253 lines
9.4 KiB
Python
253 lines
9.4 KiB
Python
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""" This module implements python API for the FIEMAP ioctl. The FIEMAP ioctl
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allows to find holes and mapped areas in a file. """
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# Note, a lot of code in this module is not very readable, because it deals
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# with the rather complex FIEMAP ioctl. To understand the code, you need to
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# know the FIEMAP interface, which is documented in the
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# Documentation/filesystems/fiemap.txt file in the Linux kernel sources.
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# Disable the following pylint recommendations:
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# * Too many instance attributes (R0902)
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# pylint: disable=R0902
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import os
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import struct
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import array
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import fcntl
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from mic.utils.misc import get_block_size
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# Format string for 'struct fiemap'
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_FIEMAP_FORMAT = "=QQLLLL"
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# sizeof(struct fiemap)
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_FIEMAP_SIZE = struct.calcsize(_FIEMAP_FORMAT)
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# Format string for 'struct fiemap_extent'
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_FIEMAP_EXTENT_FORMAT = "=QQQQQLLLL"
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# sizeof(struct fiemap_extent)
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_FIEMAP_EXTENT_SIZE = struct.calcsize(_FIEMAP_EXTENT_FORMAT)
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# The FIEMAP ioctl number
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_FIEMAP_IOCTL = 0xC020660B
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# Minimum buffer which is required for 'class Fiemap' to operate
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MIN_BUFFER_SIZE = _FIEMAP_SIZE + _FIEMAP_EXTENT_SIZE
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# The default buffer size for 'class Fiemap'
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DEFAULT_BUFFER_SIZE = 256 * 1024
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class Error(Exception):
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""" A class for exceptions generated by this module. We currently support
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only one type of exceptions, and we basically throw human-readable problem
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description in case of errors. """
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pass
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class Fiemap:
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""" This class provides API to the FIEMAP ioctl. Namely, it allows to
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iterate over all mapped blocks and over all holes. """
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def _open_image_file(self):
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""" Open the image file. """
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try:
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self._f_image = open(self._image_path, 'rb')
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except IOError as err:
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raise Error("cannot open image file '%s': %s" \
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% (self._image_path, err))
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self._f_image_needs_close = True
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def __init__(self, image, buf_size = DEFAULT_BUFFER_SIZE):
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""" Initialize a class instance. The 'image' argument is full path to
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the file to operate on, or a file object to operate on.
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The 'buf_size' argument is the size of the buffer for 'struct
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fiemap_extent' elements which will be used when invoking the FIEMAP
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ioctl. The larger is the buffer, the less times the FIEMAP ioctl will
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be invoked. """
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self._f_image_needs_close = False
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if hasattr(image, "fileno"):
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self._f_image = image
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self._image_path = image.name
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else:
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self._image_path = image
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self._open_image_file()
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# Validate 'buf_size'
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if buf_size < MIN_BUFFER_SIZE:
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raise Error("too small buffer (%d bytes), minimum is %d bytes" \
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% (buf_size, MIN_BUFFER_SIZE))
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# How many 'struct fiemap_extent' elements fit the buffer
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buf_size -= _FIEMAP_SIZE
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self._fiemap_extent_cnt = buf_size / _FIEMAP_EXTENT_SIZE
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self._buf_size = self._fiemap_extent_cnt * _FIEMAP_EXTENT_SIZE
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self._buf_size += _FIEMAP_SIZE
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# Allocate a mutable buffer for the FIEMAP ioctl
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self._buf = array.array('B', [0] * self._buf_size)
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self.image_size = os.fstat(self._f_image.fileno()).st_size
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try:
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self.block_size = get_block_size(self._f_image)
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except IOError as err:
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raise Error("cannot get block size for '%s': %s" \
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% (self._image_path, err))
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self.blocks_cnt = self.image_size + self.block_size - 1
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self.blocks_cnt /= self.block_size
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# Synchronize the image file to make sure FIEMAP returns correct values
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try:
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self._f_image.flush()
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except IOError as err:
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raise Error("cannot flush image file '%s': %s" \
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% (self._image_path, err))
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try:
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os.fsync(self._f_image.fileno()),
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except OSError as err:
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raise Error("cannot synchronize image file '%s': %s " \
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% (self._image_path, err.strerror))
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# Check if the FIEMAP ioctl is supported
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self.block_is_mapped(0)
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def __del__(self):
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""" The class destructor which closes the opened files. """
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if self._f_image_needs_close:
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self._f_image.close()
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def _invoke_fiemap(self, block, count):
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""" Invoke the FIEMAP ioctl for 'count' blocks of the file starting from
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block number 'block'.
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The full result of the operation is stored in 'self._buf' on exit.
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Returns the unpacked 'struct fiemap' data structure in form of a python
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list (just like 'struct.upack()'). """
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if block < 0 or block >= self.blocks_cnt:
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raise Error("bad block number %d, should be within [0, %d]" \
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% (block, self.blocks_cnt))
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# Initialize the 'struct fiemap' part of the buffer
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struct.pack_into(_FIEMAP_FORMAT, self._buf, 0, block * self.block_size,
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count * self.block_size, 0, 0,
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self._fiemap_extent_cnt, 0)
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try:
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fcntl.ioctl(self._f_image, _FIEMAP_IOCTL, self._buf, 1)
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except IOError as err:
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error_msg = "the FIEMAP ioctl failed for '%s': %s" \
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% (self._image_path, err)
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if err.errno == os.errno.EPERM or err.errno == os.errno.EACCES:
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# The FIEMAP ioctl was added in kernel version 2.6.28 in 2008
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error_msg += " (looks like your kernel does not support FIEMAP)"
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raise Error(error_msg)
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return struct.unpack(_FIEMAP_FORMAT, self._buf[:_FIEMAP_SIZE])
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def block_is_mapped(self, block):
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""" This function returns 'True' if block number 'block' of the image
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file is mapped and 'False' otherwise. """
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struct_fiemap = self._invoke_fiemap(block, 1)
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# The 3rd element of 'struct_fiemap' is the 'fm_mapped_extents' field.
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# If it contains zero, the block is not mapped, otherwise it is
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# mapped.
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return bool(struct_fiemap[3])
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def block_is_unmapped(self, block):
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""" This function returns 'True' if block number 'block' of the image
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file is not mapped (hole) and 'False' otherwise. """
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return not self.block_is_mapped(block)
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def _unpack_fiemap_extent(self, index):
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""" Unpack a 'struct fiemap_extent' structure object number 'index'
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from the internal 'self._buf' buffer. """
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offset = _FIEMAP_SIZE + _FIEMAP_EXTENT_SIZE * index
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return struct.unpack(_FIEMAP_EXTENT_FORMAT,
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self._buf[offset : offset + _FIEMAP_EXTENT_SIZE])
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def _do_get_mapped_ranges(self, start, count):
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""" Implements most the functionality for the 'get_mapped_ranges()'
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generator: invokes the FIEMAP ioctl, walks through the mapped
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extents and yields mapped block ranges. However, the ranges may be
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consecutive (e.g., (1, 100), (100, 200)) and 'get_mapped_ranges()'
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simply merges them. """
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block = start
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while block < start + count:
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struct_fiemap = self._invoke_fiemap(block, count)
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mapped_extents = struct_fiemap[3]
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if mapped_extents == 0:
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# No more mapped blocks
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return
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extent = 0
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while extent < mapped_extents:
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fiemap_extent = self._unpack_fiemap_extent(extent)
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# Start of the extent
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extent_start = fiemap_extent[0]
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# Starting block number of the extent
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extent_block = extent_start / self.block_size
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# Length of the extent
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extent_len = fiemap_extent[2]
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# Count of blocks in the extent
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extent_count = extent_len / self.block_size
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# Extent length and offset have to be block-aligned
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assert extent_start % self.block_size == 0
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assert extent_len % self.block_size == 0
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if extent_block > start + count - 1:
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return
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first = max(extent_block, block)
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last = min(extent_block + extent_count, start + count) - 1
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yield (first, last)
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extent += 1
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block = extent_block + extent_count
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def get_mapped_ranges(self, start, count):
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""" A generator which yields ranges of mapped blocks in the file. The
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ranges are tuples of 2 elements: [first, last], where 'first' is the
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first mapped block and 'last' is the last mapped block.
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The ranges are yielded for the area of the file of size 'count' blocks,
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starting from block 'start'. """
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iterator = self._do_get_mapped_ranges(start, count)
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first_prev, last_prev = iterator.next()
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for first, last in iterator:
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if last_prev == first - 1:
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last_prev = last
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else:
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yield (first_prev, last_prev)
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first_prev, last_prev = first, last
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yield (first_prev, last_prev)
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def get_unmapped_ranges(self, start, count):
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""" Just like 'get_mapped_ranges()', but yields unmapped block ranges
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instead (holes). """
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hole_first = start
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for first, last in self._do_get_mapped_ranges(start, count):
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if first > hole_first:
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yield (hole_first, first - 1)
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hole_first = last + 1
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if hole_first < start + count:
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yield (hole_first, start + count - 1)
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