Copy UIDs during mj_copySpec.

This will produce the same mjSpec signature after a deep copy.

PiperOrigin-RevId: 743538018
Change-Id: Ic0f5b06a00e8883cb92dc98159db3bea02cb7a36

src/user/user_model.cc

@@ -297,7 +297,7 @@ void mjCModel::CopyList(std::vector<T*>& dest,
     // copy the element from the other model to this model
     if (deepcopy_) {
       source[i]->ForgetKeyframes();
-      candidate->uid = GetUid();
+      candidate->uid = source[i]->uid;
     } else {
       candidate->AddRef();
     }

src/user/user_objects.cc

@@ -840,7 +840,7 @@ mjCBody::mjCBody(mjCModel* _model) {
 
 mjCBody::mjCBody(const mjCBody& other, mjCModel* _model) {
   model = _model;
-  uid = model->GetUid();
+  uid = other.uid;
   mjSpec* origin = model->FindSpec(other.compiler);
   compiler = origin ? &origin->compiler : &model->spec.compiler;
   *this = other;
@@ -944,7 +944,7 @@ mjCBody& mjCBody::operator+=(const mjCFrame& other) {
   frames.back()->frame = other.frame;
   if (model->deepcopy_) {
     frames.back()->NameSpace(other_model);
-    frames.back()->uid = model->GetUid();
+    frames.back()->uid = other.uid;
   } else {
     frames.back()->AddRef();
   }
@@ -1038,7 +1038,7 @@ void mjCBody::CopyList(std::vector<T*>& dst, const std::vector<T*>& src,
     if (!model->deepcopy_) {
       dst.back()->AddRef();
     } else {
-      dst.back()->uid = model->GetUid();
+      dst.back()->uid = src[i]->uid;
     }
 
     // set namespace

test/user/user_api_test.cc

@@ -538,6 +538,9 @@ TEST_F(PluginTest, RecompileCompare) {
         // copy spec
         mjSpec* s_copy = mj_copySpec(s);
 
+        // compare signature
+        EXPECT_EQ(s->element->signature, s_copy->element->signature) << xml;
+
         // compile twice and compare
         mjModel* m_old = mj_compile(s, nullptr);