Fix #2592: avoid export recursion when an ancestor is bone-parented to the skinning armature

[Builder106]

Fixes #2592.

Problem

Exporting a scene where a skinned mesh has an ancestor that is bone-parented to the same armature raises RecursionError, and the export fails.

Minimal repro: Armature (bone Bone) → Empty bone-parented to Bone → Cube object-parented to the Empty, with a vertex group Bone + an Armature modifier targeting Armature. Exporting to glTF raises RecursionError: maximum recursion depth exceeded.

Cause

gather_skin (exp/nodes.py) already guards the direct case: a mesh bone-parented to a bone of its own skinning armature would make the skin's joint an ancestor of the mesh node, forming a cycle, so the skin is dropped. But the guard only checked blender_object.parent. When the bone-parented node is an ancestor (the Empty) rather than the mesh itself, the guard misses it, the skin is created, and the exported node graph contains a cycle:

Cube.skin -> joint (Bone) -> Bone.children = [Empty] -> Empty.children = [Cube] -> Cube.skin -> ...

The exporter's node traversal follows .skin and .children, so it recurses until RecursionError.

Fix

Walk the whole ancestor chain instead of only the direct parent; if any ancestor is bone-parented to the skinning armature, drop the skin (and keep the bone parenting) — identical behavior to the existing guard, just generalized past the direct parent.

Verification

Headless Blender 5.1.1 (the reported version), export_format='GLTF_SEPARATE':

scene	before	after
repro (skinned mesh, bone-parented ancestor)	RecursionError, no file written	FINISHED · skins: [] · Cube node has mesh, no skin
control (normal skinning, mesh parented to the armature)	skins length 1, Cube skinned	skins length 1, Cube skinned (unchanged)

So the cycle is broken (skin dropped, mesh + bone parenting preserved) and normal skinning is unaffected.

Two points for your call: I kept the existing name-based armature comparison for a minimal diff — happy to switch it to identity (ancestor.parent == modifiers["ARMATURE"].object) to harden against same-named linked/duplicated armatures if you'd prefer. I can also add a tests/ fixture for this topology if that's useful.

[Builder106]

Hey @julienduroure, since you filed #2592 yourself, figured this one might be worth a look. It's been about three weeks. The existing cycle-guard in gather_skin only checks the mesh's direct parent, so it misses the case where a bone-parented object is further up the ancestor chain instead of being the mesh itself, that's what still triggers the RecursionError. This generalizes the guard to walk the whole chain. I verified both the repro (RecursionError to FINISHED, skin correctly dropped) and a normal-skinning control case so regular skinning stays unaffected. There are two smaller PRs open too, #2710 and #2711, in case it's useful to look at them together.