Handle missing stanza output (#866)

Author: dragomirp

src/backups.py

@@ -187,7 +187,10 @@ def can_use_s3_repository(self) -> tuple[bool, str | None]:
                 for line in system_identifier_from_instance.splitlines()
                 if "Database system identifier" in line
             ).split(" ")[-1]
-            system_identifier_from_stanza = str(stanza.get("db")[0]["system-id"])
+            stanza_dbs = stanza.get("db")
+            system_identifier_from_stanza = (
+                str(stanza_dbs[0]["system-id"]) if len(stanza_dbs) else None
+            )
             if system_identifier_from_instance != system_identifier_from_stanza:
                 logger.debug(
                     f"can_use_s3_repository: incompatible system identifier s3={system_identifier_from_stanza}, local={system_identifier_from_instance}"

tests/unit/test_backups.py

@@ -288,6 +288,18 @@ def test_can_use_s3_repository(harness):
         ]
         assert harness.charm.backup.can_use_s3_repository() == (True, None)
 
+        # Empty db
+        _execute_command.side_effect = [
+            (
+                f'[{{"db": [], "name": "another-model.{harness.charm.cluster_name}"}}]',
+                None,
+            )
+        ]
+        assert harness.charm.backup.can_use_s3_repository() == (
+            False,
+            ANOTHER_CLUSTER_REPOSITORY_ERROR_MESSAGE,
+        )
+
 
 def test_construct_endpoint(harness):
     # Test with an AWS endpoint without region.