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-rw-r--r--synapse/handlers/federation.py109
1 files changed, 61 insertions, 48 deletions
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 360ab6fee2..500c1c16d0 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -38,7 +38,7 @@ from signedjson.sign import verify_signed_json
 from unpaddedbase64 import decode_base64
 
 from synapse import event_auth
-from synapse.api.constants import EventContentFields, EventTypes, Membership
+from synapse.api.constants import MAX_DEPTH, EventContentFields, EventTypes, Membership
 from synapse.api.errors import (
     AuthError,
     CodeMessageException,
@@ -211,7 +211,7 @@ class FederationHandler:
         current_depth: int,
         limit: int,
         *,
-        processing_start_time: int,
+        processing_start_time: Optional[int],
     ) -> bool:
         """
         Checks whether the `current_depth` is at or approaching any backfill
@@ -223,12 +223,23 @@ class FederationHandler:
             room_id: The room to backfill in.
             current_depth: The depth to check at for any upcoming backfill points.
             limit: The max number of events to request from the remote federated server.
-            processing_start_time: The time when `maybe_backfill` started
-                processing. Only used for timing.
+            processing_start_time: The time when `maybe_backfill` started processing.
+                Only used for timing. If `None`, no timing observation will be made.
         """
         backwards_extremities = [
             _BackfillPoint(event_id, depth, _BackfillPointType.BACKWARDS_EXTREMITY)
-            for event_id, depth in await self.store.get_backfill_points_in_room(room_id)
+            for event_id, depth in await self.store.get_backfill_points_in_room(
+                room_id=room_id,
+                current_depth=current_depth,
+                # We only need to end up with 5 extremities combined with the
+                # insertion event extremities to make the `/backfill` request
+                # but fetch an order of magnitude more to make sure there is
+                # enough even after we filter them by whether visible in the
+                # history. This isn't fool-proof as all backfill points within
+                # our limit could be filtered out but seems like a good amount
+                # to try with at least.
+                limit=50,
+            )
         ]
 
         insertion_events_to_be_backfilled: List[_BackfillPoint] = []
@@ -236,7 +247,12 @@ class FederationHandler:
             insertion_events_to_be_backfilled = [
                 _BackfillPoint(event_id, depth, _BackfillPointType.INSERTION_PONT)
                 for event_id, depth in await self.store.get_insertion_event_backward_extremities_in_room(
-                    room_id
+                    room_id=room_id,
+                    current_depth=current_depth,
+                    # We only need to end up with 5 extremities combined with
+                    # the backfill points to make the `/backfill` request ...
+                    # (see the other comment above for more context).
+                    limit=50,
                 )
             ]
         logger.debug(
@@ -245,10 +261,6 @@ class FederationHandler:
             insertion_events_to_be_backfilled,
         )
 
-        if not backwards_extremities and not insertion_events_to_be_backfilled:
-            logger.debug("Not backfilling as no extremeties found.")
-            return False
-
         # 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_backfill_points: List[_BackfillPoint] = sorted(
@@ -269,6 +281,33 @@ class FederationHandler:
             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 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(
+                room_id=room_id,
+                # We use `MAX_DEPTH` so that we find all backfill points next
+                # time (all events are below the `MAX_DEPTH`)
+                current_depth=MAX_DEPTH,
+                limit=limit,
+                # We don't want to start another timing observation from this
+                # nested recursive call. The top-most call can record the time
+                # overall otherwise the smaller one will throw off the results.
+                processing_start_time=None,
+            )
+
+        # Even after recursing with `MAX_DEPTH`, we didn't find any
+        # backward extremities to backfill from.
+        if not sorted_backfill_points:
+            logger.debug(
+                "_maybe_backfill_inner: Not backfilling as no backward extremeties found."
+            )
+            return False
+
         # If we're approaching an extremity we trigger a backfill, otherwise we
         # no-op.
         #
@@ -278,47 +317,16 @@ class FederationHandler:
         # chose more than one times the limit in case of failure, but choosing a
         # much larger factor will result in triggering a backfill request much
         # earlier than necessary.
-        #
-        # XXX: shouldn't we do this *after* the filter by depth below? Again, we don't
-        # care about events that have happened after our current position.
-        #
-        max_depth = sorted_backfill_points[0].depth
-        if current_depth - 2 * limit > max_depth:
+        max_depth_of_backfill_points = sorted_backfill_points[0].depth
+        if current_depth - 2 * limit > max_depth_of_backfill_points:
             logger.debug(
                 "Not backfilling as we don't need to. %d < %d - 2 * %d",
-                max_depth,
+                max_depth_of_backfill_points,
                 current_depth,
                 limit,
             )
             return False
 
-        # We ignore extremities that have a greater depth than our current depth
-        # as:
-        #    1. we don't really care about getting events that have happened
-        #       after our current position; and
-        #    2. we have likely previously tried and failed to backfill from that
-        #       extremity, so to avoid getting "stuck" requesting the same
-        #       backfill repeatedly we drop those extremities.
-        #
-        # However, we need to check that the filtered extremities are non-empty.
-        # If they are empty 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.
-        #
-        filtered_sorted_backfill_points = [
-            t for t in sorted_backfill_points if t.depth <= current_depth
-        ]
-        if filtered_sorted_backfill_points:
-            logger.debug(
-                "_maybe_backfill_inner: backfill points before current depth: %s",
-                filtered_sorted_backfill_points,
-            )
-            sorted_backfill_points = filtered_sorted_backfill_points
-        else:
-            logger.debug(
-                "_maybe_backfill_inner: all backfill points are *after* current depth. Backfilling anyway."
-            )
-
         # For performance's sake, we only want to paginate from a particular extremity
         # if we can actually see the events we'll get. Otherwise, we'd just spend a lot
         # of resources to get redacted events. We check each extremity in turn and
@@ -452,10 +460,15 @@ class FederationHandler:
 
             return False
 
-        processing_end_time = self.clock.time_msec()
-        backfill_processing_before_timer.observe(
-            (processing_end_time - processing_start_time) / 1000
-        )
+        # If we have the `processing_start_time`, then we can make an
+        # observation. We wouldn't have the `processing_start_time` in the case
+        # where `_maybe_backfill_inner` is recursively called to find any
+        # backfill points regardless of `current_depth`.
+        if processing_start_time is not None:
+            processing_end_time = self.clock.time_msec()
+            backfill_processing_before_timer.observe(
+                (processing_end_time - processing_start_time) / 1000
+            )
 
         success = await try_backfill(likely_domains)
         if success: