element.py: Introduce artifact versions

Fixes #49

1 files changed, 29 insertions, 0 deletions

diff --git a/buildstream/element.py b/buildstream/element.py
index a4fa696a1..a1a07bb78 100644
--- a/buildstream/element.py
+++ b/buildstream/element.py
@@ -511,6 +511,19 @@ class Element(Plugin):
 
         return None
 
+    def get_artifact_version(self):
+        """Return the element plugin's artifact version
+
+        Returns:
+           (int): Artifact version
+
+        Elements must implement this method if they change their unique
+        key structure in an incompatible way or trigger a change in the
+        build output for the same unique key. Every further change
+        requires bumping the returned version.
+        """
+        return 0
+
     #############################################################
     #                  Abstract Element Methods                 #
     #############################################################
@@ -801,6 +814,20 @@ class Element(Plugin):
 
         return True
 
+    # _get_core_artifact_version():
+    #
+    # Return the artifact version of core BuildStream.
+    #
+    # Returns
+    #    (int): Artifact version
+    #
+    # The returned version requires a bump whenever the cache key
+    # algorithm, the artifact structure, or the artifact metadata
+    # changes in an incompatible way.
+    #
+    def _get_core_artifact_version(self):
+        return 0
+
     # __calculate_cache_key():
     #
     # Calculates the cache key
@@ -825,6 +852,8 @@ class Element(Plugin):
         context = self.get_context()
         project = self.get_project()
         return utils._generate_key({
+            'artifact-version': "{}.{}".format(self._get_core_artifact_version(),
+                                               self.get_artifact_version()),
             'context': context._get_cache_key(),
             'project': project._get_cache_key(),
             'element': self.get_unique_key(),