xref: /third_party/protobuf/python/google/protobuf/internal/decoder.py

# Protocol Buffers - Google's data interchange format
# Copyright 2008 Google Inc.  All rights reserved.
#
# Use of this source code is governed by a BSD-style
# license that can be found in the LICENSE file or at
# https://developers.google.com/open-source/licenses/bsd

"""Code for decoding protocol buffer primitives.

This code is very similar to encoder.py -- read the docs for that module first.

A "decoder" is a function with the signature:
  Decode(buffer, pos, end, message, field_dict)
The arguments are:
  buffer:     The string containing the encoded message.
  pos:        The current position in the string.
  end:        The position in the string where the current message ends.  May be
              less than len(buffer) if we're reading a sub-message.
  message:    The message object into which we're parsing.
  field_dict: message._fields (avoids a hashtable lookup).
The decoder reads the field and stores it into field_dict, returning the new
buffer position.  A decoder for a repeated field may proactively decode all of
the elements of that field, if they appear consecutively.

Note that decoders may throw any of the following:
  IndexError:  Indicates a truncated message.
  struct.error:  Unpacking of a fixed-width field failed.
  message.DecodeError:  Other errors.

Decoders are expected to raise an exception if they are called with pos > end.
This allows callers to be lax about bounds checking:  it's fineto read past
"end" as long as you are sure that someone else will notice and throw an
exception later on.

Something up the call stack is expected to catch IndexError and struct.error
and convert them to message.DecodeError.

Decoders are constructed using decoder constructors with the signature:
  MakeDecoder(field_number, is_repeated, is_packed, key, new_default)
The arguments are:
  field_number:  The field number of the field we want to decode.
  is_repeated:   Is the field a repeated field? (bool)
  is_packed:     Is the field a packed field? (bool)
  key:           The key to use when looking up the field within field_dict.
                 (This is actually the FieldDescriptor but nothing in this
                 file should depend on that.)
  new_default:   A function which takes a message object as a parameter and
                 returns a new instance of the default value for this field.
                 (This is called for repeated fields and sub-messages, when an
                 instance does not already exist.)

As with encoders, we define a decoder constructor for every type of field.
Then, for every field of every message class we construct an actual decoder.
That decoder goes into a dict indexed by tag, so when we decode a message
we repeatedly read a tag, look up the corresponding decoder, and invoke it.
"""

__author__ = 'kenton@google.com (Kenton Varda)'

import math
import struct

from google.protobuf import message
from google.protobuf.internal import containers
from google.protobuf.internal import encoder
from google.protobuf.internal import wire_format


# This is not for optimization, but rather to avoid conflicts with local
# variables named "message".
_DecodeError = message.DecodeError


def _VarintDecoder(mask, result_type):
  """Return an encoder for a basic varint value (does not include tag).

  Decoded values will be bitwise-anded with the given mask before being
  returned, e.g. to limit them to 32 bits.  The returned decoder does not
  take the usual "end" parameter -- the caller is expected to do bounds checking
  after the fact (often the caller can defer such checking until later).  The
  decoder returns a (value, new_pos) pair.
  """

  def DecodeVarint(buffer, pos: int=None):
    result = 0
    shift = 0
    while 1:
      if pos is None:
        # Read from BytesIO
        try:
          b = buffer.read(1)[0]
        except IndexError as e:
          if shift == 0:
            # End of BytesIO.
            return None
          else:
            raise ValueError('Fail to read varint %s' % str(e))
      else:
        b = buffer[pos]
        pos += 1
      result |= ((b & 0x7f) << shift)
      if not (b & 0x80):
        result &= mask
        result = result_type(result)
        return result if pos is None else (result, pos)
      shift += 7
      if shift >= 64:
        raise _DecodeError('Too many bytes when decoding varint.')

  return DecodeVarint


def _SignedVarintDecoder(bits, result_type):
  """Like _VarintDecoder() but decodes signed values."""

  signbit = 1 << (bits - 1)
  mask = (1 << bits) - 1

  def DecodeVarint(buffer, pos):
    result = 0
    shift = 0
    while 1:
      b = buffer[pos]
      result |= ((b & 0x7f) << shift)
      pos += 1
      if not (b & 0x80):
        result &= mask
        result = (result ^ signbit) - signbit
        result = result_type(result)
        return (result, pos)
      shift += 7
      if shift >= 64:
        raise _DecodeError('Too many bytes when decoding varint.')
  return DecodeVarint

# All 32-bit and 64-bit values are represented as int.
_DecodeVarint = _VarintDecoder((1 << 64) - 1, int)
_DecodeSignedVarint = _SignedVarintDecoder(64, int)

# Use these versions for values which must be limited to 32 bits.
_DecodeVarint32 = _VarintDecoder((1 << 32) - 1, int)
_DecodeSignedVarint32 = _SignedVarintDecoder(32, int)


def ReadTag(buffer, pos):
  """Read a tag from the memoryview, and return a (tag_bytes, new_pos) tuple.

  We return the raw bytes of the tag rather than decoding them.  The raw
  bytes can then be used to look up the proper decoder.  This effectively allows
  us to trade some work that would be done in pure-python (decoding a varint)
  for work that is done in C (searching for a byte string in a hash table).
  In a low-level language it would be much cheaper to decode the varint and
  use that, but not in Python.

  Args:
    buffer: memoryview object of the encoded bytes
    pos: int of the current position to start from

  Returns:
    Tuple[bytes, int] of the tag data and new position.
  """
  start = pos
  while buffer[pos] & 0x80:
    pos += 1
  pos += 1

  tag_bytes = buffer[start:pos].tobytes()
  return tag_bytes, pos


# --------------------------------------------------------------------


def _SimpleDecoder(wire_type, decode_value):
  """Return a constructor for a decoder for fields of a particular type.

  Args:
      wire_type:  The field's wire type.
      decode_value:  A function which decodes an individual value, e.g.
        _DecodeVarint()
  """

  def SpecificDecoder(field_number, is_repeated, is_packed, key, new_default,
                      clear_if_default=False):
    if is_packed:
      local_DecodeVarint = _DecodeVarint
      def DecodePackedField(buffer, pos, end, message, field_dict):
        value = field_dict.get(key)
        if value is None:
          value = field_dict.setdefault(key, new_default(message))
        (endpoint, pos) = local_DecodeVarint(buffer, pos)
        endpoint += pos
        if endpoint > end:
          raise _DecodeError('Truncated message.')
        while pos < endpoint:
          (element, pos) = decode_value(buffer, pos)
          value.append(element)
        if pos > endpoint:
          del value[-1]   # Discard corrupt value.
          raise _DecodeError('Packed element was truncated.')
        return pos
      return DecodePackedField
    elif is_repeated:
      tag_bytes = encoder.TagBytes(field_number, wire_type)
      tag_len = len(tag_bytes)
      def DecodeRepeatedField(buffer, pos, end, message, field_dict):
        value = field_dict.get(key)
        if value is None:
          value = field_dict.setdefault(key, new_default(message))
        while 1:
          (element, new_pos) = decode_value(buffer, pos)
          value.append(element)
          # Predict that the next tag is another copy of the same repeated
          # field.
          pos = new_pos + tag_len
          if buffer[new_pos:pos] != tag_bytes or new_pos >= end:
            # Prediction failed.  Return.
            if new_pos > end:
              raise _DecodeError('Truncated message.')
            return new_pos
      return DecodeRepeatedField
    else:
      def DecodeField(buffer, pos, end, message, field_dict):
        (new_value, pos) = decode_value(buffer, pos)
        if pos > end:
          raise _DecodeError('Truncated message.')
        if clear_if_default and not new_value:
          field_dict.pop(key, None)
        else:
          field_dict[key] = new_value
        return pos
      return DecodeField

  return SpecificDecoder


def _ModifiedDecoder(wire_type, decode_value, modify_value):
  """Like SimpleDecoder but additionally invokes modify_value on every value
  before storing it.  Usually modify_value is ZigZagDecode.
  """

  # Reusing _SimpleDecoder is slightly slower than copying a bunch of code, but
  # not enough to make a significant difference.

  def InnerDecode(buffer, pos):
    (result, new_pos) = decode_value(buffer, pos)
    return (modify_value(result), new_pos)
  return _SimpleDecoder(wire_type, InnerDecode)


def _StructPackDecoder(wire_type, format):
  """Return a constructor for a decoder for a fixed-width field.

  Args:
      wire_type:  The field's wire type.
      format:  The format string to pass to struct.unpack().
  """

  value_size = struct.calcsize(format)
  local_unpack = struct.unpack

  # Reusing _SimpleDecoder is slightly slower than copying a bunch of code, but
  # not enough to make a significant difference.

  # Note that we expect someone up-stack to catch struct.error and convert
  # it to _DecodeError -- this way we don't have to set up exception-
  # handling blocks every time we parse one value.

  def InnerDecode(buffer, pos):
    new_pos = pos + value_size
    result = local_unpack(format, buffer[pos:new_pos])[0]
    return (result, new_pos)
  return _SimpleDecoder(wire_type, InnerDecode)


def _FloatDecoder():
  """Returns a decoder for a float field.

  This code works around a bug in struct.unpack for non-finite 32-bit
  floating-point values.
  """

  local_unpack = struct.unpack

  def InnerDecode(buffer, pos):
    """Decode serialized float to a float and new position.

    Args:
      buffer: memoryview of the serialized bytes
      pos: int, position in the memory view to start at.

    Returns:
      Tuple[float, int] of the deserialized float value and new position
      in the serialized data.
    """
    # We expect a 32-bit value in little-endian byte order.  Bit 1 is the sign
    # bit, bits 2-9 represent the exponent, and bits 10-32 are the significand.
    new_pos = pos + 4
    float_bytes = buffer[pos:new_pos].tobytes()

    # If this value has all its exponent bits set, then it's non-finite.
    # In Python 2.4, struct.unpack will convert it to a finite 64-bit value.
    # To avoid that, we parse it specially.
    if (float_bytes[3:4] in b'\x7F\xFF' and float_bytes[2:3] >= b'\x80'):
      # If at least one significand bit is set...
      if float_bytes[0:3] != b'\x00\x00\x80':
        return (math.nan, new_pos)
      # If sign bit is set...
      if float_bytes[3:4] == b'\xFF':
        return (-math.inf, new_pos)
      return (math.inf, new_pos)

    # Note that we expect someone up-stack to catch struct.error and convert
    # it to _DecodeError -- this way we don't have to set up exception-
    # handling blocks every time we parse one value.
    result = local_unpack('<f', float_bytes)[0]
    return (result, new_pos)
  return _SimpleDecoder(wire_format.WIRETYPE_FIXED32, InnerDecode)


def _DoubleDecoder():
  """Returns a decoder for a double field.

  This code works around a bug in struct.unpack for not-a-number.
  """

  local_unpack = struct.unpack

  def InnerDecode(buffer, pos):
    """Decode serialized double to a double and new position.

    Args:
      buffer: memoryview of the serialized bytes.
      pos: int, position in the memory view to start at.

    Returns:
      Tuple[float, int] of the decoded double value and new position
      in the serialized data.
    """
    # We expect a 64-bit value in little-endian byte order.  Bit 1 is the sign
    # bit, bits 2-12 represent the exponent, and bits 13-64 are the significand.
    new_pos = pos + 8
    double_bytes = buffer[pos:new_pos].tobytes()

    # If this value has all its exponent bits set and at least one significand
    # bit set, it's not a number.  In Python 2.4, struct.unpack will treat it
    # as inf or -inf.  To avoid that, we treat it specially.
    if ((double_bytes[7:8] in b'\x7F\xFF')
        and (double_bytes[6:7] >= b'\xF0')
        and (double_bytes[0:7] != b'\x00\x00\x00\x00\x00\x00\xF0')):
      return (math.nan, new_pos)

    # Note that we expect someone up-stack to catch struct.error and convert
    # it to _DecodeError -- this way we don't have to set up exception-
    # handling blocks every time we parse one value.
    result = local_unpack('<d', double_bytes)[0]
    return (result, new_pos)
  return _SimpleDecoder(wire_format.WIRETYPE_FIXED64, InnerDecode)


def EnumDecoder(field_number, is_repeated, is_packed, key, new_default,
                clear_if_default=False):
  """Returns a decoder for enum field."""
  enum_type = key.enum_type
  if is_packed:
    local_DecodeVarint = _DecodeVarint
    def DecodePackedField(buffer, pos, end, message, field_dict):
      """Decode serialized packed enum to its value and a new position.

      Args:
        buffer: memoryview of the serialized bytes.
        pos: int, position in the memory view to start at.
        end: int, end position of serialized data
        message: Message object to store unknown fields in
        field_dict: Map[Descriptor, Any] to store decoded values in.

      Returns:
        int, new position in serialized data.
      """
      value = field_dict.get(key)
      if value is None:
        value = field_dict.setdefault(key, new_default(message))
      (endpoint, pos) = local_DecodeVarint(buffer, pos)
      endpoint += pos
      if endpoint > end:
        raise _DecodeError('Truncated message.')
      while pos < endpoint:
        value_start_pos = pos
        (element, pos) = _DecodeSignedVarint32(buffer, pos)
        # pylint: disable=protected-access
        if element in enum_type.values_by_number:
          value.append(element)
        else:
          if not message._unknown_fields:
            message._unknown_fields = []
          tag_bytes = encoder.TagBytes(field_number,
                                       wire_format.WIRETYPE_VARINT)

          message._unknown_fields.append(
              (tag_bytes, buffer[value_start_pos:pos].tobytes()))
          # pylint: enable=protected-access
      if pos > endpoint:
        if element in enum_type.values_by_number:
          del value[-1]   # Discard corrupt value.
        else:
          del message._unknown_fields[-1]
          # pylint: enable=protected-access
        raise _DecodeError('Packed element was truncated.')
      return pos
    return DecodePackedField
  elif is_repeated:
    tag_bytes = encoder.TagBytes(field_number, wire_format.WIRETYPE_VARINT)
    tag_len = len(tag_bytes)
    def DecodeRepeatedField(buffer, pos, end, message, field_dict):
      """Decode serialized repeated enum to its value and a new position.

      Args:
        buffer: memoryview of the serialized bytes.
        pos: int, position in the memory view to start at.
        end: int, end position of serialized data
        message: Message object to store unknown fields in
        field_dict: Map[Descriptor, Any] to store decoded values in.

      Returns:
        int, new position in serialized data.
      """
      value = field_dict.get(key)
      if value is None:
        value = field_dict.setdefault(key, new_default(message))
      while 1:
        (element, new_pos) = _DecodeSignedVarint32(buffer, pos)
        # pylint: disable=protected-access
        if element in enum_type.values_by_number:
          value.append(element)
        else:
          if not message._unknown_fields:
            message._unknown_fields = []
          message._unknown_fields.append(
              (tag_bytes, buffer[pos:new_pos].tobytes()))
        # pylint: enable=protected-access
        # Predict that the next tag is another copy of the same repeated
        # field.
        pos = new_pos + tag_len
        if buffer[new_pos:pos] != tag_bytes or new_pos >= end:
          # Prediction failed.  Return.
          if new_pos > end:
            raise _DecodeError('Truncated message.')
          return new_pos
    return DecodeRepeatedField
  else:
    def DecodeField(buffer, pos, end, message, field_dict):
      """Decode serialized repeated enum to its value and a new position.

      Args:
        buffer: memoryview of the serialized bytes.
        pos: int, position in the memory view to start at.
        end: int, end position of serialized data
        message: Message object to store unknown fields in
        field_dict: Map[Descriptor, Any] to store decoded values in.

      Returns:
        int, new position in serialized data.
      """
      value_start_pos = pos
      (enum_value, pos) = _DecodeSignedVarint32(buffer, pos)
      if pos > end:
        raise _DecodeError('Truncated message.')
      if clear_if_default and not enum_value:
        field_dict.pop(key, None)
        return pos
      # pylint: disable=protected-access
      if enum_value in enum_type.values_by_number:
        field_dict[key] = enum_value
      else:
        if not message._unknown_fields:
          message._unknown_fields = []
        tag_bytes = encoder.TagBytes(field_number,
                                     wire_format.WIRETYPE_VARINT)
        message._unknown_fields.append(
            (tag_bytes, buffer[value_start_pos:pos].tobytes()))
        # pylint: enable=protected-access
      return pos
    return DecodeField


# --------------------------------------------------------------------


Int32Decoder = _SimpleDecoder(
    wire_format.WIRETYPE_VARINT, _DecodeSignedVarint32)

Int64Decoder = _SimpleDecoder(
    wire_format.WIRETYPE_VARINT, _DecodeSignedVarint)

UInt32Decoder = _SimpleDecoder(wire_format.WIRETYPE_VARINT, _DecodeVarint32)
UInt64Decoder = _SimpleDecoder(wire_format.WIRETYPE_VARINT, _DecodeVarint)

SInt32Decoder = _ModifiedDecoder(
    wire_format.WIRETYPE_VARINT, _DecodeVarint32, wire_format.ZigZagDecode)
SInt64Decoder = _ModifiedDecoder(
    wire_format.WIRETYPE_VARINT, _DecodeVarint, wire_format.ZigZagDecode)

# Note that Python conveniently guarantees that when using the '<' prefix on
# formats, they will also have the same size across all platforms (as opposed
# to without the prefix, where their sizes depend on the C compiler's basic
# type sizes).
Fixed32Decoder  = _StructPackDecoder(wire_format.WIRETYPE_FIXED32, '<I')
Fixed64Decoder  = _StructPackDecoder(wire_format.WIRETYPE_FIXED64, '<Q')
SFixed32Decoder = _StructPackDecoder(wire_format.WIRETYPE_FIXED32, '<i')
SFixed64Decoder = _StructPackDecoder(wire_format.WIRETYPE_FIXED64, '<q')
FloatDecoder = _FloatDecoder()
DoubleDecoder = _DoubleDecoder()

BoolDecoder = _ModifiedDecoder(
    wire_format.WIRETYPE_VARINT, _DecodeVarint, bool)


def StringDecoder(field_number, is_repeated, is_packed, key, new_default,
                  clear_if_default=False):
  """Returns a decoder for a string field."""

  local_DecodeVarint = _DecodeVarint

  def _ConvertToUnicode(memview):
    """Convert byte to unicode."""
    byte_str = memview.tobytes()
    try:
      value = str(byte_str, 'utf-8')
    except UnicodeDecodeError as e:
      # add more information to the error message and re-raise it.
      e.reason = '%s in field: %s' % (e, key.full_name)
      raise

    return value

  assert not is_packed
  if is_repeated:
    tag_bytes = encoder.TagBytes(field_number,
                                 wire_format.WIRETYPE_LENGTH_DELIMITED)
    tag_len = len(tag_bytes)
    def DecodeRepeatedField(buffer, pos, end, message, field_dict):
      value = field_dict.get(key)
      if value is None:
        value = field_dict.setdefault(key, new_default(message))
      while 1:
        (size, pos) = local_DecodeVarint(buffer, pos)
        new_pos = pos + size
        if new_pos > end:
          raise _DecodeError('Truncated string.')
        value.append(_ConvertToUnicode(buffer[pos:new_pos]))
        # Predict that the next tag is another copy of the same repeated field.
        pos = new_pos + tag_len
        if buffer[new_pos:pos] != tag_bytes or new_pos == end:
          # Prediction failed.  Return.
          return new_pos
    return DecodeRepeatedField
  else:
    def DecodeField(buffer, pos, end, message, field_dict):
      (size, pos) = local_DecodeVarint(buffer, pos)
      new_pos = pos + size
      if new_pos > end:
        raise _DecodeError('Truncated string.')
      if clear_if_default and not size:
        field_dict.pop(key, None)
      else:
        field_dict[key] = _ConvertToUnicode(buffer[pos:new_pos])
      return new_pos
    return DecodeField


def BytesDecoder(field_number, is_repeated, is_packed, key, new_default,
                 clear_if_default=False):
  """Returns a decoder for a bytes field."""

  local_DecodeVarint = _DecodeVarint

  assert not is_packed
  if is_repeated:
    tag_bytes = encoder.TagBytes(field_number,
                                 wire_format.WIRETYPE_LENGTH_DELIMITED)
    tag_len = len(tag_bytes)
    def DecodeRepeatedField(buffer, pos, end, message, field_dict):
      value = field_dict.get(key)
      if value is None:
        value = field_dict.setdefault(key, new_default(message))
      while 1:
        (size, pos) = local_DecodeVarint(buffer, pos)
        new_pos = pos + size
        if new_pos > end:
          raise _DecodeError('Truncated string.')
        value.append(buffer[pos:new_pos].tobytes())
        # Predict that the next tag is another copy of the same repeated field.
        pos = new_pos + tag_len
        if buffer[new_pos:pos] != tag_bytes or new_pos == end:
          # Prediction failed.  Return.
          return new_pos
    return DecodeRepeatedField
  else:
    def DecodeField(buffer, pos, end, message, field_dict):
      (size, pos) = local_DecodeVarint(buffer, pos)
      new_pos = pos + size
      if new_pos > end:
        raise _DecodeError('Truncated string.')
      if clear_if_default and not size:
        field_dict.pop(key, None)
      else:
        field_dict[key] = buffer[pos:new_pos].tobytes()
      return new_pos
    return DecodeField


def GroupDecoder(field_number, is_repeated, is_packed, key, new_default):
  """Returns a decoder for a group field."""

  end_tag_bytes = encoder.TagBytes(field_number,
                                   wire_format.WIRETYPE_END_GROUP)
  end_tag_len = len(end_tag_bytes)

  assert not is_packed
  if is_repeated:
    tag_bytes = encoder.TagBytes(field_number,
                                 wire_format.WIRETYPE_START_GROUP)
    tag_len = len(tag_bytes)
    def DecodeRepeatedField(buffer, pos, end, message, field_dict):
      value = field_dict.get(key)
      if value is None:
        value = field_dict.setdefault(key, new_default(message))
      while 1:
        value = field_dict.get(key)
        if value is None:
          value = field_dict.setdefault(key, new_default(message))
        # Read sub-message.
        current_depth += 1
        if current_depth > _recursion_limit:
          raise _DecodeError(
              'Error parsing message: too many levels of nesting.'
          )
        pos = value.add()._InternalParse(buffer, pos, end)
        current_depth -= 1
        # Read end tag.
        new_pos = pos+end_tag_len
        if buffer[pos:new_pos] != end_tag_bytes or new_pos > end:
          raise _DecodeError('Missing group end tag.')
        # Predict that the next tag is another copy of the same repeated field.
        pos = new_pos + tag_len
        if buffer[new_pos:pos] != tag_bytes or new_pos == end:
          # Prediction failed.  Return.
          return new_pos
    return DecodeRepeatedField
  else:
    def DecodeField(buffer, pos, end, message, field_dict):
      value = field_dict.get(key)
      if value is None:
        value = field_dict.setdefault(key, new_default(message))
      # Read sub-message.
            current_depth += 1
      if current_depth > _recursion_limit:
        raise _DecodeError('Error parsing message: too many levels of nesting.')
      pos = value._InternalParse(buffer, pos, end)
      current_depth -= 1
      # Read end tag.
      new_pos = pos+end_tag_len
      if buffer[pos:new_pos] != end_tag_bytes or new_pos > end:
        raise _DecodeError('Missing group end tag.')
      return new_pos
    return DecodeField


def MessageDecoder(field_number, is_repeated, is_packed, key, new_default):
  """Returns a decoder for a message field."""

  local_DecodeVarint = _DecodeVarint

  assert not is_packed
  if is_repeated:
    tag_bytes = encoder.TagBytes(field_number,
                                 wire_format.WIRETYPE_LENGTH_DELIMITED)
    tag_len = len(tag_bytes)
    def DecodeRepeatedField(buffer, pos, end, message, field_dict):
      value = field_dict.get(key)
      if value is None:
        value = field_dict.setdefault(key, new_default(message))
      while 1:
        # Read length.
        (size, pos) = local_DecodeVarint(buffer, pos)
        new_pos = pos + size
        if new_pos > end:
          raise _DecodeError('Truncated message.')
        # Read sub-message.
        current_depth += 1
        if current_depth > _recursion_limit:
          raise _DecodeError(
              'Error parsing message: too many levels of nesting.'
          )
        if value.add()._InternalParse(buffer, pos, new_pos) != new_pos:
          # The only reason _InternalParse would return early is if it
          # encountered an end-group tag.
          raise _DecodeError('Unexpected end-group tag.')
        current_depth -= 1
        # Predict that the next tag is another copy of the same repeated field.
        pos = new_pos + tag_len
        if buffer[new_pos:pos] != tag_bytes or new_pos == end:
          # Prediction failed.  Return.
          return new_pos
    return DecodeRepeatedField
  else:
    def DecodeField(buffer, pos, end, message, field_dict):
      value = field_dict.get(key)
      if value is None:
        value = field_dict.setdefault(key, new_default(message))
      # Read length.
      (size, pos) = local_DecodeVarint(buffer, pos)
      new_pos = pos + size
      if new_pos > end:
        raise _DecodeError('Truncated message.')
      # Read sub-message.
      current_depth += 1
      if current_depth > _recursion_limit:
        raise _DecodeError('Error parsing message: too many levels of nesting.')
      if value._InternalParse(buffer, pos, new_pos) != new_pos:
        # The only reason _InternalParse would return early is if it encountered
        # an end-group tag.
        raise _DecodeError('Unexpected end-group tag.')
      current_depth -= 1
      return new_pos
    return DecodeField


# --------------------------------------------------------------------

MESSAGE_SET_ITEM_TAG = encoder.TagBytes(1, wire_format.WIRETYPE_START_GROUP)

def MessageSetItemDecoder(descriptor):
  """Returns a decoder for a MessageSet item.

  The parameter is the message Descriptor.

  The message set message looks like this:
    message MessageSet {
      repeated group Item = 1 {
        required int32 type_id = 2;
        required string message = 3;
      }
    }
  """

  type_id_tag_bytes = encoder.TagBytes(2, wire_format.WIRETYPE_VARINT)
  message_tag_bytes = encoder.TagBytes(3, wire_format.WIRETYPE_LENGTH_DELIMITED)
  item_end_tag_bytes = encoder.TagBytes(1, wire_format.WIRETYPE_END_GROUP)

  local_ReadTag = ReadTag
  local_DecodeVarint = _DecodeVarint
  local_SkipField = SkipField

  def DecodeItem(buffer, pos, end, message, field_dict):
    """Decode serialized message set to its value and new position.

    Args:
      buffer: memoryview of the serialized bytes.
      pos: int, position in the memory view to start at.
      end: int, end position of serialized data
      message: Message object to store unknown fields in
      field_dict: Map[Descriptor, Any] to store decoded values in.

    Returns:
      int, new position in serialized data.
    """
    message_set_item_start = pos
    type_id = -1
    message_start = -1
    message_end = -1

    # Technically, type_id and message can appear in any order, so we need
    # a little loop here.
    while 1:
      (tag_bytes, pos) = local_ReadTag(buffer, pos)
      if tag_bytes == type_id_tag_bytes:
        (type_id, pos) = local_DecodeVarint(buffer, pos)
      elif tag_bytes == message_tag_bytes:
        (size, message_start) = local_DecodeVarint(buffer, pos)
        pos = message_end = message_start + size
      elif tag_bytes == item_end_tag_bytes:
        break
      else:
        pos = SkipField(buffer, pos, end, tag_bytes)
        if pos == -1:
          raise _DecodeError('Missing group end tag.')

    if pos > end:
      raise _DecodeError('Truncated message.')

    if type_id == -1:
      raise _DecodeError('MessageSet item missing type_id.')
    if message_start == -1:
      raise _DecodeError('MessageSet item missing message.')

    extension = message.Extensions._FindExtensionByNumber(type_id)
    # pylint: disable=protected-access
    if extension is not None:
      value = field_dict.get(extension)
      if value is None:
        message_type = extension.message_type
        if not hasattr(message_type, '_concrete_class'):
          message_factory.GetMessageClass(message_type)
        value = field_dict.setdefault(
            extension, message_type._concrete_class())
      if value._InternalParse(buffer, message_start,message_end) != message_end:
        # The only reason _InternalParse would return early is if it encountered
        # an end-group tag.
        raise _DecodeError('Unexpected end-group tag.')
    else:
      if not message._unknown_fields:
        message._unknown_fields = []
      message._unknown_fields.append(
          (MESSAGE_SET_ITEM_TAG, buffer[message_set_item_start:pos].tobytes()))
      # pylint: enable=protected-access

    return pos

  return DecodeItem


def UnknownMessageSetItemDecoder():
  """Returns a decoder for a Unknown MessageSet item."""

  type_id_tag_bytes = encoder.TagBytes(2, wire_format.WIRETYPE_VARINT)
  message_tag_bytes = encoder.TagBytes(3, wire_format.WIRETYPE_LENGTH_DELIMITED)
  item_end_tag_bytes = encoder.TagBytes(1, wire_format.WIRETYPE_END_GROUP)

  def DecodeUnknownItem(buffer):
    pos = 0
    end = len(buffer)
    message_start = -1
    message_end = -1
    while 1:
      (tag_bytes, pos) = ReadTag(buffer, pos)
      if tag_bytes == type_id_tag_bytes:
        (type_id, pos) = _DecodeVarint(buffer, pos)
      elif tag_bytes == message_tag_bytes:
        (size, message_start) = _DecodeVarint(buffer, pos)
        pos = message_end = message_start + size
      elif tag_bytes == item_end_tag_bytes:
        break
      else:
        pos = SkipField(buffer, pos, end, tag_bytes)
        if pos == -1:
          raise _DecodeError('Missing group end tag.')

    if pos > end:
      raise _DecodeError('Truncated message.')

    if type_id == -1:
      raise _DecodeError('MessageSet item missing type_id.')
    if message_start == -1:
      raise _DecodeError('MessageSet item missing message.')

    return (type_id, buffer[message_start:message_end].tobytes())

  return DecodeUnknownItem

# --------------------------------------------------------------------

def MapDecoder(field_descriptor, new_default, is_message_map):
  """Returns a decoder for a map field."""

  key = field_descriptor
  tag_bytes = encoder.TagBytes(field_descriptor.number,
                               wire_format.WIRETYPE_LENGTH_DELIMITED)
  tag_len = len(tag_bytes)
  local_DecodeVarint = _DecodeVarint
  # Can't read _concrete_class yet; might not be initialized.
  message_type = field_descriptor.message_type

  def DecodeMap(buffer, pos, end, message, field_dict):
    submsg = message_type._concrete_class()
    value = field_dict.get(key)
    if value is None:
      value = field_dict.setdefault(key, new_default(message))
    while 1:
      # Read length.
      (size, pos) = local_DecodeVarint(buffer, pos)
      new_pos = pos + size
      if new_pos > end:
        raise _DecodeError('Truncated message.')
      # Read sub-message.
      submsg.Clear()
      if submsg._InternalParse(buffer, pos, new_pos) != new_pos:
        # The only reason _InternalParse would return early is if it
        # encountered an end-group tag.
        raise _DecodeError('Unexpected end-group tag.')

      if is_message_map:
        value[submsg.key].CopyFrom(submsg.value)
      else:
        value[submsg.key] = submsg.value

      # Predict that the next tag is another copy of the same repeated field.
      pos = new_pos + tag_len
      if buffer[new_pos:pos] != tag_bytes or new_pos == end:
        # Prediction failed.  Return.
        return new_pos

  return DecodeMap

# --------------------------------------------------------------------
# Optimization is not as heavy here because calls to SkipField() are rare,
# except for handling end-group tags.

def _SkipVarint(buffer, pos, end):
  """Skip a varint value.  Returns the new position."""
  # Previously ord(buffer[pos]) raised IndexError when pos is out of range.
  # With this code, ord(b'') raises TypeError.  Both are handled in
  # python_message.py to generate a 'Truncated message' error.
  while ord(buffer[pos:pos+1].tobytes()) & 0x80:
    pos += 1
  pos += 1
  if pos > end:
    raise _DecodeError('Truncated message.')
  return pos

def _SkipFixed64(buffer, pos, end):
  """Skip a fixed64 value.  Returns the new position."""

  pos += 8
  if pos > end:
    raise _DecodeError('Truncated message.')
  return pos


def _DecodeFixed64(buffer, pos):
  """Decode a fixed64."""
  new_pos = pos + 8
  return (struct.unpack('<Q', buffer[pos:new_pos])[0], new_pos)


def _SkipLengthDelimited(buffer, pos, end):
  """Skip a length-delimited value.  Returns the new position."""

  (size, pos) = _DecodeVarint(buffer, pos)
  pos += size
  if pos > end:
    raise _DecodeError('Truncated message.')
  return pos


def _SkipGroup(buffer, pos, end):
  """Skip sub-group.  Returns the new position."""

  while 1:
    (tag_bytes, pos) = ReadTag(buffer, pos)
    new_pos = SkipField(buffer, pos, end, tag_bytes)
    if new_pos == -1:
      return pos
    pos = new_pos


def _DecodeUnknownFieldSet(buffer, pos, end_pos=None):
  """Decode UnknownFieldSet.  Returns the UnknownFieldSet and new position."""

  unknown_field_set = containers.UnknownFieldSet()
  while end_pos is None or pos < end_pos:
    (tag_bytes, pos) = ReadTag(buffer, pos)
    (tag, _) = _DecodeVarint(tag_bytes, 0)
    field_number, wire_type = wire_format.UnpackTag(tag)
    if wire_type == wire_format.WIRETYPE_END_GROUP:
      break
    (data, pos) = _DecodeUnknownField(buffer, pos, wire_type)
    # pylint: disable=protected-access
    unknown_field_set._add(field_number, wire_type, data)

  return (unknown_field_set, pos)


def _DecodeUnknownField(buffer, pos, wire_type):
  """Decode a unknown field.  Returns the UnknownField and new position."""

  if wire_type == wire_format.WIRETYPE_VARINT:
    (data, pos) = _DecodeVarint(buffer, pos)
  elif wire_type == wire_format.WIRETYPE_FIXED64:
    (data, pos) = _DecodeFixed64(buffer, pos)
  elif wire_type == wire_format.WIRETYPE_FIXED32:
    (data, pos) = _DecodeFixed32(buffer, pos)
  elif wire_type == wire_format.WIRETYPE_LENGTH_DELIMITED:
    (size, pos) = _DecodeVarint(buffer, pos)
    data = buffer[pos:pos+size].tobytes()
    pos += size
  elif wire_type == wire_format.WIRETYPE_START_GROUP:
    current_depth += 1
    if current_depth >= _recursion_limit:
      raise _DecodeError('Error parsing message: too many levels of nesting.')
    (data, pos) = _DecodeUnknownFieldSet(buffer, pos)
    current_depth -= 1
  elif wire_type == wire_format.WIRETYPE_END_GROUP:
    return (0, -1)
  else:
    raise _DecodeError('Wrong wire type in tag.')

  return (data, pos)


def _EndGroup(buffer, pos, end):
  """Skipping an END_GROUP tag returns -1 to tell the parent loop to break."""

  return -1


def _SkipFixed32(buffer, pos, end):
  """Skip a fixed32 value.  Returns the new position."""

  pos += 4
  if pos > end:
    raise _DecodeError('Truncated message.')
  return pos


def _DecodeFixed32(buffer, pos):
  """Decode a fixed32."""

  new_pos = pos + 4
  return (struct.unpack('<I', buffer[pos:new_pos])[0], new_pos)

DEFAULT_RECURSION_LIMIT = 100
_recursion_limit = DEFAULT_RECURSION_LIMIT


def SetRecursionLimit(new_limit):
  global _recursion_limit
  _recursion_limit = new_limit


def _RaiseInvalidWireType(buffer, pos, end):
  """Skip function for unknown wire types.  Raises an exception."""

  raise _DecodeError('Tag had invalid wire type.')

def _FieldSkipper():
  """Constructs the SkipField function."""

  WIRETYPE_TO_SKIPPER = [
      _SkipVarint,
      _SkipFixed64,
      _SkipLengthDelimited,
      _SkipGroup,
      _EndGroup,
      _SkipFixed32,
      _RaiseInvalidWireType,
      _RaiseInvalidWireType,
      ]

  wiretype_mask = wire_format.TAG_TYPE_MASK

  def SkipField(buffer, pos, end, tag_bytes):
    """Skips a field with the specified tag.

    |pos| should point to the byte immediately after the tag.

    Returns:
        The new position (after the tag value), or -1 if the tag is an end-group
        tag (in which case the calling loop should break).
    """

    # The wire type is always in the first byte since varints are little-endian.
    wire_type = ord(tag_bytes[0:1]) & wiretype_mask
    return WIRETYPE_TO_SKIPPER[wire_type](buffer, pos, end)

  return SkipField

SkipField = _FieldSkipper()