gh-117657: Fix race involving immortalizing objects (#119927)

The free-threaded build currently immortalizes objects that use deferred
reference counting (see gh-117783). This typically happens once the
first non-main thread is created, but the behavior can be suppressed for
tests, in subinterpreters, or during a compile() call.

This fixes a race condition involving the tracking of whether the
behavior is suppressed.

1 files changed, 7 insertions, 7 deletions

diff --git a/Python/gc_free_threading.c b/Python/gc_free_threading.c
index e6bd012c40e..d005b79ff40 100644
--- a/Python/gc_free_threading.c
+++ b/Python/gc_free_threading.c
@@ -703,11 +703,9 @@ _PyGC_Init(PyInterpreterState *interp)
 {
     GCState *gcstate = &interp->gc;
 
-    if (_Py_IsMainInterpreter(interp)) {
-        // gh-117783: immortalize objects that would use deferred refcounting
-        // once the first non-main thread is created.
-        gcstate->immortalize.enable_on_thread_created = 1;
-    }
+    // gh-117783: immortalize objects that would use deferred refcounting
+    // once the first non-main thread is created (but not in subinterpreters).
+    gcstate->immortalize = _Py_IsMainInterpreter(interp) ? 0 : -1;
 
     gcstate->garbage = PyList_New(0);
     if (gcstate->garbage == NULL) {
@@ -1808,8 +1806,10 @@ _PyGC_ImmortalizeDeferredObjects(PyInterpreterState *interp)
 {
     struct visitor_args args;
     _PyEval_StopTheWorld(interp);
-    gc_visit_heaps(interp, &immortalize_visitor, &args);
-    interp->gc.immortalize.enabled = 1;
+    if (interp->gc.immortalize == 0) {
+        gc_visit_heaps(interp, &immortalize_visitor, &args);
+        interp->gc.immortalize = 1;
+    }
     _PyEval_StartTheWorld(interp);
 }