Throw an error if a child attached by reference to a parent is compiled.

Fixes #2482.

PiperOrigin-RevId: 735665098
Change-Id: Ia3436fdd2ff572f8f0c1ded434742a4a9186f2f7

src/user/user_model.cc

@@ -3972,6 +3972,9 @@ mjModel* mjCModel::Compile(const mjVFS* vfs, mjModel** m) {
   warningtext[0] = 0;
 
   try {
+    if (attached_) {
+      throw mjCError(0, "cannot compile child spec if attached by reference to a parent spec");
+    }
     if (setjmp(error_jmp_buf) != 0) {
       // TryCompile resulted in an mju_error which was converted to a longjmp.
       std::string error_msg = errortext;

src/user/user_model.h

@@ -317,6 +317,9 @@ class mjCModel : public mjCModel_, private mjSpec {
   // set deepcopy flag
   void SetDeepCopy(bool deepcopy) { deepcopy_ = deepcopy; }
 
+  // set attached flag
+  void SetAttached(bool deepcopy) { attached_ |= !deepcopy; }
+
  private:
   // settings for each defaults class
   std::vector<mjCDef*> defaults_;
@@ -428,5 +431,6 @@ class mjCModel : public mjCModel_, private mjSpec {
   mjCError errInfo;   // last error info
   std::vector<mjKeyInfo> key_pending_;  // attached keyframes
   bool deepcopy_;     // copy objects when attaching
+  bool attached_ = false;  // true if model is attached to a parent model
 };
 #endif  // MUJOCO_SRC_USER_USER_MODEL_H_

src/user/user_objects.cc

@@ -941,6 +941,7 @@ mjCBody& mjCBody::operator+=(const mjCFrame& other) {
   }
 
   // attach referencing elements
+  other_model->SetAttached(model->deepcopy_);
   *model += *other_model;
 
   // leave the source model in a clean state
@@ -1947,6 +1948,7 @@ mjCFrame& mjCFrame::operator+=(const mjCBody& other) {
   last_attached = &body->bodies.back()->spec;
 
   // attach referencing elements
+  other_model->SetAttached(model->deepcopy_);
   *model += *other_model;
 
   // leave the source model in a clean state