CI: add tooling for schema validation

The config files have fairly extensive documentation but no
formal description; adding JSON-Schema into the mix makes it
possible to write a machine-checkable description.

ci/configvalidator.py

@@ -0,0 +1,110 @@
+#! /usr/bin/env python3
+#
+# SPDX-FileCopyrightText: 2020 Adriaan de Groot <groot@kde.org>
+# SPDX-License-Identifier: BSD-2-Clause
+# License-Filename: LICENSES/BSD2
+#
+usage = """
+Validates a Calamares config file -- YAML syntax -- against a schema.
+
+The schema is also written in YAML syntax, but the schema itself
+is JSON-schema. This is possible because all JSON is YAML, and most
+YAML is JSON. The limited subset of YAML that Calamares uses is
+JSON-representable, anyway.
+
+Usage:
+    configvalidator.py <schema> <file> ...
+"""
+
+# The schemata originally lived outside the Calamares repository,
+# without documented tooling. By putting them in the repository
+# with the example files and explicit tooling, there's a better
+# chance of them catching problems and acting as documentation.
+
+dependencies = """
+Dependencies for this tool are: py-yaml and py-jsonschema.
+
+    https://pyyaml.org/
+    https://github.com/Julian/jsonschema
+
+Simple installation is `pip install pyyaml jsonschema`
+"""
+
+ERR_IMPORT, ERR_USAGE, ERR_FILE_NOT_FOUND, ERR_SYNTAX, ERR_INVALID = range(1,6)
+
+### DEPENDENCIES
+#
+#
+try:
+    from jsonschema import validate, SchemaError, ValidationError
+    from yaml import safe_load, YAMLError
+except ImportError as e:
+    print(e)
+    print(dependencies)
+    exit(ERR_IMPORT)
+
+from os.path import exists
+import sys
+
+### INPUT VALIDATION
+#
+#
+if len(sys.argv) < 3:
+    print(usage)
+    exit(ERR_USAGE)
+
+schema_file_name = sys.argv[1]
+config_file_names = sys.argv[2:]
+
+if not exists(schema_file_name):
+    print(usage)
+    print("\nSchema file '{}' does not exist.".format(schema_file_name))
+    exit(ERR_FILE_NOT_FOUND)
+for f in config_file_names:
+    if not exists(f):
+        print(usage)
+        print("\nYAML file '{}' does not exist.".format(f))
+        exit(ERR_FILE_NOT_FOUND)
+
+### FILES SYNTAX CHECK
+#
+#
+with open(schema_file_name, "r") as data:
+    try:
+        schema = safe_load(data)
+    except YAMLError as e:
+        print("Schema error: {} {}.".format(e.problem, e.problem_mark))
+        print("\nSchema file '{}' is invalid YAML.".format(schema_file_name))
+        exit(ERR_SYNTAX)
+
+try:
+    validate(instance={}, schema=schema)
+except SchemaError as e:
+    print(e.message)
+    print("\nSchema file '{}' is invalid JSON-Schema.".format(schema_file_name))
+    exit(ERR_INVALID)
+except ValidationError:
+    # Just means that empty isn't valid, but the Schema itself is
+    pass
+
+configs = []
+for f in config_file_names:
+    config = None
+    with open(f, "r") as data:
+        try:
+            config = safe_load(data)
+        except YAMLError as e:
+            print("YAML error: {} {}.".format(e.problem, e.problem_mark))
+            print("\nYAML file '{}' is invalid.".format(f))
+            exit(ERR_SYNTAX)
+    if config is None:
+        print("YAML file '{}' is empty.".format(f))
+    configs.append(config)
+
+### SCHEMA VALIDATION
+#
+#
+# Here a ValidationError from jsonschema carries a lot of useful information,
+# so just let it go.
+for c in configs:
+    validate(instance=c, schema=schema)