_stream.py: Added internal _track_cross_junction_filter()

Replaces Pipeline method `track_cross_junction_filter()`.

This changes the error domain for invalid cross junction tracking, so
updating the following two test cases:

  * testing/_sourcetests/track.py
  * tests/frontend/track.py

3 files changed, 49 insertions, 3 deletions

diff --git a/src/buildstream/_stream.py b/src/buildstream/_stream.py
index 3bbf27971..2625fddb1 100644
--- a/src/buildstream/_stream.py
+++ b/src/buildstream/_stream.py
@@ -47,6 +47,7 @@ from ._scheduler import (
 from .element import Element
 from ._pipeline import Pipeline
 from ._profile import Topics, PROFILER
+from ._project import ProjectRefStorage
 from ._state import State
 from .types import _KeyStrength, _PipelineSelection, _Scope
 from .plugin import Plugin
@@ -1300,11 +1301,56 @@ class Stream:
 
         for project, project_elements in track_projects.items():
             selected = self._pipeline.get_selection(project_elements, selection)
-            selected = self._pipeline.track_cross_junction_filter(project, selected, cross_junctions)
+            selected = self._track_cross_junction_filter(project, selected, cross_junctions)
             track_selected.extend(selected)
 
         return self._pipeline.except_elements(elements, track_selected, except_elements)
 
+    # _track_cross_junction_filter()
+    #
+    # Filters out elements which are across junction boundaries,
+    # otherwise asserts that there are no such elements.
+    #
+    # This is currently assumed to be only relevant for element
+    # lists targetted at tracking.
+    #
+    # Args:
+    #    project (Project): Project used for cross_junction filtering.
+    #                       All elements are expected to belong to that project.
+    #    elements (list of Element): The list of elements to filter
+    #    cross_junction_requested (bool): Whether the user requested
+    #                                     cross junction tracking
+    #
+    # Returns:
+    #    (list of Element): The filtered or asserted result
+    #
+    def _track_cross_junction_filter(self, project, elements, cross_junction_requested):
+
+        # First filter out cross junctioned elements
+        if not cross_junction_requested:
+            elements = [element for element in elements if element._get_project() is project]
+
+        # We can track anything if the toplevel project uses project.refs
+        #
+        if self._project.ref_storage == ProjectRefStorage.PROJECT_REFS:
+            return elements
+
+        # Ideally, we would want to report every cross junction element but not
+        # their dependencies, unless those cross junction elements dependencies
+        # were also explicitly requested on the command line.
+        #
+        # But this is too hard, lets shoot for a simple error.
+        for element in elements:
+            element_project = element._get_project()
+            if element_project is not self._project:
+                detail = (
+                    "Requested to track sources across junction boundaries\n"
+                    + "in a project which does not use project.refs ref-storage."
+                )
+                raise StreamError("Untrackable sources", detail=detail, reason="untrackable-sources")
+
+        return elements
+
     # _load()
     #
     # A convenience method for loading element lists
diff --git a/src/buildstream/testing/_sourcetests/track.py b/src/buildstream/testing/_sourcetests/track.py
index 38ef217f0..638cbb9b1 100644
--- a/src/buildstream/testing/_sourcetests/track.py
+++ b/src/buildstream/testing/_sourcetests/track.py
@@ -228,7 +228,7 @@ def test_cross_junction(cli, tmpdir, datafiles, ref_storage, kind):
 
     if ref_storage == "inline":
         # This is not allowed to track cross junction without project.refs.
-        result.assert_main_error(ErrorDomain.PIPELINE, "untrackable-sources")
+        result.assert_main_error(ErrorDomain.STREAM, "untrackable-sources")
     else:
         result.assert_success()
 
diff --git a/tests/frontend/track.py b/tests/frontend/track.py
index bd8444973..3dd686de0 100644
--- a/tests/frontend/track.py
+++ b/tests/frontend/track.py
@@ -222,7 +222,7 @@ def test_track_cross_junction(cli, tmpdir, datafiles, cross_junction, ref_storag
             # Cross junction tracking is not allowed when the toplevel project
             # is using inline ref storage.
             #
-            result.assert_main_error(ErrorDomain.PIPELINE, "untrackable-sources")
+            result.assert_main_error(ErrorDomain.STREAM, "untrackable-sources")
         else:
             #
             # No cross juction tracking was requested