filemap.py 文件源码

def _probe_seek_hole(self):
        """
        Check whether the system implements 'SEEK_HOLE' and 'SEEK_DATA'.
        Unfortunately, there seems to be no clean way for detecting this,
        because often the system just fakes them by just assuming that all
        files are fully mapped, so 'SEEK_HOLE' always returns EOF and
        'SEEK_DATA' always returns the requested offset.

        I could not invent a better way of detecting the fake 'SEEK_HOLE'
        implementation than just to create a temporary file in the same
        directory where the image file resides. It would be nice to change this
        to something better.
        """

        directory = os.path.dirname(self._image_path)

        try:
            tmp_obj = tempfile.TemporaryFile("w+", dir=directory)
        except IOError as err:
            raise ErrorNotSupp("cannot create a temporary in \"%s\": %s"
                              % (directory, err))

        try:
            os.ftruncate(tmp_obj.fileno(), self.block_size)
        except OSError as err:
            raise ErrorNotSupp("cannot truncate temporary file in \"%s\": %s"
                               % (directory, err))

        offs = _lseek(tmp_obj, 0, _SEEK_HOLE)
        if offs != 0:
            # We are dealing with the stub 'SEEK_HOLE' implementation which
            # always returns EOF.
            self._log.debug("lseek(0, SEEK_HOLE) returned %d" % offs)
            raise ErrorNotSupp("the file-system does not support "
                               "\"SEEK_HOLE\" and \"SEEK_DATA\" but only "
                               "provides a stub implementation")

        tmp_obj.close()