wspy/deflate_frame.py

import zlib

from extension import Extension
from frame import ControlFrame


class DeflateFrame(Extension):
    """
    This is an implementation of the "deflate-frame" extension, as defined by
    http://tools.ietf.org/html/draft-tyoshino-hybi-websocket-perframe-deflate-06.

    Supported parameters are:
    - max_window_size: maximum size for the LZ77 sliding window.
    - no_context_takeover: disallows usage of LZ77 sliding window from
                           previously built frames for the current frame.

    Note that the deflate and inflate hooks modify the RSV1 bit and payload of
    existing `Frame` objects.
    """

    name = 'deflate-frame'
    rsv1 = True
    defaults = {'max_window_bits': 15, 'no_context_takeover': False}

    def __init__(self, defaults={}, request={}):
        Extension.__init__(self, defaults, request)

        mwb = self.defaults['max_window_bits']
        cto = self.defaults['no_context_takeover']

        if not isinstance(mwb, int):
            raise ValueError('"max_window_bits" must be an integer')
        elif mwb > 15:
            raise ValueError('"max_window_bits" may not be larger than 15')

        if cto is not False and cto is not True:
            raise ValueError('"no_context_takeover" must have no value')

    class Hook(Extension.Hook):
        def __init__(self, extension, **kwargs):
            Extension.Hook.__init__(self, extension, **kwargs)

            if not self.no_context_takeover:
                self.defl = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION,
                                            zlib.DEFLATED,
                                            -self.max_window_bits)

            other_wbits = self.extension.request.get('max_window_bits', 15)
            self.dec = zlib.decompressobj(-other_wbits)

        def send(self, frame):
            if not frame.rsv1 and not isinstance(frame, ControlFrame):
                frame.rsv1 = True
                frame.payload = self.deflate(frame.payload)

            return frame

        def recv(self, frame):
            if frame.rsv1:
                if isinstance(frame, ControlFrame):
                    raise ValueError('received compressed control frame')

                frame.rsv1 = False
                frame.payload = self.inflate(frame.payload)

            return frame

        def deflate(self, data):
            if self.no_context_takeover:
                defl = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION,
                                        zlib.DEFLATED, -self.max_window_bits)
                # FIXME: why the '\x00' below? This was borrowed from
                # https://github.com/fancycode/tornado/blob/bc317b6dcf63608ff004ff1f57073be0504b6550/tornado/websocket.py#L91
                return defl.compress(data) + defl.flush(zlib.Z_FINISH) + '\x00'

            compressed = self.defl.compress(data)
            compressed += self.defl.flush(zlib.Z_SYNC_FLUSH)
            assert compressed[-4:] == '\x00\x00\xff\xff'
            return compressed[:-4]

        def inflate(self, data):
            data = self.dec.decompress(str(data + '\x00\x00\xff\xff'))
            assert not self.dec.unused_data
            return data


class WebkitDeflateFrame(DeflateFrame):
    name = 'x-webkit-deflate-frame'