source.py: Remove the reliance on consistency to get whether a source is cached

This removes the need to use consistency in Sources, by asking
explicitely whether the source is cached or not.

This introduces a new public method on source: `is_cached` that needs
implementation and that should return whether the source has a local
copy or not.

- On fetch, also reset whether the source was cached or set if as
  cached when we know it was.

- Validate the cache's source after fetching it

  This doesn't need to be run in the scheduler's process and can be
  offloaded to the child, which will allow better multiprocessing

1 files changed, 7 insertions, 1 deletions

diff --git a/src/buildstream/element.py b/src/buildstream/element.py
index 44c2bf847..b0a4da928 100644
--- a/src/buildstream/element.py
+++ b/src/buildstream/element.py
@@ -1727,7 +1727,13 @@ class Element(Plugin):
     #
     # Indicates that fetching the sources for this element has been done.
     #
-    def _fetch_done(self):
+    # Args:
+    #   fetched_original (bool): Whether the original sources had been asked (and fetched) or not
+    #
+    def _fetch_done(self, fetched_original):
+        for source in self.__sources:
+            source._fetch_done(fetched_original)
+
         # We are not updating the state recursively here since fetching can
         # never end up in updating them.