Add Strict Metaschema Validation (#1101)

Improve schema validation.

This adds strict validation of config module definitions at testing
time, with plumbing included for future runtime validation. This
eliminates a class of bugs resulting from schemas that have definitions
that are incorrect, but get interpreted by jsonschema as
"additionalProperties" that are therefore ignored.

- Add strict meta-schema for jsonschema unit test validation
- Separate schema from module metadata structure
- Improve type annotations for various functions and data types

Cleanup:
- Remove unused jsonschema "required" elements 
- Eliminate manual memoization in schema.py:get_schema(),
        reference module.__doc__ directly

1 files changed, 6 insertions, 4 deletions

diff --git a/cloudinit/config/cc_write_files.py b/cloudinit/config/cc_write_files.py
index 41c75fa2..55f8c684 100644
--- a/cloudinit/config/cc_write_files.py
+++ b/cloudinit/config/cc_write_files.py
@@ -10,8 +10,7 @@ import base64
 import os
 from textwrap import dedent
 
-from cloudinit.config.schema import (
-    get_schema_doc, validate_cloudconfig_schema)
+from cloudinit.config.schema import get_meta_doc, validate_cloudconfig_schema
 from cloudinit import log as logging
 from cloudinit.settings import PER_INSTANCE
 from cloudinit import util
@@ -38,7 +37,7 @@ supported_encoding_types = [
     'gz', 'gzip', 'gz+base64', 'gzip+base64', 'gz+b64', 'gzip+b64', 'b64',
     'base64']
 
-schema = {
+meta = {
     'id': 'cc_write_files',
     'name': 'Write Files',
     'title': 'write arbitrary files',
@@ -111,6 +110,9 @@ schema = {
           defer: true
         """)],
     'frequency': frequency,
+}
+
+schema = {
     'type': 'object',
     'properties': {
         'write_files': {
@@ -187,7 +189,7 @@ schema = {
     }
 }
 
-__doc__ = get_schema_doc(schema)  # Supplement python help()
+__doc__ = get_meta_doc(meta, schema)  # Supplement python help()
 
 
 def handle(name, cfg, _cloud, log, _args):