check GIL when performing reference counting operations in debug mode

include/nanobind/nb_lib.h

@@ -398,5 +398,13 @@ NB_CORE bool load_u64(PyObject *o, uint8_t flags, uint64_t *out) noexcept;
 NB_CORE bool load_f32(PyObject *o, uint8_t flags, float *out) noexcept;
 NB_CORE bool load_f64(PyObject *o, uint8_t flags, double *out) noexcept;
 
+// ========================================================================
+
+/// Increase the reference count of 'o', and check that the GIL is held
+NB_CORE void incref_checked(PyObject *o) noexcept;
+
+/// Decrease the reference count of 'o', and check that the GIL is held
+NB_CORE void decref_checked(PyObject *o) noexcept;
+
 NAMESPACE_END(detail)
 NAMESPACE_END(NB_NAMESPACE)

include/nanobind/nb_types.h

@@ -156,8 +156,23 @@ class handle : public detail::api<handle> {
     NB_INLINE handle(const PyObject *ptr) : m_ptr((PyObject *) ptr) { }
     NB_INLINE handle(const PyTypeObject *ptr) : m_ptr((PyObject *) ptr) { }
 
-    const handle& inc_ref() const & noexcept { Py_XINCREF(m_ptr); return *this; }
-    const handle& dec_ref() const & noexcept { Py_XDECREF(m_ptr); return *this; }
+    const handle& inc_ref() const & noexcept {
+#if defined(NDEBUG)
+        Py_XINCREF(m_ptr);
+#else
+        detail::incref_checked(m_ptr);
+#endif
+        return *this;
+    }
+
+    const handle& dec_ref() const & noexcept {
+#if defined(NDEBUG)
+        Py_XDECREF(m_ptr);
+#else
+        detail::decref_checked(m_ptr);
+#endif
+        return *this;
+    }
 
     NB_INLINE operator bool() const { return m_ptr != nullptr; }
     NB_INLINE PyObject *ptr() const { return m_ptr; }

src/common.cpp

@@ -865,5 +865,26 @@ bool load_i64(PyObject *o, uint8_t flags, int64_t *out) noexcept {
     return load_int(o, flags, out);
 }
 
+
+// ========================================================================
+
+void incref_checked(PyObject *o) noexcept {
+    if (!o)
+        return;
+    if (!PyGILState_Check())
+        fail("nanobind::detail::incref_check(): attempted to change the "
+             "reference count of a Python object while the GIL was not held.");
+    Py_INCREF(o);
+}
+
+void decref_checked(PyObject *o) noexcept {
+    if (!o)
+        return;
+    if (!PyGILState_Check())
+        fail("nanobind::detail::decref_check(): attempted to change the "
+             "reference count of a Python object while the GIL was not held.");
+    Py_DECREF(o);
+}
+
 NAMESPACE_END(detail)
 NAMESPACE_END(NB_NAMESPACE)