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

3 files changed, 30 insertions, 6 deletions

diff --git a/src/buildstream/_stream.py b/src/buildstream/_stream.py
index a9c1c7261..91f37cbf3 100644
--- a/src/buildstream/_stream.py
+++ b/src/buildstream/_stream.py
@@ -377,6 +377,10 @@ class Stream():
                        track_except_targets=except_targets,
                        track_cross_junctions=cross_junctions)
 
+        # FIXME: this can be refactored after element._update_state is simplified/removed
+        elements = [element for element in elements if element._schedule_tracking()]
+        self._pipeline.resolve_elements(elements)
+
         self._scheduler.clear_queues()
         track_queue = TrackQueue(self._scheduler)
         self._add_queue(track_queue, track=True)
@@ -1248,11 +1252,7 @@ class Stream():
                                                         track_selected,
                                                         track_except_elements)
 
-        for element in track_selected:
-            element._schedule_tracking()
-
         if not targets:
-            self._pipeline.resolve_elements(track_selected)
             return [], track_selected
 
         # ArtifactCache.setup_remotes expects all projects to be fully loaded
diff --git a/src/buildstream/element.py b/src/buildstream/element.py
index 9a0a71a97..5fa8f14df 100644
--- a/src/buildstream/element.py
+++ b/src/buildstream/element.py
@@ -1331,8 +1331,22 @@ class Element(Plugin):
     # and reinterrogation of element state after tracking of elements
     # succeeds.
     #
-    def _schedule_tracking(self):
-        self.__tracking_scheduled = True
+    # This method should return the value of `__tracking_scheduled` to report
+    # to callers that the element was marked for tracking.
+    #
+    # If `__tracking_scheduled` is not already determined then set it to `True`
+    # if at least one source advertises that it can be tracked.
+    #
+    # Returns:
+    #    (bool): value of the `__tracking_scheduled` attribute
+    #
+    def _schedule_tracking(self) -> bool:
+        # if the tracking schedule is already determined then this can be skipped
+        if not self.__tracking_scheduled:
+            # Tracking does not make sense in cases where no sources can be tracked.
+            if any(source._is_trackable() for source in self.__sources):
+                self.__tracking_scheduled = True
+        return self.__tracking_scheduled
 
     # _tracking_done():
     #
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,