diff options
Diffstat (limited to '')
| -rwxr-xr-x | synapse/app/homeserver.py | 4 | ||||
| -rw-r--r-- | synapse/replication/expire_cache.py | 60 | ||||
| -rw-r--r-- | synapse/replication/presence_resource.py | 59 | ||||
| -rw-r--r-- | synapse/replication/pusher_resource.py | 54 | ||||
| -rw-r--r-- | synapse/replication/resource.py | 576 |
5 files changed, 0 insertions, 753 deletions
diff --git a/synapse/app/homeserver.py b/synapse/app/homeserver.py index 990eb477e5..6f5924d2c7 100755 --- a/synapse/app/homeserver.py +++ b/synapse/app/homeserver.py @@ -55,7 +55,6 @@ from synapse.crypto import context_factory from synapse.util.logcontext import LoggingContext, PreserveLoggingContext from synapse.metrics import register_memory_metrics, get_metrics_for from synapse.metrics.resource import MetricsResource, METRICS_PREFIX -from synapse.replication.resource import ReplicationResource, REPLICATION_PREFIX from synapse.replication.tcp.resource import ReplicationStreamProtocolFactory from synapse.federation.transport.server import TransportLayerServer @@ -167,9 +166,6 @@ class SynapseHomeServer(HomeServer): if name == "metrics" and self.get_config().enable_metrics: resources[METRICS_PREFIX] = MetricsResource(self) - if name == "replication": - resources[REPLICATION_PREFIX] = ReplicationResource(self) - if WEB_CLIENT_PREFIX in resources: root_resource = RootRedirect(WEB_CLIENT_PREFIX) else: diff --git a/synapse/replication/expire_cache.py b/synapse/replication/expire_cache.py deleted file mode 100644 index c05a50d7a6..0000000000 --- a/synapse/replication/expire_cache.py +++ /dev/null @@ -1,60 +0,0 @@ -# Copyright 2016 OpenMarket Ltd -# -# Licensed under the Apache License, Version 2.0 (the "License"); -# you may not use this file except in compliance with the License. -# You may obtain a copy of the License at -# -# http://www.apache.org/licenses/LICENSE-2.0 -# -# Unless required by applicable law or agreed to in writing, software -# distributed under the License is distributed on an "AS IS" BASIS, -# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. -# See the License for the specific language governing permissions and -# limitations under the License. - -from synapse.http.server import respond_with_json_bytes, request_handler -from synapse.http.servlet import parse_json_object_from_request - -from twisted.web.resource import Resource -from twisted.web.server import NOT_DONE_YET - - -class ExpireCacheResource(Resource): - """ - HTTP endpoint for expiring storage caches. - - POST /_synapse/replication/expire_cache HTTP/1.1 - Content-Type: application/json - - { - "invalidate": [ - { - "name": "func_name", - "keys": ["key1", "key2"] - } - ] - } - """ - - def __init__(self, hs): - Resource.__init__(self) # Resource is old-style, so no super() - - self.store = hs.get_datastore() - self.version_string = hs.version_string - self.clock = hs.get_clock() - - def render_POST(self, request): - self._async_render_POST(request) - return NOT_DONE_YET - - @request_handler() - def _async_render_POST(self, request): - content = parse_json_object_from_request(request) - - for row in content["invalidate"]: - name = row["name"] - keys = tuple(row["keys"]) - - getattr(self.store, name).invalidate(keys) - - respond_with_json_bytes(request, 200, "{}") diff --git a/synapse/replication/presence_resource.py b/synapse/replication/presence_resource.py deleted file mode 100644 index fc18130ab4..0000000000 --- a/synapse/replication/presence_resource.py +++ /dev/null @@ -1,59 +0,0 @@ -# Copyright 2016 OpenMarket Ltd -# -# Licensed under the Apache License, Version 2.0 (the "License"); -# you may not use this file except in compliance with the License. -# You may obtain a copy of the License at -# -# http://www.apache.org/licenses/LICENSE-2.0 -# -# Unless required by applicable law or agreed to in writing, software -# distributed under the License is distributed on an "AS IS" BASIS, -# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. -# See the License for the specific language governing permissions and -# limitations under the License. - -from synapse.http.server import respond_with_json_bytes, request_handler -from synapse.http.servlet import parse_json_object_from_request - -from twisted.web.resource import Resource -from twisted.web.server import NOT_DONE_YET -from twisted.internet import defer - - -class PresenceResource(Resource): - """ - HTTP endpoint for marking users as syncing. - - POST /_synapse/replication/presence HTTP/1.1 - Content-Type: application/json - - { - "process_id": "<process_id>", - "syncing_users": ["<user_id>"] - } - """ - - def __init__(self, hs): - Resource.__init__(self) # Resource is old-style, so no super() - - self.version_string = hs.version_string - self.clock = hs.get_clock() - self.presence_handler = hs.get_presence_handler() - - def render_POST(self, request): - self._async_render_POST(request) - return NOT_DONE_YET - - @request_handler() - @defer.inlineCallbacks - def _async_render_POST(self, request): - content = parse_json_object_from_request(request) - - process_id = content["process_id"] - syncing_user_ids = content["syncing_users"] - - yield self.presence_handler.update_external_syncs( - process_id, set(syncing_user_ids) - ) - - respond_with_json_bytes(request, 200, "{}") diff --git a/synapse/replication/pusher_resource.py b/synapse/replication/pusher_resource.py deleted file mode 100644 index 9b01ab3c13..0000000000 --- a/synapse/replication/pusher_resource.py +++ /dev/null @@ -1,54 +0,0 @@ -# Copyright 2016 OpenMarket Ltd -# -# Licensed under the Apache License, Version 2.0 (the "License"); -# you may not use this file except in compliance with the License. -# You may obtain a copy of the License at -# -# http://www.apache.org/licenses/LICENSE-2.0 -# -# Unless required by applicable law or agreed to in writing, software -# distributed under the License is distributed on an "AS IS" BASIS, -# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. -# See the License for the specific language governing permissions and -# limitations under the License. - -from synapse.http.server import respond_with_json_bytes, request_handler -from synapse.http.servlet import parse_json_object_from_request - -from twisted.web.resource import Resource -from twisted.web.server import NOT_DONE_YET -from twisted.internet import defer - - -class PusherResource(Resource): - """ - HTTP endpoint for deleting rejected pushers - """ - - def __init__(self, hs): - Resource.__init__(self) # Resource is old-style, so no super() - - self.version_string = hs.version_string - self.store = hs.get_datastore() - self.notifier = hs.get_notifier() - self.clock = hs.get_clock() - - def render_POST(self, request): - self._async_render_POST(request) - return NOT_DONE_YET - - @request_handler() - @defer.inlineCallbacks - def _async_render_POST(self, request): - content = parse_json_object_from_request(request) - - for remove in content["remove"]: - yield self.store.delete_pusher_by_app_id_pushkey_user_id( - remove["app_id"], - remove["push_key"], - remove["user_id"], - ) - - self.notifier.on_new_replication_data() - - respond_with_json_bytes(request, 200, "{}") diff --git a/synapse/replication/resource.py b/synapse/replication/resource.py deleted file mode 100644 index abd3fe7665..0000000000 --- a/synapse/replication/resource.py +++ /dev/null @@ -1,576 +0,0 @@ -# -*- coding: utf-8 -*- -# Copyright 2015 OpenMarket Ltd -# -# Licensed under the Apache License, Version 2.0 (the "License"); -# you may not use this file except in compliance with the License. -# You may obtain a copy of the License at -# -# http://www.apache.org/licenses/LICENSE-2.0 -# -# Unless required by applicable law or agreed to in writing, software -# distributed under the License is distributed on an "AS IS" BASIS, -# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. -# See the License for the specific language governing permissions and -# limitations under the License. - -from synapse.http.servlet import parse_integer, parse_string -from synapse.http.server import request_handler, finish_request -from synapse.replication.pusher_resource import PusherResource -from synapse.replication.presence_resource import PresenceResource -from synapse.replication.expire_cache import ExpireCacheResource -from synapse.api.errors import SynapseError - -from twisted.web.resource import Resource -from twisted.web.server import NOT_DONE_YET -from twisted.internet import defer - -import ujson as json - -import collections -import logging - -logger = logging.getLogger(__name__) - -REPLICATION_PREFIX = "/_synapse/replication" - -STREAM_NAMES = ( - ("events",), - ("presence",), - ("typing",), - ("receipts",), - ("user_account_data", "room_account_data", "tag_account_data",), - ("backfill",), - ("push_rules",), - ("pushers",), - ("caches",), - ("to_device",), - ("public_rooms",), - ("federation",), - ("device_lists",), -) - - -class ReplicationResource(Resource): - """ - HTTP endpoint for extracting data from synapse. - - The streams of data returned by the endpoint are controlled by the - parameters given to the API. To return a given stream pass a query - parameter with a position in the stream to return data from or the - special value "-1" to return data from the start of the stream. - - If there is no data for any of the supplied streams after the given - position then the request will block until there is data for one - of the streams. This allows clients to long-poll this API. - - The possible streams are: - - * "streams": A special stream returing the positions of other streams. - * "events": The new events seen on the server. - * "presence": Presence updates. - * "typing": Typing updates. - * "receipts": Receipt updates. - * "user_account_data": Top-level per user account data. - * "room_account_data: Per room per user account data. - * "tag_account_data": Per room per user tags. - * "backfill": Old events that have been backfilled from other servers. - * "push_rules": Per user changes to push rules. - * "pushers": Per user changes to their pushers. - * "caches": Cache invalidations. - - The API takes two additional query parameters: - - * "timeout": How long to wait before returning an empty response. - * "limit": The maximum number of rows to return for the selected streams. - - The response is a JSON object with keys for each stream with updates. Under - each key is a JSON object with: - - * "position": The current position of the stream. - * "field_names": The names of the fields in each row. - * "rows": The updates as an array of arrays. - - There are a number of ways this API could be used: - - 1) To replicate the contents of the backing database to another database. - 2) To be notified when the contents of a shared backing database changes. - 3) To "tail" the activity happening on a server for debugging. - - In the first case the client would track all of the streams and store it's - own copy of the data. - - In the second case the client might theoretically just be able to follow - the "streams" stream to track where the other streams are. However in - practise it will probably need to get the contents of the streams in - order to expire the any in-memory caches. Whether it gets the contents - of the streams from this replication API or directly from the backing - store is a matter of taste. - - In the third case the client would use the "streams" stream to find what - streams are available and their current positions. Then it can start - long-polling this replication API for new data on those streams. - """ - - def __init__(self, hs): - Resource.__init__(self) # Resource is old-style, so no super() - - self.version_string = hs.version_string - self.store = hs.get_datastore() - self.sources = hs.get_event_sources() - self.presence_handler = hs.get_presence_handler() - self.typing_handler = hs.get_typing_handler() - self.federation_sender = hs.get_federation_sender() - self.notifier = hs.notifier - self.clock = hs.get_clock() - self.config = hs.get_config() - - self.putChild("remove_pushers", PusherResource(hs)) - self.putChild("syncing_users", PresenceResource(hs)) - self.putChild("expire_cache", ExpireCacheResource(hs)) - - def render_GET(self, request): - self._async_render_GET(request) - return NOT_DONE_YET - - @defer.inlineCallbacks - def current_replication_token(self): - stream_token = yield self.sources.get_current_token() - backfill_token = yield self.store.get_current_backfill_token() - push_rules_token, room_stream_token = self.store.get_push_rules_stream_token() - pushers_token = self.store.get_pushers_stream_token() - caches_token = self.store.get_cache_stream_token() - public_rooms_token = self.store.get_current_public_room_stream_id() - federation_token = self.federation_sender.get_current_token() - device_list_token = self.store.get_device_stream_token() - - defer.returnValue(_ReplicationToken( - room_stream_token, - int(stream_token.presence_key), - int(stream_token.typing_key), - int(stream_token.receipt_key), - int(stream_token.account_data_key), - backfill_token, - push_rules_token, - pushers_token, - 0, # State stream is no longer a thing - caches_token, - int(stream_token.to_device_key), - int(public_rooms_token), - int(federation_token), - int(device_list_token), - )) - - @request_handler() - @defer.inlineCallbacks - def _async_render_GET(self, request): - limit = parse_integer(request, "limit", 100) - timeout = parse_integer(request, "timeout", 10 * 1000) - - request.setHeader(b"Content-Type", b"application/json") - - request_streams = { - name: parse_integer(request, name) - for names in STREAM_NAMES for name in names - } - request_streams["streams"] = parse_string(request, "streams") - - federation_ack = parse_integer(request, "federation_ack", None) - - def replicate(): - return self.replicate( - request_streams, limit, - federation_ack=federation_ack - ) - - writer = yield self.notifier.wait_for_replication(replicate, timeout) - result = writer.finish() - - for stream_name, stream_content in result.items(): - logger.info( - "Replicating %d rows of %s from %s -> %s", - len(stream_content["rows"]), - stream_name, - request_streams.get(stream_name), - stream_content["position"], - ) - - request.write(json.dumps(result, ensure_ascii=False)) - finish_request(request) - - @defer.inlineCallbacks - def replicate(self, request_streams, limit, federation_ack=None): - writer = _Writer() - current_token = yield self.current_replication_token() - logger.debug("Replicating up to %r", current_token) - - if limit == 0: - raise SynapseError(400, "Limit cannot be 0") - - yield self.account_data(writer, current_token, limit, request_streams) - yield self.events(writer, current_token, limit, request_streams) - # TODO: implement limit - yield self.presence(writer, current_token, request_streams) - yield self.typing(writer, current_token, request_streams) - yield self.receipts(writer, current_token, limit, request_streams) - yield self.push_rules(writer, current_token, limit, request_streams) - yield self.pushers(writer, current_token, limit, request_streams) - yield self.caches(writer, current_token, limit, request_streams) - yield self.to_device(writer, current_token, limit, request_streams) - yield self.public_rooms(writer, current_token, limit, request_streams) - yield self.device_lists(writer, current_token, limit, request_streams) - self.federation(writer, current_token, limit, request_streams, federation_ack) - self.streams(writer, current_token, request_streams) - - logger.debug("Replicated %d rows", writer.total) - defer.returnValue(writer) - - def streams(self, writer, current_token, request_streams): - request_token = request_streams.get("streams") - - streams = [] - - if request_token is not None: - if request_token == "-1": - for names, position in zip(STREAM_NAMES, current_token): - streams.extend((name, position) for name in names) - else: - items = zip( - STREAM_NAMES, - current_token, - _ReplicationToken(request_token) - ) - for names, current_id, last_id in items: - if last_id < current_id: - streams.extend((name, current_id) for name in names) - - if streams: - writer.write_header_and_rows( - "streams", streams, ("name", "position"), - position=str(current_token) - ) - - @defer.inlineCallbacks - def events(self, writer, current_token, limit, request_streams): - request_events = request_streams.get("events") - request_backfill = request_streams.get("backfill") - - if request_events is not None or request_backfill is not None: - if request_events is None: - request_events = current_token.events - if request_backfill is None: - request_backfill = current_token.backfill - - no_new_tokens = ( - request_events == current_token.events - and request_backfill == current_token.backfill - ) - if no_new_tokens: - return - - res = yield self.store.get_all_new_events( - request_backfill, request_events, - current_token.backfill, current_token.events, - limit - ) - - upto_events_token = _position_from_rows( - res.new_forward_events, current_token.events - ) - - upto_backfill_token = _position_from_rows( - res.new_backfill_events, current_token.backfill - ) - - if request_events != upto_events_token: - writer.write_header_and_rows("events", res.new_forward_events, ( - "position", "event_id", "room_id", "type", "state_key", - ), position=upto_events_token) - - if request_backfill != upto_backfill_token: - writer.write_header_and_rows("backfill", res.new_backfill_events, ( - "position", "event_id", "room_id", "type", "state_key", "redacts", - ), position=upto_backfill_token) - - writer.write_header_and_rows( - "forward_ex_outliers", res.forward_ex_outliers, - ("position", "event_id", "state_group"), - ) - writer.write_header_and_rows( - "backward_ex_outliers", res.backward_ex_outliers, - ("position", "event_id", "state_group"), - ) - - @defer.inlineCallbacks - def presence(self, writer, current_token, request_streams): - current_position = current_token.presence - - request_presence = request_streams.get("presence") - - if request_presence is not None and request_presence != current_position: - presence_rows = yield self.presence_handler.get_all_presence_updates( - request_presence, current_position - ) - upto_token = _position_from_rows(presence_rows, current_position) - writer.write_header_and_rows("presence", presence_rows, ( - "position", "user_id", "state", "last_active_ts", - "last_federation_update_ts", "last_user_sync_ts", - "status_msg", "currently_active", - ), position=upto_token) - - @defer.inlineCallbacks - def typing(self, writer, current_token, request_streams): - current_position = current_token.typing - - request_typing = request_streams.get("typing") - - if request_typing is not None and request_typing != current_position: - # If they have a higher token than current max, we can assume that - # they had been talking to a previous instance of the master. Since - # we reset the token on restart, the best (but hacky) thing we can - # do is to simply resend down all the typing notifications. - if request_typing > current_position: - request_typing = 0 - - typing_rows = yield self.typing_handler.get_all_typing_updates( - request_typing, current_position - ) - upto_token = _position_from_rows(typing_rows, current_position) - writer.write_header_and_rows("typing", typing_rows, ( - "position", "room_id", "typing" - ), position=upto_token) - - @defer.inlineCallbacks - def receipts(self, writer, current_token, limit, request_streams): - current_position = current_token.receipts - - request_receipts = request_streams.get("receipts") - - if request_receipts is not None and request_receipts != current_position: - receipts_rows = yield self.store.get_all_updated_receipts( - request_receipts, current_position, limit - ) - upto_token = _position_from_rows(receipts_rows, current_position) - writer.write_header_and_rows("receipts", receipts_rows, ( - "position", "room_id", "receipt_type", "user_id", "event_id", "data" - ), position=upto_token) - - @defer.inlineCallbacks - def account_data(self, writer, current_token, limit, request_streams): - current_position = current_token.account_data - - user_account_data = request_streams.get("user_account_data") - room_account_data = request_streams.get("room_account_data") - tag_account_data = request_streams.get("tag_account_data") - - if user_account_data is not None or room_account_data is not None: - if user_account_data is None: - user_account_data = current_position - if room_account_data is None: - room_account_data = current_position - - no_new_tokens = ( - user_account_data == current_position - and room_account_data == current_position - ) - if no_new_tokens: - return - - user_rows, room_rows = yield self.store.get_all_updated_account_data( - user_account_data, room_account_data, current_position, limit - ) - - upto_users_token = _position_from_rows(user_rows, current_position) - upto_rooms_token = _position_from_rows(room_rows, current_position) - - writer.write_header_and_rows("user_account_data", user_rows, ( - "position", "user_id", "type", "content" - ), position=upto_users_token) - writer.write_header_and_rows("room_account_data", room_rows, ( - "position", "user_id", "room_id", "type", "content" - ), position=upto_rooms_token) - - if tag_account_data is not None: - tag_rows = yield self.store.get_all_updated_tags( - tag_account_data, current_position, limit - ) - upto_tag_token = _position_from_rows(tag_rows, current_position) - writer.write_header_and_rows("tag_account_data", tag_rows, ( - "position", "user_id", "room_id", "tags" - ), position=upto_tag_token) - - @defer.inlineCallbacks - def push_rules(self, writer, current_token, limit, request_streams): - current_position = current_token.push_rules - - push_rules = request_streams.get("push_rules") - - if push_rules is not None and push_rules != current_position: - rows = yield self.store.get_all_push_rule_updates( - push_rules, current_position, limit - ) - upto_token = _position_from_rows(rows, current_position) - writer.write_header_and_rows("push_rules", rows, ( - "position", "event_stream_ordering", "user_id", "rule_id", "op", - "priority_class", "priority", "conditions", "actions" - ), position=upto_token) - - @defer.inlineCallbacks - def pushers(self, writer, current_token, limit, request_streams): - current_position = current_token.pushers - - pushers = request_streams.get("pushers") - - if pushers is not None and pushers != current_position: - updated, deleted = yield self.store.get_all_updated_pushers( - pushers, current_position, limit - ) - upto_token = _position_from_rows(updated, current_position) - writer.write_header_and_rows("pushers", updated, ( - "position", "user_id", "access_token", "profile_tag", "kind", - "app_id", "app_display_name", "device_display_name", "pushkey", - "ts", "lang", "data" - ), position=upto_token) - writer.write_header_and_rows("deleted_pushers", deleted, ( - "position", "user_id", "app_id", "pushkey" - ), position=upto_token) - - @defer.inlineCallbacks - def caches(self, writer, current_token, limit, request_streams): - current_position = current_token.caches - - caches = request_streams.get("caches") - - if caches is not None and caches != current_position: - updated_caches = yield self.store.get_all_updated_caches( - caches, current_position, limit - ) - upto_token = _position_from_rows(updated_caches, current_position) - writer.write_header_and_rows("caches", updated_caches, ( - "position", "cache_func", "keys", "invalidation_ts" - ), position=upto_token) - - @defer.inlineCallbacks - def to_device(self, writer, current_token, limit, request_streams): - current_position = current_token.to_device - - to_device = request_streams.get("to_device") - - if to_device is not None and to_device != current_position: - to_device_rows = yield self.store.get_all_new_device_messages( - to_device, current_position, limit - ) - upto_token = _position_from_rows(to_device_rows, current_position) - writer.write_header_and_rows("to_device", to_device_rows, ( - "position", "user_id", "device_id", "message_json" - ), position=upto_token) - - @defer.inlineCallbacks - def public_rooms(self, writer, current_token, limit, request_streams): - current_position = current_token.public_rooms - - public_rooms = request_streams.get("public_rooms") - - if public_rooms is not None and public_rooms != current_position: - public_rooms_rows = yield self.store.get_all_new_public_rooms( - public_rooms, current_position, limit - ) - upto_token = _position_from_rows(public_rooms_rows, current_position) - writer.write_header_and_rows("public_rooms", public_rooms_rows, ( - "position", "room_id", "visibility", "appservice_id", "network_id", - ), position=upto_token) - - def federation(self, writer, current_token, limit, request_streams, federation_ack): - if self.config.send_federation: - return - - current_position = current_token.federation - - federation = request_streams.get("federation") - - if federation is not None and federation != current_position: - federation_rows = self.federation_sender.get_replication_rows( - federation, current_position, limit, federation_ack=federation_ack, - ) - upto_token = _position_from_rows(federation_rows, current_position) - writer.write_header_and_rows("federation", federation_rows, ( - "position", "type", "content", - ), position=upto_token) - - @defer.inlineCallbacks - def device_lists(self, writer, current_token, limit, request_streams): - current_position = current_token.device_lists - - device_lists = request_streams.get("device_lists") - - if device_lists is not None and device_lists != current_position: - changes = yield self.store.get_all_device_list_changes_for_remotes( - device_lists, current_position, - ) - writer.write_header_and_rows("device_lists", changes, ( - "position", "user_id", "destination", - ), position=current_position) - - -class _Writer(object): - """Writes the streams as a JSON object as the response to the request""" - def __init__(self): - self.streams = {} - self.total = 0 - - def write_header_and_rows(self, name, rows, fields, position=None): - if position is None: - if rows: - position = rows[-1][0] - else: - return - - self.streams[name] = { - "position": position if type(position) is int else str(position), - "field_names": fields, - "rows": rows, - } - - self.total += len(rows) - - def __nonzero__(self): - return bool(self.total) - - def finish(self): - return self.streams - - -class _ReplicationToken(collections.namedtuple("_ReplicationToken", ( - "events", "presence", "typing", "receipts", "account_data", "backfill", - "push_rules", "pushers", "state", "caches", "to_device", "public_rooms", - "federation", "device_lists", -))): - __slots__ = [] - - def __new__(cls, *args): - if len(args) == 1: - streams = [int(value) for value in args[0].split("_")] - if len(streams) < len(cls._fields): - streams.extend([0] * (len(cls._fields) - len(streams))) - return cls(*streams) - else: - return super(_ReplicationToken, cls).__new__(cls, *args) - - def __str__(self): - return "_".join(str(value) for value in self) - - -def _position_from_rows(rows, current_position): - """Calculates a position to return for a stream. Ideally we want to return the - position of the last row, as that will be the most correct. However, if there - are no rows we fall back to using the current position to stop us from - repeatedly hitting the storage layer unncessarily thinking there are updates. - (Not all advances of the token correspond to an actual update) - - We can't just always return the current position, as we often limit the - number of rows we replicate, and so the stream may lag. The assumption is - that if the storage layer returns no new rows then we are not lagging and - we are at the `current_position`. - """ - if rows: - return rows[-1][0] - return current_position |