only save remote cross-signing keys if they're different from the current ones (#9634)

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

changelog.d/9634.misc

@@ -0,0 +1 @@
+Only save remote cross-signing and device keys if they're different from the current ones.

synapse/handlers/device.py

@@ -907,6 +907,7 @@ class DeviceListUpdater:
         master_key = result.get("master_key")
         self_signing_key = result.get("self_signing_key")
 
+        ignore_devices = False
         # If the remote server has more than ~1000 devices for this user
         # we assume that something is going horribly wrong (e.g. a bot
         # that logs in and creates a new device every time it tries to
@@ -925,6 +926,12 @@ class DeviceListUpdater:
                 len(devices),
             )
             devices = []
+            ignore_devices = True
+        else:
+            cached_devices = await self.store.get_cached_devices_for_user(user_id)
+            if cached_devices == {d["device_id"]: d for d in devices}:
+                devices = []
+                ignore_devices = True
 
         for device in devices:
             logger.debug(
@@ -934,7 +941,10 @@ class DeviceListUpdater:
                 stream_id,
             )
 
-        await self.store.update_remote_device_list_cache(user_id, devices, stream_id)
+        if not ignore_devices:
+            await self.store.update_remote_device_list_cache(
+                user_id, devices, stream_id
+            )
         device_ids = [device["device_id"] for device in devices]
 
         # Handle cross-signing keys.
@@ -945,7 +955,8 @@ class DeviceListUpdater:
         )
         device_ids = device_ids + cross_signing_device_ids
 
-        await self.device_handler.notify_device_update(user_id, device_ids)
+        if device_ids:
+            await self.device_handler.notify_device_update(user_id, device_ids)
 
         # We clobber the seen updates since we've re-synced from a given
         # point.
@@ -973,14 +984,17 @@ class DeviceListUpdater:
         """
         device_ids = []
 
-        if master_key:
+        current_keys_map = await self.store.get_e2e_cross_signing_keys_bulk([user_id])
+        current_keys = current_keys_map.get(user_id) or {}
+
+        if master_key and master_key != current_keys.get("master"):
             await self.store.set_e2e_cross_signing_key(user_id, "master", master_key)
             _, verify_key = get_verify_key_from_cross_signing_key(master_key)
             # verify_key is a VerifyKey from signedjson, which uses
             # .version to denote the portion of the key ID after the
             # algorithm and colon, which is the device ID
             device_ids.append(verify_key.version)
-        if self_signing_key:
+        if self_signing_key and self_signing_key != current_keys.get("self_signing"):
             await self.store.set_e2e_cross_signing_key(
                 user_id, "self_signing", self_signing_key
             )