Synnefo » snf-ganeti

#

#

# 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 small utilities

"""

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

from cStringIO import StringIO

from ganeti import logger

from ganeti import errors

from ganeti import constants

_locksheld = []

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

debug = False

class RunResult(object):

  """Simple class for holding the result of running external programs.

  Instance variables:

    exit_code: the exit code of the program, or None (if the program

               didn't exit())

    signal: numeric signal that caused the program to finish, or None

            (if the program wasn't terminated by a signal)

    stdout: the standard output of the program

    stderr: the standard error of the program

    failed: a Boolean value which is True in case the program was

            terminated by a signal or exited with a non-zero exit code

    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 debug and self.failed:

      logger.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 _GetLockFile(subsystem):

  """Compute the file name for a given lock name."""

  return "%s/ganeti_lock_%s" % (constants.LOCK_DIR, subsystem)

def Lock(name, max_retries=None, debug=False, autoclean=True):

  """Lock a given subsystem.

  In case the lock is already held by an alive process, the function

  will sleep indefintely and poll with a one second interval.

  When the optional integer argument 'max_retries' is passed with a

  non-zero value, the function will sleep only for this number of

  times, and then it will will raise a LockError if the lock can't be

  acquired. Passing in a negative number will cause only one try to

  get the lock. Passing a positive number will make the function retry

  for approximately that number of seconds.

  """

  lockfile = _GetLockFile(name)

  if name in _locksheld:

    raise errors.LockError('Lock "%s" already held!' % (name,))

  errcount = 0

  cleanupcount = 0

  retries = 0

  while True:

    try:

      fd = os.open(lockfile, os.O_CREAT | os.O_EXCL | os.O_RDWR | os.O_SYNC)

      break

    except OSError, creat_err:

      if creat_err.errno != errno.EEXIST:

        raise errors.LockError("Can't create the lock file. Error '%s'." %

                               str(creat_err))

      try:

        pf = open(lockfile, 'r')

      except IOError, open_err:

        errcount += 1

        if errcount >= 5:

          raise errors.LockError("Lock file exists but cannot be opened."

                                 " Error: '%s'." % str(open_err))

        time.sleep(1)

        continue

      try:

        pid = int(pf.read())

      except ValueError:

        raise errors.LockError("Invalid pid string in %s" %

                               (lockfile,))

      if not IsProcessAlive(pid):

        if autoclean:

          cleanupcount += 1

          if cleanupcount >= 5:

            raise errors.LockError, ("Too many stale lock cleanups! Check"

                                     " what process is dying.")

          logger.Error('Stale lockfile %s for pid %d, autocleaned.' %

                       (lockfile, pid))

          RemoveFile(lockfile)

          continue

        else:

          raise errors.LockError("Stale lockfile %s for pid %d?" %

                                 (lockfile, pid))

      if max_retries and max_retries <= retries:

        raise errors.LockError("Can't acquire lock during the specified"

                               " time, aborting.")

      if retries == 5 and (debug or sys.stdin.isatty()):

        logger.ToStderr("Waiting for '%s' lock from pid %d..." % (name, pid))

      time.sleep(1)

      retries += 1

      continue

  os.write(fd, '%d\n' % (os.getpid(),))

  os.close(fd)

  _locksheld.append(name)

def Unlock(name):

  """Unlock a given subsystem.

  """

  lockfile = _GetLockFile(name)

  try:

    fd = os.open(lockfile, os.O_RDONLY)

  except OSError:

    raise errors.LockError('Lock "%s" not held.' % (name,))

  f = os.fdopen(fd, 'r')

  pid_str = f.read()

  try:

    pid = int(pid_str)

  except ValueError:

    raise errors.LockError('Unable to determine PID of locking process.')

  if pid != os.getpid():

    raise errors.LockError('Lock not held by me (%d != %d)' %

                           (os.getpid(), pid,))

  os.unlink(lockfile)

  _locksheld.remove(name)

def LockCleanup():

  """Remove all locks.

  """

  for lock in _locksheld:

    Unlock(lock)

def RunCmd(cmd):

  """Execute a (shell) command.

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

  closed.

  @param cmd: Command to run

  @type  cmd: string or list

  @return: `RunResult` instance

  @rtype: RunResult

  """

  if isinstance(cmd, list):

    cmd = [str(val) for val in cmd]

    strcmd = " ".join(cmd)

    shell = False

  else:

    strcmd = cmd

    shell = True

  env = os.environ.copy()

  env["LC_ALL"] = "C"

  poller = select.poll()

  child = subprocess.Popen(cmd, shell=shell,

                           stderr=subprocess.PIPE,

                           stdout=subprocess.PIPE,

                           stdin=subprocess.PIPE,

                           close_fds=True, env=env)

  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:

    for fd, event in poller.poll():

      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()

  if status >= 0:

    exitcode = status

    signal = None

  else:

    exitcode = None

    signal = -status

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

def RunCmdUnlocked(cmd):

  """Execute a shell command without the 'cmd' lock.

  This variant of `RunCmd()` drops the 'cmd' lock before running the

  command and re-aquires it afterwards, thus it can be used to call

  other ganeti commands.

  The argument and return values are the same as for the `RunCmd()`

  function.

  Args:

    cmd - command to run. (str)

  Returns:

    `RunResult`

  """

  Unlock('cmd')

  ret = RunCmd(cmd)

  Lock('cmd')

  return ret

def RemoveFile(filename):

  """Remove a file ignoring some errors.

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

  errors are passed.

  """

  try:

    os.unlink(filename)

  except OSError, err:

    if err.errno not in (errno.ENOENT, errno.EISDIR):

      raise

def _FingerprintFile(filename):

  """Compute the fingerprint of a file.

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

  instead.

  Args:

    filename - Filename (str)

  """

  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.

  Args:

    files - array of filenames.  ( [str, ...] )

  Return value:

    dictionary of filename: fingerprint for the files that exist

  """

  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 `target` and `template`, ensure target

  has all the keys from template. Missing keys are added with values

  from template.

  Args:

    target   - the dictionary to check

    template - template dictionary

    logname  - a caller-chosen string to identify the debug log

               entry; if None, no logging will be done

  Returns value:

    None

  """

  missing = []

  for k in template:

    if k not in target:

      missing.append(k)

      target[k] = template[k]

  if missing and logname:

    logger.Debug('%s missing keys %s' %

                 (logname, ', '.join(missing)))

def IsProcessAlive(pid):

  """Check if a given pid exists on the system.

  Returns: true or false, depending on if the pid exists or not

  Remarks: zombie processes treated as not alive

  """

  try:

    f = open("/proc/%d/status" % pid)

  except IOError, err:

    if err.errno in (errno.ENOENT, errno.ENOTDIR):

      return False

  alive = True

  try:

    data = f.readlines()

    if len(data) > 1:

      state = data[1].split()

      if len(state) > 1 and state[1] == "Z":

        alive = False

  finally:

    f.close()

  return alive

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 ['test1.example.com', 'test2.example.com', ...]. Against this

  list, 'test1' as well as 'test1.example' will match, but not

  'test1.ex'. A multiple match will be considered as no match at all

  (e.g. 'test1' against ['test1.example.com', 'test1.example.org']).

  Args:

    key: the name to be searched

    name_list: the list of strings against which to search the key

  Returns:

    None if there is no match *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 socket.gethostname()

    """

    return socket.gethostname()

  @staticmethod

  def LookupHostname(hostname):

    """Look up hostname

    Args:

      hostname: hostname to look up

    Returns:

      a tuple (name, aliases, ipaddrs) as returned by socket.gethostbyname_ex

      in case of errors in resolving, we raise a ResolverError

    """

    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

  Returns:

     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:

      logger.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

  Returns:

     True if it does, false otherwise.

  """

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

def NiceSort(name_list):

  """Sort a list of strings based on digit and non-digit groupings.

  Given a list of names ['a1', 'a10', 'a11', 'a2'] this function will

  sort the list in the logical order ['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.

  Return value

    - a copy of the list sorted according to 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 CheckDaemonAlive(pid_file, process_string):

  """Check wether the specified daemon is alive.

  Args:

   - pid_file: file to read the daemon pid from, the file is

               expected to contain only a single line containing

               only the PID

   - process_string: a substring that we expect to find in

                     the command line of the daemon process

  Returns:

   - True if the daemon is judged to be alive (that is:

      - the PID file exists, is readable and contains a number

      - a process of the specified PID is running

      - that process contains the specified string in its

        command line

      - the process is not in state Z (zombie))

   - False otherwise

  """

  try:

    pid_file = file(pid_file, 'r')

    try:

      pid = int(pid_file.readline())

    finally:

      pid_file.close()

    cmdline_file_path = "/proc/%s/cmdline" % (pid)

    cmdline_file = open(cmdline_file_path, 'r')

    try:

      cmdline = cmdline_file.readline()

    finally:

      cmdline_file.close()

    if not process_string in cmdline:

      return False

    stat_file_path =  "/proc/%s/stat" % (pid)

    stat_file = open(stat_file_path, 'r')

    try:

      process_state = stat_file.readline().split()[2]

    finally:

      stat_file.close()

    if process_state == 'Z':

      return False

  except (IndexError, IOError, ValueError):

    return False

  return True

def TryConvert(fn, val):

  """Try to convert a value ignoring errors.

  This function tries to apply function `fn` to `val`. If no

  ValueError or TypeError exceptions are raised, it will return the

  result, else it will return the original value. Any other exceptions

  are propagated to the caller.

  """

  try:

    nv = fn(val)

  except (ValueError, TypeError), err:

    nv = val

  return nv

def IsValidIP(ip):

  """Verifies the syntax of an IP address.

  This function checks if the ip address passes is valid or not based

  on syntax (not ip range, class calculations or anything).

  """

  unit = "(0|[1-9]\d{0,2})"

  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.

  """

  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.

  """

  for word in args:

    if not IsValidShellParam(word):

      raise errors.ProgrammerError("Shell argument '%s' contains"

                                   " invalid characters" % word)

  return template % args

def FormatUnit(value):

  """Formats an incoming number of MiB with the appropriate unit.

  Value needs to be passed as a numeric type. Return value is always a string.

  """

  if value < 1024:

    return "%dM" % round(value, 0)

  elif value < (1024 * 1024):

    return "%0.1fG" % round(float(value) / 1024, 1)

  else:

    return "%0.1fT" % round(float(value) / 1024 / 1024, 1)

def ParseUnit(input_string):

  """Tries to extract number and scale from the given string.

  Input must be in the format 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]+)?$', 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.

  Args:

    file_name: Path to authorized_keys file

    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.

  Args:

    file_name: Path to authorized_keys file

    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.

  """

  # 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:

        written = False

        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 RemoveEtcHostsEntry(file_name, hostname):

  """Removes a hostname from /etc/hosts.

  IP addresses without names are removed from the file.

  """

  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 CreateBackup(file_name):

  """Creates a backup of a file.

  Returns: the path to the newly created backup file.

  """

  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.

  """

  if _re_shell_unquoted.match(value):

    return value

  else:

    return "'%s'" % value.replace("'", "'\\''")

def ShellQuoteArgs(args):

  """Quotes all given shell arguments and concatenates using 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).

  Try to do a TCP connect(2) from an optional source IP to the

  specified target IP and the specified target port. If the optional

  parameter live_port_needed is set to true, requires the remote end

  to accept the connection. The timeout is specified in seconds and

  defaults to 10 seconds. If the source optional argument is not

  passed, the source address selection is left to the kernel,

  otherwise we try to connect using the passed address (failures to

  bind other than EADDRNOTAVAIL will be ignored).

  """

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

  sucess = 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 ListVisibleFiles(path):

  """Returns a list of all visible files in a directory.

  """

  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.

  """

  f = open("/proc/sys/kernel/random/uuid", "r")

  try:

    return f.read(128).rstrip("\n")

  finally:

    f.close()

def WriteFile(file_name, fn=None, data=None,

              mode=None, uid=-1, gid=-1,

              atime=None, mtime=None, close=True,

              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.

  Args:

    file_name: New filename

    fn: Content writing function, called with file descriptor as parameter

    data: Content as string

    mode: File mode

    uid: Owner

    gid: Group

    atime: Access time

    mtime: Modification time

    close: Whether to close file after writing it

    prewrite: Function object called before writing content

    postwrite: Function object called after writing content

  Returns:

    None if "close" parameter evaluates to True, otherwise file descriptor.

  """

  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")

  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):