pvc/node-daemon/pvcnoded/objects/VMInstance.py

#!/usr/bin/env python3

# VMInstance.py - Class implementing a PVC virtual machine in pvcnoded
# Part of the Parallel Virtual Cluster (PVC) system
#
#    Copyright (C) 2018-2021 Joshua M. Boniface <joshua@boniface.me>
#
#    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, version 3.
#
#    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, see <https://www.gnu.org/licenses/>.
#
###############################################################################

import uuid
import time
import libvirt
import json

from threading import Thread

from xml.etree import ElementTree

from re import match

import daemon_lib.common as common

import pvcnoded.objects.VMConsoleWatcherInstance as VMConsoleWatcherInstance

import daemon_lib.common as daemon_common


class VMInstance(object):
    # Initialization function
    def __init__(self, domuuid, zkhandler, config, logger, this_node):
        # Passed-in variables on creation
        self.domuuid = domuuid
        self.zkhandler = zkhandler
        self.config = config
        self.logger = logger
        self.this_node = this_node

        # Get data from zookeeper
        self.domname = self.zkhandler.read(('domain', domuuid))
        self.state = self.zkhandler.read(('domain.state', domuuid))
        self.node = self.zkhandler.read(('domain.node', domuuid))
        self.lastnode = self.zkhandler.read(('domain.last_node', domuuid))
        self.last_currentnode = self.zkhandler.read(('domain.node', domuuid))
        self.last_lastnode = self.zkhandler.read(('domain.last_node', domuuid))
        try:
            self.migration_method = self.zkhandler.read(('domain.meta.migrate_method', self.domuuid))
        except Exception:
            self.migration_method = 'none'

        # These will all be set later
        self.instart = False
        self.inrestart = False
        self.inmigrate = False
        self.inreceive = False
        self.inshutdown = False
        self.instop = False

        # State thread
        self.state_thread = None

        # Libvirt domuuid
        self.dom = self.lookupByUUID(self.domuuid)

        # Log watcher instance
        self.console_log_instance = VMConsoleWatcherInstance.VMConsoleWatcherInstance(self.domuuid, self.domname, self.zkhandler, self.config, self.logger, self.this_node)

        # Watch for changes to the state field in Zookeeper
        @self.zkhandler.zk_conn.DataWatch(self.zkhandler.schema.path('domain.state', self.domuuid))
        def watch_state(data, stat, event=""):
            if event and event.type == 'DELETED':
                # The key has been deleted after existing before; terminate this watcher
                # because this class instance is about to be reaped in Daemon.py
                return False

            # Perform a management command
            self.logger.out('Updating state of VM {}'.format(self.domuuid), state='i')
            self.state_thread = Thread(target=self.manage_vm_state, args=(), kwargs={})
            self.state_thread.start()

    # Get data functions
    def getstate(self):
        return self.state

    def getnode(self):
        return self.node

    def getlastnode(self):
        return self.lastnode

    def getdom(self):
        return self.dom

    def getmemory(self):
        try:
            if self.dom is not None:
                memory = int(self.dom.info()[2] / 1024)
            else:
                domain_information = daemon_common.getInformationFromXML(self.zkhandler, self.domuuid)
                memory = int(domain_information['memory'])
        except Exception:
            memory = 0

        return memory

    def getvcpus(self):
        try:
            vcpus = int(self.dom.info()[3])
        except Exception:
            vcpus = 0

        return vcpus

    # Manage local node domain_list
    def addDomainToList(self):
        if self.domuuid not in self.this_node.domain_list:
            try:
                # Add the domain to the domain_list array
                self.this_node.domain_list.append(self.domuuid)
                # Push the change up to Zookeeper
                self.zkhandler.write([
                    (('node.running_domains', self.this_node.name), ' '.join(self.this_node.domain_list))
                ])
            except Exception as e:
                self.logger.out('Error adding domain to list: {}'.format(e), state='e')

    def removeDomainFromList(self):
        if self.domuuid in self.this_node.domain_list:
            try:
                # Remove the domain from the domain_list array
                self.this_node.domain_list.remove(self.domuuid)
                # Push the change up to Zookeeper
                self.zkhandler.write([
                    (('node.running_domains', self.this_node.name), ' '.join(self.this_node.domain_list))
                ])
            except Exception as e:
                self.logger.out('Error removing domain from list: {}'.format(e), state='e')

    # Update the VNC live data
    def update_vnc(self):
        if self.dom is not None:
            live_xml = ElementTree.fromstring(self.dom.XMLDesc(0))
            graphics = live_xml.find('./devices/graphics')
            if graphics is not None:
                self.logger.out('Updating VNC data', state='i', prefix='Domain {}'.format(self.domuuid))
                port = graphics.get('port', '')
                listen = graphics.get('listen', '')
                self.zkhandler.write([
                    (('domain.console.vnc', self.domuuid), '{}:{}'.format(listen, port))
                ])
            else:
                self.zkhandler.write([
                    (('domain.console.vnc', self.domuuid), '')
                ])
        else:
            self.zkhandler.write([
                (('domain.console.vnc', self.domuuid), '')
            ])

    # Attach a device to the running domain
    def attach_device(self, xml_spec):
        if not self.dom:
            self.logger.out('Cannot attach device to non-running domain', state='w', prefix='Domain {}'.format(self.domuuid))
            return False

        try:
            self.logger.out('Attaching new device to VM', state='i', prefix='Domain {}'.format(self.domuuid))
            self.dom.attachDevice(xml_spec)
            return True
        except Exception as e:
            self.logger.out('Failed to attach device: {}'.format(e), state='e', prefix='Domain {}'.format(self.domuuid))
            return False

    # Detach a device from the running domain
    def detach_device(self, xml_spec):
        if not self.dom:
            self.logger.out('Cannot detach device from non-running domain', state='w', prefix='Domain {}'.format(self.domuuid))
            return False

        try:
            self.logger.out('Detaching device from VM', state='i', prefix='Domain {}'.format(self.domuuid))
            self.dom.detachDevice(xml_spec)
            return True
        except Exception as e:
            self.logger.out('Failed to detach device: {}'.format(e), state='e', prefix='Domain {}'.format(self.domuuid))
            return False

    # Start up the VM
    def start_vm(self):
        # Start the log watcher
        self.console_log_instance.start()

        self.logger.out('Starting VM', state='i', prefix='Domain {}'.format(self.domuuid))
        self.instart = True

        # Start up a new Libvirt connection
        libvirt_name = "qemu:///system"
        lv_conn = libvirt.open(libvirt_name)
        if lv_conn is None:
            self.logger.out('Failed to open local libvirt connection', state='e', prefix='Domain {}'.format(self.domuuid))
            self.instart = False
            return

        # Try to get the current state in case it's already running
        try:
            self.dom = self.lookupByUUID(self.domuuid)
            curstate = self.dom.state()[0]
        except Exception:
            curstate = 'notstart'

        # Handle situations where the VM crashed or the node unexpectedly rebooted
        if self.getdom() is None or self.getdom().state()[0] != libvirt.VIR_DOMAIN_RUNNING:
            # Flush locks
            self.logger.out('Flushing RBD locks', state='i', prefix='Domain {}'.format(self.domuuid))
            VMInstance.flush_locks(self.zkhandler, self.logger, self.domuuid, self.this_node)
            if self.zkhandler.read(('domain.state', self.domuuid)) == 'fail':
                lv_conn.close()
                self.dom = None
                self.instart = False
                return

        if curstate == libvirt.VIR_DOMAIN_RUNNING:
            # If it is running just update the model
            self.addDomainToList()
            self.zkhandler.write([
                (('domain.failed_reason', self.domuuid), '')
            ])
        else:
            # Or try to create it
            try:
                # Grab the domain information from Zookeeper
                xmlconfig = self.zkhandler.read(('domain.xml', self.domuuid))
                dom = lv_conn.createXML(xmlconfig, 0)
                self.addDomainToList()
                self.logger.out('Successfully started VM', state='o', prefix='Domain {}'.format(self.domuuid))
                self.dom = dom
                self.zkhandler.write([
                    (('domain.failed_reason', self.domuuid), '')
                ])
            except libvirt.libvirtError as e:
                self.logger.out('Failed to create VM', state='e', prefix='Domain {}'.format(self.domuuid))
                self.zkhandler.write([
                    (('domain.state', self.domuuid), 'fail'),
                    (('domain.failed_reason', self.domuuid), str(e))
                ])
                lv_conn.close()
                self.dom = None
                self.instart = False
                return

        lv_conn.close()

        self.instart = False

    # Restart the VM
    def restart_vm(self):
        self.logger.out('Restarting VM', state='i', prefix='Domain {}'.format(self.domuuid))
        self.inrestart = True

        # Start up a new Libvirt connection
        libvirt_name = "qemu:///system"
        lv_conn = libvirt.open(libvirt_name)
        if lv_conn is None:
            self.logger.out('Failed to open local libvirt connection', state='e', prefix='Domain {}'.format(self.domuuid))
            self.inrestart = False
            return

        self.shutdown_vm()
        time.sleep(0.2)
        self.start_vm()
        self.addDomainToList()

        self.zkhandler.write([
            (('domain.state', self.domuuid), 'start')
        ])
        lv_conn.close()
        self.inrestart = False

    # Stop the VM forcibly without updating state
    def terminate_vm(self):
        self.logger.out('Terminating VM', state='i', prefix='Domain {}'.format(self.domuuid))
        self.instop = True
        try:
            self.dom.destroy()
            time.sleep(0.2)
            try:
                if self.getdom().state()[0] == libvirt.VIR_DOMAIN_RUNNING:
                    # It didn't terminate, try again
                    self.dom.destroy()
            except libvirt.libvirtError:
                pass
        except AttributeError:
            self.logger.out('Failed to terminate VM', state='e', prefix='Domain {}'.format(self.domuuid))
        self.removeDomainFromList()
        self.logger.out('Successfully terminated VM', state='o', prefix='Domain {}'.format(self.domuuid))
        self.dom = None
        self.instop = False

        # Stop the log watcher
        self.console_log_instance.stop()

    # Stop the VM forcibly
    def stop_vm(self):
        self.logger.out('Forcibly stopping VM', state='i', prefix='Domain {}'.format(self.domuuid))
        self.instop = True
        try:
            self.dom.destroy()
            time.sleep(0.2)
            try:
                if self.getdom().state()[0] == libvirt.VIR_DOMAIN_RUNNING:
                    # It didn't terminate, try again
                    self.dom.destroy()
            except libvirt.libvirtError:
                pass
        except AttributeError:
            self.logger.out('Failed to stop VM', state='e', prefix='Domain {}'.format(self.domuuid))
        self.removeDomainFromList()

        if self.inrestart is False:
            self.zkhandler.write([
                (('domain.state', self.domuuid), 'stop')
            ])

        self.logger.out('Successfully stopped VM', state='o', prefix='Domain {}'.format(self.domuuid))
        self.dom = None
        self.instop = False

        # Stop the log watcher
        self.console_log_instance.stop()

    # Shutdown the VM gracefully
    def shutdown_vm(self):
        self.logger.out('Gracefully stopping VM', state='i', prefix='Domain {}'.format(self.domuuid))
        is_aborted = False
        self.inshutdown = True
        self.dom.shutdown()
        tick = 0
        while True:
            tick += 1
            time.sleep(1)

            # Abort shutdown if the state changes to start
            current_state = self.zkhandler.read(('domain.state', self.domuuid))
            if current_state not in ['shutdown', 'restart', 'migrate']:
                self.logger.out('Aborting VM shutdown due to state change', state='i', prefix='Domain {}'.format(self.domuuid))
                is_aborted = True
                break

            try:
                lvdomstate = self.dom.state()[0]
            except Exception:
                lvdomstate = None

            if lvdomstate != libvirt.VIR_DOMAIN_RUNNING:
                self.removeDomainFromList()
                self.zkhandler.write([
                    (('domain.state', self.domuuid), 'stop')
                ])
                self.logger.out('Successfully shutdown VM', state='o', prefix='Domain {}'.format(self.domuuid))
                self.dom = None
                # Stop the log watcher
                self.console_log_instance.stop()
                break

            if tick >= self.config['vm_shutdown_timeout']:
                self.logger.out('Shutdown timeout ({}s) expired, forcing off'.format(self.config['vm_shutdown_timeout']), state='e', prefix='Domain {}'.format(self.domuuid))
                self.zkhandler.write([
                    (('domain.state', self.domuuid), 'stop')
                ])
                break

        self.inshutdown = False

        if is_aborted:
            self.manage_vm_state()

        if self.inrestart:
            # Wait to prevent race conditions
            time.sleep(1)
            self.zkhandler.write([
                (('domain.state', self.domuuid), 'start')
            ])

    # Migrate the VM to a target host
    def migrate_vm(self, force_live=False, force_shutdown=False):
        # Wait for any previous migration
        while self.inmigrate:
            time.sleep(0.1)

        if self.migration_method == 'live':
            force_live = True
        elif self.migration_method == 'shutdown':
            force_shutdown = True

        self.inmigrate = True
        self.logger.out('Migrating VM to node "{}"'.format(self.node), state='i', prefix='Domain {}'.format(self.domuuid))

        # Used for sanity checking later
        target_node = self.zkhandler.read(('domain.node', self.domuuid))

        aborted = False

        def abort_migrate(reason):
            self.zkhandler.write([
                (('domain.state', self.domuuid), 'start'),
                (('domain.node', self.domuuid), self.this_node.name),
                (('domain.last_node', self.domuuid), self.last_lastnode)
            ])
            migrate_lock_node.release()
            migrate_lock_state.release()
            self.inmigrate = False
            self.logger.out('Aborted migration: {}'.format(reason), state='i', prefix='Domain {}'.format(self.domuuid))

        # Acquire exclusive lock on the domain node key
        migrate_lock_node = self.zkhandler.exclusivelock(('domain.node', self.domuuid))
        migrate_lock_state = self.zkhandler.exclusivelock(('domain.state', self.domuuid))
        migrate_lock_node.acquire()
        migrate_lock_state.acquire()

        # Don't try to migrate a node to itself, set back to start
        if self.node == self.lastnode or self.node == self.this_node.name:
            abort_migrate('Target node matches the current active node during initial check')
            return

        time.sleep(0.5)  # Initial delay for the first writer to grab the lock

        def migrate_live():
            self.logger.out('Setting up live migration', state='i', prefix='Domain {}'.format(self.domuuid))
            # Set up destination connection
            dest_lv = 'qemu+tcp://{}.{}/system'.format(self.node, self.config['cluster_domain'])
            dest_tcp = 'tcp://{}.{}'.format(self.node, self.config['cluster_domain'])
            try:
                self.logger.out('Opening remote libvirt connection', state='i', prefix='Domain {}'.format(self.domuuid))
                # Open a connection to the destination
                dest_lv_conn = libvirt.open(dest_lv)
                if not dest_lv_conn:
                    raise
            except Exception:
                self.logger.out('Failed to open connection to {}; aborting live migration.'.format(dest_lv), state='e', prefix='Domain {}'.format(self.domuuid))
                return False

            try:
                self.logger.out('Live migrating VM', state='i', prefix='Domain {}'.format(self.domuuid))
                # Send the live migration; force the destination URI to ensure we transit over the cluster network
                target_dom = self.dom.migrate(dest_lv_conn, libvirt.VIR_MIGRATE_LIVE, None, dest_tcp, 0)
                if not target_dom:
                    raise
            except Exception as e:
                self.logger.out('Failed to send VM to {} - aborting live migration; error: {}'.format(dest_lv, e), state='e', prefix='Domain {}'.format(self.domuuid))
                dest_lv_conn.close()
                return False

            self.logger.out('Successfully migrated VM', state='o', prefix='Domain {}'.format(self.domuuid))
            dest_lv_conn.close()
            self.console_log_instance.stop()
            self.removeDomainFromList()
            return True

        def migrate_shutdown():
            self.logger.out('Shutting down VM for offline migration', state='i', prefix='Domain {}'.format(self.domuuid))
            self.shutdown_vm()
            return True

        self.logger.out('Acquiring lock for migration phase B', state='i', prefix='Domain {}'.format(self.domuuid))
        lock = self.zkhandler.exclusivelock(('domain.migrate.sync_lock', self.domuuid))
        try:
            lock.acquire(timeout=30.0)
        except Exception:
            abort_migrate('Timed out waiting for peer')
            return
        self.logger.out('Acquired lock for migration phase B', state='o', prefix='Domain {}'.format(self.domuuid))
        migrate_live_result = False
        # Do a final verification
        if self.node == self.lastnode or self.node == self.this_node.name:
            abort_migrate('Target node matches the current active node during final check')
            return
        if self.node != target_node:
            abort_migrate('Target node changed during preparation')
            return
        if not force_shutdown:
            # A live migrate is attemped 3 times in succession
            ticks = 0
            while True:
                ticks += 1
                self.logger.out('Attempting live migration try {}'.format(ticks), state='i', prefix='Domain {}'.format(self.domuuid))
                migrate_live_result = migrate_live()
                if migrate_live_result:
                    break
                time.sleep(0.5)
                if ticks > 2:
                    break
        else:
            migrate_live_result = False
        if not migrate_live_result:
            if force_live:
                self.logger.out('Could not live migrate VM while live migration enforced', state='e', prefix='Domain {}'.format(self.domuuid))
                aborted = True
            else:
                migrate_shutdown()
        if aborted:
            abort_migrate('Live migration failed and is required')
            return
        self.last_currentnode = self.zkhandler.read(('domain.node', self.domuuid))
        self.last_lastnode = self.zkhandler.read(('domain.last_node', self.domuuid))
        migrate_lock_node.release()
        migrate_lock_state.release()
        lock.release()

        self.inmigrate = False
        return

    # Receive the migration from another host
    def receive_migrate(self):
        # Wait for any previous migration
        while self.inreceive:
            time.sleep(0.1)

        self.inreceive = True

        self.logger.out('Receiving VM migration from node "{}"'.format(self.last_currentnode), state='i', prefix='Domain {}'.format(self.domuuid))

        # Ensure our lock key is populated
        self.zkhandler.write([
            (('domain.migrate.sync_lock', self.domuuid), self.domuuid)
        ])

        self.logger.out('Acquiring lock for migration phase A', state='i', prefix='Domain {}'.format(self.domuuid))
        lock = self.zkhandler.exclusivelock(('domain.migrate.sync_lock', self.domuuid))
        try:
            lock.acquire(timeout=30.0)
        except Exception:
            self.logger.out('Failed to acquire exclusive lock for VM', state='w')
            return
        self.logger.out('Acquired lock for migration phase A', state='o', prefix='Domain {}'.format(self.domuuid))
        # Exactly twice the amount of time that the other side is waiting
        time.sleep(1)
        lock.release()

        self.logger.out('Acquiring lock for phase C', state='i', prefix='Domain {}'.format(self.domuuid))
        lock.acquire()
        self.logger.out('Acquired lock for migration phase C', state='o', prefix='Domain {}'.format(self.domuuid))
        # Set the updated data
        self.last_currentnode = self.zkhandler.read(('domain.node', self.domuuid))
        self.last_lastnode = self.zkhandler.read(('domain.last_node', self.domuuid))
        self.state = self.zkhandler.read(('domain.state', self.domuuid))
        self.dom = self.lookupByUUID(self.domuuid)
        if self.dom:
            lvdomstate = self.dom.state()[0]
            if lvdomstate == libvirt.VIR_DOMAIN_RUNNING:
                # VM has been received and started
                self.addDomainToList()
                self.zkhandler.write([
                    (('domain.state', self.domuuid), 'start')
                ])
                self.logger.out('Successfully received migrated VM', state='o', prefix='Domain {}'.format(self.domuuid))
            else:
                # The receive somehow failed
                self.zkhandler.write([
                    (('domain.state', self.domuuid), 'fail'),
                    (('domain.failed_reason', self.domuuid), 'Failed to receive migration')
                ])
                self.logger.out('Failed to receive migrated VM', state='e', prefix='Domain {}'.format(self.domuuid))
        else:
            if self.node == self.this_node.name:
                if self.state in ['start']:
                    # The receive was aborted
                    self.logger.out('Receive aborted via state change', state='w', prefix='Domain {}'.format(self.domuuid))
                elif self.state in ['stop']:
                    # The send was shutdown-based
                    self.zkhandler.write([
                        (('domain.state', self.domuuid), 'start')
                    ])
                else:
                    # The send failed or was aborted
                    self.logger.out('Migrate aborted or failed; VM in state {}'.format(self.state), state='w', prefix='Domain {}'.format(self.domuuid))
        lock.release()

        self.zkhandler.write([
            (('domain.migrate.sync_lock', self.domuuid), '')
        ])
        self.inreceive = False
        return

    #
    # Main function to manage a VM (taking only self)
    #
    def manage_vm_state(self):
        # Update the current values from zookeeper
        self.state = self.zkhandler.read(('domain.state', self.domuuid))
        self.node = self.zkhandler.read(('domain.node', self.domuuid))
        self.lastnode = self.zkhandler.read(('domain.last_node', self.domuuid))
        self.migration_method = self.zkhandler.read(('domain.meta.migrate_method', self.domuuid))

        # Check the current state of the VM
        try:
            if self.dom is not None:
                running, reason = self.dom.state()
            else:
                raise
        except Exception:
            running = libvirt.VIR_DOMAIN_NOSTATE

        self.logger.out('VM state change for "{}": {} {}'.format(self.domuuid, self.state, self.node), state='i')

        #######################
        # Handle state changes
        #######################
        # Valid states are:
        #   start
        #   migrate
        #   migrate-live
        #   restart
        #   shutdown
        #   stop
        # States we don't (need to) handle are:
        #   disable
        #   provision

        # Conditional pass one - Are we already performing an action
        if self.instart is False \
                and self.inrestart is False \
                and self.inmigrate is False \
                and self.inreceive is False \
                and self.inshutdown is False \
                and self.instop is False:
            # Conditional pass two - Is this VM configured to run on this node
            if self.node == self.this_node.name:
                # Conditional pass three - Is this VM currently running on this node
                if running == libvirt.VIR_DOMAIN_RUNNING:
                    # VM is already running and should be
                    if self.state == "start":
                        # Start the log watcher
                        self.console_log_instance.start()
                        # Add domain to running list
                        self.addDomainToList()
                    # VM is already running and should be but stuck in migrate state
                    elif self.state == "migrate" or self.state == "migrate-live":
                        # Start the log watcher
                        self.console_log_instance.start()
                        self.zkhandler.write([
                            (('domain.state', self.domuuid), 'start')
                        ])
                        # Add domain to running list
                        self.addDomainToList()
                    # VM should be restarted
                    elif self.state == "restart":
                        self.restart_vm()
                    # VM should be shut down
                    elif self.state == "shutdown":
                        self.shutdown_vm()
                    # VM should be stopped
                    elif self.state == "stop":
                        self.stop_vm()
                else:
                    # VM should be started
                    if self.state == "start":
                        # Start the domain
                        self.start_vm()
                    # VM should be migrated to this node
                    elif self.state == "migrate" or self.state == "migrate-live":
                        # Receive the migration
                        self.receive_migrate()
                    # VM should be restarted (i.e. started since it isn't running)
                    if self.state == "restart":
                        self.zkhandler.write([
                            (('domain.state', self.domuuid), 'start')
                        ])
                    # VM should be shut down; ensure it's gone from this node's domain_list
                    elif self.state == "shutdown":
                        self.removeDomainFromList()
                        # Stop the log watcher
                        self.console_log_instance.stop()
                    # VM should be stoped; ensure it's gone from this node's domain_list
                    elif self.state == "stop":
                        self.removeDomainFromList()
                        # Stop the log watcher
                        self.console_log_instance.stop()
                # Update the VNC information
                self.update_vnc()
            else:
                # Conditional pass three - Is this VM currently running on this node
                if running == libvirt.VIR_DOMAIN_RUNNING:
                    # VM should be migrated away from this node
                    if self.state == "migrate":
                        self.migrate_vm(force_live=False)
                    # VM should be migrated away from this node, live only (no shutdown fallback)
                    elif self.state == "migrate-live":
                        self.migrate_vm(force_live=True)
                    # VM should be shutdown gracefully
                    elif self.state == 'shutdown':
                        self.shutdown_vm()
                    # VM should be forcibly terminated
                    else:
                        self.terminate_vm()

        self.state_thread = None
        return

    # This function is a wrapper for libvirt.lookupByUUID which fixes some problems
    # 1. Takes a text UUID and handles converting it to bytes
    # 2. Try's it and returns a sensible value if not
    def lookupByUUID(self, tuuid):
        # Don't do anything if the VM shouldn't live on this node
        if self.node != self.this_node.name:
            return None

        lv_conn = None
        libvirt_name = "qemu:///system"

        # Convert the text UUID to bytes
        buuid = uuid.UUID(tuuid).bytes

        # Try
        try:
            # Open a libvirt connection
            lv_conn = libvirt.open(libvirt_name)
            if lv_conn is None:
                self.logger.out('Failed to open local libvirt connection', state='e', prefix='Domain {}'.format(self.domuuid))
                return None

            # Lookup the UUID
            dom = lv_conn.lookupByUUID(buuid)

        # Fail
        except Exception:
            dom = None

        # After everything
        finally:
            # Close the libvirt connection
            if lv_conn is not None:
                lv_conn.close()

        # Return the dom object (or None)
        return dom

    # Flush the locks of a VM based on UUID
    @staticmethod
    def flush_locks(zkhandler, logger, dom_uuid, this_node=None):
        logger.out('Flushing RBD locks for VM "{}"'.format(dom_uuid), state='i')
        # Get the list of RBD images
        rbd_list = zkhandler.read(('domain.storage.volumes', dom_uuid)).split(',')

        for rbd in rbd_list:
            # Check if a lock exists
            lock_list_retcode, lock_list_stdout, lock_list_stderr = common.run_os_command('rbd lock list --format json {}'.format(rbd))
            if lock_list_retcode != 0:
                logger.out('Failed to obtain lock list for volume "{}"'.format(rbd), state='e')
                continue

            try:
                lock_list = json.loads(lock_list_stdout)
            except Exception as e:
                logger.out('Failed to parse lock list for volume "{}": {}'.format(rbd, e), state='e')
                continue

            # If there's at least one lock
            if lock_list:
                # Loop through the locks
                for lock in lock_list:
                    if this_node is not None and zkhandler.read(('domain.state', dom_uuid)) != 'stop' and lock['address'].split(':')[0] != this_node.storage_ipaddr:
                        logger.out('RBD lock does not belong to this host (lock owner: {}): freeing this lock would be unsafe, aborting'.format(lock['address'].split(':')[0], state='e'))
                        zkhandler.write([
                            (('domain.state', dom_uuid), 'fail'),
                            (('domain.failed_reason', dom_uuid), 'Could not safely free RBD lock {} ({}) on volume {}; stop VM and flush locks manually'.format(lock['id'], lock['address'], rbd)),
                        ])
                        break
                    # Free the lock
                    lock_remove_retcode, lock_remove_stdout, lock_remove_stderr = common.run_os_command('rbd lock remove {} "{}" "{}"'.format(rbd, lock['id'], lock['locker']))
                    if lock_remove_retcode != 0:
                        logger.out('Failed to free RBD lock "{}" on volume "{}": {}'.format(lock['id'], rbd, lock_remove_stderr), state='e')
                        zkhandler.write([
                            (('domain.state', dom_uuid), 'fail'),
                            (('domain.failed_reason', dom_uuid), 'Could not free RBD lock {} ({}) on volume {}: {}'.format(lock['id'], lock['address'], rbd, lock_remove_stderr)),
                        ])
                        break
                    logger.out('Freed RBD lock "{}" on volume "{}"'.format(lock['id'], rbd), state='o')

        return True


# Primary command function
def vm_command(zkhandler, logger, this_node, data):
    # Get the command and args
    command, dom_uuid, *args = data.split()

    if match('success-.*', command) or match('failure-.*', command):
        return

    logger.out('Getting command "{}" for domain "{}"'.format(command, dom_uuid), state='i')

    # Verify that the VM is set to run on this node
    domain = this_node.d_domain.get(dom_uuid, None)
    if domain is None:
        return False

    if domain.getnode() != this_node.name:
        return

    # Lock the command queue
    zk_lock = zkhandler.writelock('base.cmd.domain')
    with zk_lock:
        # Flushing VM RBD locks
        if command == 'flush_locks':
            result = VMInstance.flush_locks(zkhandler, logger, dom_uuid, this_node)
        # Attaching a device
        elif command == 'attach_device':
            xml_spec = ' '.join(args)
            result = domain.attach_device(xml_spec)
        # Detaching a device
        elif command == 'detach_device':
            xml_spec = ' '.join(args)
            result = domain.detach_device(xml_spec)
        # Command not defined
        else:
            result = False

        # Command succeeded
        if result:
            # Update the command queue
            zkhandler.write([
                ('base.cmd.domain', 'success-{}'.format(data))
            ])
        # Command failed
        else:
            # Update the command queue
            zkhandler.write([
                ('base.cmd.domain', 'failure-{}'.format(data))
            ])

        # Wait 1 seconds before we free the lock, to ensure the client hits the lock
        time.sleep(1)