162 lines
6.4 KiB
Python
162 lines
6.4 KiB
Python
"""
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h2/frame_buffer
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~~~~~~~~~~~~~~~
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A data structure that provides a way to iterate over a byte buffer in terms of
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frames.
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"""
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from __future__ import annotations
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from hyperframe.exceptions import InvalidDataError, InvalidFrameError
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from hyperframe.frame import ContinuationFrame, Frame, HeadersFrame, PushPromiseFrame
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from .exceptions import FrameDataMissingError, FrameTooLargeError, ProtocolError
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# To avoid a DOS attack based on sending loads of continuation frames, we limit
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# the maximum number we're perpared to receive. In this case, we'll set the
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# limit to 64, which means the largest encoded header block we can receive by
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# default is 262144 bytes long, and the largest possible *at all* is 1073741760
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# bytes long.
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#
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# This value seems reasonable for now, but in future we may want to evaluate
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# making it configurable.
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CONTINUATION_BACKLOG = 64
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class FrameBuffer:
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"""
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A buffer data structure for HTTP/2 data that allows iteraton in terms of
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H2 frames.
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"""
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def __init__(self, server: bool = False) -> None:
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self._data = bytearray()
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self.max_frame_size = 0
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self._preamble = b"PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n" if server else b""
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self._preamble_len = len(self._preamble)
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self._headers_buffer: list[HeadersFrame | ContinuationFrame | PushPromiseFrame] = []
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def add_data(self, data: bytes) -> None:
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"""
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Add more data to the frame buffer.
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:param data: A bytestring containing the byte buffer.
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"""
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if self._preamble_len:
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data_len = len(data)
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of_which_preamble = min(self._preamble_len, data_len)
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if self._preamble[:of_which_preamble] != data[:of_which_preamble]:
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msg = "Invalid HTTP/2 preamble."
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raise ProtocolError(msg)
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data = data[of_which_preamble:]
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self._preamble_len -= of_which_preamble
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self._preamble = self._preamble[of_which_preamble:]
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self._data += data
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def _validate_frame_length(self, length: int) -> None:
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"""
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Confirm that the frame is an appropriate length.
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"""
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if length > self.max_frame_size:
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msg = f"Received overlong frame: length {length}, max {self.max_frame_size}"
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raise FrameTooLargeError(msg)
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def _update_header_buffer(self, f: Frame | None) -> Frame | None:
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"""
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Updates the internal header buffer. Returns a frame that should replace
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the current one. May throw exceptions if this frame is invalid.
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"""
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# Check if we're in the middle of a headers block. If we are, this
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# frame *must* be a CONTINUATION frame with the same stream ID as the
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# leading HEADERS or PUSH_PROMISE frame. Anything else is a
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# ProtocolError. If the frame *is* valid, append it to the header
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# buffer.
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if self._headers_buffer:
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stream_id = self._headers_buffer[0].stream_id
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valid_frame = (
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f is not None and
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isinstance(f, ContinuationFrame) and
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f.stream_id == stream_id
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)
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if not valid_frame:
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msg = "Invalid frame during header block."
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raise ProtocolError(msg)
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assert isinstance(f, ContinuationFrame)
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# Append the frame to the buffer.
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self._headers_buffer.append(f)
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if len(self._headers_buffer) > CONTINUATION_BACKLOG:
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msg = "Too many continuation frames received."
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raise ProtocolError(msg)
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# If this is the end of the header block, then we want to build a
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# mutant HEADERS frame that's massive. Use the original one we got,
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# then set END_HEADERS and set its data appopriately. If it's not
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# the end of the block, lose the current frame: we can't yield it.
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if "END_HEADERS" in f.flags:
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f = self._headers_buffer[0]
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f.flags.add("END_HEADERS")
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f.data = b"".join(x.data for x in self._headers_buffer)
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self._headers_buffer = []
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else:
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f = None
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elif (isinstance(f, (HeadersFrame, PushPromiseFrame)) and
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"END_HEADERS" not in f.flags):
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# This is the start of a headers block! Save the frame off and then
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# act like we didn't receive one.
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self._headers_buffer.append(f)
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f = None
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return f
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# The methods below support the iterator protocol.
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def __iter__(self) -> FrameBuffer:
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return self
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def __next__(self) -> Frame:
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# First, check that we have enough data to successfully parse the
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# next frame header. If not, bail. Otherwise, parse it.
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if len(self._data) < 9:
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raise StopIteration
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try:
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f, length = Frame.parse_frame_header(memoryview(self._data[:9]))
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except (InvalidDataError, InvalidFrameError) as err: # pragma: no cover
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msg = f"Received frame with invalid header: {err!s}"
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raise ProtocolError(msg) from err
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# Next, check that we have enough length to parse the frame body. If
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# not, bail, leaving the frame header data in the buffer for next time.
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if len(self._data) < length + 9:
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raise StopIteration
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# Confirm the frame has an appropriate length.
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self._validate_frame_length(length)
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# Try to parse the frame body
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try:
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f.parse_body(memoryview(self._data[9:9+length]))
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except InvalidDataError as err:
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msg = "Received frame with non-compliant data"
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raise ProtocolError(msg) from err
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except InvalidFrameError as err:
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msg = "Frame data missing or invalid"
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raise FrameDataMissingError(msg) from err
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# At this point, as we know we'll use or discard the entire frame, we
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# can update the data.
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self._data = self._data[9+length:]
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# Pass the frame through the header buffer.
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new_frame = self._update_header_buffer(f)
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# If we got a frame we didn't understand or shouldn't yield, rather
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# than return None it'd be better if we just tried to get the next
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# frame in the sequence instead. Recurse back into ourselves to do
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# that. This is safe because the amount of work we have to do here is
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# strictly bounded by the length of the buffer.
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return new_frame if new_frame is not None else self.__next__()
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