plugin.py: Added BST_FORMAT_VERSION class attribute to plugins

Plugins can now advertize their respective format version with
this class attribute.

At instantiation time, the plugin will take care of asserting
that the project's versioning requirements are met.

Part of a fix for issue #69

1 files changed, 33 insertions, 3 deletions

diff --git a/buildstream/plugin.py b/buildstream/plugin.py
index 23a272389..7253c8398 100644
--- a/buildstream/plugin.py
+++ b/buildstream/plugin.py
@@ -48,6 +48,21 @@ class Plugin():
     BST_REQUIRED_VERSION_MINOR = 0
     """Minimum required minor version"""
 
+    BST_FORMAT_VERSION = 0
+    """The plugin's YAML format version
+
+    This should be set to ``1`` the first time any new configuration
+    is understood by your :func:`~buildstream.plugin.Plugin.configure`
+    implementation and subsequently bumped every time your
+    configuration is enhanced.
+
+    .. note::
+
+       Plugins are expected to maintain backward compatibility
+       in the format and configurations they expose. The versioning
+       is intended to track availability of new features only.
+    """
+
     def __init__(self, name, context, project, provenance, type_tag):
 
         self.name = name
@@ -67,6 +82,22 @@ class Plugin():
         self.__unique_id = _plugin_register(self)  # Unique ID
         self.__log = None               # The log handle when running a task
 
+        # Infer the kind identifier
+        modulename = type(self).__module__
+        self.__kind = modulename.split('.')[-1]
+
+        # Ugly special case to determine the minimum required version
+        # from the project.
+        if type_tag == 'element':
+            version = project._element_format_versions.get(self.get_kind(), 0)
+        else:
+            version = project._source_format_versions.get(self.get_kind(), 0)
+
+        # Raise PluginError on format version mismatch here
+        if self.BST_FORMAT_VERSION < version:
+            raise PluginError("{}: Format version {} is too old for requested version {}"
+                              .format(self, self.BST_FORMAT_VERSION, version))
+
         self.debug("Created: {}".format(self))
 
     def __del__(self):
@@ -77,7 +108,7 @@ class Plugin():
 
     def __str__(self):
         return "{kind} {typetag} at {provenance}".format(
-            kind=self.get_kind(),
+            kind=self.__kind,
             typetag=self.__type_tag,
             provenance=self.__provenance)
 
@@ -87,8 +118,7 @@ class Plugin():
         Returns:
            (str): The kind of this plugin
         """
-        modulename = type(self).__module__
-        return modulename.split('.')[-1]
+        return self.__kind
 
     def get_context(self):
         """Fetches the context