Fix GIL-not-held bug in mjr_readPixels.

PiperOrigin-RevId: 441813459
Change-Id: I2f15cb77ca243b890fd0825ebbc9b910d3ece56f

python/mujoco/functions.h

@@ -192,6 +192,14 @@ static constexpr void Def(
            ::pybind11::call_guard<::pybind11::gil_scoped_release>());
 }
 
+template <typename MjTraits, typename Func>
+MUJOCO_ALWAYS_INLINE
+static constexpr void DefWithGil(::pybind11::module_& m, Func&& func) {
+  WithNamedArgs(MjTraits::param_names).def(
+      m, MjTraits::name, util::UnwrapArgs(std::forward<Func>(func)),
+      ::pybind11::doc(MjTraits::doc));
+}
+
 // Should only be invoked via the DEF_WITH_OMITTED_PY_ARGS macro.
 template <typename MjTraits, typename OmittedArgsTuple>
 struct DefWithOmittedPyArgsImpl {

python/mujoco/render.cc

@@ -13,6 +13,7 @@
 // limitations under the License.
 
 #include <array>
+#include <cstdint>
 
 #include <Eigen/Core>
 #include <mjrender.h>
@@ -252,14 +253,19 @@ PYBIND11_MODULE(_render, pymodule) {
   Def<traits::mjr_uploadHField>(pymodule);
   Def<traits::mjr_restoreBuffer>(pymodule);
   Def<traits::mjr_setBuffer>(pymodule);
-  Def<traits::mjr_readPixels>(
+  DefWithGil<traits::mjr_readPixels>(
-      pymodule, [](std::optional<py::array_t<uint8_t>> rgb,
+      pymodule, [](std::optional<py::array_t<std::uint8_t>> rgb,
                    std::optional<py::array_t<float>> depth,
                    const raw::MjrRect* viewport, const raw::MjrContext* con) {
-        return InterceptMjErrors(::mjr_readPixels)(
+        std::uint8_t* const rgb_data =
-            rgb.has_value() ? rgb->mutable_data() : nullptr,
+            rgb.has_value() ? rgb->mutable_data() : nullptr;
-            depth.has_value() ? depth->mutable_data() : nullptr, *viewport,
+        float* const depth_data =
-            con);
+            depth.has_value() ? depth->mutable_data() : nullptr;
+        {
+          py::gil_scoped_release no_gil;
+          return InterceptMjErrors(::mjr_readPixels)(rgb_data, depth_data,
+                                                     *viewport, con);
+        }
       });
   Def<traits::mjr_drawPixels>(
       pymodule,