`ast.Node` shouldn't be used after COPY ( should regard COPY as MOVE )

[AsterDY]

Background

Since ast.Node introduces lazy-load parsing mechanism, many APIs may change underlying pointer of a node. Therefore, once a node is passed-by-value to another place, the old one should never used --- in other words, Passing a node by value equals to the semantic of MOVE.

Reproducible codes

func TestNodeMove(t *testing.T) {
	old, _ := sonic.GetFromString(`{"a":1,"b":2}`)
	_ = old.Get("a")
	container := ast.NewObject([]ast.Pair{
		{Key: "data", Value: old}, // old is moved to container here
		{Key: "extra", Value: ast.NewString("hello")},
	})
	_, _ = container.MarshalJSON() // MarshalJSON() change underlying data in old
	_, err := old.Interface()      // thus old is invalid !!!
	if err != nil {
		t.Fatal(err)
	}
}

result

--- FAIL: TestNodeMove (0.00s)
    /ast_test.go:36: "Syntax error at index 13: eof\n\n\t{\"a\":1,\"b\":2}\n\t.............^\n"

How to resolve this problem if I still want to use old node?

The main idea is to UPDATE the old node's reference once you move it. Given above example, if you still want to use the old node, you can change your code like below:

func TestNodeMoveValid(t *testing.T) {
	raw, _ := sonic.GetFromString(`{"a":1,"b":2}`)
	old := &raw
	_ = old.Get("a") // lazy-loaded "a" here

	container := ast.NewObject([]ast.Pair{
		{Key: "data", Value: *old}, // old is MOVED to container here
		{Key: "extra", Value: ast.NewString("hello")},
	})

	old = container.Get("data") // update the reference of old to container's "data"

	_, _ = container.MarshalJSON() // MarshalJSON() change underlying data in old, both "a" and "b" loaded

	_, err := old.Interface() // old is still valid now
	if err != nil {
		t.Fatal(err)
	}
}

[JimWen]

Why not explicitly managing references when nodes are moved, and ensuring that any invalid accesses raise errors, the behavior becomes more predictable and clear. Users will no longer be able to mistakenly operate on invalid nodes, improving code safety and reliability.