client/bin/partition.py

"""
APIs to write tests and control files that handle partition creation, deletion
and formatting.

@copyright: Google 2006-2008
@author: Martin Bligh (mbligh@google.com)
"""

# pylint: disable=missing-docstring

import os, re, string, sys, fcntl, logging
from autotest_lib.client.bin import os_dep, utils
from autotest_lib.client.common_lib import error


class FsOptions(object):
    """
    A class encapsulating a filesystem test's parameters.
    """
    # NOTE(gps): This class could grow or be merged with something else in the
    # future that actually uses the encapsulated data (say to run mkfs) rather
    # than just being a container.

    __slots__ = ('fstype', 'mkfs_flags', 'mount_options', 'fs_tag')

    def __init__(self, fstype, fs_tag, mkfs_flags=None, mount_options=None):
        """
        Fill in our properties.

        @param fstype: The filesystem type ('ext2', 'ext4', 'xfs', etc.)
        @param fs_tag: A short name for this filesystem test to use
                in the results.
        @param mkfs_flags: Optional. Additional command line options to mkfs.
        @param mount_options: Optional. The options to pass to mount -o.
        """

        if not fstype or not fs_tag:
            raise ValueError('A filesystem and fs_tag are required.')
        self.fstype = fstype
        self.fs_tag = fs_tag
        self.mkfs_flags = mkfs_flags or ""
        self.mount_options = mount_options or ""


    def __str__(self):
        val = ('FsOptions(fstype=%r, mkfs_flags=%r, '
               'mount_options=%r, fs_tag=%r)' %
               (self.fstype, self.mkfs_flags,
                self.mount_options, self.fs_tag))
        return val


def partname_to_device(part):
    """ Converts a partition name to its associated device """
    return os.path.join(os.sep, 'dev', part)


def list_mount_devices():
    devices = []
    # list mounted filesystems
    for line in utils.system_output('mount').splitlines():
        devices.append(line.split()[0])
    # list mounted swap devices
    for line in utils.system_output('swapon -s').splitlines():
        if line.startswith('/'):        # skip header line
            devices.append(line.split()[0])
    return devices


def list_mount_points():
    mountpoints = []
    for line in utils.system_output('mount').splitlines():
        mountpoints.append(line.split()[2])
    return mountpoints


def get_iosched_path(device_name, component):
    return '/sys/block/%s/queue/%s' % (device_name, component)


def wipe_filesystem(job, mountpoint):
    wipe_cmd = 'rm -rf %s/*' % mountpoint
    try:
        utils.system(wipe_cmd)
    except:
        job.record('FAIL', None, wipe_cmd, error.format_error())
        raise
    else:
        job.record('GOOD', None, wipe_cmd)


def is_linux_fs_type(device):
    """
    Checks if specified partition is type 83

    @param device: the device, e.g. /dev/sda3

    @return: False if the supplied partition name is not type 83 linux, True
            otherwise
    """
    disk_device = device.rstrip('0123456789')

    # Parse fdisk output to get partition info.  Ugly but it works.
    fdisk_fd = os.popen("/sbin/fdisk -l -u '%s'" % disk_device)
    fdisk_lines = fdisk_fd.readlines()
    fdisk_fd.close()
    for line in fdisk_lines:
        if not line.startswith(device):
            continue
        info_tuple = line.split()
        # The Id will be in one of two fields depending on if the boot flag
        # was set.  Caveat: this assumes no boot partition will be 83 blocks.
        for fsinfo in info_tuple[4:6]:
            if fsinfo == '83':  # hex 83 is the linux fs partition type
                return True
    return False


def get_partition_list(job, min_blocks=0, filter_func=None, exclude_swap=True,
                       open_func=open):
    """
    Get a list of partition objects for all disk partitions on the system.

    Loopback devices and unnumbered (whole disk) devices are always excluded.

    @param job: The job instance to pass to the partition object
            constructor.
    @param min_blocks: The minimum number of blocks for a partition to
            be considered.
    @param filter_func: A callable that returns True if a partition is
            desired. It will be passed one parameter:
            The partition name (hdc3, etc.).
            Some useful filter functions are already defined in this module.
    @param exclude_swap: If True any partition actively in use as a swap
            device will be excluded.
    @param __open: Reserved for unit testing.

    @return: A list of L{partition} objects.
    """
    active_swap_devices = set()
    if exclude_swap:
        for swapline in open_func('/proc/swaps'):
            if swapline.startswith('/'):
                active_swap_devices.add(swapline.split()[0])

    partitions = []
    for partline in open_func('/proc/partitions').readlines():
        fields = partline.strip().split()
        if len(fields) != 4 or partline.startswith('major'):
            continue
        (major, minor, blocks, partname) = fields
        blocks = int(blocks)

        # The partition name better end with a digit, else it's not a partition
        if not partname[-1].isdigit():
            continue

        # We don't want the loopback device in the partition list
        if 'loop' in partname:
            continue

        device = partname_to_device(partname)
        if exclude_swap and device in active_swap_devices:
            logging.debug('Skipping %s - Active swap.', partname)
            continue

        if min_blocks and blocks < min_blocks:
            logging.debug('Skipping %s - Too small.', partname)
            continue

        if filter_func and not filter_func(partname):
            logging.debug('Skipping %s - Filter func.', partname)
            continue

        partitions.append(partition(job, device))

    return partitions


def get_mount_info(partition_list):
    """
    Picks up mount point information about the machine mounts. By default, we
    try to associate mount points with UUIDs, because in newer distros the
    partitions are uniquely identified using them.
    """
    mount_info = set()
    for p in partition_list:
        try:
            uuid = utils.system_output('blkid -p -s UUID -o value %s' % p.device)
        except error.CmdError:
            # fall back to using the partition
            uuid = p.device
        mount_info.add((uuid, p.get_mountpoint()))

    return mount_info


def filter_partition_list(partitions, devnames):
    """
    Pick and choose which partition to keep.

    filter_partition_list accepts a list of partition objects and a list
    of strings.  If a partition has the device name of the strings it
    is returned in a list.

    @param partitions: A list of L{partition} objects
    @param devnames: A list of devnames of the form '/dev/hdc3' that
                    specifies which partitions to include in the returned list.

    @return: A list of L{partition} objects specified by devnames, in the
             order devnames specified
    """

    filtered_list = []
    for p in partitions:
        for d in devnames:
            if p.device == d and p not in filtered_list:
                filtered_list.append(p)

    return filtered_list


def get_unmounted_partition_list(root_part, job=None, min_blocks=0,
                                 filter_func=None, exclude_swap=True,
                                 open_func=open):
    """
    Return a list of partition objects that are not mounted.

    @param root_part: The root device name (without the '/dev/' prefix, example
            'hda2') that will be filtered from the partition list.

            Reasoning: in Linux /proc/mounts will never directly mention the
            root partition as being mounted on / instead it will say that
            /dev/root is mounted on /. Thus require this argument to filter out
            the root_part from the ones checked to be mounted.
    @param job, min_blocks, filter_func, exclude_swap, open_func: Forwarded
            to get_partition_list().
    @return List of L{partition} objects that are not mounted.
    """
    partitions = get_partition_list(job=job, min_blocks=min_blocks,
        filter_func=filter_func, exclude_swap=exclude_swap, open_func=open_func)

    unmounted = []
    for part in partitions:
        if (part.device != partname_to_device(root_part) and
            not part.get_mountpoint(open_func=open_func)):
            unmounted.append(part)

    return unmounted


def parallel(partitions, method_name, *args, **dargs):
    """
    Run a partition method (with appropriate arguments) in parallel,
    across a list of partition objects
    """
    if not partitions:
        return
    job = partitions[0].job
    flist = []
    if (not hasattr(partition, method_name) or
                               not callable(getattr(partition, method_name))):
        err = "partition.parallel got invalid method %s" % method_name
        raise RuntimeError(err)

    for p in partitions:
        print_args = list(args)
        print_args += ['%s=%s' % (key, dargs[key]) for key in dargs.keys()]
        logging.debug('%s.%s(%s)', str(p), method_name,
                                     ', '.join(print_args))
        sys.stdout.flush()
        def _run_named_method(function, part=p):
            getattr(part, method_name)(*args, **dargs)
        flist.append((_run_named_method, ()))
    job.parallel(*flist)


def filesystems():
    """
    Return a list of all available filesystems
    """
    return [re.sub('(nodev)?\s*', '', fs) for fs in open('/proc/filesystems')]


def unmount_partition(device):
    """
    Unmount a mounted partition

    @param device: e.g. /dev/sda1, /dev/hda1
    """
    p = partition(job=None, device=device)
    p.unmount(record=False)


def is_valid_partition(device):
    """
    Checks if a partition is valid

    @param device: e.g. /dev/sda1, /dev/hda1
    """
    parts = get_partition_list(job=None)
    p_list = [ p.device for p in parts ]
    if device in p_list:
        return True

    return False


def is_valid_disk(device):
    """
    Checks if a disk is valid

    @param device: e.g. /dev/sda, /dev/hda
    """
    partitions = []
    for partline in open('/proc/partitions').readlines():
        fields = partline.strip().split()
        if len(fields) != 4 or partline.startswith('major'):
            continue
        (major, minor, blocks, partname) = fields
        blocks = int(blocks)

        if not partname[-1].isdigit():
            # Disk name does not end in number, AFAIK
            # so use it as a reference to a disk
            if device.strip("/dev/") == partname:
                return True

    return False


def run_test_on_partitions(job, test, partitions, mountpoint_func,
                           tag, fs_opt, do_fsck=True, **dargs):
    """
    Run a test that requires multiple partitions.  Filesystems will be
    made on the partitions and mounted, then the test will run, then the
    filesystems will be unmounted and optionally fsck'd.

    @param job: A job instance to run the test
    @param test: A string containing the name of the test
    @param partitions: A list of partition objects, these are passed to the
            test as partitions=
    @param mountpoint_func: A callable that returns a mountpoint given a
            partition instance
    @param tag: A string tag to make this test unique (Required for control
            files that make multiple calls to this routine with the same value
            of 'test'.)
    @param fs_opt: An FsOptions instance that describes what filesystem to make
    @param do_fsck: include fsck in post-test partition cleanup.
    @param dargs: Dictionary of arguments to be passed to job.run_test() and
            eventually the test
    """
    # setup the filesystem parameters for all the partitions
    for p in partitions:
        p.set_fs_options(fs_opt)

    # make and mount all the partitions in parallel
    parallel(partitions, 'setup_before_test', mountpoint_func=mountpoint_func)

    mountpoint = mountpoint_func(partitions[0])

    # run the test against all the partitions
    job.run_test(test, tag=tag, partitions=partitions, dir=mountpoint, **dargs)

    parallel(partitions, 'unmount')  # unmount all partitions in parallel
    if do_fsck:
        parallel(partitions, 'fsck')  # fsck all partitions in parallel
    # else fsck is done by caller


class partition(object):
    """
    Class for handling partitions and filesystems
    """

    def __init__(self, job, device, loop_size=0, mountpoint=None):
        """
        @param job: A L{client.bin.job} instance.
        @param device: The device in question (e.g."/dev/hda2"). If device is a
                file it will be mounted as loopback.
        @param loop_size: Size of loopback device (in MB). Defaults to 0.
        """
        self.device = device
        self.name = os.path.basename(device)
        self.job = job
        self.loop = loop_size
        self.fstype = None
        self.mountpoint = mountpoint
        self.mkfs_flags = None
        self.mount_options = None
        self.fs_tag = None
        if self.loop:
            cmd = 'dd if=/dev/zero of=%s bs=1M count=%d' % (device, loop_size)
            utils.system(cmd)


    def __repr__(self):
        return '<Partition: %s>' % self.device


    def set_fs_options(self, fs_options):
        """
        Set filesystem options

            @param fs_options: A L{FsOptions} object
        """

        self.fstype = fs_options.fstype
        self.mkfs_flags = fs_options.mkfs_flags
        self.mount_options = fs_options.mount_options
        self.fs_tag = fs_options.fs_tag


    def run_test(self, test, **dargs):
        self.job.run_test(test, dir=self.get_mountpoint(), **dargs)


    def setup_before_test(self, mountpoint_func):
        """
        Prepare a partition for running a test.  Unmounts any
        filesystem that's currently mounted on the partition, makes a
        new filesystem (according to this partition's filesystem
        options) and mounts it where directed by mountpoint_func.

        @param mountpoint_func: A callable that returns a path as a string,
                given a partition instance.
        """
        mountpoint = mountpoint_func(self)
        if not mountpoint:
            raise ValueError('Don\'t know where to put this partition')
        self.unmount(ignore_status=True, record=False)
        self.mkfs()
        if not os.path.isdir(mountpoint):
            os.makedirs(mountpoint)
        self.mount(mountpoint)


    def run_test_on_partition(self, test, mountpoint_func, **dargs):
        """
        Executes a test fs-style (umount,mkfs,mount,test)

        Here we unmarshal the args to set up tags before running the test.
        Tests are also run by first umounting, mkfsing and then mounting
        before executing the test.

        @param test: name of test to run
        @param mountpoint_func: function to return mount point string
        """
        tag = dargs.get('tag')
        if tag:
            tag = '%s.%s' % (self.name, tag)
        elif self.fs_tag:
            tag = '%s.%s' % (self.name, self.fs_tag)
        else:
            tag = self.name

        # If there's a 'suffix' argument, append it to the tag and remove it
        suffix = dargs.pop('suffix', None)
        if suffix:
            tag = '%s.%s' % (tag, suffix)

        dargs['tag'] = test + '.' + tag

        def _make_partition_and_run_test(test_tag, dir=None, **dargs):
            self.setup_before_test(mountpoint_func)
            try:
                self.job.run_test(test, tag=test_tag, dir=mountpoint, **dargs)
            finally:
                self.unmount()
                self.fsck()


        mountpoint = mountpoint_func(self)

        # The tag is the tag for the group (get stripped off by run_group)
        # The test_tag is the tag for the test itself
        self.job.run_group(_make_partition_and_run_test,
                           test_tag=tag, dir=mountpoint, **dargs)


    def get_mountpoint(self, open_func=open, filename=None):
        """
        Find the mount point of this partition object.

        @param open_func: the function to use for opening the file containing
                the mounted partitions information
        @param filename: where to look for the mounted partitions information
                (default None which means it will search /proc/mounts and/or
                /etc/mtab)

        @returns a string with the mount point of the partition or None if not
                mounted
        """
        if filename:
            for line in open_func(filename).readlines():
                parts = line.split()
                if parts[0] == self.device or parts[1] == self.mountpoint:
                    return parts[1] # The mountpoint where it's mounted
            return None

        # no specific file given, look in /proc/mounts
        res = self.get_mountpoint(open_func=open_func, filename='/proc/mounts')
        if not res:
            # sometimes the root partition is reported as /dev/root in
            # /proc/mounts in this case, try /etc/mtab
            res = self.get_mountpoint(open_func=open_func, filename='/etc/mtab')

            # trust /etc/mtab only about /
            if res != '/':
                res = None

        return res


    def mkfs_exec(self, fstype):
        """
        Return the proper mkfs executable based on fs
        """
        if fstype == 'ext4':
            if os.path.exists('/sbin/mkfs.ext4'):
                return 'mkfs'
            # If ext4 supported e2fsprogs is not installed we use the
            # autotest supplied one in tools dir which is statically linked"""
            auto_mkfs = os.path.join(self.job.toolsdir, 'mkfs.ext4dev')
            if os.path.exists(auto_mkfs):
                return auto_mkfs
        else:
            return 'mkfs'

        raise NameError('Error creating partition for filesystem type %s' %
                        fstype)


    def mkfs(self, fstype=None, args='', record=True):
        """
        Format a partition to filesystem type

        @param fstype: the filesystem type, e.g.. "ext3", "ext2"
        @param args: arguments to be passed to mkfs command.
        @param record: if set, output result of mkfs operation to autotest
                output
        """

        if list_mount_devices().count(self.device):
            raise NameError('Attempted to format mounted device %s' %
                             self.device)

        if not fstype:
            if self.fstype:
                fstype = self.fstype
            else:
                fstype = 'ext2'

        if self.mkfs_flags:
            args += ' ' + self.mkfs_flags
        if fstype == 'xfs':
            args += ' -f'

        if self.loop:
            # BAH. Inconsistent mkfs syntax SUCKS.
            if fstype.startswith('ext'):
                args += ' -F'
            elif fstype == 'reiserfs':
                args += ' -f'

        # If there isn't already a '-t <type>' argument, add one.
        if not "-t" in args:
            args = "-t %s %s" % (fstype, args)

        args = args.strip()

        mkfs_cmd = "%s %s %s" % (self.mkfs_exec(fstype), args, self.device)

        sys.stdout.flush()
        try:
            # We throw away the output here - we only need it on error, in
            # which case it's in the exception
            utils.system_output("yes | %s" % mkfs_cmd)
        except error.CmdError, e:
            logging.error(e.result_obj)
            if record:
                self.job.record('FAIL', None, mkfs_cmd, error.format_error())
            raise
        except:
            if record:
                self.job.record('FAIL', None, mkfs_cmd, error.format_error())
            raise
        else:
            if record:
                self.job.record('GOOD', None, mkfs_cmd)
            self.fstype = fstype


    def get_fsck_exec(self):
        """
        Return the proper mkfs executable based on self.fstype
        """
        if self.fstype == 'ext4':
            if os.path.exists('/sbin/fsck.ext4'):
                return 'fsck'
            # If ext4 supported e2fsprogs is not installed we use the
            # autotest supplied one in tools dir which is statically linked"""
            auto_fsck = os.path.join(self.job.toolsdir, 'fsck.ext4dev')
            if os.path.exists(auto_fsck):
                return auto_fsck
        else:
            return 'fsck'

        raise NameError('Error creating partition for filesystem type %s' %
                        self.fstype)


    def fsck(self, args='-fy', record=True):
        """
        Run filesystem check

        @param args: arguments to filesystem check tool. Default is "-n"
                which works on most tools.
        """

        # I hate reiserfstools.
        # Requires an explit Yes for some inane reason
        fsck_cmd = '%s %s %s' % (self.get_fsck_exec(), self.device, args)
        if self.fstype == 'reiserfs':
            fsck_cmd = 'yes "Yes" | ' + fsck_cmd
        sys.stdout.flush()
        try:
            utils.system_output(fsck_cmd)
        except:
            if record:
                self.job.record('FAIL', None, fsck_cmd, error.format_error())
            raise error.TestError('Fsck found errors with the underlying '
                                  'file system')
        else:
            if record:
                self.job.record('GOOD', None, fsck_cmd)


    def mount(self, mountpoint=None, fstype=None, args='', record=True):
        """
        Mount this partition to a mount point

        @param mountpoint: If you have not provided a mountpoint to partition
                object or want to use a different one, you may specify it here.
        @param fstype: Filesystem type. If not provided partition object value
                will be used.
        @param args: Arguments to be passed to "mount" command.
        @param record: If True, output result of mount operation to autotest
                output.
        """

        if fstype is None:
            fstype = self.fstype
        else:
            assert(self.fstype is None or self.fstype == fstype);

        if self.mount_options:
            args += ' -o  ' + self.mount_options
        if fstype:
            args += ' -t ' + fstype
        if self.loop:
            args += ' -o loop'
        args = args.lstrip()

        if not mountpoint and not self.mountpoint:
            raise ValueError("No mountpoint specified and no default "
                             "provided to this partition object")
        if not mountpoint:
            mountpoint = self.mountpoint

        mount_cmd = "mount %s %s %s" % (args, self.device, mountpoint)

        if list_mount_devices().count(self.device):
            err = 'Attempted to mount mounted device'
            self.job.record('FAIL', None, mount_cmd, err)
            raise NameError(err)
        if list_mount_points().count(mountpoint):
            err = 'Attempted to mount busy mountpoint'
            self.job.record('FAIL', None, mount_cmd, err)
            raise NameError(err)

        mtab = open('/etc/mtab')
        # We have to get an exclusive lock here - mount/umount are racy
        fcntl.flock(mtab.fileno(), fcntl.LOCK_EX)
        sys.stdout.flush()
        try:
            utils.system(mount_cmd)
            mtab.close()
        except:
            mtab.close()
            if record:
                self.job.record('FAIL', None, mount_cmd, error.format_error())
            raise
        else:
            if record:
                self.job.record('GOOD', None, mount_cmd)
            self.fstype = fstype


    def unmount_force(self):
        """
        Kill all other jobs accessing this partition. Use fuser and ps to find
        all mounts on this mountpoint and unmount them.

        @return: true for success or false for any errors
        """

        logging.debug("Standard umount failed, will try forcing. Users:")
        try:
            cmd = 'fuser ' + self.get_mountpoint()
            logging.debug(cmd)
            fuser = utils.system_output(cmd)
            logging.debug(fuser)
            users = re.sub('.*:', '', fuser).split()
            for user in users:
                m = re.match('(\d+)(.*)', user)
                (pid, usage) = (m.group(1), m.group(2))
                try:
                    ps = utils.system_output('ps -p %s | sed 1d' % pid)
                    logging.debug('%s %s %s', usage, pid, ps)
                except Exception:
                    pass
                utils.system('ls -l ' + self.device)
                umount_cmd = "umount -f " + self.device
                utils.system(umount_cmd)
                return True
        except error.CmdError:
            logging.debug('Umount_force failed for %s', self.device)
            return False



    def unmount(self, ignore_status=False, record=True):
        """
        Umount this partition.

        It's easier said than done to umount a partition.
        We need to lock the mtab file to make sure we don't have any
        locking problems if we are umounting in paralllel.

        If there turns out to be a problem with the simple umount we
        end up calling umount_force to get more  agressive.

        @param ignore_status: should we notice the umount status
        @param record: if True, output result of umount operation to
                autotest output
        """

        mountpoint = self.get_mountpoint()
        if not mountpoint:
            # It's not even mounted to start with
            if record and not ignore_status:
                msg = 'umount for dev %s has no mountpoint' % self.device
                self.job.record('FAIL', None, msg, 'Not mounted')
            return

        umount_cmd = "umount " + mountpoint
        mtab = open('/etc/mtab')

        # We have to get an exclusive lock here - mount/umount are racy
        fcntl.flock(mtab.fileno(), fcntl.LOCK_EX)
        sys.stdout.flush()
        try:
            utils.system(umount_cmd)
            mtab.close()
            if record:
                self.job.record('GOOD', None, umount_cmd)
        except (error.CmdError, IOError):
            mtab.close()

            # Try the forceful umount
            if self.unmount_force():
                return

            # If we are here we cannot umount this partition
            if record and not ignore_status:
                self.job.record('FAIL', None, umount_cmd, error.format_error())
            raise


    def wipe(self):
        """
        Delete all files of a given partition filesystem.
        """
        wipe_filesystem(self.job, self.get_mountpoint())


    def get_io_scheduler_list(self, device_name):
        names = open(self.__sched_path(device_name)).read()
        return names.translate(string.maketrans('[]', '  ')).split()


    def get_io_scheduler(self, device_name):
        return re.split('[\[\]]',
                        open(self.__sched_path(device_name)).read())[1]


    def set_io_scheduler(self, device_name, name):
        if name not in self.get_io_scheduler_list(device_name):
            raise NameError('No such IO scheduler: %s' % name)
        f = open(self.__sched_path(device_name), 'w')
        f.write(name)
        f.close()


    def __sched_path(self, device_name):
        return '/sys/block/%s/queue/scheduler' % device_name


class virtual_partition:
    """
    Handles block device emulation using file images of disks.
    It's important to note that this API can be used only if
    we have the following programs present on the client machine:

     * dd
     * losetup
     * truncate
    """
    def __init__(self, file_img, file_size):
        """
        Creates a virtual partition, keeping record of the device created
        under /dev/mapper (device attribute) so test writers can use it
        on their filesystem tests.

        @param file_img: Path to the desired disk image file.
        @param file_size: Size of the desired image in Bytes.
        """
        logging.debug('Sanity check before attempting to create virtual '
                      'partition')
        try:
            os_dep.commands('dd', 'losetup', 'truncate')
        except ValueError, e:
            e_msg = 'Unable to create virtual partition: %s' % e
            raise error.AutotestError(e_msg)

        logging.debug('Creating virtual partition')
        self.size = file_size
        self.img = self._create_disk_img(file_img)
        self.loop = self._attach_img_loop()
        self.device = self.loop
        logging.debug('Virtual partition successfuly created')
        logging.debug('Image disk: %s', self.img)
        logging.debug('Loopback device: %s', self.loop)
        logging.debug('Device path: %s', self.device)


    def destroy(self):
        """
        Removes the virtual partition from /dev/mapper, detaches the image file
        from the loopback device and removes the image file.
        """
        logging.debug('Removing virtual partition - device %s', self.device)
        self._detach_img_loop()
        self._remove_disk_img()


    def _create_disk_img(self, img_path):
        """
        Creates a disk image using dd.

        @param img_path: Path to the desired image file.
        @param size: Size of the desired image in MB.
        @returns: Path of the image created.
        """
        logging.debug('Creating disk image %s, size = %d MB',
                      img_path, self.size)
        try:
            cmd = 'truncate %s --size %dM' % (img_path, self.size)
            utils.run(cmd)
        except error.CmdError, e:
            e_msg = 'Error creating disk image %s: %s' % (img_path, e)
            raise error.AutotestError(e_msg)
        return img_path


    def _attach_img_loop(self):
        """
        Attaches a file image to a loopback device using losetup.
        @returns: Path of the loopback device associated.
        """
        logging.debug('Attaching image %s to a loop device', self.img)
        try:
            cmd = 'losetup -f'
            loop_path = utils.system_output(cmd)
            cmd = 'losetup -f %s' % self.img
            utils.run(cmd)
        except error.CmdError, e:
            e_msg = ('Error attaching image %s to a loop device: %s' %
                     (self.img, e))
            raise error.AutotestError(e_msg)
        return loop_path


    def _detach_img_loop(self):
        """
        Detaches the image file from the loopback device.
        """
        logging.debug('Detaching image %s from loop device %s', self.img,
                      self.loop)
        try:
            cmd = 'losetup -d %s' % self.loop
            utils.run(cmd)
        except error.CmdError, e:
            e_msg = ('Error detaching image %s from loop device %s: %s' %
                    (self.img, self.loop, e))
            raise error.AutotestError(e_msg)


    def _remove_disk_img(self):
        """
        Removes the disk image.
        """
        logging.debug('Removing disk image %s', self.img)
        try:
            os.remove(self.img)
        except:
            e_msg = 'Error removing image file %s' % self.img
            raise error.AutotestError(e_msg)


# import a site partition module to allow it to override functions
try:
    from autotest_lib.client.bin.site_partition import *
except ImportError:
    pass