fix: resolve intermittent NullPointerException in LetExprNode during concurrent execution

pkl-core/src/main/java/org/pkl/core/ast/expression/binary/LetExprNode.java

@@ -5,7 +5,7 @@
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
- *     https://www.apache.org/licenses/LICENSE-2.0
+ * https://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
@@ -50,14 +50,10 @@ public LetExprNode(
 
   @Override
   public Object executeGeneric(VirtualFrame frame) {
-    if (functionNode == null) {
-      CompilerDirectives.transferToInterpreterAndInvalidate();
-      functionNode = unresolvedFunctionNode.execute(frame);
-      callNode = insert(DirectCallNode.create(functionNode.getCallTarget()));
-      if (isCustomThisScope) {
-        // deferred until execution time s.t. nodes of inlined type aliases get the right frame slot
-        customThisSlot = VmUtils.findCustomThisSlot(frame);
-      }
+    // Usamos callNode como flag de inicialização para garantir que
+    // todos os campos dependentes estejam prontos antes de prosseguir.
+    if (callNode == null) {
+      initialize(frame);
     }
 
     var function =
@@ -72,4 +68,29 @@ public Object executeGeneric(VirtualFrame frame) {
 
     return callNode.call(function.getThisValue(), function, value);
   }
+
+  /**
+   * Inicializa o nó de chamada e resolve a função de forma thread-safe.
+   */
+  private synchronized void initialize(VirtualFrame frame) {
+    if (callNode != null) {
+      return;
+    }
+
+    CompilerDirectives.transferToInterpreterAndInvalidate();
+
+    // Resolvemos a função primeiro
+    var resolvedFunction = unresolvedFunctionNode.execute(frame);
+
+    if (isCustomThisScope) {
+      customThisSlot = VmUtils.findCustomThisSlot(frame);
+    }
+
+    // Atribuímos functionNode antes do callNode
+    this.functionNode = resolvedFunction;
+
+    // Inserimos o callNode por último. 
+    // Quando este campo deixar de ser nulo, os outros já estarão visíveis.
+    this.callNode = insert(DirectCallNode.create(resolvedFunction.getCallTarget()));
+  }
 }