gh-109860: Use a New Thread State When Switching Interpreters, When Necessary (gh-110245)

In a few places we switch to another interpreter without knowing if it has a thread state associated with the current thread.  For the main interpreter there wasn't much of a problem, but for subinterpreters we were *mostly* okay re-using the tstate created with the interpreter (located via PyInterpreterState_ThreadHead()).  There was a good chance that tstate wasn't actually in use by another thread.

However, there are no guarantees of that.  Furthermore, re-using an already used tstate is currently fragile.  To address this, now we create a new thread state in each of those places and use it.

One consequence of this change is that PyInterpreterState_ThreadHead() may not return NULL (though that won't happen for the main interpreter).

1 files changed, 4 insertions, 2 deletions

diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index c032376..eb10aa3 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -655,7 +655,8 @@ pycore_create_interpreter(_PyRuntimeState *runtime,
         return status;
     }
 
-    PyThreadState *tstate = _PyThreadState_New(interp);
+    PyThreadState *tstate = _PyThreadState_New(interp,
+                                               _PyThreadState_WHENCE_INTERP);
     if (tstate == NULL) {
         return _PyStatus_ERR("can't make first thread");
     }
@@ -2050,7 +2051,8 @@ new_interpreter(PyThreadState **tstate_p, const PyInterpreterConfig *config)
         return _PyStatus_OK();
     }
 
-    PyThreadState *tstate = _PyThreadState_New(interp);
+    PyThreadState *tstate = _PyThreadState_New(interp,
+                                               _PyThreadState_WHENCE_INTERP);
     if (tstate == NULL) {
         PyInterpreterState_Delete(interp);
         *tstate_p = NULL;