fix(canvas): reject invalid edge splits before mutation

components/canvas/canvas-connection-validation.ts

@@ -0,0 +1,76 @@
+import type { Connection, Edge as RFEdge, Node as RFNode } from "@xyflow/react";
+
+import {
+  validateCanvasConnectionPolicy,
+  type CanvasConnectionValidationReason,
+} from "@/lib/canvas-connection-policy";
+
+export function validateCanvasConnection(
+  connection: Connection,
+  nodes: RFNode[],
+  edges: RFEdge[],
+  edgeToReplaceId?: string,
+): CanvasConnectionValidationReason | null {
+  if (!connection.source || !connection.target) return "incomplete";
+  if (connection.source === connection.target) return "self-loop";
+
+  const sourceNode = nodes.find((node) => node.id === connection.source);
+  const targetNode = nodes.find((node) => node.id === connection.target);
+  if (!sourceNode || !targetNode) return "unknown-node";
+
+  return validateCanvasConnectionByType({
+    sourceType: sourceNode.type ?? "",
+    targetType: targetNode.type ?? "",
+    targetNodeId: connection.target,
+    edges,
+    edgeToReplaceId,
+  });
+}
+
+export function validateCanvasConnectionByType(args: {
+  sourceType: string;
+  targetType: string;
+  targetNodeId: string;
+  edges: RFEdge[];
+  edgeToReplaceId?: string;
+}): CanvasConnectionValidationReason | null {
+  const targetIncomingCount = args.edges.filter(
+    (edge) => edge.target === args.targetNodeId && edge.id !== args.edgeToReplaceId,
+  ).length;
+
+  return validateCanvasConnectionPolicy({
+    sourceType: args.sourceType,
+    targetType: args.targetType,
+    targetIncomingCount,
+  });
+}
+
+export function validateCanvasEdgeSplit(args: {
+  nodes: RFNode[];
+  edges: RFEdge[];
+  splitEdge: RFEdge;
+  middleNode: RFNode;
+}): CanvasConnectionValidationReason | null {
+  const sourceNode = args.nodes.find((node) => node.id === args.splitEdge.source);
+  const targetNode = args.nodes.find((node) => node.id === args.splitEdge.target);
+
+  if (!sourceNode || !targetNode) {
+    return "unknown-node";
+  }
+
+  return (
+    validateCanvasConnectionByType({
+      sourceType: sourceNode.type ?? "",
+      targetType: args.middleNode.type ?? "",
+      targetNodeId: args.middleNode.id,
+      edges: args.edges,
+    }) ??
+    validateCanvasConnectionByType({
+      sourceType: args.middleNode.type ?? "",
+      targetType: targetNode.type ?? "",
+      targetNodeId: targetNode.id,
+      edges: args.edges,
+      edgeToReplaceId: args.splitEdge.id,
+    })
+  );
+}