# -*- coding: utf-8 -*-
# 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
"""

from __future__ import print_function

import base64
import datetime
import json
import netrc
import os
import re
import socket
import sys
import warnings
from cStringIO import StringIO

import httplib2
try:
  from oauth2client import gce
except ImportError:  # Newer oauth2client versions put it in .contrib
  # pylint: disable=import-error,no-name-in-module
  from oauth2client.contrib import gce
import six
from six.moves import html_parser as HTMLParser
from six.moves import http_client as httplib
from six.moves import http_cookiejar as cookielib
from six.moves import urllib

from chromite.lib import constants
from chromite.lib import cros_logging as logging
from chromite.lib import git
from chromite.lib import retry_util
from chromite.lib import timeout_util
from chromite.lib import cros_build_lib
from chromite.utils import memoize


_GAE_VERSION = 'GAE_VERSION'


class ErrorParser(HTMLParser.HTMLParser):
  """Class to parse GOB error message reported as HTML.

  Only data inside <div id='af-error-container'> section is retrieved from the
  GOB error message. Retrieved data is processed as follows:

  - newlines are removed
  - each <br> tag is replaced with '\n'
  - each <p> tag is replaced with '\n\n'
  """

  def __init__(self):
    HTMLParser.HTMLParser.__init__(self)
    self.in_div = False
    self.err_data = ''

  def handle_starttag(self, tag, attrs):
    tag_id = [x[1] for x in attrs if x[0] == 'id']
    if tag == 'div' and tag_id and tag_id[0] == 'af-error-container':
      self.in_div = True
      return

    if self.in_div:
      if tag == 'p':
        self.err_data += '\n\n'
        return

      if tag == 'br':
        self.err_data += '\n'
        return

  def handle_endtag(self, tag):
    if tag == 'div':
      self.in_div = False

  def handle_data(self, data):
    if self.in_div:
      self.err_data += data.replace('\n', '')

  def ParsedDiv(self):
    return self.err_data.strip()


@memoize.Memoize
def _GetNetRC():
  try:
    return netrc.netrc(None)
  except (IOError, netrc.NetrcParseError):
    try:
      return netrc.netrc(os.devnull)
    except IOError:
      return None


def _NetRCAuthenticators(host):
  """Returns the authenticators, if any, for the given |host|.

  Args:
    host: A hostname
  """
  net_rc = _GetNetRC()
  if net_rc:
    return net_rc.authenticators(host)


@memoize.Memoize
def _GetAppCredentials():
  """Returns the singleton Appengine credentials for gerrit code review."""
  return gce.AppAssertionCredentials(
      scope='https://www.googleapis.com/auth/gerritcodereview')


TRY_LIMIT = 11
SLEEP = 0.5
REQUEST_TIMEOUT_SECONDS = 120  # 2 minutes.

# Controls the transport protocol used to communicate with Gerrit servers using
# git. This is parameterized primarily to enable cros_test_lib.GerritTestCase.
GIT_PROTOCOL = 'https'

# The GOB conflict errors which could be ignorable.
GOB_CONFLICT_ERRORS = (
    r'change is closed',
    r'Cannot reduce vote on labels for closed change'
)

GOB_CONFLICT_ERRORS_RE = re.compile('|'.join(GOB_CONFLICT_ERRORS),
                                    re.IGNORECASE)

GOB_ERROR_REASON_CLOSED_CHANGE = 'CLOSED CHANGE'


class GOBError(Exception):
  """Exception class for errors commuicating with the gerrit-on-borg service."""
  def __init__(self, *args, **kwargs):
    super(GOBError, self).__init__(*args)
    self.http_status = kwargs.pop('http_status', None)
    self.reason = kwargs.pop('reason', None)

    if self.http_status is not None:
      self.message += '(http_status): %d' % self.http_status
    if self.reason is not None:
      self.message += '(reason): %s' % self.reason

class InternalGOBError(GOBError):
  """Exception class for GOB errors with status >= 500"""


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.parse.quote(first_param)] if first_param else []
  q.extend(['%s:%s' % (key, val) for key, val in param_dict.items()])
  return '+'.join(q)


def GetCookies(host, path, cookie_paths=None):
  """Returns cookies that should be set on a request.

  Used by CreateHttpConn for any requests that do not already specify a Cookie
  header. All requests made by this library are HTTPS.

  Args:
    host: The hostname of the Gerrit service.
    path: The path on the Gerrit service, already including /a/ if applicable.
    cookie_paths: Files to look in for cookies. Defaults to looking in the
      standard places where GoB places cookies.

  Returns:
    A dict of cookie name to value, with no URL encoding applied.
  """
  cookies = {}
  if cookie_paths is None:
    cookie_paths = (constants.GOB_COOKIE_PATH, constants.GITCOOKIES_PATH)
  for cookie_path in cookie_paths:
    if os.path.isfile(cookie_path):
      with open(cookie_path) as f:
        for line in f:
          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 cookielib.domain_match(host, domain) and path.startswith(xpath):
            cookies[key] = value
  return cookies


def CreateHttpConn(host, path, reqtype='GET', headers=None, body=None):
  """Opens an https connection to a gerrit service, and sends a request."""
  path = '/a/' + path.lstrip('/')
  headers = headers or {}
  bare_host = host.partition(':')[0]
  auth = _NetRCAuthenticators(bare_host)
  if auth:
    headers.setdefault(
        'Authorization',
        'Basic %s' % base64.b64encode('%s:%s' % (auth[0], auth[2])))
  elif _InAppengine():
    # TODO(phobbs) how can we choose to only run this on GCE / AppEngine?
    credentials = _GetAppCredentials()
    try:
      headers.setdefault(
          'Authorization',
          'Bearer %s' % credentials.get_access_token().access_token)
    except gce.HttpAccessTokenRefreshError as e:
      logging.debug('Failed to retreive gce access token: %s', e)
    # Not in an Appengine or GCE environment.
    except httplib2.ServerNotFoundError as e:
      pass

  if not 'Authorization' in headers:
    logging.debug('No netrc file or Appengine credentials found.')
  if 'Cookie' not in headers:
    cookies = GetCookies(host, path)
    headers['Cookie'] = '; '.join('%s=%s' % (n, v) for n, v in cookies.items())

  if 'User-Agent' not in headers:
    # We may not be in a git repository.
    try:
      version = git.GetGitRepoRevision(
          os.path.dirname(os.path.realpath(__file__)))
    except cros_build_lib.RunCommandError:
      version = 'unknown'
    headers['User-Agent'] = ' '.join((
        'chromite.lib.gob_util',
        os.path.basename(sys.argv[0]),
        version,
    ))

  if body:
    body = json.JSONEncoder().encode(body)
    headers.setdefault('Content-Type', 'application/json')
  if logging.getLogger().isEnabledFor(logging.DEBUG):
    logging.debug('%s https://%s%s', reqtype, host, path)
    for key, val in headers.items():
      if key.lower() in ('authorization', 'cookie'):
        val = 'HIDDEN'
      logging.debug('%s: %s', key, val)
    if body:
      logging.debug(body)
  conn = httplib.HTTPSConnection(host)
  conn.req_host = host
  conn.req_params = {
      'url': path,
      'method': reqtype,
      'headers': headers,
      'body': body,
  }
  conn.request(**conn.req_params)
  return conn


def _InAppengine():
  """Returns whether we're in the Appengine environment."""
  return _GAE_VERSION in os.environ


def FetchUrl(host, path, reqtype='GET', headers=None, body=None,
             ignore_204=False, ignore_404=True):
  """Fetches the http response from the specified URL into a string buffer.

  Args:
    host: The hostname of the Gerrit service.
    path: The path on the Gerrit service. This will be prefixed with '/a'
          automatically.
    reqtype: The request type. Can be GET or POST.
    headers: A mapping of extra HTTP headers to pass in with the request.
    body: A string of data to send after the headers are finished.
    ignore_204: for some requests gerrit-on-borg will return 204 to confirm
                proper processing of the request. When processing responses to
                these requests we should expect this status.
    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.
  """
  @timeout_util.TimeoutDecorator(REQUEST_TIMEOUT_SECONDS)
  def _FetchUrlHelper():
    err_prefix = 'A transient error occured while querying %s:\n' % (host,)
    try:
      conn = CreateHttpConn(host, path, reqtype=reqtype, headers=headers,
                            body=body)
      response = conn.getresponse()
    except socket.error as ex:
      logging.warning('%s%s', err_prefix, str(ex))
      raise

    # Normal/good responses.
    response_body = response.read()
    if response.status == 204 and ignore_204:
      # This exception is used to confirm expected response status.
      raise GOBError(http_status=response.status, reason=response.reason)
    if response.status == 404 and ignore_404:
      return StringIO()
    elif response.status == 200:
      return StringIO(response_body)

    # Bad responses.
    logging.debug('response msg:\n%s', response.msg)
    http_version = 'HTTP/%s' % ('1.1' if response.version == 11 else '1.0')
    ep = ErrorParser()
    ep.feed(str(response_body))
    ep.close()
    parsed_div = ep.ParsedDiv()

    msg = ('%s %s %s\n%s %d %s\nResponse body: %r' %
           (reqtype, conn.req_params['url'], http_version,
            http_version, response.status, response.reason,
            response_body))

    # Ones we can retry.
    if response.status >= 500:
      # A status >=500 is assumed to be a possible transient error; retry.
      logging.warning('%s%s', err_prefix, msg)
      raise InternalGOBError(
          http_status=response.status,
          reason=response.reason)

    # Ones we cannot retry.
    home = os.environ.get('HOME', '~')
    url = 'https://%s/new-password' % host
    if response.status in (302, 303, 307):
      err_prefix = ('Redirect found; missing/bad %s/.netrc credentials or '
                    'permissions (0600)?\n See %s' % (home, url))
    elif response.status in (400,):
      err_prefix = 'Permission error; talk to the admins of the GoB instance'
    elif response.status in (401,):
      err_prefix = ('Authorization error; missing/bad %s/.netrc credentials or '
                    'permissions (0600)?\n See %s' % (home, url))
    elif response.status in (422,):
      err_prefix = ('Bad request body?')

    logging.warning(err_prefix)

    # If GOB output contained expected error message, reduce log visibility of
    # raw GOB output reported below.
    if parsed_div:
      logging.warning('GOB Error:\n%s', parsed_div)
      logging_function = logging.debug
    else:
      logging_function = logging.warning

    logging_function(msg)
    if response.status >= 400:
      # The 'X-ErrorId' header is set only on >= 400 response code.
      logging_function('X-ErrorId: %s', response.getheader('X-ErrorId'))

    try:
      logging.warning('conn.sock.getpeername(): %s', conn.sock.getpeername())
    except AttributeError:
      logging.warning('peer name unavailable')

    if response.status == httplib.CONFLICT:
      # 409 conflict
      if GOB_CONFLICT_ERRORS_RE.search(response_body):
        raise GOBError(
            http_status=response.status,
            reason=GOB_ERROR_REASON_CLOSED_CHANGE)
      else:
        raise GOBError(http_status=response.status, reason=response.reason)
    else:
      raise GOBError(http_status=response.status, reason=response.reason)

  return retry_util.RetryException(
      (socket.error, InternalGOBError, timeout_util.TimeoutError),
      TRY_LIMIT,
      _FetchUrlHelper, sleep=SLEEP, backoff_factor=2)


def FetchUrlJson(*args, **kwargs):
  """Fetch the specified URL and parse it as JSON.

  See FetchUrl for arguments.
  """
  fh = FetchUrl(*args, **kwargs)
  # The first line of the response should always be: )]}'
  s = fh.readline()
  if s and s.rstrip() != ")]}'":
    raise GOBError(http_status=200, reason='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,
                 start=None):
  """Queries a gerrit-on-borg server for changes matching query terms.

  Args:
    host: The Gerrit server hostname.
    param_dict: A dictionary of search parameters, as documented here:
        https://gerrit-review.googlesource.com/Documentation/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
    start: Offset in the result set to start at.

  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 start:
    path = '%s&S=%d' % (path, start)
  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 FetchUrlJson(host, path, ignore_404=False)


def MultiQueryChanges(host, param_dict, change_list, limit=None, o_params=None,
                      start=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.parse.quote(str(x)) for x in change_list)]
  if param_dict:
    q.append(_QueryString(param_dict))
  if limit:
    q.append('n=%d' % limit)
  if start:
    q.append('S=%s' % start)
  if o_params:
    q.extend(['o=%s' % p for p in o_params])
  path = 'changes/?%s' % '&'.join(q)
  try:
    result = FetchUrlJson(host, path, ignore_404=False)
  except GOBError as e:
    msg = '%s:\n%s' % (e.message, path)
    raise GOBError(http_status=e.http_status, reason=msg)
  return result


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


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


def _GetChangePath(change):
  """Given a change id, return a path prefix for the change."""
  return 'changes/%s' % str(change).replace('/', '%2F')


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


def GetChange(host, change):
  """Query a gerrit server for information about a single change."""
  return FetchUrlJson(host, _GetChangePath(change))


def GetChangeReview(host, change, revision=None):
  """Get the current review information for a change."""
  if revision is None:
    revision = 'current'
  path = '%s/revisions/%s/review' % (_GetChangePath(change), revision)
  return FetchUrlJson(host, path)


def GetChangeCommit(host, change, revision=None):
  """Get the current review information for a change."""
  if revision is None:
    revision = 'current'
  path = '%s/revisions/%s/commit' % (_GetChangePath(change), revision)
  return FetchUrlJson(host, path)


def GetChangeCurrentRevision(host, change):
  """Get information about the latest revision for a given change."""
  jmsg = GetChangeReview(host, change)
  if jmsg:
    return jmsg.get('current_revision')


def GetChangeDetail(host, change, o_params=None):
  """Query a gerrit server for extended information about a single change."""
  path = '%s/detail' % _GetChangePath(change)
  if o_params:
    path = '%s?%s' % (path, '&'.join(['o=%s' % p for p in o_params]))
  return FetchUrlJson(host, path)


def GetChangeReviewers(host, change):
  """Get information about all reviewers attached to a change.

  Args:
    host: The Gerrit host to interact with.
    change: The Gerrit change ID.
  """
  warnings.warn('GetChangeReviewers is deprecated; use GetReviewers instead.')
  GetReviewers(host, change)


def AbandonChange(host, change, msg=''):
  """Abandon a gerrit change."""
  path = '%s/abandon' % _GetChangePath(change)
  body = {'message': msg}
  return FetchUrlJson(host, path, reqtype='POST', body=body, ignore_404=False)


def RestoreChange(host, change, msg=''):
  """Restore a previously abandoned change."""
  path = '%s/restore' % _GetChangePath(change)
  body = {'message': msg}
  return FetchUrlJson(host, path, reqtype='POST', body=body, ignore_404=False)


def DeleteDraft(host, change):
  """Delete a gerrit draft change."""
  path = _GetChangePath(change)
  try:
    FetchUrl(host, path, reqtype='DELETE', ignore_204=True, ignore_404=False)
  except GOBError as e:
    # On success, gerrit returns status 204; anything else is an error.
    if e.http_status != 204:
      raise
  else:
    raise GOBError(
        http_status=200,
        reason='Unexpectedly received a 200 http status while deleting draft '
               ' %r' % change)


def SubmitChange(host, change, revision=None, wait_for_merge=True):
  """Submits a gerrit change via Gerrit."""
  if revision is None:
    revision = 'current'
  path = '%s/revisions/%s/submit' % (_GetChangePath(change), revision)
  body = {'wait_for_merge': wait_for_merge}
  return FetchUrlJson(host, path, reqtype='POST', body=body, ignore_404=False)


def CheckChange(host, change, sha1=None):
  """Performs consistency checks on the change, and fixes inconsistencies.

  This is useful for forcing Gerrit to check whether a change has already been
  merged into the git repo. Namely, if |sha1| is provided and the change is in
  'NEW' status, Gerrit will check if a change with that |sha1| is in the repo
  and mark the change as 'MERGED' if it exists.

  Args:
    host: The Gerrit host to interact with.
    change: The Gerrit change ID.
    sha1: An optional hint of the commit's SHA1 in Git.
  """
  path = '%s/check' % (_GetChangePath(change),)
  if sha1:
    body, headers = {'expect_merged_as': sha1}, {}
  else:
    body, headers = {}, {'Content-Length': '0'}

  return FetchUrlJson(host, path, reqtype='POST',
                      body=body, ignore_404=False,
                      headers=headers)


def GetAssignee(host, change):
  """Get assignee for a change."""
  path = '%s/assignee' % _GetChangePath(change)
  return FetchUrlJson(host, path)


def AddAssignee(host, change, assignee):
  """Add reviewers to a change.

  Args:
    host: The Gerrit host to interact with.
    change: The Gerrit change ID.
    assignee: Gerrit account email as a string
  """
  path = '%s/assignee' % _GetChangePath(change)
  body = {'assignee': assignee}
  return  FetchUrlJson(host, path, reqtype='PUT', body=body, ignore_404=False)


def MarkPrivate(host, change):
  """Marks the given CL as private.

  Args:
    host: The gob host to interact with.
    change: CL number on the given host.
  """
  path = '%s/private' % _GetChangePath(change)
  try:
    FetchUrlJson(host, path, reqtype='POST', ignore_404=False)
  except GOBError as e:
    # 201: created -- change was successfully marked private.
    if e.http_status != 201:
      raise
  else:
    raise GOBError(
        http_status=200,
        reason='Change was already marked private',
    )


def MarkNotPrivate(host, change):
  """Sets the private bit on given CL to False.

  Args:
    host: The gob host to interact with.
    change: CL number on the given host.
  """
  path = '%s/private.delete' % _GetChangePath(change)
  try:
    FetchUrlJson(host, path, reqtype='POST', ignore_404=False, ignore_204=True)
  except GOBError as e:
    if e.http_status == 204:
      # 204: no content -- change was successfully marked not private.
      pass
    elif e.http_status == 409:
      raise GOBError(
          http_status=e.http_status,
          reason='Change was already marked not private',
      )
    else:
      raise
  else:
    raise GOBError(
        http_status=200,
        reason='Got unexpected 200 when marking change not private.',
    )


def GetReviewers(host, change):
  """Get information about all reviewers attached to a change.

  Args:
    host: The Gerrit host to interact with.
    change: The Gerrit change ID.
  """
  path = '%s/reviewers' % _GetChangePath(change)
  return FetchUrlJson(host, path)


def AddReviewers(host, change, add=None):
  """Add reviewers to a change."""
  if not add:
    return
  if isinstance(add, six.string_types):
    add = (add,)
  path = '%s/reviewers' % _GetChangePath(change)
  for r in add:
    body = {'reviewer': r}
    jmsg = FetchUrlJson(host, path, reqtype='POST', body=body, ignore_404=False)
  return jmsg


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


def SetReview(host, change, revision=None, msg=None, labels=None, notify=None):
  """Set labels and/or add a message to a code review."""
  if revision is None:
    revision = 'current'
  if not msg and not labels:
    return
  path = '%s/revisions/%s/review' % (_GetChangePath(change), revision)
  body = {}
  if msg:
    body['message'] = msg
  if labels:
    body['labels'] = labels
  if notify:
    body['notify'] = notify
  response = FetchUrlJson(host, path, reqtype='POST', body=body)
  if response is None:
    raise GOBError(
        http_status=404,
        reason='CL %s not found in %s' % (change, host))
  if labels:
    for key, val in labels.items():
      if ('labels' not in response or key not in response['labels'] or
          int(response['labels'][key] != int(val))):
        raise GOBError(
            http_status=200,
            reason='Unable to set "%s" label on change %s.' % (key, change))


def SetTopic(host, change, topic):
  """Set |topic| for a change. If |topic| is empty, it will be deleted"""
  path = '%s/topic' % _GetChangePath(change)
  body = {'topic': topic}
  return FetchUrlJson(host, path, reqtype='PUT', body=body, ignore_404=False)


def SetHashtags(host, change, add, remove):
  """Adds and / or removes hashtags from a change.

  Args:
    host: Hostname (without protocol prefix) of the gerrit server.
    change: A gerrit change number.
    add: a list of hashtags to be added.
    remove: a list of hashtags to be removed.
  """
  path = '%s/hashtags' % _GetChangePath(change)
  return FetchUrlJson(host, path, reqtype='POST',
                      body={'add': add, 'remove': remove},
                      ignore_404=False)


def ResetReviewLabels(host, change, label, value='0', revision=None,
                      message=None, notify=None):
  """Reset the value of a given label for all reviewers on a change."""
  if revision is None:
    revision = 'current'
  # This is tricky when working on the "current" revision, because there's
  # always the risk that the "current" revision will change in between API
  # calls.  So, the code dereferences the "current" revision down to a literal
  # sha1 at the beginning and uses it for all subsequent calls.  As a sanity
  # check, the "current" revision is dereferenced again at the end, and if it
  # differs from the previous "current" revision, an exception is raised.
  current = (revision == 'current')
  jmsg = GetChangeDetail(
      host, change, o_params=['CURRENT_REVISION', 'CURRENT_COMMIT'])
  if current:
    revision = jmsg['current_revision']
  value = str(value)
  path = '%s/revisions/%s/review' % (_GetChangePath(change), revision)
  message = message or (
      '%s label set to %s programmatically by chromite.' % (label, value))
  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
      response = FetchUrlJson(host, path, reqtype='POST', body=body)
      if str(response['labels'][label]) != value:
        username = review.get('email', jmsg.get('name', ''))
        raise GOBError(
            http_status=200,
            reason='Unable to set %s label for user "%s" on change %s.' % (
                label, username, change))
  if current:
    new_revision = GetChangeCurrentRevision(host, change)
    if not new_revision:
      raise GOBError(
          http_status=200,
          reason='Could not get review information for change "%s"' % change)
    elif new_revision != revision:
      raise GOBError(
          http_status=200,
          reason='While resetting labels on change "%s", a new patchset was '
                 'uploaded.' % change)


def GetTipOfTrunkRevision(git_url):
  """Returns the current git revision on the master branch."""
  parsed_url = urllib.parse.urlparse(git_url)
  path = parsed_url[2].rstrip('/') + '/+log/master?n=1&format=JSON'
  j = FetchUrlJson(parsed_url[1], path, ignore_404=False)
  if not j:
    raise GOBError(
        reason='Could not find revision information from %s' % git_url)
  try:
    return j['log'][0]['commit']
  except (IndexError, KeyError, TypeError):
    msg = ('The json returned by https://%s%s has an unfamiliar structure:\n'
           '%s\n' % (parsed_url[1], path, j))
    raise GOBError(reason=msg)


def GetCommitDate(git_url, commit):
  """Returns the date of a particular git commit.

  The returned object is naive in the sense that it doesn't carry any timezone
  information - you should assume UTC.

  Args:
    git_url: URL for the repository to get the commit date from.
    commit: A git commit identifier (e.g. a sha1).

  Returns:
    A datetime object.
  """
  parsed_url = urllib.parse.urlparse(git_url)
  path = '%s/+log/%s?n=1&format=JSON' % (parsed_url.path.rstrip('/'), commit)
  j = FetchUrlJson(parsed_url.netloc, path, ignore_404=False)
  if not j:
    raise GOBError(
        reason='Could not find revision information from %s' % git_url)
  try:
    commit_timestr = j['log'][0]['committer']['time']
  except (IndexError, KeyError, TypeError):
    msg = ('The json returned by https://%s%s has an unfamiliar structure:\n'
           '%s\n' % (parsed_url.netloc, path, j))
    raise GOBError(reason=msg)
  try:
    # We're parsing a string of the form 'Tue Dec 02 17:48:06 2014'.
    return datetime.datetime.strptime(commit_timestr,
                                      constants.GOB_COMMIT_TIME_FORMAT)
  except ValueError:
    raise GOBError(reason='Failed parsing commit time "%s"' % commit_timestr)


def GetAccount(host):
  """Get information about the user account."""
  return FetchUrlJson(host, 'accounts/self')