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112 lines
3.9 KiB
Python
112 lines
3.9 KiB
Python
2 years ago
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# SPDX-FileCopyrightText: 2015 Eric Larson
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#
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# SPDX-License-Identifier: Apache-2.0
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from tempfile import NamedTemporaryFile
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import mmap
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class CallbackFileWrapper(object):
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"""
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Small wrapper around a fp object which will tee everything read into a
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buffer, and when that file is closed it will execute a callback with the
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contents of that buffer.
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All attributes are proxied to the underlying file object.
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This class uses members with a double underscore (__) leading prefix so as
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not to accidentally shadow an attribute.
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The data is stored in a temporary file until it is all available. As long
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as the temporary files directory is disk-based (sometimes it's a
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memory-backed-``tmpfs`` on Linux), data will be unloaded to disk if memory
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pressure is high. For small files the disk usually won't be used at all,
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it'll all be in the filesystem memory cache, so there should be no
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performance impact.
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"""
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def __init__(self, fp, callback):
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self.__buf = NamedTemporaryFile("rb+", delete=True)
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self.__fp = fp
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self.__callback = callback
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def __getattr__(self, name):
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# The vaguaries of garbage collection means that self.__fp is
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# not always set. By using __getattribute__ and the private
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# name[0] allows looking up the attribute value and raising an
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# AttributeError when it doesn't exist. This stop thigns from
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# infinitely recursing calls to getattr in the case where
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# self.__fp hasn't been set.
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#
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# [0] https://docs.python.org/2/reference/expressions.html#atom-identifiers
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fp = self.__getattribute__("_CallbackFileWrapper__fp")
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return getattr(fp, name)
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def __is_fp_closed(self):
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try:
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return self.__fp.fp is None
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except AttributeError:
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pass
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try:
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return self.__fp.closed
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except AttributeError:
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pass
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# We just don't cache it then.
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# TODO: Add some logging here...
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return False
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def _close(self):
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if self.__callback:
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if self.__buf.tell() == 0:
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# Empty file:
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result = b""
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else:
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# Return the data without actually loading it into memory,
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# relying on Python's buffer API and mmap(). mmap() just gives
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# a view directly into the filesystem's memory cache, so it
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# doesn't result in duplicate memory use.
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self.__buf.seek(0, 0)
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result = memoryview(
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mmap.mmap(self.__buf.fileno(), 0, access=mmap.ACCESS_READ)
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)
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self.__callback(result)
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# We assign this to None here, because otherwise we can get into
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# really tricky problems where the CPython interpreter dead locks
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# because the callback is holding a reference to something which
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# has a __del__ method. Setting this to None breaks the cycle
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# and allows the garbage collector to do it's thing normally.
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self.__callback = None
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# Closing the temporary file releases memory and frees disk space.
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# Important when caching big files.
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self.__buf.close()
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def read(self, amt=None):
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data = self.__fp.read(amt)
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if data:
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# We may be dealing with b'', a sign that things are over:
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# it's passed e.g. after we've already closed self.__buf.
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self.__buf.write(data)
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if self.__is_fp_closed():
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self._close()
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return data
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def _safe_read(self, amt):
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data = self.__fp._safe_read(amt)
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if amt == 2 and data == b"\r\n":
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# urllib executes this read to toss the CRLF at the end
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# of the chunk.
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return data
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self.__buf.write(data)
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if self.__is_fp_closed():
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self._close()
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return data
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