# -*- 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
from unicodedata import normalize

from constantly import NamedConstant, Names

from zope.interface import implementer, directlyProvides, provider

from twisted.internet import interfaces, defer, error, fdesc, threads
from twisted.internet.abstract import isIPv6Address, isIPAddress
from twisted.internet.address import (
    _ProcessAddress, HostnameAddress, IPv4Address, IPv6Address
)
from twisted.internet.interfaces import (
    IStreamServerEndpointStringParser,
    IStreamClientEndpointStringParserWithReactor, IResolutionReceiver
)
from twisted.internet.protocol import ClientFactory, Factory
from twisted.internet.protocol import ProcessProtocol, Protocol
from twisted.internet.stdio import StandardIO, PipeAddress
from twisted.internet.task import LoopingCall
from twisted.plugin import IPlugin, getPlugins
from twisted.python import log
from twisted.python.compat import nativeString, unicode, _matchingString
from twisted.python.components import proxyForInterface
from twisted.python.failure import Failure
from twisted.python.filepath import FilePath
from twisted.python.compat import iterbytes
from twisted.internet.defer import Deferred
from twisted.python.systemd import ListenFDs

from ._idna import _idnaBytes, _idnaText

try:
    from twisted.protocols.tls import TLSMemoryBIOFactory
    from twisted.internet.ssl import (
        optionsForClientTLS, PrivateCertificate, Certificate, KeyPair,
        CertificateOptions, trustRootFromCertificates
    )
    from OpenSSL.SSL import Error as SSLError
except ImportError:
    TLSMemoryBIOFactory = None

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



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,
                      interfaces.IHandshakeListener]:
            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()


    def handshakeCompleted(self):
        """
        Proxy L{interfaces.IHandshakeListener} to our
        C{self._wrappedProtocol}.
        """
        self._wrappedProtocol.handshakeCompleted()



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: L{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} if the wrapped factory raises an
        exception or returns L{None}.

        @return: An instance of L{_WrappingProtocol} or L{None}
        """
        try:
            proto = self._wrappedFactory.buildProtocol(addr)
            if proto is None:
                raise error.NoProtocol()
        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

    @ivar _stdio: a callable, like L{stdio.StandardIO}, which takes an
        L{IProtocol} provider and a C{reactor} keyword argument (interface
        dependent upon your platform).
    """

    _stdio = StandardIO

    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(self._stdio,
                             stdioProtocolFactory.buildProtocol(PipeAddress()),
                             reactor=self._reactor)



class _IProcessTransportWithConsumerAndProducer(interfaces.IProcessTransport,
                                                interfaces.IConsumer,
                                                interfaces.IPushProducer):
    """
    An L{_IProcessTransportWithConsumerAndProducer} combines various interfaces
    to work around the issue that L{interfaces.IProcessTransport} is
    incompletely defined and doesn't specify flow-control interfaces, and that
    L{proxyForInterface} doesn't allow for multiple interfaces.
    """



class _ProcessEndpointTransport(
        proxyForInterface(_IProcessTransportWithConsumerAndProducer,
                          '_process')):
    """
    An L{ITransport}, L{IProcessTransport}, L{IConsumer}, and L{IPushProducer}
    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
    """



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 C{::} (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 = staticmethod(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.

    @cvar _DEFAULT_ATTEMPT_DELAY: The default time to use between attempts, in
        seconds, when no C{attemptDelay} is given to
        L{HostnameEndpoint.__init__}.

    @ivar _hostText: the textual representation of the hostname passed to the
        constructor.  Used to pass to the reactor's hostname resolver.
    @type _hostText: L{unicode}

    @ivar _hostBytes: the encoded bytes-representation of the hostname passed
        to the constructor.  Used to construct the L{HostnameAddress}
        associated with this endpoint.
    @type _hostBytes: L{bytes}

    @ivar _hostStr: the native-string representation of the hostname passed to
        the constructor, used for exception construction
    @type _hostStr: native L{str}

    @ivar _badHostname: a flag - hopefully false!  - indicating that an invalid
        hostname was passed to the constructor.  This might be a textual
        hostname that isn't valid IDNA, or non-ASCII bytes.
    @type _badHostname: L{bool}
    """
    _DEFAULT_ATTEMPT_DELAY = 0.3

    def __init__(self, reactor, host, port, timeout=30, bindAddress=None,
                 attemptDelay=None):
        """
        Create a L{HostnameEndpoint}.

        @param reactor: The reactor to use for connections and delayed calls.
        @type reactor: provider of L{IReactorTCP}, L{IReactorTime} and either
            L{IReactorPluggableNameResolver} or L{IReactorPluggableResolver}.

        @param host: A hostname to connect to.
        @type host: L{bytes} or L{unicode}

        @param port: The port number to connect to.
        @type port: L{int}

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

        @param bindAddress: the local address of the network interface to make
            the connections from.
        @type bindAddress: L{bytes}

        @param attemptDelay: The number of seconds to delay between connection
            attempts.
        @type attemptDelay: L{float}

        @see: L{twisted.internet.interfaces.IReactorTCP.connectTCP}
        """
        self._reactor = reactor
        [self._badHostname, self._hostBytes, self._hostText] = (
            self._hostAsBytesAndText(host)
        )
        self._hostStr = self._hostBytes if bytes is str else self._hostText
        self._port = port
        self._timeout = timeout
        self._bindAddress = bindAddress
        if attemptDelay is None:
            attemptDelay = self._DEFAULT_ATTEMPT_DELAY
        self._attemptDelay = attemptDelay


    @staticmethod
    def _hostAsBytesAndText(host):
        """
        For various reasons (documented in the C{@ivar}'s in the class
        docstring) we need both a textual and a binary representation of the
        hostname given to the constructor.  For compatibility and convenience,
        we accept both textual and binary representations of the hostname, save
        the form that was passed, and convert into the other form.  This is
        mostly just because L{HostnameAddress} chose somewhat poorly to define
        its attribute as bytes; hopefully we can find a compatible way to clean
        this up in the future and just operate in terms of text internally.

        @param host: A hostname to convert.
        @type host: L{bytes} or C{str}

        @return: a 3-tuple of C{(invalid, bytes, text)} where C{invalid} is a
            boolean indicating the validity of the hostname, C{bytes} is a
            binary representation of C{host}, and C{text} is a textual
            representation of C{host}.
        """
        if isinstance(host, bytes):
            if isIPAddress(host) or isIPv6Address(host):
                return False, host, host.decode("ascii")
            else:
                try:
                    return False, host, _idnaText(host)
                except UnicodeError:
                    # Convert the host to _some_ kind of text, to handle below.
                    host = host.decode("charmap")
        else:
            host = normalize('NFC', host)
            if isIPAddress(host) or isIPv6Address(host):
                return False, host.encode("ascii"), host
            else:
                try:
                    return False, _idnaBytes(host), host
                except UnicodeError:
                    pass
        # `host` has been converted to text by this point either way; it's
        # invalid as a hostname, and so may contain unprintable characters and
        # such. escape it with backslashes so the user can get _some_ guess as
        # to what went wrong.
        asciibytes = host.encode('ascii', 'backslashreplace')
        return True, asciibytes, asciibytes.decode('ascii')


    def connect(self, protocolFactory):
        """
        Attempts a connection to each address returned by gai, and returns a
        connection which is established first.
        """
        if self._badHostname:
            return defer.fail(
                ValueError("invalid hostname: {}".format(self._hostStr))
            )
        d = Deferred()
        addresses = []
        @provider(IResolutionReceiver)
        class EndpointReceiver(object):
            @staticmethod
            def resolutionBegan(resolutionInProgress):
                pass
            @staticmethod
            def addressResolved(address):
                addresses.append(address)
            @staticmethod
            def resolutionComplete():
                d.callback(addresses)
        self._reactor.nameResolver.resolveHostName(
            EndpointReceiver, self._hostText, portNumber=self._port
        )
        d.addErrback(lambda ignored: defer.fail(error.DNSLookupError(
            "Couldn't find the hostname '{}'".format(self._hostStr))))
        @d.addCallback
        def resolvedAddressesToEndpoints(addresses):
            # Yield an endpoint for every address resolved from the name.
            for eachAddress in addresses:
                if isinstance(eachAddress, IPv6Address):
                    yield TCP6ClientEndpoint(
                        self._reactor, eachAddress.host, eachAddress.port,
                        self._timeout, self._bindAddress
                    )
                if isinstance(eachAddress, IPv4Address):
                    yield TCP4ClientEndpoint(
                        self._reactor, eachAddress.host, eachAddress.port,
                        self._timeout, self._bindAddress
                    )
        d.addCallback(list)

        def _canceller(d):
            # This canceller must remain defined outside of
            # `startConnectionAttempts`, because Deferred should not
            # participate in cycles with their cancellers; that would create a
            # potentially problematic circular reference and possibly
            # gc.garbage.
            d.errback(error.ConnectingCancelledError(
                HostnameAddress(self._hostBytes, self._port)))

        @d.addCallback
        def startConnectionAttempts(endpoints):
            """
            Given a sequence of endpoints obtained via name resolution, start
            connecting to a new one every C{self._attemptDelay} seconds until
            one of the connections succeeds, all of them fail, or the attempt
            is cancelled.

            @param endpoints: a list of all the endpoints we might try to
                connect to, as determined by name resolution.
            @type endpoints: L{list} of L{IStreamServerEndpoint}

            @return: a Deferred that fires with the result of the
                C{endpoint.connect} method that completes the fastest, or fails
                with the first connection error it encountered if none of them
                succeed.
            @rtype: L{Deferred} failing with L{error.ConnectingCancelledError}
                or firing with L{IProtocol}
            """
            if not endpoints:
                raise error.DNSLookupError(
                    "no results for hostname lookup: {}".format(self._hostStr)
                )
            iterEndpoints = iter(endpoints)
            pending = []
            failures = []
            winner = defer.Deferred(canceller=_canceller)

            def checkDone():
                if pending or checkDone.completed or checkDone.endpointsLeft:
                    return
                winner.errback(failures.pop())
            checkDone.completed = False
            checkDone.endpointsLeft = True

            @LoopingCall
            def iterateEndpoint():
                endpoint = next(iterEndpoints, None)
                if endpoint is None:
                    # The list of endpoints ends.
                    checkDone.endpointsLeft = False
                    checkDone()
                    return

                eachAttempt = endpoint.connect(protocolFactory)
                pending.append(eachAttempt)
                @eachAttempt.addBoth
                def noLongerPending(result):
                    pending.remove(eachAttempt)
                    return result
                @eachAttempt.addCallback
                def succeeded(result):
                    winner.callback(result)
                @eachAttempt.addErrback
                def failed(reason):
                    failures.append(reason)
                    checkDone()

            iterateEndpoint.clock = self._reactor
            iterateEndpoint.start(self._attemptDelay)

            @winner.addBoth
            def cancelRemainingPending(result):
                checkDone.completed = True
                for remaining in pending[:]:
                    remaining.cancel()
                if iterateEndpoint.running:
                    iterateEndpoint.stop()
                return result
            return winner

        return d



@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{interfaces.IOpenSSLContextFactory}.

        @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{interfaces.IOpenSSLContextFactory}.

        @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 L{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 L{None}

    @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 L{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 L{None}

    @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{twisted.internet.interfaces.IReactorUNIX.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 L{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 L{None}

    @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{str}

    @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{str}

    @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)
    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-----)',
            nativeString(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{IStreamServerEndpointStringParser.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{IStreamServerEndpointStringParser.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{IStreamServerEndpointStringParser.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}.
    @type description: L{str} or L{bytes}

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

            _STRING, 'hello'
            _OP, ':'
            _STRING, 'greet=ing'
            _OP, '='
            _STRING, 'world'
    """
    empty = _matchingString(u'', description)
    colon = _matchingString(u':', description)
    equals = _matchingString(u'=', description)
    backslash = _matchingString(u'\x5c', description)
    current = empty

    ops = colon + equals
    nextOps = {colon: colon + equals, equals: colon}
    iterdesc = iter(iterbytes(description))
    for n in iterdesc:
        if n in iterbytes(ops):
            yield _STRING, current
            yield _OP, n
            current = empty
            ops = nextOps[n]
        elif n == backslash:
            current += next(iterdesc)
        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 = [], {}
    colon = _matchingString(u':', description)
    def add(sofar):
        if len(sofar) == 1:
            args.append(sofar[0])
        else:
            kw[nativeString(sofar[0])] = sofar[1]
    sofar = ()
    for (type, value) in _tokenize(description):
        if type is _STRING:
            sofar += (value,)
        elif value == colon:
            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,
    }


def _parseServer(description, factory):
    """
    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}

    @return: a 3-tuple of (plugin or name, arguments, keyword arguments)
    """
    args, kw = _parse(description)
    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
        plugin = _matchPluginToPrefix(
            getPlugins(IStreamServerEndpointStringParser), endpointType
        )
        return (plugin, args[1:], kw)
    return (endpointType.upper(),) + parser(factory, *args[1:], **kw)



def _matchPluginToPrefix(plugins, endpointType):
    """
    Match plugin to prefix.
    """
    endpointType = endpointType.lower()
    for plugin in plugins:
        if (_matchingString(plugin.prefix.lower(),
                            endpointType) == endpointType):
            return plugin
    raise ValueError("Unknown endpoint type: '%s'" % (endpointType,))



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, "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, "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, "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,
            "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, "unix:/var/run/finger")
        serverFromString(reactor, "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{str}

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

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

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

    @since: 10.2
    """
    nameOrPlugin, args, kw = _parseServer(description, None)
    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 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, "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, "ssl:443:privateKey=%s" %
                         (quoteStringArgument(myPathName),))

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

    @type argument: C{str}

    @return: The quoted argument.

    @rtype: C{str}
    """
    backslash, colon = '\\:'
    for c in backslash, colon:
        argument = argument.replace(c, backslash + c)
    return argument



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{unicode} or L{bytes} pointing at a directory to
        load .pem files from, or L{None}.

    @return: an L{IOpenSSLTrustRoot} provider.
    """
    caCerts = {}
    for child in directoryPath.children():
        if not child.asTextMode().basename().split(u'.')[-1].lower() == u'pem':
            continue
        try:
            data = child.getContent()
        except IOError:
            # Permission denied, corrupt disk, we don't care.
            continue
        try:
            theCert = Certificate.loadPEM(data)
        except SSLError:
            # Duplicate certificate, invalid certificate, etc.  We don't care.
            pass
        else:
            caCerts[theCert.digest()] = theCert
    return trustRootFromCertificates(caCerts.values())



def _parseTrustRootPath(pathName):
    """
    Parse a string referring to a directory full of certificate authorities
    into a trust root.

    @param pathName: path name
    @type pathName: L{unicode} or L{bytes} or L{None}

    @return: L{None} or L{IOpenSSLTrustRoot}
    """
    if pathName is None:
        return None
    return _loadCAsFromDir(FilePath(pathName))



def _privateCertFromPaths(certificatePath, keyPath):
    """
    Parse a certificate path and key path, either or both of which might be
    L{None}, into a certificate object.

    @param certificatePath: the certificate path
    @type certificatePath: L{bytes} or L{unicode} or L{None}

    @param keyPath: the private key path
    @type keyPath: L{bytes} or L{unicode} or L{None}

    @return: a L{PrivateCertificate} or L{None}
    """
    if certificatePath is None:
        return None
    certBytes = FilePath(certificatePath).getContent()
    if keyPath is None:
        return PrivateCertificate.loadPEM(certBytes)
    else:
        return PrivateCertificate.fromCertificateAndKeyPair(
            Certificate.loadPEM(certBytes),
            KeyPair.load(FilePath(keyPath).getContent(), 1)
        )



def _parseClientSSLOptions(kwargs):
    """
    Parse common arguments for SSL endpoints, creating an L{CertificateOptions}
    instance.

    @param kwargs: A dict of keyword arguments to be parsed, potentially
        containing keys C{certKey}, C{privateKey}, C{caCertsDir}, and
        C{hostname}.  See L{_parseClientSSL}.
    @type kwargs: L{dict}

    @return: The remaining arguments, including a new key C{sslContextFactory}.
    """
    hostname = kwargs.pop('hostname', None)
    clientCertificate = _privateCertFromPaths(kwargs.pop('certKey', None),
                                              kwargs.pop('privateKey', None))
    trustRoot = _parseTrustRootPath(kwargs.pop('caCertsDir', None))
    if hostname is not None:
        configuration = optionsForClientTLS(
            _idnaText(hostname), trustRoot=trustRoot,
            clientCertificate=clientCertificate
        )
    else:
        # _really_ though, you should specify a hostname.
        if clientCertificate is not None:
            privateKeyOpenSSL = clientCertificate.privateKey.original
            certificateOpenSSL = clientCertificate.original
        else:
            privateKeyOpenSSL = None
            certificateOpenSSL = None
        configuration = CertificateOptions(
            trustRoot=trustRoot,
            privateKey=privateKeyOpenSSL,
            certificate=certificateOpenSSL,
        )
    kwargs['sslContextFactory'] = configuration
    return kwargs



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: L{str}

    @param hostname: The hostname to use for validating the server's
        certificate.
    @type hostname: L{unicode}

    @return: The coerced values as a L{dict}.
    """
    kwargs = _parseClientTCP(*args, **kwargs)
    return _parseClientSSLOptions(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, "tcp:host=www.example.com:port=80")

    or, without specifying host and port keywords::

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

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

        clientFromString(reactor, "tcp:host=www.example.com:80")
        clientFromString(reactor, "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, "ssl:web.example.com:443:"
                                  "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, "ssl:host=web.example.com:port=443:"
                                  "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, "tcp:www.example.com:80:"
                                  "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, "unix:/var/foo/bar")
        clientFromString(reactor, "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{str}

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

    @since: 10.2
    """
    args, kwargs = _parse(description)
    aname = args.pop(0)
    name = aname.upper()
    if name not in _clientParsers:
        plugin = _matchPluginToPrefix(
            getPlugins(IStreamClientEndpointStringParserWithReactor), name
        )
        return plugin.parseStreamClient(reactor, *args, **kwargs)
    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())



@implementer(interfaces.IStreamClientEndpoint)
class _WrapperEndpoint(object):
    """
    An endpoint that wraps another endpoint.
    """

    def __init__(self, wrappedEndpoint, wrapperFactory):
        """
        Construct a L{_WrapperEndpoint}.
        """
        self._wrappedEndpoint = wrappedEndpoint
        self._wrapperFactory = wrapperFactory


    def connect(self, protocolFactory):
        """
        Connect the given protocol factory and unwrap its result.
        """
        return self._wrappedEndpoint.connect(
            self._wrapperFactory(protocolFactory)
        ).addCallback(lambda protocol: protocol.wrappedProtocol)



@implementer(interfaces.IStreamServerEndpoint)
class _WrapperServerEndpoint(object):
    """
    A server endpoint that wraps another server endpoint.
    """

    def __init__(self, wrappedEndpoint, wrapperFactory):
        """
        Construct a L{_WrapperServerEndpoint}.
        """
        self._wrappedEndpoint = wrappedEndpoint
        self._wrapperFactory = wrapperFactory


    def listen(self, protocolFactory):
        """
        Connect the given protocol factory and unwrap its result.
        """
        return self._wrappedEndpoint.listen(
            self._wrapperFactory(protocolFactory)
        )



def wrapClientTLS(connectionCreator, wrappedEndpoint):
    """
    Wrap an endpoint which upgrades to TLS as soon as the connection is
    established.

    @since: 16.0

    @param connectionCreator: The TLS options to use when connecting; see
        L{twisted.internet.ssl.optionsForClientTLS} for how to construct this.
    @type connectionCreator:
        L{twisted.internet.interfaces.IOpenSSLClientConnectionCreator}

    @param wrappedEndpoint: The endpoint to wrap.
    @type wrappedEndpoint: An L{IStreamClientEndpoint} provider.

    @return: an endpoint that provides transport level encryption layered on
        top of C{wrappedEndpoint}
    @rtype: L{twisted.internet.interfaces.IStreamClientEndpoint}
    """
    if TLSMemoryBIOFactory is None:
        raise NotImplementedError(
            "OpenSSL not available. Try `pip install twisted[tls]`."
        )
    return _WrapperEndpoint(
        wrappedEndpoint,
        lambda protocolFactory:
        TLSMemoryBIOFactory(connectionCreator, True, protocolFactory)
    )



def _parseClientTLS(reactor, host, port, timeout=b'30', bindAddress=None,
                    certificate=None, privateKey=None, trustRoots=None,
                    endpoint=None, **kwargs):
    """
    Internal method to construct an endpoint from string parameters.

    @param reactor: The reactor passed to L{clientFromString}.

    @param host: The hostname to connect to.
    @type host: L{bytes} or L{unicode}

    @param port: The port to connect to.
    @type port: L{bytes} or L{unicode}

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

    @param bindAddress: The address to which to bind outgoing connections.
    @type bindAddress: L{bytes} or L{unicode}

    @param certificate: a string representing a filesystem path to a
        PEM-encoded certificate.
    @type certificate: L{bytes} or L{unicode}

    @param privateKey: a string representing a filesystem path to a PEM-encoded
        certificate.
    @type privateKey: L{bytes} or L{unicode}

    @param endpoint: an optional string endpoint description of an endpoint to
        wrap; if this is passed then C{host} is used only for certificate
        verification.
    @type endpoint: L{bytes} or L{unicode}

    @return: a client TLS endpoint
    @rtype: L{IStreamClientEndpoint}
    """
    if kwargs:
        raise TypeError('unrecognized keyword arguments present',
                        list(kwargs.keys()))
    host = host if isinstance(host, unicode) else host.decode("utf-8")
    bindAddress = (bindAddress
                   if isinstance(bindAddress, unicode) or bindAddress is None
                   else bindAddress.decode("utf-8"))
    port = int(port)
    timeout = int(timeout)
    return wrapClientTLS(
        optionsForClientTLS(
            host, trustRoot=_parseTrustRootPath(trustRoots),
            clientCertificate=_privateCertFromPaths(certificate,
                                                    privateKey)),
        clientFromString(reactor, endpoint) if endpoint is not None
        else HostnameEndpoint(reactor, _idnaBytes(host), port, timeout,
                              bindAddress)
    )



@implementer(IPlugin, IStreamClientEndpointStringParserWithReactor)
class _TLSClientEndpointParser(object):
    """
    Stream client endpoint string parser for L{wrapClientTLS} with
    L{HostnameEndpoint}.

    @ivar prefix: See
        L{IStreamClientEndpointStringParserWithReactor.prefix}.
    """
    prefix = 'tls'

    @staticmethod
    def parseStreamClient(reactor, *args, **kwargs):
        """
        Redirects to another function L{_parseClientTLS}; tricks zope.interface
        into believing the interface is correctly implemented, since the
        signature is (C{reactor}, C{*args}, C{**kwargs}).  See
        L{_parseClientTLS} for the specific signature description for this
        endpoint parser.

        @param reactor: The reactor passed to L{clientFromString}.

        @param args: The positional arguments in the endpoint description.
        @type args: L{tuple}

        @param kwargs: The named arguments in the endpoint description.
        @type kwargs: L{dict}

        @return: a client TLS endpoint
        @rtype: L{IStreamClientEndpoint}
        """
        return _parseClientTLS(reactor, *args, **kwargs)
