[Python] Ensure returned cel::Value does not depend on cel::Program lifetime by calling Clone

cel_expr_python/cel_test.py

@@ -885,6 +885,22 @@ def testProtoMessageToCelValueError(self):
     self.assertEqual(res.type(), cel.Type.ERROR)
     self.assertIn("Custom pool error", str(res.value()))
 
+  def testValueOutlivesExpression(self):
+    expr: cel.Expression = self.env.compile("'hello ' + 'world'")
+    val: cel.Value = expr.eval()
+    del expr
+    self.assertEqual(val.value(), "hello world")
+
+    expr = self.env.compile("b'hello'")
+    val = expr.eval()
+    del expr
+    self.assertEqual(val.value(), b"hello")
+
+    expr = self.env.compile("['a', 'b', 'c']")
+    val = expr.eval()
+    del expr
+    self.assertEqual(val.plain_value(), ["a", "b", "c"])
+
 
 class CompatibleNumber:
 

cel_expr_python/py_cel_expression.cc

@@ -202,6 +202,7 @@ absl::StatusOr<PyCelValue> PyCelExpression::Eval(
         result,
         program->Evaluate(arena->GetArena(), *activation.GetActivation(),
                           std::move(options)));
+    result = result.Clone(arena->GetArena());
   }
   return PyCelValue(result, arena, std::move(env));
 }