backend.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.


"""Functions used by the node daemon"""


import os
import os.path
import shutil
import time
import stat
import errno
import re
import subprocess
import random
import logging
import tempfile
import zlib
import base64

from ganeti import errors
from ganeti import utils
from ganeti import ssh
from ganeti import hypervisor
from ganeti import constants
from ganeti import bdev
from ganeti import objects
from ganeti import ssconf


class RPCFail(Exception):
  """Class denoting RPC failure.

  Its argument is the error message.

  """

def _Fail(msg, *args, **kwargs):
  """Log an error and the raise an RPCFail exception.

  This exception is then handled specially in the ganeti daemon and
  turned into a 'failed' return type. As such, this function is a
  useful shortcut for logging the error and returning it to the master
  daemon.

  @type msg: string
  @param msg: the text of the exception
  @raise RPCFail

  """
  if args:
    msg = msg % args
  if "exc" in kwargs and kwargs["exc"]:
    logging.exception(msg)
  else:
    logging.error(msg)
  raise RPCFail(msg)


def _GetConfig():
  """Simple wrapper to return a SimpleStore.

  @rtype: L{ssconf.SimpleStore}
  @return: a SimpleStore instance

  """
  return ssconf.SimpleStore()


def _GetSshRunner(cluster_name):
  """Simple wrapper to return an SshRunner.

  @type cluster_name: str
  @param cluster_name: the cluster name, which is needed
      by the SshRunner constructor
  @rtype: L{ssh.SshRunner}
  @return: an SshRunner instance

  """
  return ssh.SshRunner(cluster_name)


def _Decompress(data):
  """Unpacks data compressed by the RPC client.

  @type data: list or tuple
  @param data: Data sent by RPC client
  @rtype: str
  @return: Decompressed data

  """
  assert isinstance(data, (list, tuple))
  assert len(data) == 2
  (encoding, content) = data
  if encoding == constants.RPC_ENCODING_NONE:
    return content
  elif encoding == constants.RPC_ENCODING_ZLIB_BASE64:
    return zlib.decompress(base64.b64decode(content))
  else:
    raise AssertionError("Unknown data encoding")


def _CleanDirectory(path, exclude=None):
  """Removes all regular files in a directory.

  @type path: str
  @param path: the directory to clean
  @type exclude: list
  @param exclude: list of files to be excluded, defaults
      to the empty list

  """
  if not os.path.isdir(path):
    return
  if exclude is None:
    exclude = []
  else:
    # Normalize excluded paths
    exclude = [os.path.normpath(i) for i in exclude]

  for rel_name in utils.ListVisibleFiles(path):
    full_name = os.path.normpath(os.path.join(path, rel_name))
    if full_name in exclude:
      continue
    if os.path.isfile(full_name) and not os.path.islink(full_name):
      utils.RemoveFile(full_name)


def JobQueuePurge():
  """Removes job queue files and archived jobs.

  @rtype: None

  """
  _CleanDirectory(constants.QUEUE_DIR, exclude=[constants.JOB_QUEUE_LOCK_FILE])
  _CleanDirectory(constants.JOB_QUEUE_ARCHIVE_DIR)


def GetMasterInfo():
  """Returns master information.

  This is an utility function to compute master information, either
  for consumption here or from the node daemon.

  @rtype: tuple
  @return: (master_netdev, master_ip, master_name) if we have a good
      configuration, otherwise (None, None, None)

  """
  try:
    cfg = _GetConfig()
    master_netdev = cfg.GetMasterNetdev()
    master_ip = cfg.GetMasterIP()
    master_node = cfg.GetMasterNode()
  except errors.ConfigurationError, err:
    logging.exception("Cluster configuration incomplete")
    return (None, None, None)
  return (master_netdev, master_ip, master_node)


def StartMaster(start_daemons):
  """Activate local node as master node.

  The function will always try activate the IP address of the master
  (unless someone else has it). It will also start the master daemons,
  based on the start_daemons parameter.

  @type start_daemons: boolean
  @param start_daemons: whther to also start the master
      daemons (ganeti-masterd and ganeti-rapi)
  @rtype: None

  """
  master_netdev, master_ip, _ = GetMasterInfo()
  if not master_netdev:
    return False, "Cluster configuration incomplete, cannot read ssconf files"

  payload = []
  if utils.TcpPing(master_ip, constants.DEFAULT_NODED_PORT):
    if utils.OwnIpAddress(master_ip):
      # we already have the ip:
      logging.debug("Master IP already configured, doing nothing")
    else:
      msg = "Someone else has the master ip, not activating"
      logging.error(msg)
      payload.append(msg)
  else:
    result = utils.RunCmd(["ip", "address", "add", "%s/32" % master_ip,
                           "dev", master_netdev, "label",
                           "%s:0" % master_netdev])
    if result.failed:
      msg = "Can't activate master IP: %s" % result.output
      logging.error(msg)
      payload.append(msg)

    result = utils.RunCmd(["arping", "-q", "-U", "-c 3", "-I", master_netdev,
                           "-s", master_ip, master_ip])
    # we'll ignore the exit code of arping

  # and now start the master and rapi daemons
  if start_daemons:
    for daemon in 'ganeti-masterd', 'ganeti-rapi':
      result = utils.RunCmd([daemon])
      if result.failed:
        msg = "Can't start daemon %s: %s" % (daemon, result.output)
        logging.error(msg)
        payload.append(msg)

  return not payload, "; ".join(payload)


def StopMaster(stop_daemons):
  """Deactivate this node as master.

  The function will always try to deactivate the IP address of the
  master. It will also stop the master daemons depending on the
  stop_daemons parameter.

  @type stop_daemons: boolean
  @param stop_daemons: whether to also stop the master daemons
      (ganeti-masterd and ganeti-rapi)
  @rtype: None

  """
  # TODO: log and report back to the caller the error failures; we
  # need to decide in which case we fail the RPC for this
  master_netdev, master_ip, _ = GetMasterInfo()
  if not master_netdev:
    return False, "Cluster configuration incomplete, cannot read ssconf files"

  result = utils.RunCmd(["ip", "address", "del", "%s/32" % master_ip,
                         "dev", master_netdev])
  if result.failed:
    logging.error("Can't remove the master IP, error: %s", result.output)
    # but otherwise ignore the failure

  if stop_daemons:
    # stop/kill the rapi and the master daemon
    for daemon in constants.RAPI_PID, constants.MASTERD_PID:
      utils.KillProcess(utils.ReadPidFile(utils.DaemonPidFileName(daemon)))

  return True, None


def AddNode(dsa, dsapub, rsa, rsapub, sshkey, sshpub):
  """Joins this node to the cluster.

  This does the following:
      - updates the hostkeys of the machine (rsa and dsa)
      - adds the ssh private key to the user
      - adds the ssh public key to the users' authorized_keys file

  @type dsa: str
  @param dsa: the DSA private key to write
  @type dsapub: str
  @param dsapub: the DSA public key to write
  @type rsa: str
  @param rsa: the RSA private key to write
  @type rsapub: str
  @param rsapub: the RSA public key to write
  @type sshkey: str
  @param sshkey: the SSH private key to write
  @type sshpub: str
  @param sshpub: the SSH public key to write
  @rtype: boolean
  @return: the success of the operation

  """
  sshd_keys =  [(constants.SSH_HOST_RSA_PRIV, rsa, 0600),
                (constants.SSH_HOST_RSA_PUB, rsapub, 0644),
                (constants.SSH_HOST_DSA_PRIV, dsa, 0600),
                (constants.SSH_HOST_DSA_PUB, dsapub, 0644)]
  for name, content, mode in sshd_keys:
    utils.WriteFile(name, data=content, mode=mode)

  try:
    priv_key, pub_key, auth_keys = ssh.GetUserFiles(constants.GANETI_RUNAS,
                                                    mkdir=True)
  except errors.OpExecError, err:
    _Fail("Error while processing user ssh files: %s", err, exc=True)

  for name, content in [(priv_key, sshkey), (pub_key, sshpub)]:
    utils.WriteFile(name, data=content, mode=0600)

  utils.AddAuthorizedKey(auth_keys, sshpub)

  utils.RunCmd([constants.SSH_INITD_SCRIPT, "restart"])

  return (True, "Node added successfully")


def LeaveCluster():
  """Cleans up and remove the current node.

  This function cleans up and prepares the current node to be removed
  from the cluster.

  If processing is successful, then it raises an
  L{errors.QuitGanetiException} which is used as a special case to
  shutdown the node daemon.

  """
  _CleanDirectory(constants.DATA_DIR)
  JobQueuePurge()

  try:
    priv_key, pub_key, auth_keys = ssh.GetUserFiles(constants.GANETI_RUNAS)

    f = open(pub_key, 'r')
    try:
      utils.RemoveAuthorizedKey(auth_keys, f.read(8192))
    finally:
      f.close()

    utils.RemoveFile(priv_key)
    utils.RemoveFile(pub_key)
  except errors.OpExecError:
    logging.exception("Error while processing ssh files")

  # Raise a custom exception (handled in ganeti-noded)
  raise errors.QuitGanetiException(True, 'Shutdown scheduled')


def GetNodeInfo(vgname, hypervisor_type):
  """Gives back a hash with different informations about the node.

  @type vgname: C{string}
  @param vgname: the name of the volume group to ask for disk space information
  @type hypervisor_type: C{str}
  @param hypervisor_type: the name of the hypervisor to ask for
      memory information
  @rtype: C{dict}
  @return: dictionary with the following keys:
      - vg_size is the size of the configured volume group in MiB
      - vg_free is the free size of the volume group in MiB
      - memory_dom0 is the memory allocated for domain0 in MiB
      - memory_free is the currently available (free) ram in MiB
      - memory_total is the total number of ram in MiB

  """
  outputarray = {}
  vginfo = _GetVGInfo(vgname)
  outputarray['vg_size'] = vginfo['vg_size']
  outputarray['vg_free'] = vginfo['vg_free']

  hyper = hypervisor.GetHypervisor(hypervisor_type)
  hyp_info = hyper.GetNodeInfo()
  if hyp_info is not None:
    outputarray.update(hyp_info)

  f = open("/proc/sys/kernel/random/boot_id", 'r')
  try:
    outputarray["bootid"] = f.read(128).rstrip("\n")
  finally:
    f.close()

  return True, outputarray


def VerifyNode(what, cluster_name):
  """Verify the status of the local node.

  Based on the input L{what} parameter, various checks are done on the
  local node.

  If the I{filelist} key is present, this list of
  files is checksummed and the file/checksum pairs are returned.

  If the I{nodelist} key is present, we check that we have
  connectivity via ssh with the target nodes (and check the hostname
  report).

  If the I{node-net-test} key is present, we check that we have
  connectivity to the given nodes via both primary IP and, if
  applicable, secondary IPs.

  @type what: C{dict}
  @param what: a dictionary of things to check:
      - filelist: list of files for which to compute checksums
      - nodelist: list of nodes we should check ssh communication with
      - node-net-test: list of nodes we should check node daemon port
        connectivity with
      - hypervisor: list with hypervisors to run the verify for
  @rtype: dict
  @return: a dictionary with the same keys as the input dict, and
      values representing the result of the checks

  """
  result = {}

  if constants.NV_HYPERVISOR in what:
    result[constants.NV_HYPERVISOR] = tmp = {}
    for hv_name in what[constants.NV_HYPERVISOR]:
      tmp[hv_name] = hypervisor.GetHypervisor(hv_name).Verify()

  if constants.NV_FILELIST in what:
    result[constants.NV_FILELIST] = utils.FingerprintFiles(
      what[constants.NV_FILELIST])

  if constants.NV_NODELIST in what:
    result[constants.NV_NODELIST] = tmp = {}
    random.shuffle(what[constants.NV_NODELIST])
    for node in what[constants.NV_NODELIST]:
      success, message = _GetSshRunner(cluster_name).VerifyNodeHostname(node)
      if not success:
        tmp[node] = message

  if constants.NV_NODENETTEST in what:
    result[constants.NV_NODENETTEST] = tmp = {}
    my_name = utils.HostInfo().name
    my_pip = my_sip = None
    for name, pip, sip in what[constants.NV_NODENETTEST]:
      if name == my_name:
        my_pip = pip
        my_sip = sip
        break
    if not my_pip:
      tmp[my_name] = ("Can't find my own primary/secondary IP"
                      " in the node list")
    else:
      port = utils.GetNodeDaemonPort()
      for name, pip, sip in what[constants.NV_NODENETTEST]:
        fail = []
        if not utils.TcpPing(pip, port, source=my_pip):
          fail.append("primary")
        if sip != pip:
          if not utils.TcpPing(sip, port, source=my_sip):
            fail.append("secondary")
        if fail:
          tmp[name] = ("failure using the %s interface(s)" %
                       " and ".join(fail))

  if constants.NV_LVLIST in what:
    result[constants.NV_LVLIST] = GetVolumeList(what[constants.NV_LVLIST])

  if constants.NV_INSTANCELIST in what:
    result[constants.NV_INSTANCELIST] = GetInstanceList(
      what[constants.NV_INSTANCELIST])

  if constants.NV_VGLIST in what:
    result[constants.NV_VGLIST] = utils.ListVolumeGroups()

  if constants.NV_VERSION in what:
    result[constants.NV_VERSION] = (constants.PROTOCOL_VERSION,
                                    constants.RELEASE_VERSION)

  if constants.NV_HVINFO in what:
    hyper = hypervisor.GetHypervisor(what[constants.NV_HVINFO])
    result[constants.NV_HVINFO] = hyper.GetNodeInfo()

  if constants.NV_DRBDLIST in what:
    try:
      used_minors = bdev.DRBD8.GetUsedDevs().keys()
    except errors.BlockDeviceError, err:
      logging.warning("Can't get used minors list", exc_info=True)
      used_minors = str(err)
    result[constants.NV_DRBDLIST] = used_minors

  return True, result


def GetVolumeList(vg_name):
  """Compute list of logical volumes and their size.

  @type vg_name: str
  @param vg_name: the volume group whose LVs we should list
  @rtype: dict
  @return:
      dictionary of all partions (key) with value being a tuple of
      their size (in MiB), inactive and online status::

        {'test1': ('20.06', True, True)}

      in case of errors, a string is returned with the error
      details.

  """
  lvs = {}
  sep = '|'
  result = utils.RunCmd(["lvs", "--noheadings", "--units=m", "--nosuffix",
                         "--separator=%s" % sep,
                         "-olv_name,lv_size,lv_attr", vg_name])
  if result.failed:
    _Fail("Failed to list logical volumes, lvs output: %s", result.output)

  valid_line_re = re.compile("^ *([^|]+)\|([0-9.]+)\|([^|]{6})\|?$")
  for line in result.stdout.splitlines():
    line = line.strip()
    match = valid_line_re.match(line)
    if not match:
      logging.error("Invalid line returned from lvs output: '%s'", line)
      continue
    name, size, attr = match.groups()
    inactive = attr[4] == '-'
    online = attr[5] == 'o'
    lvs[name] = (size, inactive, online)

  return lvs


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

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

  """
  return True, utils.ListVolumeGroups()


def NodeVolumes():
  """List all volumes on this node.

  @rtype: list
  @return:
    A list of dictionaries, each having four keys:
      - name: the logical volume name,
      - size: the size of the logical volume
      - dev: the physical device on which the LV lives
      - vg: the volume group to which it belongs

    In case of errors, we return an empty list and log the
    error.

    Note that since a logical volume can live on multiple physical
    volumes, the resulting list might include a logical volume
    multiple times.

  """
  result = utils.RunCmd(["lvs", "--noheadings", "--units=m", "--nosuffix",
                         "--separator=|",
                         "--options=lv_name,lv_size,devices,vg_name"])
  if result.failed:
    logging.error("Failed to list logical volumes, lvs output: %s",
                  result.output)
    return []

  def parse_dev(dev):
    if '(' in dev:
      return dev.split('(')[0]
    else:
      return dev

  def map_line(line):
    return {
      'name': line[0].strip(),
      'size': line[1].strip(),
      'dev': parse_dev(line[2].strip()),
      'vg': line[3].strip(),
    }

  return [map_line(line.split('|')) for line in result.stdout.splitlines()
          if line.count('|') >= 3]


def BridgesExist(bridges_list):
  """Check if a list of bridges exist on the current node.

  @rtype: boolean
  @return: C{True} if all of them exist, C{False} otherwise

  """
  missing = []
  for bridge in bridges_list:
    if not utils.BridgeExists(bridge):
      missing.append(bridge)

  if missing:
    return False, "Missing bridges %s" % (", ".join(missing),)

  return True, None


def GetInstanceList(hypervisor_list):
  """Provides a list of instances.

  @type hypervisor_list: list
  @param hypervisor_list: the list of hypervisors to query information

  @rtype: list
  @return: a list of all running instances on the current node
    - instance1.example.com
    - instance2.example.com

  """
  results = []
  for hname in hypervisor_list:
    try:
      names = hypervisor.GetHypervisor(hname).ListInstances()
      results.extend(names)
    except errors.HypervisorError, err:
      _Fail("Error enumerating instances (hypervisor %s): %s",
            hname, err, exc=True)

  return results


def GetInstanceInfo(instance, hname):
  """Gives back the informations about an instance as a dictionary.

  @type instance: string
  @param instance: the instance name
  @type hname: string
  @param hname: the hypervisor type of the instance

  @rtype: dict
  @return: dictionary with the following keys:
      - memory: memory size of instance (int)
      - state: xen state of instance (string)
      - time: cpu time of instance (float)

  """
  output = {}

  iinfo = hypervisor.GetHypervisor(hname).GetInstanceInfo(instance)
  if iinfo is not None:
    output['memory'] = iinfo[2]
    output['state'] = iinfo[4]
    output['time'] = iinfo[5]

  return True, output


def GetInstanceMigratable(instance):
  """Gives whether an instance can be migrated.

  @type instance: L{objects.Instance}
  @param instance: object representing the instance to be checked.

  @rtype: tuple
  @return: tuple of (result, description) where:
      - result: whether the instance can be migrated or not
      - description: a description of the issue, if relevant

  """
  hyper = hypervisor.GetHypervisor(instance.hypervisor)
  if instance.name not in hyper.ListInstances():
    return (False, 'not running')

  for idx in range(len(instance.disks)):
    link_name = _GetBlockDevSymlinkPath(instance.name, idx)
    if not os.path.islink(link_name):
      return (False, 'not restarted since ganeti 1.2.5')

  return (True, '')


def GetAllInstancesInfo(hypervisor_list):
  """Gather data about all instances.

  This is the equivalent of L{GetInstanceInfo}, except that it
  computes data for all instances at once, thus being faster if one
  needs data about more than one instance.

  @type hypervisor_list: list
  @param hypervisor_list: list of hypervisors to query for instance data

  @rtype: dict
  @return: dictionary of instance: data, with data having the following keys:
      - memory: memory size of instance (int)
      - state: xen state of instance (string)
      - time: cpu time of instance (float)
      - vcpus: the number of vcpus

  """
  output = {}

  for hname in hypervisor_list:
    iinfo = hypervisor.GetHypervisor(hname).GetAllInstancesInfo()
    if iinfo:
      for name, inst_id, memory, vcpus, state, times in iinfo:
        value = {
          'memory': memory,
          'vcpus': vcpus,
          'state': state,
          'time': times,
          }
        if name in output:
          # we only check static parameters, like memory and vcpus,
          # and not state and time which can change between the
          # invocations of the different hypervisors
          for key in 'memory', 'vcpus':
            if value[key] != output[name][key]:
              _Fail("Instance %s is running twice"
                    " with different parameters", name)
        output[name] = value

  return True, output


def InstanceOsAdd(instance, reinstall):
  """Add an OS to an instance.

  @type instance: L{objects.Instance}
  @param instance: Instance whose OS is to be installed
  @type reinstall: boolean
  @param reinstall: whether this is an instance reinstall
  @rtype: boolean
  @return: the success of the operation

  """
  try:
    inst_os = OSFromDisk(instance.os)
  except errors.InvalidOS, err:
    os_name, os_dir, os_err = err.args
    if os_dir is None:
      return (False, "Can't find OS '%s': %s" % (os_name, os_err))
    else:
      return (False, "Error parsing OS '%s' in directory %s: %s" %
              (os_name, os_dir, os_err))

  create_env = OSEnvironment(instance)
  if reinstall:
    create_env['INSTANCE_REINSTALL'] = "1"

  logfile = "%s/add-%s-%s-%d.log" % (constants.LOG_OS_DIR, instance.os,
                                     instance.name, int(time.time()))

  result = utils.RunCmd([inst_os.create_script], env=create_env,
                        cwd=inst_os.path, output=logfile,)
  if result.failed:
    logging.error("os create command '%s' returned error: %s, logfile: %s,"
                  " output: %s", result.cmd, result.fail_reason, logfile,
                  result.output)
    lines = [utils.SafeEncode(val)
             for val in utils.TailFile(logfile, lines=20)]
    return (False, "OS create script failed (%s), last lines in the"
            " log file:\n%s" % (result.fail_reason, "\n".join(lines)))

  return (True, "Successfully installed")


def RunRenameInstance(instance, old_name):
  """Run the OS rename script for an instance.

  @type instance: L{objects.Instance}
  @param instance: Instance whose OS is to be installed
  @type old_name: string
  @param old_name: previous instance name
  @rtype: boolean
  @return: the success of the operation

  """
  inst_os = OSFromDisk(instance.os)

  rename_env = OSEnvironment(instance)
  rename_env['OLD_INSTANCE_NAME'] = old_name

  logfile = "%s/rename-%s-%s-%s-%d.log" % (constants.LOG_OS_DIR, instance.os,
                                           old_name,
                                           instance.name, int(time.time()))

  result = utils.RunCmd([inst_os.rename_script], env=rename_env,
                        cwd=inst_os.path, output=logfile)

  if result.failed:
    logging.error("os create command '%s' returned error: %s output: %s",
                  result.cmd, result.fail_reason, result.output)
    lines = [utils.SafeEncode(val)
             for val in utils.TailFile(logfile, lines=20)]
    return (False, "OS rename script failed (%s), last lines in the"
            " log file:\n%s" % (result.fail_reason, "\n".join(lines)))

  return (True, "Rename successful")


def _GetVGInfo(vg_name):
  """Get informations about the volume group.

  @type vg_name: str
  @param vg_name: the volume group which we query
  @rtype: dict
  @return:
    A dictionary with the following keys:
      - C{vg_size} is the total size of the volume group in MiB
      - C{vg_free} is the free size of the volume group in MiB
      - C{pv_count} are the number of physical disks in that VG

    If an error occurs during gathering of data, we return the same dict
    with keys all set to None.

  """
  retdic = dict.fromkeys(["vg_size", "vg_free", "pv_count"])

  retval = utils.RunCmd(["vgs", "-ovg_size,vg_free,pv_count", "--noheadings",
                         "--nosuffix", "--units=m", "--separator=:", vg_name])

  if retval.failed:
    logging.error("volume group %s not present", vg_name)
    return retdic
  valarr = retval.stdout.strip().rstrip(':').split(':')
  if len(valarr) == 3:
    try:
      retdic = {
        "vg_size": int(round(float(valarr[0]), 0)),
        "vg_free": int(round(float(valarr[1]), 0)),
        "pv_count": int(valarr[2]),
        }
    except ValueError, err:
      logging.exception("Fail to parse vgs output")
  else:
    logging.error("vgs output has the wrong number of fields (expected"
                  " three): %s", str(valarr))
  return retdic


def _GetBlockDevSymlinkPath(instance_name, idx):
  return os.path.join(constants.DISK_LINKS_DIR,
                      "%s:%d" % (instance_name, idx))


def _SymlinkBlockDev(instance_name, device_path, idx):
  """Set up symlinks to a instance's block device.

  This is an auxiliary function run when an instance is start (on the primary
  node) or when an instance is migrated (on the target node).


  @param instance_name: the name of the target instance
  @param device_path: path of the physical block device, on the node
  @param idx: the disk index
  @return: absolute path to the disk's symlink

  """
  link_name = _GetBlockDevSymlinkPath(instance_name, idx)
  try:
    os.symlink(device_path, link_name)
  except OSError, err:
    if err.errno == errno.EEXIST:
      if (not os.path.islink(link_name) or
          os.readlink(link_name) != device_path):
        os.remove(link_name)
        os.symlink(device_path, link_name)
    else:
      raise

  return link_name


def _RemoveBlockDevLinks(instance_name, disks):
  """Remove the block device symlinks belonging to the given instance.

  """
  for idx, disk in enumerate(disks):
    link_name = _GetBlockDevSymlinkPath(instance_name, idx)
    if os.path.islink(link_name):
      try:
        os.remove(link_name)
      except OSError:
        logging.exception("Can't remove symlink '%s'", link_name)


def _GatherAndLinkBlockDevs(instance):
  """Set up an instance's block device(s).

  This is run on the primary node at instance startup. The block
  devices must be already assembled.

  @type instance: L{objects.Instance}
  @param instance: the instance whose disks we shoul assemble
  @rtype: list
  @return: list of (disk_object, device_path)

  """
  block_devices = []
  for idx, disk in enumerate(instance.disks):
    device = _RecursiveFindBD(disk)
    if device is None:
      raise errors.BlockDeviceError("Block device '%s' is not set up." %
                                    str(disk))
    device.Open()
    try:
      link_name = _SymlinkBlockDev(instance.name, device.dev_path, idx)
    except OSError, e:
      raise errors.BlockDeviceError("Cannot create block device symlink: %s" %
                                    e.strerror)

    block_devices.append((disk, link_name))

  return block_devices


def StartInstance(instance):
  """Start an instance.

  @type instance: L{objects.Instance}
  @param instance: the instance object
  @rtype: boolean
  @return: whether the startup was successful or not

  """
  running_instances = GetInstanceList([instance.hypervisor])

  if instance.name in running_instances:
    return (True, "Already running")

  try:
    block_devices = _GatherAndLinkBlockDevs(instance)
    hyper = hypervisor.GetHypervisor(instance.hypervisor)
    hyper.StartInstance(instance, block_devices)
  except errors.BlockDeviceError, err:
    _Fail("Block device error: %s", err, exc=True)
  except errors.HypervisorError, err:
    _RemoveBlockDevLinks(instance.name, instance.disks)
    _Fail("Hypervisor error: %s", err, exc=True)

  return (True, "Instance started successfully")


def InstanceShutdown(instance):
  """Shut an instance down.

  @note: this functions uses polling with a hardcoded timeout.

  @type instance: L{objects.Instance}
  @param instance: the instance object
  @rtype: boolean
  @return: whether the startup was successful or not

  """
  hv_name = instance.hypervisor
  running_instances = GetInstanceList([hv_name])

  if instance.name not in running_instances:
    return (True, "Instance already stopped")

  hyper = hypervisor.GetHypervisor(hv_name)
  try:
    hyper.StopInstance(instance)
  except errors.HypervisorError, err:
    _Fail("Failed to stop instance %s: %s", instance.name, err)

  # test every 10secs for 2min

  time.sleep(1)
  for dummy in range(11):
    if instance.name not in GetInstanceList([hv_name]):
      break
    time.sleep(10)
  else:
    # the shutdown did not succeed
    logging.error("Shutdown of '%s' unsuccessful, using destroy",
                  instance.name)

    try:
      hyper.StopInstance(instance, force=True)
    except errors.HypervisorError, err:
      _Fail("Failed to force stop instance %s: %s", instance.name, err)

    time.sleep(1)
    if instance.name in GetInstanceList([hv_name]):
      _Fail("Could not shutdown instance %s even by destroy", instance.name)

  _RemoveBlockDevLinks(instance.name, instance.disks)

  return (True, "Instance has been shutdown successfully")


def InstanceReboot(instance, reboot_type):
  """Reboot an instance.

  @type instance: L{objects.Instance}
  @param instance: the instance object to reboot
  @type reboot_type: str
  @param reboot_type: the type of reboot, one the following
    constants:
      - L{constants.INSTANCE_REBOOT_SOFT}: only reboot the
        instance OS, do not recreate the VM
      - L{constants.INSTANCE_REBOOT_HARD}: tear down and
        restart the VM (at the hypervisor level)
      - the other reboot type (L{constants.INSTANCE_REBOOT_HARD})
        is not accepted here, since that mode is handled
        differently
  @rtype: boolean
  @return: the success of the operation

  """
  running_instances = GetInstanceList([instance.hypervisor])

  if instance.name not in running_instances:
    _Fail("Cannot reboot instance %s that is not running", instance.name)

  hyper = hypervisor.GetHypervisor(instance.hypervisor)
  if reboot_type == constants.INSTANCE_REBOOT_SOFT:
    try:
      hyper.RebootInstance(instance)
    except errors.HypervisorError, err:
      _Fail("Failed to soft reboot instance %s: %s", instance.name, err)
  elif reboot_type == constants.INSTANCE_REBOOT_HARD:
    try:
      stop_result = InstanceShutdown(instance)
      if not stop_result[0]:
        return stop_result
      return StartInstance(instance)
    except errors.HypervisorError, err:
      _Fail("Failed to hard reboot instance %s: %s", instance.name, err)
  else:
    _Fail("Invalid reboot_type received: %s", reboot_type)

  return (True, "Reboot successful")


def MigrationInfo(instance):
  """Gather information about an instance to be migrated.

  @type instance: L{objects.Instance}
  @param instance: the instance definition

  """
  hyper = hypervisor.GetHypervisor(instance.hypervisor)
  try:
    info = hyper.MigrationInfo(instance)
  except errors.HypervisorError, err:
    _Fail("Failed to fetch migration information: %s", err, exc=True)
  return (True, info)


def AcceptInstance(instance, info, target):
  """Prepare the node to accept an instance.

  @type instance: L{objects.Instance}
  @param instance: the instance definition
  @type info: string/data (opaque)
  @param info: migration information, from the source node
  @type target: string
  @param target: target host (usually ip), on this node

  """
  hyper = hypervisor.GetHypervisor(instance.hypervisor)
  try:
    hyper.AcceptInstance(instance, info, target)
  except errors.HypervisorError, err:
    _Fail("Failed to accept instance: %s", err, exc=True)
  return (True, "Accept successfull")


def FinalizeMigration(instance, info, success):
  """Finalize any preparation to accept an instance.

  @type instance: L{objects.Instance}
  @param instance: the instance definition
  @type info: string/data (opaque)
  @param info: migration information, from the source node
  @type success: boolean
  @param success: whether the migration was a success or a failure

  """
  hyper = hypervisor.GetHypervisor(instance.hypervisor)
  try:
    hyper.FinalizeMigration(instance, info, success)
  except errors.HypervisorError, err:
    _Fail("Failed to finalize migration: %s", err, exc=True)
  return (True, "Migration Finalized")


def MigrateInstance(instance, target, live):
  """Migrates an instance to another node.

  @type instance: L{objects.Instance}
  @param instance: the instance definition
  @type target: string
  @param target: the target node name
  @type live: boolean
  @param live: whether the migration should be done live or not (the
      interpretation of this parameter is left to the hypervisor)
  @rtype: tuple
  @return: a tuple of (success, msg) where:
      - succes is a boolean denoting the success/failure of the operation
      - msg is a string with details in case of failure

  """
  hyper = hypervisor.GetHypervisor(instance.hypervisor)

  try:
    hyper.MigrateInstance(instance.name, target, live)
  except errors.HypervisorError, err:
    _Fail("Failed to migrate instance: %s", err, exc=True)
  return (True, "Migration successfull")


def BlockdevCreate(disk, size, owner, on_primary, info):
  """Creates a block device for an instance.