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

Fixes #2482.

PiperOrigin-RevId: 735665098
Change-Id: Ia3436fdd2ff572f8f0c1ded434742a4a9186f2f7

doc/programming/modeledit.rst

@@ -147,8 +147,10 @@ or :ref:`attach a frame to a body<mjs_attachFrame>`:
 
 Note that in the above examples, the parent and child models have different values for ``compiler.degree``,
 corresponding to the :ref:`compiler/angle<compiler-angle>` attribute, specifying the units in which angles are
-interperted. Compiler options are carried over during attachment, so the child model will be compiled using X, while the
-parent will be compiled using Y.
+interperted. Compiler flags are carried over during attachment, so the child model will be compiled using the child
+flags, while the parent will be compiled using the parent flags.
+
+Note also that once a child is attached by reference to a parent, the child cannot be compiled on its own.
 
 .. _meDefault:
 

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

test/user/user_api_test.cc

@@ -2544,5 +2544,30 @@ TEST_F(MujocoTest, ApplyNameSpaceToDefaults) {
   mj_deleteVFS(vfs.get());
 }
 
+TEST_F(MujocoTest, ErrorWhenCompilingOrphanedSpec) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <worldbody>
+      <body name="a"/>
+    </worldbody>
+  </mujoco>
+  )";
+  std::array<char, 1024> er;
+  mjSpec* child = mj_parseXMLString(xml, 0, er.data(), er.size());
+  EXPECT_THAT(child, NotNull()) << er.data();
+  mjSpec* parent = mj_makeSpec();
+  EXPECT_THAT(parent, NotNull());
+  mjsBody* body = mjs_findBody(child, "a");
+  EXPECT_THAT(body, NotNull());
+  mjsFrame* frame = mjs_addFrame(mjs_findBody(parent, "world"), nullptr);
+  EXPECT_THAT(frame, NotNull());
+  mjs_attachBody(frame, body, "child-", "");
+  mj_deleteSpec(parent);
+  mjModel* model = mj_compile(child, 0);
+  EXPECT_THAT(model, IsNull());
+  EXPECT_THAT(mjs_getError(child), HasSubstr("by reference to a parent"));
+  mj_deleteSpec(child);
+}
+
 }  // namespace
 }  // namespace mujoco