Allow junctions to inject dependencies from current project to junctioned onevalentindavid/junction-replacements-1.4

There are cases where downstream projects want to overwrite upstream
elements.

So far, either the downstream project just allows overlaps and hopes
that ABI is compatible. There are also files left from original
elements when not overwritten.

Or downstream adds an "overlay" with local source to the
junctions. However on the latter, elements in the overlay are
considered as part of upstream, which means in cannot depend on
element outside of upstream (unless cyclic junction maybe?).

Here we add a feature to junctions to allow injecting elements from
current project to junctions.

This is useful for example in the case of GNOME SDK needing to override
GLib, GObjectIntrospection, libsoup, etc. from Freedesktop SDK.

2 files changed, 14 insertions, 0 deletions

diff --git a/buildstream/_loader/loader.py b/buildstream/_loader/loader.py
index ec929eadb..dc15109c8 100644
--- a/buildstream/_loader/loader.py
+++ b/buildstream/_loader/loader.py
@@ -217,6 +217,13 @@ class Loader():
         if ticker:
             ticker(filename)
 
+        if self.project.junction and filename in self.project.junction.replacements:
+            replacement = _yaml.node_get(self.project.junction.replacements, str, filename)
+            elt = self._parent._load_file(replacement,
+                                          rewritable, ticker, fetch_subprojects, provenance=provenance)
+            self._elements[filename] = elt
+            return elt
+
         # Load the data and process any conditional statements therein
         fullpath = os.path.join(self._basedir, filename)
         try:
@@ -311,6 +318,8 @@ class Loader():
             validated = {}
 
         element = self._elements[element_name]
+        if getattr(element, '_loader', self) != self:
+            return element._loader._check_circular_deps(element.name, check_elements, validated)
 
         # element name must be unique across projects
         # to be usable as key for the check_elements and validated dicts
@@ -353,6 +362,8 @@ class Loader():
             visited = {}
 
         element = self._elements[element_name]
+        if getattr(element, '_loader', self) != self:
+            return element._loader._sort_dependencies(element.name, visited)
 
         # element name must be unique across projects
         # to be usable as key for the visited dict
@@ -424,6 +435,8 @@ class Loader():
     def _collect_element(self, element_name):
 
         element = self._elements[element_name]
+        if element._loader != self:
+            return element._loader._collect_element(element.name)
 
         # Return the already built one, if we already built it
         meta_element = self._meta_elements.get(element_name)
diff --git a/buildstream/plugins/elements/junction.py b/buildstream/plugins/elements/junction.py
index d2c62fe48..f87ac2273 100644
--- a/buildstream/plugins/elements/junction.py
+++ b/buildstream/plugins/elements/junction.py
@@ -142,6 +142,7 @@ class JunctionElement(Element):
     def configure(self, node):
         self.path = self.node_get_member(node, str, 'path', default='')
         self.options = self.node_get_member(node, Mapping, 'options', default={})
+        self.replacements = self.node_get_member(node, Mapping, 'replacements', default={})
 
     def preflight(self):
         pass