util: avoid overwriting config_file with None on failed jinja render

When render_jinja_payload_from_file returns None (e.g. due to an
undefined attribute error), read_conf previously assigned config_file
to None, causing load_yaml to crash with an AttributeError. Only update
config_file when a valid rendered payload is returned.

Fixes GH-7007

Author: sundeep8967

cloudinit/util.py

@@ -327,17 +327,19 @@ def read_conf(fname, *, instance_data_file=None) -> Dict:
 
     if instance_data_file and os.path.exists(instance_data_file):
         try:
-            config_file = render_jinja_payload_from_file(
+            rendered_config = render_jinja_payload_from_file(
                 config_file,
                 fname,
                 instance_data_file,
             )
-            LOG.debug(
-                "Applied instance data in '%s' to "
-                "configuration loaded from '%s'",
-                instance_data_file,
-                fname,
-            )
+            if rendered_config is not None:
+                config_file = rendered_config
+                LOG.debug(
+                    "Applied instance data in '%s' to "
+                    "configuration loaded from '%s'",
+                    instance_data_file,
+                    fname,
+                )
         except JinjaSyntaxParsingException as e:
             LOG.warning(
                 "Failed to render templated yaml config file '%s'. %s",

tests/unittests/test_util.py

@@ -480,6 +480,23 @@ def test_read_conf_with_failed_instance_data_json(self, mocker, caplog):
         assert "Could not apply Jinja template" in caplog.text
         assert conf == {"a": "{{c}}"}
 
+    def test_read_conf_with_failed_jinja_render_undefined_attribute(
+        self, mocker, caplog
+    ):
+        mocker.patch("os.path.exists", return_value=True)
+        mocker.patch(
+            "cloudinit.util.load_text_file",
+            return_value='## template: jinja\n{"a": "{{ ds.meta_data.placement.region }}"}',
+        )
+        mocker.patch(
+            "cloudinit.handlers.jinja_template.load_text_file",
+            return_value='{"ds": {"meta_data": {}}}',
+        )
+        conf = util.read_conf("cfg_path", instance_data_file="vars_path")
+        assert "Ignoring jinja template for cfg_path" in caplog.text
+        assert conf == {"a": "{{ ds.meta_data.placement.region }}"}
+
+
     @pytest.mark.parametrize(
         "template",
         [