scripts/cros_sdk.py

#!/usr/bin/env python
# Copyright (c) 2012 The Chromium OS Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

"""This script fetches and prepares an SDK chroot."""

import errno
import glob
import os
import pwd
import signal
import sys
import time
import urlparse

from chromite.cbuildbot import constants
from chromite.lib import cgroups
from chromite.lib import commandline
from chromite.lib import cros_build_lib
from chromite.lib import locking
from chromite.lib import namespaces
from chromite.lib import osutils
from chromite.lib import retry_util
from chromite.lib import toolchain

cros_build_lib.STRICT_SUDO = True


COMPRESSION_PREFERENCE = ('xz', 'bz2')

# TODO(zbehan): Remove the dependency on these, reimplement them in python
MAKE_CHROOT = [os.path.join(constants.SOURCE_ROOT,
                            'src/scripts/sdk_lib/make_chroot.sh')]
ENTER_CHROOT = [os.path.join(constants.SOURCE_ROOT,
                             'src/scripts/sdk_lib/enter_chroot.sh')]

# Proxy simulator configuration.
PROXY_HOST_IP = '192.168.240.1'
PROXY_PORT = 8080
PROXY_GUEST_IP = '192.168.240.2'
PROXY_NETMASK = 30
PROXY_VETH_PREFIX = 'veth'
PROXY_CONNECT_PORTS = (80, 443, 9418)
PROXY_APACHE_FALLBACK_USERS = ('www-data', 'apache', 'nobody')
PROXY_APACHE_MPMS = ('event', 'worker', 'prefork')
PROXY_APACHE_FALLBACK_PATH = ':'.join(
    '/usr/lib/apache2/mpm-%s' % mpm for mpm in PROXY_APACHE_MPMS
)
PROXY_APACHE_MODULE_GLOBS = ('/usr/lib*/apache2/modules', '/usr/lib*/apache2')

# We need these tools to run. Very common tools (tar,..) are omitted.
NEEDED_TOOLS = ('curl', 'xz')

# Tools needed for --proxy-sim only.
PROXY_NEEDED_TOOLS = ('ip',)


def GetArchStageTarballs(version):
  """Returns the URL for a given arch/version"""
  extension = {'bz2':'tbz2', 'xz':'tar.xz'}
  return [toolchain.GetSdkURL(suburl='cros-sdk-%s.%s'
                              % (version, extension[compressor]))
          for compressor in COMPRESSION_PREFERENCE]


def GetStage3Urls(version):
  return [toolchain.GetSdkURL(suburl='stage3-amd64-%s.tar.%s' % (version, ext))
          for ext in COMPRESSION_PREFERENCE]


def FetchRemoteTarballs(storage_dir, urls):
  """Fetches a tarball given by url, and place it in |storage_dir|.

  Args:
    storage_dir: Path where to save the tarball.
    urls: List of URLs to try to download. Download will stop on first success.

  Returns:
    Full path to the downloaded file
  """

  # Note we track content length ourselves since certain versions of curl
  # fail if asked to resume a complete file.
  # pylint: disable=C0301,W0631
  # https://sourceforge.net/tracker/?func=detail&atid=100976&aid=3482927&group_id=976
  for url in urls:
    # http://www.logilab.org/ticket/8766
    # pylint: disable=E1101
    parsed = urlparse.urlparse(url)
    tarball_name = os.path.basename(parsed.path)
    if parsed.scheme in ('', 'file'):
      if os.path.exists(parsed.path):
        return parsed.path
      continue
    content_length = 0
    print 'Attempting download: %s' % url
    result = retry_util.RunCurl(
          ['-I', url], redirect_stdout=True, redirect_stderr=True,
          print_cmd=False)
    successful = False
    for header in result.output.splitlines():
      # We must walk the output to find the string '200 OK' for use cases where
      # a proxy is involved and may have pushed down the actual header.
      if header.find('200 OK') != -1:
        successful = True
      elif header.lower().startswith("content-length:"):
        content_length = int(header.split(":", 1)[-1].strip())
        if successful:
          break
    if successful:
      break
  else:
    raise Exception('No valid URLs found!')

  tarball_dest = os.path.join(storage_dir, tarball_name)
  current_size = 0
  if os.path.exists(tarball_dest):
    current_size = os.path.getsize(tarball_dest)
    if current_size > content_length:
      osutils.SafeUnlink(tarball_dest)
      current_size = 0

  if current_size < content_length:
    retry_util.RunCurl(
        ['-f', '-L', '-y', '30', '-C', '-', '--output', tarball_dest, url],
        print_cmd=False)

  # Cleanup old tarballs now since we've successfull fetched; only cleanup
  # the tarballs for our prefix, or unknown ones.
  ignored_prefix = ('stage3-' if tarball_name.startswith('cros-sdk-')
                    else 'cros-sdk-')
  for filename in os.listdir(storage_dir):
    if filename == tarball_name or filename.startswith(ignored_prefix):
      continue

    print 'Cleaning up old tarball: %s' % (filename,)
    osutils.SafeUnlink(os.path.join(storage_dir, filename))

  return tarball_dest


def CreateChroot(chroot_path, sdk_tarball, cache_dir, nousepkg=False):
  """Creates a new chroot from a given SDK"""

  cmd = MAKE_CHROOT + ['--stage3_path', sdk_tarball,
                       '--chroot', chroot_path,
                       '--cache_dir', cache_dir]
  if nousepkg:
    cmd.append('--nousepkg')

  try:
    cros_build_lib.RunCommand(cmd, print_cmd=False)
  except cros_build_lib.RunCommandError:
    raise SystemExit('Running %r failed!' % cmd)


def DeleteChroot(chroot_path):
  """Deletes an existing chroot"""
  cmd = MAKE_CHROOT + ['--chroot', chroot_path,
                       '--delete']
  try:
    cros_build_lib.RunCommand(cmd, print_cmd=False)
  except cros_build_lib.RunCommandError:
    raise SystemExit('Running %r failed!' % cmd)


def EnterChroot(chroot_path, cache_dir, chrome_root, chrome_root_mount,
                additional_args):
  """Enters an existing SDK chroot"""
  st = os.statvfs(os.path.join(chroot_path, 'usr', 'bin', 'sudo'))
  # The os.ST_NOSUID constant wasn't added until python-3.2.
  if st.f_flag & 0x2:
    cros_build_lib.Die('chroot cannot be in a nosuid mount')

  cmd = ENTER_CHROOT + ['--chroot', chroot_path, '--cache_dir', cache_dir]
  if chrome_root:
    cmd.extend(['--chrome_root', chrome_root])
  if chrome_root_mount:
    cmd.extend(['--chrome_root_mount', chrome_root_mount])
  if len(additional_args) > 0:
    cmd.append('--')
    cmd.extend(additional_args)

  ret = cros_build_lib.RunCommand(cmd, print_cmd=False, error_code_ok=True)
  # If we were in interactive mode, ignore the exit code; it'll be whatever
  # they last ran w/in the chroot and won't matter to us one way or another.
  # Note this does allow chroot entrance to fail and be ignored during
  # interactive; this is however a rare case and the user will immediately
  # see it (nor will they be checking the exit code manually).
  if ret.returncode != 0 and additional_args:
    raise SystemExit('Running %r failed with exit code %i'
                     % (cmd, ret.returncode))


def _SudoCommand():
  """Get the 'sudo' command, along with all needed environment variables."""

  # Pass in the ENVIRONMENT_WHITELIST and ENV_PASSTHRU variables so that
  # scripts in the chroot know what variables to pass through.
  cmd = ['sudo']
  for key in constants.CHROOT_ENVIRONMENT_WHITELIST + constants.ENV_PASSTHRU:
    value = os.environ.get(key)
    if value is not None:
      cmd += ['%s=%s' % (key, value)]

  # Pass in the path to the depot_tools so that users can access them from
  # within the chroot.
  cmd += ['DEPOT_TOOLS=%s' % constants.DEPOT_TOOLS_DIR]

  return cmd


def _ReportMissing(missing):
  """Report missing utilities, then exit.

  Args:
    missing: List of missing utilities, as returned by
             osutils.FindMissingBinaries.  If non-empty, will not return.
  """

  if missing:
    raise SystemExit(
        'The tool(s) %s were not found.\n'
        'Please install the appropriate package in your host.\n'
        'Example(ubuntu):\n'
        '  sudo apt-get install <packagename>'
        % ', '.join(missing))


def _ProxySimSetup(options):
  """Set up proxy simulator, and return only in the child environment.

  TODO: Ideally, this should support multiple concurrent invocations of
  cros_sdk --proxy-sim; currently, such invocations will conflict with each
  other due to the veth device names and IP addresses.  Either this code would
  need to generate fresh, unused names for all of these before forking, or it
  would need to support multiple concurrent cros_sdk invocations sharing one
  proxy and allowing it to exit when unused (without counting on any local
  service-management infrastructure on the host).
  """

  may_need_mpm = False
  apache_bin = osutils.Which('apache2')
  if apache_bin is None:
    apache_bin = osutils.Which('apache2', PROXY_APACHE_FALLBACK_PATH)
    if apache_bin is None:
      _ReportMissing(('apache2',))
  else:
    may_need_mpm = True

  # Module names and .so names included for ease of grepping.
  apache_modules = [('proxy_module', 'mod_proxy.so'),
                    ('proxy_connect_module', 'mod_proxy_connect.so'),
                    ('proxy_http_module', 'mod_proxy_http.so'),
                    ('proxy_ftp_module', 'mod_proxy_ftp.so')]

  # Find the apache module directory, and make sure it has the modules we need.
  module_dirs = {}
  for g in PROXY_APACHE_MODULE_GLOBS:
    for mod, so in apache_modules:
      for f in glob.glob(os.path.join(g, so)):
        module_dirs.setdefault(os.path.dirname(f), []).append(so)
  for apache_module_path, modules_found in module_dirs.iteritems():
    if len(modules_found) == len(apache_modules):
      break
  else:
    # Appease cros lint, which doesn't understand that this else block will not
    # fall through to the subsequent code which relies on apache_module_path.
    apache_module_path = None
    raise SystemExit(
        'Could not find apache module path containing all required modules: %s'
            % ', '.join(so for mod, so in apache_modules))

  def check_add_module(name):
    so = 'mod_%s.so' % name
    if os.access(os.path.join(apache_module_path, so), os.F_OK):
      mod = '%s_module' % name
      apache_modules.append((mod, so))
      return True
    return False

  check_add_module('authz_core')
  if may_need_mpm:
    for mpm in PROXY_APACHE_MPMS:
      if check_add_module('mpm_%s' % mpm):
        break

  veth_host = '%s-host' % PROXY_VETH_PREFIX
  veth_guest = '%s-guest' % PROXY_VETH_PREFIX

  # Set up pipes from parent to child and vice versa.
  # The child writes a byte to the parent after calling unshare, so that the
  # parent can then assign the guest end of the veth interface to the child's
  # new network namespace.  The parent then writes a byte to the child after
  # assigning the guest interface, so that the child can then configure that
  # interface.  In both cases, if we get back an EOF when reading from the
  # pipe, we assume the other end exited with an error message, so just exit.
  parent_readfd, child_writefd = os.pipe()
  child_readfd, parent_writefd = os.pipe()
  SUCCESS_FLAG = '+'

  pid = os.fork()
  if not pid:
    os.close(parent_readfd)
    os.close(parent_writefd)

    namespaces.Unshare(namespaces.CLONE_NEWNET)
    os.write(child_writefd, SUCCESS_FLAG)
    os.close(child_writefd)
    if os.read(child_readfd, 1) != SUCCESS_FLAG:
      # Parent failed; it will have already have outputted an error message.
      sys.exit(1)
    os.close(child_readfd)

    # Set up child side of the network.
    commands = (
      ('ip', 'address', 'add',
       '%s/%u' % (PROXY_GUEST_IP, PROXY_NETMASK),
       'dev', veth_guest),
      ('ip', 'link', 'set', veth_guest, 'up'),
    )
    try:
      for cmd in commands:
        cros_build_lib.RunCommand(cmd, print_cmd=False)
    except cros_build_lib.RunCommandError:
      raise SystemExit('Running %r failed!' % (cmd,))

    proxy_url = 'http://%s:%u' % (PROXY_HOST_IP, PROXY_PORT)
    for proto in ('http', 'https', 'ftp'):
      os.environ[proto + '_proxy'] = proxy_url
    for v in ('all_proxy', 'RSYNC_PROXY', 'no_proxy'):
      os.environ.pop(v, None)
    return

  os.close(child_readfd)
  os.close(child_writefd)

  if os.read(parent_readfd, 1) != SUCCESS_FLAG:
    # Child failed; it will have already have outputted an error message.
    sys.exit(1)
  os.close(parent_readfd)

  # Set up parent side of the network.
  uid = int(os.environ.get('SUDO_UID', '0'))
  gid = int(os.environ.get('SUDO_GID', '0'))
  if uid == 0 or gid == 0:
    for username in PROXY_APACHE_FALLBACK_USERS:
      try:
        pwnam = pwd.getpwnam(username)
        uid, gid = pwnam.pw_uid, pwnam.pw_gid
        break
      except KeyError:
        continue
    if uid == 0 or gid == 0:
      raise SystemExit('Could not find a non-root user to run Apache as')

  chroot_parent, chroot_base = os.path.split(options.chroot)
  pid_file = os.path.join(chroot_parent, '.%s-apache-proxy.pid' % chroot_base)
  log_file = os.path.join(chroot_parent, '.%s-apache-proxy.log' % chroot_base)

  apache_directives = [
    'User #%u' % uid,
    'Group #%u' % gid,
    'PidFile %s' % pid_file,
    'ErrorLog %s' % log_file,
    'Listen %s:%u' % (PROXY_HOST_IP, PROXY_PORT),
    'ServerName %s' % PROXY_HOST_IP,
    'ProxyRequests On',
    'AllowCONNECT %s' % ' '.join(map(str, PROXY_CONNECT_PORTS)),
  ] + [
    'LoadModule %s %s' % (mod, os.path.join(apache_module_path, so))
    for (mod, so) in apache_modules
  ]
  commands = (
    ('ip', 'link', 'add', 'name', veth_host,
     'type', 'veth', 'peer', 'name', veth_guest),
    ('ip', 'address', 'add',
     '%s/%u' % (PROXY_HOST_IP, PROXY_NETMASK),
     'dev', veth_host),
    ('ip', 'link', 'set', veth_host, 'up'),
    [apache_bin, '-f', '/dev/null']
        + [arg for d in apache_directives for arg in ('-C', d)],
    ('ip', 'link', 'set', veth_guest, 'netns', str(pid)),
  )
  cmd = None # Make cros lint happy.
  try:
    for cmd in commands:
      cros_build_lib.RunCommand(cmd, print_cmd=False)
  except cros_build_lib.RunCommandError:
    # Clean up existing interfaces, if any.
    cmd_cleanup = ('ip', 'link', 'del', veth_host)
    try:
      cros_build_lib.RunCommand(cmd_cleanup, print_cmd=False)
    except cros_build_lib.RunCommandError:
      cros_build_lib.Error('running %r failed', cmd_cleanup)
    raise SystemExit('Running %r failed!' % (cmd,))
  os.write(parent_writefd, SUCCESS_FLAG)
  os.close(parent_writefd)

  _ExitAsStatus(os.waitpid(pid, 0)[1])


def _ExitAsStatus(status):
  """Exit the same way as |status|.

  If the status field says it was killed by a signal, then we'll do that to
  ourselves.  Otherwise we'll exit with the exit code.

  See http://www.cons.org/cracauer/sigint.html for more details.

  Args:
    status: A status as returned by os.wait type funcs.
  """
  sig_status = status & 0xff
  exit_status = (status >> 8) & 0xff

  if sig_status:
    # Kill ourselves with the same signal.
    pid = os.getpid()
    os.kill(pid, sig_status)
    time.sleep(0.1)

    # Still here?  Maybe the signal was masked.
    signal.signal(sig_status, signal.SIG_DFL)
    os.kill(pid, sig_status)
    time.sleep(0.1)

    # Still here?  Just exit.
    exit_status = 127

  # Exit with the code we want.
  sys.exit(exit_status)


def _ReapChildren(pid):
  """Reap all children that get reparented to us until we see |pid| exit.

  Args:
    pid: The main child to watch for.

  Returns:
    The wait status of the |pid| child.
  """
  pid_status = 0

  while True:
    try:
      (wpid, status) = os.wait()
      if pid == wpid:
        # Save the status of our main child so we can exit with it below.
        pid_status = status
    except OSError as e:
      if e.errno == errno.ECHILD:
        break
      else:
        raise

  return pid_status


def _CreatePidNamespace():
  """Start a new pid namespace

  This will launch all the right manager processes.  The child that returns
  will be isolated in a new pid namespace.

  If functionality is not available, then it will return w/out doing anything.

  Returns:
    The last pid outside of the namespace.
  """
  first_pid = os.getpid()

  try:
    # First create the namespace.
    namespaces.Unshare(namespaces.CLONE_NEWPID)
  except OSError as e:
    if e.errno == errno.EINVAL:
      # For older kernels, or the functionality is disabled in the config,
      # return silently.  We don't want to hard require this stuff.
      return first_pid
    else:
      # For all other errors, abort.  They shouldn't happen.
      raise

  # Now that we're in the new pid namespace, fork.  The parent is the master
  # of it in the original namespace, so it only monitors the child inside it.
  # It is only allowed to fork once too.
  pid = os.fork()
  if pid:
    # Reap the children as the parent of the new namespace.
    _ExitAsStatus(_ReapChildren(pid))
  else:
    # The child needs its own proc mount as it'll be different.
    osutils.Mount('proc', '/proc', 'proc',
                  osutils.MS_NOSUID | osutils.MS_NODEV | osutils.MS_NOEXEC |
                  osutils.MS_RELATIME)

    pid = os.fork()
    if pid:
      # Watch all of the children.  We need to act as the master inside the
      # namespace and reap old processes.
      _ExitAsStatus(_ReapChildren(pid))

  # The grandchild will return and take over the rest of the sdk steps.
  return first_pid


def _ReExecuteIfNeeded(argv):
  """Re-execute cros_sdk as root.

  Also unshare the mount namespace so as to ensure that processes outside
  the chroot can't mess with our mounts.
  """
  if os.geteuid() != 0:
    cmd = _SudoCommand() + ['--'] + argv
    os.execvp(cmd[0], cmd)
  else:
    # We must set up the cgroups mounts before we enter our own namespace.
    # This way it is a shared resource in the root mount namespace.
    cgroups.Cgroup.InitSystem()
    namespaces.Unshare(namespaces.CLONE_NEWNS | namespaces.CLONE_NEWUTS)


def _CreateParser(sdk_latest_version, bootstrap_latest_version):
  """Generate and return the parser with all the options."""
  usage = """usage: %prog [options] [VAR1=val1 .. VARn=valn -- args]

This script is used for manipulating local chroot environments; creating,
deleting, downloading, etc.  If given --enter (or no args), it defaults
to an interactive bash shell within the chroot.

If given args those are passed to the chroot environment, and executed."""

  parser = commandline.OptionParser(usage=usage, caching=True)

  # Global options.
  default_chroot = os.path.join(constants.SOURCE_ROOT,
                                constants.DEFAULT_CHROOT_DIR)
  parser.add_option(
      '--chroot', dest='chroot', default=default_chroot, type='path',
      help=('SDK chroot dir name [%s]' % constants.DEFAULT_CHROOT_DIR))

  parser.add_option('--chrome_root', default=None, type='path',
                    help='Mount this chrome root into the SDK chroot')
  parser.add_option('--chrome_root_mount', default=None, type='path',
                    help='Mount chrome into this path inside SDK chroot')
  parser.add_option('--nousepkg', action='store_true', default=False,
                    help='Do not use binary packages when creating a chroot.')
  parser.add_option('-u', '--url',
                    dest='sdk_url', default=None,
                    help=('''Use sdk tarball located at this url.
                             Use file:// for local files.'''))
  parser.add_option('--sdk-version', default=None,
                    help='Use this sdk version.  For prebuilt, current is %r'
                         ', for bootstrapping it is %r.'
                          % (sdk_latest_version, bootstrap_latest_version))

  # Commands.
  group = parser.add_option_group('Commands')
  group.add_option(
      '--enter', action='store_true', default=False,
      help='Enter the SDK chroot.  Implies --create.')
  group.add_option(
      '--create', action='store_true',default=False,
      help='Create the chroot only if it does not already exist.  '
      'Implies --download.')
  group.add_option(
      '--bootstrap', action='store_true', default=False,
      help='Build everything from scratch, including the sdk.  '
      'Use this only if you need to validate a change '
      'that affects SDK creation itself (toolchain and '
      'build are typically the only folk who need this).  '
      'Note this will quite heavily slow down the build.  '
      'This option implies --create --nousepkg.')
  group.add_option(
      '-r', '--replace', action='store_true', default=False,
      help='Replace an existing SDK chroot.  Basically an alias '
      'for --delete --create.')
  group.add_option(
      '--delete', action='store_true', default=False,
      help='Delete the current SDK chroot if it exists.')
  group.add_option(
      '--download', action='store_true', default=False,
      help='Download the sdk.')
  commands = group

  # Namespace options.
  group = parser.add_option_group('Namespaces')
  group.add_option('--proxy-sim', action='store_true', default=False,
                   help='Simulate a restrictive network requiring an outbound'
                        ' proxy.')
  group.add_option('--no-ns-pid', dest='ns_pid',
                   default=True, action='store_false',
                   help='Do not create a new PID namespace.')

  # Internal options.
  group = parser.add_option_group(
      'Internal Chromium OS Build Team Options',
      'Caution: these are for meant for the Chromium OS build team only')
  group.add_option('--buildbot-log-version', default=False, action='store_true',
                   help='Log SDK version for buildbot consumption')

  return (parser, commands)


def main(argv):
  conf = cros_build_lib.LoadKeyValueFile(
      os.path.join(constants.SOURCE_ROOT, constants.SDK_VERSION_FILE),
      ignore_missing=True)
  sdk_latest_version = conf.get('SDK_LATEST_VERSION', '<unknown>')
  bootstrap_latest_version = conf.get('BOOTSTRAP_LATEST_VERSION', '<unknown>')
  parser, commands = _CreateParser(sdk_latest_version, bootstrap_latest_version)
  options, chroot_command = parser.parse_args(argv)

  # Some sanity checks first, before we ask for sudo credentials.
  cros_build_lib.AssertOutsideChroot()

  host = os.uname()[4]
  if host != 'x86_64':
    parser.error(
        "cros_sdk is currently only supported on x86_64; you're running"
        " %s.  Please find a x86_64 machine." % (host,))

  _ReportMissing(osutils.FindMissingBinaries(NEEDED_TOOLS))
  if options.proxy_sim:
    _ReportMissing(osutils.FindMissingBinaries(PROXY_NEEDED_TOOLS))

  _ReExecuteIfNeeded([sys.argv[0]] + argv)
  if options.ns_pid:
    first_pid = _CreatePidNamespace()
  else:
    first_pid = None

  # Expand out the aliases...
  if options.replace:
    options.delete = options.create = True

  if options.bootstrap:
    options.create = True

  # If a command is not given, default to enter.
  options.enter |= not any(getattr(options, x.dest)
                           for x in commands.option_list)
  options.enter |= bool(chroot_command)

  if options.enter and options.delete and not options.create:
    parser.error("Trying to enter the chroot when --delete "
                 "was specified makes no sense.")

  # Finally, discern if we need to create the chroot.
  chroot_exists = os.path.exists(options.chroot)
  if options.create or options.enter:
    # Only create if it's being wiped, or if it doesn't exist.
    if not options.delete and chroot_exists:
      options.create = False
    else:
      options.download = True

  # Finally, flip create if necessary.
  if options.enter:
    options.create |= not chroot_exists

  if not options.sdk_version:
    sdk_version = (bootstrap_latest_version if options.bootstrap
                   else sdk_latest_version)
  else:
    sdk_version = options.sdk_version
  if options.buildbot_log_version:
    cros_build_lib.PrintBuildbotStepText(sdk_version)

  # Based on selections, fetch the tarball.
  if options.sdk_url:
    urls = [options.sdk_url]
  elif options.bootstrap:
    urls = GetStage3Urls(sdk_version)
  else:
    urls = GetArchStageTarballs(sdk_version)

  lock_path = os.path.dirname(options.chroot)
  lock_path = os.path.join(lock_path,
                           '.%s_lock' % os.path.basename(options.chroot))
  with cgroups.SimpleContainChildren('cros_sdk', pid=first_pid):
    with locking.FileLock(lock_path, 'chroot lock') as lock:

      if options.proxy_sim:
        _ProxySimSetup(options)

      if options.delete and os.path.exists(options.chroot):
        lock.write_lock()
        DeleteChroot(options.chroot)

      sdk_cache = os.path.join(options.cache_dir, 'sdks')
      distfiles_cache = os.path.join(options.cache_dir, 'distfiles')
      osutils.SafeMakedirsNonRoot(options.cache_dir)

      for target in (sdk_cache, distfiles_cache):
        src = os.path.join(constants.SOURCE_ROOT, os.path.basename(target))
        if not os.path.exists(src):
          osutils.SafeMakedirs(target)
          continue
        lock.write_lock(
            "Upgrade to %r needed but chroot is locked; please exit "
            "all instances so this upgrade can finish." % src)
        if not os.path.exists(src):
          # Note that while waiting for the write lock, src may've vanished;
          # it's a rare race during the upgrade process that's a byproduct
          # of us avoiding taking a write lock to do the src check.  If we
          # took a write lock for that check, it would effectively limit
          # all cros_sdk for a chroot to a single instance.
          osutils.SafeMakedirs(target)
        elif not os.path.exists(target):
          # Upgrade occurred, but a reversion, or something whacky
          # occurred writing to the old location.  Wipe and continue.
          os.rename(src, target)
        else:
          # Upgrade occurred once already, but either a reversion or
          # some before/after separate cros_sdk usage is at play.
          # Wipe and continue.
          osutils.RmDir(src)

      if options.download:
        lock.write_lock()
        sdk_tarball = FetchRemoteTarballs(sdk_cache, urls)

      if options.create:
        lock.write_lock()
        CreateChroot(options.chroot, sdk_tarball, options.cache_dir,
                     nousepkg=(options.bootstrap or options.nousepkg))

      if options.enter:
        lock.read_lock()
        EnterChroot(options.chroot, options.cache_dir, options.chrome_root,
                    options.chrome_root_mount, chroot_command)