MjSpec.compile() should populate resulting MjModel's XML to enable mujoco.mj_saveLastXML

[hartikainen]

The feature, motivation and pitch

Currently, the mj_model resulting from mj_model = mj_spec.compile() cannot be saved with mujoco.mj_saveLastXML(mj_model) because the XML information is not stored in the model. It would be useful to be able to save the XML of an MjModel without carrying the MjSpec around.

It feels like this would be as simple as storing the MjSpec.to_xml() into the MjModel, but perhaps I'm overlooking something and this is actually more complicated than I think.

Alternatives

I can save the compiled model with mujoco.mj_saveModel. For debugging purposes, it would be nice to be able to look into the XML. This I can also do by keeping the spec around but it would be more convienient to not have to worry about the spec.

Additional context

Here's a simple test case that fails:

  def test_spec_model_saveLastXML(self):
    spec_1 = mujoco.MjSpec()
    spec_2 = mujoco.MjSpec()
    spec_1.from_string("<mujoco></mujoco>")
    model = spec_1.compile()
    with tempfile.NamedTemporaryFile(suffix=".xml", mode="w+t") as fp:
      mujoco.mj_saveLastXML(fp.name, model)
      spec_2.from_file(fp.name)

    self.assertEqual(spec_1.to_xml(), spec_2.to_xml())

Which gives:

======================================================================
ERROR: test_spec_model_saveLastXML (specs_test.SpecsTest.test_spec_model_saveLastXML)
specs_test.SpecsTest.test_spec_model_saveLastXML
----------------------------------------------------------------------
Traceback (most recent call last):
  File "mujoco/python/mujoco/specs_test.py", line 802, in test_spec_model_saveLastXML
    mujoco.mj_saveLastXML(fp.name, model)
mujoco.FatalError: No XML model loaded

----------------------------------------------------------------------

[yuvaltassa]

In what way is mj_saveXML not what you want?