gh-112175: Add `eval_breaker` to `PyThreadState` (#115194)

This change adds an `eval_breaker` field to `PyThreadState`. The primary
motivation is for performance in free-threaded builds: with thread-local eval
breakers, we can stop a specific thread (e.g., for an async exception) without
interrupting other threads.

The source of truth for the global instrumentation version is stored in the
`instrumentation_version` field in PyInterpreterState. Threads usually read the
version from their local `eval_breaker`, where it continues to be colocated
with the eval breaker bits.

1 files changed, 6 insertions, 3 deletions

diff --git a/Python/gc_free_threading.c b/Python/gc_free_threading.c
index a758c99..2993ef4 100644
--- a/Python/gc_free_threading.c
+++ b/Python/gc_free_threading.c
@@ -981,7 +981,7 @@ record_allocation(PyThreadState *tstate)
         if (gc_should_collect(gcstate) &&
             !_Py_atomic_load_int_relaxed(&gcstate->collecting))
         {
-            _Py_ScheduleGC(tstate->interp);
+            _Py_ScheduleGC(tstate);
         }
     }
 }
@@ -1564,9 +1564,12 @@ PyObject_IS_GC(PyObject *obj)
 }
 
 void
-_Py_ScheduleGC(PyInterpreterState *interp)
+_Py_ScheduleGC(PyThreadState *tstate)
 {
-    _Py_set_eval_breaker_bit(interp, _PY_GC_SCHEDULED_BIT, 1);
+    if (!_Py_eval_breaker_bit_is_set(tstate, _PY_GC_SCHEDULED_BIT))
+    {
+        _Py_set_eval_breaker_bit(tstate, _PY_GC_SCHEDULED_BIT);
+    }
 }
 
 void