\:/key.pem:certKey=C\\:/cert.pem")

    finally, the 'unix' prefix may be used to specify a filesystem UNIX socket,
    optionally with a 'mode' argument to specify the mode of the socket file
    created by C{listen}::

        serverFromString(reactor, b"unix:/var/run/finger")
        serverFromString(reactor, b"unix:/var/run/finger:mode=660")

    This function is also extensible; new endpoint types may be registered as
    L{IStreamServerEndpointStringParser} plugins.  See that interface for more
    information.

    @param reactor: The server endpoint will be constructed with this reactor.

    @param description: The strports description to parse.
    @type description: L{bytes}

    @return: A new endpoint which can be used to listen with the parameters
        given by by C{description}.

    @rtype: L{IStreamServerEndpoint<twisted.internet.interfaces.IStreamServerEndpoint>}

    @raise ValueError: when the 'description' string cannot be parsed.

    @since: 10.2
    """
    return _serverFromStringLegacy(reactor, description, _NO_DEFAULT)



def quoteStringArgument(argument):
    """
    Quote an argument to L{serverFromString} and L{clientFromString}.  Since
    arguments are separated with colons and colons are escaped with
    backslashes, some care is necessary if, for example, you have a pathname,
    you may be tempted to interpolate into a string like this::

        serverFromString(reactor, b"ssl:443:privateKey=%s" % (myPathName,))

    This may appear to work, but will have portability issues (Windows
    pathnames, for example).  Usually you should just construct the appropriate
    endpoint type rather than interpolating strings, which in this case would
    be L{SSL4ServerEndpoint}.  There are some use-cases where you may need to
    generate such a string, though; for example, a tool to manipulate a
    configuration file which has strports descriptions in it.  To be correct in
    those cases, do this instead::

        serverFromString(reactor, b"ssl:443:privateKey=%s" %
                         (quoteStringArgument(myPathName),))

    @param argument: The part of the endpoint description string you want to
        pass through.

    @type argument: C{bytes}

    @return: The quoted argument.

    @rtype: C{bytes}
    """
    return argument.replace(b'\\', b'\\\\').replace(b':', b'\\:')



def _parseClientTCP(*args, **kwargs):
    """
    Perform any argument value coercion necessary for TCP client parameters.

    Valid positional arguments to this function are host and port.

    Valid keyword arguments to this function are all L{IReactorTCP.connectTCP}
    arguments.

    @return: The coerced values as a C{dict}.
    """

    if len(args) == 2:
        kwargs['port'] = int(args[1])
        kwargs['host'] = args[0]
    elif len(args) == 1:
        if 'host' in kwargs:
            kwargs['port'] = int(args[0])
        else:
            kwargs['host'] = args[0]

    try:
        kwargs['port'] = int(kwargs['port'])
    except KeyError:
        pass

    try:
        kwargs['timeout'] = int(kwargs['timeout'])
    except KeyError:
        pass

    try:
        kwargs['bindAddress'] = (kwargs['bindAddress'], 0)
    except KeyError:
        pass

    return kwargs



def _loadCAsFromDir(directoryPath):
    """
    Load certificate-authority certificate objects in a given directory.

    @param directoryPath: a L{FilePath} pointing at a directory to load .pem
        files from.

    @return: a C{list} of L{OpenSSL.crypto.X509} objects.
    """
    from twisted.internet import ssl

    caCerts = {}
    for child in directoryPath.children():
        if not child.basename().split('.')[-1].lower() == 'pem':
            continue
        try:
            data = child.getContent()
        except IOError:
            # Permission denied, corrupt disk, we don't care.
            continue
        try:
            theCert = ssl.Certificate.loadPEM(data)
        except ssl.SSL.Error:
            # Duplicate certificate, invalid certificate, etc.  We don't care.
            pass
        else:
            caCerts[theCert.digest()] = theCert.original
    return caCerts.values()



def _parseClientSSL(*args, **kwargs):
    """
    Perform any argument value coercion necessary for SSL client parameters.

    Valid keyword arguments to this function are all L{IReactorSSL.connectSSL}
    arguments except for C{contextFactory}.  Instead, C{certKey} (the path name
    of the certificate file) C{privateKey} (the path name of the private key
    associated with the certificate) are accepted and used to construct a
    context factory.

    Valid positional arguments to this function are host and port.

    @param caCertsDir: The one parameter which is not part of
        L{IReactorSSL.connectSSL}'s signature, this is a path name used to
        construct a list of certificate authority certificates.  The directory
        will be scanned for files ending in C{.pem}, all of which will be
        considered valid certificate authorities for this connection.

    @type caCertsDir: C{str}

    @return: The coerced values as a C{dict}.
    """
    from twisted.internet import ssl
    kwargs = _parseClientTCP(*args, **kwargs)
    certKey = kwargs.pop('certKey', None)
    privateKey = kwargs.pop('privateKey', None)
    caCertsDir = kwargs.pop('caCertsDir', None)
    if certKey is not None:
        certx509 = ssl.Certificate.loadPEM(
            FilePath(certKey).getContent()).original
    else:
        certx509 = None
    if privateKey is not None:
        privateKey = ssl.PrivateCertificate.loadPEM(
            FilePath(privateKey).getContent()).privateKey.original
    else:
        privateKey = None
    if caCertsDir is not None:
        verify = True
        caCerts = _loadCAsFromDir(FilePath(caCertsDir))
    else:
        verify = False
        caCerts = None
    kwargs['sslContextFactory'] = ssl.CertificateOptions(
        method=ssl.SSL.SSLv23_METHOD,
        certificate=certx509,
        privateKey=privateKey,
        verify=verify,
        caCerts=caCerts
    )
    return kwargs



def _parseClientUNIX(*args, **kwargs):
    """
    Perform any argument value coercion necessary for UNIX client parameters.

    Valid keyword arguments to this function are all L{IReactorUNIX.connectUNIX}
    keyword arguments except for C{checkPID}.  Instead, C{lockfile} is accepted
    and has the same meaning.  Also C{path} is used instead of C{address}.

    Valid positional arguments to this function are C{path}.

    @return: The coerced values as a C{dict}.
    """
    if len(args) == 1:
        kwargs['path'] = args[0]

    try:
        kwargs['checkPID'] = bool(int(kwargs.pop('lockfile')))
    except KeyError:
        pass
    try:
        kwargs['timeout'] = int(kwargs['timeout'])
    except KeyError:
        pass
    return kwargs

_clientParsers = {
    'TCP': _parseClientTCP,
    'SSL': _parseClientSSL,
    'UNIX': _parseClientUNIX,
    }



def clientFromString(reactor, description):
    """
    Construct a client endpoint from a description string.

    Client description strings are much like server description strings,
    although they take all of their arguments as keywords, aside from host and
    port.

    You can create a TCP client endpoint with the 'host' and 'port' arguments,
    like so::

        clientFromString(reactor, b"tcp:host=www.example.com:port=80")

    or, without specifying host and port keywords::

        clientFromString(reactor, b"tcp:www.example.com:80")

    Or you can specify only one or the other, as in the following 2 examples::

        clientFromString(reactor, b"tcp:host=www.example.com:80")
        clientFromString(reactor, b"tcp:www.example.com:port=80")

    or an SSL client endpoint with those arguments, plus the arguments used by
    the server SSL, for a client certificate::

        clientFromString(reactor, b"ssl:web.example.com:443:"
                                  b"privateKey=foo.pem:certKey=foo.pem")

    to specify your certificate trust roots, you can identify a directory with
    PEM files in it with the C{caCertsDir} argument::

        clientFromString(reactor, b"ssl:host=web.example.com:port=443:"
                                  b"caCertsDir=/etc/ssl/certs")

    Both TCP and SSL client endpoint description strings can include a
    'bindAddress' keyword argument, whose value should be a local IPv4
    address. This fixes the client socket to that IP address::

        clientFromString(reactor, b"tcp:www.example.com:80:"
                                  b"bindAddress=192.0.2.100")

    NB: Fixed client ports are not currently supported in TCP or SSL
    client endpoints. The client socket will always use an ephemeral
    port assigned by the operating system

    You can create a UNIX client endpoint with the 'path' argument and optional
    'lockfile' and 'timeout' arguments::

        clientFromString(
            reactor, b"unix:path=/var/foo/bar:lockfile=1:timeout=9")

    or, with the path as a positional argument with or without optional
    arguments as in the following 2 examples::

        clientFromString(reactor, b"unix:/var/foo/bar")
        clientFromString(reactor, b"unix:/var/foo/bar:lockfile=1:timeout=9")

    This function is also extensible; new endpoint types may be registered as
    L{IStreamClientEndpointStringParserWithReactor} plugins.  See that
    interface for more information.

    @param reactor: The client endpoint will be constructed with this reactor.

    @param description: The strports description to parse.
    @type description: L{bytes}

    @return: A new endpoint which can be used to connect with the parameters
        given by by C{description}.
    @rtype: L{IStreamClientEndpoint<twisted.internet.interfaces.IStreamClientEndpoint>}

    @since: 10.2
    """
    args, kwargs = _parse(description)
    aname = args.pop(0)
    name = aname.upper()
    for plugin in getPlugins(IStreamClientEndpointStringParserWithReactor):
        if plugin.prefix.upper() == name:
            return plugin.parseStreamClient(reactor, *args, **kwargs)
    for plugin in getPlugins(IStreamClientEndpointStringParser):
        if plugin.prefix.upper() == name:
            return plugin.parseStreamClient(*args, **kwargs)
    if name not in _clientParsers:
        raise ValueError("Unknown endpoint type: %r" % (aname,))
    kwargs = _clientParsers[name](*args, **kwargs)
    return _endpointClientFactories[name](reactor, **kwargs)



def connectProtocol(endpoint, protocol):
    """
    Connect a protocol instance to an endpoint.

    This allows using a client endpoint without having to create a factory.

    @param endpoint: A client endpoint to connect to.

    @param protocol: A protocol instance.

    @return: The result of calling C{connect} on the endpoint, i.e. a
        L{Deferred} that will fire with the protocol when connected, or an
        appropriate error.

    @since: 13.1
    """
    class OneShotFactory(Factory):
        def buildProtocol(self, addr):
            return protocol
    return endpoint.connect(OneShotFactory())



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