Add _is_trackable() method to Source()

This method reports whether the source can be tracked. This would be
false for sources advertising BST_KEY_REQUIRES_STAGE.

Element tracking can be skipped if none of the held sources can be
tracked. This is determined by the value of the
`Element.__tracking_scheduled` attribute which is set in
`Element._schedule_tracking()`. This is set to `True` if at least one
source can be tracked.

Also remove some of the tracking handling from `_stream._load` to
`_stream.track` where it is more relevant.

closes #1186

1 files changed, 10 insertions, 0 deletions

diff --git a/src/buildstream/source.py b/src/buildstream/source.py
index 5321363a4..05a1ae464 100644
--- a/src/buildstream/source.py
+++ b/src/buildstream/source.py
@@ -1087,6 +1087,16 @@ class Source(Plugin):
 
         return new_ref
 
+    # _is_trackable()
+    #
+    # Returns:
+    #   (bool): Whether this source is trackable
+    #
+    def _is_trackable(self) -> bool:
+        """Report whether this source can be tracked."""
+        # sources that require staging to generate keys cannot be tracked
+        return not self.BST_KEY_REQUIRES_STAGE
+
     # _requires_previous_sources()
     #
     # If a plugin requires access to previous sources at track or fetch time,