src/mod_pywebsocket/util.py

# Copyright 2011, Google Inc.
# All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are
# met:
#
#     * Redistributions of source code must retain the above copyright
# notice, this list of conditions and the following disclaimer.
#     * Redistributions in binary form must reproduce the above
# copyright notice, this list of conditions and the following disclaimer
# in the documentation and/or other materials provided with the
# distribution.
#     * Neither the name of Google Inc. nor the names of its
# contributors may be used to endorse or promote products derived from
# this software without specific prior written permission.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.


"""WebSocket utilities.
"""


import array
import errno

# Import hash classes from a module available and recommended for each Python
# version and re-export those symbol. Use sha and md5 module in Python 2.4, and
# hashlib module in Python 2.6.
try:
    import hashlib
    md5_hash = hashlib.md5
    sha1_hash = hashlib.sha1
except ImportError:
    import md5
    import sha
    md5_hash = md5.md5
    sha1_hash = sha.sha

import StringIO
import logging
import os
import re
import socket
import traceback
import zlib


def get_stack_trace():
    """Get the current stack trace as string.

    This is needed to support Python 2.3.
    TODO: Remove this when we only support Python 2.4 and above.
          Use traceback.format_exc instead.
    """

    out = StringIO.StringIO()
    traceback.print_exc(file=out)
    return out.getvalue()


def prepend_message_to_exception(message, exc):
    """Prepend message to the exception."""

    exc.args = (message + str(exc),)
    return


def __translate_interp(interp, cygwin_path):
    """Translate interp program path for Win32 python to run cygwin program
    (e.g. perl).  Note that it doesn't support path that contains space,
    which is typically true for Unix, where #!-script is written.
    For Win32 python, cygwin_path is a directory of cygwin binaries.

    Args:
      interp: interp command line
      cygwin_path: directory name of cygwin binary, or None
    Returns:
      translated interp command line.
    """
    if not cygwin_path:
        return interp
    m = re.match('^[^ ]*/([^ ]+)( .*)?', interp)
    if m:
        cmd = os.path.join(cygwin_path, m.group(1))
        return cmd + m.group(2)
    return interp


def get_script_interp(script_path, cygwin_path=None):
    """Gets #!-interpreter command line from the script.

    It also fixes command path.  When Cygwin Python is used, e.g. in WebKit,
    it could run "/usr/bin/perl -wT hello.pl".
    When Win32 Python is used, e.g. in Chromium, it couldn't.  So, fix
    "/usr/bin/perl" to "<cygwin_path>\perl.exe".

    Args:
      script_path: pathname of the script
      cygwin_path: directory name of cygwin binary, or None
    Returns:
      #!-interpreter command line, or None if it is not #!-script.
    """
    fp = open(script_path)
    line = fp.readline()
    fp.close()
    m = re.match('^#!(.*)', line)
    if m:
        return __translate_interp(m.group(1), cygwin_path)
    return None


def wrap_popen3_for_win(cygwin_path):
    """Wrap popen3 to support #!-script on Windows.

    Args:
      cygwin_path:  path for cygwin binary if command path is needed to be
                    translated.  None if no translation required.
    """

    __orig_popen3 = os.popen3

    def __wrap_popen3(cmd, mode='t', bufsize=-1):
        cmdline = cmd.split(' ')
        interp = get_script_interp(cmdline[0], cygwin_path)
        if interp:
            cmd = interp + ' ' + cmd
        return __orig_popen3(cmd, mode, bufsize)

    os.popen3 = __wrap_popen3


def hexify(s):
    return ' '.join(map(lambda x: '%02x' % ord(x), s))


def get_class_logger(o):
    return logging.getLogger(
        '%s.%s' % (o.__class__.__module__, o.__class__.__name__))


class NoopMasker(object):
    """A masking object that has the same interface as RepeatedXorMasker but
    just returns the string passed in without making any change.
    """

    def __init__(self):
        pass

    def mask(self, s):
        return s


class RepeatedXorMasker(object):
    """A masking object that applies XOR on the string given to mask method
    with the masking bytes given to the constructor repeatedly. This object
    remembers the position in the masking bytes the last mask method call
    ended and resumes from that point on the next mask method call.
    """

    def __init__(self, mask):
        self._mask = map(ord, mask)
        self._mask_size = len(self._mask)
        self._count = 0

    def mask(self, s):
        result = array.array('B')
        result.fromstring(s)
        # Use temporary local variables to eliminate the cost to access
        # attributes
        count = self._count
        mask = self._mask
        mask_size = self._mask_size
        for i in xrange(len(result)):
            result[i] ^= mask[count]
            count = (count + 1) % mask_size
        self._count = count

        return result.tostring()


class DeflateRequest(object):
    """A wrapper class for request object to intercept send and recv to perform
    deflate compression and decompression transparently.
    """

    def __init__(self, request):
        self._request = request
        self.connection = DeflateConnection(request.connection)

    def __getattribute__(self, name):
        if name in ('_request', 'connection'):
            return object.__getattribute__(self, name)
        return self._request.__getattribute__(name)

    def __setattr__(self, name, value):
        if name in ('_request', 'connection'):
            return object.__setattr__(self, name, value)
        return self._request.__setattr__(name, value)


# By making wbits option negative, we can suppress CMF/FLG (2 octet) and
# ADLER32 (4 octet) fields of zlib so that we can use zlib module just as
# deflate library. DICTID won't be added as far as we don't set dictionary.
# LZ77 window of 32K will be used for both compression and decompression.
# For decompression, we can just use 32K to cover any windows size. For
# compression, we use 32K so receivers must use 32K.
#
# Compression level is Z_DEFAULT_COMPRESSION. We don't have to match level
# to decode.
#
# See zconf.h, deflate.cc, inflate.cc of zlib library, and zlibmodule.c of
# Python. See also RFC1950 (ZLIB 3.3).


class _Deflater(object):

    def __init__(self, window_bits):
        self._logger = get_class_logger(self)

        self._compress = zlib.compressobj(
            zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -window_bits)

    def compress_and_flush(self, bytes):
        compressed_bytes = self._compress.compress(bytes)
        compressed_bytes += self._compress.flush(zlib.Z_SYNC_FLUSH)
        self._logger.debug('Compress input %r', bytes)
        self._logger.debug('Compress result %r', compressed_bytes)
        return compressed_bytes


class _Inflater(object):

    def __init__(self):
        self._logger = get_class_logger(self)

        self._unconsumed = ''

        self.reset()

    def decompress(self, size):
        if not (size == -1 or size > 0):
            raise Exception('size must be -1 or positive')

        data = ''

        while True:
            if size == -1:
                data += self._decompress.decompress(self._unconsumed)
                # See Python bug http://bugs.python.org/issue12050 to
                # understand why the same code cannot be used for updating
                # self._unconsumed for here and else block.
                self._unconsumed = ''
            else:
                data += self._decompress.decompress(
                    self._unconsumed, size - len(data))
                self._unconsumed = self._decompress.unconsumed_tail
            if self._decompress.unused_data:
                # Encountered a last block (i.e. a block with BFINAL = 1) and
                # found a new stream (unused_data). We cannot use the same
                # zlib.Decompress object for the new stream. Create a new
                # Decompress object to decompress the new one.
                #
                # It's fine to ignore unconsumed_tail if unused_data is not
                # empty.
                self._unconsumed = self._decompress.unused_data
                self.reset()
                if size >= 0 and len(data) == size:
                    # data is filled. Don't call decompress again.
                    break
                else:
                    # Re-invoke Decompress.decompress to try to decompress all
                    # available bytes before invoking read which blocks until
                    # any new byte is available.
                    continue
            else:
                # Here, since unused_data is empty, even if unconsumed_tail is
                # not empty, bytes of requested length are already in data. We
                # don't have to "continue" here.
                break

        if data:
            self._logger.debug('Decompressed %r', data)
        return data

    def append(self, data):
        self._logger.debug('Appended %r', data)
        self._unconsumed += data

    def reset(self):
        self._logger.debug('Reset')
        self._decompress = zlib.decompressobj(-zlib.MAX_WBITS)


# Compresses/decompresses given octets using the method introduced in RFC1979.


class _RFC1979Deflater(object):
    """A compressor class that applies DEFLATE to given byte sequence and
    flushes using the algorithm described in the RFC1979 section 2.1.
    """

    def __init__(self, window_bits, no_context_takeover):
        self._deflater = None
        if window_bits is None:
            window_bits = zlib.MAX_WBITS
        self._window_bits = window_bits
        self._no_context_takeover = no_context_takeover

    def filter(self, bytes):
        if self._deflater is None or self._no_context_takeover:
            self._deflater = _Deflater(self._window_bits)

        # Strip last 4 octets which is LEN and NLEN field of a non-compressed
        # block added for Z_SYNC_FLUSH.
        return self._deflater.compress_and_flush(bytes)[:-4]


class _RFC1979Inflater(object):
    """A decompressor class for byte sequence compressed and flushed following
    the algorithm described in the RFC1979 section 2.1.
    """

    def __init__(self):
        self._inflater = _Inflater()

    def filter(self, bytes):
        # Restore stripped LEN and NLEN field of a non-compressed block added
        # for Z_SYNC_FLUSH.
        self._inflater.append(bytes + '\x00\x00\xff\xff')
        return self._inflater.decompress(-1)


class DeflateSocket(object):
    """A wrapper class for socket object to intercept send and recv to perform
    deflate compression and decompression transparently.
    """

    # Size of the buffer passed to recv to receive compressed data.
    _RECV_SIZE = 4096

    def __init__(self, socket):
        self._socket = socket

        self._logger = get_class_logger(self)

        self._deflater = _Deflater(zlib.MAX_WBITS)
        self._inflater = _Inflater()

    def recv(self, size):
        """Receives data from the socket specified on the construction up
        to the specified size. Once any data is available, returns it even
        if it's smaller than the specified size.
        """

        # TODO(tyoshino): Allow call with size=0. It should block until any
        # decompressed data is available.
        if size <= 0:
            raise Exception('Non-positive size passed')
        while True:
            data = self._inflater.decompress(size)
            if len(data) != 0:
                return data

            read_data = self._socket.recv(DeflateSocket._RECV_SIZE)
            if not read_data:
                return ''
            self._inflater.append(read_data)

    def sendall(self, bytes):
        self.send(bytes)

    def send(self, bytes):
        self._socket.sendall(self._deflater.compress_and_flush(bytes))
        return len(bytes)


class DeflateConnection(object):
    """A wrapper class for request object to intercept write and read to
    perform deflate compression and decompression transparently.
    """

    def __init__(self, connection):
        self._connection = connection

        self._logger = get_class_logger(self)

        self._deflater = _Deflater(zlib.MAX_WBITS)
        self._inflater = _Inflater()

    def get_remote_addr(self):
        return self._connection.remote_addr
    remote_addr = property(get_remote_addr)

    def put_bytes(self, bytes):
        self.write(bytes)

    def read(self, size=-1):
        """Reads at most size bytes. Blocks until there's at least one byte
        available.
        """

        # TODO(tyoshino): Allow call with size=0.
        if not (size == -1 or size > 0):
            raise Exception('size must be -1 or positive')

        data = ''
        while True:
            if size == -1:
                data += self._inflater.decompress(-1)
            else:
                data += self._inflater.decompress(size - len(data))

            if size >= 0 and len(data) != 0:
                break

            # TODO(tyoshino): Make this read efficient by some workaround.
            #
            # In 3.0.3 and prior of mod_python, read blocks until length bytes
            # was read. We don't know the exact size to read while using
            # deflate, so read byte-by-byte.
            #
            # _StandaloneRequest.read that ultimately performs
            # socket._fileobject.read also blocks until length bytes was read
            read_data = self._connection.read(1)
            if not read_data:
                break
            self._inflater.append(read_data)
        return data

    def write(self, bytes):
        self._connection.write(self._deflater.compress_and_flush(bytes))


def _is_ewouldblock_errno(error_number):
    """Returns True iff error_number indicates that receive operation would
    block. To make this portable, we check availability of errno and then
    compare them.
    """

    for error_name in ['WSAEWOULDBLOCK', 'EWOULDBLOCK', 'EAGAIN']:
        if (error_name in dir(errno) and
            error_number == getattr(errno, error_name)):
            return True
    return False


def drain_received_data(raw_socket):
    # Set the socket non-blocking.
    original_timeout = raw_socket.gettimeout()
    raw_socket.settimeout(0.0)

    drained_data = []

    # Drain until the socket is closed or no data is immediately
    # available for read.
    while True:
        try:
            data = raw_socket.recv(1)
            if not data:
                break
            drained_data.append(data)
        except socket.error, e:
            # e can be either a pair (errno, string) or just a string (or
            # something else) telling what went wrong. We suppress only
            # the errors that indicates that the socket blocks. Those
            # exceptions can be parsed as a pair (errno, string).
            try:
                error_number, message = e
            except:
                # Failed to parse socket.error.
                raise e

            if _is_ewouldblock_errno(error_number):
                break
            else:
                raise e

    # Rollback timeout value.
    raw_socket.settimeout(original_timeout)

    return ''.join(drained_data)


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