Fix deleting of stale marker for device lists (#6819)

We were in fact only deleting stale marker when we got an incremental
update, rather than when we did a full resync.

changelog.d/6819.misc

@@ -0,0 +1 @@
+Detect unknown remote devices and mark cache as stale.

synapse/storage/data_stores/main/devices.py

@@ -940,13 +940,6 @@ def _update_remote_device_list_cache_entry_txn(
  lock=False,
  )
 
- # If we're replacing the remote user's device list cache presumably
- # we've done a full resync, so we remove the entry that says we need
- # to resync
- self.db.simple_delete_txn(
- txn, table="device_lists_remote_resync", keyvalues={"user_id": user_id},
- )
-
  def update_remote_device_list_cache(self, user_id, devices, stream_id):
  """Replace the entire cache of the remote user's devices.
 
@@ -1003,6 +996,13 @@ def _update_remote_device_list_cache_txn(self, txn, user_id, devices, stream_id)
  lock=False,
  )
 
+ # If we're replacing the remote user's device list cache presumably
+ # we've done a full resync, so we remove the entry that says we need
+ # to resync
+ self.db.simple_delete_txn(
+ txn, table="device_lists_remote_resync", keyvalues={"user_id": user_id},
+ )
+
  @defer.inlineCallbacks
  def add_device_change_to_streams(self, user_id, device_ids, hosts):
  """Persist that a user's devices have been updated, and which hosts