1 files changed, 18 insertions, 4 deletions
diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py
index e4a5b64d10..a5f4ce7c8a 100644
--- a/synapse/handlers/federation_event.py
+++ b/synapse/handlers/federation_event.py
@@ -766,10 +766,24 @@ class FederationEventHandler:
         """
         logger.info("Processing pulled event %s", event)
 
-        # these should not be outliers.
-        assert (
-            not event.internal_metadata.is_outlier()
-        ), "pulled event unexpectedly flagged as outlier"
+        # This function should not be used to persist outliers (use something
+        # else) because this does a bunch of operations that aren't necessary
+        # (extra work; in particular, it makes sure we have all the prev_events
+        # and resolves the state across those prev events). If you happen to run
+        # into a situation where the event you're trying to process/backfill is
+        # marked as an `outlier`, then you should update that spot to return an
+        # `EventBase` copy that doesn't have `outlier` flag set.
+        #
+        # `EventBase` is used to represent both an event we have not yet
+        # persisted, and one that we have persisted and now keep in the cache.
+        # In an ideal world this method would only be called with the first type
+        # of event, but it turns out that's not actually the case and for
+        # example, you could get an event from cache that is marked as an
+        # `outlier` (fix up that spot though).
+        assert not event.internal_metadata.is_outlier(), (
+            "Outlier event passed to _process_pulled_event. "
+            "To persist an event as a non-outlier, make sure to pass in a copy without `event.internal_metadata.outlier = true`."
+        )
 
         event_id = event.event_id