Ensure parsing always has a valid VFS.

Explicitly free allocated memory for local resources. (Upcoming
changes to `mju_closeResource` will not work for local resources.)

PiperOrigin-RevId: 854254013
Change-Id: I5cbb8264b590327767d5500f0c2a32ebe036bc4f

src/user/user_util.h

@@ -18,9 +18,12 @@
 #include <algorithm>
 #include <cstddef>
 #include <cstdint>
+#include <deque>
+#include <functional>
 #include <optional>
 #include <string>
 #include <string_view>
+#include <utility>
 #include <vector>
 
 const double mjEPS = 1E-14;     // minimum value in various calculations
@@ -235,6 +238,15 @@ class FilePath {
   std::string path_;
 };
 
+// utility class for scoping resources to functions
+struct Cleanup {
+  using Fn = std::function<void()>;
+  ~Cleanup() { for (auto& f : cleanup) f(); }
+  void operator+=(Fn f) { cleanup.push_front(std::move(f)); }
+  std::deque<Fn> cleanup;
+};
+
+
 // read file into memory buffer
 std::vector<uint8_t> FileToMemory(const char* filename);