# 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.

"""Library to make common google storage operations more reliable."""

import collections
import contextlib
import datetime
import errno
import fnmatch
import getpass
import hashlib
import logging
import os
from pathlib import Path
import re
import shutil
import subprocess
import sys
import tempfile
import urllib.parse

from chromite.lib import cache
from chromite.lib import constants
from chromite.lib import cros_build_lib
from chromite.lib import cros_collections
from chromite.lib import osutils
from chromite.lib import path_util
from chromite.lib import retry_stats
from chromite.lib import retry_util
from chromite.lib import signals
from chromite.lib import timeout_util


# This bucket has the allAuthenticatedUsers:READER ACL.
AUTHENTICATION_BUCKET = 'gs://chromeos-authentication-bucket/'

# Public path, only really works for files.
PUBLIC_BASE_HTTPS_URL = 'https://storage.googleapis.com/'

# Private path for files.
PRIVATE_BASE_HTTPS_URL = 'https://storage.cloud.google.com/'

# Private path for directories.
# TODO(akeshet): this is a workaround for b/27653354. If that is ultimately
# fixed, revisit this workaround.
PRIVATE_BASE_HTTPS_DOWNLOAD_URL = 'https://stainless.corp.google.com/browse/'
BASE_GS_URL = 'gs://'

# Format used by "gsutil ls -l" when reporting modified time.
DATETIME_FORMAT = '%Y-%m-%dT%H:%M:%SZ'

# Regexp for parsing each line of output from "gsutil ls -l".
# This regexp is prepared for the generation and meta_generation values,
# too, even though they are not expected until we use "-a".
#
# A detailed listing looks like:
#    99908  2014-03-01T05:50:08Z  gs://bucket/foo/abc#1234  metageneration=1
#                                 gs://bucket/foo/adir/
#    99908  2014-03-04T01:16:55Z  gs://bucket/foo/def#5678  metageneration=1
# TOTAL: 2 objects, 199816 bytes (495.36 KB)
LS_LA_RE = re.compile(
    r'^\s*(?P<content_length>\d*?)\s+'
    r'(?P<creation_time>\S*?)\s+'
    r'(?P<url>[^#$]+).*?'
    r'('
    r'#(?P<generation>\d+)\s+'
    r'meta_?generation=(?P<metageneration>\d+)'
    r')?\s*$')
LS_RE = re.compile(r'^\s*(?P<content_length>)(?P<creation_time>)(?P<url>.*)'
                   r'(?P<generation>)(?P<metageneration>)\s*$')

# Format used by ContainsWildCard, which is duplicated from
# https://github.com/GoogleCloudPlatform/gsutil/blob/v4.21/gslib/storage_url.py#L307.
WILDCARD_REGEX = re.compile(r'[*?\[\]]')


def PathIsGs(path):
  """Determine if a path is a Google Storage URI."""
  return path.startswith(BASE_GS_URL)


def CanonicalizeURL(url, strict=False):
  """Convert provided URL to gs:// URL, if it follows a known format.

  Args:
    url: URL to canonicalize.
    strict: Raises exception if URL cannot be canonicalized.
  """
  for prefix in (PUBLIC_BASE_HTTPS_URL,
                 PRIVATE_BASE_HTTPS_URL,
                 PRIVATE_BASE_HTTPS_DOWNLOAD_URL,
                 'https://pantheon.corp.google.com/storage/browser/',
                 'https://commondatastorage.googleapis.com/'):
    if url.startswith(prefix):
      return url.replace(prefix, BASE_GS_URL, 1)

  if not PathIsGs(url) and strict:
    raise ValueError('Url %r cannot be canonicalized.' % url)

  return url


def GetGsURL(bucket, for_gsutil=False, public=True, suburl=''):
  """Construct a Google Storage URL

  Args:
    bucket: The Google Storage bucket to use
    for_gsutil: Do you want a URL for passing to `gsutil`?
    public: Do we want the public or private url
    suburl: A url fragment to tack onto the end

  Returns:
    The fully constructed URL
  """
  url = 'gs://%s/%s' % (bucket, suburl)

  if for_gsutil:
    return url
  else:
    return GsUrlToHttp(url, public=public)


def GsUrlToHttp(path, public=True, directory=False):
  """Convert a GS URL to a HTTP URL for the same resource.

  Because the HTTP Urls are not fixed (and may not always be simple prefix
  replacements), use this method to centralize the conversion.

  Directories need to have different URLs from files, because the Web UIs for GS
  are weird and really inconsistent. Also public directories probably
  don't work, and probably never will (permissions as well as UI).

  e.g. 'gs://chromeos-image-archive/path/file' ->
       'https://pantheon/path/file'

  Args:
    path: GS URL to convert.
    public: Is this URL for Googler access, or publicly visible?
    directory: Force this URL to be treated as a directory?
               We try to autodetect on False.

  Returns:
    https URL as a string.
  """
  assert PathIsGs(path)
  directory = directory or path.endswith('/')

  # Public HTTP URls for directories don't work'
  # assert not public or not directory,

  if public:
    return path.replace(BASE_GS_URL, PUBLIC_BASE_HTTPS_URL, 1)
  else:
    if directory:
      return path.replace(BASE_GS_URL, PRIVATE_BASE_HTTPS_DOWNLOAD_URL, 1)
    else:
      return path.replace(BASE_GS_URL, PRIVATE_BASE_HTTPS_URL, 1)


class GSContextException(Exception):
  """Base exception for all exceptions thrown by GSContext."""


# Since the underlying code uses run, some callers might be trying to
# catch cros_build_lib.RunCommandError themselves.  Extend that class so that
# code continues to work.
class GSCommandError(GSContextException, cros_build_lib.RunCommandError):
  """Thrown when an error happened we couldn't decode."""


class GSContextPreconditionFailed(GSContextException):
  """Thrown when google storage returns code=PreconditionFailed."""


class GSNoSuchKey(GSContextException):
  """Thrown when google storage returns code=NoSuchKey."""


# Detailed results of GSContext.Stat.
#
# The fields directory correspond to gsutil stat results.
#
#  Field name        Type         Example
#   creation_time     datetime     Sat, 23 Aug 2014 06:53:20 GMT
#   content_length    int          74
#   content_type      string       application/octet-stream
#   hash_crc32c       string       BBPMPA==
#   hash_md5          string       ms+qSYvgI9SjXn8tW/5UpQ==
#   etag              string       CNCgocbmqMACEAE=
#   generation        int          1408776800850000
#   metageneration    int          1
#
# Note: We omit a few stat fields as they are not always available, and we
# have no callers that want this currently.
#
#   content_language  string/None  en   # This field may be None.
GSStatResult = collections.namedtuple(
    'GSStatResult',
    ('creation_time', 'content_length', 'content_type', 'hash_crc32c',
     'hash_md5', 'etag', 'generation', 'metageneration'))


# Detailed results of GSContext.List.
GSListResult = collections.namedtuple(
    'GSListResult',
    ('url', 'creation_time', 'content_length', 'generation', 'metageneration'))


ErrorDetails = cros_collections.Collection(
    'ErrorDetails',
    type=None, message_pattern='', retriable=None, exception=None)


class GSCounter(object):
  """A counter class for Google Storage."""

  def __init__(self, ctx, path):
    """Create a counter object.

    Args:
      ctx: A GSContext object.
      path: The path to the counter in Google Storage.
    """
    self.ctx = ctx
    self.path = path

  def Get(self):
    """Get the current value of a counter."""
    try:
      return int(self.ctx.Cat(self.path))
    except GSNoSuchKey:
      return 0

  def AtomicCounterOperation(self, default_value, operation):
    """Atomically set the counter value using |operation|.

    Args:
      default_value: Default value to use for counter, if counter
                     does not exist.
      operation: Function that takes the current counter value as a
                 parameter, and returns the new desired value.

    Returns:
      The new counter value. None if value could not be set.
    """
    generation, _ = self.ctx.GetGeneration(self.path)
    for _ in range(self.ctx.retries + 1):
      try:
        value = default_value if generation == 0 else operation(self.Get())
        self.ctx.Copy('-', self.path, input=str(value), version=generation)
        return value
      except (GSContextPreconditionFailed, GSNoSuchKey):
        # GSContextPreconditionFailed is thrown if another builder is also
        # trying to update the counter and we lost the race. GSNoSuchKey is
        # thrown if another builder deleted the counter. In either case, fetch
        # the generation again, and, if it has changed, try the copy again.
        new_generation, _ = self.ctx.GetGeneration(self.path)
        if new_generation == generation:
          raise
        generation = new_generation

  def Increment(self):
    """Increment the counter.

    Returns:
      The new counter value. None if value could not be set.
    """
    return self.AtomicCounterOperation(1, lambda x: x + 1)

  def Decrement(self):
    """Decrement the counter.

    Returns:
      The new counter value. None if value could not be set.
    """
    return self.AtomicCounterOperation(-1, lambda x: x - 1)

  def Reset(self):
    """Reset the counter to zero.

    Returns:
      The new counter value. None if value could not be set.
    """
    return self.AtomicCounterOperation(0, lambda x: 0)

  def StreakIncrement(self):
    """Increment the counter if it is positive, otherwise set it to 1.

    Returns:
      The new counter value. None if value could not be set.
    """
    return self.AtomicCounterOperation(1, lambda x: x + 1 if x > 0 else 1)

  def StreakDecrement(self):
    """Decrement the counter if it is negative, otherwise set it to -1.

    Returns:
      The new counter value. None if value could not be set.
    """
    return self.AtomicCounterOperation(-1, lambda x: x - 1 if x < 0 else -1)


class GSContext(object):
  """A class to wrap common google storage operations."""

  # Error messages that indicate an invalid BOTO config.
  AUTHORIZATION_ERRORS = ('no configured', 'none configured',
                          'detail=Authorization', '401 Anonymous caller')

  DEFAULT_BOTO_FILE = os.path.expanduser('~/.boto')
  DEFAULT_GSUTIL_TRACKER_DIR = os.path.expanduser('~/.gsutil/tracker-files')
  # This is set for ease of testing.
  _DEFAULT_GSUTIL_BIN = None
  _DEFAULT_GSUTIL_BUILDER_BIN = '/b/build/third_party/gsutil/gsutil'
  _CRCMOD_METHOD = None
  # How many times to retry uploads.
  DEFAULT_RETRIES = 3

  # Multiplier for how long to sleep (in seconds) between retries; will delay
  # (1*sleep) the first time, then (2*sleep), continuing via attempt * sleep.
  DEFAULT_SLEEP_TIME = 60

  GSUTIL_VERSION = '4.60'
  GSUTIL_TAR = 'gsutil_%s.tar.gz' % GSUTIL_VERSION
  GSUTIL_URL = (PUBLIC_BASE_HTTPS_URL +
                'chromeos-mirror/gentoo/distfiles/%s' % GSUTIL_TAR)
  GSUTIL_API_SELECTOR = 'JSON'

  RESUMABLE_UPLOAD_ERROR = (b'Too many resumable upload attempts failed '
                            b'without progress')
  RESUMABLE_DOWNLOAD_ERROR = (b'Too many resumable download attempts failed '
                              b'without progress')

  # TODO: Below is a list of known flaky errors that we should
  # retry. The list needs to be extended.
  RESUMABLE_ERROR_MESSAGE = (
      RESUMABLE_DOWNLOAD_ERROR,
      RESUMABLE_UPLOAD_ERROR,
      b'ResumableUploadException',
      b'ResumableUploadAbortException',
      b'ResumableDownloadException',
      b'ssl.SSLError: The read operation timed out',
      # TODO: Error messages may change in different library versions,
      # use regexes to match resumable error messages.
      b"ssl.SSLError: ('The read operation timed out',)",
      b'ssl.SSLError: _ssl.c:495: The handshake operation timed out',
      b'Unable to find the server',
      b"doesn't match cloud-supplied digest",
      b'ssl.SSLError: [Errno 8]',
      b'EOF occurred in violation of protocol',
      # TODO(nxia): crbug.com/775330 narrow down the criteria for retrying
      b'AccessDeniedException',
  )

  # We have seen flaky errors with 5xx return codes
  # See b/17376491 for the "JSON decoding" error.
  # We have seen transient Oauth 2.0 credential errors (crbug.com/414345).
  TRANSIENT_ERROR_MESSAGE = (
      b'ServiceException: 5',
      b'Failure: No JSON object could be decoded',
      b'Oauth 2.0 User Account',
      b'InvalidAccessKeyId',
      b'socket.error: [Errno 104] Connection reset by peer',
      b'Received bad request from server',
      b"can't start new thread",
  )

  @classmethod
  def InitializeCache(cls, cache_dir=None, cache_user=None):
    """Setup the gsutil cache if needed."""
    if cls._DEFAULT_GSUTIL_BIN is not None:
      return

    if cache_dir is None:
      cache_dir = path_util.GetCacheDir()
    if cache_dir is not None:
      common_path = os.path.join(cache_dir, constants.COMMON_CACHE)
      tar_cache = cache.TarballCache(common_path, cache_user=cache_user)
      key = (cls.GSUTIL_TAR,)
      # The common cache will not be LRU, removing the need to hold a read
      # lock on the cached gsutil.
      ref = tar_cache.Lookup(key)
      ref.SetDefault(cls.GSUTIL_URL)
      cls._DEFAULT_GSUTIL_BIN = os.path.join(ref.path, 'gsutil', 'gsutil')
      cls._DetermineCrcmodStrategy(Path(ref.path))
    else:
      # Check if the default gsutil path for builders exists. If
      # not, try locating gsutil. If none exists, simply use 'gsutil'.
      gsutil_bin = cls._DEFAULT_GSUTIL_BUILDER_BIN
      if not os.path.exists(gsutil_bin):
        gsutil_bin = osutils.Which('gsutil')
      if gsutil_bin is None:
        gsutil_bin = 'gsutil'
      cls._DEFAULT_GSUTIL_BIN = gsutil_bin

  @classmethod
  def _DetermineCrcmodStrategy(cls, path: Path):
    """Figure out how we'll get a compiled crcmod.

    The compiled crcmod code is much faster than the python implementation, and
    enables some more features (otherwise gsutil internally disables them).
    Try to compile the module on demand in the crcmod tree bundled with gsutil.

    If that's not working, we'll try to fall back to vpython when available.

    For more details, see:
    https://cloud.google.com/storage/docs/gsutil/addlhelp/CRC32CandInstallingcrcmod

    Args:
      path: The path to our local copy of gsutil.

    Returns:
      'import' if the active python has it available already (whoo!).
      'bundle' if we were able to build the bundled copy in the gsutil source.
      'vpython' if we need to use vpython to pull a wheel on the fly.
      'missing' if we weren't able to find a compiled version.
    """
    if cls._CRCMOD_METHOD is not None:
      return cls._CRCMOD_METHOD

    # See if the system includes one in which case we're done.
    # We'll use the active python interp to run gsutil directly.
    try:
      from crcmod.crcmod import _usingExtension
      if _usingExtension:
        cls._CRCMOD_METHOD = 'import'
        return cls._CRCMOD_METHOD
    except ImportError:
      pass

    # Try to compile the bundled copy.
    if cls._SetupBundledCrcmod(path):
      cls._CRCMOD_METHOD = 'bundle'
      return cls._CRCMOD_METHOD

    # Give vpython a chance if it exists.
    if cls._SetupVpython(path):
      cls._CRCMOD_METHOD = 'vpython'
      return cls._CRCMOD_METHOD

    cls._CRCMOD_METHOD = 'missing'
    return cls._CRCMOD_METHOD

  @classmethod
  def _SetupBundledCrcmod(cls, path: Path) -> bool:
    """Try to setup a compiled crcmod for gsutil."""
    src_root = path / 'gsutil' / 'third_party' / 'crcmod'

    # See if module already exists.
    try:
      next(src_root.glob('python3/crcmod/_crcfunext*.so'))
      return True
    except StopIteration:
      pass

    # Try to build it once.
    flag = src_root / '.chromite.tried.build'
    if flag.exists():
      return False
    # Flag things now regardless of how the attempt below works out.
    try:
      osutils.Touch(flag)
    except IOError as e:
      # If the gsutil dir was cached previously as root, but now we're
      # non-root, just flag it and return.
      if e.errno == errno.EACCES:
        logging.debug('Skipping gsutil crcmod compile due to permissions')
        cros_build_lib.sudo_run(['touch', str(flag)], debug_level=logging.DEBUG)
        return False
      else:
        raise

    # See if the local copy has one.
    logging.debug('Attempting to compile local crcmod for gsutil')
    with osutils.TempDir(prefix='chromite.gsutil.crcmod') as tempdir:
      tempdir = Path(tempdir)
      result = cros_build_lib.run(
          [sys.executable, 'setup.py', 'build',
           '--build-base', str(tempdir),
           '--build-platlib', str(tempdir)],
          cwd=src_root, capture_output=True, check=False,
          debug_level=logging.DEBUG)
      if result.returncode:
        return False

      # Locate the module in the build dir.
      copied = False
      for mod_path in tempdir.glob('crcmod/_crcfunext*.so'):
        dst_mod_path = src_root / 'python3' / 'crcmod' / mod_path.name
        try:
          shutil.copy2(mod_path, dst_mod_path)
          copied = True
        except shutil.Error:
          pass

      if not copied:
        # If the module compile failed (missing compiler/headers/whatever),
        # then the setup.py build command above would have passed, but there
        # won't actually be a _crcfunext.so module.  Check for it here to
        # disambiguate other errors from shutil.copy2.
        logging.debug('No crcmod module produced (missing host compiler?)')

      return copied

  @classmethod
  def _SetupVpython(cls, path: Path) -> bool:
    """Setup a vpython spec to use later on."""
    if not osutils.Which('vpython3'):
      return False

    spec = path / 'gsutil' / 'gsutil.vpython3'
    if spec.exists():
      return True
    data = b"""python_version: "3.8"
wheel: <
  name: "infra/python/wheels/crcmod/${vpython_platform}"
  version: "version:1.7"
>
"""
    # TODO(vapier): Drop str() once WriteFile accepts Path objects.
    spec = str(spec)
    try:
      osutils.WriteFile(spec, data, mode='wb', atomic=True)
    except OSError:
      # If the cache already existed, as root, but hadn't had the spec written,
      # do it as root now.
      osutils.WriteFile(spec, data, mode='wb', atomic=True, sudo=True)
    return True

  def __init__(self, boto_file=None, cache_dir=None, acl=None,
               dry_run=False, gsutil_bin=None, init_boto=False, retries=None,
               sleep=None, cache_user=None):
    """Constructor.

    Args:
      boto_file: Fully qualified path to user's .boto credential file.
      cache_dir: The absolute path to the cache directory. Use the default
        fallback if not given.
      acl: If given, a canned ACL. It is not valid to pass in an ACL file
        here, because most gsutil commands do not accept ACL files. If you
        would like to use an ACL file, use the SetACL command instead.
      dry_run: Testing mode that prints commands that would be run.
      gsutil_bin: If given, the absolute path to the gsutil binary.  Else
        the default fallback will be used.
      init_boto: If set to True, GSContext will check during __init__ if a
        valid boto config is configured, and if not, will attempt to ask the
        user to interactively set up the boto config.
      retries: Number of times to retry a command before failing.
      sleep: Amount of time to sleep between failures.
      cache_user: user for creating cache_dir for gsutil. Default is None.
    """
    if gsutil_bin is None:
      self.InitializeCache(cache_dir=cache_dir, cache_user=cache_user)
      gsutil_bin = self._DEFAULT_GSUTIL_BIN
      if self._CRCMOD_METHOD == 'vpython':
        self._gsutil_bin = ['vpython3', gsutil_bin]
      else:
        self._gsutil_bin = [sys.executable, gsutil_bin]
    else:
      self._CheckFile('gsutil not found', gsutil_bin)
      self._gsutil_bin = [gsutil_bin]

    # The version of gsutil is retrieved on demand and cached here.
    self._gsutil_version = None

    # Increase the number of retries. With 10 retries, Boto will try a total of
    # 11 times and wait up to 2**11 seconds (~30 minutes) in total, not
    # not including the time spent actually uploading or downloading.
    self.gsutil_flags = ['-o', 'Boto:num_retries=10']

    # Set HTTP proxy if environment variable http_proxy is set
    # (crbug.com/325032).
    if 'http_proxy' in os.environ:
      url = urllib.parse.urlparse(os.environ['http_proxy'])
      if not url.hostname or (not url.username and url.password):
        logging.warning('GS_ERROR: Ignoring env variable http_proxy because it '
                        'is not properly set: %s', os.environ['http_proxy'])
      else:
        self.gsutil_flags += ['-o', 'Boto:proxy=%s' % url.hostname]
        if url.username:
          self.gsutil_flags += ['-o', 'Boto:proxy_user=%s' % url.username]
        if url.password:
          self.gsutil_flags += ['-o', 'Boto:proxy_pass=%s' % url.password]
        if url.port:
          self.gsutil_flags += ['-o', 'Boto:proxy_port=%d' % url.port]

    # Prefer boto_file if specified, else prefer the env then the default.
    if boto_file is None:
      boto_file = os.environ.get('BOTO_CONFIG')
    if boto_file is None and os.path.isfile(self.DEFAULT_BOTO_FILE):
      # Only set boto file to DEFAULT_BOTO_FILE if it exists.
      boto_file = self.DEFAULT_BOTO_FILE

    self.boto_file = boto_file

    self.acl = acl

    self.dry_run = dry_run
    self.retries = self.DEFAULT_RETRIES if retries is None else int(retries)
    self._sleep_time = self.DEFAULT_SLEEP_TIME if sleep is None else int(sleep)

    if init_boto and not dry_run:
      # We can't really expect gsutil to even be present in dry_run mode.
      self._InitBoto()

  @property
  def gsutil_version(self):
    """Return the version of the gsutil in this context."""
    if not self._gsutil_version:
      if self.dry_run:
        self._gsutil_version = self.GSUTIL_VERSION
      else:
        cmd = ['-q', 'version']

        # gsutil has been known to return version to stderr in the past, so
        # use stderr=subprocess.STDOUT.
        result = self.DoCommand(cmd, stdout=True, stderr=subprocess.STDOUT)

        # Expect output like: 'gsutil version 3.35' or 'gsutil version: 4.5'.
        match = re.search(r'^\s*gsutil\s+version:?\s+([\d.]+)', result.output,
                          re.IGNORECASE)
        if match:
          self._gsutil_version = match.group(1)
        else:
          raise GSContextException('Unexpected output format from "%s":\n%s.' %
                                   (result.cmdstr, result.output))

    return self._gsutil_version

  def _CheckFile(self, errmsg, afile):
    """Pre-flight check for valid inputs.

    Args:
      errmsg: Error message to display.
      afile: Fully qualified path to test file existance.
    """
    if not os.path.isfile(afile):
      raise GSContextException('%s, %s is not a file' % (errmsg, afile))

  def _TestGSLs(self):
    """Quick test of gsutil functionality."""
    # The bucket in question is readable by any authenticated account.
    # If we can list its contents, we have valid authentication.
    cmd = ['ls', AUTHENTICATION_BUCKET]
    result = self.DoCommand(cmd, retries=0, debug_level=logging.DEBUG,
                            capture_output=True, check=False)

    # Did we fail with an authentication error?
    if (result.returncode == 1 and
        any(e in result.error for e in self.AUTHORIZATION_ERRORS)):
      logging.warning('gsutil authentication failure msg: %s', result.error)
      return False

    return True

  def _ConfigureBotoConfig(self):
    """Make sure we can access protected bits in GS."""
    print('Configuring gsutil. **Please use your @google.com account.**')
    try:
      if not self.boto_file:
        self.boto_file = self.DEFAULT_BOTO_FILE
      self.DoCommand(['config'], retries=0, debug_level=logging.CRITICAL,
                     print_cmd=False)
    finally:
      if (os.path.exists(self.boto_file) and not
          os.path.getsize(self.boto_file)):
        os.remove(self.boto_file)
        raise GSContextException('GS config could not be set up.')

  def _InitBoto(self):
    if not self._TestGSLs():
      self._ConfigureBotoConfig()

  def Cat(self, path, **kwargs):
    """Returns the contents of a GS object."""
    kwargs.setdefault('stdout', True)
    encoding = kwargs.setdefault('encoding', None)
    errors = kwargs.setdefault('errors', None)
    if not PathIsGs(path):
      # gsutil doesn't support cat-ting a local path, so read it ourselves.
      mode = 'rb' if encoding is None else 'r'
      try:
        return osutils.ReadFile(path, mode=mode, encoding=encoding,
                                errors=errors)
      except Exception as e:
        if getattr(e, 'errno', None) == errno.ENOENT:
          raise GSNoSuchKey('Cat Error: file %s does not exist' % path)
        else:
          raise GSContextException(str(e))
    elif self.dry_run:
      return b'' if encoding is None else ''
    else:
      return self.DoCommand(['cat', path], **kwargs).output

  def StreamingCat(self, path, chunksize=0x100000):
    """Returns the content of a GS file as a stream.

    Unlike Cat or Copy, this function doesn't support any internal retry or
    validation by computing checksum of downloaded data. Users should perform
    their own validation, or use Cat() instead.

    Args:
      path: Full gs:// path of the src file.
      chunksize: At most how much data read from upstream and yield to callers
        at a time. The default value is 1 MB.

    Yields:
      The file content, chunk by chunk, as bytes.
    """
    assert PathIsGs(path)

    if self.dry_run:
      return (lambda: (yield ''))()

    env = None
    if self.boto_file and os.path.isfile(self.boto_file):
      env = os.environ.copy()
      env['BOTO_CONFIG'] = self.boto_file

    cmd = self._gsutil_bin + self.gsutil_flags + ['cat', path]
    proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, env=env)

    def read_content():
      try:
        while True:
          data = proc.stdout.read(chunksize)
          if not data and proc.poll() is not None:
            break
          if data:
            yield data

        rc = proc.poll()
        if rc:
          raise GSCommandError(
              'Cannot stream cat %s from Google Storage!' % path, rc, None)
      finally:
        if proc.returncode is None:
          proc.stdout.close()
          proc.terminate()

    return read_content()

  def CopyInto(self, local_path, remote_dir, filename=None, **kwargs):
    """Upload a local file into a directory in google storage.

    Args:
      local_path: Local file path to copy.
      remote_dir: Full gs:// url of the directory to transfer the file into.
      filename: If given, the filename to place the content at; if not given,
        it's discerned from basename(local_path).
      **kwargs: See Copy() for documentation.

    Returns:
      The generation of the remote file.
    """
    filename = filename if filename is not None else local_path
    # Basename it even if an explicit filename was given; we don't want
    # people using filename as a multi-directory path fragment.
    return self.Copy(local_path,
                     '%s/%s' % (remote_dir, os.path.basename(filename)),
                     **kwargs)

  @staticmethod
  def GetTrackerFilenames(dest_path):
    """Returns a list of gsutil tracker filenames.

    Tracker files are used by gsutil to resume downloads/uploads. This
    function does not handle parallel uploads.

    Args:
      dest_path: Either a GS path or an absolute local path.

    Returns:
      The list of potential tracker filenames.
    """
    dest = urllib.parse.urlsplit(dest_path)
    filenames = []
    if dest.scheme == 'gs':
      prefix = 'upload'
      bucket_name = dest.netloc
      object_name = dest.path.lstrip('/')
      filenames.append(
          re.sub(r'[/\\]', '_', 'resumable_upload__%s__%s__%s.url' %
                 (bucket_name, object_name, GSContext.GSUTIL_API_SELECTOR)))
    else:
      prefix = 'download'
      filenames.append(
          re.sub(r'[/\\]', '_', 'resumable_download__%s__%s.etag' %
                 (dest.path, GSContext.GSUTIL_API_SELECTOR)))

    hashed_filenames = []
    for filename in filenames:
      m = hashlib.sha1(filename.encode())
      hashed_filenames.append('%s_TRACKER_%s.%s' %
                              (prefix, m.hexdigest(), filename[-16:]))

    return hashed_filenames

  def _RetryFilter(self, e):
    """Returns whether to retry RunCommandError exception |e|.

    Args:
      e: Exception object to filter. Exception may be re-raised as
         as different type, if _RetryFilter determines a more appropriate
         exception type based on the contents of |e|.
    """
    error_details = self._MatchKnownError(e)
    if error_details.exception:
      raise error_details.exception
    return error_details.retriable

  def _MatchKnownError(self, e):
    """Function to match known RunCommandError exceptions.

    Args:
      e: Exception object to filter.

    Returns:
      An ErrorDetails instance with details about the message pattern found.
    """
    if not retry_util.ShouldRetryCommandCommon(e):
      if not isinstance(e, cros_build_lib.RunCommandError):
        error_type = 'unknown'
      else:
        error_type = 'failed_to_launch'
      return ErrorDetails(type=error_type, retriable=False)

    # e is guaranteed by above filter to be a RunCommandError
    if e.result.returncode < 0:
      sig_name = signals.StrSignal(-e.result.returncode)
      logging.info('Child process received signal %d; not retrying.', sig_name)
      return ErrorDetails(type='received_signal', message_pattern=sig_name,
                          retriable=False)

    error = e.result.error
    if error:
      # Since the captured error will use the encoding the user requested,
      # normalize to bytes for testing below.
      if isinstance(error, str):
        error = error.encode('utf-8')

      # gsutil usually prints PreconditionException when a precondition fails.
      # It may also print "ResumableUploadAbortException: 412 Precondition
      # Failed", so the logic needs to be a little more general.
      if (b'PreconditionException' in error or
          b'412 Precondition Failed' in error):
        return ErrorDetails(type='precondition_exception', retriable=False,
                            exception=GSContextPreconditionFailed(e))

      # If the file does not exist, one of the following errors occurs. The
      # "stat" command leaves off the "CommandException: " prefix, but it also
      # outputs to stdout instead of stderr and so will not be caught here
      # regardless.
      if (b'CommandException: No URLs matched' in error or
          b'NotFoundException:' in error or
          b'One or more URLs matched no objects' in error):
        return ErrorDetails(type='no_such_key', retriable=False,
                            exception=GSNoSuchKey(e))

      logging.warning('GS_ERROR: %s ', error)

      # Temporary fix: remove the gsutil tracker files so that our retry
      # can hit a different backend. This should be removed after the
      # bug is fixed by the Google Storage team (see crbug.com/308300).
      resumable_error = _FirstSubstring(error, self.RESUMABLE_ERROR_MESSAGE)
      if resumable_error:
        # Only remove the tracker files if we try to upload/download a file.
        if 'cp' in e.result.cmd[:-2]:
          # Assume a command: gsutil [options] cp [options] src_path dest_path
          # dest_path needs to be a fully qualified local path, which is already
          # required for GSContext.Copy().
          tracker_filenames = self.GetTrackerFilenames(e.result.cmd[-1])
          logging.info('Potential list of tracker files: %s',
                       tracker_filenames)
          for tracker_filename in tracker_filenames:
            tracker_file_path = os.path.join(self.DEFAULT_GSUTIL_TRACKER_DIR,
                                             tracker_filename)
            if os.path.exists(tracker_file_path):
              logging.info('Deleting gsutil tracker file %s before retrying.',
                           tracker_file_path)
              logging.info('The content of the tracker file: %s',
                           osutils.ReadFile(tracker_file_path))
              osutils.SafeUnlink(tracker_file_path)
        return ErrorDetails(type='resumable',
                            message_pattern=resumable_error.decode('utf-8'),
                            retriable=True)

      transient_error = _FirstSubstring(error, self.TRANSIENT_ERROR_MESSAGE)
      if transient_error:
        return ErrorDetails(type='transient',
                            message_pattern=transient_error.decode('utf-8'),
                            retriable=True)

    return ErrorDetails(type='unknown', retriable=False)

  # TODO(mtennant): Make a private method.
  def DoCommand(self, gsutil_cmd, headers=(), retries=None, version=None,
                parallel=False, **kwargs):
    """Run a gsutil command, suppressing output, and setting retry/sleep.

    Args:
      gsutil_cmd: The (mostly) constructed gsutil subcommand to run.
      headers: A list of raw headers to pass down.
      parallel: Whether gsutil should enable parallel copy/update of multiple
        files. NOTE: This option causes gsutil to use significantly more
        memory, even if gsutil is only uploading one file.
      retries: How many times to retry this command (defaults to setting given
        at object creation).
      version: If given, the generation; essentially the timestamp of the last
        update.  Note this is not the same as sequence-number; it's
        monotonically increasing bucket wide rather than reset per file.
        The usage of this is if we intend to replace/update only if the version
        is what we expect.  This is useful for distributed reasons- for example,
        to ensure you don't overwrite someone else's creation, a version of
        0 states "only update if no version exists".

    Returns:
      A RunCommandResult object.
    """
    kwargs = kwargs.copy()
    kwargs.setdefault('stderr', True)
    kwargs.setdefault('encoding', 'utf-8')

    cmd = self._gsutil_bin + self.gsutil_flags
    for header in headers:
      cmd += ['-h', header]
    if version is not None:
      cmd += ['-h', 'x-goog-if-generation-match:%d' % int(version)]

    # Enable parallel copy/update of multiple files if stdin is not to
    # be piped to the command. This does not split a single file into
    # smaller components for upload.
    if parallel and kwargs.get('input') is None:
      cmd += ['-m']

    cmd.extend(gsutil_cmd)

    if retries is None:
      retries = self.retries

    extra_env = kwargs.pop('extra_env', {})
    if self.boto_file and os.path.isfile(self.boto_file):
      extra_env.setdefault('BOTO_CONFIG', self.boto_file)

    if self.dry_run:
      logging.debug("%s: would've run: %s", self.__class__.__name__,
                    cros_build_lib.CmdToStr(cmd))
    else:
      try:
        return retry_stats.RetryWithStats(retry_stats.GSUTIL,
                                          self._RetryFilter,
                                          retries, cros_build_lib.run,
                                          cmd, sleep=self._sleep_time,
                                          extra_env=extra_env, **kwargs)
      except cros_build_lib.RunCommandError as e:
        raise GSCommandError(e.msg, e.result, e.exception)

  def Copy(self, src_path, dest_path, acl=None, recursive=False,
           skip_symlinks=True, auto_compress=False, **kwargs):
    """Copy to/from GS bucket.

    Canned ACL permissions can be specified on the gsutil cp command line.

    More info:
    https://developers.google.com/storage/docs/accesscontrol#applyacls

    Args:
      src_path: Fully qualified local path or full gs:// path of the src file.
      dest_path: Fully qualified local path or full gs:// path of the dest
                 file.
      acl: One of the google storage canned_acls to apply.
      recursive: Whether to copy recursively.
      skip_symlinks: Skip symbolic links when copying recursively.
      auto_compress: Automatically compress with gzip when uploading.

    Returns:
      The generation of the remote file.

    Raises:
      RunCommandError if the command failed despite retries.
    """
    # -v causes gs://bucket/path#generation to be listed in output.
    cmd = ['cp', '-v']

    # Certain versions of gsutil (at least 4.3) assume the source of a copy is
    # a directory if the -r option is used. If it's really a file, gsutil will
    # look like it's uploading it but not actually do anything. We'll work
    # around that problem by surpressing the -r flag if we detect the source
    # is a local file.
    if recursive and not os.path.isfile(src_path):
      cmd.append('-r')
      if skip_symlinks:
        cmd.append('-e')

    if auto_compress:
      cmd.append('-Z')

    acl = self.acl if acl is None else acl
    if acl is not None:
      cmd += ['-a', acl]

    with cros_build_lib.ContextManagerStack() as stack:
      # Write the input into a tempfile if possible. This is needed so that
      # gsutil can retry failed requests.  We allow the input to be a string
      # or bytes regardless of the output encoding.
      if src_path == '-' and kwargs.get('input') is not None:
        f = stack.Add(tempfile.NamedTemporaryFile, mode='wb')
        data = kwargs['input']
        if isinstance(data, str):
          data = data.encode('utf-8')
        f.write(data)
        f.flush()
        del kwargs['input']
        src_path = f.name

      cmd += ['--', src_path, dest_path]

      if not (PathIsGs(src_path) or PathIsGs(dest_path)):
        # Don't retry on local copies.
        kwargs.setdefault('retries', 0)

      kwargs['capture_output'] = True
      try:
        result = self.DoCommand(cmd, **kwargs)
        if self.dry_run:
          return None

        # Now we parse the output for the current generation number.  Example:
        #   Created: gs://chromeos-throw-away-bucket/foo#1360630664537000.1
        m = re.search(r'Created: .*#(\d+)([.](\d+))?\n', result.error)
        if m:
          return int(m.group(1))
        else:
          return None
      except GSNoSuchKey as e:
        # If the source was a local file, the error is a quirk of gsutil 4.5
        # and should be ignored. If the source was remote, there might
        # legitimately be no such file. See crbug.com/393419.
        if os.path.isfile(src_path):
          return None

        # Temp log for crbug.com/642986, should be removed when the bug
        # is fixed.
        logging.warning('Copy Error: src %s dest %s: %s '
                        '(Temp log for crbug.com/642986)',
                        src_path, dest_path, e)
        raise

  def CreateWithContents(self, gs_uri, contents, **kwargs):
    """Creates the specified file with specified contents.

    Args:
      gs_uri: The URI of a file on Google Storage.
      contents: String or bytes with contents to write to the file.
      kwargs: See additional options that Copy takes.

    Raises:
      See Copy.
    """
    self.Copy('-', gs_uri, input=contents, **kwargs)

  # TODO: Merge LS() and List()?
  def LS(self, path, **kwargs):
    """Does a directory listing of the given gs path.

    Args:
      path: The path to get a listing of.
      kwargs: See options that DoCommand takes.

    Returns:
      A list of paths that matched |path|.  Might be more than one if a
      directory or path include wildcards/etc...
    """
    if self.dry_run:
      return []

    if not PathIsGs(path):
      # gsutil doesn't support listing a local path, so just run 'ls'.
      kwargs.pop('retries', None)
      kwargs.pop('headers', None)
      kwargs['capture_output'] = True
      kwargs.setdefault('encoding', 'utf-8')
      result = cros_build_lib.run(['ls', path], **kwargs)
      return result.output.splitlines()
    else:
      return [x.url for x in self.List(path, **kwargs)]

  def List(self, path, details=False, **kwargs):
    """Does a directory listing of the given gs path.

    Args:
      path: The path to get a listing of.
      details: Whether to include size/timestamp info.
      kwargs: See options that DoCommand takes.

    Returns:
      A list of GSListResult objects that matched |path|.  Might be more
      than one if a directory or path include wildcards/etc...
    """
    ret = []
    if self.dry_run:
      return ret

    cmd = ['ls']
    if details:
      cmd += ['-l']
    cmd += ['--']
    if isinstance(path, str):
      cmd.append(path)
    else:
      cmd.extend(path)

    # We always request the extended details as the overhead compared to a plain
    # listing is negligible.
    kwargs['stdout'] = True
    lines = self.DoCommand(cmd, **kwargs).output.splitlines()

    if details:
      # The last line is expected to be a summary line.  Ignore it.
      lines = lines[:-1]
      ls_re = LS_LA_RE
    else:
      ls_re = LS_RE

    # Handle optional fields.
    intify = lambda x: int(x) if x else None

    # Parse out each result and build up the results list.
    for line in lines:
      match = ls_re.search(line)
      if not match:
        raise GSContextException('unable to parse line: %s' % line)
      if match.group('creation_time'):
        timestamp = datetime.datetime.strptime(match.group('creation_time'),
                                               DATETIME_FORMAT)
      else:
        timestamp = None

      ret.append(GSListResult(
          content_length=intify(match.group('content_length')),
          creation_time=timestamp,
          url=match.group('url'),
          generation=intify(match.group('generation')),
          metageneration=intify(match.group('metageneration'))))

    return ret

  def GetSize(self, path, **kwargs):
    """Returns size of a single object (local or GS)."""
    if not PathIsGs(path):
      return os.path.getsize(path)
    else:
      return self.Stat(path, **kwargs).content_length

  def Move(self, src_path, dest_path, **kwargs):
    """Move/rename to/from GS bucket.

    Args:
      src_path: Fully qualified local path or full gs:// path of the src file.
      dest_path: Fully qualified local path or full gs:// path of the dest file.
      kwargs: See options that DoCommand takes.
    """
    cmd = ['mv', '--', src_path, dest_path]
    return self.DoCommand(cmd, **kwargs)

  def SetACL(self, path, acl=None, **kwargs):
    """Set access on a file already in google storage.

    Args:
      path: gs:// url that will have acl applied to it.
      acl: An ACL permissions file or canned ACL.
      kwargs: See options that DoCommand takes.
    """
    if acl is None:
      if not self.acl:
        raise GSContextException(
            'SetAcl invoked w/out a specified acl, nor a default acl.')
      acl = self.acl

    cmd = ['acl', 'set', '--', acl]
    if isinstance(path, str):
      cmd.append(path)
    else:
      cmd.extend(path)

    self.DoCommand(cmd, **kwargs)

  def ChangeACL(self, upload_url, acl_args_file=None, acl_args=None, **kwargs):
    """Change access on a file already in google storage with "acl ch".

    Args:
      upload_url: gs:// url that will have acl applied to it.
      acl_args_file: A file with arguments to the gsutil acl ch command. The
                     arguments can be spread across multiple lines. Comments
                     start with a # character and extend to the end of the
                     line. Exactly one of this argument or acl_args must be
                     set.
      acl_args: A list of arguments for the gsutil acl ch command. Exactly
                one of this argument or acl_args must be set.
      kwargs: See options that DoCommand takes.
    """
    if acl_args_file and acl_args:
      raise GSContextException(
          'ChangeACL invoked with both acl_args and acl_args set.')
    if not acl_args_file and not acl_args:
      raise GSContextException(
          'ChangeACL invoked with neither acl_args nor acl_args set.')

    if acl_args_file:
      lines = osutils.ReadFile(acl_args_file).splitlines()
      # Strip out comments.
      lines = [x.split('#', 1)[0].strip() for x in lines]
      acl_args = ' '.join([x for x in lines if x]).split()

    # Some versions of gsutil bubble up precondition failures even when we
    # didn't request it due to how ACL changes happen internally to gsutil.
    # https://crbug.com/763450
    # We keep the retry limit a bit low because DoCommand already has its
    # own level of retries.
    retry_util.RetryException(
        GSContextPreconditionFailed, 3, self.DoCommand,
        ['acl', 'ch'] + acl_args + [upload_url], **kwargs)

  def Exists(self, path, **kwargs):
    """Checks whether the given object exists.

    Args:
      path: Local path or gs:// url to check.
      kwargs: Flags to pass to DoCommand.

    Returns:
      True if the path exists; otherwise returns False.
    """
    if not PathIsGs(path):
      return os.path.exists(path)

    try:
      self.Stat(path, **kwargs)
    except GSNoSuchKey:
      return False

    return True

  def Remove(self, path, recursive=False, ignore_missing=False, **kwargs):
    """Remove the specified file.

    Args:
      path: Full gs:// url of the file to delete.
      recursive: Remove recursively starting at path.
      ignore_missing: Whether to suppress errors about missing files.
      kwargs: Flags to pass to DoCommand.
    """
    cmd = ['rm']
    if 'recurse' in kwargs:
      raise TypeError('"recurse" has been renamed to "recursive"')
    if recursive:
      cmd.append('-R')
    cmd.append('--')
    if isinstance(path, str):
      cmd.append(path)
    else:
      cmd.extend(path)
    try:
      self.DoCommand(cmd, **kwargs)
    except GSNoSuchKey:
      if not ignore_missing:
        raise

  def GetGeneration(self, path):
    """Get the generation and metageneration of the given |path|.

    Returns:
      A tuple of the generation and metageneration.
    """
    try:
      res = self.Stat(path)
    except GSNoSuchKey:
      return 0, 0

    return res.generation, res.metageneration

  def Stat(self, path, **kwargs):
    """Stat a GS file, and get detailed information.

    Args:
      path: A GS path for files to Stat. Wildcards are NOT supported.
      kwargs: Flags to pass to DoCommand.

    Returns:
      A GSStatResult object with all fields populated.

    Raises:
      Assorted GSContextException exceptions.
    """
    try:
      res = self.DoCommand(['stat', '--', path], stdout=True, **kwargs)
    except GSCommandError as e:
      # Because the 'gsutil stat' command logs errors itself (instead of
      # raising errors internally like other commands), we have to look
      # for errors ourselves.  See the related bug report here:
      # https://github.com/GoogleCloudPlatform/gsutil/issues/288
      # Example line:
      # No URLs matched gs://bucket/file
      if e.result.error and e.result.error.startswith('No URLs matched'):
        raise GSNoSuchKey('Stat Error: No URLs matched %s.' % path)

      # No idea what this is, so just choke.
      raise

    # In dryrun mode, DoCommand doesn't return an object, so we need to fake
    # out the behavior ourselves.
    if self.dry_run:
      return GSStatResult(
          creation_time=datetime.datetime.now(),
          content_length=0,
          content_type='application/octet-stream',
          hash_crc32c='AAAAAA==',
          hash_md5='',
          etag='',
          generation=0,
          metageneration=0)

    # We expect Stat output like the following. However, the Content-Language
    # line appears to be optional based on how the file in question was
    # created.
    #
    # gs://bucket/path/file:
    #     Creation time:      Sat, 23 Aug 2014 06:53:20 GMT
    #     Content-Language:   en
    #     Content-Length:     74
    #     Content-Type:       application/octet-stream
    #     Hash (crc32c):      BBPMPA==
    #     Hash (md5):         ms+qSYvgI9SjXn8tW/5UpQ==
    #     ETag:               CNCgocbmqMACEAE=
    #     Generation:         1408776800850000
    #     Metageneration:     1

    if not res.output.startswith('gs://'):
      raise GSContextException('Unexpected stat output: %s' % res.output)

    def _GetField(name, optional=False):
      m = re.search(r'%s:\s*(.+)' % re.escape(name), res.output)
      if m:
        return m.group(1)
      elif optional:
        return None
      else:
        raise GSContextException('Field "%s" missing in "%s"' %
                                 (name, res.output))

    return GSStatResult(
        creation_time=datetime.datetime.strptime(
            _GetField('Creation time'), '%a, %d %b %Y %H:%M:%S %Z'),
        content_length=int(_GetField('Content-Length')),
        content_type=_GetField('Content-Type'),
        hash_crc32c=_GetField('Hash (crc32c)'),
        hash_md5=_GetField('Hash (md5)', optional=True),
        etag=_GetField('ETag'),
        generation=int(_GetField('Generation')),
        metageneration=int(_GetField('Metageneration')))

  def Counter(self, path):
    """Return a GSCounter object pointing at a |path| in Google Storage.

    Args:
      path: The path to the counter in Google Storage.
    """
    return GSCounter(self, path)

  def WaitForGsPaths(self, paths, timeout, period=10):
    """Wait until a list of files exist in GS.

    Args:
      paths: The list of files to wait for.
      timeout: Max seconds to wait for file to appear.
      period: How often to check for files while waiting.

    Raises:
      timeout_util.TimeoutError if the timeout is reached.
    """
    # Copy the list of URIs to wait for, so we don't modify the callers context.
    pending_paths = paths[:]

    def _CheckForExistence():
      pending_paths[:] = [x for x in pending_paths if not self.Exists(x)]

    def _Retry(_return_value):
      # Retry, if there are any pending paths left.
      return pending_paths

    timeout_util.WaitForSuccess(_Retry, _CheckForExistence,
                                timeout=timeout, period=period)

  def ContainsWildcard(self, url):
    """Checks whether url_string contains a wildcard.

    Args:
      url: URL string to check.

    Returns:
      True if |url| contains a wildcard.
    """
    return bool(WILDCARD_REGEX.search(url))

  def GetGsNamesWithWait(self, pattern, url, timeout=600, period=10,
                         is_regex_pattern=False):
    """Returns the google storage names specified by the given pattern.

    This method polls Google Storage until the target files specified by the
    pattern is available or until the timeout occurs. Because we may not know
    the exact name of the target files, the method accepts a filename pattern,
    to identify whether a file whose name matches the pattern exists
    (e.g. use pattern '*_full_*' to search for the full payload
    'chromeos_R17-1413.0.0-a1_x86-mario_full_dev.bin'). Returns the name only
    if found before the timeout.

    Warning: GS listing are not perfect, and are eventually consistent. Doing a
    search for file existence is a 'best effort'. Calling code should be aware
    and ready to handle that.

    Args:
      pattern: a path pattern (glob or regex) identifying the files we need.
      url: URL of the Google Storage bucket.
      timeout: how many seconds are we allowed to keep trying.
      period: how many seconds to wait between attempts.
      is_regex_pattern: Whether the pattern is a regex (otherwise a glob).

    Returns:
      The list of files matching the pattern in Google Storage bucket or None
      if the files are not found and hit the timeout_util.TimeoutError.
    """
    def _GetGsName():
      uploaded_list = [os.path.basename(p.url) for p in self.List(url)]

      if is_regex_pattern:
        filter_re = re.compile(pattern)
        matching_names = [f for f in uploaded_list
                          if filter_re.search(f) is not None]
      else:
        matching_names = fnmatch.filter(uploaded_list, pattern)

      return matching_names

    try:
      matching_names = None
      if not (is_regex_pattern or self.ContainsWildcard(pattern)):
        try:
          self.WaitForGsPaths(['%s/%s' % (url, pattern)], timeout)
          return [os.path.basename(pattern)]
        except GSCommandError:
          pass

      if not matching_names:
        matching_names = timeout_util.WaitForSuccess(
            lambda x: not x, _GetGsName, timeout=timeout, period=period)

      logging.debug('matching_names=%s, is_regex_pattern=%r',
                    matching_names, is_regex_pattern)
      return matching_names
    except timeout_util.TimeoutError:
      return None


def _FirstMatch(predicate, elems):
  """Returns the first element matching the given |predicate|.

  Args:
    predicate: A function which takes an element and returns a bool
    elems: A sequence of elements.
  """
  matches = [x for x in elems if predicate(x)]
  return matches[0] if matches else None


def _FirstSubstring(superstring, haystack):
  """Returns the first elem of |haystack| which is a substring of |superstring|.

  Args:
    superstring: A string to search for substrings of.
    haystack: A sequence of strings to search through.
  """
  return _FirstMatch(lambda s: s in superstring, haystack)


@contextlib.contextmanager
def TemporaryURL(prefix):
  """Context manager to generate a random URL.

  At the end, the URL will be deleted.
  """
  url = '%s/chromite-temp/%s/%s/%s' % (constants.TRASH_BUCKET, prefix,
                                       getpass.getuser(),
                                       cros_build_lib.GetRandomString())
  ctx = GSContext()
  ctx.Remove(url, ignore_missing=True, recursive=True)
  try:
    yield url
  finally:
    ctx.Remove(url, ignore_missing=True, recursive=True)