Bg update to populate new events table columns (#13215)

These columns were added back in Synapse 1.52, and have been populated for new
events since then. It's now (beyond) time to back-populate them for existing
events.

synapse/storage/schema/main/delta/72/03bg_populate_events_columns.py

@@ -0,0 +1,47 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import json
+
+from synapse.storage.types import Cursor
+
+
+def run_create(cur: Cursor, database_engine, *args, **kwargs):
+    """Add a bg update to populate the `state_key` and `rejection_reason` columns of `events`"""
+
+    # we know that any new events will have the columns populated (and that has been
+    # the case since schema_version 68, so there is no chance of rolling back now).
+    #
+    # So, we only need to make sure that existing rows are updated. We read the
+    # current min and max stream orderings, since that is guaranteed to include all
+    # the events that were stored before the new columns were added.
+    cur.execute("SELECT MIN(stream_ordering), MAX(stream_ordering) FROM events")
+    (min_stream_ordering, max_stream_ordering) = cur.fetchone()
+
+    if min_stream_ordering is None:
+        # no rows, nothing to do.
+        return
+
+    cur.execute(
+        "INSERT into background_updates (ordering, update_name, progress_json)"
+        " VALUES (7203, 'events_populate_state_key_rejections', ?)",
+        (
+            json.dumps(
+                {
+                    "min_stream_ordering_exclusive": min_stream_ordering - 1,
+                    "max_stream_ordering_inclusive": max_stream_ordering,
+                }
+            ),
+        ),
+    )