# Copyright (C) 2012-2015  Codethink Limited
# Copyright (C) 2011, 2012  Lars Wirzenius
#
# 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 2 of the License.
#
# 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 <http://www.gnu.org/licenses/>.


import contextlib
import errno
import fcntl
import logging
import os
import re
import select
import shutil
import stat
import subprocess
import sys
import tempfile
import time


def get_data_path(relative_path):
    '''Return path to a data file in the morphlib Python package.

    ``relative_path`` is the name of the data file, relative to the
    `extensions/` directory.

    '''

    extensions_dir = os.path.dirname(__file__)
    return os.path.join(extensions_dir, relative_path)


def get_data(relative_path): # pragma: no cover
    '''Return contents of a data file from the morphlib Python package.

    ``relative_path`` is the name of the data file, relative to the
    `extensions/` directory.

    '''

    with open(get_data_path(relative_path)) as f:
        return f.read()


def runcmd(argv, *args, **kwargs):
    '''Run external command or pipeline.

    Example: ``runcmd(['grep', 'foo'], ['wc', '-l'],
                      feed_stdin='foo\nbar\n')``

    Return the standard output of the command.

    Raise ``ExtensionError`` if external command returns
    non-zero exit code. ``*args`` and ``**kwargs`` are passed
    onto ``subprocess.Popen``.

    '''

    our_options = (
        ('ignore_fail', False),
        ('log_error', True),
    )
    opts = {}
    for name, default in our_options:
        opts[name] = default
        if name in kwargs:
            opts[name] = kwargs[name]
            del kwargs[name]

    exit, out, err = runcmd_unchecked(argv, *args, **kwargs)
    if exit != 0:
        msg = 'Command failed: %s\n%s' % (' '.join(argv), err)
        if opts['ignore_fail']:
            if opts['log_error']:
                logging.info(msg)
        else:
            if opts['log_error']:
                logging.error(msg)
            raise ExtensionError(msg)
    return out

def runcmd_unchecked(argv, *argvs, **kwargs):
    '''Run external command or pipeline.

    Return the exit code, and contents of standard output and error
    of the command.

    See also ``runcmd``.

    '''

    argvs = [argv] + list(argvs)
    logging.debug('run external command: %s' % repr(argvs))

    def pop_kwarg(name, default):
        if name in kwargs:
            value = kwargs[name]
            del kwargs[name]
            return value
        else:
            return default

    feed_stdin = pop_kwarg('feed_stdin', '')
    pipe_stdin = pop_kwarg('stdin', subprocess.PIPE)
    pipe_stdout = pop_kwarg('stdout', subprocess.PIPE)
    pipe_stderr = pop_kwarg('stderr', subprocess.PIPE)

    try:
        pipeline = _build_pipeline(argvs,
                                   pipe_stdin,
                                   pipe_stdout,
                                   pipe_stderr,
                                   kwargs)
        return _run_pipeline(pipeline, feed_stdin, pipe_stdin,
                              pipe_stdout, pipe_stderr)
    except OSError, e: # pragma: no cover
        if e.errno == errno.ENOENT and e.filename is None:
            e.filename = argv[0]
            raise e
        else:
            raise

def _build_pipeline(argvs, pipe_stdin, pipe_stdout, pipe_stderr, kwargs):
    procs = []
    for i, argv in enumerate(argvs):
        if i == 0 and i == len(argvs) - 1:
            stdin = pipe_stdin
            stdout = pipe_stdout
            stderr = pipe_stderr
        elif i == 0:
            stdin = pipe_stdin
            stdout = subprocess.PIPE
            stderr = pipe_stderr
        elif i == len(argvs) - 1:
            stdin = procs[-1].stdout
            stdout = pipe_stdout
            stderr = pipe_stderr
        else:
            stdin = procs[-1].stdout
            stdout = subprocess.PIPE
            stderr = pipe_stderr
        p = subprocess.Popen(argv, stdin=stdin, stdout=stdout,
                             stderr=stderr, close_fds=True, **kwargs)
        procs.append(p)

    return procs

def _run_pipeline(procs, feed_stdin, pipe_stdin, pipe_stdout, pipe_stderr):

    stdout_eof = False
    stderr_eof = False
    out = []
    err = []
    pos = 0
    io_size = 1024

    def set_nonblocking(fd):
        flags = fcntl.fcntl(fd, fcntl.F_GETFL, 0)
        flags = flags | os.O_NONBLOCK
        fcntl.fcntl(fd, fcntl.F_SETFL, flags)

    if feed_stdin and pipe_stdin == subprocess.PIPE:
        set_nonblocking(procs[0].stdin.fileno())
    if pipe_stdout == subprocess.PIPE:
        set_nonblocking(procs[-1].stdout.fileno())
    if pipe_stderr == subprocess.PIPE:
        set_nonblocking(procs[-1].stderr.fileno())

    def still_running():
        for p in procs:
            p.poll()
        for p in procs:
            if p.returncode is None:
                return True
        if pipe_stdout == subprocess.PIPE and not stdout_eof:
            return True
        if pipe_stderr == subprocess.PIPE and not stderr_eof:
            return True # pragma: no cover
        return False

    while still_running():
        rlist = []
        if not stdout_eof and pipe_stdout == subprocess.PIPE:
            rlist.append(procs[-1].stdout)
        if not stderr_eof and pipe_stderr == subprocess.PIPE:
            rlist.append(procs[-1].stderr)

        wlist = []
        if pipe_stdin == subprocess.PIPE and pos < len(feed_stdin):
            wlist.append(procs[0].stdin)

        if rlist or wlist:
            try:
                r, w, x = select.select(rlist, wlist, [])
            except select.error, e:
                err, msg = e.args
                if err == errno.EINTR:
                    break
                raise
        else:
            break # Let's not busywait waiting for processes to die.

        if procs[0].stdin in w and pos < len(feed_stdin):
            data = feed_stdin[pos : pos+io_size]
            procs[0].stdin.write(data)
            pos += len(data)
            if pos >= len(feed_stdin):
                procs[0].stdin.close()

        if procs[-1].stdout in r:
            data = procs[-1].stdout.read(io_size)
            if data:
                out.append(data)
            else:
                stdout_eof = True

        if procs[-1].stderr in r:
            data = procs[-1].stderr.read(io_size)
            if data:
                err.append(data)
            else:
                stderr_eof = True

    while still_running():
        for p in procs:
            if p.returncode is None:
                p.wait()

    errorcodes = [p.returncode for p in procs if p.returncode != 0] or [0]
    return errorcodes[-1], ''.join(out), ''.join(err)


def shell_quote(s):
    '''Return a shell-quoted version of s.'''

    lower_ascii = 'abcdefghijklmnopqrstuvwxyz'
    upper_ascii = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
    digits = '0123456789'
    punctuation = '-_/=.,:'
    safe = set(lower_ascii + upper_ascii + digits + punctuation)

    quoted = []
    for c in s:
        if c in safe:
            quoted.append(c)
        elif c == "'":
            quoted.append('"\'"')
        else:
            quoted.append("'%c'" % c)

    return ''.join(quoted)


def ssh_runcmd(target, argv, **kwargs):
    '''Run command in argv on remote host target.

    This is similar to runcmd, but the command is run on the remote
    machine. The command is given as an argv array; elements in the
    array are automatically quoted so they get passed to the other
    side correctly.

    An optional ``tty=`` parameter can be passed to ``ssh_runcmd`` in
    order to force or disable pseudo-tty allocation. This is often
    required to run ``sudo`` on another machine and might be useful
    in other situations as well. Supported values are ``tty=True`` for
    forcing tty allocation, ``tty=False`` for disabling it and
    ``tty=None`` for not passing anything tty related to ssh.

    With the ``tty`` option,
    ``cliapp.runcmd(['ssh', '-tt', 'user@host', '--', 'sudo', 'ls'])``
    can be written as
    ``cliapp.ssh_runcmd('user@host', ['sudo', 'ls'], tty=True)``
    which is more intuitive.

    The target is given as-is to ssh, and may use any syntax ssh
    accepts.

    Environment variables may or may not be passed to the remote
    machine: this is dependent on the ssh and sshd configurations.
    Invoke env(1) explicitly to pass in the variables you need to
    exist on the other end.

    Pipelines are not supported.

    '''

    tty = kwargs.get('tty', None)
    if tty:
        ssh_cmd = ['ssh', '-tt', target, '--']
    elif tty is False:
        ssh_cmd = ['ssh', '-T', target, '--']
    else:
        ssh_cmd = ['ssh', target, '--']
    if 'tty' in kwargs:
        del kwargs['tty']

    local_argv = ssh_cmd + map(shell_quote, argv)
    return runcmd(local_argv, **kwargs)

def write_from_dict(filepath, d, validate=lambda x, y: True):
    """Takes a dictionary and appends the contents to a file

    An optional validation callback can be passed to perform validation on
    each value in the dictionary.

    e.g.

        def validation_callback(dictionary_key, dictionary_value):
            if not dictionary_value.isdigit():
                raise Exception('value contains non-digit character(s)')

    Any callback supplied to this function should raise an exception
    if validation fails.

    """

    # Sort items asciibetically
    # the output of the deployment should not depend
    # on the locale of the machine running the deployment
    items = sorted(d.iteritems(), key=lambda (k, v): [ord(c) for c in v])

    for (k, v) in items:
        validate(k, v)

    with open(filepath, 'a') as f:
        for (_, v) in items:
            f.write('%s\n' % v)

        os.fchown(f.fileno(), 0, 0)
        os.fchmod(f.fileno(), 0644)


def parse_environment_pairs(env, pairs):
    '''Add key=value pairs to the environment dict.

    Given a dict and a list of strings of the form key=value,
    set dict[key] = value, unless key is already set in the
    environment, at which point raise an exception.

    This does not modify the passed in dict.

    Returns the extended dict.

    '''

    extra_env = dict(p.split('=', 1) for p in pairs)
    conflicting = [k for k in extra_env if k in env]
    if conflicting:
        raise EnvironmentAlreadySetError(conflicting)

    # Return a dict that is the union of the two
    # This is not the most performant, since it creates
    # 3 unnecessary lists, but I felt this was the most
    # easy to read. Using itertools.chain may be more efficicent
    return dict(env.items() + extra_env.items())


class ExtensionError(Exception):

    def __init__(self, msg):
        self.msg = msg

    def __str__(self):
        return self.msg


class Fstab(object):
    '''Small helper class for parsing and adding lines to /etc/fstab.'''

    # There is an existing Python helper library for editing of /etc/fstab.
    # However it is unmaintained and has an incompatible license (GPL3).
    #
    # https://code.launchpad.net/~computer-janitor-hackers/python-fstab/trunk

    def __init__(self, filepath='/etc/fstab'):
        if os.path.exists(filepath):
            with open(filepath, 'r') as f:
                self.text= f.read()
        else:
            self.text = ''
        self.filepath = filepath
        self.lines_added = 0

    def get_mounts(self):
        '''Return list of mount devices and targets in /etc/fstab.

        Return value is a dict of target -> device.
        '''
        mounts = dict()
        for line in self.text.splitlines():
            words = line.split()
            if len(words) >= 2 and not words[0].startswith('#'):
                device, target = words[0:2]
                mounts[target] = device
        return mounts

    def add_line(self, line):
        '''Add a new entry to /etc/fstab.

        Lines are appended, and separated from any entries made by configure
        extensions with a comment.

        '''
        if self.lines_added == 0:
            if len(self.text) == 0 or self.text[-1] is not '\n':
                self.text += '\n'
            self.text += '# Morph default system layout\n'
        self.lines_added += 1

        self.text += line + '\n'

    def write(self):
        '''Rewrite the fstab file to include all new entries.'''
        with tempfile.NamedTemporaryFile(delete=False) as f:
            f.write(self.text)
            tmp = f.name
        shutil.move(os.path.abspath(tmp), os.path.abspath(self.filepath))


class Extension(object):

    '''A base class for deployment extensions.

    A subclass should subclass this class, and add a
    ``process_args`` method.

    Note that it is not necessary to subclass this class for write
    extensions. This class is here just to collect common code for
    write extensions.

    '''

    def setup_logging(self):
        '''Direct all logging output to MORPH_LOG_FD, if set.

        This file descriptor is read by Morph and written into its own log
        file.

        '''
        log_write_fd = int(os.environ.get('MORPH_LOG_FD', 0))

        if log_write_fd == 0:
            return

        formatter = logging.Formatter('%(message)s')

        handler = logging.StreamHandler(os.fdopen(log_write_fd, 'w'))
        handler.setFormatter(formatter)

        logger = logging.getLogger()
        logger.addHandler(handler)
        logger.setLevel(logging.DEBUG)

    def process_args(self, args):
        raise NotImplementedError()

    def run(self, args=None):
        if args is None:
            args = sys.argv[1:]
        try:
            self.setup_logging()
            self.process_args(args)
        except ExtensionError as e:
            sys.stdout.write('ERROR: %s\n' % e)
            sys.exit(1)

    def status(self, **kwargs):
        '''Provide status output.

        The ``msg`` keyword argument is the actual message,
        the rest are values for fields in the message as interpolated
        by %.

        '''
        sys.stdout.write('%s\n' % (kwargs['msg'] % kwargs))
        sys.stdout.flush()


class WriteExtension(Extension):

    '''A base class for deployment write extensions.

    A subclass should subclass this class, and add a
    ``process_args`` method.

    Note that it is not necessary to subclass this class for write
    extensions. This class is here just to collect common code for
    write extensions.

    '''

    def check_for_btrfs_in_deployment_host_kernel(self):
        with open('/proc/filesystems') as f:
            text = f.read()
        return '\tbtrfs\n' in text

    def require_btrfs_in_deployment_host_kernel(self):
        if not self.check_for_btrfs_in_deployment_host_kernel():
            raise ExtensionError(
                'Error: Btrfs is required for this deployment, but was not '
                'detected in the kernel of the machine that is running Morph.')

    def create_local_system(self, temp_root, raw_disk):
        '''Create a raw system image locally.'''

        with self.created_disk_image(raw_disk):
            self.format_btrfs(raw_disk)
            self.create_system(temp_root, raw_disk)

    @contextlib.contextmanager
    def created_disk_image(self, location):
        size = self.get_disk_size()
        if not size:
            raise ExtensionError('DISK_SIZE is not defined')
        self.create_raw_disk_image(location, size)
        try:
            yield
        except BaseException:
            os.unlink(location)
            raise

    def format_btrfs(self, raw_disk):
        try:
            self.mkfs_btrfs(raw_disk)
        except BaseException:
            sys.stderr.write('Error creating disk image')
            raise

    def create_system(self, temp_root, raw_disk):
        with self.mount(raw_disk) as mp:
            try:
                self.create_btrfs_system_layout(
                    temp_root, mp, version_label='factory',
                    disk_uuid=self.get_uuid(raw_disk))
            except BaseException as e:
                sys.stderr.write('Error creating Btrfs system layout')
                raise

    def _parse_size(self, size):
        '''Parse a size from a string.

        Return size in bytes.

        '''

        m = re.match('^(\d+)([kmgKMG]?)$', size)
        if not m:
            return None

        factors = {
            '': 1,
            'k': 1024,
            'm': 1024**2,
            'g': 1024**3,
        }
        factor = factors[m.group(2).lower()]

        return int(m.group(1)) * factor

    def _parse_size_from_environment(self, env_var, default):
        '''Parse a size from an environment variable.'''

        size = os.environ.get(env_var, default)
        if size is None:
            return None
        bytes = self._parse_size(size)
        if bytes is None:
            raise ExtensionError('Cannot parse %s value %s' % (env_var, size))
        return bytes

    def get_disk_size(self):
        '''Parse disk size from environment.'''
        return self._parse_size_from_environment('DISK_SIZE', None)

    def get_ram_size(self):
        '''Parse RAM size from environment.'''
        return self._parse_size_from_environment('RAM_SIZE', '1G')

    def get_vcpu_count(self):
        '''Parse the virtual cpu count from environment.'''
        return self._parse_size_from_environment('VCPUS', '1')

    def create_raw_disk_image(self, filename, size):
        '''Create a raw disk image.'''

        self.status(msg='Creating empty disk image')
        with open(filename, 'wb') as f:
            if size > 0:
                f.seek(size-1)
                f.write('\0')

    def mkfs_btrfs(self, location):
        '''Create a btrfs filesystem on the disk.'''

        self.status(msg='Creating btrfs filesystem')
        try:
            # The following command disables some new filesystem features. We
            # need to do this because at the time of writing, SYSLINUX has not
            # been updated to understand these new features and will fail to
            # boot if the kernel is on a filesystem where they are enabled.
            subprocess.check_output(
                ['mkfs.btrfs','-f', '-L', 'baserock',
                '--features', '^extref',
                '--features', '^skinny-metadata',
                '--features', '^mixed-bg',
                '--nodesize', '4096',
                location])
        except subprocess.CalledProcessError as e:
            if 'unrecognized option \'--features\'' in e.output:
                # Old versions of mkfs.btrfs (including v0.20, present in many
                # Baserock releases) don't support the --features option, but
                # also don't enable the new features by default. So we can
                # still create a bootable system in this situation.
                logging.debug(
                    'Assuming mkfs.btrfs failure was because the tool is too '
                    'old to have --features flag.')
                subprocess.check_call(
                    ['mkfs.btrfs','-f', '-L', 'baserock', location])
            else:
                raise

    def get_uuid(self, location):
        '''Get the UUID of a block device's file system.'''
        # Requires util-linux blkid; busybox one ignores options and
        # lies by exiting successfully.
        return subprocess.check_output(['blkid', '-s', 'UUID', '-o', 'value',
                                        location]).strip()

    @contextlib.contextmanager
    def mount(self, location):
        self.status(msg='Mounting filesystem')
        try:
            mount_point = tempfile.mkdtemp()
            if self.is_device(location):
                subprocess.check_call(['mount', location, mount_point])
            else:
                subprocess.check_call(['mount', '-o', 'loop',
                                       location, mount_point])
        except BaseException as e:
            sys.stderr.write('Error mounting filesystem')
            os.rmdir(mount_point)
            raise
        try:
            yield mount_point
        finally:
            self.status(msg='Unmounting filesystem')
            subprocess.check_call(['umount', mount_point])
            os.rmdir(mount_point)

    def create_btrfs_system_layout(self, temp_root, mountpoint, version_label,
                                   disk_uuid):
        '''Separate base OS versions from state using subvolumes.

        '''
        initramfs = self.find_initramfs(temp_root)
        version_root = os.path.join(mountpoint, 'systems', version_label)
        state_root = os.path.join(mountpoint, 'state')

        os.makedirs(version_root)
        os.makedirs(state_root)

        self.create_orig(version_root, temp_root)
        system_dir = os.path.join(version_root, 'orig')

        state_dirs = self.complete_fstab_for_btrfs_layout(system_dir,
                                                          disk_uuid)

        for state_dir in state_dirs:
            self.create_state_subvolume(system_dir, mountpoint, state_dir)

        self.create_run(version_root)

        os.symlink(
                version_label, os.path.join(mountpoint, 'systems', 'default'))

        if self.bootloader_config_is_wanted():
            self.install_kernel(version_root, temp_root)
            if self.get_dtb_path() != '':
                self.install_dtb(version_root, temp_root)
            self.install_syslinux_menu(mountpoint, version_root)
            if initramfs is not None:
                self.install_initramfs(initramfs, version_root)
                self.generate_bootloader_config(mountpoint, disk_uuid)
            else:
                self.generate_bootloader_config(mountpoint)
            self.install_bootloader(mountpoint)

    def create_orig(self, version_root, temp_root):
        '''Create the default "factory" system.'''

        orig = os.path.join(version_root, 'orig')

        self.status(msg='Creating orig subvolume')
        subprocess.check_call(['btrfs', 'subvolume', 'create', orig])
        self.status(msg='Copying files to orig subvolume')
        subprocess.check_call(['cp', '-a', temp_root + '/.', orig + '/.'])

    def create_run(self, version_root):
        '''Create the 'run' snapshot.'''

        self.status(msg='Creating run subvolume')
        orig = os.path.join(version_root, 'orig')
        run = os.path.join(version_root, 'run')
        subprocess.check_call(
            ['btrfs', 'subvolume', 'snapshot', orig, run])

    def create_state_subvolume(self, system_dir, mountpoint, state_subdir):
        '''Create a shared state subvolume.

        We need to move any files added to the temporary rootfs by the
        configure extensions to their correct home. For example, they might
        have added keys in `/root/.ssh` which we now need to transfer to
        `/state/root/.ssh`.

        '''
        self.status(msg='Creating %s subvolume' % state_subdir)
        subvolume = os.path.join(mountpoint, 'state', state_subdir)
        subprocess.check_call(['btrfs', 'subvolume', 'create', subvolume])
        os.chmod(subvolume, 0o755)

        existing_state_dir = os.path.join(system_dir, state_subdir)
        files = []
        if os.path.exists(existing_state_dir):
            files = os.listdir(existing_state_dir)
        if len(files) > 0:
            self.status(msg='Moving existing data to %s subvolume' % subvolume)
        for filename in files:
            filepath = os.path.join(existing_state_dir, filename)
            subprocess.check_call(['mv', filepath, subvolume])

    def complete_fstab_for_btrfs_layout(self, system_dir, rootfs_uuid=None):
        '''Fill in /etc/fstab entries for the default Btrfs disk layout.

        In the future we should move this code out of the write extension and
        in to a configure extension. To do that, though, we need some way of
        informing the configure extension what layout should be used. Right now
        a configure extension doesn't know if the system is going to end up as
        a Btrfs disk image, a tarfile or something else and so it can't come
        up with a sensible default fstab.

        Configuration extensions can already create any /etc/fstab that they
        like. This function only fills in entries that are missing, so if for
        example the user configured /home to be on a separate partition, that
        decision will be honoured and /state/home will not be created.

        '''
        shared_state_dirs = {'home', 'root', 'opt', 'srv', 'var'}

        fstab = Fstab(os.path.join(system_dir, 'etc', 'fstab'))
        existing_mounts = fstab.get_mounts()

        if '/' in existing_mounts:
            root_device = existing_mounts['/']
        else:
            root_device = (self.get_root_device() if rootfs_uuid is None else
                           'UUID=%s' % rootfs_uuid)
            fstab.add_line('%s  / btrfs defaults,rw,noatime 0 1' % root_device)

        state_dirs_to_create = set()
        for state_dir in shared_state_dirs:
            if '/' + state_dir not in existing_mounts:
                state_dirs_to_create.add(state_dir)
                state_subvol = os.path.join('/state', state_dir)
                fstab.add_line(
                        '%s  /%s  btrfs subvol=%s,defaults,rw,noatime 0 2' %
                        (root_device, state_dir, state_subvol))

        fstab.write()
        return state_dirs_to_create

    def find_initramfs(self, temp_root):
        '''Check whether the rootfs has an initramfs.

        Uses the INITRAMFS_PATH option to locate it.
        '''
        if 'INITRAMFS_PATH' in os.environ:
            initramfs = os.path.join(temp_root, os.environ['INITRAMFS_PATH'])
            if not os.path.exists(initramfs):
                raise ExtensionError('INITRAMFS_PATH specified, '
                                     'but file does not exist')
            return initramfs
        return None

    def install_initramfs(self, initramfs_path, version_root):
        '''Install the initramfs outside of 'orig' or 'run' subvolumes.

        This is required because syslinux doesn't traverse subvolumes when
        loading the kernel or initramfs.
        '''
        self.status(msg='Installing initramfs')
        initramfs_dest = os.path.join(version_root, 'initramfs')
        subprocess.check_call(['cp', '-a', initramfs_path, initramfs_dest])

    def install_kernel(self, version_root, temp_root):
        '''Install the kernel outside of 'orig' or 'run' subvolumes'''

        self.status(msg='Installing kernel')
        image_names = ['vmlinuz', 'zImage', 'uImage']
        kernel_dest = os.path.join(version_root, 'kernel')
        for name in image_names:
            try_path = os.path.join(temp_root, 'boot', name)
            if os.path.exists(try_path):
                subprocess.check_call(['cp', '-a', try_path, kernel_dest])
                break

    def install_dtb(self, version_root, temp_root):
        '''Install the device tree outside of 'orig' or 'run' subvolumes'''

        self.status(msg='Installing devicetree')
        device_tree_path = self.get_dtb_path()
        dtb_dest = os.path.join(version_root, 'dtb')
        try_path = os.path.join(temp_root, device_tree_path)
        if os.path.exists(try_path):
            subprocess.check_call(['cp', '-a', try_path, dtb_dest])
        else:
            logging.error("Failed to find device tree %s", device_tree_path)
            raise ExtensionError(
                'Failed to find device tree %s' % device_tree_path)

    def get_dtb_path(self):
        return os.environ.get('DTB_PATH', '')

    def get_bootloader_install(self):
        # Do we actually want to install the bootloader?
        # Set this to "none" to prevent the install
        return os.environ.get('BOOTLOADER_INSTALL', 'extlinux')

    def get_bootloader_config_format(self):
        # The config format for the bootloader,
        # if not set we default to extlinux for x86
        return os.environ.get('BOOTLOADER_CONFIG_FORMAT', 'extlinux')

    def get_extra_kernel_args(self):
        return os.environ.get('KERNEL_ARGS', '')

    def get_root_device(self):
        return os.environ.get('ROOT_DEVICE', '/dev/sda')

    def generate_bootloader_config(self, real_root, disk_uuid=None):
        '''Install extlinux on the newly created disk image.'''
        config_function_dict = {
            'extlinux': self.generate_extlinux_config,
        }

        config_type = self.get_bootloader_config_format()
        if config_type in config_function_dict:
            config_function_dict[config_type](real_root, disk_uuid)
        else:
            raise ExtensionError(
                'Invalid BOOTLOADER_CONFIG_FORMAT %s' % config_type)

    def generate_extlinux_config(self, real_root, disk_uuid=None):
        '''Install extlinux on the newly created disk image.'''

        self.status(msg='Creating extlinux.conf')
        config = os.path.join(real_root, 'extlinux.conf')

        ''' Please also update the documentation in the following files
            if you change these default kernel args:
            - kvm.write.help
            - rawdisk.write.help
            - virtualbox-ssh.write.help '''
        kernel_args = (
            'rw ' # ro ought to work, but we don't test that regularly
            'init=/sbin/init ' # default, but it doesn't hurt to be explicit
            'rootfstype=btrfs ' # required when using initramfs, also boots
                                # faster when specified without initramfs
            'rootflags=subvol=systems/default/run ') # boot runtime subvol
        kernel_args += 'root=%s ' % (self.get_root_device()
                                     if disk_uuid is None
                                     else 'UUID=%s' % disk_uuid)
        kernel_args += self.get_extra_kernel_args()
        with open(config, 'w') as f:
            f.write('default linux\n')
            f.write('timeout 1\n')
            f.write('label linux\n')
            f.write('kernel /systems/default/kernel\n')
            if disk_uuid is not None:
                f.write('initrd /systems/default/initramfs\n')
            if self.get_dtb_path() != '':
                f.write('devicetree /systems/default/dtb\n')
            f.write('append %s\n' % kernel_args)

    def install_bootloader(self, real_root):
        install_function_dict = {
            'extlinux': self.install_bootloader_extlinux,
        }

        install_type = self.get_bootloader_install()
        if install_type in install_function_dict:
            install_function_dict[install_type](real_root)
        elif install_type != 'none':
            raise ExtensionError(
                'Invalid BOOTLOADER_INSTALL %s' % install_type)

    def install_bootloader_extlinux(self, real_root):
        self.status(msg='Installing extlinux')
        subprocess.check_call(['extlinux', '--install', real_root])

        # FIXME this hack seems to be necessary to let extlinux finish
        subprocess.check_call(['sync'])
        time.sleep(2)

    def install_syslinux_menu(self, real_root, version_root):
        '''Make syslinux/extlinux menu binary available.

        The syslinux boot menu is compiled to a file named menu.c32. Extlinux
        searches a few places for this file but it does not know to look inside
        our subvolume, so we copy it to the filesystem root.

        If the file is not available, the bootloader will still work but will
        not be able to show a menu.

        '''
        menu_file = os.path.join(version_root, 'orig',
            'usr', 'share', 'syslinux', 'menu.c32')
        if os.path.isfile(menu_file):
            self.status(msg='Copying menu.c32')
            shutil.copy(menu_file, real_root)

    def parse_attach_disks(self):
        '''Parse $ATTACH_DISKS into list of disks to attach.'''

        if 'ATTACH_DISKS' in os.environ:
            s = os.environ['ATTACH_DISKS']
            return s.split(':')
        else:
            return []

    def bootloader_config_is_wanted(self):
        '''Does the user want to generate a bootloader config?

        The user may set $BOOTLOADER_CONFIG_FORMAT to the desired
        format. 'extlinux' is the only allowed value, and is the default
        value for x86-32 and x86-64.

        '''

        def is_x86(arch):
            return (arch == 'x86_64' or
                    (arch.startswith('i') and arch.endswith('86')))

        value = os.environ.get('BOOTLOADER_CONFIG_FORMAT', '')
        if value == '':
            if not is_x86(os.uname()[-1]):
                return False

        return True

    def get_environment_boolean(self, variable):
        '''Parse a yes/no boolean passed through the environment.'''

        value = os.environ.get(variable, 'no').lower()
        if value in ['no', '0', 'false']:
            return False
        elif value in ['yes', '1', 'true']:
            return True
        else:
            raise ExtensionError('Unexpected value for %s: %s' %
                                 (variable, value))

    def check_ssh_connectivity(self, ssh_host):
        try:
            output = ssh_runcmd(ssh_host, ['echo', 'test'])
        except subprocess.CalledProcessError as e:
            logging.error("Error checking SSH connectivity: %s", str(e))
            raise ExtensionError(
                'Unable to SSH to %s: %s' % (ssh_host, e))

        if output.strip() != 'test':
            raise ExtensionError(
                'Unexpected output from remote machine: %s' % output.strip())

    def is_device(self, location):
        try:
            st = os.stat(location)
            return stat.S_ISBLK(st.st_mode)
        except OSError as e:
            if e.errno == errno.ENOENT:
                return False
            raise