element.py: Force resolve intermediate dependency cache keys

When a cache key is discovered (e.g.: due to a workspace build completing),
then we have no guarantee that reverse dependency cache keys will be
updated in order.

As such, we must ensure that `Element._update_state()` serializes this
and ensures that cache keys for intermediate dependencies get resolved.

This fixes #919

1 files changed, 12 insertions, 0 deletions

diff --git a/buildstream/element.py b/buildstream/element.py
index bc939bc92..92c9d0181 100644
--- a/buildstream/element.py
+++ b/buildstream/element.py
@@ -1092,6 +1092,18 @@ class Element(Plugin):
                     return
 
         if self.__strict_cache_key is None:
+
+            # We cannot make the assumption that dependency cache keys
+            # have already been resolved if possible.
+            #
+            # If a cache key was recently discovered, we need to be sure
+            # that the interemediate dependencies get their cache keys
+            # resolved before calculating this element's cache key.
+            #
+            for e in self.dependencies(Scope.BUILD):
+                if e.__strict_cache_key is None:
+                    e._update_state()
+
             dependencies = [
                 e.__strict_cache_key for e in self.dependencies(Scope.BUILD)
             ]