From c10b78469f84aa67828b6c6160ed4724405db615 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Fri, 12 Dec 2025 14:22:00 +0000
Subject: [PATCH] Prune sliding_sync_connection_required_state

---
 .../storage/databases/main/sliding_sync.py    | 38 +++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/synapse/storage/databases/main/sliding_sync.py b/synapse/storage/databases/main/sliding_sync.py
index c66002dae4..3c32550e97 100644
--- a/synapse/storage/databases/main/sliding_sync.py
+++ b/synapse/storage/databases/main/sliding_sync.py
@@ -450,6 +450,9 @@ class SlidingSyncStore(SQLBaseStore):
 
         # Now that we have seen the client has received and used the connection
         # position, we can delete all the other connection positions.
+        #
+        # Note: the rest of the code here assumes this is the only remaining
+        # connection position.
         sql = """
             DELETE FROM sliding_sync_connection_positions
             WHERE connection_key = ? AND connection_position != ?
@@ -515,6 +518,41 @@ class SlidingSyncStore(SQLBaseStore):
                 required_state_map=required_state_map[required_state_id],
             )
 
+        # Clean up any required state IDs that are no longer used by any
+        # connection position on this connection.
+        #
+        # We store the required state config per-connection per-room. Since this
+        # can be a lot of data, we deduplicate the required state JSON and store
+        # it separately, with multiple rooms referencing the same required state
+        # ID. Over time as the required state configs change, some required
+        # state IDs may no longer be referenced by any room config, so we need
+        # to clean them up.
+        #
+        # We do this by noting that we have pulled out *all* rows from
+        # `sliding_sync_connection_required_state` for this connection above. We
+        # have also pulled out all referenced required state IDs for *this*
+        # connection position, which is the only connection position that
+        # remains (we deleted the others above).
+        #
+        # Thus we can compute the unused required state IDs by looking for any
+        # required state IDs that are not referenced by the remaining connection
+        # position.
+        used_required_state_ids = {
+            required_state_id for _, _, required_state_id in room_config_rows
+        }
+
+        unused_required_state_ids = required_state_map.keys() - used_required_state_ids
+        if unused_required_state_ids:
+            self.db_pool.simple_delete_many_batch_txn(
+                txn,
+                table="sliding_sync_connection_required_state",
+                keys=("connection_key", "required_state_id"),
+                values=[
+                    (connection_key, required_state_id)
+                    for required_state_id in unused_required_state_ids
+                ],
+            )
+
         # Now look up the per-room stream data.
         rooms: dict[str, HaveSentRoom[str]] = {}
         receipts: dict[str, HaveSentRoom[str]] = {}