Fix 1.38 interface-bound input upgrade (#2968)

MaterialX 1.39 validates node inputs as having exactly one binding. 1.38 nodegraph implementations allowed having a default value and an interfacename on implementation inputs. During the 1.38 to 1.39 upgrade we remove the stale value if connection is present.

(This is needed to use some of our Houdini 19.5 assets with MaterialX 1.39)

Author: tdavidovicNV

source/MaterialXCore/Version.cpp

@@ -1088,6 +1088,21 @@ void Document::upgradeVersion()
             }
         }
 
+        // In MaterialX 1.39, each node input may have only one binding.  Legacy
+        // 1.38 nodegraph implementations sometimes kept a default value on an
+        // input that was also connected to a nodegraph interface, so preserve the
+        // binding and let the declaration provide any default value.
+        for (ElementPtr elem : traverseTree())
+        {
+            InputPtr input = elem->asA<Input>();
+            if (input && input->getParent()->isA<Node>() && input->hasValue() &&
+                (input->hasNodeName() || input->hasNodeGraphString() ||
+                 input->hasInterfaceName() || input->hasOutputString()))
+            {
+                input->removeAttribute(ValueElement::VALUE_ATTRIBUTE);
+            }
+        }
+
         // Update all nodes.
         vector<NodePtr> unusedNodes;
         for (ElementPtr elem : traverseTree())