Fix AttributeError in egl.GLContext.__del__ on partial construction

[shoemoney]

Summary

• Pre-initializes self._context = None as the first statement of GLContext.__init__ in python/mujoco/egl/__init__.py, so free() (called from __del__) is safe on a partially-constructed instance.
• Adds a regression test asserting that free()/__del__ do not raise AttributeError on a GLContext that never ran __init__.

Why

On a headless host whose EGL driver lacks the PLATFORM_DEVICE extension, create_initialized_egl_device_display() returns EGL_NO_DISPLAY and GLContext.__init__ raises ImportError at line 97-101, before self._context is ever assigned (it's first set at line 114). When the partially-constructed object is later garbage collected, __del__ calls free(), which unconditionally reads self._context:

def free(self):
  """Frees resources associated with this context."""
  global EGL_DISPLAY
  if self._context:   # AttributeError here if __init__ never got this far
    ...

This raises AttributeError: 'GLContext' object has no attribute '_context' from __del__, surfaced via sys.unraisablehook as a noisy Exception ignored in: <function GLContext.__del__ ...> traceback -- which masks the real, actionable ImportError that told the user EGL is unavailable. Reported in #991.

This is the same bug class fixed for Renderer.__init__/renderer.py in #3225 (issue #3213): pre-initialize the guarded attribute to None before anything that can raise, so __del__ is always safe regardless of how far __init__ got.

The fix is a single line:

def __init__(self, max_width, max_height):
  self._context = None
  del max_width, max_height  # unused
  ...

Test

hasattr(mujoco, 'GLContext')-style tests (used by render_test.py/renderer_test.py) are skipped in this repo's own CI, which runs the Python test job with MUJOCO_GL=disable. Reliably forcing GLContext.__init__ itself to fail the way a real headless host without PLATFORM_DEVICE support would additionally requires an EGL driver that's present but specifically lacks that extension -- not a precondition a test environment can create on demand.

So python/mujoco/egl_gl_context_test.py instead patches create_initialized_egl_device_display (via unittest.mock.patch.object) to return EGL.EGL_NO_DISPLAY, exactly as it would on such a host, and drives a real GLContext(640, 480) call through its real __init__ code path. The test asserts __init__ still raises ImportError (the real, actionable error), then garbage-collects the partially-constructed object under sys.unraisablehook and asserts nothing was raised from __del__. It skips only when the mujoco.egl module itself can't be imported (no libEGL/libOpenGL on the host -- an environment precondition orthogonal to this bug, and the same reason this repo's own CI runs with MUJOCO_GL=disable).

Scope

This PR fixes only the missing-attribute-on-__del__ bug. It does not touch the unrelated EGL_DISPLAY-goes-stale-after-atexit-termination bug that #2510 (open) fixes in the same file's free()/make_current() -- that PR adds EGL_DISPLAY and to the if self._context: check and swaps atexit.register(EGL.eglTerminate, ...) for a wrapper that nulls EGL_DISPLAY after termination; it does not pre-initialize self._context, so it does not fix this bug, and this PR's one-line change does not touch either of the lines #2510 touches.

Validation

Built MuJoCo from source (CMake + Ninja, Release, no IPO/LTO) and the Python bindings against it (Python 3.12), then ran pytest -v --pyargs mujoco with MUJOCO_GL=disable -- matching this repo's own CI invocation (.github/workflows/build.yml, "Test Python bindings" step) -- on both clean main and this branch:

• Clean main (baseline): 380 passed, 5 failed, 14 skipped.
• This branch: 380 passed, 5 failed, 15 skipped.

The 5 failures are identical on both and are all plugin-loading failures unrelated to this change (my local build didn't install/copy the plugin shared libraries the way CI's copy_plugins_posix step does):
bindings_test.py::MuJoCoBindingsTest::test_copy_mjdata_with_plugin, test_deepcopy_mjdata_with_plugin, test_load_plugin, specs_test.py::SpecsTest::test_delete_unused_plugin, test_plugin. None of these touch python/mujoco/egl/.

The one extra skip on this branch is the new egl_gl_context_test.py, which skips because this machine (macOS) has no libEGL/libOpenGL for mujoco.egl to import -- same reason the existing render_test.py/renderer_test.py GL tests are skipped here and in this repo's own CI (MUJOCO_GL=disable).

I additionally verified the new test's logic outside of pytest by stubbing the OpenGL.EGL import chain in-process so mujoco.egl becomes importable on a machine without a real EGL driver, then running the test body directly against both the pre-fix and post-fix egl/__init__.py: it fails with the exact AttributeError: 'GLContext' object has no attribute '_context' from __del__ on pre-fix code, and passes on post-fix code.