Delete unreferenced state groups in background (#18254)

This PR fixes #18154 to avoid de-deltaing state groups which resulted in
DB size temporarily increasing until the DB was `VACUUM`'ed. As a
result, less state groups will get deleted now.
It also attempts to improve performance by not duplicating work when
processing state groups it has already processed in previous iterations.

### Pull Request Checklist

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before submitting your pull request -->

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- Be a short description of your change which makes sense to users.
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---------

Co-authored-by: Erik Johnston <erikj@element.io>

1 files changed, 30 insertions, 0 deletions

diff --git a/synapse/_scripts/synapse_port_db.py b/synapse/_scripts/synapse_port_db.py

index 1bb9940180..438b2ff8a0 100755
--- a/synapse/_scripts/synapse_port_db.py
+++ b/synapse/_scripts/synapse_port_db.py
@@ -192,6 +192,11 @@ APPEND_ONLY_TABLES = [
 
 
 IGNORED_TABLES = {
+    # Porting the auto generated sequence in this table is non-trivial.
+    # None of the entries in this list are mandatory for Synapse to keep working.
+    # If state group disk space is an issue after the port, the
+    # `mark_unreferenced_state_groups_for_deletion_bg_update` background task can be run again.
+    "state_groups_pending_deletion",
     # We don't port these tables, as they're a faff and we can regenerate
     # them anyway.
     "user_directory",
@@ -217,6 +222,15 @@ IGNORED_TABLES = {
 }
 
 
+# These background updates will not be applied upon creation of the postgres database.
+IGNORED_BACKGROUND_UPDATES = {
+    # Reapplying this background update to the postgres database is unnecessary after
+    # already having waited for the SQLite database to complete all running background
+    # updates.
+    "mark_unreferenced_state_groups_for_deletion_bg_update",
+}
+
+
 # Error returned by the run function. Used at the top-level part of the script to
 # handle errors and return codes.
 end_error: Optional[str] = None
@@ -688,6 +702,20 @@ class Porter:
         # 0 means off. 1 means full. 2 means incremental.
         return autovacuum_setting != 0
 
+    async def remove_ignored_background_updates_from_database(self) -> None:
+        def _remove_delete_unreferenced_state_groups_bg_updates(
+            txn: LoggingTransaction,
+        ) -> None:
+            txn.execute(
+                "DELETE FROM background_updates WHERE update_name = ANY(?)",
+                (list(IGNORED_BACKGROUND_UPDATES),),
+            )
+
+        await self.postgres_store.db_pool.runInteraction(
+            "remove_delete_unreferenced_state_groups_bg_updates",
+            _remove_delete_unreferenced_state_groups_bg_updates,
+        )
+
     async def run(self) -> None:
         """Ports the SQLite database to a PostgreSQL database.
 
@@ -733,6 +761,8 @@ class Porter:
                 self.hs_config.database.get_single_database()
             )
 
+            await self.remove_ignored_background_updates_from_database()
+
             await self.run_background_updates_on_postgres()
 
             self.progress.set_state("Creating port tables")