oslo.versionedobjects/nova/utils.py

# vim: tabstop=4 shiftwidth=4 softtabstop=4

# Copyright 2010 United States Government as represented by the
# Administrator of the National Aeronautics and Space Administration.
# Copyright 2011 Justin Santa Barbara
# All Rights Reserved.
#
#    Licensed under the Apache License, Version 2.0 (the "License"); you may
#    not use this file except in compliance with the License. You may obtain
#    a copy of the License at
#
#         http://www.apache.org/licenses/LICENSE-2.0
#
#    Unless required by applicable law or agreed to in writing, software
#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
#    License for the specific language governing permissions and limitations
#    under the License.

"""Utilities and helper functions."""

import contextlib
import datetime
import functools
import hashlib
import inspect
import os
import pyclbr
import random
import re
import shutil
import socket
import struct
import sys
import tempfile
import time
from xml.sax import saxutils

import eventlet
import netaddr

from oslo.config import cfg

from nova import exception
from nova.openstack.common import excutils
from nova.openstack.common import importutils
from nova.openstack.common import lockutils
from nova.openstack.common import log as logging
from nova.openstack.common import processutils
from nova.openstack.common.rpc import common as rpc_common
from nova.openstack.common import timeutils

notify_decorator = 'nova.openstack.common.notifier.api.notify_decorator'

monkey_patch_opts = [
    cfg.BoolOpt('monkey_patch',
                default=False,
                help='Whether to log monkey patching'),
    cfg.ListOpt('monkey_patch_modules',
                default=[
                  'nova.api.ec2.cloud:%s' % (notify_decorator),
                  'nova.compute.api:%s' % (notify_decorator)
                  ],
                help='List of modules/decorators to monkey patch'),
]
utils_opts = [
    cfg.IntOpt('password_length',
               default=12,
               help='Length of generated instance admin passwords'),
    cfg.StrOpt('instance_usage_audit_period',
               default='month',
               help='time period to generate instance usages for.  '
                    'Time period must be hour, day, month or year'),
    cfg.StrOpt('rootwrap_config',
               default="/etc/nova/rootwrap.conf",
               help='Path to the rootwrap configuration file to use for '
                    'running commands as root'),
    cfg.StrOpt('tempdir',
               default=None,
               help='Explicitly specify the temporary working directory'),
]
CONF = cfg.CONF
CONF.register_opts(monkey_patch_opts)
CONF.register_opts(utils_opts)

LOG = logging.getLogger(__name__)

# Used for looking up extensions of text
# to their 'multiplied' byte amount
BYTE_MULTIPLIERS = {
    '': 1,
    't': 1024 ** 4,
    'g': 1024 ** 3,
    'm': 1024 ** 2,
    'k': 1024,
}

# used in limits
TIME_UNITS = {
    'SECOND': 1,
    'MINUTE': 60,
    'HOUR': 3600,
    'DAY': 84400
}

synchronized = lockutils.synchronized_with_prefix('nova-')


def vpn_ping(address, port, timeout=0.05, session_id=None):
    """Sends a vpn negotiation packet and returns the server session.

    Returns False on a failure. Basic packet structure is below.

    Client packet (14 bytes)::

         0 1      8 9  13
        +-+--------+-----+
        |x| cli_id |?????|
        +-+--------+-----+
        x = packet identifier 0x38
        cli_id = 64 bit identifier
        ? = unknown, probably flags/padding

    Server packet (26 bytes)::

         0 1      8 9  13 14    21 2225
        +-+--------+-----+--------+----+
        |x| srv_id |?????| cli_id |????|
        +-+--------+-----+--------+----+
        x = packet identifier 0x40
        cli_id = 64 bit identifier
        ? = unknown, probably flags/padding
        bit 9 was 1 and the rest were 0 in testing

    """
    if session_id is None:
        session_id = random.randint(0, 0xffffffffffffffff)
    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    data = struct.pack('!BQxxxxx', 0x38, session_id)
    sock.sendto(data, (address, port))
    sock.settimeout(timeout)
    try:
        received = sock.recv(2048)
    except socket.timeout:
        return False
    finally:
        sock.close()
    fmt = '!BQxxxxxQxxxx'
    if len(received) != struct.calcsize(fmt):
        LOG.warn(_('Expected to receive %(exp)s bytes, but actually %(act)s') %
                 dict(exp=struct.calcsize(fmt), act=len(received)))
        return False
    (identifier, server_sess, client_sess) = struct.unpack(fmt, received)
    if identifier == 0x40 and client_sess == session_id:
        return server_sess


def execute(*cmd, **kwargs):
    """Convenience wrapper around oslo's execute() method."""
    if 'run_as_root' in kwargs and not 'root_helper' in kwargs:
        kwargs['root_helper'] = 'sudo nova-rootwrap %s' % CONF.rootwrap_config
    return processutils.execute(*cmd, **kwargs)


def trycmd(*args, **kwargs):
    """Convenience wrapper around oslo's trycmd() method."""
    if 'run_as_root' in kwargs and not 'root_helper' in kwargs:
        kwargs['root_helper'] = 'sudo nova-rootwrap %s' % CONF.rootwrap_config
    return processutils.trycmd(*args, **kwargs)


def novadir():
    import nova
    return os.path.abspath(nova.__file__).split('nova/__init__.py')[0]


def debug(arg):
    LOG.debug(_('debug in callback: %s'), arg)
    return arg


def generate_uid(topic, size=8):
    characters = '01234567890abcdefghijklmnopqrstuvwxyz'
    choices = [random.choice(characters) for _x in xrange(size)]
    return '%s-%s' % (topic, ''.join(choices))


# Default symbols to use for passwords. Avoids visually confusing characters.
# ~6 bits per symbol
DEFAULT_PASSWORD_SYMBOLS = ('23456789',  # Removed: 0,1
                            'ABCDEFGHJKLMNPQRSTUVWXYZ',   # Removed: I, O
                            'abcdefghijkmnopqrstuvwxyz')  # Removed: l


# ~5 bits per symbol
EASIER_PASSWORD_SYMBOLS = ('23456789',  # Removed: 0, 1
                           'ABCDEFGHJKLMNPQRSTUVWXYZ')  # Removed: I, O


def last_completed_audit_period(unit=None, before=None):
    """This method gives you the most recently *completed* audit period.

    arguments:
            units: string, one of 'hour', 'day', 'month', 'year'
                    Periods normally begin at the beginning (UTC) of the
                    period unit (So a 'day' period begins at midnight UTC,
                    a 'month' unit on the 1st, a 'year' on Jan, 1)
                    unit string may be appended with an optional offset
                    like so:  'day@18'  This will begin the period at 18:00
                    UTC.  'month@15' starts a monthly period on the 15th,
                    and year@3 begins a yearly one on March 1st.
            before: Give the audit period most recently completed before
                    <timestamp>. Defaults to now.


    returns:  2 tuple of datetimes (begin, end)
              The begin timestamp of this audit period is the same as the
              end of the previous.
    """
    if not unit:
        unit = CONF.instance_usage_audit_period

    offset = 0
    if '@' in unit:
        unit, offset = unit.split("@", 1)
        offset = int(offset)

    if before is not None:
        rightnow = before
    else:
        rightnow = timeutils.utcnow()
    if unit not in ('month', 'day', 'year', 'hour'):
        raise ValueError('Time period must be hour, day, month or year')
    if unit == 'month':
        if offset == 0:
            offset = 1
        end = datetime.datetime(day=offset,
                                month=rightnow.month,
                                year=rightnow.year)
        if end >= rightnow:
            year = rightnow.year
            if 1 >= rightnow.month:
                year -= 1
                month = 12 + (rightnow.month - 1)
            else:
                month = rightnow.month - 1
            end = datetime.datetime(day=offset,
                                    month=month,
                                    year=year)
        year = end.year
        if 1 >= end.month:
            year -= 1
            month = 12 + (end.month - 1)
        else:
            month = end.month - 1
        begin = datetime.datetime(day=offset, month=month, year=year)

    elif unit == 'year':
        if offset == 0:
            offset = 1
        end = datetime.datetime(day=1, month=offset, year=rightnow.year)
        if end >= rightnow:
            end = datetime.datetime(day=1,
                                    month=offset,
                                    year=rightnow.year - 1)
            begin = datetime.datetime(day=1,
                                      month=offset,
                                      year=rightnow.year - 2)
        else:
            begin = datetime.datetime(day=1,
                                      month=offset,
                                      year=rightnow.year - 1)

    elif unit == 'day':
        end = datetime.datetime(hour=offset,
                               day=rightnow.day,
                               month=rightnow.month,
                               year=rightnow.year)
        if end >= rightnow:
            end = end - datetime.timedelta(days=1)
        begin = end - datetime.timedelta(days=1)

    elif unit == 'hour':
        end = rightnow.replace(minute=offset, second=0, microsecond=0)
        if end >= rightnow:
            end = end - datetime.timedelta(hours=1)
        begin = end - datetime.timedelta(hours=1)

    return (begin, end)


def generate_password(length=None, symbolgroups=DEFAULT_PASSWORD_SYMBOLS):
    """Generate a random password from the supplied symbol groups.

    At least one symbol from each group will be included. Unpredictable
    results if length is less than the number of symbol groups.

    Believed to be reasonably secure (with a reasonable password length!)

    """
    if length is None:
        length = CONF.password_length

    r = random.SystemRandom()

    # NOTE(jerdfelt): Some password policies require at least one character
    # from each group of symbols, so start off with one random character
    # from each symbol group
    password = [r.choice(s) for s in symbolgroups]
    # If length < len(symbolgroups), the leading characters will only
    # be from the first length groups. Try our best to not be predictable
    # by shuffling and then truncating.
    r.shuffle(password)
    password = password[:length]
    length -= len(password)

    # then fill with random characters from all symbol groups
    symbols = ''.join(symbolgroups)
    password.extend([r.choice(symbols) for _i in xrange(length)])

    # finally shuffle to ensure first x characters aren't from a
    # predictable group
    r.shuffle(password)

    return ''.join(password)


def last_octet(address):
    return int(address.split('.')[-1])


def get_my_ipv4_address():
    """Run ip route/addr commands to figure out the best ipv4
    """
    LOCALHOST = '127.0.0.1'
    try:
        out = execute('ip', '-f', 'inet', '-o', 'route', 'show',
                      run_as_root=True)

        # Find the default route
        regex_default = ('default\s*via\s*'
                         '(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})'
                         '\s*dev\s*(\w*)\s*')
        default_routes = re.findall(regex_default, out[0])
        if not default_routes:
            return LOCALHOST
        gateway, iface = default_routes[0]

        # Find the right subnet for the gateway/interface for
        # the default route
        route = ('(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})\/(\d{1,2})'
              '\s*dev\s*(\w*)\s*')
        for match in re.finditer(route, out[0]):
            subnet = netaddr.IPNetwork(match.group(1) + "/" + match.group(2))
            if (match.group(3) == iface and
                    netaddr.IPAddress(gateway) in subnet):
                try:
                    return _get_ipv4_address_for_interface(iface)
                except exception.NovaException:
                    pass
    except Exception as ex:
        LOG.error(_("Couldn't get IPv4 : %(ex)s") % {'ex': ex})
    return LOCALHOST


def _get_ipv4_address_for_interface(iface):
    """Run ip addr show for an interface and grab its ipv4 addresses
    """
    try:
        out = execute('ip', '-f', 'inet', '-o', 'addr', 'show', iface,
                      run_as_root=True)
        regexp_address = re.compile('inet\s*'
                                    '(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})')
        address = [m.group(1) for m in regexp_address.finditer(out[0])
                   if m.group(1) != '127.0.0.1']
        if address:
            return address[0]
        else:
            msg = _('IPv4 address is not found.: %s') % out[0]
            raise exception.NovaException(msg)
    except Exception as ex:
        msg = _("Couldn't get IPv4 of %(interface)s"
                " : %(ex)s") % {'interface': iface, 'ex': ex}
        LOG.error(msg)
        raise exception.NovaException(msg)


def get_my_linklocal(interface):
    try:
        if_str = execute('ip', '-f', 'inet6', '-o', 'addr', 'show', interface)
        condition = '\s+inet6\s+([0-9a-f:]+)/\d+\s+scope\s+link'
        links = [re.search(condition, x) for x in if_str[0].split('\n')]
        address = [w.group(1) for w in links if w is not None]
        if address[0] is not None:
            return address[0]
        else:
            msg = _('Link Local address is not found.:%s') % if_str
            raise exception.NovaException(msg)
    except Exception as ex:
        msg = _("Couldn't get Link Local IP of %(interface)s"
                " :%(ex)s") % locals()
        raise exception.NovaException(msg)


def parse_mailmap(mailmap='.mailmap'):
    mapping = {}
    if os.path.exists(mailmap):
        fp = open(mailmap, 'r')
        for l in fp:
            l = l.strip()
            if not l.startswith('#') and ' ' in l:
                canonical_email, alias = l.split(' ')
                mapping[alias.lower()] = canonical_email.lower()
    return mapping


def str_dict_replace(s, mapping):
    for s1, s2 in mapping.iteritems():
        s = s.replace(s1, s2)
    return s


class LazyPluggable(object):
    """A pluggable backend loaded lazily based on some value."""

    def __init__(self, pivot, config_group=None, **backends):
        self.__backends = backends
        self.__pivot = pivot
        self.__backend = None
        self.__config_group = config_group

    def __get_backend(self):
        if not self.__backend:
            if self.__config_group is None:
                backend_name = CONF[self.__pivot]
            else:
                backend_name = CONF[self.__config_group][self.__pivot]
            if backend_name not in self.__backends:
                msg = _('Invalid backend: %s') % backend_name
                raise exception.NovaException(msg)

            backend = self.__backends[backend_name]
            if isinstance(backend, tuple):
                name = backend[0]
                fromlist = backend[1]
            else:
                name = backend
                fromlist = backend

            self.__backend = __import__(name, None, None, fromlist)
        return self.__backend

    def __getattr__(self, key):
        backend = self.__get_backend()
        return getattr(backend, key)


def xhtml_escape(value):
    """Escapes a string so it is valid within XML or XHTML.

    """
    return saxutils.escape(value, {'"': '&quot;', "'": '&apos;'})


def utf8(value):
    """Try to turn a string into utf-8 if possible.

    Code is directly from the utf8 function in
    http://github.com/facebook/tornado/blob/master/tornado/escape.py

    """
    if isinstance(value, unicode):
        return value.encode('utf-8')
    assert isinstance(value, str)
    return value


def get_from_path(items, path):
    """Returns a list of items matching the specified path.

    Takes an XPath-like expression e.g. prop1/prop2/prop3, and for each item
    in items, looks up items[prop1][prop2][prop3].  Like XPath, if any of the
    intermediate results are lists it will treat each list item individually.
    A 'None' in items or any child expressions will be ignored, this function
    will not throw because of None (anywhere) in items.  The returned list
    will contain no None values.

    """
    if path is None:
        raise exception.NovaException('Invalid mini_xpath')

    (first_token, sep, remainder) = path.partition('/')

    if first_token == '':
        raise exception.NovaException('Invalid mini_xpath')

    results = []

    if items is None:
        return results

    if not isinstance(items, list):
        # Wrap single objects in a list
        items = [items]

    for item in items:
        if item is None:
            continue
        get_method = getattr(item, 'get', None)
        if get_method is None:
            continue
        child = get_method(first_token)
        if child is None:
            continue
        if isinstance(child, list):
            # Flatten intermediate lists
            for x in child:
                results.append(x)
        else:
            results.append(child)

    if not sep:
        # No more tokens
        return results
    else:
        return get_from_path(results, remainder)


def flatten_dict(dict_, flattened=None):
    """Recursively flatten a nested dictionary."""
    flattened = flattened or {}
    for key, value in dict_.iteritems():
        if hasattr(value, 'iteritems'):
            flatten_dict(value, flattened)
        else:
            flattened[key] = value
    return flattened


def partition_dict(dict_, keys):
    """Return two dicts, one with `keys` the other with everything else."""
    intersection = {}
    difference = {}
    for key, value in dict_.iteritems():
        if key in keys:
            intersection[key] = value
        else:
            difference[key] = value
    return intersection, difference


def map_dict_keys(dict_, key_map):
    """Return a dict in which the dictionaries keys are mapped to new keys."""
    mapped = {}
    for key, value in dict_.iteritems():
        mapped_key = key_map[key] if key in key_map else key
        mapped[mapped_key] = value
    return mapped


def subset_dict(dict_, keys):
    """Return a dict that only contains a subset of keys."""
    subset = partition_dict(dict_, keys)[0]
    return subset


def diff_dict(orig, new):
    """
    Return a dict describing how to change orig to new.  The keys
    correspond to values that have changed; the value will be a list
    of one or two elements.  The first element of the list will be
    either '+' or '-', indicating whether the key was updated or
    deleted; if the key was updated, the list will contain a second
    element, giving the updated value.
    """
    # Figure out what keys went away
    result = dict((k, ['-']) for k in set(orig.keys()) - set(new.keys()))
    # Compute the updates
    for key, value in new.items():
        if key not in orig or value != orig[key]:
            result[key] = ['+', value]
    return result


def check_isinstance(obj, cls):
    """Checks that obj is of type cls, and lets PyLint infer types."""
    if isinstance(obj, cls):
        return obj
    raise Exception(_('Expected object of type: %s') % (str(cls)))


def parse_server_string(server_str):
    """
    Parses the given server_string and returns a list of host and port.
    If it's not a combination of host part and port, the port element
    is a null string. If the input is invalid expression, return a null
    list.
    """
    try:
        # First of all, exclude pure IPv6 address (w/o port).
        if netaddr.valid_ipv6(server_str):
            return (server_str, '')

        # Next, check if this is IPv6 address with a port number combination.
        if server_str.find("]:") != -1:
            (address, port) = server_str.replace('[', '', 1).split(']:')
            return (address, port)

        # Third, check if this is a combination of an address and a port
        if server_str.find(':') == -1:
            return (server_str, '')

        # This must be a combination of an address and a port
        (address, port) = server_str.split(':')
        return (address, port)

    except Exception:
        LOG.error(_('Invalid server_string: %s'), server_str)
        return ('', '')


def is_int_like(val):
    """Check if a value looks like an int."""
    try:
        return str(int(val)) == str(val)
    except Exception:
        return False


def is_valid_ipv4(address):
    """Verify that address represents a valid IPv4 address."""
    try:
        return netaddr.valid_ipv4(address)
    except Exception:
        return False


def is_valid_ipv6(address):
    try:
        return netaddr.valid_ipv6(address)
    except Exception:
        return False


def is_valid_ipv6_cidr(address):
    try:
        str(netaddr.IPNetwork(address, version=6).cidr)
        return True
    except Exception:
        return False


def get_shortened_ipv6(address):
    addr = netaddr.IPAddress(address, version=6)
    return str(addr.ipv6())


def get_shortened_ipv6_cidr(address):
    net = netaddr.IPNetwork(address, version=6)
    return str(net.cidr)


def is_valid_cidr(address):
    """Check if address is valid

    The provided address can be a IPv6 or a IPv4
    CIDR address.
    """
    try:
        # Validate the correct CIDR Address
        netaddr.IPNetwork(address)
    except netaddr.core.AddrFormatError:
        return False
    except UnboundLocalError:
        # NOTE(MotoKen): work around bug in netaddr 0.7.5 (see detail in
        # https://github.com/drkjam/netaddr/issues/2)
        return False

    # Prior validation partially verify /xx part
    # Verify it here
    ip_segment = address.split('/')

    if (len(ip_segment) <= 1 or
            ip_segment[1] == ''):
        return False

    return True


def get_ip_version(network):
    """Returns the IP version of a network (IPv4 or IPv6).

    Raises AddrFormatError if invalid network.
    """
    if netaddr.IPNetwork(network).version == 6:
        return "IPv6"
    elif netaddr.IPNetwork(network).version == 4:
        return "IPv4"


def monkey_patch():
    """If the Flags.monkey_patch set as True,
    this function patches a decorator
    for all functions in specified modules.
    You can set decorators for each modules
    using CONF.monkey_patch_modules.
    The format is "Module path:Decorator function".
    Example:
      'nova.api.ec2.cloud:nova.openstack.common.notifier.api.notify_decorator'

    Parameters of the decorator is as follows.
    (See nova.openstack.common.notifier.api.notify_decorator)

    name - name of the function
    function - object of the function
    """
    # If CONF.monkey_patch is not True, this function do nothing.
    if not CONF.monkey_patch:
        return
    # Get list of modules and decorators
    for module_and_decorator in CONF.monkey_patch_modules:
        module, decorator_name = module_and_decorator.split(':')
        # import decorator function
        decorator = importutils.import_class(decorator_name)
        __import__(module)
        # Retrieve module information using pyclbr
        module_data = pyclbr.readmodule_ex(module)
        for key in module_data.keys():
            # set the decorator for the class methods
            if isinstance(module_data[key], pyclbr.Class):
                clz = importutils.import_class("%s.%s" % (module, key))
                for method, func in inspect.getmembers(clz, inspect.ismethod):
                    setattr(clz, method,
                        decorator("%s.%s.%s" % (module, key, method), func))
            # set the decorator for the function
            if isinstance(module_data[key], pyclbr.Function):
                func = importutils.import_class("%s.%s" % (module, key))
                setattr(sys.modules[module], key,
                    decorator("%s.%s" % (module, key), func))


def convert_to_list_dict(lst, label):
    """Convert a value or list into a list of dicts."""
    if not lst:
        return None
    if not isinstance(lst, list):
        lst = [lst]
    return [{label: x} for x in lst]


def timefunc(func):
    """Decorator that logs how long a particular function took to execute."""
    @functools.wraps(func)
    def inner(*args, **kwargs):
        start_time = time.time()
        try:
            return func(*args, **kwargs)
        finally:
            total_time = time.time() - start_time
            LOG.debug(_("timefunc: '%(name)s' took %(total_time).2f secs") %
                      dict(name=func.__name__, total_time=total_time))
    return inner


def make_dev_path(dev, partition=None, base='/dev'):
    """Return a path to a particular device.

    >>> make_dev_path('xvdc')
    /dev/xvdc

    >>> make_dev_path('xvdc', 1)
    /dev/xvdc1
    """
    path = os.path.join(base, dev)
    if partition:
        path += str(partition)
    return path


def total_seconds(td):
    """Local total_seconds implementation for compatibility with python 2.6."""
    if hasattr(td, 'total_seconds'):
        return td.total_seconds()
    else:
        return ((td.days * 86400 + td.seconds) * 10 ** 6 +
                td.microseconds) / 10.0 ** 6


def sanitize_hostname(hostname):
    """Return a hostname which conforms to RFC-952 and RFC-1123 specs."""
    if isinstance(hostname, unicode):
        hostname = hostname.encode('latin-1', 'ignore')

    hostname = re.sub('[ _]', '-', hostname)
    hostname = re.sub('[^\w.-]+', '', hostname)
    hostname = hostname.lower()
    hostname = hostname.strip('.-')

    return hostname


def read_cached_file(filename, cache_info, reload_func=None):
    """Read from a file if it has been modified.

    :param cache_info: dictionary to hold opaque cache.
    :param reload_func: optional function to be called with data when
                        file is reloaded due to a modification.

    :returns: data from file

    """
    mtime = os.path.getmtime(filename)
    if not cache_info or mtime != cache_info.get('mtime'):
        LOG.debug(_("Reloading cached file %s") % filename)
        with open(filename) as fap:
            cache_info['data'] = fap.read()
        cache_info['mtime'] = mtime
        if reload_func:
            reload_func(cache_info['data'])
    return cache_info['data']


def hash_file(file_like_object):
    """Generate a hash for the contents of a file."""
    checksum = hashlib.sha1()
    for chunk in iter(lambda: file_like_object.read(32768), b''):
        checksum.update(chunk)
    return checksum.hexdigest()


@contextlib.contextmanager
def temporary_mutation(obj, **kwargs):
    """Temporarily set the attr on a particular object to a given value then
    revert when finished.

    One use of this is to temporarily set the read_deleted flag on a context
    object:

        with temporary_mutation(context, read_deleted="yes"):
            do_something_that_needed_deleted_objects()
    """
    def is_dict_like(thing):
        return hasattr(thing, 'has_key')

    def get(thing, attr, default):
        if is_dict_like(thing):
            return thing.get(attr, default)
        else:
            return getattr(thing, attr, default)

    def set_value(thing, attr, val):
        if is_dict_like(thing):
            thing[attr] = val
        else:
            setattr(thing, attr, val)

    def delete(thing, attr):
        if is_dict_like(thing):
            del thing[attr]
        else:
            delattr(thing, attr)

    NOT_PRESENT = object()

    old_values = {}
    for attr, new_value in kwargs.items():
        old_values[attr] = get(obj, attr, NOT_PRESENT)
        set_value(obj, attr, new_value)

    try:
        yield
    finally:
        for attr, old_value in old_values.items():
            if old_value is NOT_PRESENT:
                delete(obj, attr)
            else:
                set_value(obj, attr, old_value)


def generate_mac_address():
    """Generate an Ethernet MAC address."""
    # NOTE(vish): We would prefer to use 0xfe here to ensure that linux
    #             bridge mac addresses don't change, but it appears to
    #             conflict with libvirt, so we use the next highest octet
    #             that has the unicast and locally administered bits set
    #             properly: 0xfa.
    #             Discussion: https://bugs.launchpad.net/nova/+bug/921838
    mac = [0xfa, 0x16, 0x3e,
           random.randint(0x00, 0xff),
           random.randint(0x00, 0xff),
           random.randint(0x00, 0xff)]
    return ':'.join(map(lambda x: "%02x" % x, mac))


def read_file_as_root(file_path):
    """Secure helper to read file as root."""
    try:
        out, _err = execute('cat', file_path, run_as_root=True)
        return out
    except processutils.ProcessExecutionError:
        raise exception.FileNotFound(file_path=file_path)


@contextlib.contextmanager
def temporary_chown(path, owner_uid=None):
    """Temporarily chown a path.

    :params owner_uid: UID of temporary owner (defaults to current user)
    """
    if owner_uid is None:
        owner_uid = os.getuid()

    orig_uid = os.stat(path).st_uid

    if orig_uid != owner_uid:
        execute('chown', owner_uid, path, run_as_root=True)
    try:
        yield
    finally:
        if orig_uid != owner_uid:
            execute('chown', orig_uid, path, run_as_root=True)


@contextlib.contextmanager
def tempdir(**kwargs):
    argdict = kwargs.copy()
    if 'dir' not in argdict:
        argdict['dir'] = CONF.tempdir
    tmpdir = tempfile.mkdtemp(**argdict)
    try:
        yield tmpdir
    finally:
        try:
            shutil.rmtree(tmpdir)
        except OSError as e:
            LOG.error(_('Could not remove tmpdir: %s'), str(e))


def walk_class_hierarchy(clazz, encountered=None):
    """Walk class hierarchy, yielding most derived classes first."""
    if not encountered:
        encountered = []
    for subclass in clazz.__subclasses__():
        if subclass not in encountered:
            encountered.append(subclass)
            # drill down to leaves first
            for subsubclass in walk_class_hierarchy(subclass, encountered):
                yield subsubclass
            yield subclass


class UndoManager(object):
    """Provides a mechanism to facilitate rolling back a series of actions
    when an exception is raised.
    """
    def __init__(self):
        self.undo_stack = []

    def undo_with(self, undo_func):
        self.undo_stack.append(undo_func)

    def _rollback(self):
        for undo_func in reversed(self.undo_stack):
            undo_func()

    def rollback_and_reraise(self, msg=None, **kwargs):
        """Rollback a series of actions then re-raise the exception.

        .. note:: (sirp) This should only be called within an
                  exception handler.
        """
        with excutils.save_and_reraise_exception():
            if msg:
                LOG.exception(msg, **kwargs)

            self._rollback()


def mkfs(fs, path, label=None):
    """Format a file or block device

    :param fs: Filesystem type (examples include 'swap', 'ext3', 'ext4'
               'btrfs', etc.)
    :param path: Path to file or block device to format
    :param label: Volume label to use
    """
    if fs == 'swap':
        args = ['mkswap']
    else:
        args = ['mkfs', '-t', fs]
    #add -F to force no interactive execute on non-block device.
    if fs in ('ext3', 'ext4'):
        args.extend(['-F'])
    if label:
        if fs in ('msdos', 'vfat'):
            label_opt = '-n'
        else:
            label_opt = '-L'
        args.extend([label_opt, label])
    args.append(path)
    execute(*args)


def last_bytes(file_like_object, num):
    """Return num bytes from the end of the file, and remaining byte count.

    :param file_like_object: The file to read
    :param num: The number of bytes to return

    :returns (data, remaining)
    """

    try:
        file_like_object.seek(-num, os.SEEK_END)
    except IOError as e:
        if e.errno == 22:
            file_like_object.seek(0, os.SEEK_SET)
        else:
            raise

    remaining = file_like_object.tell()
    return (file_like_object.read(), remaining)


def metadata_to_dict(metadata):
    result = {}
    for item in metadata:
        if not item.get('deleted'):
            result[item['key']] = item['value']
    return result


def dict_to_metadata(metadata):
    result = []
    for key, value in metadata.iteritems():
        result.append(dict(key=key, value=value))
    return result


def instance_meta(instance):
    if isinstance(instance['metadata'], dict):
        return instance['metadata']
    else:
        return metadata_to_dict(instance['metadata'])


def instance_sys_meta(instance):
    if isinstance(instance['system_metadata'], dict):
        return instance['system_metadata']
    else:
        return metadata_to_dict(instance['system_metadata'])


def get_wrapped_function(function):
    """Get the method at the bottom of a stack of decorators."""
    if not hasattr(function, 'func_closure') or not function.func_closure:
        return function

    def _get_wrapped_function(function):
        if not hasattr(function, 'func_closure') or not function.func_closure:
            return None

        for closure in function.func_closure:
            func = closure.cell_contents

            deeper_func = _get_wrapped_function(func)
            if deeper_func:
                return deeper_func
            elif hasattr(closure.cell_contents, '__call__'):
                return closure.cell_contents

    return _get_wrapped_function(function)


class ExceptionHelper(object):
    """Class to wrap another and translate the ClientExceptions raised by its
    function calls to the actual ones.
    """

    def __init__(self, target):
        self._target = target

    def __getattr__(self, name):
        func = getattr(self._target, name)

        @functools.wraps(func)
        def wrapper(*args, **kwargs):
            try:
                return func(*args, **kwargs)
            except rpc_common.ClientException as e:
                raise (e._exc_info[1], None, e._exc_info[2])
        return wrapper


def check_string_length(value, name, min_length=0, max_length=None):
    """Check the length of specified string
    :param value: the value of the string
    :param name: the name of the string
    :param min_length: the min_length of the string
    :param max_length: the max_length of the string
    """
    if not isinstance(value, basestring):
        msg = _("%s is not a string or unicode") % name
        raise exception.InvalidInput(message=msg)

    if len(value) < min_length:
        msg = _("%(name)s has less than %(min_length)s "
                    "characters.") % locals()
        raise exception.InvalidInput(message=msg)

    if max_length and len(value) > max_length:
        msg = _("%(name)s has more than %(max_length)s "
                    "characters.") % locals()
        raise exception.InvalidInput(message=msg)


def spawn_n(func, *args, **kwargs):
    """Passthrough method for eventlet.spawn_n.

    This utility exists so that it can be stubbed for testing without
    interfering with the service spawns.
    """
    eventlet.spawn_n(func, *args, **kwargs)


def is_none_string(val):
    """
    Check if a string represents a None value.
    """
    if not isinstance(val, basestring):
        return False

    return val.lower() == 'none'