def get_device_name_for_instance(context, instance, bdms, device):
    """Validates (or generates) a device name for instance.

    If device is not set, it will generate a unique device appropriate
    for the instance. It uses the block device mapping table to find
    valid device names. If the device name is valid but applicable to
    a different backend (for example /dev/vdc is specified but the
    backend uses /dev/xvdc), the device name will be converted to the
    appropriate format.
    """
    req_prefix = None
    req_letter = None

    if device:
        try:
            req_prefix, req_letter = block_device.match_device(device)
        except (TypeError, AttributeError, ValueError):
            raise exception.InvalidDevicePath(path=device)

    mappings = block_device.instance_block_mapping(instance, bdms)

    try:
        prefix = block_device.match_device(mappings['root'])[0]
    except (TypeError, AttributeError, ValueError):
        raise exception.InvalidDevicePath(path=mappings['root'])

    # NOTE(vish): remove this when xenapi is setting default_root_device
    if driver.compute_driver_matches('xenapi.XenAPIDriver'):
        prefix = '/dev/xvd'

    if req_prefix != prefix:
        LOG.debug(_("Using %(prefix)s instead of %(req_prefix)s"),
                  {'prefix': prefix, 'req_prefix': req_prefix})

    used_letters = set()
    for device_path in mappings.itervalues():
        letter = block_device.strip_prefix(device_path)
        # NOTE(vish): delete numbers in case we have something like
        #             /dev/sda1
        letter = re.sub("\d+", "", letter)
        used_letters.add(letter)

    # NOTE(vish): remove this when xenapi is properly setting
    #             default_ephemeral_device and default_swap_device
    if driver.compute_driver_matches('xenapi.XenAPIDriver'):
        instance_type = flavors.extract_flavor(instance)
        if instance_type['ephemeral_gb']:
            used_letters.add('b')

        if instance_type['swap']:
            used_letters.add('c')

    if not req_letter:
        req_letter = _get_unused_letter(used_letters)

    if req_letter in used_letters:
        raise exception.DevicePathInUse(path=device)

    device_name = prefix + req_letter
    return device_name

def get_device_name_for_instance(context, instance, device):
    # NOTE(vish): this will generate a unique device name that is not
    #             in use already. It is a reasonable guess at where
    #             it will show up in a linux guest, but it may not
    #             always be correct
    req_prefix = None
    req_letters = None
    if device:
        try:
            match = re.match("(^/dev/x{0,1}[a-z]d)([a-z]+)$", device)
            req_prefix, req_letters = match.groups()
        except (TypeError, AttributeError, ValueError):
            raise exception.InvalidDevicePath(path=device)
    bdms = db.block_device_mapping_get_all_by_instance(context, instance["uuid"])
    mappings = block_device.instance_block_mapping(instance, bdms)
    try:
        match = re.match("(^/dev/x{0,1}[a-z]d)[a-z]+[0-9]*$", mappings["root"])
        prefix = match.groups()[0]
    except (TypeError, AttributeError, ValueError):
        raise exception.InvalidDevicePath(path=mappings["root"])
    if not req_prefix:
        req_prefix = prefix
    letters_list = []
    for _name, device in mappings.iteritems():
        letter = block_device.strip_prefix(device)
        # NOTE(vish): delete numbers in case we have something like
        #             /dev/sda1
        letter = re.sub("\d+", "", letter)
        letters_list.append(letter)
    used_letters = set(letters_list)
    if not req_letters:
        req_letters = _get_unused_letters(used_letters)
    if req_letters in used_letters:
        raise exception.DevicePathInUse(path=device)
    return req_prefix + req_letters

def get_device_name_for_instance(context, instance, bdms, device):
    """Validates (or generates) a device name for instance.

    This method is a wrapper for get_next_device_name that gets the list
    of used devices and the root device from a block device mapping.
    """
    mappings = block_device.instance_block_mapping(instance, bdms)
    return get_next_device_name(instance, mappings.values(), mappings["root"], device)

def get_device_name_for_instance(context, instance, device):
    """Validates (or generates) a device name for instance.

    If device is not set, it will generate a unique device appropriate
    for the instance. It uses the block device mapping table to find
    valid device names. If the device name is valid but applicable to
    a different backend (for example /dev/vdc is specified but the
    backend uses /dev/xvdc), the device name will be converted to the
    appropriate format.
    """
    req_prefix = None
    req_letters = None
    if device:
        try:
            req_prefix, req_letters = block_device.match_device(device)
        except (TypeError, AttributeError, ValueError):
            raise exception.InvalidDevicePath(path=device)
    bdms = db.block_device_mapping_get_all_by_instance(context,
                instance['uuid'])
    mappings = block_device.instance_block_mapping(instance, bdms)
    try:
        prefix = block_device.match_device(mappings['root'])[0]
    except (TypeError, AttributeError, ValueError):
        raise exception.InvalidDevicePath(path=mappings['root'])
    # NOTE(vish): remove this when xenapi is setting default_root_device
    if (FLAGS.connection_type == 'xenapi' or
        FLAGS.compute_driver.endswith('xenapi.XenAPIDriver')):
        prefix = '/dev/xvd'
    if req_prefix != prefix:
        LOG.debug(_("Using %(prefix)s instead of %(req_prefix)s") % locals())
    letters_list = []
    for _name, device in mappings.iteritems():
        letter = block_device.strip_prefix(device)
        # NOTE(vish): delete numbers in case we have something like
        #             /dev/sda1
        letter = re.sub("\d+", "", letter)
        letters_list.append(letter)
    used_letters = set(letters_list)

    # NOTE(vish): remove this when xenapi is properly setting
    #             default_ephemeral_device and default_swap_device
    if (FLAGS.connection_type == 'xenapi' or
        FLAGS.compute_driver.endswith('xenapi.XenAPIDriver')):
        instance_type_id = instance['instance_type_id']
        instance_type = instance_types.get_instance_type(instance_type_id)
        if instance_type['ephemeral_gb']:
            used_letters.update('b')
        if instance_type['swap']:
            used_letters.update('c')

    if not req_letters:
        req_letters = _get_unused_letters(used_letters)
    if req_letters in used_letters:
        raise exception.DevicePathInUse(path=device)
    return prefix + req_letters

def get_device_name_for_instance(context, instance, device):
    """Validates (or generates) a device name for instance.

    If device is not set, it will generate a unique device appropriate
    for the instance. It uses the block device mapping table to find
    valid device names. If the device name is valid but applicable to
    a different backend (for example /dev/vdc is specified but the
    backend uses /dev/xvdc), the device name will be converted to the
    appropriate format.
    """
    req_prefix = None
    req_letters = None
    if device:
        try:
            req_prefix, req_letters = block_device.match_device(device)
        except (TypeError, AttributeError, ValueError):
            raise exception.InvalidDevicePath(path=device)
    bdms = db.block_device_mapping_get_all_by_instance(context,
                instance['uuid'])
    mappings = block_device.instance_block_mapping(instance, bdms)
    try:
        prefix = block_device.match_device(mappings['root'])[0]
    except (TypeError, AttributeError, ValueError):
        raise exception.InvalidDevicePath(path=mappings['root'])
    if req_prefix != prefix:
        LOG.debug(_("Using %(prefix)s instead of %(req_prefix)s") % locals())
    letters_list = []
    for _name, device in mappings.iteritems():
        letter = block_device.strip_prefix(device)
        # NOTE(vish): delete numbers in case we have something like
        #             /dev/sda1
        letter = re.sub("\d+", "", letter)
        letters_list.append(letter)
    used_letters = set(letters_list)
    if not req_letters:
        req_letters = _get_unused_letters(used_letters)
    if req_letters in used_letters:
        raise exception.DevicePathInUse(path=device)
    return prefix + req_letters

def _format_instance_mapping(ctxt, instance):
    bdms = objects.BlockDeviceMappingList.get_by_instance_uuid(ctxt, instance.uuid)
    return block_device.instance_block_mapping(instance, bdms)

def _format_instance_mapping(conductor_api, ctxt, instance):
    bdms = conductor_api.block_device_mapping_get_all_by_instance(
               ctxt, instance)
    return block_device.instance_block_mapping(instance, bdms)

def _format_instance_mapping(ctxt, instance):
    bdms = db.block_device_mapping_get_all_by_instance(ctxt, instance['uuid'])
    return block_device.instance_block_mapping(instance, bdms)