Implement file version and a check when using it

If the configuration file structure changes over time, it might
introduce incompatibilities.

This patch introduces versioning of configuration file.

Configuration files needs to add a 'version' entry into their 'header'
like this:

header:
  version: '0.9'

After loading this file, the version is checked agains kas version.

Every version 'M.m.p' of kas is allowed to load configuration files with
version 'M.m' and all versions backwards up to and including
the '__compatible_version__' from kas/__version__.py

Signed-off-by: Claudius Heine <ch@denx.de>

kas/__init__.py

@@ -24,6 +24,7 @@
 """
 
 from .__version__ import __version__
+from .__version__ import __compatible_version__
 
 __license__ = 'MIT'
 __copyright__ = 'Copyright (c) Siemens AG, 2017'

kas/__version__.py

@@ -26,3 +26,4 @@ __license__ = 'MIT'
 __copyright__ = 'Copyright (c) Siemens AG, 2017'
 
 __version__ = '0.9.0'
+__compatible_version__ = '0.9'

kas/includehandler.py

@@ -30,11 +30,70 @@ import sys
 import collections
 import functools
 import logging
+from distutils.version import StrictVersion
+
+from . import __version__, __compatible_version__
 
 __license__ = 'MIT'
 __copyright__ = 'Copyright (c) Siemens AG, 2017'
 
 
+def load_config(filename):
+    """
+        Load the configuration file and test if version is supported.
+    """
+    (_, ext) = os.path.splitext(filename)
+    config = None
+    if ext == '.json':
+        import json
+        with open(filename, 'rb') as fds:
+            config = json.load(fds)
+    elif ext == '.yml':
+        import yaml
+        with open(filename, 'rb') as fds:
+            config = yaml.safe_load(fds)
+    else:
+        logging.error('Config file extension not recognized: %s',
+                      filename)
+        sys.exit(1)
+
+    file_version_string = config.get('header', {}).get('version', None)
+
+    if file_version_string is None:
+        logging.error('Version missing: %s', filename)
+        sys.exit(1)
+
+    try:
+        if not isinstance(file_version_string, str):
+            logging.error('Version has to be a string: %s',
+                          filename)
+            sys.exit(1)
+
+        file_version = StrictVersion()
+        file_version.parse(file_version_string)
+        kas_version = StrictVersion()
+        kas_version.parse(__version__)
+        lower_version = StrictVersion()
+        lower_version.parse(__compatible_version__)
+
+        # Remove patch version, because we provide limited forwards
+        # compatibility:
+        if file_version.version[2] > 0:
+            file_version.prerelease = None
+            file_version.version = tuple(list(file_version.version[:2]) + [0])
+
+        if file_version < lower_version or kas_version < file_version:
+            logging.error('This version of kas is compatible with version %s '
+                          'to %s, file has version %s: %s',
+                          lower_version, kas_version, file_version, filename)
+            sys.exit(1)
+    except ValueError:
+        logging.exception('Not expected version format: %s', filename)
+        raise
+
+    return config
+
+
 class IncludeException(Exception):
     """
         Class for exceptions that appear in the include mechanism.
@@ -88,19 +147,6 @@ class GlobalIncludes(IncludeHandler):
     def get_config(self, repos=None):
         repos = repos or {}
 
-        def _internal_file_parser(filename):
-            (_, ext) = os.path.splitext(filename)
-            if ext == '.json':
-                import json
-                with open(filename, 'rb') as fds:
-                    return json.load(fds)
-            elif ext == '.yml':
-                import yaml
-                with open(filename, 'rb') as fds:
-                    return yaml.safe_load(fds)
-            logging.error('Config file extension not recognized: %s', ext)
-            sys.exit(1)
-
         def _internal_include_handler(filename):
             """
             Recursively load include files and find missing repos.
@@ -129,7 +175,7 @@ class GlobalIncludes(IncludeHandler):
             """
             missing_repos = []
             configs = []
-            current_config = _internal_file_parser(filename)
+            current_config = load_config(filename)
             if not isinstance(current_config, collections.Mapping):
                 raise IncludeException('Configuration file does not contain a '
                                        'dictionary as base type')