Re-apply http://codereview.chromium.org/1622012

net/tools/testserver/chromiumsync.py

+#!/usr/bin/python2.4
+# Copyright (c) 2010 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""An implementation of the server side of the Chromium sync protocol.
+
+The details of the protocol are described mostly by comments in the protocol
+buffer definition at chrome/browser/sync/protocol/sync.proto.
+"""
+
+import operator
+import random
+import threading
+
+import autofill_specifics_pb2
+import bookmark_specifics_pb2
+import preference_specifics_pb2
+import theme_specifics_pb2
+import typed_url_specifics_pb2
+import sync_pb2
+
+# An enumeration of the various kinds of data that can be synced.
+# Over the wire, this enumeration is not used: a sync object's type is
+# inferred by which EntitySpecifics extension it has.  But in the context
+# of a program, it is useful to have an enumeration.
+ALL_TYPES = (
+    TOP_LEVEL,  # The type of the 'Google Chrome' folder.
+    BOOKMARK,
+    AUTOFILL,
+    TYPED_URL,
+    PREFERENCE,
+    # PASSWORD,  # Disabled since there's no specifics proto.
+    # SESSION,
+    THEME) = range(6)
+
+# Given a sync type from ALL_TYPES, find the extension token corresponding
+# to that datatype.  Note that TOP_LEVEL has no such token.
+SYNC_TYPE_TO_EXTENSION = {
+    BOOKMARK: bookmark_specifics_pb2.bookmark,
+    AUTOFILL: autofill_specifics_pb2.autofill,
+    TYPED_URL: typed_url_specifics_pb2.typed_url,
+    PREFERENCE: preference_specifics_pb2.preference,
+    # PASSWORD: password_specifics_pb2.password,  # Disabled
+    # SESSION: session_specifics_pb2.session,     # Disabled
+    THEME: theme_specifics_pb2.theme,
+    }
+
+# The parent ID used to indicate a top-level node.
+ROOT_ID = '0'
+
+def GetEntryType(entry):
+  """Extract the sync type from a SyncEntry.
+
+  Args:
+    entry: A SyncEntity protobuf object whose type to determine.
+  Returns:
+    A value from ALL_TYPES if the entry's type can be determined, or None
+    if the type cannot be determined.
+  """
+  if entry.server_defined_unique_tag == 'google_chrome':
+    return TOP_LEVEL
+  entry_types = GetEntryTypesFromSpecifics(entry.specifics)
+  if not entry_types:
+    return None
+  # It is presupposed that the entry has at most one specifics extension
+  # present.  If there is more than one, either there's a bug, or else
+  # the caller should use GetEntryTypes.
+  if len(entry_types) > 1:
+    raise 'GetEntryType called with multiple extensions present.'
+  return entry_types[0]
+
+def GetEntryTypesFromSpecifics(specifics):
+  """Determine the sync types indicated by an EntitySpecifics's extension(s).
+
+  If the specifics have more than one recognized extension (as commonly
+  happens with the requested_types field of GetUpdatesMessage), all types
+  will be returned.  Callers must handle the possibility of the returned
+  value having more than one item.
+
+  Args:
+    specifics: A EntitySpecifics protobuf message whose extensions to
+      enumerate.
+  Returns:
+    A list of the sync types (values from ALL_TYPES) assocated with each
+    recognized extension of the specifics message.
+  """
+  entry_types = []
+  for data_type, extension in SYNC_TYPE_TO_EXTENSION.iteritems():
+    if specifics.HasExtension(extension):
+      entry_types.append(data_type)
+  return entry_types
+
+def GetRequestedTypes(get_updates_message):
+  """Determine the sync types requested by a client GetUpdates operation."""
+  types = GetEntryTypesFromSpecifics(
+      get_updates_message.requested_types)
+  if types:
+    types.append(TOP_LEVEL)
+  return types
+
+def GetDefaultEntitySpecifics(data_type):
+  """Get an EntitySpecifics having a sync type's default extension value.
+  """
+  specifics = sync_pb2.EntitySpecifics()
+  if data_type in SYNC_TYPE_TO_EXTENSION:
+    extension_handle = SYNC_TYPE_TO_EXTENSION[data_type]
+    specifics.Extensions[extension_handle].SetInParent()
+  return specifics
+
+def DeepCopyOfProto(proto):
+  """Return a deep copy of a protocol buffer."""
+  new_proto = type(proto)()
+  new_proto.MergeFrom(proto)
+  return new_proto
+
+
+class PermanentItem(object):
+  """A specification of one server-created permanent item.
+
+  Attributes:
+    tag: A known-to-the-client value that uniquely identifies a server-created
+      permanent item.
+    name: The human-readable display name for this item.
+    parent_tag: The tag of the permanent item's parent.  If ROOT_ID, indicates
+      a top-level item.  Otherwise, this must be the tag value of some other
+      server-created permanent item.
+    sync_type: A value from ALL_TYPES, giving the datatype of this permanent
+      item.  This controls which types of client GetUpdates requests will
+      cause the permanent item to be created and returned.
+  """
+
+  def __init__(self, tag, name, parent_tag, sync_type):
+    self.tag = tag
+    self.name = name
+    self.parent_tag = parent_tag
+    self.sync_type = sync_type
+
+class SyncDataModel(object):
+  """Models the account state of one sync user.
+  """
+  _BATCH_SIZE = 100
+
+  # Specify all the permanent items that a model might need.
+  _PERMANENT_ITEM_SPECS = [
+      PermanentItem('google_chrome', name='Google Chrome',
+                    parent_tag=ROOT_ID, sync_type=TOP_LEVEL),
+      PermanentItem('google_chrome_bookmarks', name='Bookmarks',
+                    parent_tag='google_chrome', sync_type=BOOKMARK),
+      PermanentItem('bookmark_bar', name='Bookmark Bar',
+                    parent_tag='google_chrome_bookmarks', sync_type=BOOKMARK),
+      PermanentItem('other_bookmarks', name='Other Bookmarks',
+                    parent_tag='google_chrome_bookmarks', sync_type=BOOKMARK),
+      PermanentItem('google_chrome_preferences', name='Preferences',
+                    parent_tag='google_chrome', sync_type=PREFERENCE),
+      PermanentItem('google_chrome_autofill', name='Autofill',
+                    parent_tag='google_chrome', sync_type=AUTOFILL),
+      # TODO(nick): Disabled since the protocol does not support them yet.
+      # PermanentItem('google_chrome_passwords', name='Passwords',
+      #               parent_tag='google_chrome', sync_type=PASSWORD),
+      # PermanentItem('google_chrome_sessions', name='Sessions',
+      #               parent_tag='google_chrome', SESSION),
+      PermanentItem('google_chrome_themes', name='Themes',
+                    parent_tag='google_chrome', sync_type=THEME),
+      PermanentItem('google_chrome_typed_urls', name='Typed URLs',
+                    parent_tag='google_chrome', sync_type=TYPED_URL),
+      ]
+
+  def __init__(self):
+    self._version = 0
+
+    # Monotonically increasing version number.  The next object change will
+    # take on this value + 1.
+    self._entries = {}
+
+    # TODO(nick): uuid.uuid1() is better, but python 2.5 only.
+    self.store_birthday = '%0.30f' % random.random()
+
+  def _SaveEntry(self, entry):
+    """Insert or update an entry in the change log, and give it a new version.
+
+    The ID fields of this entry are assumed to be valid server IDs.  This
+    entry will be updated with a new version number and sync_timestamp.
+
+    Args:
+      entry: The entry to be added or updated.
+    """
+    self._version = self._version + 1
+    entry.version = self._version
+    entry.sync_timestamp = self._version
+
+    # Preserve the originator info, which the client is not required to send
+    # when updating.
+    base_entry = self._entries.get(entry.id_string)
+    if base_entry:
+      entry.originator_cache_guid = base_entry.originator_cache_guid
+      entry.originator_client_item_id = base_entry.originator_client_item_id
+
+    self._entries[entry.id_string] = DeepCopyOfProto(entry)
+
+  def _ServerTagToId(self, tag):
+    """Determine the server ID from a server-unique tag.
+
+    The resulting value is guaranteed not to collide with the other ID
+    generation methods.
+
+    Args:
+      tag: The unique, known-to-the-client tag of a server-generated item.
+    """
+    if tag and tag != ROOT_ID:
+      return '<server tag>%s' % tag
+    else:
+      return tag
+
+  def _ClientTagToId(self, tag):
+    """Determine the server ID from a client-unique tag.
+
+    The resulting value is guaranteed not to collide with the other ID
+    generation methods.
+
+    Args:
+      tag: The unique, opaque-to-the-server tag of a client-tagged item.
+    """
+    return '<client tag>%s' % tag
+
+  def _ClientIdToId(self, client_guid, client_item_id):
+    """Compute a unique server ID from a client-local ID tag.
+
+    The resulting value is guaranteed not to collide with the other ID
+    generation methods.
+
+    Args:
+      client_guid: A globally unique ID that identifies the client which
+        created this item.
+      client_item_id: An ID that uniquely identifies this item on the client
+        which created it.
+    """
+    # Using the client ID info is not required here (we could instead generate
+    # a random ID), but it's useful for debugging.
+    return '<server ID originally>%s/%s' % (client_guid, client_item_id)
+
+  def _WritePosition(self, entry, parent_id, prev_id=None):
+    """Convert from a relative position into an absolute, numeric position.
+
+    Clients specify positions using the predecessor-based references; the
+    server stores and reports item positions using sparse integer values.
+    This method converts from the former to the latter.
+
+    Args:
+      entry: The entry for which to compute a position.  Its ID field are
+        assumed to be server IDs.  This entry will have its parent_id_string
+        and position_in_parent fields updated; its insert_after_item_id field
+        will be cleared.
+      parent_id: The ID of the entry intended as the new parent.
+      prev_id: The ID of the entry intended as the new predecessor.  If this
+        is None, or an ID of an object which is not a child of the new parent,
+        the entry will be positioned at the end (right) of the ordering.  If
+        the empty ID (''), this will be positioned at the front (left) of the
+        ordering.  Otherwise, the entry will be given a position_in_parent
+        value placing it just after (to the right of) the new predecessor.
+    """
+    PREFERRED_GAP = 2 ** 20
+    # Compute values at the beginning or end.
+    def ExtendRange(current_limit_entry, sign_multiplier):
+      if current_limit_entry.id_string == entry.id_string:
+        step = 0
+      else:
+        step = sign_multiplier * PREFERRED_GAP
+      return current_limit_entry.position_in_parent + step
+
+    siblings = [x for x in self._entries.values()
+                if x.parent_id_string == parent_id and not x.deleted]
+    siblings = sorted(siblings, key=operator.attrgetter('position_in_parent'))
+    if prev_id == entry.id_string:
+      prev_id = ''
+    if not siblings:
+      # First item in this container; start in the middle.
+      entry.position_in_parent = 0
+    elif prev_id == '':
+      # A special value in the protocol.  Insert at first position.
+      entry.position_in_parent = ExtendRange(siblings[0], -1)
+    else:
+      # Consider items along with their successors.
+      for a, b in zip(siblings, siblings[1:]):
+        if a.id_string != prev_id:
+          continue
+        elif b.id_string == entry.id_string:
+          # We're already in place; don't change anything.
+          entry.position_in_parent = b.position_in_parent
+        else:
+          # Interpolate new position between two others.
+          entry.position_in_parent = (
+              a.position_in_parent * 7 + b.position_in_parent) / 8
+        break
+      else:
+        # Insert at end. Includes the case where prev_id is None.
+        entry.position_in_parent = ExtendRange(siblings[-1], +1)
+
+    entry.parent_id_string = parent_id
+    entry.ClearField('insert_after_item_id')
+
+  def _ItemExists(self, id_string):
+    """Determine whether an item exists in the changelog."""
+    return id_string in self._entries
+
+  def _CreatePermanentItem(self, spec):
+    """Create one permanent item from its spec, if it doesn't exist.
+
+    The resulting item is added to the changelog.
+
+    Args:
+      spec: A PermanentItem object holding the properties of the item to create.
+    """
+    id_string = self._ServerTagToId(spec.tag)
+    if self._ItemExists(id_string):
+      return
+    print 'Creating permanent item: %s' % spec.name
+    entry = sync_pb2.SyncEntity()
+    entry.id_string = id_string
+    entry.non_unique_name = spec.name
+    entry.name = spec.name
+    entry.server_defined_unique_tag = spec.tag
+    entry.folder = True
+    entry.deleted = False
+    entry.specifics.CopyFrom(GetDefaultEntitySpecifics(spec.sync_type))
+    self._WritePosition(entry, self._ServerTagToId(spec.parent_tag))
+    self._SaveEntry(entry)
+
+  def _CreatePermanentItems(self, requested_types):
+    """Ensure creation of all permanent items for a given set of sync types.
+
+    Args:
+      requested_types: A list of sync data types from ALL_TYPES.
+        Permanent items of only these types will be created.
+    """
+    for spec in self._PERMANENT_ITEM_SPECS:
+      if spec.sync_type in requested_types:
+        self._CreatePermanentItem(spec)
+
+  def GetChangesFromTimestamp(self, requested_types, timestamp):
+    """Get entries which have changed since a given timestamp, oldest first.
+
+    The returned entries are limited to being _BATCH_SIZE many.  The entries
+    are returned in strict version order.
+
+    Args:
+      requested_types: A list of sync data types from ALL_TYPES.
+        Only items of these types will be retrieved; others will be filtered
+        out.
+      timestamp: A timestamp / version number.  Only items that have changed
+        more recently than this value will be retrieved; older items will
+        be filtered out.
+    Returns:
+      A tuple of (version, entries).  Version is a new timestamp value, which
+      should be used as the starting point for the next query.  Entries is the
+      batch of entries meeting the current timestamp query.
+    """
+    if timestamp == 0:
+      self._CreatePermanentItems(requested_types)
+    change_log = sorted(self._entries.values(),
+                        key=operator.attrgetter('version'))
+    new_changes = [x for x in change_log if x.version > timestamp]
+    # Pick batch_size new changes, and then filter them.  This matches
+    # the RPC behavior of the production sync server.
+    batch = new_changes[:self._BATCH_SIZE]
+    if not batch:
+      # Client is up to date.
+      return (timestamp, [])
+
+    # Restrict batch to requested types.  Tombstones are untyped
+    # and will always get included.
+    filtered = []
+    for x in batch:
+      if (GetEntryType(x) in requested_types) or x.deleted:
+        filtered.append(DeepCopyOfProto(x))
+    # The new client timestamp is the timestamp of the last item in the
+    # batch, even if that item was filtered out.
+    return (batch[-1].version, filtered)
+
+  def _CheckVersionForCommit(self, entry):
+    """Perform an optimistic concurrency check on the version number.
+
+    Clients are only allowed to commit if they report having seen the most
+    recent version of an object.
+
+    Args:
+      entry: A sync entity from the client.  It is assumed that ID fields
+        have been converted to server IDs.
+    Returns:
+      A boolean value indicating whether the client's version matches the
+      newest server version for the given entry.
+    """
+    if entry.id_string in self._entries:
+      if (self._entries[entry.id_string].version != entry.version and
+          not self._entries[entry.id_string].deleted):
+        # Version mismatch that is not a tombstone recreation.
+        return False
+    else:
+      if entry.version != 0:
+        # Edit to an item that does not exist.
+        return False
+    return True
+
+  def _CheckParentIdForCommit(self, entry):
+    """Check that the parent ID referenced in a SyncEntity actually exists.
+
+    Args:
+      entry: A sync entity from the client.  It is assumed that ID fields
+        have been converted to server IDs.
+    Returns:
+      A boolean value indicating whether the entity's parent ID is an object
+      that actually exists (and is not deleted) in the current account state.
+    """
+    if entry.parent_id_string == ROOT_ID:
+      # This is generally allowed.
+      return True
+    if entry.parent_id_string not in self._entries:
+      print 'Warning: Client sent unknown ID.  Should never happen.'
+      return False
+    if entry.parent_id_string == entry.id_string:
+      print 'Warning: Client sent circular reference.  Should never happen.'
+      return False
+    if self._entries[entry.parent_id_string].deleted:
+      # This can happen in a race condition between two clients.
+      return False
+    if not self._entries[entry.parent_id_string].folder:
+      print 'Warning: Client sent non-folder parent.  Should never happen.'
+      return False
+    return True
+
+  def _RewriteIdsAsServerIds(self, entry, cache_guid, commit_session):
+    """Convert ID fields in a client sync entry to server IDs.
+
+    A commit batch sent by a client may contain new items for which the
+    server has not generated IDs yet.  And within a commit batch, later
+    items are allowed to refer to earlier items.  This method will
+    generate server IDs for new items, as well as rewrite references
+    to items whose server IDs were generated earlier in the batch.
+
+    Args:
+      entry: The client sync entry to modify.
+      cache_guid: The globally unique ID of the client that sent this
+        commit request.
+      commit_session: A dictionary mapping the original IDs to the new server
+        IDs, for any items committed earlier in the batch.
+    """
+    if entry.version == 0:
+      if entry.HasField('client_defined_unique_tag'):
+        # When present, this should determine the item's ID.
+        new_id = self._ClientTagToId(entry.client_defined_unique_tag)
+      else:
+        new_id = self._ClientIdToId(cache_guid, entry.id_string)
+        entry.originator_cache_guid = cache_guid
+        entry.originator_client_item_id = entry.id_string
+      commit_session[entry.id_string] = new_id  # Remember the remapping.
+      entry.id_string = new_id
+    if entry.parent_id_string in commit_session:
+      entry.parent_id_string = commit_session[entry.parent_id_string]
+    if entry.insert_after_item_id in commit_session:
+      entry.insert_after_item_id = commit_session[entry.insert_after_item_id]
+
+  def CommitEntry(self, entry, cache_guid, commit_session):
+    """Attempt to commit one entry to the user's account.
+
+    Args:
+      entry: A SyncEntity protobuf representing desired object changes.
+      cache_guid: A string value uniquely identifying the client; this
+        is used for ID generation and will determine the originator_cache_guid
+        if the entry is new.
+      commit_session: A dictionary mapping client IDs to server IDs for any
+        objects committed earlier this session.  If the entry gets a new ID
+        during commit, the change will be recorded here.
+    Returns:
+      A SyncEntity reflecting the post-commit value of the entry, or None
+      if the entry was not committed due to an error.
+    """
+    entry = DeepCopyOfProto(entry)
+
+    # Generate server IDs for this entry, and write generated server IDs
+    # from earlier entries into the message's fields, as appropriate.  The
+    # ID generation state is stored in 'commit_session'.
+    self._RewriteIdsAsServerIds(entry, cache_guid, commit_session)
+
+    # Perform the optimistic concurrency check on the entry's version number.
+    # Clients are not allowed to commit unless they indicate that they've seen
+    # the most recent version of an object.
+    if not self._CheckVersionForCommit(entry):
+      return None
+
+    # Check the validity of the parent ID; it must exist at this point.
+    # TODO(nick): Implement cycle detection and resolution.
+    if not self._CheckParentIdForCommit(entry):
+      return None
+
+    # At this point, the commit is definitely going to happen.
+
+    # Deletion works by storing a limited record for an entry, called a
+    # tombstone.  A sync server must track deleted IDs forever, since it does
+    # not keep track of client knowledge (there's no deletion ACK event).
+    if entry.deleted:
+      # Only the ID, version and deletion state are preserved on a tombstone.
+      # TODO(nick): Does the production server not preserve the type?  Not
+      # doing so means that tombstones cannot be filtered based on
+      # requested_types at GetUpdates time.
+      tombstone = sync_pb2.SyncEntity()
+      tombstone.id_string = entry.id_string
+      tombstone.deleted = True
+      tombstone.name = ''  # 'name' is a required field; we're stuck with it.
+      entry = tombstone
+    else:
+      # Comments in sync.proto detail how the representation of positional
+      # ordering works: the 'insert_after_item_id' field specifies a
+      # predecessor during Commit operations, but the 'position_in_parent'
+      # field provides an absolute ordering in GetUpdates contexts.  Here
+      # we convert from the former to the latter.  Specifically, we'll
+      # generate a numeric position placing the item just after the object
+      # identified by 'insert_after_item_id', and then clear the
+      # 'insert_after_item_id' field so that it's not sent back to the client
+      # during later GetUpdates requests.
+      if entry.HasField('insert_after_item_id'):
+        self._WritePosition(entry, entry.parent_id_string,
+                            entry.insert_after_item_id)
+      else:
+        self._WritePosition(entry, entry.parent_id_string)
+
+    # Preserve the originator info, which the client is not required to send
+    # when updating.
+    base_entry = self._entries.get(entry.id_string)
+    if base_entry and not entry.HasField("originator_cache_guid"):
+      entry.originator_cache_guid = base_entry.originator_cache_guid
+      entry.originator_client_item_id = base_entry.originator_client_item_id
+
+    # Commit the change.  This also updates the version number.
+    self._SaveEntry(entry)
+    # TODO(nick): Handle recursive deletion.
+    return entry
+
+class TestServer(object):
+  """An object to handle requests for one (and only one) Chrome Sync account.
+
+  TestServer consumes the sync command messages that are the outermost
+  layers of the protocol, performs the corresponding actions on its
+  SyncDataModel, and constructs an appropropriate response message.
+  """
+
+  def __init__(self):
+    # The implementation supports exactly one account; its state is here.
+    self.account = SyncDataModel()
+    self.account_lock = threading.Lock()
+
+  def HandleCommand(self, raw_request):
+    """Decode and handle a sync command from a raw input of bytes.
+
+    This is the main entry point for this class.  It is safe to call this
+    method from multiple threads.
+
+    Args:
+      raw_request: An iterable byte sequence to be interpreted as a sync
+        protocol command.
+    Returns:
+      A tuple (response_code, raw_response); the first value is an HTTP
+      result code, while the second value is a string of bytes which is the
+      serialized reply to the command.
+    """
+    self.account_lock.acquire()
+    try:
+      request = sync_pb2.ClientToServerMessage()
+      request.MergeFromString(raw_request)
+      contents = request.message_contents
+
+      response = sync_pb2.ClientToServerResponse()
+      response.error_code = sync_pb2.ClientToServerResponse.SUCCESS
+      response.store_birthday = self.account.store_birthday
+
+      if contents == sync_pb2.ClientToServerMessage.AUTHENTICATE:
+        print 'Authenticate'
+        # We accept any authentication token, and support only one account.
+        # TODO(nick): Mock out the GAIA authentication as well; hook up here.
+        response.authenticate.user.email = 'syncjuser@chromium'
+        response.authenticate.user.display_name = 'Sync J User'
+      elif contents == sync_pb2.ClientToServerMessage.COMMIT:
+        print 'Commit'
+        self.HandleCommit(request.commit, response.commit)
+      elif contents == sync_pb2.ClientToServerMessage.GET_UPDATES:
+        print ('GetUpdates from timestamp %d' %
+            request.get_updates.from_timestamp)
+        self.HandleGetUpdates(request.get_updates, response.get_updates)
+      return (200, response.SerializeToString())
+    finally:
+      self.account_lock.release()
+
+  def HandleCommit(self, commit_message, commit_response):
+    """Respond to a Commit request by updating the user's account state.
+
+    Commit attempts stop after the first error, returning a CONFLICT result
+    for any unattempted entries.
+
+    Args:
+      commit_message: A sync_pb.CommitMessage protobuf holding the content
+        of the client's request.
+      commit_response: A sync_pb.CommitResponse protobuf into which a reply
+        to the client request will be written.
+    """
+    commit_response.SetInParent()
+    batch_failure = False
+    session = {}  # Tracks ID renaming during the commit operation.
+    guid = commit_message.cache_guid
+    for entry in commit_message.entries:
+      server_entry = None
+      if not batch_failure:
+        # Try to commit the change to the account.
+        server_entry = self.account.CommitEntry(entry, guid, session)
+
+      # An entryresponse is returned in both success and failure cases.
+      reply = commit_response.entryresponse.add()
+      if not server_entry:
+        reply.response_type = sync_pb2.CommitResponse.CONFLICT
+        reply.error_message = 'Conflict.'
+        batch_failure = True  # One failure halts the batch.
+      else:
+        reply.response_type = sync_pb2.CommitResponse.SUCCESS
+        # These are the properties that the server is allowed to override
+        # during commit; the client wants to know their values at the end
+        # of the operation.
+        reply.id_string = server_entry.id_string
+        if not server_entry.deleted:
+          reply.parent_id_string = server_entry.parent_id_string
+          reply.position_in_parent = server_entry.position_in_parent
+          reply.version = server_entry.version
+          reply.name = server_entry.name
+          reply.non_unique_name = server_entry.non_unique_name
+
+  def HandleGetUpdates(self, update_request, update_response):
+    """Respond to a GetUpdates request by querying the user's account.
+
+    Args:
+      update_request: A sync_pb.GetUpdatesMessage protobuf holding the content
+        of the client's request.
+      update_response: A sync_pb.GetUpdatesResponse protobuf into which a reply
+        to the client request will be written.
+    """
+    update_response.SetInParent()
+    requested_types = GetRequestedTypes(update_request)
+    new_timestamp, entries = self.account.GetChangesFromTimestamp(
+        requested_types, update_request.from_timestamp)
+
+    # If the client is up to date, we are careful not to set the
+    # new_timestamp field.
+    if new_timestamp != update_request.from_timestamp:
+      update_response.new_timestamp = new_timestamp
+      for e in entries:
+        reply = update_response.entries.add()
+        reply.CopyFrom(e)