# 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.

import argparse
import collections
import contextlib
import json
import logging
import os
import distutils.version

from chromite import cros
from chromite.lib import cache
from chromite.lib import chrome_util
from chromite.lib import commandline
from chromite.lib import cros_build_lib
from chromite.lib import git
from chromite.lib import gs
from chromite.lib import osutils
from chromite.lib import stats
from chromite.buildbot import cbuildbot_config
from chromite.buildbot import constants


COMMAND_NAME = 'chrome-sdk'


def Log(*args, **kwargs):
  """Conditional logging.

  Arguments:
    silent: If set to True, then logs with level DEBUG.  logs with level INFO
      otherwise.  Defaults to False.
  """
  silent = kwargs.pop('silent', False)
  level = logging.DEBUG if silent else logging.INFO
  logging.log(level, *args, **kwargs)


class SDKError(Exception):
  """Raised by SDKFetcher."""


class SDKFetcher(object):
  """Functionality for fetching an SDK environment.

  For the version of ChromeOS specified, the class downloads and caches
  SDK components.
  """
  SDK_VERSION_ENV = '%SDK_VERSION'
  SDK_BOARD_ENV = '%SDK_BOARD'

  SDKContext = collections.namedtuple(
      'SDKContext', ['version', 'metadata', 'key_map'])

  TARBALL_CACHE = 'tarballs'
  MISC_CACHE = 'misc'

  TARGET_TOOLCHAIN_KEY = 'target_toolchain'

  def __init__(self, cache_dir, board, silent=False):
    """Initialize the class.

    Arguments:
      cache_dir: The toplevel cache dir to use.
      board: The board to manage the SDK for.
    """
    self.gs_ctx = gs.GSContext.Cached(cache_dir, init_boto=True)
    self.cache_base = os.path.join(cache_dir, COMMAND_NAME)
    self.tarball_cache = cache.TarballCache(
        os.path.join(self.cache_base, self.TARBALL_CACHE))
    self.misc_cache = cache.DiskCache(
        os.path.join(self.cache_base, self.MISC_CACHE))
    self.board = board
    self.config = cbuildbot_config.FindCanonicalConfigForBoard(board)
    self.gs_base = '%s/%s' % (constants.DEFAULT_ARCHIVE_BUCKET,
                              self.config['name'])
    self.silent = silent

  def _UpdateTarball(self, url, ref):
    """Worker function to fetch tarballs"""
    with osutils.TempDir(base_dir=self.tarball_cache.staging_dir) as tempdir:
      local_path = os.path.join(tempdir, os.path.basename(url))
      Log('SDK: Fetching %s', url, silent=self.silent)
      self.gs_ctx.Copy(url, tempdir, debug_level=logging.DEBUG)
      ref.SetDefault(local_path, lock=True)

  def _GetMetadata(self, version):
    """Return metadata (in the form of a dict) for a given version."""
    raw_json = None
    full_version = self.GetFullVersion(version)
    version_base = os.path.join(self.gs_base, full_version)
    with self.misc_cache.Lookup(
        (self.board, version, constants.METADATA_JSON)) as ref:
      if ref.Exists(lock=True):
        raw_json = osutils.ReadFile(ref.path)
      else:
        raw_json = self.gs_ctx.Cat(
            os.path.join(version_base, constants.METADATA_JSON),
                         debug_level=logging.DEBUG).output
        ref.AssignText(raw_json)

    return json.loads(raw_json)

  def _GetChromeLKGM(self, chrome_src_dir):
    """Get ChromeOS LKGM checked into the Chrome tree.

    Returns:
      Version number in format '3929.0.0'.
    """
    version = osutils.ReadFile(os.path.join(
        chrome_src_dir, constants.PATH_TO_CHROME_LKGM))
    return version

  def _GetRepoCheckoutVersion(self, repo_root):
    """Get the version specified in chromeos_version.sh.

    Returns:
      Version number in format '3929.0.0'.
    """
    chromeos_version_sh = os.path.join(repo_root, constants.VERSION_FILE)
    sourced_env = osutils.SourceEnvironment(
        chromeos_version_sh, ['CHROMEOS_VERSION_STRING'],
        env={'CHROMEOS_OFFICIAL': '1'})
    return sourced_env['CHROMEOS_VERSION_STRING']

  def _GetNewestManifestVersion(self):
    """Gets the latest uploaded SDK version.

    Returns:
      Version number in the format '3929.0.0'.
    """
    branch = git.GetChromiteTrackingBranch()
    version_file = '%s/LATEST-%s' % (self.gs_base, branch)
    full_version = self.gs_ctx.Cat(version_file).output
    assert full_version.startswith('R')
    return full_version.split('-')[1]

  def GetDefaultVersion(self):
    """Get the default SDK version to use.

    If we are in an existing SDK shell, use the SDK version of the shell.
    Otherwise, use what we defaulted to last time.  If there was no last time,
    returns None.
    """
    if os.environ.get(self.SDK_BOARD_ENV) == self.board:
      sdk_version = os.environ.get(self.SDK_VERSION_ENV)
      if sdk_version is not None:
        return sdk_version

    with self.misc_cache.Lookup((self.board, 'latest')) as ref:
      if ref.Exists(lock=True):
        version = osutils.ReadFile(ref.path).strip()
        # Deal with the old version format.
        if version.startswith('R'):
          version = version.split('-')[1]
        return version
      else:
        return None

  def _SetDefaultVersion(self, version):
    """Set the new default version."""
    with self.misc_cache.Lookup((self.board, 'latest')) as ref:
      ref.AssignText(version)

  def UpdateDefaultVersion(self):
    """Update the version that we default to using.

    Returns:
      A tuple of the form (version, updated), where |version| is the
      version number in the format '3929.0.0', and |updated| indicates
      whether the version was indeed updated.
    """
    checkout = commandline.DetermineCheckout(os.getcwd())
    current = self.GetDefaultVersion() or '0'
    if checkout.chrome_src_dir:
      target = self._GetChromeLKGM(checkout.chrome_src_dir)
    elif checkout.type == commandline.CHECKOUT_TYPE_REPO:
      target = self._GetRepoCheckoutVersion(checkout.root)
      if target != current:
        lv_cls = distutils.version.LooseVersion
        if lv_cls(target) > lv_cls(current):
          # Hit the network for the newest uploaded version for the branch.
          newest = self._GetNewestManifestVersion()
          # The SDK for the version of the checkout has not been uploaded yet,
          # so fall back to the latest uploaded SDK.
          if lv_cls(target) > lv_cls(newest):
            target = newest
    else:
      target = self._GetNewestManifestVersion()
    self._SetDefaultVersion(target)
    return target, target != current

  def GetFullVersion(self, version):
    """Add the release branch to a ChromeOS platform version.

    Arguments:
      version: A ChromeOS platform number of the form XXXX.XX.XX, i.e.,
        3918.0.0.

    Returns:
      The version with release branch prepended.  I.e., R28-3918.0.0.
    """
    def DebugGsLs(*args, **kwargs):
      kwargs.setdefault('retries', 0)
      kwargs.setdefault('debug_level', logging.DEBUG)
      kwargs.setdefault('error_code_ok', True)
      return self.gs_ctx.LS(*args, **kwargs)

    assert not version.startswith('R')

    with self.misc_cache.Lookup(('full-version', version)) as ref:
      if ref.Exists(lock=True):
        return osutils.ReadFile(ref.path).strip()
      else:
        # First, assume crbug.com/230190 has been fixed, and we can just use
        # the bare version.  The 'ls' we do here is a relatively cheap check
        # compared to the pattern matching we do below.
        # TODO(rcui): Rename this function to VerifyVersion()
        lines = DebugGsLs(
            os.path.join(self.gs_base, version) + '/').output.splitlines()
        full_version = version
        if not lines:
          # TODO(rcui): Remove this code when crbug.com/230190 is fixed.
          lines = DebugGsLs(os.path.join(
              self.gs_base, 'R*-%s' % version) + '/').output.splitlines()
          if not lines:
            raise SDKError('Invalid version %s' % version)
          real_path = lines[0]
          if not real_path.startswith(self.gs_base):
            raise AssertionError('%s does not start with %s'
                                 % (real_path, self.gs_base))
          real_path = real_path[len(self.gs_base) + 1:]
          full_version = real_path.partition('/')[0]
          if not full_version.endswith(version):
            raise AssertionError('%s does not end with %s'
                                 % (full_version, version))

        ref.AssignText(full_version)
        return full_version

  @contextlib.contextmanager
  def Prepare(self, components, version=None):
    """Ensures the components of an SDK exist and are read-locked.

    For a given SDK version, pulls down missing components, and provides a
    context where the components are read-locked, which prevents the cache from
    deleting them during its purge operations.

    Arguments:
      gs_ctx: GSContext object.
      components: A list of specific components(tarballs) to prepare.
      version: The version to prepare.  If not set, uses the version returned by
        GetDefaultVersion().  If there is no default version set (this is the
        first time we are being executed), then we update the default version.

    Yields: An SDKFetcher.SDKContext namedtuple object.  The attributes of the
      object are:

      version: The version that was prepared.
      metadata: A dictionary containing the build metadata for the SDK version.
      key_map: Dictionary that contains CacheReference objects for the SDK
        artifacts, indexed by cache key.
    """
    if version is None:
      version = self.GetDefaultVersion()
      if version is None:
        version, _ = self.UpdateDefaultVersion()
    components = list(components)

    key_map = {}
    fetch_urls = {}

    metadata = self._GetMetadata(version)
    # Fetch toolchains from separate location.
    if self.TARGET_TOOLCHAIN_KEY in components:
      tc_tuple = metadata['toolchain-tuple'][0]
      fetch_urls[self.TARGET_TOOLCHAIN_KEY] = os.path.join(
          'gs://', constants.SDK_GS_BUCKET,
          metadata['toolchain-url'] % {'target': tc_tuple})
      components.remove(self.TARGET_TOOLCHAIN_KEY)

    full_version = self.GetFullVersion(version)
    version_base = os.path.join(self.gs_base, full_version)
    fetch_urls.update((t, os.path.join(version_base, t)) for t in components)
    try:
      for key, url in fetch_urls.iteritems():
        cache_key = (self.board, version, key)
        ref = self.tarball_cache.Lookup(cache_key)
        key_map[key] = ref
        ref.Acquire()
        if not ref.Exists(lock=True):
          # TODO(rcui): Parallelize this.  Requires acquiring locks *before*
          # generating worker processes; therefore the functionality needs to
          # be moved into the DiskCache class itself -
          # i.e.,DiskCache.ParallelSetDefault().
          self._UpdateTarball(url, ref)

      yield self.SDKContext(version, metadata, key_map)
    finally:
      # TODO(rcui): Move to using cros_build_lib.ContextManagerStack()
      cros_build_lib.SafeRun([ref.Release for ref in key_map.itervalues()])


class GomaError(Exception):
  """Indicates error with setting up Goma."""


class ClangError(Exception):
  """Indicates error with setting up Clang."""
  pass


@cros.CommandDecorator(COMMAND_NAME)
class ChromeSDKCommand(cros.CrosCommand):
  """
  Pulls down SDK components for building and testing Chrome for ChromeOS,
  sets up the environment for building Chrome, and runs a command in the
  environment, starting a bash session if no command is specified.

  The bash session environment is set up by a user-configurable rc file located
  at ~/chromite/chrome_sdk.bashrc.
  """

  # Note, this URL is not accessible outside of corp.
  _GOMA_URL = ('https://clients5.google.com/cxx-compiler-service/'
               'download/goma_ctl.sh')

  _CLANG_DIR = 'third_party/llvm-build/Release+Asserts/bin'
  _CLANG_UPDATE_SH = 'tools/clang/scripts/update.sh'

  EBUILD_ENV = (
      'CXX',
      'CC',
      'AR',
      'AS',
      'LD',
      'RANLIB',
      'GOLD_SET',
      'GYP_DEFINES',
  )

  SDK_GOMA_PORT_ENV = 'SDK_GOMA_PORT'
  SDK_GOMA_DIR_ENV = 'SDK_GOMA_DIR'

  GOMACC_PORT_CMD  = ['./gomacc', 'port']
  FETCH_GOMA_CMD  = ['wget', _GOMA_URL]

  # Override base class property to enable stats upload.
  upload_stats = True

  @property
  def upload_stats_timeout(self):
    # Give a longer timeout for interactive SDK shell invocations, since the
    # user will not notice a longer wait because it's happening in the
    # background.
    if self.options.cmd:
      return super(ChromeSDKCommand, self).upload_stats_timeout
    else:
      return stats.StatsUploader.UPLOAD_TIMEOUT

  @staticmethod
  def ValidateVersion(version):
    if version.startswith('R') or len(version.split('.')) != 3:
      raise argparse.ArgumentTypeError(
          '--version should be in the format 3912.0.0')
    return version

  @classmethod
  def AddParser(cls, parser):
    super(ChromeSDKCommand, cls).AddParser(parser)
    parser.add_argument(
        '--board', required=True, help='The board SDK to use.')
    parser.add_argument(
        '--bashrc', type=osutils.ExpandPath,
        default=constants.CHROME_SDK_BASHRC,
        help='A bashrc file used to set up the SDK shell environment. '
             'Defaults to %s.' % constants.CHROME_SDK_BASHRC)
    parser.add_argument(
        '--chroot', type=osutils.ExpandPath,
        help='Path to a ChromeOS chroot to use.  If set, '
             '<chroot>/build/<board> will be used as the sysroot that Chrome '
             'is built against.  The version shown in the SDK shell prompt '
             'will then have an asterisk prepended to it.')
    parser.add_argument(
        '--chrome-src', type=osutils.ExpandPath,
        help='Specifies the location of a Chrome src/ directory.  Required if '
             'running with --clang if not running from a Chrome checkout.')
    parser.add_argument(
        '--clang', action='store_true', default=False,
        help='Sets up the environment for building with clang.  Due to a bug '
             'with ninja, requires --make and possibly --chrome-src to be set.')
    parser.add_argument(
        '--internal', action='store_true', default=False,
        help='Sets up SDK for building official (internal) Chrome '
             'Chrome, rather than Chromium.')
    parser.add_argument(
        '--make', action='store_true', default=False,
        help='If set, gyp_chromium will generate Make files instead of Ninja '
             'files.  Note: Make files are spread out through the source tree, '
             'and not concentrated in the out_<board> directory, so you can '
             'only have one Make config running at a time.')
    parser.add_argument(
        '--nogoma', action='store_false', default=True, dest='goma',
        help="Disables Goma in the shell by removing it from the PATH.")
    parser.add_argument(
        '--version', default=None, type=cls.ValidateVersion,
        help="Specify version of SDK to use, in the format '3912.0.0'.  "
             "Defaults to determining version based on the type of checkout "
             "(Chrome or ChromeOS) you are executing from.")
    parser.add_argument(
        'cmd', nargs='*', default=None,
        help='The command to execute in the SDK environment.  Defaults to '
              'starting a bash shell.')

  def __init__(self, options):
    cros.CrosCommand.__init__(self, options)
    self.board = options.board
    # Lazy initialized.
    self.sdk = None
    # Initialized later based on options passed in.
    self.silent = True

  @staticmethod
  def _CreatePS1(board, version, chroot=None):
    """Returns PS1 string that sets commandline and xterm window caption.

    If a chroot path is set, then indicate we are using the sysroot from there
    instead of the stock sysroot by prepending an asterisk to the version.

    Arguments:
      board, version: The SDK board and version.
      chroot: The path to the chroot, if set.
    """
    custom = '*' if chroot else ''
    sdk_version = '(sdk %s %s%s)' % (board, custom, version)
    label = '\\u@\\h: \\w'
    window_caption = "\\[\\e]0;%(sdk_version)s %(label)s \\a\\]"
    command_line = "%(sdk_version)s \\[\\e[1;33m\\]%(label)s \\$ \\[\\e[m\\]"
    ps1 = window_caption + command_line
    return (ps1 % {'sdk_version': sdk_version,
                   'label': label})

  def _FixGoldPath(self, var_contents, toolchain_path):
    """Point to the gold linker in the toolchain tarball.

    Accepts an already set environment variable in the form of '<cmd>
    -B<gold_path>', and overrides the gold_path to the correct path in the
    extracted toolchain tarball.

    Arguments:
      var_contents: The contents of the environment variable.
      toolchain_path: Path to the extracted toolchain tarball contents.

    Returns:
      Environment string that has correct gold path.
    """
    cmd, _, gold_path = var_contents.partition(' -B')
    gold_path = os.path.join(toolchain_path, gold_path.lstrip('/'))
    return '%s -B%s' % (cmd, gold_path)

  def _SetupTCEnvironment(self, sdk_ctx, options, env, goma_dir=None):
    """Sets up toolchain-related environment variables."""
    target_tc = sdk_ctx.key_map[self.sdk.TARGET_TOOLCHAIN_KEY].path
    tc_bin = os.path.join(target_tc, 'bin')
    env['PATH'] = '%s:%s' % (tc_bin, os.environ['PATH'])

    for var in ('CXX', 'CC', 'LD'):
      env[var] = self._FixGoldPath(env[var], target_tc)

    if options.clang:
      # clang++ requires C++ header paths to be explicitly specified.
      # See discussion on crbug.com/86037.
      tc_tuple = sdk_ctx.metadata['toolchain-tuple'][0]
      gcc_path = os.path.join(tc_bin, '%s-gcc' % tc_tuple)
      gcc_version = cros_build_lib.DebugRunCommand(
          [gcc_path, '-dumpversion'], redirect_stdout=True).output.strip()
      gcc_lib = 'usr/lib/gcc/%(tuple)s/%(ver)s/include/g++-v%(major_ver)s' % {
          'tuple': tc_tuple,
          'ver': gcc_version,
          'major_ver': gcc_version[0],
      }
      tc_gcc_lib = os.path.join(target_tc, gcc_lib)
      includes = []
      for p in ('',  tc_tuple, 'backward'):
        includes.append('-isystem %s' % os.path.join(tc_gcc_lib, p))
      env['CC'] = 'clang'
      env['CXX'] = 'clang++ %s' % ' '.join(includes)

    env['CXX_host'] = 'g++'
    env['CC_host'] = 'gcc'

    if options.clang:
      clang_path = os.path.join(options.chrome_src, self._CLANG_DIR)
      env['PATH'] = '%s:%s' % (clang_path, env['PATH'])

    # The Goma path must appear before all the local compiler paths.
    if goma_dir:
      env['PATH'] = '%s:%s' % (goma_dir, env['PATH'])

  def _SetupEnvironment(self, board, sdk_ctx, options, goma_dir=None,
                        goma_port=None):
    """Sets environment variables to export to the SDK shell."""
    if options.chroot:
      sysroot = os.path.join(options.chroot, 'build', board)
      if not os.path.isdir(sysroot) and not options.cmd:
        logging.warning("Because --chroot is set, expected a sysroot to be at "
                        "%s, but couldn't find one.", sysroot)
    else:
      sysroot = sdk_ctx.key_map[constants.CHROME_SYSROOT_TAR].path

    environment = os.path.join(sdk_ctx.key_map[constants.CHROME_ENV_TAR].path,
                               'environment')
    env = osutils.SourceEnvironment(environment, self.EBUILD_ENV)
    self._SetupTCEnvironment(sdk_ctx, options, env, goma_dir=goma_dir)

    # Add managed components to the PATH.
    env['PATH'] = '%s:%s' % (constants.CHROMITE_BIN_DIR, env['PATH'])

    # Export internally referenced variables.
    os.environ[self.sdk.SDK_VERSION_ENV] = sdk_ctx.version
    os.environ[self.sdk.SDK_BOARD_ENV] = board
    # Export the board/version info in a more accessible way, so developers can
    # reference them in their chrome_sdk.bashrc files, as well as within the
    # chrome-sdk shell.
    for var in [self.sdk.SDK_VERSION_ENV, self.sdk.SDK_BOARD_ENV]:
      env[var.lstrip('%')] = os.environ[var]

    # Export Goma information.
    if goma_dir:
      env[self.SDK_GOMA_DIR_ENV] = goma_dir
      env[self.SDK_GOMA_PORT_ENV] = goma_port

    # SYSROOT is necessary for Goma and the sysroot wrapper.
    env['SYSROOT'] = sysroot
    gyp_dict = chrome_util.ProcessGypDefines(env['GYP_DEFINES'])
    gyp_dict['sysroot'] = sysroot
    gyp_dict.pop('order_text_section', None)
    if options.clang:
      gyp_dict['clang'] = 1
      gyp_dict['werror'] = ''
      gyp_dict['clang_use_chrome_plugins'] = 0
      gyp_dict['linux_use_tcmalloc'] = 0
    if options.internal:
      gyp_dict['branding'] = 'Chrome'
      gyp_dict['buildtype'] = 'Official'
    else:
      gyp_dict.pop('branding', None)
      gyp_dict.pop('buildtype', None)

    env['GYP_DEFINES'] = chrome_util.DictToGypDefines(gyp_dict)

    # PS1 sets the command line prompt and xterm window caption.
    env['PS1'] = self._CreatePS1(
        self.board, self.sdk.GetFullVersion(sdk_ctx.version),
        chroot=options.chroot)

    out_dir = 'out_%s' % self.board
    env['builddir_name'] = out_dir
    env['GYP_GENERATORS'] = 'make' if options.make else 'ninja'
    env['GYP_GENERATOR_FLAGS'] = 'output_dir=%s' % out_dir
    return env

  @staticmethod
  def _VerifyGoma(user_rc):
    """Verify that the user has no goma installations set up in user_rc.

    If the user does have a goma installation set up, verify that it's for
    ChromeOS.

    Arguments:
      user_rc: User-supplied rc file.
    """
    user_env = osutils.SourceEnvironment(user_rc, ['PATH'])
    goma_ctl = osutils.Which('goma_ctl.sh', user_env.get('PATH'))
    if goma_ctl is not None:
      logging.warning(
          '%s is adding Goma to the PATH.  Using that Goma instead of the '
          'managed Goma install.', user_rc)
      manifest = os.path.join(os.path.dirname(goma_ctl), 'MANIFEST')
      platform_env = osutils.SourceEnvironment(manifest, ['PLATFORM'])
      platform = platform_env.get('PLATFORM')
      if platform is not None and platform != 'chromeos':
        logging.warning('Found %s version of Goma in PATH.', platform)
        logging.warning('Goma will not work')

  @staticmethod
  def _VerifyChromiteBin(user_rc):
    """Verify that the user has not set a chromite bin/ dir in user_rc.

    Arguments:
      user_rc: User-supplied rc file.
    """
    user_env = osutils.SourceEnvironment(user_rc, ['PATH'])
    chromite_bin = osutils.Which('parallel_emerge', user_env.get('PATH'))
    if chromite_bin is not None:
      logging.warning(
          '%s is adding chromite/bin to the PATH.  Remove it from the PATH to '
          'use the the default Chromite.', user_rc)

  @staticmethod
  def _VerifyClang(user_rc):
    """Verify that the user has not set a clang bin/ dir in user_rc.

    Arguments:
      user_rc: User-supplied rc file.
    """
    user_env = osutils.SourceEnvironment(user_rc, ['PATH'])
    clang_bin = osutils.Which('clang', user_env.get('PATH'))
    if clang_bin is not None:
      clang_dir = os.path.dirname(clang_bin)
      if not osutils.Which('goma_ctl.sh', clang_dir):
        logging.warning(
            '%s is adding Clang to the PATH.  Because of this, Goma is being '
            'bypassed.  Remove it from the PATH to use Goma with the default '
            'Clang.', user_rc)

  @contextlib.contextmanager
  def _GetRCFile(self, env, user_rc):
    """Returns path to dynamically created bashrc file.

    The bashrc file sets the environment variables contained in |env|, as well
    as sources the user-editable chrome_sdk.bashrc file in the user's home
    directory.  That rc file is created if it doesn't already exist.

    Arguments:
      env: A dictionary of environment variables that will be set by the rc
        file.
      user_rc: User-supplied rc file.
    """
    if not os.path.exists(user_rc):
      osutils.Touch(user_rc, makedirs=True)

    self._VerifyGoma(user_rc)
    self._VerifyChromiteBin(user_rc)
    if self.options.clang:
      self._VerifyClang(user_rc)

    # We need a temporary rc file to 'wrap' the user configuration file,
    # because running with '--rcfile' causes bash to ignore bash special
    # variables passed through subprocess.Popen, such as PS1.  So we set them
    # here.
    #
    # Having a wrapper rc file will also allow us to inject bash functions into
    # the environment, not just variables.
    with osutils.TempDir() as tempdir:
      # Only source the user's ~/.bashrc if running in interactive mode.
      contents = [
          '[[ -e ~/.bashrc && $- == *i* ]] && . ~/.bashrc\n',
      ]

      for key, value in env.iteritems():
        contents.append("export %s='%s'\n" % (key, value))
      contents.append('. "%s"\n' % user_rc)

      rc_file = os.path.join(tempdir, 'rcfile')
      osutils.WriteFile(rc_file, contents)
      yield rc_file

  def _GomaPort(self, goma_dir):
    """Returns current active Goma port."""
    port = cros_build_lib.RunCommandCaptureOutput(
        self.GOMACC_PORT_CMD, cwd=goma_dir,
        debug_level=logging.DEBUG, error_code_ok=True).output.strip()
    return port

  def _FetchGoma(self):
    """Fetch, install, and start Goma, using cached version if it exists.

    Returns:
      A tuple (dir, port) containing the path to the cached goma/ dir and the
      Goma port.
    """
    common_path = os.path.join(self.options.cache_dir, constants.COMMON_CACHE)
    common_cache = cache.DiskCache(common_path)

    ref = common_cache.Lookup(('goma',))
    if not ref.Exists():
      Log('Installing Goma.', silent=self.silent)
      with osutils.TempDir() as tempdir:
        goma_dir = os.path.join(tempdir, 'goma')
        os.mkdir(goma_dir)
        result = cros_build_lib.DebugRunCommand(
            self.FETCH_GOMA_CMD, cwd=goma_dir, error_code_ok=True)
        if result.returncode:
          raise GomaError('Failed to fetch Goma')
        cros_build_lib.DebugRunCommand(
            ['bash', 'goma_ctl.sh', 'update'], cwd=goma_dir, input='2\n')
        ref.SetDefault(goma_dir)
    goma_dir =  os.path.join(ref.path, 'goma')

    port = self._GomaPort(goma_dir)
    if not port:
      Log('Starting Goma.', silent=self.silent)
      cros_build_lib.DebugRunCommand(
          ['./goma_ctl.sh', 'ensure_start'], cwd=goma_dir)
      port = self._GomaPort(goma_dir)
      Log('Goma is started on port %s', port, silent=self.silent)
      if not port:
        raise GomaError('No Goma port detected')
    return goma_dir, port

  def _SetupClang(self):
    """Install clang if needed."""
    clang_path = os.path.join(self.options.chrome_src, self._CLANG_DIR)
    if not os.path.exists(clang_path):
      try:
        update_sh = os.path.join(self.options.chrome_src, self._CLANG_UPDATE_SH)
        if not os.path.isfile(update_sh):
          raise ClangError('%s not found.' % update_sh)
        results = cros_build_lib.DebugRunCommand(
            [update_sh], cwd=self.options.chrome_src, error_code_ok=True)
        if results.returncode:
          raise ClangError('Clang update failed with error code %s' %
                           (results.returncode,))
        if not os.path.exists(clang_path):
          raise ClangError('%s not found.' % clang_path)
      except ClangError as e:
        logging.error('Encountered errors while installing/updating clang: %s',
                      e)

  def Run(self):
    """Perform the command."""
    if os.environ.get(SDKFetcher.SDK_VERSION_ENV) is not None:
      cros_build_lib.Die('Already in an SDK shell.')

    if self.options.clang and not self.options.make:
      cros_build_lib.Die('--clang requires --make to be set.')

    if not self.options.chrome_src:
      checkout = commandline.DetermineCheckout(os.getcwd())
      self.options.chrome_src = checkout.chrome_src_dir
    else:
      checkout = commandline.DetermineCheckout(self.options.chrome_src)
      if not checkout.chrome_src_dir:
        cros_build_lib.Die('Chrome checkout not found at %s',
                           self.options.chrome_src)
      self.options.chrome_src = checkout.chrome_src_dir

    if self.options.clang and not self.options.chrome_src:
      cros_build_lib.Die('--clang requires --chrome-src to be set.')

    self.silent = bool(self.options.cmd)
    # Lazy initialize because SDKFetcher creates a GSContext() object in its
    # constructor, which may block on user input.
    self.sdk = SDKFetcher(self.options.cache_dir, self.options.board,
                          silent=self.silent)

    if not self.sdk.config['internal']:
      cros_build_lib.Die('External boards are not supported due to '
                         'http://crbug.com/236500')

    prepare_version = self.options.version
    if not prepare_version:
      prepare_version, _ = self.sdk.UpdateDefaultVersion()

    components = [self.sdk.TARGET_TOOLCHAIN_KEY, constants.CHROME_ENV_TAR]
    if not self.options.chroot:
      components.append(constants.CHROME_SYSROOT_TAR)

    goma_dir = None
    goma_port = None
    if self.options.goma:
      try:
        goma_dir, goma_port = self._FetchGoma()
      except GomaError as e:
        logging.error('Goma: %s.  Bypass by running with --nogoma.', e)

    if self.options.clang:
      self._SetupClang()

    with self.sdk.Prepare(components, version=prepare_version) as ctx:
      env = self._SetupEnvironment(self.options.board, ctx, self.options,
                                   goma_dir=goma_dir, goma_port=goma_port)
      with self._GetRCFile(env, self.options.bashrc) as rcfile:
        bash_cmd = ['/bin/bash']

        extra_env = None
        if not self.options.cmd:
          bash_cmd.extend(['--rcfile', rcfile, '-i'])
        else:
          # The '"$@"' expands out to the properly quoted positional args
          # coming after the '--'.
          bash_cmd.extend(['-c', '"$@"', '--'])
          bash_cmd.extend(self.options.cmd)
          # When run in noninteractive mode, bash sources the rc file set in
          # BASH_ENV, and ignores the --rcfile flag.
          extra_env = {'BASH_ENV': rcfile}

        cros_build_lib.RunCommand(
            bash_cmd, print_cmd=False, debug_level=logging.CRITICAL,
            error_code_ok=True, extra_env=extra_env)