gh-112723: Call `PyThreadState_Clear()` from the correct interpreter (#112776)

The `PyThreadState_Clear()` function must only be called with the GIL
held and must be called from the same interpreter as the passed in
thread state. Otherwise, any Python objects on the thread state may be
destroyed using the wrong interpreter, leading to memory corruption.

This is also important for `Py_GIL_DISABLED` builds because free lists
will be associated with PyThreadStates and cleared in
`PyThreadState_Clear()`.

This fixes two places that called `PyThreadState_Clear()` from the wrong
interpreter and adds an assertion to `PyThreadState_Clear()`.

1 files changed, 2 insertions, 0 deletions

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 4bb54c9..4e9e134 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -547,7 +547,9 @@ interp_create(PyObject *self, PyObject *args, PyObject *kwds)
         return NULL;
     }
 
+    PyThreadState_Swap(tstate);
     PyThreadState_Clear(tstate);
+    PyThreadState_Swap(save_tstate);
     PyThreadState_Delete(tstate);
 
     _PyInterpreterState_RequireIDRef(interp, 1);