fix: make create-replication action fail cleanly without async-replication relation (#1676)

* fix: make create-replication action fail cleanly without async-replication relation

Signed-off-by: SAY-5 <saiasish.cnp@gmail.com>

* fix(test): scope PropertyMock to prevent class-level leak

Signed-off-by: SAY-5 <saiasish.cnp@gmail.com>

---------

Signed-off-by: SAY-5 <saiasish.cnp@gmail.com>
Co-authored-by: Dragomir Penev <dragomir.penev@canonical.com>

Author: SAY-5

src/relations/async_replication.py

@@ -567,6 +567,13 @@ def _on_create_replication(self, event: ActionEvent) -> None:
             event.fail("There is already a replication set up.")
             return
 
+        if self._relation is None:
+            event.fail(
+                "No async-replication relation has been established."
+                " Create the offer and relate the two clusters before running this action."
+            )
+            return
+
         if self._relation.name == REPLICATION_CONSUMER_RELATION:
             event.fail("This action must be run in the cluster where the offer was created.")
             return

tests/unit/test_async_replication.py

@@ -571,6 +571,25 @@ def test_on_create_replication():
 
     assert result is None
 
+    # 5. No async-replication relation established (regression for #1675).
+    mock_charm = MagicMock()
+    mock_event = MagicMock()
+
+    relation = PostgreSQLAsyncReplication(mock_charm)
+
+    relation._get_primary_cluster = MagicMock(return_value=None)
+
+    with patch.object(
+        PostgreSQLAsyncReplication, "_relation", new_callable=PropertyMock, return_value=None
+    ):
+        result = relation._on_create_replication(mock_event)
+
+    assert result is None
+    mock_event.fail.assert_called_once_with(
+        "No async-replication relation has been established."
+        " Create the offer and relate the two clusters before running this action."
+    )
+
 
 def test_promote_to_primary():
     # 1.