xbuddy.py

# Copyright (c) 2013 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 ConfigParser
import datetime
import operator
import os
import re
import shutil
import time
import threading

import build_util
import artifact_info
import common_util
import devserver_constants
import downloader
import gsutil_util
import log_util

# Module-local log function.
def _Log(message, *args):
  return log_util.LogWithTag('XBUDDY', message, *args)

# xBuddy config constants
CONFIG_FILE = 'xbuddy_config.ini'
SHADOW_CONFIG_FILE = 'shadow_xbuddy_config.ini'
PATH_REWRITES = 'PATH_REWRITES'
GENERAL = 'GENERAL'

# Path for shadow config in chroot.
CHROOT_SHADOW_DIR = '/mnt/host/source/src/platform/dev'

# XBuddy aliases
TEST = 'test'
BASE = 'base'
DEV = 'dev'
FULL = 'full_payload'
RECOVERY = 'recovery'
STATEFUL = 'stateful'
AUTOTEST = 'autotest'

# Local build constants
ANY = "ANY"
LATEST = "latest"
LOCAL = "local"
REMOTE = "remote"

# TODO(sosa): Fix a lot of assumptions about these aliases. There is too much
# implicit logic here that's unnecessary. What should be done:
# 1) Collapse Alias logic to one set of aliases for xbuddy (not local/remote).
# 2) Do not use zip when creating these dicts. Better to not rely on ordering.
# 3) Move alias/artifact mapping to a central module rather than having it here.
# 4) Be explicit when things are missing i.e. no dev images in image.zip.

LOCAL_ALIASES = [
  TEST,
  DEV,
  BASE,
  FULL,
  STATEFUL,
  ANY,
]

LOCAL_FILE_NAMES = [
  devserver_constants.TEST_IMAGE_FILE,
  devserver_constants.IMAGE_FILE,
  devserver_constants.BASE_IMAGE_FILE,
  devserver_constants.UPDATE_FILE,
  devserver_constants.STATEFUL_FILE,
  None, # For ANY.
]

LOCAL_ALIAS_TO_FILENAME = dict(zip(LOCAL_ALIASES, LOCAL_FILE_NAMES))

# Google Storage constants
GS_ALIASES = [
  TEST,
  DEV,
  BASE,
  RECOVERY,
  FULL,
  STATEFUL,
  AUTOTEST,
]

GS_FILE_NAMES = [
  devserver_constants.TEST_IMAGE_FILE,
  devserver_constants.BASE_IMAGE_FILE,
  devserver_constants.RECOVERY_IMAGE_FILE,
  devserver_constants.UPDATE_FILE,
  devserver_constants.STATEFUL_FILE,
  devserver_constants.AUTOTEST_DIR,
]

ARTIFACTS = [
  artifact_info.TEST_IMAGE,
  artifact_info.BASE_IMAGE,
  artifact_info.RECOVERY_IMAGE,
  artifact_info.FULL_PAYLOAD,
  artifact_info.STATEFUL_PAYLOAD,
  artifact_info.AUTOTEST,
]

GS_ALIAS_TO_FILENAME = dict(zip(GS_ALIASES, GS_FILE_NAMES))
GS_ALIAS_TO_ARTIFACT = dict(zip(GS_ALIASES, ARTIFACTS))

LATEST_OFFICIAL = "latest-official"

RELEASE = "-release"


class XBuddyException(Exception):
  """Exception classes used by this module."""
  pass


# no __init__ method
#pylint: disable=W0232
class Timestamp():
  """Class to translate build path strings and timestamp filenames."""

  _TIMESTAMP_DELIMITER = 'SLASH'
  XBUDDY_TIMESTAMP_DIR = 'xbuddy_UpdateTimestamps'

  @staticmethod
  def TimestampToBuild(timestamp_filename):
    return timestamp_filename.replace(Timestamp._TIMESTAMP_DELIMITER, '/')

  @staticmethod
  def BuildToTimestamp(build_path):
    return build_path.replace('/', Timestamp._TIMESTAMP_DELIMITER)

  @staticmethod
  def UpdateTimestamp(timestamp_dir, build_id):
    """Update timestamp file of build with build_id."""
    common_util.MkDirP(timestamp_dir)
    _Log("Updating timestamp for %s", build_id)
    time_file = os.path.join(timestamp_dir,
                             Timestamp.BuildToTimestamp(build_id))
    with file(time_file, 'a'):
      os.utime(time_file, None)
#pylint: enable=W0232


class XBuddy(build_util.BuildObject):
  """Class that manages image retrieval and caching by the devserver.

  Image retrieval by xBuddy path:
    XBuddy accesses images and artifacts that it stores using an xBuddy
    path of the form: board/version/alias
    The primary xbuddy.Get call retrieves the correct artifact or url to where
    the artifacts can be found.

  Image caching:
    Images and other artifacts are stored identically to how they would have
    been if devserver's stage rpc was called and the xBuddy cache replaces
    build versions on a LRU basis. Timestamps are maintained by last accessed
    times of representative files in the a directory in the static serve
    directory (XBUDDY_TIMESTAMP_DIR).

  Private class members:
    _true_values: used for interpreting boolean values
    _staging_thread_count: track download requests
    _timestamp_folder: directory with empty files standing in as timestamps
                        for each image currently cached by xBuddy
  """
  _true_values = ['true', 't', 'yes', 'y']

  # Number of threads that are staging images.
  _staging_thread_count = 0
  # Lock used to lock increasing/decreasing count.
  _staging_thread_count_lock = threading.Lock()

  def __init__(self, manage_builds=False, board=None, images_dir=None,
               **kwargs):
    super(XBuddy, self).__init__(**kwargs)

    self.config = self._ReadConfig()
    self._manage_builds = manage_builds or self._ManageBuilds()
    self._board = board
    self._timestamp_folder = os.path.join(self.static_dir,
                                          Timestamp.XBUDDY_TIMESTAMP_DIR)
    if images_dir:
      self.images_dir = images_dir
    else:
      self.images_dir = os.path.join(self.GetSourceRoot(), 'src/build/images')

    common_util.MkDirP(self._timestamp_folder)

  @classmethod
  def ParseBoolean(cls, boolean_string):
    """Evaluate a string to a boolean value"""
    if boolean_string:
      return boolean_string.lower() in cls._true_values
    else:
      return False

  def _ReadConfig(self):
    """Read xbuddy config from ini files.

    Reads the base config from xbuddy_config.ini, and then merges in the
    shadow config from shadow_xbuddy_config.ini

    Returns:
      The merged configuration.
    Raises:
      XBuddyException if the config file is missing.
    """
    xbuddy_config = ConfigParser.ConfigParser()
    config_file = os.path.join(self.devserver_dir, CONFIG_FILE)
    if os.path.exists(config_file):
      xbuddy_config.read(config_file)
    else:
      raise XBuddyException('%s not found' % (CONFIG_FILE))

    # Read the shadow file if there is one.
    if os.path.isdir(CHROOT_SHADOW_DIR):
      shadow_config_file = os.path.join(CHROOT_SHADOW_DIR, SHADOW_CONFIG_FILE)
    else:
      shadow_config_file = os.path.join(self.devserver_dir, SHADOW_CONFIG_FILE)

    _Log('Using shadow config file stored at %s', shadow_config_file)
    if os.path.exists(shadow_config_file):
      shadow_xbuddy_config = ConfigParser.ConfigParser()
      shadow_xbuddy_config.read(shadow_config_file)

      # Merge shadow config in.
      sections = shadow_xbuddy_config.sections()
      for s in sections:
        if not xbuddy_config.has_section(s):
          xbuddy_config.add_section(s)
        options = shadow_xbuddy_config.options(s)
        for o in options:
          val = shadow_xbuddy_config.get(s, o)
          xbuddy_config.set(s, o, val)

    return xbuddy_config

  def _ManageBuilds(self):
    """Checks if xBuddy is managing local builds using the current config."""
    try:
      return self.ParseBoolean(self.config.get(GENERAL, 'manage_builds'))
    except ConfigParser.Error:
      return False

  def _Capacity(self):
    """Gets the xbuddy capacity from the current config."""
    try:
      return int(self.config.get(GENERAL, 'capacity'))
    except ConfigParser.Error:
      return 5

  def _LookupAlias(self, alias, board):
    """Given the full xbuddy config, look up an alias for path rewrite.

    Args:
      alias: The xbuddy path that could be one of the aliases in the
        rewrite table.
      board: The board to fill in with when paths are rewritten. Can be from
        the update request xml or the default board from devserver.
    Returns:
      If a rewrite is found, a string with the current board substituted in.
      If no rewrite is found, just return the original string.
    """
    if alias == '':
      alias = 'update_default'

    try:
      val = self.config.get(PATH_REWRITES, alias)
    except ConfigParser.Error:
      # No alias lookup found. Return original path.
      return alias

    if not val.strip():
      # The found value was an empty string.
      return alias
    else:
      # Fill in the board.
      rewrite = val.replace("BOARD", "%(board)s") % {
          'board': board}
      _Log("Path was rewritten to %s", rewrite)
      return rewrite

  def _LookupOfficial(self, board, suffix=RELEASE):
    """Check LATEST-master for the version number of interest."""
    _Log("Checking gs for latest %s-%s image", board, suffix)
    latest_addr = devserver_constants.GS_LATEST_MASTER % {'board':board,
                                                          'suffix':suffix}
    cmd = 'gsutil cat %s' % latest_addr
    msg = 'Failed to find build at %s' % latest_addr
    # Full release + version is in the LATEST file.
    version = gsutil_util.GSUtilRun(cmd, msg)

    return devserver_constants.IMAGE_DIR % {'board':board,
                                            'suffix':suffix,
                                            'version':version}
  def _LookupChannel(self, board, channel='stable'):
    """Check the channel folder for the version number of interest."""
    # Get all names in channel dir. Get 10 highest directories by version.
    _Log("Checking channel '%s' for latest '%s' image", channel, board)
    channel_dir = devserver_constants.GS_CHANNEL_DIR % {'channel':channel,
                                                        'board':board}
    latest_version = gsutil_util.GetLatestVersionFromGSDir(
        channel_dir, with_release=False)

    # Figure out release number from the version number.
    image_url = devserver_constants.IMAGE_DIR % {
        'board':board,
        'suffix':RELEASE,
        'version':'R*' + latest_version}
    image_dir = os.path.join(devserver_constants.GS_IMAGE_DIR, image_url)

    # There should only be one match on cros-image-archive.
    full_version = gsutil_util.GetLatestVersionFromGSDir(image_dir)

    return devserver_constants.IMAGE_DIR % {'board':board,
                                            'suffix':RELEASE,
                                            'version':full_version}

  def _LookupVersion(self, board, version):
    """Search GS image releases for the highest match to a version prefix."""
    # Build the pattern for GS to match.
    _Log("Checking gs for latest '%s' image with prefix '%s'", board, version)
    image_url = devserver_constants.IMAGE_DIR % {'board':board,
                                                 'suffix':RELEASE,
                                                 'version':version + '*'}
    image_dir = os.path.join(devserver_constants.GS_IMAGE_DIR, image_url)

    # Grab the newest version of the ones matched.
    full_version = gsutil_util.GetLatestVersionFromGSDir(image_dir)
    return devserver_constants.IMAGE_DIR % {'board':board,
                                            'suffix':RELEASE,
                                            'version':full_version}

  def _RemoteBuildId(self, board, version):
    """Returns the remote build_id for the given board and version.

    Raises:
      XBuddyException: If we failed to resolve the version to a valid build_id.
    """
    build_id_as_is = devserver_constants.IMAGE_DIR % {'board':board,
                                                      'suffix':'',
                                                      'version':version}
    build_id_release = devserver_constants.IMAGE_DIR % {'board':board,
                                                        'suffix':RELEASE,
                                                        'version':version}
    # Return the first path that exists. We assume that what the user typed
    # is better than with a default suffix added i.e. x86-generic/blah is
    # more valuable than x86-generic-release/blah.
    for build_id in build_id_as_is, build_id_release:
      cmd = 'gsutil ls %s/%s' % (devserver_constants.GS_IMAGE_DIR, build_id)
      try:
        version = gsutil_util.GSUtilRun(cmd, None)
        return build_id
      except gsutil_util.GSUtilError:
        continue
    else:
      raise XBuddyException('Could not find remote build_id for %s %s' % (
          board, version))

  def _ResolveVersionToBuildId(self, board, version):
    """Handle version aliases for remote payloads in GS.

    Args:
      board: as specified in the original call. (i.e. x86-generic, parrot)
      version: as entered in the original call. can be
        {TBD, 0. some custom alias as defined in a config file}
        1. latest
        2. latest-{channel}
        3. latest-official-{board suffix}
        4. version prefix (i.e. RX-Y.X, RX-Y, RX)

    Returns:
      Location where the image dir is actually found on GS (build_id)

    Raises:
      XBuddyException: If we failed to resolve the version to a valid url.
    """
    # Only the last segment of the alias is variable relative to the rest.
    version_tuple = version.rsplit('-', 1)

    if re.match(devserver_constants.VERSION_RE, version):
      return self._RemoteBuildId(board, version)
    elif version == LATEST_OFFICIAL:
      # latest-official --> LATEST build in board-release
      return self._LookupOfficial(board)
    elif version_tuple[0] == LATEST_OFFICIAL:
      # latest-official-{suffix} --> LATEST build in board-{suffix}
      return self._LookupOfficial(board, version_tuple[1])
    elif version == LATEST:
      # latest --> latest build on stable channel
      return self._LookupChannel(board)
    elif version_tuple[0] == LATEST:
      if re.match(devserver_constants.VERSION_RE, version_tuple[1]):
        # latest-R* --> most recent qualifying build
        return self._LookupVersion(board, version_tuple[1])
      else:
        # latest-{channel} --> latest build within that channel
        return self._LookupChannel(board, version_tuple[1])
    else:
      # The given version doesn't match any known patterns.
      raise XBuddyException("Version %s unknown. Can't find on GS." % version)

  @staticmethod
  def _Symlink(link, target):
    """Symlinks link to target, and removes whatever link was there before."""
    _Log("Linking to %s from %s", link, target)
    if os.path.lexists(link):
      os.unlink(link)
    os.symlink(target, link)

  def _GetLatestLocalVersion(self, board):
    """Get the version of the latest image built for board by build_image

    Updates the symlink reference within the xBuddy static dir to point to
    the real image dir in the local /build/images directory.

    Args:
      board: board that image was built for.

    Returns:
      The discovered version of the image.

    Raises:
      XBuddyException if neither test nor dev image was found in latest built
      directory.
    """
    latest_local_dir = self.GetLatestImageDir(board)
    if not latest_local_dir or not os.path.exists(latest_local_dir):
      raise XBuddyException('No builds found for %s. Did you run build_image?' %
                            board)

    # Assume that the version number is the name of the directory.
    return os.path.basename(latest_local_dir.rstrip('/'))

  @staticmethod
  def _FindAny(local_dir):
    """Returns the image_type for ANY given the local_dir."""
    dev_image = os.path.join(local_dir, devserver_constants.IMAGE_FILE)
    test_image = os.path.join(local_dir, devserver_constants.TEST_IMAGE_FILE)
    if os.path.exists(dev_image):
      return 'dev'

    if os.path.exists(test_image):
      return 'test'

    raise XBuddyException('No images found in %s' % local_dir)

  @staticmethod
  def _InterpretPath(path):
    """Split and return the pieces of an xBuddy path name

    Args:
      path: the path xBuddy Get was called with.

    Return:
      tuple of (image_type, board, version, whether the path is local)

    Raises:
      XBuddyException: if the path can't be resolved into valid components
    """
    path_list = filter(None, path.split('/'))

    # Required parts of path parsing.
    try:
      # Determine if image is explicitly local or remote.
      is_local = True
      if path_list[0] in (REMOTE, LOCAL):
        is_local = (path_list.pop(0) == LOCAL)

      # Set board.
      board = path_list.pop(0)

      # Set defaults.
      version = LATEST
      image_type = GS_ALIASES[0]
    except IndexError:
      msg = "Specify at least the board in your xBuddy call. Your path: %s"
      raise XBuddyException(msg % os.path.join(path_list))

    # Read as much of the xBuddy path as possible.
    try:
      # Override default if terminal is a valid artifact alias or a version.
      terminal = path_list[-1]
      if terminal in GS_ALIASES + LOCAL_ALIASES:
        image_type = terminal
        version = path_list[-2]
      else:
        version = terminal
    except IndexError:
      # This path doesn't have an alias or a version. That's fine.
      _Log("Some parts of the path not specified. Using defaults.")

    _Log("Get artifact '%s' in '%s/%s'. Locally? %s",
         image_type, board, version, is_local)

    return image_type, board, version, is_local

  def _SyncRegistryWithBuildImages(self):
    """ Crawl images_dir for build_ids of images generated from build_image.

    This will find images and symlink them in xBuddy's static dir so that
    xBuddy's cache can serve them.
    If xBuddy's _manage_builds option is on, then a timestamp will also be
    generated, and xBuddy will clear them from the directory they are in, as
    necessary.
    """
    build_ids = []
    for b in os.listdir(self.images_dir):
      # Ensure we have directories to track all boards in build/images
      common_util.MkDirP(os.path.join(self.static_dir, b))
      board_dir = os.path.join(self.images_dir, b)
      build_ids.extend(['/'.join([b, v]) for v
                        in os.listdir(board_dir) if not v == LATEST])

    # Check currently registered images.
    for f in os.listdir(self._timestamp_folder):
      build_id = Timestamp.TimestampToBuild(f)
      if build_id in build_ids:
        build_ids.remove(build_id)

    # Symlink undiscovered images, and update timestamps if manage_builds is on.
    for build_id in build_ids:
      link = os.path.join(self.static_dir, build_id)
      target = os.path.join(self.images_dir, build_id)
      XBuddy._Symlink(link, target)
      if self._manage_builds:
        Timestamp.UpdateTimestamp(self._timestamp_folder, build_id)

  def _ListBuildTimes(self):
    """ Returns the currently cached builds and their last access timestamp.

    Returns:
      list of tuples that matches xBuddy build/version to timestamps in long
    """
    # Update currently cached builds.
    build_dict = {}

    for f in os.listdir(self._timestamp_folder):
      last_accessed = os.path.getmtime(os.path.join(self._timestamp_folder, f))
      build_id = Timestamp.TimestampToBuild(f)
      stale_time = datetime.timedelta(seconds=(time.time() - last_accessed))
      build_dict[build_id] = stale_time
    return_tup = sorted(build_dict.iteritems(), key=operator.itemgetter(1))
    return return_tup

  def _Download(self, gs_url, artifacts):
    """Download the artifacts from the given gs_url.

    Raises:
      build_artifact.ArtifactDownloadError: If we failed to download the
                                            artifact.
    """
    with XBuddy._staging_thread_count_lock:
      XBuddy._staging_thread_count += 1
    try:
      _Log("Downloading %s from %s", artifacts, gs_url)
      downloader.Downloader(self.static_dir, gs_url).Download(artifacts, [])
    finally:
      with XBuddy._staging_thread_count_lock:
        XBuddy._staging_thread_count -= 1

  def CleanCache(self):
    """Delete all builds besides the newest N builds"""
    if not self._manage_builds:
      return
    cached_builds = [e[0] for e in self._ListBuildTimes()]
    _Log('In cache now: %s', cached_builds)

    for b in range(self._Capacity(), len(cached_builds)):
      b_path = cached_builds[b]
      _Log("Clearing '%s' from cache", b_path)

      time_file = os.path.join(self._timestamp_folder,
                               Timestamp.BuildToTimestamp(b_path))
      os.unlink(time_file)
      clear_dir = os.path.join(self.static_dir, b_path)
      try:
        # Handle symlinks, in the case of links to local builds if enabled.
        if os.path.islink(clear_dir):
          target = os.readlink(clear_dir)
          _Log('Deleting locally built image at %s', target)

          os.unlink(clear_dir)
          if os.path.exists(target):
            shutil.rmtree(target)
        elif os.path.exists(clear_dir):
          _Log('Deleting downloaded image at %s', clear_dir)
          shutil.rmtree(clear_dir)

      except Exception as err:
        raise XBuddyException('Failed to clear %s: %s' % (clear_dir, err))

  def _GetFromGS(self, build_id, image_type):
    """Check if the artifact is available locally. Download from GS if not.

    Raises:
        build_artifact.ArtifactDownloadError: If we failed to download the
                                              artifact.
    """
    gs_url = os.path.join(devserver_constants.GS_IMAGE_DIR,
                          build_id)

    # Stage image if not found in cache.
    file_name = GS_ALIAS_TO_FILENAME[image_type]
    file_loc = os.path.join(self.static_dir, build_id, file_name)
    cached = os.path.exists(file_loc)

    if not cached:
      artifact = GS_ALIAS_TO_ARTIFACT[image_type]
      self._Download(gs_url, [artifact])
    else:
      _Log('Image already cached.')

  def _GetArtifact(self, path_list, board=None, lookup_only=False):
    """Interpret an xBuddy path and return directory/file_name to resource.

    Note board can be passed that in but by default if self._board is set,
    that is used rather than board.

    Returns:
    build_id to the directory
    file_name of the artifact

    Raises:
      XBuddyException: if the path could not be translated
      build_artifact.ArtifactDownloadError: if we failed to download the
                                            artifact.
    """
    path = '/'.join(path_list)
    # Rewrite the path if there is an appropriate default.
    path = self._LookupAlias(path, self._board if self._board else board)

    # Parse the path.
    image_type, board, version, is_local = self._InterpretPath(path)

    if is_local:
      # Get a local image.
      if version == LATEST:
        # Get the latest local image for the given board.
        version = self._GetLatestLocalVersion(board)

      build_id = os.path.join(board, version)
      artifact_dir = os.path.join(self.static_dir, build_id)
      if image_type == ANY:
        image_type = self._FindAny(artifact_dir)

      file_name = LOCAL_ALIAS_TO_FILENAME[image_type]
      artifact_path = os.path.join(artifact_dir, file_name)
      if not os.path.exists(artifact_path):
        raise XBuddyException('Local %s artifact not in static_dir at %s' %
                              (image_type, artifact_path))

    else:
      # Get a remote image.
      if image_type not in GS_ALIASES:
        raise XBuddyException('Bad remote image type: %s. Use one of: %s' %
                              (image_type, GS_ALIASES))
      build_id = self._ResolveVersionToBuildId(board, version)
      _Log('Resolved version %s to %s.', version, build_id)
      file_name = GS_ALIAS_TO_FILENAME[image_type]
      if not lookup_only:
        self._GetFromGS(build_id, image_type)

    return build_id, file_name

  ############################ BEGIN PUBLIC METHODS

  def List(self):
    """Lists the currently available images & time since last access."""
    self._SyncRegistryWithBuildImages()
    builds = self._ListBuildTimes()
    return_string = ''
    for build, timestamp in builds:
      return_string += '<b>' + build + '</b>       '
      return_string += '(time since last access: ' + str(timestamp) + ')<br>'
    return return_string

  def Capacity(self):
    """Returns the number of images cached by xBuddy."""
    return str(self._Capacity())

  def Translate(self, path_list, board=None):
    """Translates an xBuddy path to a real path to artifact if it exists.

    Equivalent to the Get call, minus downloading and updating timestamps,

    Returns:
      build_id: Path to the image or update directory on the devserver.
        e.g. 'x86-generic/R26-4000.0.0'
        The returned path is always the path to the directory within
        static_dir, so it is always the build_id of the image.
      file_name: The file name of the artifact. Can take any of the file
        values in devserver_constants.
        e.g. 'chromiumos_test_image.bin' or 'update.gz' if the path list
        specified 'test' or 'full_payload' artifacts, respectively.

    Raises:
      XBuddyException: if the path couldn't be translated
    """
    self._SyncRegistryWithBuildImages()
    build_id, file_name = self._GetArtifact(path_list, board=board,
                                            lookup_only=True)

    _Log('Returning path to payload: %s/%s', build_id, file_name)
    return build_id, file_name

  def StageTestAritfactsForUpdate(self, path_list):
    """Stages test artifacts for update and returns build_id.

    Raises:
      XBuddyException: if the path could not be translated
      build_artifact.ArtifactDownloadError: if we failed to download the test
                                            artifacts.
    """
    build_id, file_name = self.Translate(path_list)
    if file_name == devserver_constants.TEST_IMAGE_FILE:
      gs_url = os.path.join(devserver_constants.GS_IMAGE_DIR,
                            build_id)
      artifacts = [FULL, STATEFUL]
      self._Download(gs_url, artifacts)
      return build_id

  def Get(self, path_list):
    """The full xBuddy call, returns resource specified by path_list.

    Please see devserver.py:xbuddy for full documentation.

    Args:
      path_list: [board, version, alias] as split from the xbuddy call url

    Returns:
      build_id: Path to the image or update directory on the devserver.
          e.g. 'x86-generic/R26-4000.0.0'
          The returned path is always the path to the directory within
          static_dir, so it is always the build_id of the image.
      file_name: The file name of the artifact. Can take any of the file
          values in devserver_constants.
          e.g. 'chromiumos_test_image.bin' or 'update.gz' if the path list
          specified 'test' or 'full_payload' artifacts, respectively.

    Raises:
      XBuddyException: if the path could not be translated
      build_artifact.ArtifactDownloadError: if we failed to download the
                                            artifact.
    """
    self._SyncRegistryWithBuildImages()
    build_id, file_name = self._GetArtifact(path_list)
    Timestamp.UpdateTimestamp(self._timestamp_folder, build_id)
    #TODO (joyc): run in sep thread
    self.CleanCache()

    _Log('Returning path to payload: %s/%s', build_id, file_name)
    return build_id, file_name