text: """
  A high-speed, production ready, thread pooled, generic HTTP server.
  
  For those of you wanting to understand internals of this module, here's the
  basic call flow. The server's listening thread runs a very tight loop,
  sticking incoming connections onto a Queue::
  
      server = HTTPServer(...)
      server.start()
      ->  while True:
              tick()
              # This blocks until a request comes in:
              child = socket.accept()
              conn = HTTPConnection(child, ...)
              server.requests.put(conn)
  
  Worker threads are kept in a pool and poll the Queue, popping off and then
  handling each connection in turn. Each connection can consist of an arbitrary
  number of requests and their responses, so we run a nested loop::
  
      while True:
          conn = server.requests.get()
          conn.communicate()
          ->  while True:
                  req = HTTPRequest(...)
                  req.parse_request()
                  ->  # Read the Request-Line, e.g. "GET /page HTTP/1.1"
                      req.rfile.readline()
                      read_headers(req.rfile, req.inheaders)
                  req.respond()
                  ->  response = app(...)
                      try:
                          for chunk in response:
                              if chunk:
                                  req.write(chunk)
                      finally:
                          if hasattr(response, "close"):
                              response.close()
                  if req.close_connection:
                      return
  
  For running a server you can invoke :func:`start() <HTTPServer.start()>` (it
  will run the server forever) or use invoking :func:`prepare()
  <HTTPServer.prepare()>` and :func:`serve() <HTTPServer.serve()>` like this::
  
      server = HTTPServer(...)
      server.prepare()
      try:
          threading.Thread(target=server.serve).start()
  
          # waiting/detecting some appropriate stop condition here
          ...
  
      finally:
          server.stop()
  
  And now for a trivial doctest to exercise the test suite
  
  >>> 'HTTPServer' in globals()
  True
  
  """
  
  from __future__ import absolute_import, division, print_function
  __metaclass__ = type
  
  import os
  import io
  import re
  import email.utils
  import socket
  import sys
  import time
  import traceback as traceback_
  import logging
  import platform
  
  try:
      from functools import lru_cache
  except ImportError:
      from backports.functools_lru_cache import lru_cache
  
  import six
  from six.moves import queue
  from six.moves import urllib
  
  from . import connections, errors, __version__
  from ._compat import bton, ntou
  from ._compat import IS_PPC
  from .workers import threadpool
  from .makefile import MakeFile, StreamWriter
  
  
  __all__ = (
      'HTTPRequest', 'HTTPConnection', 'HTTPServer',
      'SizeCheckWrapper', 'KnownLengthRFile', 'ChunkedRFile',
      'Gateway', 'get_ssl_adapter_class',
  )
  
  
  IS_WINDOWS = platform.system() == 'Windows'
  """Flag indicating whether the app is running under Windows."""
  
  
  IS_GAE = os.getenv('SERVER_SOFTWARE', '').startswith('Google App Engine/')
  """Flag indicating whether the app is running in GAE env.
  
  Ref:
  https://cloud.google.com/appengine/docs/standard/python/tools
  /using-local-server#detecting_application_runtime_environment
  """
  
  
  IS_UID_GID_RESOLVABLE = not IS_WINDOWS and not IS_GAE
  """Indicates whether UID/GID resolution's available under current platform."""
  
  
  if IS_UID_GID_RESOLVABLE:
      try:
          import grp
          import pwd
      except ImportError:
          """Unavailable in the current env.
  
          This shouldn't be happening normally.
          All of the known cases are excluded via the if clause.
          """
          IS_UID_GID_RESOLVABLE = False
          grp, pwd = None, None
      import struct
  
  
  if IS_WINDOWS and hasattr(socket, 'AF_INET6'):
      if not hasattr(socket, 'IPPROTO_IPV6'):
          socket.IPPROTO_IPV6 = 41
      if not hasattr(socket, 'IPV6_V6ONLY'):
          socket.IPV6_V6ONLY = 27
  
  
  if not hasattr(socket, 'SO_PEERCRED'):
      """
      NOTE: the value for SO_PEERCRED can be architecture specific, in
      which case the getsockopt() will hopefully fail. The arch
      specific value could be derived from platform.processor()
      """
      socket.SO_PEERCRED = 21 if IS_PPC else 17
  
  
  LF = b'\n'
  CRLF = b'\r\n'
  TAB = b'\t'
  SPACE = b' '
  COLON = b':'
  SEMICOLON = b';'
  EMPTY = b''
  ASTERISK = b'*'
  FORWARD_SLASH = b'/'
  QUOTED_SLASH = b'%2F'
  QUOTED_SLASH_REGEX = re.compile(b'(?i)' + QUOTED_SLASH)
  
  
  comma_separated_headers = [
      b'Accept', b'Accept-Charset', b'Accept-Encoding',
      b'Accept-Language', b'Accept-Ranges', b'Allow', b'Cache-Control',
      b'Connection', b'Content-Encoding', b'Content-Language', b'Expect',
      b'If-Match', b'If-None-Match', b'Pragma', b'Proxy-Authenticate', b'TE',
      b'Trailer', b'Transfer-Encoding', b'Upgrade', b'Vary', b'Via', b'Warning',
      b'WWW-Authenticate',
  ]
  
  
  if not hasattr(logging, 'statistics'):
      logging.statistics = {}
  
  
  class HeaderReader:
      """Object for reading headers from an HTTP request.
  
      Interface and default implementation.
      """
  
      def __call__(self, rfile, hdict=None):
          """
          Read headers from the given stream into the given header dict.
  
          If hdict is None, a new header dict is created. Returns the populated
          header dict.
  
          Headers which are repeated are folded together using a comma if their
          specification so dictates.
  
          This function raises ValueError when the read bytes violate the HTTP
          spec.
          You should probably return "400 Bad Request" if this happens.
          """
          if hdict is None:
              hdict = {}
  
          while True:
              line = rfile.readline()
              if not line:
                  # No more data--illegal end of headers
                  raise ValueError('Illegal end of headers.')
  
              if line == CRLF:
                  # Normal end of headers
                  break
              if not line.endswith(CRLF):
                  raise ValueError('HTTP requires CRLF terminators')
  
              if line[0] in (SPACE, TAB):
                  # It's a continuation line.
                  v = line.strip()
              else:
                  try:
                      k, v = line.split(COLON, 1)
                  except ValueError:
                      raise ValueError('Illegal header line.')
                  v = v.strip()
                  k = self._transform_key(k)
                  hname = k
  
              if not self._allow_header(k):
                  continue
  
              if k in comma_separated_headers:
                  existing = hdict.get(hname)
                  if existing:
                      v = b', '.join((existing, v))
              hdict[hname] = v
  
          return hdict
  
      def _allow_header(self, key_name):
          return True
  
      def _transform_key(self, key_name):
          # TODO: what about TE and WWW-Authenticate?
          return key_name.strip().title()
  
  
  class DropUnderscoreHeaderReader(HeaderReader):
      """Custom HeaderReader to exclude any headers with underscores in them."""
  
      def _allow_header(self, key_name):
          orig = super(DropUnderscoreHeaderReader, self)._allow_header(key_name)
          return orig and '_' not in key_name
  
  
  class SizeCheckWrapper:
      """Wraps a file-like object, raising MaxSizeExceeded if too large."""
  
      def __init__(self, rfile, maxlen):
          """Initialize SizeCheckWrapper instance.
  
          Args:
              rfile (file): file of a limited size
              maxlen (int): maximum length of the file being read
          """
          self.rfile = rfile
          self.maxlen = maxlen
          self.bytes_read = 0
  
      def _check_length(self):
          if self.maxlen and self.bytes_read > self.maxlen:
              raise errors.MaxSizeExceeded()
  
      def read(self, size=None):
          """Read a chunk from rfile buffer and return it.
  
          Args:
              size (int): amount of data to read
  
          Returns:
              bytes: Chunk from rfile, limited by size if specified.
  
          """
          data = self.rfile.read(size)
          self.bytes_read += len(data)
          self._check_length()
          return data
  
      def readline(self, size=None):
          """Read a single line from rfile buffer and return it.
  
          Args:
              size (int): minimum amount of data to read
  
          Returns:
              bytes: One line from rfile.
  
          """
          if size is not None:
              data = self.rfile.readline(size)
              self.bytes_read += len(data)
              self._check_length()
              return data
  
          # User didn't specify a size ...
          # We read the line in chunks to make sure it's not a 100MB line !
          res = []
          while True:
              data = self.rfile.readline(256)
              self.bytes_read += len(data)
              self._check_length()
              res.append(data)
              # See https://github.com/cherrypy/cherrypy/issues/421
              if len(data) < 256 or data[-1:] == LF:
                  return EMPTY.join(res)
  
      def readlines(self, sizehint=0):
          """Read all lines from rfile buffer and return them.
  
          Args:
              sizehint (int): hint of minimum amount of data to read
  
          Returns:
              list[bytes]: Lines of bytes read from rfile.
  
          """
          # Shamelessly stolen from StringIO
          total = 0
          lines = []
          line = self.readline(sizehint)
          while line:
              lines.append(line)
              total += len(line)
              if 0 < sizehint <= total:
                  break
              line = self.readline(sizehint)
          return lines
  
      def close(self):
          """Release resources allocated for rfile."""
          self.rfile.close()
  
      def __iter__(self):
          """Return file iterator."""
          return self
  
      def __next__(self):
          """Generate next file chunk."""
          data = next(self.rfile)
          self.bytes_read += len(data)
          self._check_length()
          return data
  
      next = __next__
  
  
  class KnownLengthRFile:
      """Wraps a file-like object, returning an empty string when exhausted."""
  
      def __init__(self, rfile, content_length):
          """Initialize KnownLengthRFile instance.
  
          Args:
              rfile (file): file of a known size
              content_length (int): length of the file being read
  
          """
          self.rfile = rfile
          self.remaining = content_length
  
      def read(self, size=None):
          """Read a chunk from rfile buffer and return it.
  
          Args:
              size (int): amount of data to read
  
          Returns:
              bytes: Chunk from rfile, limited by size if specified.
  
          """
          if self.remaining == 0:
              return b''
          if size is None:
              size = self.remaining
          else:
              size = min(size, self.remaining)
  
          data = self.rfile.read(size)
          self.remaining -= len(data)
          return data
  
      def readline(self, size=None):
          """Read a single line from rfile buffer and return it.
  
          Args:
              size (int): minimum amount of data to read
  
          Returns:
              bytes: One line from rfile.
  
          """
          if self.remaining == 0:
              return b''
          if size is None:
              size = self.remaining
          else:
              size = min(size, self.remaining)
  
          data = self.rfile.readline(size)
          self.remaining -= len(data)
          return data
  
      def readlines(self, sizehint=0):
          """Read all lines from rfile buffer and return them.
  
          Args:
              sizehint (int): hint of minimum amount of data to read
  
          Returns:
              list[bytes]: Lines of bytes read from rfile.
  
          """
          # Shamelessly stolen from StringIO
          total = 0
          lines = []
          line = self.readline(sizehint)
          while line:
              lines.append(line)
              total += len(line)
              if 0 < sizehint <= total:
                  break
              line = self.readline(sizehint)
          return lines
  
      def close(self):
          """Release resources allocated for rfile."""
          self.rfile.close()
  
      def __iter__(self):
          """Return file iterator."""
          return self
  
      def __next__(self):
          """Generate next file chunk."""
          data = next(self.rfile)
          self.remaining -= len(data)
          return data
  
      next = __next__
  
  
  class ChunkedRFile:
      """Wraps a file-like object, returning an empty string when exhausted.
  
      This class is intended to provide a conforming wsgi.input value for
      request entities that have been encoded with the 'chunked' transfer
      encoding.
      """
  
      def __init__(self, rfile, maxlen, bufsize=8192):
          """Initialize ChunkedRFile instance.
  
          Args:
              rfile (file): file encoded with the 'chunked' transfer encoding
              maxlen (int): maximum length of the file being read
              bufsize (int): size of the buffer used to read the file
          """
          self.rfile = rfile
          self.maxlen = maxlen
          self.bytes_read = 0
          self.buffer = EMPTY
          self.bufsize = bufsize
          self.closed = False
  
      def _fetch(self):
          if self.closed:
              return
  
          line = self.rfile.readline()
          self.bytes_read += len(line)
  
          if self.maxlen and self.bytes_read > self.maxlen:
              raise errors.MaxSizeExceeded(
                  'Request Entity Too Large', self.maxlen,
              )
  
          line = line.strip().split(SEMICOLON, 1)
  
          try:
              chunk_size = line.pop(0)
              chunk_size = int(chunk_size, 16)
          except ValueError:
              raise ValueError('Bad chunked transfer size: ' + repr(chunk_size))
  
          if chunk_size <= 0:
              self.closed = True
              return
  
  #            if line: chunk_extension = line[0]
  
          if self.maxlen and self.bytes_read + chunk_size > self.maxlen:
              raise IOError('Request Entity Too Large')
  
          chunk = self.rfile.read(chunk_size)
          self.bytes_read += len(chunk)
          self.buffer += chunk
  
          crlf = self.rfile.read(2)
          if crlf != CRLF:
              raise ValueError(
                  "Bad chunked transfer coding (expected '\\r\\n', "
                  'got ' + repr(crlf) + ')',
              )
  
      def read(self, size=None):
          """Read a chunk from rfile buffer and return it.
  
          Args:
              size (int): amount of data to read
  
          Returns:
              bytes: Chunk from rfile, limited by size if specified.
  
          """
          data = EMPTY
  
          if size == 0:
              return data
  
          while True:
              if size and len(data) >= size:
                  return data
  
              if not self.buffer:
                  self._fetch()
                  if not self.buffer:
                      # EOF
                      return data
  
              if size:
                  remaining = size - len(data)
                  data += self.buffer[:remaining]
                  self.buffer = self.buffer[remaining:]
              else:
                  data += self.buffer
                  self.buffer = EMPTY
  
      def readline(self, size=None):
          """Read a single line from rfile buffer and return it.
  
          Args:
              size (int): minimum amount of data to read
  
          Returns:
              bytes: One line from rfile.
  
          """
          data = EMPTY
  
          if size == 0:
              return data
  
          while True:
              if size and len(data) >= size:
                  return data
  
              if not self.buffer:
                  self._fetch()
                  if not self.buffer:
                      # EOF
                      return data
  
              newline_pos = self.buffer.find(LF)
              if size:
                  if newline_pos == -1:
                      remaining = size - len(data)
                      data += self.buffer[:remaining]
                      self.buffer = self.buffer[remaining:]
                  else:
                      remaining = min(size - len(data), newline_pos)
                      data += self.buffer[:remaining]
                      self.buffer = self.buffer[remaining:]
              else:
                  if newline_pos == -1:
                      data += self.buffer
                      self.buffer = EMPTY
                  else:
                      data += self.buffer[:newline_pos]
                      self.buffer = self.buffer[newline_pos:]
  
      def readlines(self, sizehint=0):
          """Read all lines from rfile buffer and return them.
  
          Args:
              sizehint (int): hint of minimum amount of data to read
  
          Returns:
              list[bytes]: Lines of bytes read from rfile.
  
          """
          # Shamelessly stolen from StringIO
          total = 0
          lines = []
          line = self.readline(sizehint)
          while line:
              lines.append(line)
              total += len(line)
              if 0 < sizehint <= total:
                  break
              line = self.readline(sizehint)
          return lines
  
      def read_trailer_lines(self):
          """Read HTTP headers and yield them.
  
          Returns:
              Generator: yields CRLF separated lines.
  
          """
          if not self.closed:
              raise ValueError(
                  'Cannot read trailers until the request body has been read.',
              )
  
          while True:
              line = self.rfile.readline()
              if not line:
                  # No more data--illegal end of headers
                  raise ValueError('Illegal end of headers.')
  
              self.bytes_read += len(line)
              if self.maxlen and self.bytes_read > self.maxlen:
                  raise IOError('Request Entity Too Large')
  
              if line == CRLF:
                  # Normal end of headers
                  break
              if not line.endswith(CRLF):
                  raise ValueError('HTTP requires CRLF terminators')
  
              yield line
  
      def close(self):
          """Release resources allocated for rfile."""
          self.rfile.close()
  
  
  class HTTPRequest:
      """An HTTP Request (and response).
  
      A single HTTP connection may consist of multiple request/response pairs.
      """
  
      server = None
      """The HTTPServer object which is receiving this request."""
  
      conn = None
      """The HTTPConnection object on which this request connected."""
  
      inheaders = {}
      """A dict of request headers."""
  
      outheaders = []
      """A list of header tuples to write in the response."""
  
      ready = False
      """When True, the request has been parsed and is ready to begin generating
      the response. When False, signals the calling Connection that the response
      should not be generated and the connection should close."""
  
      close_connection = False
      """Signals the calling Connection that the request should close. This does
      not imply an error! The client and/or server may each request that the
      connection be closed."""
  
      chunked_write = False
      """If True, output will be encoded with the "chunked" transfer-coding.
  
      This value is set automatically inside send_headers."""
  
      header_reader = HeaderReader()
      """
      A HeaderReader instance or compatible reader.
      """
  
      def __init__(self, server, conn, proxy_mode=False, strict_mode=True):
          """Initialize HTTP request container instance.
  
          Args:
              server (HTTPServer): web server object receiving this request
              conn (HTTPConnection): HTTP connection object for this request
              proxy_mode (bool): whether this HTTPServer should behave as a PROXY
              server for certain requests
              strict_mode (bool): whether we should return a 400 Bad Request when
              we encounter a request that a HTTP compliant client should not be
              making
          """
          self.server = server
          self.conn = conn
  
          self.ready = False
          self.started_request = False
          self.scheme = b'http'
          if self.server.ssl_adapter is not None:
              self.scheme = b'https'
          # Use the lowest-common protocol in case read_request_line errors.
          self.response_protocol = 'HTTP/1.0'
          self.inheaders = {}
  
          self.status = ''
          self.outheaders = []
          self.sent_headers = False
          self.close_connection = self.__class__.close_connection
          self.chunked_read = False
          self.chunked_write = self.__class__.chunked_write
          self.proxy_mode = proxy_mode
          self.strict_mode = strict_mode
  
      def parse_request(self):
          """Parse the next HTTP request start-line and message-headers."""
          self.rfile = SizeCheckWrapper(
              self.conn.rfile,
              self.server.max_request_header_size,
          )
          try:
              success = self.read_request_line()
          except errors.MaxSizeExceeded:
              self.simple_response(
                  '414 Request-URI Too Long',
                  'The Request-URI sent with the request exceeds the maximum '
                  'allowed bytes.',
              )
              return
          else:
              if not success:
                  return
  
          try:
              success = self.read_request_headers()
          except errors.MaxSizeExceeded:
              self.simple_response(
                  '413 Request Entity Too Large',
                  'The headers sent with the request exceed the maximum '
                  'allowed bytes.',
              )
              return
          else:
              if not success:
                  return
  
          self.ready = True
  
      def read_request_line(self):
          """Read and parse first line of the HTTP request.
  
          Returns:
              bool: True if the request line is valid or False if it's malformed.
  
          """
          # HTTP/1.1 connections are persistent by default. If a client
          # requests a page, then idles (leaves the connection open),
          # then rfile.readline() will raise socket.error("timed out").
          # Note that it does this based on the value given to settimeout(),
          # and doesn't need the client to request or acknowledge the close
          # (although your TCP stack might suffer for it: cf Apache's history
          # with FIN_WAIT_2).
          request_line = self.rfile.readline()
  
          # Set started_request to True so communicate() knows to send 408
          # from here on out.
          self.started_request = True
          if not request_line:
              return False
  
          if request_line == CRLF:
              # RFC 2616 sec 4.1: "...if the server is reading the protocol
              # stream at the beginning of a message and receives a CRLF
              # first, it should ignore the CRLF."
              # But only ignore one leading line! else we enable a DoS.
              request_line = self.rfile.readline()
              if not request_line:
                  return False
  
          if not request_line.endswith(CRLF):
              self.simple_response(
                  '400 Bad Request', 'HTTP requires CRLF terminators',
              )
              return False
  
          try:
              method, uri, req_protocol = request_line.strip().split(SPACE, 2)
              if not req_protocol.startswith(b'HTTP/'):
                  self.simple_response(
                      '400 Bad Request', 'Malformed Request-Line: bad protocol',
                  )
                  return False
              rp = req_protocol[5:].split(b'.', 1)
              if len(rp) != 2:
                  self.simple_response(
                      '400 Bad Request', 'Malformed Request-Line: bad version',
                  )
                  return False
              rp = tuple(map(int, rp))  # Minor.Major must be threat as integers
              if rp > (1, 1):
                  self.simple_response(
                      '505 HTTP Version Not Supported', 'Cannot fulfill request',
                  )
                  return False
          except (ValueError, IndexError):
              self.simple_response('400 Bad Request', 'Malformed Request-Line')
              return False
  
          self.uri = uri
          self.method = method.upper()
  
          if self.strict_mode and method != self.method:
              resp = (
                  'Malformed method name: According to RFC 2616 '
                  '(section 5.1.1) and its successors '
                  'RFC 7230 (section 3.1.1) and RFC 7231 (section 4.1) '
                  'method names are case-sensitive and uppercase.'
              )
              self.simple_response('400 Bad Request', resp)
              return False
  
          try:
              if six.PY2:  # FIXME: Figure out better way to do this
                  # Ref: https://stackoverflow.com/a/196392/595220 (like this?)
                  """This is a dummy check for unicode in URI."""
                  ntou(bton(uri, 'ascii'), 'ascii')
              scheme, authority, path, qs, fragment = urllib.parse.urlsplit(uri)
          except UnicodeError:
              self.simple_response('400 Bad Request', 'Malformed Request-URI')
              return False
  
          uri_is_absolute_form = (scheme or authority)
  
          if self.method == b'OPTIONS':
              # TODO: cover this branch with tests
              path = (
                  uri
                  # https://tools.ietf.org/html/rfc7230#section-5.3.4
                  if (self.proxy_mode and uri_is_absolute_form)
                  else path
              )
          elif self.method == b'CONNECT':
              # TODO: cover this branch with tests
              if not self.proxy_mode:
                  self.simple_response('405 Method Not Allowed')
                  return False
  
              # `urlsplit()` above parses "example.com:3128" as path part of URI.
              # this is a workaround, which makes it detect netloc correctly
              uri_split = urllib.parse.urlsplit(b'//' + uri)
              _scheme, _authority, _path, _qs, _fragment = uri_split
              _port = EMPTY
              try:
                  _port = uri_split.port
              except ValueError:
                  pass
  
              # FIXME: use third-party validation to make checks against RFC
              # the validation doesn't take into account, that urllib parses
              # invalid URIs without raising errors
              # https://tools.ietf.org/html/rfc7230#section-5.3.3
              invalid_path = (
                  _authority != uri
                  or not _port
                  or any((_scheme, _path, _qs, _fragment))
              )
              if invalid_path:
                  self.simple_response(
                      '400 Bad Request',
                      'Invalid path in Request-URI: request-'
                      'target must match authority-form.',
                  )
                  return False
  
              authority = path = _authority
              scheme = qs = fragment = EMPTY
          else:
              disallowed_absolute = (
                  self.strict_mode
                  and not self.proxy_mode
                  and uri_is_absolute_form
              )
              if disallowed_absolute:
                  # https://tools.ietf.org/html/rfc7230#section-5.3.2
                  # (absolute form)
                  """Absolute URI is only allowed within proxies."""
                  self.simple_response(
                      '400 Bad Request',
                      'Absolute URI not allowed if server is not a proxy.',
                  )
                  return False
  
              invalid_path = (
                  self.strict_mode
                  and not uri.startswith(FORWARD_SLASH)
                  and not uri_is_absolute_form
              )
              if invalid_path:
                  # https://tools.ietf.org/html/rfc7230#section-5.3.1
                  # (origin_form) and
                  """Path should start with a forward slash."""
                  resp = (
                      'Invalid path in Request-URI: request-target must contain '
                      'origin-form which starts with absolute-path (URI '
                      'starting with a slash "/").'
                  )
                  self.simple_response('400 Bad Request', resp)
                  return False
  
              if fragment:
                  self.simple_response(
                      '400 Bad Request',
                      'Illegal #fragment in Request-URI.',
                  )
                  return False
  
              if path is None:
                  # FIXME: It looks like this case cannot happen
                  self.simple_response(
                      '400 Bad Request',
                      'Invalid path in Request-URI.',
                  )
                  return False
  
              # Unquote the path+params (e.g. "/this%20path" -> "/this path").
              # https://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html#sec5.1.2
              #
              # But note that "...a URI must be separated into its components
              # before the escaped characters within those components can be
              # safely decoded." https://www.ietf.org/rfc/rfc2396.txt, sec 2.4.2
              # Therefore, "/this%2Fpath" becomes "/this%2Fpath", not
              # "/this/path".
              try:
                  # TODO: Figure out whether exception can really happen here.
                  # It looks like it's caught on urlsplit() call above.
                  atoms = [
                      urllib.parse.unquote_to_bytes(x)
                      for x in QUOTED_SLASH_REGEX.split(path)
                  ]
              except ValueError as ex:
                  self.simple_response('400 Bad Request', ex.args[0])
                  return False
              path = QUOTED_SLASH.join(atoms)
  
          if not path.startswith(FORWARD_SLASH):
              path = FORWARD_SLASH + path
  
          if scheme is not EMPTY:
              self.scheme = scheme
          self.authority = authority
          self.path = path
  
          # Note that, like wsgiref and most other HTTP servers,
          # we "% HEX HEX"-unquote the path but not the query string.
          self.qs = qs
  
          # Compare request and server HTTP protocol versions, in case our
          # server does not support the requested protocol. Limit our output
          # to min(req, server). We want the following output:
          #     request    server     actual written   supported response
          #     protocol   protocol  response protocol    feature set
          # a     1.0        1.0           1.0                1.0
          # b     1.0        1.1           1.1                1.0
          # c     1.1        1.0           1.0                1.0
          # d     1.1        1.1           1.1                1.1
          # Notice that, in (b), the response will be "HTTP/1.1" even though
          # the client only understands 1.0. RFC 2616 10.5.6 says we should
          # only return 505 if the _major_ version is different.
          sp = int(self.server.protocol[5]), int(self.server.protocol[7])
  
          if sp[0] != rp[0]:
              self.simple_response('505 HTTP Version Not Supported')
              return False
  
          self.request_protocol = req_protocol
          self.response_protocol = 'HTTP/%s.%s' % min(rp, sp)
  
          return True
  
      def read_request_headers(self):
          """Read self.rfile into self.inheaders. Return success."""
          # then all the http headers
          try:
              self.header_reader(self.rfile, self.inheaders)
          except ValueError as ex:
              self.simple_response('400 Bad Request', ex.args[0])
              return False
  
          mrbs = self.server.max_request_body_size
  
          try:
              cl = int(self.inheaders.get(b'Content-Length', 0))
          except ValueError:
              self.simple_response(
                  '400 Bad Request',
                  'Malformed Content-Length Header.',
              )
              return False
  
          if mrbs and cl > mrbs:
              self.simple_response(
                  '413 Request Entity Too Large',
                  'The entity sent with the request exceeds the maximum '
                  'allowed bytes.',
              )
              return False
  
          # Persistent connection support
          if self.response_protocol == 'HTTP/1.1':
              # Both server and client are HTTP/1.1
              if self.inheaders.get(b'Connection', b'') == b'close':
                  self.close_connection = True
          else:
              # Either the server or client (or both) are HTTP/1.0
              if self.inheaders.get(b'Connection', b'') != b'Keep-Alive':
                  self.close_connection = True
  
          # Transfer-Encoding support
          te = None
          if self.response_protocol == 'HTTP/1.1':
              te = self.inheaders.get(b'Transfer-Encoding')
              if te:
                  te = [x.strip().lower() for x in te.split(b',') if x.strip()]
  
          self.chunked_read = False
  
          if te:
              for enc in te:
                  if enc == b'chunked':
                      self.chunked_read = True
                  else:
                      # Note that, even if we see "chunked", we must reject
                      # if there is an extension we don't recognize.
                      self.simple_response('501 Unimplemented')
                      self.close_connection = True
                      return False
  
          # From PEP 333:
          # "Servers and gateways that implement HTTP 1.1 must provide
          # transparent support for HTTP 1.1's "expect/continue" mechanism.
          # This may be done in any of several ways:
          #   1. Respond to requests containing an Expect: 100-continue request
          #      with an immediate "100 Continue" response, and proceed normally.
          #   2. Proceed with the request normally, but provide the application
          #      with a wsgi.input stream that will send the "100 Continue"
          #      response if/when the application first attempts to read from
          #      the input stream. The read request must then remain blocked
          #      until the client responds.
          #   3. Wait until the client decides that the server does not support
          #      expect/continue, and sends the request body on its own.
          #      (This is suboptimal, and is not recommended.)
          #
          # We used to do 3, but are now doing 1. Maybe we'll do 2 someday,
          # but it seems like it would be a big slowdown for such a rare case.
          if self.inheaders.get(b'Expect', b'') == b'100-continue':
              # Don't use simple_response here, because it emits headers
              # we don't want. See
              # https://github.com/cherrypy/cherrypy/issues/951
              msg = self.server.protocol.encode('ascii')
              msg += b' 100 Continue\r\n\r\n'
              try:
                  self.conn.wfile.write(msg)
              except socket.error as ex:
                  if ex.args[0] not in errors.socket_errors_to_ignore:
                      raise
          return True
  
      def respond(self):
          """Call the gateway and write its iterable output."""
          mrbs = self.server.max_request_body_size
          if self.chunked_read:
              self.rfile = ChunkedRFile(self.conn.rfile, mrbs)
          else:
              cl = int(self.inheaders.get(b'Content-Length', 0))
              if mrbs and mrbs < cl:
                  if not self.sent_headers:
                      self.simple_response(
                          '413 Request Entity Too Large',
                          'The entity sent with the request exceeds the '
                          'maximum allowed bytes.',
                      )
                  return
              self.rfile = KnownLengthRFile(self.conn.rfile, cl)
  
          self.server.gateway(self).respond()
          self.ready and self.ensure_headers_sent()
  
          if self.chunked_write:
              self.conn.wfile.write(b'0\r\n\r\n')
  
      def simple_response(self, status, msg=''):
          """Write a simple response back to the client."""
          status = str(status)
          proto_status = '%s %s\r\n' % (self.server.protocol, status)
          content_length = 'Content-Length: %s\r\n' % len(msg)
          content_type = 'Content-Type: text/plain\r\n'
          buf = [
              proto_status.encode('ISO-8859-1'),
              content_length.encode('ISO-8859-1'),
              content_type.encode('ISO-8859-1'),
          ]
  
          if status[:3] in ('413', '414'):
              # Request Entity Too Large / Request-URI Too Long
              self.close_connection = True
              if self.response_protocol == 'HTTP/1.1':
                  # This will not be true for 414, since read_request_line
                  # usually raises 414 before reading the whole line, and we
                  # therefore cannot know the proper response_protocol.
                  buf.append(b'Connection: close\r\n')
              else:
                  # HTTP/1.0 had no 413/414 status nor Connection header.
                  # Emit 400 instead and trust the message body is enough.
                  status = '400 Bad Request'
  
          buf.append(CRLF)
          if msg:
              if isinstance(msg, six.text_type):
                  msg = msg.encode('ISO-8859-1')
              buf.append(msg)
  
          try:
              self.conn.wfile.write(EMPTY.join(buf))
          except socket.error as ex:
              if ex.args[0] not in errors.socket_errors_to_ignore:
                  raise
  
      def ensure_headers_sent(self):
          """Ensure headers are sent to the client if not already sent."""
          if not self.sent_headers:
              self.sent_headers = True
              self.send_headers()
  
      def write(self, chunk):
          """Write unbuffered data to the client."""
          if self.chunked_write and chunk:
              chunk_size_hex = hex(len(chunk))[2:].encode('ascii')
              buf = [chunk_size_hex, CRLF, chunk, CRLF]
              self.conn.wfile.write(EMPTY.join(buf))
          else:
              self.conn.wfile.write(chunk)
  
      def send_headers(self):
          """Assert, process, and send the HTTP response message-headers.
  
          You must set self.status, and self.outheaders before calling this.
          """
          hkeys = [key.lower() for key, value in self.outheaders]
          status = int(self.status[:3])
  
          if status == 413:
              # Request Entity Too Large. Close conn to avoid garbage.
              self.close_connection = True
          elif b'content-length' not in hkeys:
              # "All 1xx (informational), 204 (no content),
              # and 304 (not modified) responses MUST NOT
              # include a message-body." So no point chunking.
              if status < 200 or status in (204, 205, 304):
                  pass
              else:
                  needs_chunked = (
                      self.response_protocol == 'HTTP/1.1'
                      and self.method != b'HEAD'
                  )
                  if needs_chunked:
                      # Use the chunked transfer-coding
                      self.chunked_write = True
                      self.outheaders.append((b'Transfer-Encoding', b'chunked'))
                  else:
                      # Closing the conn is the only way to determine len.
                      self.close_connection = True
  
          if b'connection' not in hkeys:
              if self.response_protocol == 'HTTP/1.1':
                  # Both server and client are HTTP/1.1 or better
                  if self.close_connection:
                      self.outheaders.append((b'Connection', b'close'))
              else:
                  # Server and/or client are HTTP/1.0
                  if not self.close_connection:
                      self.outheaders.append((b'Connection', b'Keep-Alive'))
  
          if (not self.close_connection) and (not self.chunked_read):
              # Read any remaining request body data on the socket.
              # "If an origin server receives a request that does not include an
              # Expect request-header field with the "100-continue" expectation,
              # the request includes a request body, and the server responds
              # with a final status code before reading the entire request body
              # from the transport connection, then the server SHOULD NOT close
              # the transport connection until it has read the entire request,
              # or until the client closes the connection. Otherwise, the client
              # might not reliably receive the response message. However, this
              # requirement is not be construed as preventing a server from
              # defending itself against denial-of-service attacks, or from
              # badly broken client implementations."
              remaining = getattr(self.rfile, 'remaining', 0)
              if remaining > 0:
                  self.rfile.read(remaining)
  
          if b'date' not in hkeys:
              self.outheaders.append((
                  b'Date',
                  email.utils.formatdate(usegmt=True).encode('ISO-8859-1'),
              ))
  
          if b'server' not in hkeys:
              self.outheaders.append((
                  b'Server',
                  self.server.server_name.encode('ISO-8859-1'),
              ))
  
          proto = self.server.protocol.encode('ascii')
          buf = [proto + SPACE + self.status + CRLF]
          for k, v in self.outheaders:
              buf.append(k + COLON + SPACE + v + CRLF)
          buf.append(CRLF)
          self.conn.wfile.write(EMPTY.join(buf))
  
  
  class HTTPConnection:
      """An HTTP connection (active socket)."""
  
      remote_addr = None
      remote_port = None
      ssl_env = None
      rbufsize = io.DEFAULT_BUFFER_SIZE
      wbufsize = io.DEFAULT_BUFFER_SIZE
      RequestHandlerClass = HTTPRequest
      peercreds_enabled = False
      peercreds_resolve_enabled = False
  
      # Fields set by ConnectionManager.
      closeable = False
      last_used = None
      ready_with_data = False
  
      def __init__(self, server, sock, makefile=MakeFile):
          """Initialize HTTPConnection instance.
  
          Args:
              server (HTTPServer): web server object receiving this request
              sock (socket._socketobject): the raw socket object (usually
                  TCP) for this connection
              makefile (file): a fileobject class for reading from the socket
          """
          self.server = server
          self.socket = sock
          self.rfile = makefile(sock, 'rb', self.rbufsize)
          self.wfile = makefile(sock, 'wb', self.wbufsize)
          self.requests_seen = 0
  
          self.peercreds_enabled = self.server.peercreds_enabled
          self.peercreds_resolve_enabled = self.server.peercreds_resolve_enabled
  
          # LRU cached methods:
          # Ref: https://stackoverflow.com/a/14946506/595220
          self.resolve_peer_creds = (
              lru_cache(maxsize=1)(self.resolve_peer_creds)
          )
          self.get_peer_creds = (
              lru_cache(maxsize=1)(self.get_peer_creds)
          )
  
      def communicate(self):
          """Read each request and respond appropriately.
  
          Returns true if the connection should be kept open.
          """
          request_seen = False
          try:
              req = self.RequestHandlerClass(self.server, self)
              req.parse_request()
              if self.server.stats['Enabled']:
                  self.requests_seen += 1
              if not req.ready:
                  # Something went wrong in the parsing (and the server has
                  # probably already made a simple_response). Return and
                  # let the conn close.
                  return False
  
              request_seen = True
              req.respond()
              if not req.close_connection:
                  return True
          except socket.error as ex:
              errnum = ex.args[0]
              # sadly SSL sockets return a different (longer) time out string
              timeout_errs = 'timed out', 'The read operation timed out'
              if errnum in timeout_errs:
                  # Don't error if we're between requests; only error
                  # if 1) no request has been started at all, or 2) we're
                  # in the middle of a request.
                  # See https://github.com/cherrypy/cherrypy/issues/853
                  if (not request_seen) or (req and req.started_request):
                      self._conditional_error(req, '408 Request Timeout')
              elif errnum not in errors.socket_errors_to_ignore:
                  self.server.error_log(
                      'socket.error %s' % repr(errnum),
                      level=logging.WARNING, traceback=True,
                  )
                  self._conditional_error(req, '500 Internal Server Error')
          except (KeyboardInterrupt, SystemExit):
              raise
          except errors.FatalSSLAlert:
              pass
          except errors.NoSSLError:
              self._handle_no_ssl(req)
          except Exception as ex:
              self.server.error_log(
                  repr(ex), level=logging.ERROR, traceback=True,
              )
              self._conditional_error(req, '500 Internal Server Error')
          return False
  
      linger = False
  
      def _handle_no_ssl(self, req):
          if not req or req.sent_headers:
              return
          # Unwrap wfile
          try:
              resp_sock = self.socket._sock
          except AttributeError:
              # self.socket is of OpenSSL.SSL.Connection type
              resp_sock = self.socket._socket
          self.wfile = StreamWriter(resp_sock, 'wb', self.wbufsize)
          msg = (
              'The client sent a plain HTTP request, but '
              'this server only speaks HTTPS on this port.'
          )
          req.simple_response('400 Bad Request', msg)
          self.linger = True
  
      def _conditional_error(self, req, response):
          """Respond with an error.
  
          Don't bother writing if a response
          has already started being written.
          """
          if not req or req.sent_headers:
              return
  
          try:
              req.simple_response(response)
          except errors.FatalSSLAlert:
              pass
          except errors.NoSSLError:
              self._handle_no_ssl(req)
  
      def close(self):
          """Close the socket underlying this connection."""
          self.rfile.close()
  
          if not self.linger:
              self._close_kernel_socket()
              self.socket.close()
          else:
              # On the other hand, sometimes we want to hang around for a bit
              # to make sure the client has a chance to read our entire
              # response. Skipping the close() calls here delays the FIN
              # packet until the socket object is garbage-collected later.
              # Someday, perhaps, we'll do the full lingering_close that
              # Apache does, but not today.
              pass
  
      def get_peer_creds(self):  # LRU cached on per-instance basis, see __init__
          """Return the PID/UID/GID tuple of the peer socket for UNIX sockets.
  
          This function uses SO_PEERCRED to query the UNIX PID, UID, GID
          of the peer, which is only available if the bind address is
          a UNIX domain socket.
  
          Raises:
              NotImplementedError: in case of unsupported socket type
              RuntimeError: in case of SO_PEERCRED lookup unsupported or disabled
  
          """
          PEERCRED_STRUCT_DEF = '3i'
  
          if IS_WINDOWS or self.socket.family != socket.AF_UNIX:
              raise NotImplementedError(
                  'SO_PEERCRED is only supported in Linux kernel and WSL',
              )
          elif not self.peercreds_enabled:
              raise RuntimeError(
                  'Peer creds lookup is disabled within this server',
              )
  
          try:
              peer_creds = self.socket.getsockopt(
                  # FIXME: Use LOCAL_CREDS for BSD-like OSs
                  # Ref: https://gist.github.com/LucaFilipozzi/e4f1e118202aff27af6aadebda1b5d91  # noqa
                  socket.SOL_SOCKET, socket.SO_PEERCRED,
                  struct.calcsize(PEERCRED_STRUCT_DEF),
              )
          except socket.error as socket_err:
              """Non-Linux kernels don't support SO_PEERCRED.
  
              Refs:
              http://welz.org.za/notes/on-peer-cred.html
              https://github.com/daveti/tcpSockHack
              msdn.microsoft.com/en-us/commandline/wsl/release_notes#build-15025
              """
              six.raise_from(  # 3.6+: raise RuntimeError from socket_err
                  RuntimeError,
                  socket_err,
              )
          else:
              pid, uid, gid = struct.unpack(PEERCRED_STRUCT_DEF, peer_creds)
              return pid, uid, gid
  
      @property
      def peer_pid(self):
          """Return the id of the connected peer process."""
          pid, _, _ = self.get_peer_creds()
          return pid
  
      @property
      def peer_uid(self):
          """Return the user id of the connected peer process."""
          _, uid, _ = self.get_peer_creds()
          return uid
  
      @property
      def peer_gid(self):
          """Return the group id of the connected peer process."""
          _, _, gid = self.get_peer_creds()
          return gid
  
      def resolve_peer_creds(self):  # LRU cached on per-instance basis
          """Return the username and group tuple of the peercreds if available.
  
          Raises:
              NotImplementedError: in case of unsupported OS
              RuntimeError: in case of UID/GID lookup unsupported or disabled
  
          """
          if not IS_UID_GID_RESOLVABLE:
              raise NotImplementedError(
                  'UID/GID lookup is unavailable under current platform. '
                  'It can only be done under UNIX-like OS '
                  'but not under the Google App Engine',
              )
          elif not self.peercreds_resolve_enabled:
              raise RuntimeError(
                  'UID/GID lookup is disabled within this server',
              )
  
          user = pwd.getpwuid(self.peer_uid).pw_name  # [0]
          group = grp.getgrgid(self.peer_gid).gr_name  # [0]
  
          return user, group
  
      @property
      def peer_user(self):
          """Return the username of the connected peer process."""
          user, _ = self.resolve_peer_creds()
          return user
  
      @property
      def peer_group(self):
          """Return the group of the connected peer process."""
          _, group = self.resolve_peer_creds()
          return group
  
      def _close_kernel_socket(self):
          """Close kernel socket in outdated Python versions.
  
          On old Python versions,
          Python's socket module does NOT call close on the kernel
          socket when you call socket.close(). We do so manually here
          because we want this server to send a FIN TCP segment
          immediately. Note this must be called *before* calling
          socket.close(), because the latter drops its reference to
          the kernel socket.
          """
          if six.PY2 and hasattr(self.socket, '_sock'):
              self.socket._sock.close()
  
  
  class HTTPServer:
      """An HTTP server."""
  
      _bind_addr = '127.0.0.1'
      _interrupt = None
  
      gateway = None
      """A Gateway instance."""
  
      minthreads = None
      """The minimum number of worker threads to create (default 10)."""
  
      maxthreads = None
      """The maximum number of worker threads to create.
  
      (default -1 = no limit)"""
  
      server_name = None
      """The name of the server; defaults to ``self.version``."""
  
      protocol = 'HTTP/1.1'
      """The version string to write in the Status-Line of all HTTP responses.
  
      For example, "HTTP/1.1" is the default. This also limits the supported
      features used in the response."""
  
      request_queue_size = 5
      """The 'backlog' arg to socket.listen(); max queued connections.
  
      (default 5)."""
  
      shutdown_timeout = 5
      """The total time to wait for worker threads to cleanly exit.
  
      Specified in seconds."""
  
      timeout = 10
      """The timeout in seconds for accepted connections (default 10)."""
  
      version = 'Cheroot/' + __version__
      """A version string for the HTTPServer."""
  
      software = None
      """The value to set for the SERVER_SOFTWARE entry in the WSGI environ.
  
      If None, this defaults to ``'%s Server' % self.version``.
      """
  
      ready = False
      """Internal flag which indicating the socket is accepting connections."""
  
      max_request_header_size = 0
      """The maximum size, in bytes, for request headers, or 0 for no limit."""
  
      max_request_body_size = 0
      """The maximum size, in bytes, for request bodies, or 0 for no limit."""
  
      nodelay = True
      """If True (the default since 3.1), sets the TCP_NODELAY socket option."""
  
      ConnectionClass = HTTPConnection
      """The class to use for handling HTTP connections."""
  
      ssl_adapter = None
      """An instance of ssl.Adapter (or a subclass).
  
      You must have the corresponding SSL driver library installed.
      """
  
      peercreds_enabled = False
      """If True, peer cred lookup can be performed via UNIX domain socket."""
  
      peercreds_resolve_enabled = False
      """If True, username/group will be looked up in the OS from peercreds."""
  
      keep_alive_conn_limit = 10
      """The maximum number of waiting keep-alive connections that will be kept open.
  
      Default is 10. Set to None to have unlimited connections."""
  
      def __init__(
          self, bind_addr, gateway,
          minthreads=10, maxthreads=-1, server_name=None,
          peercreds_enabled=False, peercreds_resolve_enabled=False,
      ):
          """Initialize HTTPServer instance.
  
          Args:
              bind_addr (tuple): network interface to listen to
              gateway (Gateway): gateway for processing HTTP requests
              minthreads (int): minimum number of threads for HTTP thread pool
              maxthreads (int): maximum number of threads for HTTP thread pool
              server_name (str): web server name to be advertised via Server
                  HTTP header
          """
          self.bind_addr = bind_addr
          self.gateway = gateway
  
          self.requests = threadpool.ThreadPool(
              self, min=minthreads or 1, max=maxthreads,
          )
          self.connections = connections.ConnectionManager(self)
  
          if not server_name:
              server_name = self.version
          self.server_name = server_name
          self.peercreds_enabled = peercreds_enabled
          self.peercreds_resolve_enabled = (
              peercreds_resolve_enabled and peercreds_enabled
          )
          self.clear_stats()
  
      def clear_stats(self):
          """Reset server stat counters.."""
          self._start_time = None
          self._run_time = 0
          self.stats = {
              'Enabled': False,
              'Bind Address': lambda s: repr(self.bind_addr),
              'Run time': lambda s: (not s['Enabled']) and -1 or self.runtime(),
              'Accepts': 0,
              'Accepts/sec': lambda s: s['Accepts'] / self.runtime(),
              'Queue': lambda s: getattr(self.requests, 'qsize', None),
              'Threads': lambda s: len(getattr(self.requests, '_threads', [])),
              'Threads Idle': lambda s: getattr(self.requests, 'idle', None),
              'Socket Errors': 0,
              'Requests': lambda s: (not s['Enabled']) and -1 or sum(
                  [w['Requests'](w) for w in s['Worker Threads'].values()], 0,
              ),
              'Bytes Read': lambda s: (not s['Enabled']) and -1 or sum(
                  [w['Bytes Read'](w) for w in s['Worker Threads'].values()], 0,
              ),
              'Bytes Written': lambda s: (not s['Enabled']) and -1 or sum(
                  [w['Bytes Written'](w) for w in s['Worker Threads'].values()],
                  0,
              ),
              'Work Time': lambda s: (not s['Enabled']) and -1 or sum(
                  [w['Work Time'](w) for w in s['Worker Threads'].values()], 0,
              ),
              'Read Throughput': lambda s: (not s['Enabled']) and -1 or sum(
                  [w['Bytes Read'](w) / (w['Work Time'](w) or 1e-6)
                   for w in s['Worker Threads'].values()], 0,
              ),
              'Write Throughput': lambda s: (not s['Enabled']) and -1 or sum(
                  [w['Bytes Written'](w) / (w['Work Time'](w) or 1e-6)
                   for w in s['Worker Threads'].values()], 0,
              ),
              'Worker Threads': {},
          }
          logging.statistics['Cheroot HTTPServer %d' % id(self)] = self.stats
  
      def runtime(self):
          """Return server uptime."""
          if self._start_time is None:
              return self._run_time
          else:
              return self._run_time + (time.time() - self._start_time)
  
      def __str__(self):
          """Render Server instance representing bind address."""
          return '%s.%s(%r)' % (
              self.__module__, self.__class__.__name__,
              self.bind_addr,
          )
  
      @property
      def bind_addr(self):
          """Return the interface on which to listen for connections.
  
          For TCP sockets, a (host, port) tuple. Host values may be any IPv4
          or IPv6 address, or any valid hostname. The string 'localhost' is a
          synonym for '127.0.0.1' (or '::1', if your hosts file prefers IPv6).
          The string '0.0.0.0' is a special IPv4 entry meaning "any active
          interface" (INADDR_ANY), and '::' is the similar IN6ADDR_ANY for
          IPv6. The empty string or None are not allowed.
  
          For UNIX sockets, supply the filename as a string.
  
          Systemd socket activation is automatic and doesn't require tempering
          with this variable.
          """
          return self._bind_addr
  
      @bind_addr.setter
      def bind_addr(self, value):
          """Set the interface on which to listen for connections."""
          if isinstance(value, tuple) and value[0] in ('', None):
              # Despite the socket module docs, using '' does not
              # allow AI_PASSIVE to work. Passing None instead
              # returns '0.0.0.0' like we want. In other words:
              #     host    AI_PASSIVE     result
              #      ''         Y         192.168.x.y
              #      ''         N         192.168.x.y
              #     None        Y         0.0.0.0
              #     None        N         127.0.0.1
              # But since you can get the same effect with an explicit
              # '0.0.0.0', we deny both the empty string and None as values.
              raise ValueError(
                  "Host values of '' or None are not allowed. "
                  "Use '0.0.0.0' (IPv4) or '::' (IPv6) instead "
                  'to listen on all active interfaces.',
              )
          self._bind_addr = value
  
      def safe_start(self):
          """Run the server forever, and stop it cleanly on exit."""
          try:
              self.start()
          except (KeyboardInterrupt, IOError):
              # The time.sleep call might raise
              # "IOError: [Errno 4] Interrupted function call" on KBInt.
              self.error_log('Keyboard Interrupt: shutting down')
              self.stop()
              raise
          except SystemExit:
              self.error_log('SystemExit raised: shutting down')
              self.stop()
              raise
  
      def prepare(self):
          """Prepare server to serving requests.
  
          It binds a socket's port, setups the socket to ``listen()`` and does
          other preparing things.
          """
          self._interrupt = None
  
          if self.software is None:
              self.software = '%s Server' % self.version
  
          # Select the appropriate socket
          self.socket = None
          msg = 'No socket could be created'
          if os.getenv('LISTEN_PID', None):
              # systemd socket activation
              self.socket = socket.fromfd(3, socket.AF_INET, socket.SOCK_STREAM)
          elif isinstance(self.bind_addr, (six.text_type, six.binary_type)):
              # AF_UNIX socket
              try:
                  self.bind_unix_socket(self.bind_addr)
              except socket.error as serr:
                  msg = '%s -- (%s: %s)' % (msg, self.bind_addr, serr)
                  six.raise_from(socket.error(msg), serr)
          else:
              # AF_INET or AF_INET6 socket
              # Get the correct address family for our host (allows IPv6
              # addresses)
              host, port = self.bind_addr
              try:
                  info = socket.getaddrinfo(
                      host, port, socket.AF_UNSPEC,
                      socket.SOCK_STREAM, 0, socket.AI_PASSIVE,
                  )
              except socket.gaierror:
                  sock_type = socket.AF_INET
                  bind_addr = self.bind_addr
  
                  if ':' in host:
                      sock_type = socket.AF_INET6
                      bind_addr = bind_addr + (0, 0)
  
                  info = [(sock_type, socket.SOCK_STREAM, 0, '', bind_addr)]
  
              for res in info:
                  af, socktype, proto, canonname, sa = res
                  try:
                      self.bind(af, socktype, proto)
                      break
                  except socket.error as serr:
                      msg = '%s -- (%s: %s)' % (msg, sa, serr)
                      if self.socket:
                          self.socket.close()
                      self.socket = None
  
          if not self.socket:
              raise socket.error(msg)
  
          # Timeout so KeyboardInterrupt can be caught on Win32
          self.socket.settimeout(1)
          self.socket.listen(self.request_queue_size)
  
          # Create worker threads
          self.requests.start()
  
          self.ready = True
          self._start_time = time.time()
  
      def serve(self):
          """Serve requests, after invoking :func:`prepare()`."""
          while self.ready:
              try:
                  self.tick()
              except (KeyboardInterrupt, SystemExit):
                  raise
              except Exception:
                  self.error_log(
                      'Error in HTTPServer.tick', level=logging.ERROR,
                      traceback=True,
                  )
  
              if self.interrupt:
                  while self.interrupt is True:
                      # Wait for self.stop() to complete. See _set_interrupt.
                      time.sleep(0.1)
                  if self.interrupt:
                      raise self.interrupt
  
      def start(self):
          """Run the server forever.
  
          It is shortcut for invoking :func:`prepare()` then :func:`serve()`.
          """
          # We don't have to trap KeyboardInterrupt or SystemExit here,
          # because cherrypy.server already does so, calling self.stop() for us.
          # If you're using this server with another framework, you should
          # trap those exceptions in whatever code block calls start().
          self.prepare()
          self.serve()
  
      def error_log(self, msg='', level=20, traceback=False):
          """Write error message to log.
  
          Args:
              msg (str): error message
              level (int): logging level
              traceback (bool): add traceback to output or not
          """
          # Override this in subclasses as desired
          sys.stderr.write(msg + '\n')
          sys.stderr.flush()
          if traceback:
              tblines = traceback_.format_exc()
              sys.stderr.write(tblines)
              sys.stderr.flush()
  
      def bind(self, family, type, proto=0):
          """Create (or recreate) the actual socket object."""
          sock = self.prepare_socket(
              self.bind_addr,
              family, type, proto,
              self.nodelay, self.ssl_adapter,
          )
          sock = self.socket = self.bind_socket(sock, self.bind_addr)
          self.bind_addr = self.resolve_real_bind_addr(sock)
          return sock
  
      def bind_unix_socket(self, bind_addr):
          """Create (or recreate) a UNIX socket object."""
          if IS_WINDOWS:
              """
              Trying to access socket.AF_UNIX under Windows
              causes an AttributeError.
              """
              raise ValueError(  # or RuntimeError?
                  'AF_UNIX sockets are not supported under Windows.',
              )
  
          fs_permissions = 0o777  # TODO: allow changing mode
  
          try:
              # Make possible reusing the socket...
              os.unlink(self.bind_addr)
          except OSError:
              """
              File does not exist, which is the primary goal anyway.
              """
          except TypeError as typ_err:
              err_msg = str(typ_err)
              if (
                      'remove() argument 1 must be encoded '
                      'string without null bytes, not unicode'
                      not in err_msg
                      and 'embedded NUL character' not in err_msg  # py34
                      and 'argument must be a '
                      'string without NUL characters' not in err_msg  # pypy2
              ):
                  raise
          except ValueError as val_err:
              err_msg = str(val_err)
              if (
                      'unlink: embedded null '
                      'character in path' not in err_msg
                      and 'embedded null byte' not in err_msg
                      and 'argument must be a '
                      'string without NUL characters' not in err_msg  # pypy3
              ):
                  raise
  
          sock = self.prepare_socket(
              bind_addr=bind_addr,
              family=socket.AF_UNIX, type=socket.SOCK_STREAM, proto=0,
              nodelay=self.nodelay, ssl_adapter=self.ssl_adapter,
          )
  
          try:
              """Linux way of pre-populating fs mode permissions."""
              # Allow everyone access the socket...
              os.fchmod(sock.fileno(), fs_permissions)
              FS_PERMS_SET = True
          except OSError:
              FS_PERMS_SET = False
  
          try:
              sock = self.bind_socket(sock, bind_addr)
          except socket.error:
              sock.close()
              raise
  
          bind_addr = self.resolve_real_bind_addr(sock)
  
          try:
              """FreeBSD/macOS pre-populating fs mode permissions."""
              if not FS_PERMS_SET:
                  try:
                      os.lchmod(bind_addr, fs_permissions)
                  except AttributeError:
                      os.chmod(bind_addr, fs_permissions, follow_symlinks=False)
                  FS_PERMS_SET = True
          except OSError:
              pass
  
          if not FS_PERMS_SET:
              self.error_log(
                  'Failed to set socket fs mode permissions',
                  level=logging.WARNING,
              )
  
          self.bind_addr = bind_addr
          self.socket = sock
          return sock
  
      @staticmethod
      def prepare_socket(bind_addr, family, type, proto, nodelay, ssl_adapter):
          """Create and prepare the socket object."""
          sock = socket.socket(family, type, proto)
          connections.prevent_socket_inheritance(sock)
  
          host, port = bind_addr[:2]
          IS_EPHEMERAL_PORT = port == 0
  
          if not (IS_WINDOWS or IS_EPHEMERAL_PORT):
              """Enable SO_REUSEADDR for the current socket.
  
              Skip for Windows (has different semantics)
              or ephemeral ports (can steal ports from others).
  
              Refs:
              * https://msdn.microsoft.com/en-us/library/ms740621(v=vs.85).aspx
              * https://github.com/cherrypy/cheroot/issues/114
              * https://gavv.github.io/blog/ephemeral-port-reuse/
              """
              sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
          if nodelay and not isinstance(
                  bind_addr,
                  (six.text_type, six.binary_type),
          ):
              sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)
  
          if ssl_adapter is not None:
              sock = ssl_adapter.bind(sock)
  
          # If listening on the IPV6 any address ('::' = IN6ADDR_ANY),
          # activate dual-stack. See
          # https://github.com/cherrypy/cherrypy/issues/871.
          listening_ipv6 = (
              hasattr(socket, 'AF_INET6')
              and family == socket.AF_INET6
              and host in ('::', '::0', '::0.0.0.0')
          )
          if listening_ipv6:
              try:
                  sock.setsockopt(
                      socket.IPPROTO_IPV6, socket.IPV6_V6ONLY, 0,
                  )
              except (AttributeError, socket.error):
                  # Apparently, the socket option is not available in
                  # this machine's TCP stack
                  pass
  
          return sock
  
      @staticmethod
      def bind_socket(socket_, bind_addr):
          """Bind the socket to given interface."""
          socket_.bind(bind_addr)
          return socket_
  
      @staticmethod
      def resolve_real_bind_addr(socket_):
          """Retrieve actual bind addr from bound socket."""
          # FIXME: keep requested bind_addr separate real bound_addr (port
          # is different in case of ephemeral port 0)
          bind_addr = socket_.getsockname()
          if socket_.family in (
              # Windows doesn't have socket.AF_UNIX, so not using it in check
              socket.AF_INET,
              socket.AF_INET6,
          ):
              """UNIX domain sockets are strings or bytes.
  
              In case of bytes with a leading null-byte it's an abstract socket.
              """
              return bind_addr[:2]
  
          if isinstance(bind_addr, six.binary_type):
              bind_addr = bton(bind_addr)
  
          return bind_addr
  
      def tick(self):
          """Accept a new connection and put it on the Queue."""
          if not self.ready:
              return
  
          conn = self.connections.get_conn(self.socket)
          if conn:
              try:
                  self.requests.put(conn)
              except queue.Full:
                  # Just drop the conn. TODO: write 503 back?
                  conn.close()
  
          self.connections.expire()
  
      @property
      def interrupt(self):
          """Flag interrupt of the server."""
          return self._interrupt
  
      @interrupt.setter
      def interrupt(self, interrupt):
          """Perform the shutdown of this server and save the exception."""
          self._interrupt = True
          self.stop()
          self._interrupt = interrupt
  
      def stop(self):
          """Gracefully shutdown a server that is serving forever."""
          self.ready = False
          if self._start_time is not None:
              self._run_time += (time.time() - self._start_time)
          self._start_time = None
  
          sock = getattr(self, 'socket', None)
          if sock:
              if not isinstance(
                      self.bind_addr,
                      (six.text_type, six.binary_type),
              ):
                  # Touch our own socket to make accept() return immediately.
                  try:
                      host, port = sock.getsockname()[:2]
                  except socket.error as ex:
                      if ex.args[0] not in errors.socket_errors_to_ignore:
                          # Changed to use error code and not message
                          # See
                          # https://github.com/cherrypy/cherrypy/issues/860.
                          raise
                  else:
                      # Note that we're explicitly NOT using AI_PASSIVE,
                      # here, because we want an actual IP to touch.
                      # localhost won't work if we've bound to a public IP,
                      # but it will if we bound to '0.0.0.0' (INADDR_ANY).
                      for res in socket.getaddrinfo(
                          host, port, socket.AF_UNSPEC,
                          socket.SOCK_STREAM,
                      ):
                          af, socktype, proto, canonname, sa = res
                          s = None
                          try:
                              s = socket.socket(af, socktype, proto)
                              # See
                              # https://groups.google.com/group/cherrypy-users/
                              #     browse_frm/thread/bbfe5eb39c904fe0
                              s.settimeout(1.0)
                              s.connect((host, port))
                              s.close()
                          except socket.error:
                              if s:
                                  s.close()
              if hasattr(sock, 'close'):
                  sock.close()
              self.socket = None
  
          self.connections.close()
          self.requests.stop(self.shutdown_timeout)
  
  
  class Gateway:
      """Base class to interface HTTPServer with other systems, such as WSGI."""
  
      def __init__(self, req):
          """Initialize Gateway instance with request.
  
          Args:
              req (HTTPRequest): current HTTP request
          """
          self.req = req
  
      def respond(self):
          """Process the current request. Must be overridden in a subclass."""
          raise NotImplementedError  # pragma: no cover
  
  
  # These may either be ssl.Adapter subclasses or the string names
  # of such classes (in which case they will be lazily loaded).
  ssl_adapters = {
      'builtin': 'cheroot.ssl.builtin.BuiltinSSLAdapter',
      'pyopenssl': 'cheroot.ssl.pyopenssl.pyOpenSSLAdapter',
  }
  
  
  def get_ssl_adapter_class(name='builtin'):
      """Return an SSL adapter class for the given name."""
      adapter = ssl_adapters[name.lower()]
      if isinstance(adapter, six.string_types):
          last_dot = adapter.rfind('.')
          attr_name = adapter[last_dot + 1:]
          mod_path = adapter[:last_dot]
  
          try:
              mod = sys.modules[mod_path]
              if mod is None:
                  raise KeyError()
          except KeyError:
              # The last [''] is important.
              mod = __import__(mod_path, globals(), locals(), [''])
  
          # Let an AttributeError propagate outward.
          try:
              adapter = getattr(mod, attr_name)
          except AttributeError:
              raise AttributeError("'%s' object has no attribute '%s'"
                                   % (mod_path, attr_name))
  
      return adapter
  