Add python schema validator

This CL adds a python schema validator that can validate JSON schemas.
The schema validator is used in a presubmit to validate all the schemas
in policy_templates.json. The schema validator itself supports the
minimal set of features needed for our current policies.

Bug: 873641, 856903, 856901
Change-Id: Ie577e825807e4438eca6921722b88af7c8d1781c
Reviewed-on: https://chromium-review.googlesource.com/1172433
Commit-Queue: Alexander Hendrich <hendrich@chromium.org>
Reviewed-by: Lutz Justen <ljusten@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585003}

components/policy/tools/syntax_check_policy_template_json.py

@@ -12,6 +12,7 @@ import optparse
 import os
 import re
 import sys
+from schema_validator import SchemaValidator
 
 
 LEADING_WHITESPACE = re.compile('^([ \t]*)')
@@ -79,6 +80,7 @@ class PolicyTemplateChecker(object):
     self.num_policies_in_groups = 0
     self.options = None
     self.features = []
+    self.schema_validator = SchemaValidator()
 
   def _Error(self, message, parent_element=None, identifier=None,
              offending_snippet=None):
@@ -186,17 +188,24 @@ class PolicyTemplateChecker(object):
 
   def _CheckPolicySchema(self, policy, policy_type):
     '''Checks that the 'schema' field matches the 'type' field.'''
-    self._CheckContains(policy, 'schema', dict)
-    if isinstance(policy.get('schema'), dict):
-      self._CheckContains(policy['schema'], 'type', str)
-      schema_type = policy['schema'].get('type')
+    schema = self._CheckContains(policy, 'schema', dict)
+    if schema:
+      schema_type = self._CheckContains(schema, 'type', str)
       if schema_type not in TYPE_TO_SCHEMA[policy_type]:
         self._Error('Schema type must match the existing type for policy %s' %
                     policy.get('name'))
+      if not self.schema_validator.ValidateSchema(schema):
+        self._Error('Schema is invalid for policy %s' % policy.get('name'))
+
+    if policy.has_key('validation_schema'):
+      validation_schema = policy.get('validation_schema')
+      if not self.schema_validator.ValidateSchema(validation_schema):
+        self._Error('Validation schema is invalid for policy %s' %
+                    policy.get('name'))
 
     # Checks that boolean policies are not negated (which makes them harder to
     # reason about).
-      if (schema_type == 'boolean' and
+    if (policy_type == 'main' and
         'disable' in policy.get('name').lower() and
         policy.get('name') not in LEGACY_INVERTED_POLARITY_WHITELIST):
       self._Error(('Boolean policy %s uses negative polarity, please make ' +