Some house keeping on maybe_backfill() functions (#15709)

changelog.d/15709.misc

@@ -0,0 +1 @@
+Update docstring and traces on `maybe_backfill()` functions.

synapse/handlers/federation.py

@@ -200,6 +200,7 @@ def __init__(self, hs: "HomeServer"):
             )
 
     @trace
+    @tag_args
     async def maybe_backfill(
         self, room_id: str, current_depth: int, limit: int
     ) -> bool:
@@ -214,6 +215,9 @@ async def maybe_backfill(
             limit: The number of events that the pagination request will
                 return. This is used as part of the heuristic to decide if we
                 should back paginate.
+
+        Returns:
+            True if we actually tried to backfill something, otherwise False.
         """
         # Starting the processing time here so we can include the room backfill
         # linearizer lock queue in the timing
@@ -227,6 +231,8 @@ async def maybe_backfill(
                 processing_start_time=processing_start_time,
             )
 
+    @trace
+    @tag_args
     async def _maybe_backfill_inner(
         self,
         room_id: str,
@@ -247,6 +253,9 @@ async def _maybe_backfill_inner(
             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. If `None`, no timing observation will be made.
+
+        Returns:
+            True if we actually tried to backfill something, otherwise False.
         """
         backwards_extremities = [
             _BackfillPoint(event_id, depth, _BackfillPointType.BACKWARDS_EXTREMITY)
@@ -302,6 +311,14 @@ async def _maybe_backfill_inner(
             len(sorted_backfill_points),
             sorted_backfill_points,
         )
+        set_tag(
+            SynapseTags.RESULT_PREFIX + "sorted_backfill_points",
+            str(sorted_backfill_points),
+        )
+        set_tag(
+            SynapseTags.RESULT_PREFIX + "sorted_backfill_points.length",
+            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