Patch from the async code

Author: dragomirp

src/relations/async_replication.py

@@ -544,6 +544,16 @@ def _on_async_relation_changed(self, event: RelationChangedEvent) -> None:
         if self._configure_primary_cluster(primary_cluster, event):
             return
 
+        # Patch API config if following leader.
+        if (
+            self.charm.unit.is_leader()
+            and self._is_following_promoted_cluster()
+            and (primary_endpoint := self.get_primary_cluster_endpoint())
+        ):
+            self.charm._patroni.bulk_update_parameters_controller_by_patroni({
+                "standby_cluster": {"host": primary_endpoint}
+            })
+
         # Return if this is a new unit.
         if not self.charm.unit.is_leader() and self._is_following_promoted_cluster():
             logger.debug("Early exit on_async_relation_changed: following promoted cluster.")

tests/integration/high_availability/test_async_replication.py

@@ -266,6 +266,7 @@ def test_unrelate_and_relate(first_model: str, second_model: str) -> None:
 def test_failover_in_main_cluster(first_model: str, second_model: str) -> None:
     """Test that async replication fails over correctly."""
     model_1 = Juju(model=first_model)
+    model_2 = Juju(model=second_model)
 
     rerelate_test_app(model_1, DB_APP_1, DB_TEST_APP_1)
 
@@ -274,6 +275,9 @@ def test_failover_in_main_cluster(first_model: str, second_model: str) -> None:
     model_1.wait(
         ready=wait_for_apps_status(jubilant.all_active, DB_APP_1), timeout=10 * MINUTE_SECS
     )
+    model_2.wait(
+        ready=wait_for_apps_status(jubilant.all_active, DB_APP_2), timeout=10 * MINUTE_SECS
+    )
 
     results = get_db_max_written_values(first_model, second_model, first_model, DB_TEST_APP_1)