Internal server error when calling /relations with a token from /sync

[clokep]

2023-01-12 16:41:20,025 - synapse.http.server - 124 - ERROR - GET-832933- Failed handle request via 'RelationPaginationServlet': <XForwardedForRequest at 0x7f5a7a2c98c0 method='GET' uri='/_matrix/client/v1/rooms/!test%3Amatrix.org/relations/%24foo/m.thread?dir=b&limit=20&from=sXXXX_1_YYY_ZZZ_AAA_BBB_CCC_DDD_0' clientproto='HTTP/1.1' site='8008'>
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/synapse/util/caches/descriptors.py", line 243, in _wrapped
    ret = cache.get(cache_key, callback=invalidate_callback)
  File "/usr/local/lib/python3.9/site-packages/synapse/util/caches/deferred_cache.py", line 180, in get
    raise KeyError()
KeyError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/synapse/http/server.py", line 307, in _async_render_wrapper
    callback_return = await self._async_render(request)
  File "/usr/local/lib/python3.9/site-packages/synapse/http/server.py", line 513, in _async_render
    callback_return = await raw_callback_return
  File "/usr/local/lib/python3.9/site-packages/synapse/rest/client/relations.py", line 71, in on_GET
    result = await self._relations_handler.get_relations(
  File "/usr/local/lib/python3.9/site-packages/synapse/handlers/relations.py", line 125, in get_relations
    related_events, next_token = await self._main_store.get_relations_for_event(
  File "/usr/local/lib/python3.9/site-packages/twisted/internet/defer.py", line 1697, in _inlineCallbacks
    result = context.run(gen.send, result)
  File "/usr/local/lib/python3.9/site-packages/synapse/storage/databases/main/relations.py", line 213, in get_relations_for_event
    from_token=from_token.room_key.as_historical_tuple()
  File "/usr/local/lib/python3.9/site-packages/synapse/types.py", line 569, in as_historical_tuple
    raise Exception(
Exception: Cannot call `RoomStreamToken.as_historical_tuple` on live token

https://sentry.tools.element.io/organizations/element/issues/58936/

[clokep]

Reported by @justjanne, trying to use a token from /sync when paginating /relations, which should be allowed (df36945).

[clokep]

Related to #13862, I think.

[clokep]

The way the pagination clause is generated is slightly incorrect, as #13862 kind of talks about. We need to abstract / port from /messages, something like this:

synapse/synapse/storage/databases/main/stream.py

Lines 1284 to 1315 in 84ce93c

	# The bounds for the stream tokens are complicated by the fact
	# that we need to handle the instance_map part of the tokens. We do this
	# by fetching all events between the min stream token and the maximum
	# stream token (as returned by `RoomStreamToken.get_max_stream_pos`) and
	# then filtering the results.
	if from_token.topological is not None:
	from_bound: Tuple[Optional[int], int] = from_token.as_historical_tuple()
	elif direction == "b":
	from_bound = (
	None,
	from_token.get_max_stream_pos(),
	)
	else:
	from_bound = (
	None,
	from_token.stream,
	)
	to_bound: Optional[Tuple[Optional[int], int]] = None
	if to_token:
	if to_token.topological is not None:
	to_bound = to_token.as_historical_tuple()
	elif direction == "b":
	to_bound = (
	None,
	to_token.stream,
	)
	else:
	to_bound = (
	None,
	to_token.get_max_stream_pos(),
	)

I have a branch that attempts to do this but is very bitrotted. I need to recreate it (or rebase it).