Prep work for removing outlier from internal_metadata (#9411)

* Populate `internal_metadata.outlier` based on `events` table

Rather than relying on `outlier` being in the `internal_metadata` column,
populate it based on the `events.outlier` column.

* Move `outlier` out of InternalMetadata._dict

Ultimately, this will allow us to stop writing it to the database. For now, we
have to grandfather it back in so as to maintain compatibility with older
versions of Synapse.

synapse/storage/databases/main/events.py

@@ -1270,8 +1270,10 @@ class PersistEventsStore:
                     logger.exception("")
                     raise
 
+                # update the stored internal_metadata to update the "outlier" flag.
+                # TODO: This is unused as of Synapse 1.31. Remove it once we are happy
+                #  to drop backwards-compatibility with 1.30.
                 metadata_json = json_encoder.encode(event.internal_metadata.get_dict())
-
                 sql = "UPDATE event_json SET internal_metadata = ? WHERE event_id = ?"
                 txn.execute(sql, (metadata_json, event.event_id))
 
@@ -1319,6 +1321,19 @@ class PersistEventsStore:
             d.pop("redacted_because", None)
             return d
 
+        def get_internal_metadata(event):
+            im = event.internal_metadata.get_dict()
+
+            # temporary hack for database compatibility with Synapse 1.30 and earlier:
+            # store the `outlier` flag inside the internal_metadata json as well as in
+            # the `events` table, so that if anyone rolls back to an older Synapse,
+            # things keep working. This can be removed once we are happy to drop support
+            # for that
+            if event.internal_metadata.is_outlier():
+                im["outlier"] = True
+
+            return im
+
         self.db_pool.simple_insert_many_txn(
             txn,
             table="event_json",
@@ -1327,7 +1342,7 @@ class PersistEventsStore:
                     "event_id": event.event_id,
                     "room_id": event.room_id,
                     "internal_metadata": json_encoder.encode(
-                        event.internal_metadata.get_dict()
+                        get_internal_metadata(event)
                     ),
                     "json": json_encoder.encode(event_dict(event)),
                     "format_version": event.format_version,