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, 5 deletions

diff --git a/cloudinit/config/cc_snap.py b/cloudinit/config/cc_snap.py
index 20ed7d2f..21f30b57 100644
--- a/cloudinit/config/cc_snap.py
+++ b/cloudinit/config/cc_snap.py
@@ -8,8 +8,7 @@ import sys
 from textwrap import dedent
 
 from cloudinit import log as logging
-from cloudinit.config.schema import (
-    get_schema_doc, validate_cloudconfig_schema)
+from cloudinit.config.schema import get_meta_doc, validate_cloudconfig_schema
 from cloudinit.settings import PER_INSTANCE
 from cloudinit.subp import prepend_base_command
 from cloudinit import subp
@@ -21,7 +20,7 @@ frequency = PER_INSTANCE
 
 LOG = logging.getLogger(__name__)
 
-schema = {
+meta = {
     'id': 'cc_snap',
     'name': 'Snap',
     'title': 'Install, configure and manage snapd and snap packages',
@@ -103,6 +102,9 @@ schema = {
                     signed_assertion_blob_here
     """)],
     'frequency': PER_INSTANCE,
+}
+
+schema = {
     'type': 'object',
     'properties': {
         'snap': {
@@ -139,13 +141,12 @@ schema = {
                 }
             },
             'additionalProperties': False,  # Reject keys not in schema
-            'required': [],
             'minProperties': 1
         }
     }
 }
 
-__doc__ = get_schema_doc(schema)  # Supplement python help()
+__doc__ = get_meta_doc(meta, schema)  # Supplement python help()
 
 SNAP_CMD = "snap"
 ASSERTIONS_FILE = "/var/lib/cloud/instance/snapd.assertions"