Xen and KVM: correct a typo when checking args

[ganeti-local] / lib / utils.py

#
#

# Copyright (C) 2006, 2007 Google Inc.
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful, but
# WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
# General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
# 02110-1301, USA.


"""Ganeti utility module.

This module holds functions that can be used in both daemons (all) and
the command line scripts.

"""


import sys
import os
import sha
import time
import subprocess
import re
import socket
import tempfile
import shutil
import errno
import pwd
import itertools
import select
import fcntl
import resource
import logging
import signal

from cStringIO import StringIO

from ganeti import errors
from ganeti import constants


_locksheld = []
_re_shell_unquoted = re.compile('^[-.,=:/_+@A-Za-z0-9]+$')

debug = False
debug_locks = False

#: when set to True, L{RunCmd} is disabled
no_fork = False


class RunResult(object):
  """Holds the result of running external programs.

  @type exit_code: int
  @ivar exit_code: the exit code of the program, or None (if the program
      didn't exit())
  @type signal: int or None
  @ivar signal: the signal that caused the program to finish, or None
      (if the program wasn't terminated by a signal)
  @type stdout: str
  @ivar stdout: the standard output of the program
  @type stderr: str
  @ivar stderr: the standard error of the program
  @type failed: boolean
  @ivar failed: True in case the program was
      terminated by a signal or exited with a non-zero exit code
  @ivar fail_reason: a string detailing the termination reason

  """
  __slots__ = ["exit_code", "signal", "stdout", "stderr",
               "failed", "fail_reason", "cmd"]


  def __init__(self, exit_code, signal_, stdout, stderr, cmd):
    self.cmd = cmd
    self.exit_code = exit_code
    self.signal = signal_
    self.stdout = stdout
    self.stderr = stderr
    self.failed = (signal_ is not None or exit_code != 0)

    if self.signal is not None:
      self.fail_reason = "terminated by signal %s" % self.signal
    elif self.exit_code is not None:
      self.fail_reason = "exited with exit code %s" % self.exit_code
    else:
      self.fail_reason = "unable to determine termination reason"

    if self.failed:
      logging.debug("Command '%s' failed (%s); output: %s",
                    self.cmd, self.fail_reason, self.output)

  def _GetOutput(self):
    """Returns the combined stdout and stderr for easier usage.

    """
    return self.stdout + self.stderr

  output = property(_GetOutput, None, None, "Return full output")


def RunCmd(cmd, env=None, output=None, cwd='/'):
  """Execute a (shell) command.

  The command should not read from its standard input, as it will be
  closed.

  @type  cmd: string or list
  @param cmd: Command to run
  @type env: dict
  @param env: Additional environment
  @type output: str
  @param output: if desired, the output of the command can be
      saved in a file instead of the RunResult instance; this
      parameter denotes the file name (if not None)
  @type cwd: string
  @param cwd: if specified, will be used as the working
      directory for the command; the default will be /
  @rtype: L{RunResult}
  @return: RunResult instance
  @raise erors.ProgrammerError: if we call this when forks are disabled

  """
  if no_fork:
    raise errors.ProgrammerError("utils.RunCmd() called with fork() disabled")

  if isinstance(cmd, list):
    cmd = [str(val) for val in cmd]
    strcmd = " ".join(cmd)
    shell = False
  else:
    strcmd = cmd
    shell = True
  logging.debug("RunCmd '%s'", strcmd)

  cmd_env = os.environ.copy()
  cmd_env["LC_ALL"] = "C"
  if env is not None:
    cmd_env.update(env)

  if output is None:
    out, err, status = _RunCmdPipe(cmd, cmd_env, shell, cwd)
  else:
    status = _RunCmdFile(cmd, cmd_env, shell, output, cwd)
    out = err = ""

  if status >= 0:
    exitcode = status
    signal_ = None
  else:
    exitcode = None
    signal_ = -status

  return RunResult(exitcode, signal_, out, err, strcmd)


def _RunCmdPipe(cmd, env, via_shell, cwd):
  """Run a command and return its output.

  @type  cmd: string or list
  @param cmd: Command to run
  @type env: dict
  @param env: The environment to use
  @type via_shell: bool
  @param via_shell: if we should run via the shell
  @type cwd: string
  @param cwd: the working directory for the program
  @rtype: tuple
  @return: (out, err, status)

  """
  poller = select.poll()
  child = subprocess.Popen(cmd, shell=via_shell,
                           stderr=subprocess.PIPE,
                           stdout=subprocess.PIPE,
                           stdin=subprocess.PIPE,
                           close_fds=True, env=env,
                           cwd=cwd)

  child.stdin.close()
  poller.register(child.stdout, select.POLLIN)
  poller.register(child.stderr, select.POLLIN)
  out = StringIO()
  err = StringIO()
  fdmap = {
    child.stdout.fileno(): (out, child.stdout),
    child.stderr.fileno(): (err, child.stderr),
    }
  for fd in fdmap:
    status = fcntl.fcntl(fd, fcntl.F_GETFL)
    fcntl.fcntl(fd, fcntl.F_SETFL, status | os.O_NONBLOCK)

  while fdmap:
    try:
      pollresult = poller.poll()
    except EnvironmentError, eerr:
      if eerr.errno == errno.EINTR:
        continue
      raise
    except select.error, serr:
      if serr[0] == errno.EINTR:
        continue
      raise

    for fd, event in pollresult:
      if event & select.POLLIN or event & select.POLLPRI:
        data = fdmap[fd][1].read()
        # no data from read signifies EOF (the same as POLLHUP)
        if not data:
          poller.unregister(fd)
          del fdmap[fd]
          continue
        fdmap[fd][0].write(data)
      if (event & select.POLLNVAL or event & select.POLLHUP or
          event & select.POLLERR):
        poller.unregister(fd)
        del fdmap[fd]

  out = out.getvalue()
  err = err.getvalue()

  status = child.wait()
  return out, err, status


def _RunCmdFile(cmd, env, via_shell, output, cwd):
  """Run a command and save its output to a file.

  @type  cmd: string or list
  @param cmd: Command to run
  @type env: dict
  @param env: The environment to use
  @type via_shell: bool
  @param via_shell: if we should run via the shell
  @type output: str
  @param output: the filename in which to save the output
  @type cwd: string
  @param cwd: the working directory for the program
  @rtype: int
  @return: the exit status

  """
  fh = open(output, "a")
  try:
    child = subprocess.Popen(cmd, shell=via_shell,
                             stderr=subprocess.STDOUT,
                             stdout=fh,
                             stdin=subprocess.PIPE,
                             close_fds=True, env=env,
                             cwd=cwd)

    child.stdin.close()
    status = child.wait()
  finally:
    fh.close()
  return status


def RemoveFile(filename):
  """Remove a file ignoring some errors.

  Remove a file, ignoring non-existing ones or directories. Other
  errors are passed.

  @type filename: str
  @param filename: the file to be removed

  """
  try:
    os.unlink(filename)
  except OSError, err:
    if err.errno not in (errno.ENOENT, errno.EISDIR):
      raise


def RenameFile(old, new, mkdir=False, mkdir_mode=0750):
  """Renames a file.

  @type old: string
  @param old: Original path
  @type new: string
  @param new: New path
  @type mkdir: bool
  @param mkdir: Whether to create target directory if it doesn't exist
  @type mkdir_mode: int
  @param mkdir_mode: Mode for newly created directories

  """
  try:
    return os.rename(old, new)
  except OSError, err:
    # In at least one use case of this function, the job queue, directory
    # creation is very rare. Checking for the directory before renaming is not
    # as efficient.
    if mkdir and err.errno == errno.ENOENT:
      # Create directory and try again
      os.makedirs(os.path.dirname(new), mkdir_mode)
      return os.rename(old, new)
    raise


def _FingerprintFile(filename):
  """Compute the fingerprint of a file.

  If the file does not exist, a None will be returned
  instead.

  @type filename: str
  @param filename: the filename to checksum
  @rtype: str
  @return: the hex digest of the sha checksum of the contents
      of the file

  """
  if not (os.path.exists(filename) and os.path.isfile(filename)):
    return None

  f = open(filename)

  fp = sha.sha()
  while True:
    data = f.read(4096)
    if not data:
      break

    fp.update(data)

  return fp.hexdigest()


def FingerprintFiles(files):
  """Compute fingerprints for a list of files.

  @type files: list
  @param files: the list of filename to fingerprint
  @rtype: dict
  @return: a dictionary filename: fingerprint, holding only
      existing files

  """
  ret = {}

  for filename in files:
    cksum = _FingerprintFile(filename)
    if cksum:
      ret[filename] = cksum

  return ret


def CheckDict(target, template, logname=None):
  """Ensure a dictionary has a required set of keys.

  For the given dictionaries I{target} and I{template}, ensure
  I{target} has all the keys from I{template}. Missing keys are added
  with values from template.

  @type target: dict
  @param target: the dictionary to update
  @type template: dict
  @param template: the dictionary holding the default values
  @type logname: str or None
  @param logname: if not None, causes the missing keys to be
      logged with this name

  """
  missing = []
  for k in template:
    if k not in target:
      missing.append(k)
      target[k] = template[k]

  if missing and logname:
    logging.warning('%s missing keys %s', logname, ', '.join(missing))


def IsProcessAlive(pid):
  """Check if a given pid exists on the system.

  @note: zombie status is not handled, so zombie processes
      will be returned as alive
  @type pid: int
  @param pid: the process ID to check
  @rtype: boolean
  @return: True if the process exists

  """
  if pid <= 0:
    return False

  try:
    os.stat("/proc/%d/status" % pid)
    return True
  except EnvironmentError, err:
    if err.errno in (errno.ENOENT, errno.ENOTDIR):
      return False
    raise


def ReadPidFile(pidfile):
  """Read a pid from a file.

  @type  pidfile: string
  @param pidfile: path to the file containing the pid
  @rtype: int
  @return: The process id, if the file exista and contains a valid PID,
           otherwise 0

  """
  try:
    pf = open(pidfile, 'r')
  except EnvironmentError, err:
    if err.errno != errno.ENOENT:
      logging.exception("Can't read pid file?!")
    return 0

  try:
    pid = int(pf.read())
  except ValueError, err:
    logging.info("Can't parse pid file contents", exc_info=True)
    return 0

  return pid


def MatchNameComponent(key, name_list):
  """Try to match a name against a list.

  This function will try to match a name like test1 against a list
  like C{['test1.example.com', 'test2.example.com', ...]}. Against
  this list, I{'test1'} as well as I{'test1.example'} will match, but
  not I{'test1.ex'}. A multiple match will be considered as no match
  at all (e.g. I{'test1'} against C{['test1.example.com',
  'test1.example.org']}).

  @type key: str
  @param key: the name to be searched
  @type name_list: list
  @param name_list: the list of strings against which to search the key

  @rtype: None or str
  @return: None if there is no match I{or} if there are multiple matches,
      otherwise the element from the list which matches

  """
  mo = re.compile("^%s(\..*)?$" % re.escape(key))
  names_filtered = [name for name in name_list if mo.match(name) is not None]
  if len(names_filtered) != 1:
    return None
  return names_filtered[0]


class HostInfo:
  """Class implementing resolver and hostname functionality

  """
  def __init__(self, name=None):
    """Initialize the host name object.

    If the name argument is not passed, it will use this system's
    name.

    """
    if name is None:
      name = self.SysName()

    self.query = name
    self.name, self.aliases, self.ipaddrs = self.LookupHostname(name)
    self.ip = self.ipaddrs[0]

  def ShortName(self):
    """Returns the hostname without domain.

    """
    return self.name.split('.')[0]

  @staticmethod
  def SysName():
    """Return the current system's name.

    This is simply a wrapper over C{socket.gethostname()}.

    """
    return socket.gethostname()

  @staticmethod
  def LookupHostname(hostname):
    """Look up hostname

    @type hostname: str
    @param hostname: hostname to look up

    @rtype: tuple
    @return: a tuple (name, aliases, ipaddrs) as returned by
        C{socket.gethostbyname_ex}
    @raise errors.ResolverError: in case of errors in resolving

    """
    try:
      result = socket.gethostbyname_ex(hostname)
    except socket.gaierror, err:
      # hostname not found in DNS
      raise errors.ResolverError(hostname, err.args[0], err.args[1])

    return result


def ListVolumeGroups():
  """List volume groups and their size

  @rtype: dict
  @return:
       Dictionary with keys volume name and values
       the size of the volume

  """
  command = "vgs --noheadings --units m --nosuffix -o name,size"
  result = RunCmd(command)
  retval = {}
  if result.failed:
    return retval

  for line in result.stdout.splitlines():
    try:
      name, size = line.split()
      size = int(float(size))
    except (IndexError, ValueError), err:
      logging.error("Invalid output from vgs (%s): %s", err, line)
      continue

    retval[name] = size

  return retval


def BridgeExists(bridge):
  """Check whether the given bridge exists in the system

  @type bridge: str
  @param bridge: the bridge name to check
  @rtype: boolean
  @return: True if it does

  """
  return os.path.isdir("/sys/class/net/%s/bridge" % bridge)


def CheckBEParams(beparams):
  """Checks whether the user-supplied be-params are valid,
  and converts them from string format where appropriate.

  @type beparams: dict
  @param beparams: new params dict

  """
  if beparams:
    for item in beparams:
      if item not in constants.BES_PARAMETERS:
        raise errors.OpPrereqError("Unknown backend parameter %s" % item)
      if item in (constants.BE_MEMORY, constants.BE_VCPUS):
        val = beparams[item]
        if val != constants.VALUE_DEFAULT:
          try:
            val = int(val)
          except ValueError, err:
            raise errors.OpPrereqError("Invalid %s size: %s" % (item, str(err)))
          beparams[item] = val
      if item in (constants.BE_AUTO_BALANCE):
        val = beparams[item]
        if not isinstance(val, bool):
          if val == constants.VALUE_TRUE:
            beparams[item] = True
          elif val == constants.VALUE_FALSE:
            beparams[item] = False
          else:
            raise errors.OpPrereqError("Invalid %s value: %s" % (item, val))


def NiceSort(name_list):
  """Sort a list of strings based on digit and non-digit groupings.

  Given a list of names C{['a1', 'a10', 'a11', 'a2']} this function
  will sort the list in the logical order C{['a1', 'a2', 'a10',
  'a11']}.

  The sort algorithm breaks each name in groups of either only-digits
  or no-digits. Only the first eight such groups are considered, and
  after that we just use what's left of the string.

  @type name_list: list
  @param name_list: the names to be sorted
  @rtype: list
  @return: a copy of the name list sorted with our algorithm

  """
  _SORTER_BASE = "(\D+|\d+)"
  _SORTER_FULL = "^%s%s?%s?%s?%s?%s?%s?%s?.*$" % (_SORTER_BASE, _SORTER_BASE,
                                                  _SORTER_BASE, _SORTER_BASE,
                                                  _SORTER_BASE, _SORTER_BASE,
                                                  _SORTER_BASE, _SORTER_BASE)
  _SORTER_RE = re.compile(_SORTER_FULL)
  _SORTER_NODIGIT = re.compile("^\D*$")
  def _TryInt(val):
    """Attempts to convert a variable to integer."""
    if val is None or _SORTER_NODIGIT.match(val):
      return val
    rval = int(val)
    return rval

  to_sort = [([_TryInt(grp) for grp in _SORTER_RE.match(name).groups()], name)
             for name in name_list]
  to_sort.sort()
  return [tup[1] for tup in to_sort]


def TryConvert(fn, val):
  """Try to convert a value ignoring errors.

  This function tries to apply function I{fn} to I{val}. If no
  C{ValueError} or C{TypeError} exceptions are raised, it will return
  the result, else it will return the original value. Any other
  exceptions are propagated to the caller.

  @type fn: callable
  @param fn: function to apply to the value
  @param val: the value to be converted
  @return: The converted value if the conversion was successful,
      otherwise the original value.

  """
  try:
    nv = fn(val)
  except (ValueError, TypeError), err:
    nv = val
  return nv


def IsValidIP(ip):
  """Verifies the syntax of an IPv4 address.

  This function checks if the IPv4 address passes is valid or not based
  on syntax (not IP range, class calculations, etc.).

  @type ip: str
  @param ip: the address to be checked
  @rtype: a regular expression match object
  @return: a regular epression match object, or None if the
      address is not valid

  """
  unit = "(0|[1-9]\d{0,2})"
  #TODO: convert and return only boolean
  return re.match("^%s\.%s\.%s\.%s$" % (unit, unit, unit, unit), ip)


def IsValidShellParam(word):
  """Verifies is the given word is safe from the shell's p.o.v.

  This means that we can pass this to a command via the shell and be
  sure that it doesn't alter the command line and is passed as such to
  the actual command.

  Note that we are overly restrictive here, in order to be on the safe
  side.

  @type word: str
  @param word: the word to check
  @rtype: boolean
  @return: True if the word is 'safe'

  """
  return bool(re.match("^[-a-zA-Z0-9._+/:%@]+$", word))


def BuildShellCmd(template, *args):
  """Build a safe shell command line from the given arguments.

  This function will check all arguments in the args list so that they
  are valid shell parameters (i.e. they don't contain shell
  metacharaters). If everything is ok, it will return the result of
  template % args.

  @type template: str
  @param template: the string holding the template for the
      string formatting
  @rtype: str
  @return: the expanded command line

  """
  for word in args:
    if not IsValidShellParam(word):
      raise errors.ProgrammerError("Shell argument '%s' contains"
                                   " invalid characters" % word)
  return template % args


def FormatUnit(value, units):
  """Formats an incoming number of MiB with the appropriate unit.

  @type value: int
  @param value: integer representing the value in MiB (1048576)
  @type units: char
  @param units: the type of formatting we should do:
      - 'h' for automatic scaling
      - 'm' for MiBs
      - 'g' for GiBs
      - 't' for TiBs
  @rtype: str
  @return: the formatted value (with suffix)

  """
  if units not in ('m', 'g', 't', 'h'):
    raise errors.ProgrammerError("Invalid unit specified '%s'" % str(units))

  suffix = ''

  if units == 'm' or (units == 'h' and value < 1024):
    if units == 'h':
      suffix = 'M'
    return "%d%s" % (round(value, 0), suffix)

  elif units == 'g' or (units == 'h' and value < (1024 * 1024)):
    if units == 'h':
      suffix = 'G'
    return "%0.1f%s" % (round(float(value) / 1024, 1), suffix)

  else:
    if units == 'h':
      suffix = 'T'
    return "%0.1f%s" % (round(float(value) / 1024 / 1024, 1), suffix)


def ParseUnit(input_string):
  """Tries to extract number and scale from the given string.

  Input must be in the format C{NUMBER+ [DOT NUMBER+] SPACE*
  [UNIT]}. If no unit is specified, it defaults to MiB. Return value
  is always an int in MiB.

  """
  m = re.match('^([.\d]+)\s*([a-zA-Z]+)?$', str(input_string))
  if not m:
    raise errors.UnitParseError("Invalid format")

  value = float(m.groups()[0])

  unit = m.groups()[1]
  if unit:
    lcunit = unit.lower()
  else:
    lcunit = 'm'

  if lcunit in ('m', 'mb', 'mib'):
    # Value already in MiB
    pass

  elif lcunit in ('g', 'gb', 'gib'):
    value *= 1024

  elif lcunit in ('t', 'tb', 'tib'):
    value *= 1024 * 1024

  else:
    raise errors.UnitParseError("Unknown unit: %s" % unit)

  # Make sure we round up
  if int(value) < value:
    value += 1

  # Round up to the next multiple of 4
  value = int(value)
  if value % 4:
    value += 4 - value % 4

  return value


def AddAuthorizedKey(file_name, key):
  """Adds an SSH public key to an authorized_keys file.

  @type file_name: str
  @param file_name: path to authorized_keys file
  @type key: str
  @param key: string containing key

  """
  key_fields = key.split()

  f = open(file_name, 'a+')
  try:
    nl = True
    for line in f:
      # Ignore whitespace changes
      if line.split() == key_fields:
        break
      nl = line.endswith('\n')
    else:
      if not nl:
        f.write("\n")
      f.write(key.rstrip('\r\n'))
      f.write("\n")
      f.flush()
  finally:
    f.close()


def RemoveAuthorizedKey(file_name, key):
  """Removes an SSH public key from an authorized_keys file.

  @type file_name: str
  @param file_name: path to authorized_keys file
  @type key: str
  @param key: string containing key

  """
  key_fields = key.split()

  fd, tmpname = tempfile.mkstemp(dir=os.path.dirname(file_name))
  try:
    out = os.fdopen(fd, 'w')
    try:
      f = open(file_name, 'r')
      try:
        for line in f:
          # Ignore whitespace changes while comparing lines
          if line.split() != key_fields:
            out.write(line)

        out.flush()
        os.rename(tmpname, file_name)
      finally:
        f.close()
    finally:
      out.close()
  except:
    RemoveFile(tmpname)
    raise


def SetEtcHostsEntry(file_name, ip, hostname, aliases):
  """Sets the name of an IP address and hostname in /etc/hosts.

  @type file_name: str
  @param file_name: path to the file to modify (usually C{/etc/hosts})
  @type ip: str
  @param ip: the IP address
  @type hostname: str
  @param hostname: the hostname to be added
  @type aliases: list
  @param aliases: the list of aliases to add for the hostname

  """
  # Ensure aliases are unique
  aliases = UniqueSequence([hostname] + aliases)[1:]

  fd, tmpname = tempfile.mkstemp(dir=os.path.dirname(file_name))
  try:
    out = os.fdopen(fd, 'w')
    try:
      f = open(file_name, 'r')
      try:
        for line in f:
          fields = line.split()
          if fields and not fields[0].startswith('#') and ip == fields[0]:
            continue
          out.write(line)

        out.write("%s\t%s" % (ip, hostname))
        if aliases:
          out.write(" %s" % ' '.join(aliases))
        out.write('\n')

        out.flush()
        os.fsync(out)
        os.rename(tmpname, file_name)
      finally:
        f.close()
    finally:
      out.close()
  except:
    RemoveFile(tmpname)
    raise


def AddHostToEtcHosts(hostname):
  """Wrapper around SetEtcHostsEntry.

  @type hostname: str
  @param hostname: a hostname that will be resolved and added to
      L{constants.ETC_HOSTS}

  """
  hi = HostInfo(name=hostname)
  SetEtcHostsEntry(constants.ETC_HOSTS, hi.ip, hi.name, [hi.ShortName()])


def RemoveEtcHostsEntry(file_name, hostname):
  """Removes a hostname from /etc/hosts.

  IP addresses without names are removed from the file.

  @type file_name: str
  @param file_name: path to the file to modify (usually C{/etc/hosts})
  @type hostname: str
  @param hostname: the hostname to be removed

  """
  fd, tmpname = tempfile.mkstemp(dir=os.path.dirname(file_name))
  try:
    out = os.fdopen(fd, 'w')
    try:
      f = open(file_name, 'r')
      try:
        for line in f:
          fields = line.split()
          if len(fields) > 1 and not fields[0].startswith('#'):
            names = fields[1:]
            if hostname in names:
              while hostname in names:
                names.remove(hostname)
              if names:
                out.write("%s %s\n" % (fields[0], ' '.join(names)))
              continue

          out.write(line)

        out.flush()
        os.fsync(out)
        os.rename(tmpname, file_name)
      finally:
        f.close()
    finally:
      out.close()
  except:
    RemoveFile(tmpname)
    raise


def RemoveHostFromEtcHosts(hostname):
  """Wrapper around RemoveEtcHostsEntry.

  @type hostname: str
  @param hostname: hostname that will be resolved and its
      full and shot name will be removed from
      L{constants.ETC_HOSTS}

  """
  hi = HostInfo(name=hostname)
  RemoveEtcHostsEntry(constants.ETC_HOSTS, hi.name)
  RemoveEtcHostsEntry(constants.ETC_HOSTS, hi.ShortName())


def CreateBackup(file_name):
  """Creates a backup of a file.

  @type file_name: str
  @param file_name: file to be backed up
  @rtype: str
  @return: the path to the newly created backup
  @raise errors.ProgrammerError: for invalid file names

  """
  if not os.path.isfile(file_name):
    raise errors.ProgrammerError("Can't make a backup of a non-file '%s'" %
                                file_name)

  prefix = '%s.backup-%d.' % (os.path.basename(file_name), int(time.time()))
  dir_name = os.path.dirname(file_name)

  fsrc = open(file_name, 'rb')
  try:
    (fd, backup_name) = tempfile.mkstemp(prefix=prefix, dir=dir_name)
    fdst = os.fdopen(fd, 'wb')
    try:
      shutil.copyfileobj(fsrc, fdst)
    finally:
      fdst.close()
  finally:
    fsrc.close()

  return backup_name


def ShellQuote(value):
  """Quotes shell argument according to POSIX.

  @type value: str
  @param value: the argument to be quoted
  @rtype: str
  @return: the quoted value

  """
  if _re_shell_unquoted.match(value):
    return value
  else:
    return "'%s'" % value.replace("'", "'\\''")


def ShellQuoteArgs(args):
  """Quotes a list of shell arguments.

  @type args: list
  @param args: list of arguments to be quoted
  @rtype: str
  @return: the quoted arguments concatenaned with spaces

  """
  return ' '.join([ShellQuote(i) for i in args])


def TcpPing(target, port, timeout=10, live_port_needed=False, source=None):
  """Simple ping implementation using TCP connect(2).

  Check if the given IP is reachable by doing attempting a TCP connect
  to it.

  @type target: str
  @param target: the IP or hostname to ping
  @type port: int
  @param port: the port to connect to
  @type timeout: int
  @param timeout: the timeout on the connection attemp
  @type live_port_needed: boolean
  @param live_port_needed: whether a closed port will cause the
      function to return failure, as if there was a timeout
  @type source: str or None
  @param source: if specified, will cause the connect to be made
      from this specific source address; failures to bind other
      than C{EADDRNOTAVAIL} will be ignored

  """
  sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

  success = False

  if source is not None:
    try:
      sock.bind((source, 0))
    except socket.error, (errcode, errstring):
      if errcode == errno.EADDRNOTAVAIL:
        success = False

  sock.settimeout(timeout)

  try:
    sock.connect((target, port))
    sock.close()
    success = True
  except socket.timeout:
    success = False
  except socket.error, (errcode, errstring):
    success = (not live_port_needed) and (errcode == errno.ECONNREFUSED)

  return success


def OwnIpAddress(address):
  """Check if the current host has the the given IP address.

  Currently this is done by TCP-pinging the address from the loopback
  address.

  @type address: string
  @param address: the addres to check
  @rtype: bool
  @return: True if we own the address

  """
  return TcpPing(address, constants.DEFAULT_NODED_PORT,
                 source=constants.LOCALHOST_IP_ADDRESS)


def ListVisibleFiles(path):
  """Returns a list of visible files in a directory.

  @type path: str
  @param path: the directory to enumerate
  @rtype: list
  @return: the list of all files not starting with a dot

  """
  files = [i for i in os.listdir(path) if not i.startswith(".")]
  files.sort()
  return files


def GetHomeDir(user, default=None):
  """Try to get the homedir of the given user.

  The user can be passed either as a string (denoting the name) or as
  an integer (denoting the user id). If the user is not found, the
  'default' argument is returned, which defaults to None.

  """
  try:
    if isinstance(user, basestring):
      result = pwd.getpwnam(user)
    elif isinstance(user, (int, long)):
      result = pwd.getpwuid(user)
    else:
      raise errors.ProgrammerError("Invalid type passed to GetHomeDir (%s)" %
                                   type(user))
  except KeyError:
    return default
  return result.pw_dir


def NewUUID():
  """Returns a random UUID.

  @note: This is a Linux-specific method as it uses the /proc
      filesystem.
  @rtype: str

  """
  f = open("/proc/sys/kernel/random/uuid", "r")
  try:
    return f.read(128).rstrip("\n")
  finally:
    f.close()


def GenerateSecret():
  """Generates a random secret.

  This will generate a pseudo-random secret, and return its sha digest
  (so that it can be used where an ASCII string is needed).

  @rtype: str
  @return: a sha1 hexdigest of a block of 64 random bytes

  """
  return sha.new(os.urandom(64)).hexdigest()


def ReadFile(file_name, size=None):
  """Reads a file.

  @type size: None or int
  @param size: Read at most size bytes
  @rtype: str
  @return: the (possibly partial) conent of the file

  """
  f = open(file_name, "r")
  try:
    if size is None:
      return f.read()
    else:
      return f.read(size)
  finally:
    f.close()


def WriteFile(file_name, fn=None, data=None,
              mode=None, uid=-1, gid=-1,
              atime=None, mtime=None, close=True,
              dry_run=False, backup=False,
              prewrite=None, postwrite=None):
  """(Over)write a file atomically.

  The file_name and either fn (a function taking one argument, the
  file descriptor, and which should write the data to it) or data (the
  contents of the file) must be passed. The other arguments are
  optional and allow setting the file mode, owner and group, and the
  mtime/atime of the file.

  If the function doesn't raise an exception, it has succeeded and the
  target file has the new contents. If the file has raised an
  exception, an existing target file should be unmodified and the
  temporary file should be removed.

  @type file_name: str
  @param file_name: the target filename
  @type fn: callable
  @param fn: content writing function, called with
      file descriptor as parameter
  @type data: sr
  @param data: contents of the file
  @type mode: int
  @param mode: file mode
  @type uid: int
  @param uid: the owner of the file
  @type gid: int
  @param gid: the group of the file
  @type atime: int
  @param atime: a custom access time to be set on the file
  @type mtime: int
  @param mtime: a custom modification time to be set on the file
  @type close: boolean
  @param close: whether to close file after writing it
  @type prewrite: callable
  @param prewrite: function to be called before writing content
  @type postwrite: callable
  @param postwrite: function to be called after writing content

  @rtype: None or int
  @return: None if the 'close' parameter evaluates to True,
      otherwise the file descriptor

  @raise errors.ProgrammerError: if an of the arguments are not valid

  """
  if not os.path.isabs(file_name):
    raise errors.ProgrammerError("Path passed to WriteFile is not"
                                 " absolute: '%s'" % file_name)

  if [fn, data].count(None) != 1:
    raise errors.ProgrammerError("fn or data required")

  if [atime, mtime].count(None) == 1:
    raise errors.ProgrammerError("Both atime and mtime must be either"
                                 " set or None")

  if backup and not dry_run and os.path.isfile(file_name):
    CreateBackup(file_name)

  dir_name, base_name = os.path.split(file_name)
  fd, new_name = tempfile.mkstemp('.new', base_name, dir_name)
  # here we need to make sure we remove the temp file, if any error
  # leaves it in place
  try:
    if uid != -1 or gid != -1:
      os.chown(new_name, uid, gid)
    if mode:
      os.chmod(new_name, mode)
    if callable(prewrite):
      prewrite(fd)
    if data is not None:
      os.write(fd, data)
    else:
      fn(fd)
    if callable(postwrite):
      postwrite(fd)
    os.fsync(fd)
    if atime is not None and mtime is not None:
      os.utime(new_name, (atime, mtime))
    if not dry_run:
      os.rename(new_name, file_name)
  finally:
    if close:
      os.close(fd)
      result = None
    else:
      result = fd
    RemoveFile(new_name)

  return result


def FirstFree(seq, base=0):
  """Returns the first non-existing integer from seq.

  The seq argument should be a sorted list of positive integers. The
  first time the index of an element is smaller than the element
  value, the index will be returned.

  The base argument is used to start at a different offset,
  i.e. C{[3, 4, 6]} with I{offset=3} will return 5.

  Example: C{[0, 1, 3]} will return I{2}.

  @type seq: sequence
  @param seq: the sequence to be analyzed.
  @type base: int
  @param base: use this value as the base index of the sequence
  @rtype: int
  @return: the first non-used index in the sequence

  """
  for idx, elem in enumerate(seq):
    assert elem >= base, "Passed element is higher than base offset"
    if elem > idx + base:
      # idx is not used
      return idx + base
  return None


def all(seq, pred=bool):
  "Returns True if pred(x) is True for every element in the iterable"
  for elem in itertools.ifilterfalse(pred, seq):
    return False
  return True


def any(seq, pred=bool):
  "Returns True if pred(x) is True for at least one element in the iterable"
  for elem in itertools.ifilter(pred, seq):
    return True
  return False


def UniqueSequence(seq):
  """Returns a list with unique elements.

  Element order is preserved.

  @type seq: sequence
  @param seq: the sequence with the source elementes
  @rtype: list
  @return: list of unique elements from seq

  """
  seen = set()
  return [i for i in seq if i not in seen and not seen.add(i)]


def IsValidMac(mac):
  """Predicate to check if a MAC address is valid.

  Checks wether the supplied MAC address is formally correct, only
  accepts colon separated format.

  @type mac: str
  @param mac: the MAC to be validated
  @rtype: boolean
  @return: True is the MAC seems valid

  """
  mac_check = re.compile("^([0-9a-f]{2}(:|$)){6}$")
  return mac_check.match(mac) is not None


def TestDelay(duration):
  """Sleep for a fixed amount of time.

  @type duration: float
  @param duration: the sleep duration
  @rtype: boolean
  @return: False for negative value, True otherwise

  """
  if duration < 0:
    return False
  time.sleep(duration)
  return True


def _CloseFDNoErr(fd, retries=5):
  """Close a file descriptor ignoring errors.

  @type fd: int
  @param fd: the file descriptor
  @type retries: int
  @param retries: how many retries to make, in case we get any
      other error than EBADF

  """
  try:
    os.close(fd)
  except OSError, err:
    if err.errno != errno.EBADF:
      if retries > 0:
        _CloseFDNoErr(fd, retries - 1)
    # else either it's closed already or we're out of retries, so we
    # ignore this and go on


def CloseFDs(noclose_fds=None):
  """Close file descriptors.

  This closes all file descriptors above 2 (i.e. except
  stdin/out/err).

  @type noclose_fds: list or None
  @param noclose_fds: if given, it denotes a list of file descriptor
      that should not be closed

  """
  # Default maximum for the number of available file descriptors.
  if 'SC_OPEN_MAX' in os.sysconf_names:
    try:
      MAXFD = os.sysconf('SC_OPEN_MAX')
      if MAXFD < 0:
        MAXFD = 1024
    except OSError:
      MAXFD = 1024
  else:
    MAXFD = 1024
  maxfd = resource.getrlimit(resource.RLIMIT_NOFILE)[1]
  if (maxfd == resource.RLIM_INFINITY):
    maxfd = MAXFD

  # Iterate through and close all file descriptors (except the standard ones)
  for fd in range(3, maxfd):
    if noclose_fds and fd in noclose_fds:
      continue
    _CloseFDNoErr(fd)


def Daemonize(logfile):
  """Daemonize the current process.

  This detaches the current process from the controlling terminal and
  runs it in the background as a daemon.

  @type logfile: str
  @param logfile: the logfile to which we should redirect stdout/stderr
  @rtype: int
  @returns: the value zero

  """
  UMASK = 077
  WORKDIR = "/"

  # this might fail
  pid = os.fork()
  if (pid == 0):  # The first child.
    os.setsid()
    # this might fail
    pid = os.fork() # Fork a second child.
    if (pid == 0):  # The second child.
      os.chdir(WORKDIR)
      os.umask(UMASK)
    else:
      # exit() or _exit()?  See below.
      os._exit(0) # Exit parent (the first child) of the second child.
  else:
    os._exit(0) # Exit parent of the first child.

  for fd in range(3):
    _CloseFDNoErr(fd)
  i = os.open("/dev/null", os.O_RDONLY) # stdin
  assert i == 0, "Can't close/reopen stdin"
  i = os.open(logfile, os.O_WRONLY|os.O_CREAT|os.O_APPEND, 0600) # stdout
  assert i == 1, "Can't close/reopen stdout"
  # Duplicate standard output to standard error.
  os.dup2(1, 2)
  return 0


def DaemonPidFileName(name):
  """Compute a ganeti pid file absolute path

  @type name: str
  @param name: the daemon name
  @rtype: str
  @return: the full path to the pidfile corresponding to the given
      daemon name

  """
  return os.path.join(constants.RUN_GANETI_DIR, "%s.pid" % name)


def WritePidFile(name):
  """Write the current process pidfile.

  The file will be written to L{constants.RUN_GANETI_DIR}I{/name.pid}

  @type name: str
  @param name: the daemon name to use
  @raise errors.GenericError: if the pid file already exists and
      points to a live process

  """
  pid = os.getpid()
  pidfilename = DaemonPidFileName(name)
  if IsProcessAlive(ReadPidFile(pidfilename)):
    raise errors.GenericError("%s contains a live process" % pidfilename)

  WriteFile(pidfilename, data="%d\n" % pid)


def RemovePidFile(name):
  """Remove the current process pidfile.

  Any errors are ignored.

  @type name: str
  @param name: the daemon name used to derive the pidfile name

  """
  pid = os.getpid()
  pidfilename = DaemonPidFileName(name)
  # TODO: we could check here that the file contains our pid
  try:
    RemoveFile(pidfilename)
  except:
    pass


def KillProcess(pid, signal_=signal.SIGTERM, timeout=30,
                waitpid=False):
  """Kill a process given by its pid.

  @type pid: int
  @param pid: The PID to terminate.
  @type signal_: int
  @param signal_: The signal to send, by default SIGTERM
  @type timeout: int
  @param timeout: The timeout after which, if the process is still alive,
                  a SIGKILL will be sent. If not positive, no such checking
                  will be done
  @type waitpid: boolean
  @param waitpid: If true, we should waitpid on this process after
      sending signals, since it's our own child and otherwise it
      would remain as zombie

  """
  def _helper(pid, signal_, wait):
    """Simple helper to encapsulate the kill/waitpid sequence"""
    os.kill(pid, signal_)
    if wait:
      try:
        os.waitpid(pid, os.WNOHANG)
      except OSError:
        pass

  if pid <= 0:
    # kill with pid=0 == suicide
    raise errors.ProgrammerError("Invalid pid given '%s'" % pid)

  if not IsProcessAlive(pid):
    return
  _helper(pid, signal_, waitpid)
  if timeout <= 0:
    return

  # Wait up to $timeout seconds
  end = time.time() + timeout
  wait = 0.01
  while time.time() < end and IsProcessAlive(pid):
    try:
      (result_pid, _) = os.waitpid(pid, os.WNOHANG)
      if result_pid > 0:
        break
    except OSError:
      pass
    time.sleep(wait)
    # Make wait time longer for next try
    if wait < 0.1:
      wait *= 1.5

  if IsProcessAlive(pid):
    # Kill process if it's still alive
    _helper(pid, signal.SIGKILL, waitpid)


def FindFile(name, search_path, test=os.path.exists):
  """Look for a filesystem object in a given path.

  This is an abstract method to search for filesystem object (files,
  dirs) under a given search path.

  @type name: str
  @param name: the name to look for
  @type search_path: str
  @param search_path: location to start at
  @type test: callable
  @param test: a function taking one argument that should return True
      if the a given object is valid; the default value is
      os.path.exists, causing only existing files to be returned
  @rtype: str or None
  @return: full path to the object if found, None otherwise

  """
  for dir_name in search_path:
    item_name = os.path.sep.join([dir_name, name])
    if test(item_name):
      return item_name
  return None


def CheckVolumeGroupSize(vglist, vgname, minsize):
  """Checks if the volume group list is valid.

  The function will check if a given volume group is in the list of
  volume groups and has a minimum size.

  @type vglist: dict
  @param vglist: dictionary of volume group names and their size
  @type vgname: str
  @param vgname: the volume group we should check
  @type minsize: int
  @param minsize: the minimum size we accept
  @rtype: None or str
  @return: None for success, otherwise the error message

  """
  vgsize = vglist.get(vgname, None)
  if vgsize is None:
    return "volume group '%s' missing" % vgname
  elif vgsize < minsize:
    return ("volume group '%s' too small (%s MiB required, %d MiB found)" %
            (vgname, minsize, vgsize))
  return None


def SplitTime(value):
  """Splits time as floating point number into a tuple.

  @param value: Time in seconds
  @type value: int or float
  @return: Tuple containing (seconds, microseconds)

  """
  (seconds, microseconds) = divmod(int(value * 1000000), 1000000)

  assert 0 <= seconds, \
    "Seconds must be larger than or equal to 0, but are %s" % seconds
  assert 0 <= microseconds <= 999999, \
    "Microseconds must be 0-999999, but are %s" % microseconds

  return (int(seconds), int(microseconds))


def MergeTime(timetuple):
  """Merges a tuple into time as a floating point number.

  @param timetuple: Time as tuple, (seconds, microseconds)
  @type timetuple: tuple
  @return: Time as a floating point number expressed in seconds

  """
  (seconds, microseconds) = timetuple

  assert 0 <= seconds, \
    "Seconds must be larger than or equal to 0, but are %s" % seconds
  assert 0 <= microseconds <= 999999, \
    "Microseconds must be 0-999999, but are %s" % microseconds

  return float(seconds) + (float(microseconds) * 0.000001)


def GetNodeDaemonPort():
  """Get the node daemon port for this cluster.

  Note that this routine does not read a ganeti-specific file, but
  instead uses C{socket.getservbyname} to allow pre-customization of
  this parameter outside of Ganeti.

  @rtype: int

  """
  try:
    port = socket.getservbyname("ganeti-noded", "tcp")
  except socket.error:
    port = constants.DEFAULT_NODED_PORT

  return port


def SetupLogging(logfile, debug=False, stderr_logging=False, program="",
                 multithreaded=False):
  """Configures the logging module.

  @type logfile: str
  @param logfile: the filename to which we should log
  @type debug: boolean
  @param debug: whether to enable debug messages too or
      only those at C{INFO} and above level
  @type stderr_logging: boolean
  @param stderr_logging: whether we should also log to the standard error
  @type program: str
  @param program: the name under which we should log messages
  @type multithreaded: boolean
  @param multithreaded: if True, will add the thread name to the log file
  @raise EnvironmentError: if we can't open the log file and
      stderr logging is disabled

  """
  fmt = "%(asctime)s: " + program + " pid=%(process)d"
  if multithreaded:
    fmt += "/%(threadName)s"
  if debug:
    fmt += " %(module)s:%(lineno)s"
  fmt += " %(levelname)s %(message)s"
  formatter = logging.Formatter(fmt)

  root_logger = logging.getLogger("")
  root_logger.setLevel(logging.NOTSET)

  # Remove all previously setup handlers
  for handler in root_logger.handlers:
    handler.close()
    root_logger.removeHandler(handler)

  if stderr_logging:
    stderr_handler = logging.StreamHandler()
    stderr_handler.setFormatter(formatter)
    if debug:
      stderr_handler.setLevel(logging.NOTSET)
    else:
      stderr_handler.setLevel(logging.CRITICAL)
    root_logger.addHandler(stderr_handler)

  # this can fail, if the logging directories are not setup or we have
  # a permisssion problem; in this case, it's best to log but ignore
  # the error if stderr_logging is True, and if false we re-raise the
  # exception since otherwise we could run but without any logs at all
  try:
    logfile_handler = logging.FileHandler(logfile)
    logfile_handler.setFormatter(formatter)
    if debug:
      logfile_handler.setLevel(logging.DEBUG)
    else:
      logfile_handler.setLevel(logging.INFO)
    root_logger.addHandler(logfile_handler)
  except EnvironmentError:
    if stderr_logging:
      logging.exception("Failed to enable logging to file '%s'", logfile)
    else:
      # we need to re-raise the exception
      raise


def TailFile(fname, lines=20):
  """Return the last lines from a file.

  @note: this function will only read and parse the last 4KB of
      the file; if the lines are very long, it could be that less
      than the requested number of lines are returned

  @param fname: the file name
  @type lines: int
  @param lines: the (maximum) number of lines to return

  """
  fd = open(fname, "r")
  try:
    fd.seek(0, 2)
    pos = fd.tell()
    pos = max(0, pos-4096)
    fd.seek(pos, 0)
    raw_data = fd.read()
  finally:
    fd.close()

  rows = raw_data.splitlines()
  return rows[-lines:]


def LockedMethod(fn):
  """Synchronized object access decorator.

  This decorator is intended to protect access to an object using the
  object's own lock which is hardcoded to '_lock'.

  """
  def _LockDebug(*args, **kwargs):
    if debug_locks:
      logging.debug(*args, **kwargs)

  def wrapper(self, *args, **kwargs):
    assert hasattr(self, '_lock')
    lock = self._lock
    _LockDebug("Waiting for %s", lock)
    lock.acquire()
    try:
      _LockDebug("Acquired %s", lock)
      result = fn(self, *args, **kwargs)
    finally:
      _LockDebug("Releasing %s", lock)
      lock.release()
      _LockDebug("Released %s", lock)
    return result
  return wrapper


def LockFile(fd):
  """Locks a file using POSIX locks.

  @type fd: int
  @param fd: the file descriptor we need to lock

  """
  try:
    fcntl.flock(fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
  except IOError, err:
    if err.errno == errno.EAGAIN:
      raise errors.LockError("File already locked")
    raise


class FileLock(object):
  """Utility class for file locks.

  """
  def __init__(self, filename):
    """Constructor for FileLock.

    This will open the file denoted by the I{filename} argument.

    @type filename: str
    @param filename: path to the file to be locked

    """
    self.filename = filename
    self.fd = open(self.filename, "w")

  def __del__(self):
    self.Close()

  def Close(self):
    """Close the file and release the lock.

    """
    if self.fd:
      self.fd.close()
      self.fd = None

  def _flock(self, flag, blocking, timeout, errmsg):
    """Wrapper for fcntl.flock.

    @type flag: int
    @param flag: operation flag
    @type blocking: bool
    @param blocking: whether the operation should be done in blocking mode.
    @type timeout: None or float
    @param timeout: for how long the operation should be retried (implies
                    non-blocking mode).
    @type errmsg: string
    @param errmsg: error message in case operation fails.

    """
    assert self.fd, "Lock was closed"
    assert timeout is None or timeout >= 0, \
      "If specified, timeout must be positive"

    if timeout is not None:
      flag |= fcntl.LOCK_NB
      timeout_end = time.time() + timeout

    # Blocking doesn't have effect with timeout
    elif not blocking:
      flag |= fcntl.LOCK_NB
      timeout_end = None

    retry = True
    while retry:
      try:
        fcntl.flock(self.fd, flag)
        retry = False
      except IOError, err:
        if err.errno in (errno.EAGAIN, ):
          if timeout_end is not None and time.time() < timeout_end:
            # Wait before trying again
            time.sleep(max(0.1, min(1.0, timeout)))
          else:
            raise errors.LockError(errmsg)
        else:
          logging.exception("fcntl.flock failed")
          raise

  def Exclusive(self, blocking=False, timeout=None):
    """Locks the file in exclusive mode.

    @type blocking: boolean
    @param blocking: whether to block and wait until we
        can lock the file or return immediately
    @type timeout: int or None
    @param timeout: if not None, the duration to wait for the lock
        (in blocking mode)

    """
    self._flock(fcntl.LOCK_EX, blocking, timeout,
                "Failed to lock %s in exclusive mode" % self.filename)

  def Shared(self, blocking=False, timeout=None):
    """Locks the file in shared mode.

    @type blocking: boolean
    @param blocking: whether to block and wait until we
        can lock the file or return immediately
    @type timeout: int or None
    @param timeout: if not None, the duration to wait for the lock
        (in blocking mode)

    """
    self._flock(fcntl.LOCK_SH, blocking, timeout,
                "Failed to lock %s in shared mode" % self.filename)

  def Unlock(self, blocking=True, timeout=None):
    """Unlocks the file.

    According to C{flock(2)}, unlocking can also be a nonblocking
    operation::

      To make a non-blocking request, include LOCK_NB with any of the above
      operations.

    @type blocking: boolean
    @param blocking: whether to block and wait until we
        can lock the file or return immediately
    @type timeout: int or None
    @param timeout: if not None, the duration to wait for the lock
        (in blocking mode)

    """
    self._flock(fcntl.LOCK_UN, blocking, timeout,
                "Failed to unlock %s" % self.filename)


class SignalHandler(object):
  """Generic signal handler class.

  It automatically restores the original handler when deconstructed or
  when L{Reset} is called. You can either pass your own handler
  function in or query the L{called} attribute to detect whether the
  signal was sent.

  @type signum: list
  @ivar signum: the signals we handle
  @type called: boolean
  @ivar called: tracks whether any of the signals have been raised

  """
  def __init__(self, signum):
    """Constructs a new SignalHandler instance.

    @type signum: int or list of ints
    @param signum: Single signal number or set of signal numbers

    """
    if isinstance(signum, (int, long)):
      self.signum = set([signum])
    else:
      self.signum = set(signum)

    self.called = False

    self._previous = {}
    try:
      for signum in self.signum:
        # Setup handler
        prev_handler = signal.signal(signum, self._HandleSignal)
        try:
          self._previous[signum] = prev_handler
        except:
          # Restore previous handler
          signal.signal(signum, prev_handler)
          raise
    except:
      # Reset all handlers
      self.Reset()
      # Here we have a race condition: a handler may have already been called,
      # but there's not much we can do about it at this point.
      raise

  def __del__(self):
    self.Reset()

  def Reset(self):
    """Restore previous handler.

    This will reset all the signals to their previous handlers.

    """
    for signum, prev_handler in self._previous.items():
      signal.signal(signum, prev_handler)
      # If successful, remove from dict
      del self._previous[signum]

  def Clear(self):
    """Unsets the L{called} flag.

    This function can be used in case a signal may arrive several times.

    """
    self.called = False

  def _HandleSignal(self, signum, frame):
    """Actual signal handling function.

    """
    # This is not nice and not absolutely atomic, but it appears to be the only
    # solution in Python -- there are no atomic types.
    self.called = True


class FieldSet(object):
  """A simple field set.

  Among the features are:
    - checking if a string is among a list of static string or regex objects
    - checking if a whole list of string matches
    - returning the matching groups from a regex match

  Internally, all fields are held as regular expression objects.

  """
  def __init__(self, *items):
    self.items = [re.compile("^%s$" % value) for value in items]

  def Extend(self, other_set):
    """Extend the field set with the items from another one"""
    self.items.extend(other_set.items)

  def Matches(self, field):
    """Checks if a field matches the current set

    @type field: str
    @param field: the string to match
    @return: either False or a regular expression match object

    """
    for m in itertools.ifilter(None, (val.match(field) for val in self.items)):
      return m
    return False

  def NonMatching(self, items):
    """Returns the list of fields not matching the current set

    @type items: list
    @param items: the list of fields to check
    @rtype: list
    @return: list of non-matching fields

    """
    return [val for val in items if not self.Matches(val)]