_elementsources.py: Raise SkipJob() when tracking is unimplemented by all sources

This ensures we get a SKIP message instead of a SUCCESS message when
tracking an element where all of it's sources did not implement track(),
which is the case for sources like `local`, `workspace` or `patch`.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/buildstream/_elementsources.py b/src/buildstream/_elementsources.py
index c030591f8..eed847e75 100644
--- a/src/buildstream/_elementsources.py
+++ b/src/buildstream/_elementsources.py
@@ -20,6 +20,7 @@ from contextlib import contextmanager
 from typing import TYPE_CHECKING, Iterator
 
 from . import _cachekey, utils
+from ._exceptions import SkipJob
 from ._context import Context
 from ._protos.buildstream.v2 import source_pb2
 from .plugin import Plugin
@@ -109,6 +110,12 @@ class ElementSources:
                 )
                 source.warn("Updated reference will be ignored as source has open workspace", detail=detail)
 
+        # Sources which do not implement track() will return None, produce
+        # a SKIP message in the UI if all sources produce None
+        #
+        if all(ref is None for _, ref in refs):
+            raise SkipJob("Element sources are not trackable")
+
         return refs
 
     # stage_and_cache():