Check that element is a nullptr before casting.

The bug was caused by the fact that the `mjs_as*` functions would return a pointer to a `mjC*` object, even if the input `mjsElement` pointer was null. This would cause a segfault.

PiperOrigin-RevId: 651802736
Change-Id: Ibec00d269a4befadff8a68e0513fd381c7e38a2a

python/mujoco/specs_test.py

@@ -234,5 +234,21 @@ class SpecsTest(absltest.TestCase):
     self.assertEqual(spec.sensors[1].name, 'sensor2')
     self.assertEqual(spec.sensors[2].name, 'sensor3')
 
+  def test_iterators(self):
+    spec = mujoco.MjSpec()
+    geom1 = spec.worldbody.add_geom()
+    geom2 = spec.worldbody.add_geom()
+    geom3 = spec.worldbody.add_geom()
+    geom1.name = 'geom1'
+    geom2.name = 'geom2'
+    geom3.name = 'geom3'
+    geom = spec.worldbody.first_geom()
+    i = 1
+    while geom:
+      self.assertEqual(geom.name, 'geom' + str(i))
+      geom = spec.worldbody.next_geom(geom)
+      i += 1
+
+
 if __name__ == '__main__':
   absltest.main()

src/user/user_api.cc

@@ -636,161 +636,230 @@ mjsElement* mjs_nextElement(mjSpec* s, mjsElement* element) {
 
 // return body given mjsElement
 mjsBody* mjs_asBody(mjsElement* element) {
-  return element->elemtype == mjOBJ_BODY ? &(static_cast<mjCBody*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_BODY) {
+    return &(static_cast<mjCBody*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return geom given mjsElement
 mjsGeom* mjs_asGeom(mjsElement* element) {
-  return element->elemtype == mjOBJ_GEOM ? &(static_cast<mjCGeom*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_GEOM) {
+    return &(static_cast<mjCGeom*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return joint given mjsElement
 mjsJoint* mjs_asJoint(mjsElement* element) {
-  return element->elemtype == mjOBJ_JOINT ? &(static_cast<mjCJoint*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_JOINT) {
+    return &(static_cast<mjCJoint*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // Return site given mjsElement
 mjsSite* mjs_asSite(mjsElement* element) {
-  return element->elemtype == mjOBJ_SITE ? &(static_cast<mjCSite*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_SITE) {
+    return &(static_cast<mjCSite*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return camera given mjsElement
 mjsCamera* mjs_asCamera(mjsElement* element) {
-  return element->elemtype == mjOBJ_CAMERA ? &(static_cast<mjCCamera*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_CAMERA) {
+    return &(static_cast<mjCCamera*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return light given mjsElement
 mjsLight* mjs_asLight(mjsElement* element) {
-  return element->elemtype == mjOBJ_LIGHT ? &(static_cast<mjCLight*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_LIGHT) {
+    return &(static_cast<mjCLight*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return frame given mjsElement
 mjsFrame* mjs_asFrame(mjsElement* element) {
-  return element->elemtype == mjOBJ_FRAME ? &(static_cast<mjCFrame*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_FRAME) {
+    return &(static_cast<mjCFrame*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return actuator given mjsElement
 mjsActuator* mjs_asActuator(mjsElement* element) {
-  return element->elemtype == mjOBJ_ACTUATOR ? &(static_cast<mjCActuator*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_ACTUATOR) {
+    return &(static_cast<mjCActuator*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return sensor given mjsElement
 mjsSensor* mjs_asSensor(mjsElement* element) {
-  return element->elemtype == mjOBJ_SENSOR ? &(static_cast<mjCSensor*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_SENSOR) {
+    return &(static_cast<mjCSensor*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return flex given mjsElement
 mjsFlex* mjs_asFlex(mjsElement* element) {
-  return element->elemtype == mjOBJ_FLEX ? &(static_cast<mjCFlex*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_FLEX) {
+    return &(static_cast<mjCFlex*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return pair given mjsElement
 mjsPair* mjs_asPair(mjsElement* element) {
-  return element->elemtype == mjOBJ_PAIR ? &(static_cast<mjCPair*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_PAIR) {
+    return &(static_cast<mjCPair*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return equality given mjsElement
 mjsEquality* mjs_asEquality(mjsElement* element) {
-  return element->elemtype == mjOBJ_EQUALITY ? &(static_cast<mjCEquality*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_EQUALITY) {
+    return &(static_cast<mjCEquality*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return exclude given mjsElement
 mjsExclude* mjs_asExclude(mjsElement* element) {
-  return element->elemtype == mjOBJ_EXCLUDE ? &(static_cast<mjCBodyPair*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_EXCLUDE) {
+    return &(static_cast<mjCBodyPair*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return tendon given mjsElement
 mjsTendon* mjs_asTendon(mjsElement* element) {
-  return element->elemtype == mjOBJ_TENDON ? &(static_cast<mjCTendon*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_TENDON) {
+    return &(static_cast<mjCTendon*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return numeric given mjsElement
 mjsNumeric* mjs_asNumeric(mjsElement* element) {
-  return element->elemtype == mjOBJ_NUMERIC ? &(static_cast<mjCNumeric*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_NUMERIC) {
+    return &(static_cast<mjCNumeric*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return text given mjsElement
 mjsText* mjs_asText(mjsElement* element) {
-  return element->elemtype == mjOBJ_TEXT ? &(static_cast<mjCText*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_TEXT) {
+    return &(static_cast<mjCText*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return tuple given mjsElement
 mjsTuple* mjs_asTuple(mjsElement* element) {
-  return element->elemtype == mjOBJ_TUPLE ? &(static_cast<mjCTuple*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_TUPLE) {
+    return &(static_cast<mjCTuple*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return key given mjsElement
 mjsKey* mjs_asKey(mjsElement* element) {
-  return element->elemtype == mjOBJ_KEY ? &(static_cast<mjCKey*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_KEY) {
+    return &(static_cast<mjCKey*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return mesh given mjsElement
 mjsMesh* mjs_asMesh(mjsElement* element) {
-  return element->elemtype == mjOBJ_MESH ? &(static_cast<mjCMesh*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_MESH) {
+    return &(static_cast<mjCMesh*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return hfield given mjsElement
 mjsHField* mjs_asHField(mjsElement* element) {
-  return element->elemtype == mjOBJ_HFIELD ? &(static_cast<mjCHField*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_HFIELD) {
+    return &(static_cast<mjCHField*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return skin given mjsElement
 mjsSkin* mjs_asSkin(mjsElement* element) {
-  return element->elemtype == mjOBJ_SKIN ? &(static_cast<mjCSkin*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_SKIN) {
+    return &(static_cast<mjCSkin*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return texture given mjsElement
 mjsTexture* mjs_asTexture(mjsElement* element) {
-  return element->elemtype == mjOBJ_TEXTURE ? &(static_cast<mjCTexture*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_TEXTURE) {
+    return &(static_cast<mjCTexture*>(element)->spec);
+  }
+  return  nullptr;
 }
 
 
 
 // return material given mjsElement
 mjsMaterial* mjs_asMaterial(mjsElement* element) {
-  return element->elemtype == mjOBJ_MATERIAL ? &(static_cast<mjCMaterial*>(element)->spec) : nullptr;
+  if (element && element->elemtype == mjOBJ_MATERIAL) {
+    return &(static_cast<mjCMaterial*>(element)->spec);
+  }
+  return  nullptr;
 }