Hand off the GIL when waiting on interpreter references.

We should probably use an event instead of a silly loop like this, but I
don't care at the moment.

Author: ZeroIntensity

Modules/_testcapimodule.c

@@ -2763,6 +2763,15 @@ test_weak_interpreter_ref_after_shutdown(PyObject *self, PyObject *unused)
     Py_RETURN_NONE;
 }
 
+static PyObject *
+foo(PyObject *self, PyObject *foo)
+{
+    PyInterpreterRef ref;
+    PyInterpreterRef_Get(&ref);
+    PyInterpreterRef_Close(ref);
+    Py_RETURN_NONE;
+}
+
 static PyMethodDef TestMethods[] = {
     {"set_errno",               set_errno,                       METH_VARARGS},
     {"test_config",             test_config,                     METH_NOARGS},
@@ -2861,6 +2870,7 @@ static PyMethodDef TestMethods[] = {
     {"test_thread_state_ensure_nested", test_thread_state_ensure_nested, METH_NOARGS},
     {"test_thread_state_ensure_crossinterp", test_thread_state_ensure_crossinterp, METH_NOARGS},
     {"test_weak_interpreter_ref_after_shutdown", test_weak_interpreter_ref_after_shutdown, METH_NOARGS},
+    {"foo", foo, METH_NOARGS},
     {NULL, NULL} /* sentinel */
 };
 

Python/pylifecycle.c

@@ -2127,6 +2127,7 @@ make_pre_finalization_calls(PyThreadState *tstate, int subinterpreters)
         int has_subinterpreters = subinterpreters
                                     ? runtime_has_subinterpreters(interp->runtime)
                                     : 0;
+        // TODO: The interpreter reference countdown probably isn't very efficient.
         int should_continue = (interp_has_threads(interp)
                               || interp_has_atexit_callbacks(interp)
                               || interp_has_pending_calls(interp)
@@ -2135,9 +2136,12 @@ make_pre_finalization_calls(PyThreadState *tstate, int subinterpreters)
         if (!should_continue) {
             break;
         }
+        // Temporarily let other threads execute
+        _PyThreadState_Detach(tstate);
         _PyRWMutex_Unlock(&interp->references.lock);
         _PyEval_StartTheWorldAll(interp->runtime);
         PyMutex_Unlock(&interp->ceval.pending.mutex);
+        _PyThreadState_Attach(tstate);
     }
     assert(PyMutex_IsLocked(&interp->ceval.pending.mutex));
     ASSERT_WORLD_STOPPED(interp);