gh-109793: Allow Switching Interpreters During Finalization (gh-109794)

Essentially, we should check the thread ID rather than the thread state pointer.

1 files changed, 16 insertions, 1 deletions

diff --git a/Python/pystate.c b/Python/pystate.c
index dcc6c11..570b524 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -2964,11 +2964,26 @@ _PyThreadState_MustExit(PyThreadState *tstate)
        tstate->interp->runtime to support calls from Python daemon threads.
        After Py_Finalize() has been called, tstate can be a dangling pointer:
        point to PyThreadState freed memory. */
+    unsigned long finalizing_id = _PyRuntimeState_GetFinalizingID(&_PyRuntime);
     PyThreadState *finalizing = _PyRuntimeState_GetFinalizing(&_PyRuntime);
     if (finalizing == NULL) {
+        // XXX This isn't completely safe from daemon thraeds,
+        // since tstate might be a dangling pointer.
         finalizing = _PyInterpreterState_GetFinalizing(tstate->interp);
+        finalizing_id = _PyInterpreterState_GetFinalizingID(tstate->interp);
     }
-    return (finalizing != NULL && finalizing != tstate);
+    // XXX else check &_PyRuntime._main_interpreter._initial_thread
+    if (finalizing == NULL) {
+        return 0;
+    }
+    else if (finalizing == tstate) {
+        return 0;
+    }
+    else if (finalizing_id == PyThread_get_thread_ident()) {
+        /* gh-109793: we must have switched interpreters. */
+        return 0;
+    }
+    return 1;
 }