Fix race which caused deleted devices to reappear

changelog.d/6514.bugfix

@@ -0,0 +1 @@
+Fix race which occasionally caused deleted devices to reappear.

synapse/storage/data_stores/main/client_ips.py

@@ -451,16 +451,18 @@ def _update_client_ips_batch_txn(self, txn, to_update):
                 # Technically an access token might not be associated with
                 # a device so we need to check.
                 if device_id:
-                    self.db.simple_upsert_txn(
+                    # this is always an update rather than an upsert: the row should
+                    # already exist, and if it doesn't, that may be because it has been
+                    # deleted, and we don't want to re-create it.
+                    self.db.simple_update_txn(
                         txn,
                         table="devices",
                         keyvalues={"user_id": user_id, "device_id": device_id},
-                        values={
+                        updatevalues={
                             "user_agent": user_agent,
                             "last_seen": last_seen,
                             "ip": ip,
                         },
-                        lock=False,
                     )
             except Exception as e:
                 # Failed to upsert, log and continue