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, 3 insertions, 0 deletions

diff --git a/src/buildstream/plugins/sources/_downloadablefilesource.py b/src/buildstream/plugins/sources/_downloadablefilesource.py
index 4e43ee3e3..581d32e7d 100644
--- a/src/buildstream/plugins/sources/_downloadablefilesource.py
+++ b/src/buildstream/plugins/sources/_downloadablefilesource.py
@@ -88,6 +88,9 @@ class DownloadableFileSource(Source):
     def get_unique_key(self):
         return [self.original_url, self.ref]
 
+    def is_cached(self) -> bool:
+        return os.path.isfile(self._get_mirror_file())
+
     def get_consistency(self):
         if self.ref is None:
             return Consistency.INCONSISTENT