From 09a6fb7d4dea43236b04e831940ef42c483ed073 Mon Sep 17 00:00:00 2001
From: Google DeepMind <noreply@deepmind.com>
Date: Tue, 13 Jan 2026 05:54:06 -0800
Subject: [PATCH] #mjspec Enforce that material textures array is always of
 length mjNTEXROLE.

PiperOrigin-RevId: 855684213
Change-Id: Id8a3734c75841dfb91bd2252114249f08461b7f2
---
 .../mujoco/codegen/generate_spec_bindings.py  | 24 ++++++++++++++
 python/mujoco/specs_test.py                   | 31 ++++++++++++++++---
 2 files changed, 50 insertions(+), 5 deletions(-)

diff --git a/python/mujoco/codegen/generate_spec_bindings.py b/python/mujoco/codegen/generate_spec_bindings.py
index 52763fc7..a0026e55 100644
--- a/python/mujoco/codegen/generate_spec_bindings.py
+++ b/python/mujoco/codegen/generate_spec_bindings.py
@@ -239,6 +239,30 @@ def _ptr_binding_code(
       }}
     }}, py::return_value_policy::move);"""
   elif vartype == 'mjStringVec':
+    # Special case for material.textures: must be exactly mjNTEXROLE size
+    if classname == 'mjsMaterial' and varname == 'textures':
+      return f"""\
+  {classname}.def_property(
+    "{varname}",
+    []({rawclassname}& self) -> MjTypeVec<std::string> {{
+        return MjTypeVec<std::string>(self.{fullvarname}->data(),
+                                      self.{fullvarname}->size());
+      }},
+    []({rawclassname}& self, py::object rhs) {{
+        if (py::len(rhs) != mjNTEXROLE) {{
+          throw pybind11::value_error(
+              "material.textures must have exactly " + std::to_string(mjNTEXROLE) +
+              " elements, got " + std::to_string(py::len(rhs)) + ". " +
+              "Assign a list of " + std::to_string(mjNTEXROLE) + " texture names " +
+              "(use empty strings '' for unused slots).");
+        }}
+        self.{fullvarname}->clear();
+        self.{fullvarname}->reserve(mjNTEXROLE);
+        for (auto val : rhs) {{
+          self.{fullvarname}->push_back(py::cast<std::string>(val));
+      }}
+    }}, py::return_value_policy::move);"""
+    # Default case for other mjStringVec properties
     return f"""\
   {classname}.def_property(
     "{varname}",
diff --git a/python/mujoco/specs_test.py b/python/mujoco/specs_test.py
index e92ddb59..e9ee5cc5 100644
--- a/python/mujoco/specs_test.py
+++ b/python/mujoco/specs_test.py
@@ -1002,23 +1002,44 @@ class SpecsTest(absltest.TestCase):
     material.textures[texture_index] = 'texture_name'
     self.assertEqual(material.textures[texture_index], 'texture_name')
 
-    # Assign a complete list
-    material.textures = ['', 'new_name', '', '', '']
+    # Assign a complete list (must be mjNTEXROLE = 10 elements)
+    material.textures = ['', 'new_name', '', '', '', '', '', '', '', '']
     self.assertEqual(material.textures[texture_index], 'new_name')
 
     # textures is iterable
     self.assertEqual('new_name', ''.join(material.textures))
 
-    # supports `len`
-    self.assertLen(material.textures, 5)
+    # supports `len` - should always be mjNTEXROLE (10)
+    self.assertLen(material.textures, mujoco.mjtTextureRole.mjNTEXROLE)
 
     # field checks for out-of-bound access on read and on write
     with self.assertRaises(IndexError):
-      material.textures[5] = 'x'
+      material.textures[10] = 'x'
 
     with self.assertRaises(IndexError):
       material.textures[-1] = 'x'
 
+  def test_textures(self):
+    """Tests that partial texture list assignment raises ValueError."""
+
+    spec = mujoco.MjSpec()
+    material = spec.add_material(name='mat')
+    texture = spec.add_texture(
+        name='tex', builtin=mujoco.mjtBuiltin.mjBUILTIN_FLAT, width=2, height=2
+    )
+
+    # Should raise ValueError for incorrect size (only 1 element instead of 10)
+    with self.assertRaises(ValueError) as cm:
+      material.textures = ['tex']
+    self.assertIn('must have exactly 10 elements', str(cm.exception))
+    self.assertIn('got 1', str(cm.exception))
+
+    # Should succeed with correct size (mjNTEXROLE = 10)
+    material.textures = ['tex', '', '', '', '', '', '', '', '', '']
+    spec.worldbody.add_geom(size=[0.1, 0.1, 0.1], material='mat')
+    model = spec.compile()
+    self.assertIsNotNone(model)
+
   def test_assign_texture(self):
     spec = mujoco.MjSpec()
     texture = spec.add_texture(name='texture', height=2, width=2)