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#
# This file is part of pyasn1 software.
#
# Copyright (c) 2005-2019, Ilya Etingof <etingof@gmail.com>
# License: http://snmplabs.com/pyasn1/license.html
#
import io
import os
import sys
from pyasn1 import error
from pyasn1.type import univ
_PY2 = sys.version_info < (3,)
class CachingStreamWrapper(io.IOBase):
"""Wrapper around non-seekable streams.
Note that the implementation is tied to the decoder,
not checking for dangerous arguments for the sake
of performance.
The read bytes are kept in an internal cache until
setting _markedPosition which may reset the cache.
"""
def __init__(self, raw):
self._raw = raw
self._cache = io.BytesIO()
self._markedPosition = 0
def peek(self, n):
result = self.read(n)
self._cache.seek(-len(result), os.SEEK_CUR)
return result
def seekable(self):
return True
def seek(self, n=-1, whence=os.SEEK_SET):
# Note that this not safe for seeking forward.
return self._cache.seek(n, whence)
def read(self, n=-1):
read_from_cache = self._cache.read(n)
if n != -1:
n -= len(read_from_cache)
if not n: # 0 bytes left to read
return read_from_cache
read_from_raw = self._raw.read(n)
self._cache.write(read_from_raw)
return read_from_cache + read_from_raw
@property
def markedPosition(self):
"""Position where the currently processed element starts.
This is used for back-tracking in SingleItemDecoder.__call__
and (indefLen)ValueDecoder and should not be used for other purposes.
The client is not supposed to ever seek before this position.
"""
return self._markedPosition
@markedPosition.setter
def markedPosition(self, value):
# By setting the value, we ensure we won't seek back before it.
# `value` should be the same as the current position
# We don't check for this for performance reasons.
self._markedPosition = value
# Whenever we set _marked_position, we know for sure
# that we will not return back, and thus it is
# safe to drop all cached data.
if self._cache.tell() > io.DEFAULT_BUFFER_SIZE:
self._cache = io.BytesIO(self._cache.read())
self._markedPosition = 0
def tell(self):
return self._cache.tell()
def asSeekableStream(substrate):
"""Convert object to seekable byte-stream.
Parameters
----------
substrate: :py:class:`bytes` or :py:class:`io.IOBase` or :py:class:`univ.OctetString`
Returns
-------
: :py:class:`io.IOBase`
Raises
------
: :py:class:`~pyasn1.error.PyAsn1Error`
If the supplied substrate cannot be converted to a seekable stream.
"""
if isinstance(substrate, bytes):
return io.BytesIO(substrate)
elif isinstance(substrate, univ.OctetString):
return io.BytesIO(substrate.asOctets())
try:
# Special case: impossible to set attributes on `file` built-in
if _PY2 and isinstance(substrate, file):
return io.BufferedReader(substrate)
elif substrate.seekable(): # Will fail for most invalid types
return substrate
else:
return CachingStreamWrapper(substrate)
except AttributeError:
raise error.UnsupportedSubstrateError(
"Cannot convert " + substrate.__class__.__name__ +
" to a seekable bit stream.")
def isEndOfStream(substrate):
"""Check whether we have reached the end of a stream.
Although it is more effective to read and catch exceptions, this
function
Parameters
----------
substrate: :py:class:`IOBase`
Stream to check
Returns
-------
: :py:class:`bool`
"""
if isinstance(substrate, io.BytesIO):
cp = substrate.tell()
substrate.seek(0, os.SEEK_END)
result = substrate.tell() == cp
substrate.seek(cp, os.SEEK_SET)
yield result
else:
received = substrate.read(1)
if received is None:
yield
if received:
substrate.seek(-1, os.SEEK_CUR)
yield not received
def peek(substrate, size=-1):
"""Peek the stream.
Parameters
----------
substrate: :py:class:`IOBase`
Stream to read from.
size: :py:class:`int`
How many bytes to peek (-1 = all available)
Returns
-------
: :py:class:`bytes` or :py:class:`str`
The return type depends on Python major version
"""
if hasattr(substrate, "peek"):
received = substrate.peek(size)
if received is None:
yield
while len(received) < size:
yield
yield received
else:
current_position = substrate.tell()
try:
for chunk in read(substrate, size):
yield chunk
finally:
substrate.seek(current_position)
def read(substrate, size=-1, context=None):
"""Read from the stream.
Parameters
----------
substrate: :py:class:`IOBase`
Stream to read from.
Keyword parameters
------------------
size: :py:class:`int`
How many bytes to read (-1 = all available)
context: :py:class:`dict`
Opaque caller context will be attached to exception objects created
by this function.
Yields
------
: :py:class:`bytes` or :py:class:`str` or :py:class:`SubstrateUnderrunError`
Returns read data or :py:class:`~pyasn1.error.SubstrateUnderrunError`
object if no `size` bytes is readily available in the stream. The
data type depends on Python major version
Raises
------
: :py:class:`~pyasn1.error.EndOfStreamError`
Input stream is exhausted
"""
while True:
# this will block unless stream is non-blocking
received = substrate.read(size)
if received is None: # non-blocking stream can do this
yield error.SubstrateUnderrunError(context=context)
elif size != 0 and not received: # end-of-stream
raise error.EndOfStreamError(context=context)
elif len(received) < size:
substrate.seek(-len(received), os.SEEK_CUR)
# behave like a non-blocking stream
yield error.SubstrateUnderrunError(context=context)
else:
break
yield received
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