fix: Don't break modules that use get_meta_doc() (canonical#5953)

get_meta_doc() is no longer used, but there may be custom modules that
still try to reference it. Don't break them.

cloudinit/config/schema.py

@@ -1461,6 +1461,26 @@ def handle_schema_args(name, args):
         )
 
 
+def get_meta_doc(*_args, **_kwargs) -> str:
+    """Provide a stub for backwards compatibility.
+
+    This function is no longer used, but earlier versions of modules
+    required this function for documentation purposes. This is a stub so
+    that custom modules do not break on upgrade.
+    """
+    lifecycle.log_with_downgradable_level(
+        logger=LOG,
+        version="24.4",
+        requested_level=logging.WARNING,
+        msg=(
+            "The 'get_meta_doc()' function is deprecated and will be removed "
+            "in a future version of cloud-init."
+        ),
+        args=(),
+    )
+    return ""
+
+
 def main():
     """Tool to validate schema of a cloud-config file or print schema docs."""
     parser = get_parser()

tests/unittests/config/test_schema.py

@@ -27,6 +27,7 @@
     SchemaValidationError,
     annotated_cloudconfig_file,
     get_jsonschema_validator,
+    get_meta_doc,
     get_schema,
     get_schema_dir,
     handle_schema_args,
@@ -2212,3 +2213,14 @@ def test_handle_schema_args_unknown_header(
         assert read_cfg_paths.call_args_list == [
             mock.call(fetch_existing_datasource="trust")
         ]
+
+
+class TestDeprecation:
+    def test_get_meta_doc_deprecation(self, caplog):
+        """Test that calling get_meta_doc() emits deprecation.
+
+        Ensures that custom modules calling `get_meta_doc()` can still
+        function but receive deprecation warning.
+        """
+        get_meta_doc("some", "random", "arguments", plus="kwargs")
+        assert "The 'get_meta_doc()' function is deprecated" in caplog.text