gerrit_util.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.

"""
Utilities for requesting information for a gerrit server via https.

https://gerrit-review.googlesource.com/Documentation/rest-api.html
"""

import base64
import cookielib
import httplib
import json
import logging
import netrc
import os
import re
import socket
import stat
import sys
import time
import urllib
import urlparse
from cStringIO import StringIO


LOGGER = logging.getLogger()
TRY_LIMIT = 5


# Controls the transport protocol used to communicate with gerrit.
# This is parameterized primarily to enable GerritTestCase.
GERRIT_PROTOCOL = 'https'


# Processing comments in "netrc" can trigger a bug in Windows.
# See crbug.com/664664
class safeNetrc(netrc.netrc):
  # pylint: disable=redefined-builtin
  def __init__(self, file=None):
    self._orig_parse, self._parse = self._parse, self._safe_parse
    netrc.netrc.__init__(self, file=file)

  # pylint: disable=redefined-builtin
  def _safe_parse(self, file, fp, default_netrc):
    # Buffer the file.
    sio = StringIO(''.join(l for l in fp
                           if l.strip() and not l.strip().startswith('#')))
    return self._orig_parse(file, sio, default_netrc)


class GerritError(Exception):
  """Exception class for errors commuicating with the gerrit-on-borg service."""
  def __init__(self, http_status, *args, **kwargs):
    super(GerritError, self).__init__(*args, **kwargs)
    self.http_status = http_status
    self.message = '(%d) %s' % (self.http_status, self.message)


class GerritAuthenticationError(GerritError):
  """Exception class for authentication errors during Gerrit communication."""


def _QueryString(param_dict, first_param=None):
  """Encodes query parameters in the key:val[+key:val...] format specified here:

  https://gerrit-review.googlesource.com/Documentation/rest-api-changes.html#list-changes
  """
  q = [urllib.quote(first_param)] if first_param else []
  q.extend(['%s:%s' % (key, val) for key, val in param_dict.iteritems()])
  return '+'.join(q)


def GetConnectionClass(protocol=None):
  if protocol is None:
    protocol = GERRIT_PROTOCOL
  if protocol == 'https':
    return httplib.HTTPSConnection
  elif protocol == 'http':
    return httplib.HTTPConnection
  else:
    raise RuntimeError(
        "Don't know how to work with protocol '%s'" % protocol)


class Authenticator(object):
  """Base authenticator class for authenticator implementations to subclass."""

  def get_auth_header(self, host):
    raise NotImplementedError()

  @staticmethod
  def get():
    """Returns: (Authenticator) The identified Authenticator to use.

    Probes the local system and its environment and identifies the
    Authenticator instance to use.
    """
    if GceAuthenticator.is_gce():
      return GceAuthenticator()
    return CookiesAuthenticator()


class CookiesAuthenticator(Authenticator):
  """Authenticator implementation that uses ".netrc" or ".gitcookies" for token.

  Expected case for developer workstations.
  """

  def __init__(self):
    self.netrc = self._get_netrc()
    self.gitcookies = self._get_gitcookies()

  @classmethod
  def get_new_password_message(cls, host):
    assert not host.startswith('http')
    # Assume *.googlesource.com pattern.
    parts = host.split('.')
    if not parts[0].endswith('-review'):
      parts[0] += '-review'
    url = 'https://%s/new-password' % ('.'.join(parts))
    return 'You can (re)generate your credentails by visiting %s' % url

  @classmethod
  def get_netrc_path(cls):
    path = '_netrc' if sys.platform.startswith('win') else '.netrc'
    return os.path.expanduser(os.path.join('~', path))

  @classmethod
  def _get_netrc(cls):
    path = cls.get_netrc_path()
    if not os.path.exists(path):
      return safeNetrc(os.devnull)

    try:
      return safeNetrc(path)
    except IOError:
      print >> sys.stderr, 'WARNING: Could not read netrc file %s' % path
      return safeNetrc(os.devnull)
    except netrc.NetrcParseError:
      st = os.stat(path)
      if st.st_mode & (stat.S_IRWXG | stat.S_IRWXO):
        print >> sys.stderr, (
            'WARNING: netrc file %s cannot be used because its file '
            'permissions are insecure.  netrc file permissions should be '
            '600.' % path)
      else:
        print >> sys.stderr, ('ERROR: Cannot use netrc file %s due to a '
                              'parsing error.' % path)
        raise
      return safeNetrc(os.devnull)

  @classmethod
  def get_gitcookies_path(cls):
    return os.path.join(os.environ['HOME'], '.gitcookies')

  @classmethod
  def _get_gitcookies(cls):
    gitcookies = {}
    path = cls.get_gitcookies_path()
    if not os.path.exists(path):
      return gitcookies

    try:
      f = open(path, 'rb')
    except IOError:
      return gitcookies

    with f:
      for line in f:
        try:
          fields = line.strip().split('\t')
          if line.strip().startswith('#') or len(fields) != 7:
            continue
          domain, xpath, key, value = fields[0], fields[2], fields[5], fields[6]
          if xpath == '/' and key == 'o':
            login, secret_token = value.split('=', 1)
            gitcookies[domain] = (login, secret_token)
        except (IndexError, ValueError, TypeError) as exc:
          logging.warning(exc)

    return gitcookies

  def get_auth_header(self, host):
    auth = None
    for domain, creds in self.gitcookies.iteritems():
      if cookielib.domain_match(host, domain):
        auth = (creds[0], None, creds[1])
        break

    if not auth:
      auth = self.netrc.authenticators(host)
    if auth:
      return 'Basic %s' % (base64.b64encode('%s:%s' % (auth[0], auth[2])))
    return None

# Backwards compatibility just in case somebody imports this outside of
# depot_tools.
NetrcAuthenticator = CookiesAuthenticator


class GceAuthenticator(Authenticator):
  """Authenticator implementation that uses GCE metadata service for token.
  """

  _INFO_URL = 'http://metadata.google.internal'
  _ACQUIRE_URL = ('http://metadata/computeMetadata/v1/instance/'
                  'service-accounts/default/token')
  _ACQUIRE_HEADERS = {"Metadata-Flavor": "Google"}

  _cache_is_gce = None
  _token_cache = None
  _token_expiration = None

  @classmethod
  def is_gce(cls):
    if cls._cache_is_gce is None:
      cls._cache_is_gce = cls._test_is_gce()
    return cls._cache_is_gce

  @classmethod
  def _test_is_gce(cls):
    # Based on https://cloud.google.com/compute/docs/metadata#runninggce
    try:
      resp = cls._get(cls._INFO_URL)
    except socket.error:
      # Could not resolve URL.
      return False
    return resp.getheader('Metadata-Flavor', None) == 'Google'

  @staticmethod
  def _get(url, **kwargs):
    next_delay_sec = 1
    for i in xrange(TRY_LIMIT):
      if i > 0:
        # Retry server error status codes.
        LOGGER.info('Encountered server error; retrying after %d second(s).',
                    next_delay_sec)
        time.sleep(next_delay_sec)
        next_delay_sec *= 2

      p = urlparse.urlparse(url)
      c = GetConnectionClass(protocol=p.scheme)(p.netloc)
      c.request('GET', url, **kwargs)
      resp = c.getresponse()
      LOGGER.debug('GET [%s] #%d/%d (%d)', url, i+1, TRY_LIMIT, resp.status)
      if resp.status < httplib.INTERNAL_SERVER_ERROR:
        return resp


  @classmethod
  def _get_token_dict(cls):
    if cls._token_cache:
      # If it expires within 25 seconds, refresh.
      if cls._token_expiration < time.time() - 25:
        return cls._token_cache

    resp = cls._get(cls._ACQUIRE_URL, headers=cls._ACQUIRE_HEADERS)
    if resp.status != httplib.OK:
      return None
    cls._token_cache = json.load(resp)
    cls._token_expiration = cls._token_cache['expires_in'] + time.time()
    return cls._token_cache

  def get_auth_header(self, _host):
    token_dict = self._get_token_dict()
    if not token_dict:
      return None
    return '%(token_type)s %(access_token)s' % token_dict



def CreateHttpConn(host, path, reqtype='GET', headers=None, body=None):
  """Opens an https connection to a gerrit service, and sends a request."""
  headers = headers or {}
  bare_host = host.partition(':')[0]

  auth = Authenticator.get().get_auth_header(bare_host)
  if auth:
    headers.setdefault('Authorization', auth)
  else:
    LOGGER.debug('No authorization found for %s.' % bare_host)

  url = path
  if not url.startswith('/'):
    url = '/' + url
  if 'Authorization' in headers and not url.startswith('/a/'):
    url = '/a%s' % url

  if body:
    body = json.JSONEncoder().encode(body)
    headers.setdefault('Content-Type', 'application/json')
  if LOGGER.isEnabledFor(logging.DEBUG):
    LOGGER.debug('%s %s://%s%s' % (reqtype, GERRIT_PROTOCOL, host, url))
    for key, val in headers.iteritems():
      if key == 'Authorization':
        val = 'HIDDEN'
      LOGGER.debug('%s: %s' % (key, val))
    if body:
      LOGGER.debug(body)
  conn = GetConnectionClass()(host)
  conn.req_host = host
  conn.req_params = {
      'url': url,
      'method': reqtype,
      'headers': headers,
      'body': body,
  }
  conn.request(**conn.req_params)
  return conn


def ReadHttpResponse(conn, expect_status=200, ignore_404=True):
  """Reads an http response from a connection into a string buffer.

  Args:
    conn: An HTTPSConnection or HTTPConnection created by CreateHttpConn, above.
    expect_status: Success is indicated by this status in the response.
    ignore_404: For many requests, gerrit-on-borg will return 404 if the request
                doesn't match the database contents.  In most such cases, we
                want the API to return None rather than raise an Exception.
  Returns: A string buffer containing the connection's reply.
  """

  sleep_time = 0.5
  for idx in range(TRY_LIMIT):
    response = conn.getresponse()

    # Check if this is an authentication issue.
    www_authenticate = response.getheader('www-authenticate')
    if (response.status in (httplib.UNAUTHORIZED, httplib.FOUND) and
        www_authenticate):
      auth_match = re.search('realm="([^"]+)"', www_authenticate, re.I)
      host = auth_match.group(1) if auth_match else conn.req_host
      reason = ('Authentication failed. Please make sure your .netrc file '
                'has credentials for %s' % host)
      raise GerritAuthenticationError(response.status, reason)

    # If response.status < 500 then the result is final; break retry loop.
    if response.status < 500:
      break
    # A status >=500 is assumed to be a possible transient error; retry.
    http_version = 'HTTP/%s' % ('1.1' if response.version == 11 else '1.0')
    msg = (
        'A transient error occurred while querying %s:\n'
        '%s %s %s\n'
        '%s %d %s' % (
            conn.host, conn.req_params['method'], conn.req_params['url'],
            http_version, http_version, response.status, response.reason))
    if TRY_LIMIT - idx > 1:
      msg += '\n... will retry %d more times.' % (TRY_LIMIT - idx - 1)
      time.sleep(sleep_time)
      sleep_time = sleep_time * 2
      req_host = conn.req_host
      req_params = conn.req_params
      conn = GetConnectionClass()(req_host)
      conn.req_host = req_host
      conn.req_params = req_params
      conn.request(**req_params)
    LOGGER.warn(msg)
  if ignore_404 and response.status == 404:
    return StringIO()
  if response.status != expect_status:
    reason = '%s: %s' % (response.reason, response.read())
    raise GerritError(response.status, reason)
  return StringIO(response.read())


def ReadHttpJsonResponse(conn, expect_status=200, ignore_404=True):
  """Parses an https response as json."""
  fh = ReadHttpResponse(
      conn, expect_status=expect_status, ignore_404=ignore_404)
  # The first line of the response should always be: )]}'
  s = fh.readline()
  if s and s.rstrip() != ")]}'":
    raise GerritError(200, 'Unexpected json output: %s' % s)
  s = fh.read()
  if not s:
    return None
  return json.loads(s)


def QueryChanges(host, param_dict, first_param=None, limit=None, o_params=None,
                 sortkey=None):
  """
  Queries a gerrit-on-borg server for changes matching query terms.

  Args:
    param_dict: A dictionary of search parameters, as documented here:
        http://gerrit-documentation.googlecode.com/svn/Documentation/2.6/user-search.html
    first_param: A change identifier
    limit: Maximum number of results to return.
    o_params: A list of additional output specifiers, as documented here:
        https://gerrit-review.googlesource.com/Documentation/rest-api-changes.html#list-changes
  Returns:
    A list of json-decoded query results.
  """
  # Note that no attempt is made to escape special characters; YMMV.
  if not param_dict and not first_param:
    raise RuntimeError('QueryChanges requires search parameters')
  path = 'changes/?q=%s' % _QueryString(param_dict, first_param)
  if sortkey:
    path = '%s&N=%s' % (path, sortkey)
  if limit:
    path = '%s&n=%d' % (path, limit)
  if o_params:
    path = '%s&%s' % (path, '&'.join(['o=%s' % p for p in o_params]))
  # Don't ignore 404; a query should always return a list, even if it's empty.
  return ReadHttpJsonResponse(CreateHttpConn(host, path), ignore_404=False)


def GenerateAllChanges(host, param_dict, first_param=None, limit=500,
                       o_params=None, sortkey=None):
  """
  Queries a gerrit-on-borg server for all the changes matching the query terms.

  A single query to gerrit-on-borg is limited on the number of results by the
  limit parameter on the request (see QueryChanges) and the server maximum
  limit. This function uses the "_more_changes" and "_sortkey" attributes on
  the returned changes to iterate all of them making multiple queries to the
  server, regardless the query limit.

  Args:
    param_dict, first_param: Refer to QueryChanges().
    limit: Maximum number of requested changes per query.
    o_params: Refer to QueryChanges().
    sortkey: The value of the "_sortkey" attribute where starts from. None to
        start from the first change.

  Returns:
    A generator object to the list of returned changes, possibly unbound.
  """
  more_changes = True
  while more_changes:
    page = QueryChanges(host, param_dict, first_param, limit, o_params, sortkey)
    for cl in page:
      yield cl

    more_changes = [cl for cl in page if '_more_changes' in cl]
    if len(more_changes) > 1:
      raise GerritError(
          200,
          'Received %d changes with a _more_changes attribute set but should '
          'receive at most one.' % len(more_changes))
    if more_changes:
      sortkey = more_changes[0]['_sortkey']


def MultiQueryChanges(host, param_dict, change_list, limit=None, o_params=None,
                      sortkey=None):
  """Initiate a query composed of multiple sets of query parameters."""
  if not change_list:
    raise RuntimeError(
        "MultiQueryChanges requires a list of change numbers/id's")
  q = ['q=%s' % '+OR+'.join([urllib.quote(str(x)) for x in change_list])]
  if param_dict:
    q.append(_QueryString(param_dict))
  if limit:
    q.append('n=%d' % limit)
  if sortkey:
    q.append('N=%s' % sortkey)
  if o_params:
    q.extend(['o=%s' % p for p in o_params])
  path = 'changes/?%s' % '&'.join(q)
  try:
    result = ReadHttpJsonResponse(CreateHttpConn(host, path), ignore_404=False)
  except GerritError as e:
    msg = '%s:\n%s' % (e.message, path)
    raise GerritError(e.http_status, msg)
  return result


def GetGerritFetchUrl(host):
  """Given a gerrit host name returns URL of a gerrit instance to fetch from."""
  return '%s://%s/' % (GERRIT_PROTOCOL, host)


def GetChangePageUrl(host, change_number):
  """Given a gerrit host name and change number, return change page url."""
  return '%s://%s/#/c/%d/' % (GERRIT_PROTOCOL, host, change_number)


def GetChangeUrl(host, change):
  """Given a gerrit host name and change id, return an url for the change."""
  return '%s://%s/a/changes/%s' % (GERRIT_PROTOCOL, host, change)


def GetChange(host, change):
  """Query a gerrit server for information about a single change."""
  path = 'changes/%s' % change
  return ReadHttpJsonResponse(CreateHttpConn(host, path))


def GetChangeDetail(host, change, o_params=None, ignore_404=True):
  """Query a gerrit server for extended information about a single change."""
  # TODO(tandrii): cahnge ignore_404 to False by default.
  path = 'changes/%s/detail' % change
  if o_params:
    path += '?%s' % '&'.join(['o=%s' % p for p in o_params])
  return ReadHttpJsonResponse(CreateHttpConn(host, path), ignore_404=ignore_404)


def GetChangeCommit(host, change, revision='current'):
  """Query a gerrit server for a revision associated with a change."""
  path = 'changes/%s/revisions/%s/commit?links' % (change, revision)
  return ReadHttpJsonResponse(CreateHttpConn(host, path))


def GetChangeDescriptionFromGitiles(url, revision):
  """Query Gitiles for actual commit message for a given url and ref.

  url must be obtained from call to GetChangeDetail for a specific
  revision (patchset) under 'fetch' key.
  """
  parsed = urlparse.urlparse(url)
  path = '%s/+/%s?format=json' % (parsed.path, revision)
  # Note: Gerrit instances that Chrome infrastructure uses thus far have all
  # enabled Gitiles, which allowes us to execute this call. This isn't true for
  # all Gerrit instances out there. Thus, if line below fails, consider adding a
  # fallback onto actually fetching ref from remote using pure git.
  return ReadHttpJsonResponse(CreateHttpConn(parsed.netloc, path))['message']


def GetChangeCurrentRevision(host, change):
  """Get information about the latest revision for a given change."""
  return QueryChanges(host, {}, change, o_params=('CURRENT_REVISION',))


def GetChangeRevisions(host, change):
  """Get information about all revisions associated with a change."""
  return QueryChanges(host, {}, change, o_params=('ALL_REVISIONS',))


def GetChangeReview(host, change, revision=None):
  """Get the current review information for a change."""
  if not revision:
    jmsg = GetChangeRevisions(host, change)
    if not jmsg:
      return None
    elif len(jmsg) > 1:
      raise GerritError(200, 'Multiple changes found for ChangeId %s.' % change)
    revision = jmsg[0]['current_revision']
  path = 'changes/%s/revisions/%s/review'
  return ReadHttpJsonResponse(CreateHttpConn(host, path))


def AbandonChange(host, change, msg=''):
  """Abandon a gerrit change."""
  path = 'changes/%s/abandon' % change
  body = {'message': msg} if msg else {}
  conn = CreateHttpConn(host, path, reqtype='POST', body=body)
  return ReadHttpJsonResponse(conn, ignore_404=False)


def RestoreChange(host, change, msg=''):
  """Restore a previously abandoned change."""
  path = 'changes/%s/restore' % change
  body = {'message': msg} if msg else {}
  conn = CreateHttpConn(host, path, reqtype='POST', body=body)
  return ReadHttpJsonResponse(conn, ignore_404=False)


def SubmitChange(host, change, wait_for_merge=True):
  """Submits a gerrit change via Gerrit."""
  path = 'changes/%s/submit' % change
  body = {'wait_for_merge': wait_for_merge}
  conn = CreateHttpConn(host, path, reqtype='POST', body=body)
  return ReadHttpJsonResponse(conn, ignore_404=False)


def HasPendingChangeEdit(host, change):
  conn = CreateHttpConn(host, 'changes/%s/edit' % change)
  try:
    ReadHttpResponse(conn, ignore_404=False)
  except GerritError as e:
    # On success, gerrit returns status 204; anything else is an error.
    if e.http_status != 204:
      raise
    return False
  else:
    return True


def DeletePendingChangeEdit(host, change):
  conn = CreateHttpConn(host, 'changes/%s/edit' % change, reqtype='DELETE')
  try:
    ReadHttpResponse(conn, ignore_404=False)
  except GerritError as e:
    # On success, gerrit returns status 204; if the edit was already deleted it
    # returns 404.  Anything else is an error.
    if e.http_status not in (204, 404):
      raise


def SetCommitMessage(host, change, description):
  """Updates a commit message."""
  # First, edit the commit message in a draft.
  path = 'changes/%s/edit:message' % change
  body = {'message': description}
  conn = CreateHttpConn(host, path, reqtype='PUT', body=body)
  try:
    ReadHttpResponse(conn, ignore_404=False)
  except GerritError as e:
    # On success, gerrit returns status 204; anything else is an error.
    if e.http_status != 204:
      raise
  else:
    raise GerritError(
        'Unexpectedly received a 200 http status while editing message in '
        'change %s' % change)

  # And then publish it.
  path = 'changes/%s/edit:publish' % change
  conn = CreateHttpConn(host, path, reqtype='POST', body={})
  try:
    ReadHttpResponse(conn, ignore_404=False)
  except GerritError as e:
    # On success, gerrit returns status 204; anything else is an error.
    if e.http_status != 204:
      raise
  else:
    raise GerritError(
        'Unexpectedly received a 200 http status while publishing message '
        'change in %s' % change)


def GetReviewers(host, change):
  """Get information about all reviewers attached to a change."""
  path = 'changes/%s/reviewers' % change
  return ReadHttpJsonResponse(CreateHttpConn(host, path))


def GetReview(host, change, revision):
  """Get review information about a specific revision of a change."""
  path = 'changes/%s/revisions/%s/review' % (change, revision)
  return ReadHttpJsonResponse(CreateHttpConn(host, path))


def AddReviewers(host, change, add=None, is_reviewer=True):
  """Add reviewers to a change."""
  errors = None
  if not add:
    return None
  if isinstance(add, basestring):
    add = (add,)
  path = 'changes/%s/reviewers' % change
  for r in add:
    state = 'REVIEWER' if is_reviewer else 'CC'
    body = {
      'reviewer': r,
      'state': state,
    }
    try:
      conn = CreateHttpConn(host, path, reqtype='POST', body=body)
      _ = ReadHttpJsonResponse(conn, ignore_404=False)
    except GerritError as e:
      if e.http_status == 422:  # "Unprocessable Entity"
        LOGGER.warn('Failed to add "%s" as a %s' % (r, state.lower()))
        errors = True
      else:
        raise
  return errors


def RemoveReviewers(host, change, remove=None):
  """Remove reveiewers from a change."""
  if not remove:
    return
  if isinstance(remove, basestring):
    remove = (remove,)
  for r in remove:
    path = 'changes/%s/reviewers/%s' % (change, r)
    conn = CreateHttpConn(host, path, reqtype='DELETE')
    try:
      ReadHttpResponse(conn, ignore_404=False)
    except GerritError as e:
      # On success, gerrit returns status 204; anything else is an error.
      if e.http_status != 204:
        raise
    else:
      raise GerritError(
          'Unexpectedly received a 200 http status while deleting reviewer "%s"'
          ' from change %s' % (r, change))


def SetReview(host, change, msg=None, labels=None, notify=None):
  """Set labels and/or add a message to a code review."""
  if not msg and not labels:
    return
  path = 'changes/%s/revisions/current/review' % change
  body = {}
  if msg:
    body['message'] = msg
  if labels:
    body['labels'] = labels
  if notify:
    body['notify'] = notify
  conn = CreateHttpConn(host, path, reqtype='POST', body=body)
  response = ReadHttpJsonResponse(conn)
  if labels:
    for key, val in labels.iteritems():
      if ('labels' not in response or key not in response['labels'] or
          int(response['labels'][key] != int(val))):
        raise GerritError(200, 'Unable to set "%s" label on change %s.' % (
            key, change))


def ResetReviewLabels(host, change, label, value='0', message=None,
                      notify=None):
  """Reset the value of a given label for all reviewers on a change."""
  # This is tricky, because we want to work on the "current revision", but
  # there's always the risk that "current revision" will change in between
  # API calls.  So, we check "current revision" at the beginning and end; if
  # it has changed, raise an exception.
  jmsg = GetChangeCurrentRevision(host, change)
  if not jmsg:
    raise GerritError(
        200, 'Could not get review information for change "%s"' % change)
  value = str(value)
  revision = jmsg[0]['current_revision']
  path = 'changes/%s/revisions/%s/review' % (change, revision)
  message = message or (
      '%s label set to %s programmatically.' % (label, value))
  jmsg = GetReview(host, change, revision)
  if not jmsg:
    raise GerritError(200, 'Could not get review information for revison %s '
                   'of change %s' % (revision, change))
  for review in jmsg.get('labels', {}).get(label, {}).get('all', []):
    if str(review.get('value', value)) != value:
      body = {
          'message': message,
          'labels': {label: value},
          'on_behalf_of': review['_account_id'],
      }
      if notify:
        body['notify'] = notify
      conn = CreateHttpConn(
          host, path, reqtype='POST', body=body)
      response = ReadHttpJsonResponse(conn)
      if str(response['labels'][label]) != value:
        username = review.get('email', jmsg.get('name', ''))
        raise GerritError(200, 'Unable to set %s label for user "%s"'
                       ' on change %s.' % (label, username, change))
  jmsg = GetChangeCurrentRevision(host, change)
  if not jmsg:
    raise GerritError(
        200, 'Could not get review information for change "%s"' % change)
  elif jmsg[0]['current_revision'] != revision:
    raise GerritError(200, 'While resetting labels on change "%s", '
                   'a new patchset was uploaded.' % change)