Synnefo » snf-ganeti

#

#

# Copyright (C) 2007, 2008 Google Inc.

#

# This program is free software; you can redistribute it and/or modify

# it under the terms of the GNU General Public License as published by

# the Free Software Foundation; either version 2 of the License, or

# (at your option) any later version.

#

# This program is distributed in the hope that it will be useful, but

# WITHOUT ANY WARRANTY; without even the implied warranty of

# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

# General Public License for more details.

#

# You should have received a copy of the GNU General Public License

# along with this program; if not, write to the Free Software

# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA

# 02110-1301, USA.

"""HTTP module.

"""

import logging

import mimetools

import OpenSSL

import select

import socket

import errno

from cStringIO import StringIO

from ganeti import constants

from ganeti import serializer

from ganeti import utils

HTTP_GANETI_VERSION = "Ganeti %s" % constants.RELEASE_VERSION

HTTP_OK = 200

HTTP_NO_CONTENT = 204

HTTP_NOT_MODIFIED = 304

HTTP_0_9 = "HTTP/0.9"

HTTP_1_0 = "HTTP/1.0"

HTTP_1_1 = "HTTP/1.1"

HTTP_GET = "GET"

HTTP_HEAD = "HEAD"

HTTP_POST = "POST"

HTTP_PUT = "PUT"

HTTP_DELETE = "DELETE"

HTTP_ETAG = "ETag"

HTTP_HOST = "Host"

HTTP_SERVER = "Server"

HTTP_DATE = "Date"

HTTP_USER_AGENT = "User-Agent"

HTTP_CONTENT_TYPE = "Content-Type"

HTTP_CONTENT_LENGTH = "Content-Length"

HTTP_CONNECTION = "Connection"

HTTP_KEEP_ALIVE = "Keep-Alive"

HTTP_WWW_AUTHENTICATE = "WWW-Authenticate"

HTTP_AUTHORIZATION = "Authorization"

HTTP_AUTHENTICATION_INFO = "Authentication-Info"

HTTP_ALLOW = "Allow"

_SSL_UNEXPECTED_EOF = "Unexpected EOF"

# Socket operations

(SOCKOP_SEND,

 SOCKOP_RECV,

 SOCKOP_SHUTDOWN,

 SOCKOP_HANDSHAKE) = range(4)

# send/receive quantum

SOCK_BUF_SIZE = 32768

class HttpError(Exception):

  """Internal exception for HTTP errors.

  This should only be used for internal error reporting.

  """

class HttpConnectionClosed(Exception):

  """Internal exception for a closed connection.

  This should only be used for internal error reporting. Only use

  it if there's no other way to report this condition.

  """

class HttpSessionHandshakeUnexpectedEOF(HttpError):

  """Internal exception for errors during SSL handshake.

  This should only be used for internal error reporting.

  """

class HttpSocketTimeout(Exception):

  """Internal exception for socket timeouts.

  This should only be used for internal error reporting.

  """

class HttpException(Exception):

  code = None

  message = None

  def __init__(self, message=None, headers=None):

    Exception.__init__(self)

    self.message = message

    self.headers = headers

class HttpBadRequest(HttpException):

  """400 Bad Request

  RFC2616, 10.4.1: The request could not be understood by the server

  due to malformed syntax. The client SHOULD NOT repeat the request

  without modifications.

  """

  code = 400

class HttpUnauthorized(HttpException):

  """401 Unauthorized

  RFC2616, section 10.4.2: The request requires user

  authentication. The response MUST include a WWW-Authenticate header

  field (section 14.47) containing a challenge applicable to the

  requested resource.

  """

  code = 401

class HttpForbidden(HttpException):

  """403 Forbidden

  RFC2616, 10.4.4: The server understood the request, but is refusing

  to fulfill it.  Authorization will not help and the request SHOULD

  NOT be repeated.

  """

  code = 403

class HttpNotFound(HttpException):

  """404 Not Found

  RFC2616, 10.4.5: The server has not found anything matching the

  Request-URI.  No indication is given of whether the condition is

  temporary or permanent.

  """

  code = 404

class HttpMethodNotAllowed(HttpException):

  """405 Method Not Allowed

  RFC2616, 10.4.6: The method specified in the Request-Line is not

  allowed for the resource identified by the Request-URI. The response

  MUST include an Allow header containing a list of valid methods for

  the requested resource.

  """

  code = 405

class HttpRequestTimeout(HttpException):

  """408 Request Timeout

  RFC2616, 10.4.9: The client did not produce a request within the

  time that the server was prepared to wait. The client MAY repeat the

  request without modifications at any later time.

  """

  code = 408

class HttpConflict(HttpException):

  """409 Conflict

  RFC2616, 10.4.10: The request could not be completed due to a

  conflict with the current state of the resource. This code is only

  allowed in situations where it is expected that the user might be

  able to resolve the conflict and resubmit the request.

  """

  code = 409

class HttpGone(HttpException):

  """410 Gone

  RFC2616, 10.4.11: The requested resource is no longer available at

  the server and no forwarding address is known. This condition is

  expected to be considered permanent.

  """

  code = 410

class HttpLengthRequired(HttpException):

  """411 Length Required

  RFC2616, 10.4.12: The server refuses to accept the request without a

  defined Content-Length. The client MAY repeat the request if it adds

  a valid Content-Length header field containing the length of the

  message-body in the request message.

  """

  code = 411

class HttpPreconditionFailed(HttpException):

  """412 Precondition Failed

  RFC2616, 10.4.13: The precondition given in one or more of the

  request-header fields evaluated to false when it was tested on the

  server.

  """

  code = 412

class HttpInternalServerError(HttpException):

  """500 Internal Server Error

  RFC2616, 10.5.1: The server encountered an unexpected condition

  which prevented it from fulfilling the request.

  """

  code = 500

class HttpNotImplemented(HttpException):

  """501 Not Implemented

  RFC2616, 10.5.2: The server does not support the functionality

  required to fulfill the request.

  """

  code = 501

class HttpBadGateway(HttpException):

  """502 Bad Gateway

  RFC2616, 10.5.3: The server, while acting as a gateway or proxy,

  received an invalid response from the upstream server it accessed in

  attempting to fulfill the request.

  """

  code = 502

class HttpServiceUnavailable(HttpException):

  """503 Service Unavailable

  RFC2616, 10.5.4: The server is currently unable to handle the

  request due to a temporary overloading or maintenance of the server.

  """

  code = 503

class HttpGatewayTimeout(HttpException):

  """504 Gateway Timeout

  RFC2616, 10.5.5: The server, while acting as a gateway or proxy, did

  not receive a timely response from the upstream server specified by

  the URI (e.g.  HTTP, FTP, LDAP) or some other auxiliary server

  (e.g. DNS) it needed to access in attempting to complete the

  request.

  """

  code = 504

class HttpVersionNotSupported(HttpException):

  """505 HTTP Version Not Supported

  RFC2616, 10.5.6: The server does not support, or refuses to support,

  the HTTP protocol version that was used in the request message.

  """

  code = 505

class HttpJsonConverter:

  CONTENT_TYPE = "application/json"

  def Encode(self, data):

    return serializer.DumpJson(data)

  def Decode(self, data):

    return serializer.LoadJson(data)

def WaitForSocketCondition(sock, event, timeout):

  """Waits for a condition to occur on the socket.

  @type sock: socket

  @param sock: Wait for events on this socket

  @type event: int

  @param event: ORed condition (see select module)

  @type timeout: float or None

  @param timeout: Timeout in seconds

  @rtype: int or None

  @return: None for timeout, otherwise occured conditions

  """

  check = (event | select.POLLPRI |

           select.POLLNVAL | select.POLLHUP | select.POLLERR)

  if timeout is not None:

    # Poller object expects milliseconds

    timeout *= 1000

  poller = select.poll()

  poller.register(sock, event)

  try:

    while True:

      # TODO: If the main thread receives a signal and we have no timeout, we

      # could wait forever. This should check a global "quit" flag or

      # something every so often.

      io_events = poller.poll(timeout)

      if not io_events:

        # Timeout

        return None

      for (evfd, evcond) in io_events:

        if evcond & check:

          return evcond

  finally:

    poller.unregister(sock)

def SocketOperation(sock, op, arg1, timeout):

  """Wrapper around socket functions.

  This function abstracts error handling for socket operations, especially

  for the complicated interaction with OpenSSL.

  @type sock: socket

  @param sock: Socket for the operation

  @type op: int

  @param op: Operation to execute (SOCKOP_* constants)

  @type arg1: any

  @param arg1: Parameter for function (if needed)

  @type timeout: None or float

  @param timeout: Timeout in seconds or None

  @return: Return value of socket function

  """

  # TODO: event_poll/event_check/override

  if op in (SOCKOP_SEND, SOCKOP_HANDSHAKE):

    event_poll = select.POLLOUT

    event_check = select.POLLOUT

  elif op == SOCKOP_RECV:

    event_poll = select.POLLIN

    event_check = select.POLLIN | select.POLLPRI

  elif op == SOCKOP_SHUTDOWN:

    event_poll = None

    event_check = None

    # The timeout is only used when OpenSSL requests polling for a condition.

    # It is not advisable to have no timeout for shutdown.

    assert timeout

  else:

    raise AssertionError("Invalid socket operation")

  # Handshake is only supported by SSL sockets

  if (op == SOCKOP_HANDSHAKE and

      not isinstance(sock, OpenSSL.SSL.ConnectionType)):

    return

  # No override by default

  event_override = 0

  while True:

    # Poll only for certain operations and when asked for by an override

    if event_override or op in (SOCKOP_SEND, SOCKOP_RECV, SOCKOP_HANDSHAKE):

      if event_override:

        wait_for_event = event_override

      else:

        wait_for_event = event_poll

      event = WaitForSocketCondition(sock, wait_for_event, timeout)

      if event is None:

        raise HttpSocketTimeout()

      if (op == SOCKOP_RECV and

          event & (select.POLLNVAL | select.POLLHUP | select.POLLERR)):

        return ""

      if not event & wait_for_event:

        continue

    # Reset override

    event_override = 0

    try:

      try:

        if op == SOCKOP_SEND:

          return sock.send(arg1)

        elif op == SOCKOP_RECV:

          return sock.recv(arg1)

        elif op == SOCKOP_SHUTDOWN:

          if isinstance(sock, OpenSSL.SSL.ConnectionType):

            # PyOpenSSL's shutdown() doesn't take arguments

            return sock.shutdown()

          else:

            return sock.shutdown(arg1)

        elif op == SOCKOP_HANDSHAKE:

          return sock.do_handshake()

      except OpenSSL.SSL.WantWriteError:

        # OpenSSL wants to write, poll for POLLOUT

        event_override = select.POLLOUT

        continue

      except OpenSSL.SSL.WantReadError:

        # OpenSSL wants to read, poll for POLLIN

        event_override = select.POLLIN | select.POLLPRI

        continue

      except OpenSSL.SSL.WantX509LookupError:

        continue

      except OpenSSL.SSL.ZeroReturnError, err:

        # SSL Connection has been closed. In SSL 3.0 and TLS 1.0, this only

        # occurs if a closure alert has occurred in the protocol, i.e. the

        # connection has been closed cleanly. Note that this does not

        # necessarily mean that the transport layer (e.g. a socket) has been

        # closed.

        if op == SOCKOP_SEND:

          # Can happen during a renegotiation

          raise HttpConnectionClosed(err.args)

        elif op == SOCKOP_RECV:

          return ""

        # SSL_shutdown shouldn't return SSL_ERROR_ZERO_RETURN

        raise socket.error(err.args)

      except OpenSSL.SSL.SysCallError, err:

        if op == SOCKOP_SEND:

          # arg1 is the data when writing

          if err.args and err.args[0] == -1 and arg1 == "":

            # errors when writing empty strings are expected

            # and can be ignored

            return 0

        if err.args == (-1, _SSL_UNEXPECTED_EOF):

          if op == SOCKOP_RECV:

            return ""

          elif op == SOCKOP_HANDSHAKE:

            # Can happen if peer disconnects directly after the connection is

            # opened.

            raise HttpSessionHandshakeUnexpectedEOF(err.args)

        raise socket.error(err.args)

      except OpenSSL.SSL.Error, err:

        raise socket.error(err.args)

    except socket.error, err:

      if err.args and err.args[0] == errno.EAGAIN:

        # Ignore EAGAIN

        continue

      raise

def ShutdownConnection(sock, close_timeout, write_timeout, msgreader, force):

  """Closes the connection.

  @type sock: socket

  @param sock: Socket to be shut down

  @type close_timeout: float

  @param close_timeout: How long to wait for the peer to close

      the connection

  @type write_timeout: float

  @param write_timeout: Write timeout for shutdown

  @type msgreader: http.HttpMessageReader

  @param msgreader: Request message reader, used to determine whether

      peer should close connection

  @type force: bool

  @param force: Whether to forcibly close the connection without

      waiting for peer

  """

  #print msgreader.peer_will_close, force

  if msgreader and msgreader.peer_will_close and not force:

    # Wait for peer to close

    try:

      # Check whether it's actually closed

      if not SocketOperation(sock, SOCKOP_RECV, 1, close_timeout):

        return

    except (socket.error, HttpError, HttpSocketTimeout):

      # Ignore errors at this stage

      pass

  # Close the connection from our side

  try:

    # We don't care about the return value, see NOTES in SSL_shutdown(3).

    SocketOperation(sock, SOCKOP_SHUTDOWN, socket.SHUT_RDWR,

                    write_timeout)

  except HttpSocketTimeout:

    raise HttpError("Timeout while shutting down connection")

  except socket.error, err:

    # Ignore ENOTCONN

    if not (err.args and err.args[0] == errno.ENOTCONN):

      raise HttpError("Error while shutting down connection: %s" % err)

def Handshake(sock, write_timeout):

  """Shakes peer's hands.

  @type sock: socket

  @param sock: Socket to be shut down

  @type write_timeout: float

  @param write_timeout: Write timeout for handshake

  """

  try:

    return SocketOperation(sock, SOCKOP_HANDSHAKE, None, write_timeout)

  except HttpSocketTimeout:

    raise HttpError("Timeout during SSL handshake")

  except socket.error, err:

    raise HttpError("Error in SSL handshake: %s" % err)

class HttpSslParams(object):

  """Data class for SSL key and certificate.

  """

  def __init__(self, ssl_key_path, ssl_cert_path):

    """Initializes this class.

    @type ssl_key_path: string

    @param ssl_key_path: Path to file containing SSL key in PEM format

    @type ssl_cert_path: string

    @param ssl_cert_path: Path to file containing SSL certificate

        in PEM format

    """

    self.ssl_key_pem = utils.ReadFile(ssl_key_path)

    self.ssl_cert_pem = utils.ReadFile(ssl_cert_path)

  def GetKey(self):

    return OpenSSL.crypto.load_privatekey(OpenSSL.crypto.FILETYPE_PEM,

                                          self.ssl_key_pem)

  def GetCertificate(self):

    return OpenSSL.crypto.load_certificate(OpenSSL.crypto.FILETYPE_PEM,

                                           self.ssl_cert_pem)

class HttpBase(object):

  """Base class for HTTP server and client.

  """

  def __init__(self):

    self.using_ssl = None

    self._ssl_params = None

    self._ssl_key = None

    self._ssl_cert = None

  def _CreateSocket(self, ssl_params, ssl_verify_peer):

    """Creates a TCP socket and initializes SSL if needed.

    @type ssl_params: HttpSslParams

    @param ssl_params: SSL key and certificate

    @type ssl_verify_peer: bool

    @param ssl_verify_peer: Whether to require client certificate

        and compare it with our certificate

    """

    self._ssl_params = ssl_params

    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

    # Should we enable SSL?

    self.using_ssl = ssl_params is not None

    if not self.using_ssl:

      return sock

    self._ssl_key = ssl_params.GetKey()

    self._ssl_cert = ssl_params.GetCertificate()

    ctx = OpenSSL.SSL.Context(OpenSSL.SSL.SSLv23_METHOD)

    ctx.set_options(OpenSSL.SSL.OP_NO_SSLv2)

    ctx.use_privatekey(self._ssl_key)

    ctx.use_certificate(self._ssl_cert)

    ctx.check_privatekey()

    if ssl_verify_peer:

      ctx.set_verify(OpenSSL.SSL.VERIFY_PEER |

                     OpenSSL.SSL.VERIFY_FAIL_IF_NO_PEER_CERT,

                     self._SSLVerifyCallback)

    return OpenSSL.SSL.Connection(ctx, sock)

  def _SSLVerifyCallback(self, conn, cert, errnum, errdepth, ok):

    """Verify the certificate provided by the peer

    We only compare fingerprints. The client must use the same certificate as

    we do on our side.

    """

    assert self._ssl_params, "SSL not initialized"

    return (self._ssl_cert.digest("sha1") == cert.digest("sha1") and

            self._ssl_cert.digest("md5") == cert.digest("md5"))

class HttpMessage(object):

  """Data structure for HTTP message.

  """

  def __init__(self):

    self.start_line = None

    self.headers = None

    self.body = None

    self.decoded_body = None

class HttpClientToServerStartLine(object):

  """Data structure for HTTP request start line.

  """

  def __init__(self, method, path, version):

    self.method = method

    self.path = path

    self.version = version

  def __str__(self):

    return "%s %s %s" % (self.method, self.path, self.version)

class HttpServerToClientStartLine(object):

  """Data structure for HTTP response start line.

  """

  def __init__(self, version, code, reason):

    self.version = version

    self.code = code

    self.reason = reason

  def __str__(self):

    return "%s %s %s" % (self.version, self.code, self.reason)

class HttpMessageWriter(object):

  """Writes an HTTP message to a socket.

  """

  def __init__(self, sock, msg, write_timeout):

    """Initializes this class and writes an HTTP message to a socket.

    @type sock: socket

    @param sock: Socket to be written to

    @type msg: http.HttpMessage

    @param msg: HTTP message to be written

    @type write_timeout: float

    @param write_timeout: Write timeout for socket

    """

    self._msg = msg

    self._PrepareMessage()

    buf = self._FormatMessage()

    pos = 0

    end = len(buf)

    while pos < end:

      # Send only SOCK_BUF_SIZE bytes at a time

      data = buf[pos:(pos + SOCK_BUF_SIZE)]

      sent = SocketOperation(sock, SOCKOP_SEND, data, write_timeout)

      # Remove sent bytes

      pos += sent

    assert pos == end, "Message wasn't sent completely"

  def _PrepareMessage(self):

    """Prepares the HTTP message by setting mandatory headers.

    """

    # RFC2616, section 4.3: "The presence of a message-body in a request is

    # signaled by the inclusion of a Content-Length or Transfer-Encoding header

    # field in the request's message-headers."

    if self._msg.body:

      self._msg.headers[HTTP_CONTENT_LENGTH] = len(self._msg.body)

  def _FormatMessage(self):

    """Serializes the HTTP message into a string.

    """

    buf = StringIO()

    # Add start line

    buf.write(str(self._msg.start_line))

    buf.write("\r\n")

    # Add headers

    if self._msg.start_line.version != HTTP_0_9:

      for name, value in self._msg.headers.iteritems():

        buf.write("%s: %s\r\n" % (name, value))

    buf.write("\r\n")

    # Add message body if needed

    if self.HasMessageBody():

      buf.write(self._msg.body)

    elif self._msg.body:

      logging.warning("Ignoring message body")

    return buf.getvalue()

  def HasMessageBody(self):

    """Checks whether the HTTP message contains a body.

    Can be overriden by subclasses.

    """

    return bool(self._msg.body)

class HttpMessageReader(object):

  """Reads HTTP message from socket.

  """

  # Length limits

  START_LINE_LENGTH_MAX = None

  HEADER_LENGTH_MAX = None

  # Parser state machine

  PS_START_LINE = "start-line"

  PS_HEADERS = "headers"

  PS_BODY = "entity-body"

  PS_COMPLETE = "complete"

  def __init__(self, sock, msg, read_timeout):

    """Reads an HTTP message from a socket.

    @type sock: socket

    @param sock: Socket to be read from

    @type msg: http.HttpMessage

    @param msg: Object for the read message

    @type read_timeout: float

    @param read_timeout: Read timeout for socket

    """

    self.sock = sock

    self.msg = msg

    self.start_line_buffer = None

    self.header_buffer = StringIO()

    self.body_buffer = StringIO()

    self.parser_status = self.PS_START_LINE

    self.content_length = None

    self.peer_will_close = None

    buf = ""

    eof = False

    while self.parser_status != self.PS_COMPLETE:

      # TODO: Don't read more than necessary (Content-Length), otherwise

      # data might be lost and/or an error could occur

      data = SocketOperation(sock, SOCKOP_RECV, SOCK_BUF_SIZE, read_timeout)

      if data:

        buf += data

      else:

        eof = True

      # Do some parsing and error checking while more data arrives

      buf = self._ContinueParsing(buf, eof)

      # Must be done only after the buffer has been evaluated

      # TODO: Connection-length < len(data read) and connection closed

      if (eof and

          self.parser_status in (self.PS_START_LINE,

                                 self.PS_HEADERS)):

        raise HttpError("Connection closed prematurely")

    # Parse rest

    buf = self._ContinueParsing(buf, True)

    assert self.parser_status == self.PS_COMPLETE

    assert not buf, "Parser didn't read full response"

    msg.body = self.body_buffer.getvalue()

    # TODO: Content-type, error handling

    if msg.body:

      msg.decoded_body = HttpJsonConverter().Decode(msg.body)

    else:

      msg.decoded_body = None

    if msg.decoded_body:

      logging.debug("Message body: %s", msg.decoded_body)

  def _ContinueParsing(self, buf, eof):

    """Main function for HTTP message state machine.

    @type buf: string

    @param buf: Receive buffer

    @type eof: bool

    @param eof: Whether we've reached EOF on the socket

    @rtype: string

    @return: Updated receive buffer

    """

    # TODO: Use offset instead of slicing when possible

    if self.parser_status == self.PS_START_LINE:

      # Expect start line

      while True:

        idx = buf.find("\r\n")

        # RFC2616, section 4.1: "In the interest of robustness, servers SHOULD

        # ignore any empty line(s) received where a Request-Line is expected.

        # In other words, if the server is reading the protocol stream at the

        # beginning of a message and receives a CRLF first, it should ignore

        # the CRLF."

        if idx == 0:

          # TODO: Limit number of CRLFs/empty lines for safety?

          buf = buf[:2]

          continue

        if idx > 0:

          self.start_line_buffer = buf[:idx]

          self._CheckStartLineLength(len(self.start_line_buffer))

          # Remove status line, including CRLF

          buf = buf[idx + 2:]

          self.msg.start_line = self.ParseStartLine(self.start_line_buffer)

          self.parser_status = self.PS_HEADERS

        else:

          # Check whether incoming data is getting too large, otherwise we just

          # fill our read buffer.

          self._CheckStartLineLength(len(buf))

        break

    # TODO: Handle messages without headers

    if self.parser_status == self.PS_HEADERS:

      # Wait for header end

      idx = buf.find("\r\n\r\n")

      if idx >= 0:

        self.header_buffer.write(buf[:idx + 2])

        self._CheckHeaderLength(self.header_buffer.tell())

        # Remove headers, including CRLF

        buf = buf[idx + 4:]

        self._ParseHeaders()

        self.parser_status = self.PS_BODY

      else:

        # Check whether incoming data is getting too large, otherwise we just

        # fill our read buffer.

        self._CheckHeaderLength(len(buf))

    if self.parser_status == self.PS_BODY:

      # TODO: Implement max size for body_buffer

      self.body_buffer.write(buf)

      buf = ""

      # Check whether we've read everything

      #

      # RFC2616, section 4.4: "When a message-body is included with a message,

      # the transfer-length of that body is determined by one of the following

      # [...] 5. By the server closing the connection. (Closing the connection

      # cannot be used to indicate the end of a request body, since that would

      # leave no possibility for the server to send back a response.)"

      #

      # TODO: Error when buffer length > Content-Length header

      if (eof or

          self.content_length is None or

          (self.content_length is not None and

           self.body_buffer.tell() >= self.content_length)):

        self.parser_status = self.PS_COMPLETE

    return buf

  def _CheckStartLineLength(self, length):

    """Limits the start line buffer size.

    @type length: int

    @param length: Buffer size

    """

    if (self.START_LINE_LENGTH_MAX is not None and

        length > self.START_LINE_LENGTH_MAX):

      raise HttpError("Start line longer than %d chars" %

                       self.START_LINE_LENGTH_MAX)

  def _CheckHeaderLength(self, length):

    """Limits the header buffer size.

    @type length: int

    @param length: Buffer size

    """

    if (self.HEADER_LENGTH_MAX is not None and

        length > self.HEADER_LENGTH_MAX):

      raise HttpError("Headers longer than %d chars" % self.HEADER_LENGTH_MAX)

  def ParseStartLine(self, start_line):

    """Parses the start line of a message.

    Must be overriden by subclass.

    @type start_line: string

    @param start_line: Start line string

    """

    raise NotImplementedError()

  def _WillPeerCloseConnection(self):

    """Evaluate whether peer will close the connection.

    @rtype: bool

    @return: Whether peer will close the connection

    """

    # RFC2616, section 14.10: "HTTP/1.1 defines the "close" connection option

    # for the sender to signal that the connection will be closed after

    # completion of the response. For example,

    #

    #        Connection: close

    #

    # in either the request or the response header fields indicates that the

    # connection SHOULD NOT be considered `persistent' (section 8.1) after the

    # current request/response is complete."

    hdr_connection = self.msg.headers.get(HTTP_CONNECTION, None)

    if hdr_connection:

      hdr_connection = hdr_connection.lower()

    # An HTTP/1.1 server is assumed to stay open unless explicitly closed.

    if self.msg.start_line.version == HTTP_1_1:

      return (hdr_connection and "close" in hdr_connection)

    # Some HTTP/1.0 implementations have support for persistent connections,

    # using rules different than HTTP/1.1.

    # For older HTTP, Keep-Alive indicates persistent connection.

    if self.msg.headers.get(HTTP_KEEP_ALIVE):

      return False

    # At least Akamai returns a "Connection: Keep-Alive" header, which was

    # supposed to be sent by the client.

    if hdr_connection and "keep-alive" in hdr_connection:

      return False

    return True

  def _ParseHeaders(self):

    """Parses the headers.

    This function also adjusts internal variables based on header values.

    RFC2616, section 4.3: The presence of a message-body in a request is

    signaled by the inclusion of a Content-Length or Transfer-Encoding header

    field in the request's message-headers.

    """

    # Parse headers

    self.header_buffer.seek(0, 0)

    self.msg.headers = mimetools.Message(self.header_buffer, 0)

    self.peer_will_close = self._WillPeerCloseConnection()

    # Do we have a Content-Length header?

    hdr_content_length = self.msg.headers.get(HTTP_CONTENT_LENGTH, None)

    if hdr_content_length:

      try:

        self.content_length = int(hdr_content_length)

      except ValueError:

        self.content_length = None

      if self.content_length is not None and self.content_length < 0:

        self.content_length = None

    # if the connection remains open and a content-length was not provided,

    # then assume that the connection WILL close.

    if self.content_length is None:

      self.peer_will_close = True