Preserve time when recompiling a model.

Fixes #1890.

PiperOrigin-RevId: 662084324
Change-Id: I6aed40761cee5044e63426935d102123db295b25

src/user/user_api.cc

@@ -91,13 +91,16 @@ mjModel* mj_compile(mjSpec* s, const mjVFS* vfs) {
 // recompile spec into existing model and data while preserving the state
 void mj_recompile(mjSpec* s, const mjVFS* vfs, mjModel* m, mjData* d) {
   mjCModel* modelC = static_cast<mjCModel*>(s->element);
+  mjtNum time = 0;
   if (d) {
+    time = d->time;
     modelC->SaveState(d->qpos, d->qvel, d->act);
   }
   modelC->Compile(vfs, &m);
   if (d) {
     modelC->MakeData(m, &d);
     modelC->RestoreState(d->qpos, d->qvel, d->act);
+    d->time = time;
   }
 }
 

test/user/user_api_test.cc

@@ -851,6 +851,7 @@ TEST_F(MujocoTest, PreserveState) {
   // step models
   mj_step(model, data);
   mj_step(m_expected, d_expected);
+  EXPECT_THAT(data->time, model->opt.timestep);
 
   // detach subtree
   mjsBody* body = mjs_findBody(spec, "detachable");
@@ -877,6 +878,7 @@ TEST_F(MujocoTest, PreserveState) {
   // compile new model
   mj_recompile(spec, 0, model, data);
   EXPECT_THAT(model, NotNull());
+  EXPECT_THAT(data->time, model->opt.timestep);
 
   // compare qpos
   EXPECT_EQ(model->nq, m_expected->nq);