[core] Handle missing link nodes when deserializing edges

Avoid uncaught errors when deserializing edges in case linked
nodes are missing.
Handle missing nodes the same way missing attributes are dealt with.

meshroom/core/attribute.py

@@ -339,9 +339,12 @@ class Attribute(BaseObject):
         elif self.isInput and Attribute.isLinkExpression(v):
             # value is a link to another attribute
             link = v[1:-1]
-            linkNode, linkAttr = link.split('.')
+            linkNodeName, linkAttrName = link.split('.')
             try:
-                g.addEdge(g.node(linkNode).attribute(linkAttr), self)
+                node = g.node(linkNodeName)
+                if not node:
+                    raise KeyError(f"Node '{linkNodeName}' not found")
+                g.addEdge(node.attribute(linkAttrName), self)
             except KeyError as err:
                 logging.warning('Connect Attribute from Expression failed.')
                 logging.warning('Expression: "{exp}"\nError: "{err}".'.format(exp=v, err=err))

tests/test_graphIO.py

@@ -336,3 +336,15 @@ class TestImportGraphContentFromMinimalGraphData:
             assert len(graph.nodes) == 1
             assert len(graph.compatibilityNodes) == 0
 
+    def test_connectionsToMissingNodesAreDiscarded(self):
+        graph = Graph("")
+
+        with registeredNodeTypes([SimpleNode]):
+            sampleGraphContent = dedent("""
+            {
+                "SimpleNode_1": { 
+                    "nodeType": "SimpleNode", "inputs": { "input": "{NotSerializedNode.output}" } 
+                }
+            }
+            """)
+            graph._deserialize(json.loads(sampleGraphContent))