junctions: Replace coalescing with explicit overrides

This patch removes the functionality of coalescing junction
configurations on junction element names, and replaces it with
an explicit format which allows junction declarations to override
junctions in subprojects with junction declarations from the local
project.

Changes:

  * plugins/elements/junction.py: Load the overrides dictionary

  * exceptions.py: Added new CIRCULAR_REFERENCE to LoadErrorReason.

  * _loader/loadcontext.py: Add new register_loader() function which
    is called by the Loader to register all loaders with the context,
    delegating the task of detecting conflicting junctions to the
    load context.

  * _loader/loader.py: When loading a junction, check if there is
    an override from the parent project and use that instead.

    Register with the LoadContext, and pass the LoadContext
    and provenance along when creating a Project

    Further, we now protect against circular element path references
    with a new _loader_search_provenances table.

  * _project.py: Pass new `provenance` member through the constructor
    and onwards into the child loader.

1 files changed, 4 insertions, 3 deletions

diff --git a/src/buildstream/_project.py b/src/buildstream/_project.py
index e0ddf3d41..fcfb31988 100644
--- a/src/buildstream/_project.py
+++ b/src/buildstream/_project.py
@@ -99,6 +99,7 @@ class Project:
         cli_options=None,
         default_mirror=None,
         parent_loader=None,
+        provenance=None,
         search_for_project=True,
     ):
 
@@ -164,7 +165,7 @@ class Project:
         self._project_includes = None
 
         with PROFILER.profile(Topics.LOAD_PROJECT, self.directory.replace(os.sep, "-")):
-            self._load(parent_loader=parent_loader)
+            self._load(parent_loader=parent_loader, provenance=provenance)
 
         self._partially_loaded = True
 
@@ -653,7 +654,7 @@ class Project:
     #
     # Raises: LoadError if there was a problem with the project.conf
     #
-    def _load(self, *, parent_loader=None):
+    def _load(self, *, parent_loader=None, provenance=None):
 
         # Load builtin default
         projectfile = os.path.join(self.directory, _PROJECT_CONF_FILE)
@@ -700,7 +701,7 @@ class Project:
         # Fatal warnings
         self._fatal_warnings = pre_config_node.get_str_list("fatal-warnings", default=[])
 
-        self.loader = Loader(self, parent=parent_loader)
+        self.loader = Loader(self, parent=parent_loader, provenance=provenance)
 
         self._project_includes = Includes(self.loader, copy_tree=False)