Add is_trackable() public 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.__has_trackable_sources` attribute which is set in
`Element._schedule_tracking()`.

Also remove some of the tracking handling from `_stream._load`.

closes #1186

3 files changed, 32 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..ab4cad69c 100644
--- a/src/buildstream/element.py
+++ b/src/buildstream/element.py
@@ -257,6 +257,8 @@ class Element(Plugin):
         self.__cached_remotely = None           # Whether the element is cached remotely
         # List of Sources
         self.__sources = []                     # type: List[Source]
+        # At least one of the sources can be tracked (bool)
+        self.__has_trackable_sources = None         # type: ignore
         self.__weak_cache_key = None            # Our cached weak cache key
         self.__strict_cache_key = None          # Our cached cache key for strict builds
         self.__artifacts = context.artifactcache  # Artifact cache
@@ -1331,8 +1333,25 @@ class Element(Plugin):
     # and reinterrogation of element state after tracking of elements
     # succeeds.
     #
-    def _schedule_tracking(self):
-        self.__tracking_scheduled = True
+    # If `__has_trackable_sources` is not set, then we haven't yet determined
+    # if this element has sources that can be tracked. Set it to True if
+    # at least one source advertises that it can be tracked and False
+    # otherwise. If it is True then also set `__tracking_scheduled` to True.
+    #
+    # Return the value `__has_trackable_sources` to communicate this to callers.
+    #
+    # Returns:
+    #    (bool): value of self.__has_trackable_sources
+    #
+    def _schedule_tracking(self) -> bool:
+        # Scheduling tracking does not make sense in cases where no sources
+        # can be tracked.
+        if self.__has_trackable_sources is None:
+            self.__has_trackable_sources = any(source.is_trackable() for source in self.__sources)
+
+        if self.__has_trackable_sources:
+            self.__tracking_scheduled = True
+        return self.__has_trackable_sources
 
     # _tracking_done():
     #
diff --git a/src/buildstream/source.py b/src/buildstream/source.py
index 5321363a4..25c205acc 100644
--- a/src/buildstream/source.py
+++ b/src/buildstream/source.py
@@ -563,6 +563,13 @@ class Source(Plugin):
     #############################################################
     #                       Public Methods                      #
     #############################################################
+    def is_trackable(self) -> bool:
+        """Report whether this source can be tracked."""
+        # sources that require staging to generate keys cannot be tracked
+        if self.BST_KEY_REQUIRES_STAGE:
+            return False
+        return True
+
     def get_mirror_directory(self) -> str:
         """Fetches the directory where this source should store things