Fix segfault on a mesh with convex inertia that no geom references

[VihaanAgarwal]

Fixes #3431.

What happens

A mesh declared with inertia="convex" that no geom references segfaults the compiler:

<mujoco>
  <asset>
    <mesh name="orphan" inertia="convex"
          vertex="0 0 1   1 0 0   0 1 0   -1 0 0   0 -1 0"
          face="0 1 2   0 2 3   0 3 4   0 4 1   1 4 3   1 3 2"/>
  </asset>
  <worldbody/>
</mujoco>

mujoco.MjModel.from_xml_path("orphan.xml") exits with SIGSEGV on 3.10.0 and on current main.

Root cause

mjCModel::TryCompile decides which meshes need a convex hull by walking geoms_, and one of the
conditions in that loop is geoms_[i]->mesh->spec.inertia == mjMESH_INERTIA_CONVEX. That condition
is a property of the mesh, but it is only ever evaluated for meshes some geom points at. An
unreferenced mesh is never visited, so needhull_ stays false, mjCMesh::Compile skips
MakeGraph, and graph_ stays null. ComputeVolume and ComputeInertia then do

int nf = (inertia == mjMESH_INERTIA_CONVEX) ? graph_[1] : nface();
const int* f = (inertia == mjMESH_INERTIA_CONVEX) ? GraphFaces() : face_.data();

on that null pointer, which is the crash. Note the mesh here has faces, so the face_.empty()
fallback that would otherwise build the hull does not fire either.

Fix

Move the convex-inertia condition out of the geom loop and apply it to every mesh, so a mesh that
computes its inertia from its hull always gets one. The geom loop keeps the conditions that really
are about geoms (contype, conaffinity, membership in a contact pair).

The reporter asked for "a warning about unused meshes, or silence" — this gives silence, and it
matches the intent the original condition already expressed.

Test

MjCMeshTest.UnreferencedConvexInertiaMesh in test/user/user_mesh_test.cc compiles the model
above. It dies with SIGSEGV on main and passes with this change.

Ran on macOS arm64, Release build:

• user_mesh_test 71/71 pass
• user_objects_test 90/90, user_recompile_test 320/320, user_flex_test 59/59,
  user_composite_test 4/4 all pass
• user_model_test has one failure (MujocoTest.ResolvePluginMissingInstanceThrowsError) and
  user_api_test segfaults, both of which reproduce identically on unmodified main here, so they
  are local/environmental and not from this change.

Changelog entry added under Compiler -> Bug fixes.

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[VihaanAgarwal]

Hey, I get the suspicion. I did use a coding agent to help me navigate the codebase, but I wasn't just submitting whatever it generated.

It took me a while to track this down: graph_ is null because the geom loop in TryCompile only checks the convex-inertia condition for meshes referenced by a geom. For an orphan mesh, needhull_ never gets set.

The test segfaults on main and passes with the change.

This is my first time working in a C++ codebase this large, so if the fix itself or the test placement is off, just let me know and I'll redo it.

[VihaanAgarwal]

?

[VihaanAgarwal]

Yeah, I use Claude Code. It’s 2026. If there’s an issue with the actual fix, I’m happy to address it.