This test created a subinterpreter when PYBIND11_SUBINTERPRETER_SUPPORT was off

So the fix is really this test should not be run in these older versions at all.

The hang was a GIL issue between the subinterpreters during pybind11::exception::what().

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

tests/test_embed/test_interpreter.cpp

@@ -336,6 +336,7 @@ TEST_CASE("Restart the interpreter") {
     REQUIRE(py_widget.attr("the_message").cast<std::string>() == "Hello after restart");
 }
 
+#if defined(PYBIND11_SUBINTERPRETER_SUPPORT)
 TEST_CASE("Subinterpreter") {
     py::module_::import("external_module"); // in the main interpreter
 
@@ -347,6 +348,10 @@ TEST_CASE("Subinterpreter") {
 
         REQUIRE(m.attr("add")(1, 2).cast<int>() == 3);
     }
+
+    auto main_int
+        = py::module_::import("external_module").attr("internals_at")().cast<uintptr_t>();
+
     REQUIRE(has_state_dict_internals_obj());
     REQUIRE(has_pybind11_internals_static());
 
@@ -359,7 +364,6 @@ TEST_CASE("Subinterpreter") {
     // Subinterpreters get their own copy of builtins.
     REQUIRE_FALSE(has_state_dict_internals_obj());
 
-#if defined(PYBIND11_SUBINTERPRETER_SUPPORT) && PY_VERSION_HEX >= 0x030C0000
     // internals hasn't been populated yet, but will be different for the subinterpreter
     REQUIRE_FALSE(has_pybind11_internals_static());
 
@@ -369,14 +373,12 @@ TEST_CASE("Subinterpreter") {
     py::detail::get_internals();
     REQUIRE(has_pybind11_internals_static());
     REQUIRE(get_details_as_uintptr() == ext_int);
-#else
-    // This static is still defined
-    REQUIRE(has_pybind11_internals_static());
-#endif
+    REQUIRE(main_int != ext_int);
 
     // Modules tags should be gone.
     REQUIRE_FALSE(py::hasattr(py::module_::import("__main__"), "tag"));
     {
+        REQUIRE_NOTHROW(py::module_::import("widget_module"));
         auto m = py::module_::import("widget_module");
         REQUIRE_FALSE(py::hasattr(m, "extension_module_tag"));
 
@@ -397,7 +399,6 @@ TEST_CASE("Subinterpreter") {
     REQUIRE(has_state_dict_internals_obj());
 }
 
-#if defined(PYBIND11_SUBINTERPRETER_SUPPORT)
 TEST_CASE("Multiple Subinterpreters") {
     // Make sure the module is in the main interpreter and save its pointer
     auto *main_ext = py::module_::import("external_module").ptr();
@@ -527,13 +528,7 @@ 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 +552,8 @@ 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);