loader: Handle note files missing top-level keys

Release note files should look like this:

  issues:
  - |
    Some text here.

However, it's possible to make a mistake and create a release note
missing the top-level key:

  - |
    Some text here.

If you do this, reno fails with a cryptic error which Sphinx will bubble
up with zero context:

  AttributeError: 'list' object has no attribute 'items'

Make this failure more obvious.

Change-Id: I488d14bc1134a38d4f76ed5f5368c84db8e39df3
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>
Story: 1670421
Task: 42802

releasenotes/notes/validate-note-files-1cdbdcde9ae7829b.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    Release note file validation is improved. Files missing section information
+    will now be correctly handled and rejected.

reno/loader.py

@@ -119,6 +119,17 @@ class Loader(object):
 
         cleaned_content = {}
 
+        if not isinstance(content, dict):
+            LOG.warning(
+                '%s does not appear to be structured as a YAML mapping. '
+                'Did you forget a top-level key?',
+                filename,
+            )
+            raise ValueError(
+                f'{filename} does not appear to be structured as a YAML '
+                f'mapping. Did you forget a top-level key?'
+            )
+
         for section_name, section_content in content.items():
             if section_name == self._config.prelude_section_name:
                 if not isinstance(section_content, str):

reno/tests/test_loader.py

@@ -87,3 +87,19 @@ class TestValidate(base.TestCase):
         ldr = self._make_loader(note_bodies)
         ldr.parse_note_file('note1', None)
         self.assertIn('dict', self.logger.output)
+
+    def test_invalid_note_with_missing_key(self):
+        """Test behavior when note is not structured as a mapping.
+
+        This one should be an error since we can't correct the input.
+        """
+        note_bodies = yaml.safe_load(textwrap.dedent('''
+        - |
+          This is an issue but we're missing the top-level 'issues' key.
+        '''))
+        self.assertIsInstance(note_bodies, list)
+        ldr = self._make_loader(note_bodies)
+        self.assertRaises(ValueError, ldr.parse_note_file, 'note1', None)
+        self.assertIn(
+            'does not appear to be structured as a YAML mapping',
+            self.logger.output)