loader: Validate section keys

This stops silly typos making their way into release notes.

Change-Id: I43d98cc75332677c604e53a5b7a21239505ab4a4
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>
Story: 1670421
Task: 6737

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

@@ -2,4 +2,4 @@
 features:
   - |
     Release note file validation is improved. Files missing section information
-    will now be correctly handled and rejected.
+    will now be correctly handled and rejected, as will invalid sections.

reno/loader.py

@@ -139,6 +139,15 @@ class Loader(object):
                         section_name, filename,
                     )
             else:
+                if section_name not in dict(self._config.sections):
+                    # TODO(stephenfin): Make this an error in a future release
+                    LOG.warning(
+                        'The %s section of %s is not a recognized section. '
+                        'It should be one of: %s. '
+                        'This will be an error in a future release.',
+                        section_name, filename,
+                        ', '.join(dict(self._config.sections)),
+                    )
                 if isinstance(section_content, str):
                     # A single string is OK, but wrap it with a list
                     # so the rest of the code can treat the data model

reno/tests/test_loader.py

@@ -90,6 +90,20 @@ class TestValidate(base.TestCase):
         ldr.parse_note_file('note1', None)
         self.assertIn('instead of a string', self.logger.output)
 
+    def test_invalid_note_with_unrecognized_key(self):
+        """Test behavior when note contains an unrecognized section."""
+        note_bodies = yaml.safe_load(textwrap.dedent('''
+        foobar:
+        - |
+          This is an issue but we're using an unrecognized section key.
+        '''))
+        self.assertIsInstance(note_bodies, dict)
+        ldr = self._make_loader(note_bodies)
+        ldr.parse_note_file('note1', None)
+        self.assertIn(
+            'The foobar section of note1 is not a recognized section.',
+            self.logger.output)
+
     def test_invalid_note_with_missing_key(self):
         """Test behavior when note is not structured as a mapping.