Refactor: Lazily instantiate ProvenanceInformation objectstristan/lazy-provenance

As a rule, throughout the codebase we should not be using internal
ProvenanceInformation objects in our APIs, but rather Node objects.

This is because ProvenanceInformation is generated on the fly from
a Node object, and it is needlessly expensive to instantiate one before
it is absolutely needed.

This patch unilaterally fixes the codebase to pass `provenance_node`
Node objects around as arguments rather than `provenance` ProvenanceInformation
objects.

2 files changed, 8 insertions, 14 deletions

diff --git a/src/buildstream/plugins/elements/junction.py b/src/buildstream/plugins/elements/junction.py
index 425b917ef..8693313af 100644
--- a/src/buildstream/plugins/elements/junction.py
+++ b/src/buildstream/plugins/elements/junction.py
@@ -284,25 +284,22 @@ class JunctionElement(Element):
         self.ignore_junction_remotes = node.get_bool("ignore-junction-remotes", default=False)
 
         # The overrides dictionary has the target junction
-        # to override as a key, and a tuple consisting
-        # of the local overriding junction and the provenance
-        # of the override declaration.
+        # to override as a key, and the ScalarNode of the
+        # junction name as a value
         self.overrides = {}
         overrides_node = node.get_mapping("overrides", {})
-        for key, value in overrides_node.items():
-            junction_name = value.as_str()
-            provenance = value.get_provenance()
+        for key, junction_name in overrides_node.items():
 
             # Cannot override a subproject with the project itself
             #
-            if junction_name == self.name:
+            if junction_name.as_str() == self.name:
                 raise ElementError(
                     "{}: Attempt to override subproject junction '{}' with the overriding junction '{}' itself".format(
-                        provenance, key, junction_name
+                        junction_name.get_provenance(), key, junction_name.as_str()
                     ),
                     reason="override-junction-with-self",
                 )
-            self.overrides[key] = (junction_name, provenance)
+            self.overrides[key] = junction_name
 
     def preflight(self):
         pass
diff --git a/src/buildstream/plugins/elements/link.py b/src/buildstream/plugins/elements/link.py
index e6d7f056e..e5e694579 100644
--- a/src/buildstream/plugins/elements/link.py
+++ b/src/buildstream/plugins/elements/link.py
@@ -59,12 +59,9 @@ class LinkElement(Element):
 
         node.validate_keys(["target"])
 
-        # Hold onto the provenance of the specified target,
-        # allowing the loader to raise errors with better context.
+        # Hold onto the node, keep it around for provenance.
         #
-        target_node = node.get_scalar("target")
-        self.target = target_node.as_str()
-        self.target_provenance = target_node.get_provenance()
+        self.target_node = node.get_scalar("target")
 
     def preflight(self):
         pass