148 lines
5.0 KiB
Python
148 lines
5.0 KiB
Python
#!/usr/bin/env python
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#
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# Copyright 2017 Google Inc.
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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"""Compression support for apitools."""
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from collections import deque
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from apitools.base.py import gzip
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__all__ = [
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'CompressStream',
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]
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# pylint: disable=invalid-name
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# Note: Apitools only uses the default chunksize when compressing.
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def CompressStream(in_stream, length=None, compresslevel=2,
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chunksize=16777216):
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"""Compresses an input stream into a file-like buffer.
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This reads from the input stream until either we've stored at least length
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compressed bytes, or the input stream has been exhausted.
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This supports streams of unknown size.
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Args:
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in_stream: The input stream to read from.
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length: The target number of compressed bytes to buffer in the output
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stream. If length is none, the input stream will be compressed
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until it's exhausted.
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The actual length of the output buffer can vary from the target.
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If the input stream is exhaused, the output buffer may be smaller
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than expected. If the data is incompressible, the maximum length
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can be exceeded by can be calculated to be:
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chunksize + 5 * (floor((chunksize - 1) / 16383) + 1) + 17
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This accounts for additional header data gzip adds. For the default
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16MiB chunksize, this results in the max size of the output buffer
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being:
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length + 16Mib + 5142 bytes
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compresslevel: Optional, defaults to 2. The desired compression level.
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chunksize: Optional, defaults to 16MiB. The chunk size used when
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reading data from the input stream to write into the output
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buffer.
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Returns:
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A file-like output buffer of compressed bytes, the number of bytes read
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from the input stream, and a flag denoting if the input stream was
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exhausted.
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"""
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in_read = 0
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in_exhausted = False
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out_stream = StreamingBuffer()
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with gzip.GzipFile(mode='wb',
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fileobj=out_stream,
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compresslevel=compresslevel) as compress_stream:
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# Read until we've written at least length bytes to the output stream.
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while not length or out_stream.length < length:
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data = in_stream.read(chunksize)
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data_length = len(data)
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compress_stream.write(data)
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in_read += data_length
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# If we read less than requested, the stream is exhausted.
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if data_length < chunksize:
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in_exhausted = True
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break
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return out_stream, in_read, in_exhausted
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class StreamingBuffer(object):
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"""Provides a file-like object that writes to a temporary buffer.
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When data is read from the buffer, it is permanently removed. This is
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useful when there are memory constraints preventing the entire buffer from
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being stored in memory.
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"""
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def __init__(self):
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# The buffer of byte arrays.
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self.__buf = deque()
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# The number of bytes in __buf.
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self.__size = 0
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def __len__(self):
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return self.__size
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def __nonzero__(self):
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# For 32-bit python2.x, len() cannot exceed a 32-bit number; avoid
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# accidental len() calls from httplib in the form of "if this_object:".
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return bool(self.__size)
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@property
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def length(self):
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# For 32-bit python2.x, len() cannot exceed a 32-bit number.
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return self.__size
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def write(self, data):
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# Gzip can write many 0 byte chunks for highly compressible data.
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# Prevent them from being added internally.
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if data is not None and data:
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self.__buf.append(data)
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self.__size += len(data)
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def read(self, size=None):
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"""Read at most size bytes from this buffer.
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Bytes read from this buffer are consumed and are permanently removed.
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Args:
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size: If provided, read no more than size bytes from the buffer.
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Otherwise, this reads the entire buffer.
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Returns:
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The bytes read from this buffer.
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"""
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if size is None:
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size = self.__size
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ret_list = []
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while size > 0 and self.__buf:
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data = self.__buf.popleft()
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size -= len(data)
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ret_list.append(data)
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if size < 0:
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ret_list[-1], remainder = ret_list[-1][:size], ret_list[-1][size:]
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self.__buf.appendleft(remainder)
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ret = b''.join(ret_list)
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self.__size -= len(ret)
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return ret
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