diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 8b995c0d3e..cad103d39f 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -179,60 +179,6 @@ class FederationHandler:
logger.debug("Not backfilling as no extremeties found.")
return False
- # We only want to paginate if we can actually see the events we'll get,
- # as otherwise we'll just spend a lot of resources to get redacted
- # events.
- #
- # We do this by filtering all the backwards extremities and seeing if
- # any remain. Given we don't have the extremity events themselves, we
- # need to actually check the events that reference them.
- #
- # *Note*: the spec wants us to keep backfilling until we reach the start
- # of the room in case we are allowed to see some of the history. However
- # in practice that causes more issues than its worth, as a) its
- # relatively rare for there to be any visible history and b) even when
- # there is its often sufficiently long ago that clients would stop
- # attempting to paginate before backfill reached the visible history.
- #
- # TODO: If we do do a backfill then we should filter the backwards
- # extremities to only include those that point to visible portions of
- # history.
- #
- # TODO: Correctly handle the case where we are allowed to see the
- # forward event but not the backward extremity, e.g. in the case of
- # initial join of the server where we are allowed to see the join
- # event but not anything before it. This would require looking at the
- # state *before* the event, ignoring the special casing certain event
- # types have.
-
- forward_event_ids = await self.store.get_successor_events(
- list(oldest_events_with_depth)
- )
-
- extremities_events = await self.store.get_events(
- forward_event_ids,
- redact_behaviour=EventRedactBehaviour.AS_IS,
- get_prev_content=False,
- )
-
- # We set `check_history_visibility_only` as we might otherwise get false
- # positives from users having been erased.
- filtered_extremities = await filter_events_for_server(
- self.storage,
- self.server_name,
- list(extremities_events.values()),
- redact=False,
- check_history_visibility_only=True,
- )
- logger.debug(
- "_maybe_backfill_inner: filtered_extremities %s", filtered_extremities
- )
-
- if not filtered_extremities and not insertion_events_to_be_backfilled:
- return False
-
- # TODO: insertion_events_to_be_backfilled is currently skipping the filtered_extremities checks
-
# we now have a list of potential places to backpaginate from. We prefer to
# start with the most recent (ie, max depth), so let's sort the list.
sorted_extremeties_tuples: List[Tuple[str, int]] = sorted(
@@ -292,6 +238,60 @@ class FederationHandler:
if filtered_sorted_extremeties_tuples:
sorted_extremeties_tuples = filtered_sorted_extremeties_tuples
+ # We only want to paginate if we can actually see the events we'll get,
+ # as otherwise we'll just spend a lot of resources to get redacted
+ # events.
+ #
+ # We do this by filtering all the backwards extremities and seeing if
+ # any remain. Given we don't have the extremity events themselves, we
+ # need to actually check the events that reference them.
+ #
+ # *Note*: the spec wants us to keep backfilling until we reach the start
+ # of the room in case we are allowed to see some of the history. However
+ # in practice that causes more issues than its worth, as a) its
+ # relatively rare for there to be any visible history and b) even when
+ # there is its often sufficiently long ago that clients would stop
+ # attempting to paginate before backfill reached the visible history.
+ #
+ # TODO: If we do do a backfill then we should filter the backwards
+ # extremities to only include those that point to visible portions of
+ # history.
+ #
+ # TODO: Correctly handle the case where we are allowed to see the
+ # forward event but not the backward extremity, e.g. in the case of
+ # initial join of the server where we are allowed to see the join
+ # event but not anything before it. This would require looking at the
+ # state *before* the event, ignoring the special casing certain event
+ # types have.
+
+ forward_event_ids = await self.store.get_successor_events(
+ list(oldest_events_with_depth)
+ )
+
+ extremities_events = await self.store.get_events(
+ forward_event_ids,
+ redact_behaviour=EventRedactBehaviour.AS_IS,
+ get_prev_content=False,
+ )
+
+ # We set `check_history_visibility_only` as we might otherwise get false
+ # positives from users having been erased.
+ filtered_extremities = await filter_events_for_server(
+ self.storage,
+ self.server_name,
+ list(extremities_events.values()),
+ redact=False,
+ check_history_visibility_only=True,
+ )
+ logger.debug(
+ "_maybe_backfill_inner: filtered_extremities %s", filtered_extremities
+ )
+
+ if not filtered_extremities and not insertion_events_to_be_backfilled:
+ return False
+
+ # TODO: insertion_events_to_be_backfilled is currently skipping the filtered_extremities checks
+
# We don't want to specify too many extremities as it causes the backfill
# request URI to be too long.
extremities = dict(sorted_extremeties_tuples[:5])
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