Backfill in the background if we're doing it "just because" (#15710)

Fix #15702

changelog.d/15710.feature

@@ -0,0 +1 @@
+Speed up `/messages` by backfilling in the background when there are no backward extremities where we are directly paginating.

synapse/handlers/federation.py

@@ -320,14 +320,21 @@ async def _maybe_backfill_inner(
  str(len(sorted_backfill_points)),
  )
 
- # If we have no backfill points lower than the `current_depth` then
- # either we can a) bail or b) still attempt to backfill. We opt to try
- # backfilling anyway just in case we do get relevant events.
+ # If we have no backfill points lower than the `current_depth` then either we
+ # can a) bail or b) still attempt to backfill. We opt to try backfilling anyway
+ # just in case we do get relevant events. This is good for eventual consistency
+ # sake but we don't need to block the client for something that is just as
+ # likely not to return anything relevant so we backfill in the background. The
+ # only way, this could return something relevant is if we discover a new branch
+ # of history that extends all the way back to where we are currently paginating
+ # and it's within the 100 events that are returned from `/backfill`.
  if not sorted_backfill_points and current_depth != MAX_DEPTH:
  logger.debug(
  "_maybe_backfill_inner: all backfill points are *after* current depth. Trying again with later backfill points."
  )
- return await self._maybe_backfill_inner(
+ run_as_background_process(
+ "_maybe_backfill_inner_anyway_with_max_depth",
+ self._maybe_backfill_inner,
  room_id=room_id,
  # We use `MAX_DEPTH` so that we find all backfill points next
  # time (all events are below the `MAX_DEPTH`)
@@ -338,6 +345,9 @@ async def _maybe_backfill_inner(
  # overall otherwise the smaller one will throw off the results.
  processing_start_time=None,
  )
+ # We return `False` because we're backfilling in the background and there is
+ # no new events immediately for the caller to know about yet.
+ return False
 
  # Even after recursing with `MAX_DEPTH`, we didn't find any
  # backward extremities to backfill from.