Ensure we have a valid vfs when loading a spec.

PiperOrigin-RevId: 882503274
Change-Id: Iddbd6f00c0344bb34d68bb72a640c0a8e81076a2

src/experimental/platform/model_holder.cc

@@ -61,6 +61,10 @@ std::unique_ptr<ModelHolder> ModelHolder::FromBuffer(
   return mh;
 }
 
+ModelHolder::ModelHolder() {
+  mj_defaultVFS(&vfs_);
+}
+
 ModelHolder::~ModelHolder() {
   if (data_) {
     mj_deleteData(data_);
@@ -94,7 +98,6 @@ void ModelHolder::InitFromSpec(mjSpec* spec) {
 }
 
 void ModelHolder::InitFromFile(std::string_view filepath) {
-  mj_defaultVFS(&vfs_);
   if (filepath.ends_with(".mjb")) {
     model_ = mj_loadModel(filepath.data(), &vfs_);
   } else {
@@ -108,8 +111,6 @@ void ModelHolder::InitFromFile(std::string_view filepath) {
 void ModelHolder::InitFromBuffer(std::span<const std::byte> buffer,
                                  std::string_view content_type,
                                  std::string_view filename) {
-  mj_defaultVFS(&vfs_);
-
   if (content_type == "text/xml") {
     const char* ptr = reinterpret_cast<const char*>(buffer.data());
     spec_ = mj_parseXMLString(ptr, nullptr, error_, sizeof(error_));

src/experimental/platform/model_holder.h

@@ -64,7 +64,7 @@ class ModelHolder {
   mjModel* ReleaseModel();
 
  private:
-  ModelHolder() = default;
+  ModelHolder();
   void InitFromSpec(mjSpec* spec);
   void InitFromFile(std::string_view filepath);
   void InitFromBuffer(std::span<const std::byte> buffer,