Try immortalizing the module def object

Author: b-pass

include/pybind11/pybind11.h

@@ -1484,7 +1484,7 @@ class module_ : public object {
     using slots_array = std::array<PyModuleDef_Slot, 4>;
 
     /** \rst
-        Initialized a module def for use with multi-phase module initialization.
+        Initialize a module def for use with multi-phase module initialization.
 
         ``def`` should point to a statically allocated module_def.
         ``slots`` must already contain a Py_mod_exec or Py_mod_create slot and will be filled with
@@ -1544,6 +1544,10 @@ class module_ : public object {
             }
             pybind11_fail("Internal error in module_::initialize_multiphase_module_def()");
         }
+        // since def was statically allocated, this object should be immortal.
+#ifdef _Py_IMMORTAL_REFCNT
+        Py_SET_REFCNT(m, _Py_IMMORTAL_REFCNT);
+#endif
         return reinterpret_borrow<object>(m);
     }
 };

tests/test_embed/test_interpreter.cpp

@@ -527,13 +527,7 @@ TEST_CASE("Per-Subinterpreter GIL") {
         T_REQUIRE(caught);
 
         // widget_module did provide the per_interpreter_gil tag, so it this does not throw
-        try {
-            py::module_::import("widget_module");
-            caught = false;
-        } catch (pybind11::error_already_set &) {
-            caught = true;
-        }
-        T_REQUIRE(!caught);
+        py::module_::import("widget_module");
 
         T_REQUIRE(!py::hasattr(py::module_::import("external_module"), "multi_interp"));
         py::module_::import("external_module").attr("multi_interp") = std::to_string(num);
@@ -557,8 +551,8 @@ TEST_CASE("Per-Subinterpreter GIL") {
 
         Py_EndInterpreter(sub);
 
-        PyThreadState_Swap(
-            main_tstate); // switch back so the scoped_acquire can release the GIL properly
+        // switch back so the scoped_acquire can release the GIL properly
+        PyThreadState_Swap(main_tstate);
     };
 
     std::thread t1(thread_main, 1);