Make NodePath copy-on-write, to avoid it changing accidentally by owned copies.

core/string/node_path.cpp

@@ -42,6 +42,7 @@ void NodePath::_update_hash_cache() const {
 
 void NodePath::prepend_period() {
 	if (data->path.size() && data->path[0].operator String() != ".") {
+		_copy_on_write();
 		data->path.insert(0, ".");
 		data->concatenated_path = StringName();
 		data->hash_cache_valid = false;
@@ -167,6 +168,29 @@ void NodePath::operator=(const NodePath &p_path) {
 	}
 }
 
+void NodePath::_copy_on_write() {
+	if (!data) {
+		return; // No data; nothing to do.
+	}
+
+	if (data->refcount.get() == 1) {
+		return; // Already the only owner of data.
+	}
+
+	// Make a copy.
+	Data *new_data = memnew(Data);
+	new_data->refcount.init();
+	new_data->path = data->path;
+	new_data->subpath = data->subpath;
+	new_data->concatenated_path = data->concatenated_path;
+	new_data->concatenated_subpath = data->concatenated_subpath;
+	new_data->absolute = data->absolute;
+	new_data->hash_cache_valid = data->hash_cache_valid;
+	new_data->hash_cache = data->hash_cache;
+	unref();
+	data = new_data;
+}
+
 NodePath::operator String() const {
 	if (!data) {
 		return String();
@@ -332,6 +356,7 @@ void NodePath::simplify() {
 	if (!data) {
 		return;
 	}
+	_copy_on_write();
 	for (int i = 0; i < data->path.size(); i++) {
 		if (data->path.size() == 1) {
 			break;

core/string/node_path.h

@@ -35,6 +35,10 @@
 
 #include <climits>
 
+// Represents a path to a node or property in a hierarchy of nodes
+// Note that NodePath is (effectively) const: If you hold a NodePath,
+// you can expect it to remain unchanged, even if you make copies of
+// it. This is achieved through copy-on-write (CoW).
 class [[nodiscard]] NodePath {
 	struct Data {
 		SafeRefCount refcount;
@@ -52,6 +56,10 @@ class [[nodiscard]] NodePath {
 
 	void _update_hash_cache() const;
 
+	// Copies the underlying data.
+	// Every non-const function must call this before starting to mutate data.
+	void _copy_on_write();
+
 public:
 	bool is_absolute() const;
 	int get_name_count() const;