Manual loading of plugins libraries fails via python api, but works fine via c++ api

[aaarne]

Intro

Hi!

I am a graduate student at TU Munich and the German Aerospace Center (DLR); I use MuJoCo to research elastic robots with variable stiffness actuators.

My setup

MuJoCo version: 2.3.4
API: Both (Plugin Development: C++, Using the model: Python)
Architecture: Arch Linux on 6.11.5 kernel

What's happening? What did you expect?

I have an actuator plugin compiled into a separate shared library (.so) and a model using that plugin. Different scenarios:

1. I use the model via the mujoco C++ API. I register the library via mj_loadPluginLibrary/mj_loadAllPluginLibraries or I use the simulate application and put the so where simulate wants. This works fine! (so model and plugin should be okay)
2. I use the Python API and put the plugin .so into the mujoco plugin folder, where mujoco's __init__.py finds and loads it. This also works fine. (so it should generally work using the python API)
3. I use the Python API, but keep my plugin .so in some folder where I want it. I use mujoco.mj_loadPluginLibrary/mujoco.mj_loadAllPluginLibraries to load the plugin manual via the python API. This does not work. I see that the library registers the plugin. I have compiled a printout into the .so that prints a message before registering the plugin in the mjPLUGIN_LIB_INIT, and I see that message. However, it then fails on loading the XML model with ValueError: XML Error: plugin vsa not found.

Steps for reproduction

1. Generate a plugin library compiled into a separate .so file, and a model referencing that plugin.
2. Load the library in python using the exposed functions mj_loadPluginLibrary or mj_loadAllPluginLibraries.
3. The library registers the plugins.
4. Load the XML model referencing the plugin. It will fail claiming it cannot find the plugin.

Minimal model for reproduction

<mujoco>
  <extension>
    <plugin plugin="testplugin"/>
  </extension>
  <worldbody>
    <light pos="0 0 1"/>
    <geom type="sphere" size="1" rgba="1 0 0 1"/>
  </worldbody>
</mujoco>

Code required for reproduction

import mujoco

mujoco.mj_loadPluginLibrary("somepath/mypluginlibrary.so")  
# after this call mjPLUGIN_LIB_INIT in mypluginlibrary.so is executed, so it is somehow loaded.

model = mujoco.MjModel.from_xml_string("some_model_using_testplugin_in_mypluginlibrary.xml")
# This fails with ValueError: XML Error: plugin testplugin not found, but testplugin is registered by mypluginlibrary.so

Doing the above steps via the c++ API works on the same files. It also works via the python API when removing the mj_loadPluginLibrary call and placing mypluginlibrary.so into mujoco/plugin.

Confirmations

• I searched the latest documentation thoroughly before posting.
• I searched previous Issues and Discussions, I am certain this has not been raised before.