GH-156425: Clear GC frame on early return

Lib/test/test_gc.py

@@ -1289,6 +1289,18 @@ def test_heap_size(self):
         del l
         self.assertEqual(count, _testinternalcapi.get_tracked_heap_size())
 
+    @unittest.skipIf(_testinternalcapi is None, "requires _testinternalcapi")
+    def test_clear_frame_on_early_return(self):
+        # gh-156425: Make sure that a garbage collection always clears
+        # PyInterpreterState.gc.frame when it's done.
+        thresholds = gc.get_threshold()
+        gc.enable()
+        try:
+            gc.collect(2)
+            self.assertTrue(_testinternalcapi.is_gc_frame_cleared(thresholds[0]))
+        finally:
+            gc.disable()
+
 
 class GCCallbackTests(unittest.TestCase):
     def setUp(self):

Misc/NEWS.d/next/Core_and_Builtins/2026-09-08-17-54-34.gh-issue-156425.-Rn7Tw.rst

@@ -0,0 +1,3 @@
+Fix a bug in the garbage collector where frames could be kept alive longer
+than necessary, potentially distorting profiler statistics. Patch by Sergey
+Miryanov.

Modules/_testinternalcapi.c

@@ -3256,6 +3256,59 @@ test_thread_state_ensure_from_view_interp_switch(PyObject *self, PyObject *unuse
     Py_RETURN_NONE;
 }
 
+// gh-156425: Make sure that a garbage collection always clears
+// PyInterpreterState.gc.frame when it's done.
+static PyObject *
+is_gc_frame_cleared(PyObject *self, PyObject *arg)
+{
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    PyThreadState *tstate = PyThreadState_GET();
+    assert(interp != NULL);
+    assert(tstate != NULL);
+
+    int32_t n = 0;
+    if (0 > PyLong_AsInt32(arg, &n)) {
+        return NULL;
+    }
+
+    PyObject *list = PyList_New(0);
+    if (list == NULL) {
+        return NULL;
+    }
+
+    // We create n objects, which should schedule a GC run.
+    // However, since we are inside a C call, garbage collection will
+    // not run immediately.
+    for (int32_t i = 0; i < n; i++) {
+        PyObject *b = PyLong_GetInfo();
+        if (b == NULL) {
+            Py_DECREF(list);
+            return NULL;
+        }
+        if (0 > PyList_Append(list, b)) {
+            Py_DECREF(list);
+            Py_DECREF(b);
+            return NULL;
+        }
+        Py_DECREF(b);
+    }
+
+    // Then we clear n objects, which decrements the young generation counter.
+    assert(0 == PyList_Clear(list));
+    Py_DECREF(list);
+
+    // We trigger pending garbage collection.
+    // However, the GC start conditions are not satisfied (young.threshold > young.count),
+    // so gc_select_generation() cannot select a generation and returns early.
+    assert(0 == _Py_HandlePending(tstate));
+
+    if (!interp->gc.frame) {
+        Py_RETURN_TRUE;
+    }
+
+    Py_RETURN_FALSE;
+}
+
 static PyObject *
 unicodewriter_overflow(PyObject *self, PyObject *unused)
 {
@@ -3466,6 +3519,7 @@ static PyMethodDef module_functions[] = {
     {"test_interp_guard_countdown", test_interp_guard_countdown, METH_NOARGS},
     {"test_interp_view_countdown", test_interp_view_countdown, METH_NOARGS},
     {"test_thread_state_ensure_from_view_interp_switch", test_thread_state_ensure_from_view_interp_switch, METH_NOARGS},
+    {"is_gc_frame_cleared", is_gc_frame_cleared, METH_O},
     {"unicodewriter_overflow", unicodewriter_overflow, METH_NOARGS},
     {NULL, NULL} /* sentinel */
 };

Python/gc.c

@@ -1437,6 +1437,7 @@ gc_collect_main(PyThreadState *tstate, int generation, _PyGC_Reason reason)
     PyGC_Head finalizers;  /* objects with, & reachable from, __del__ */
     PyGC_Head *gc;
     GCState *gcstate = &tstate->interp->gc;
+    Py_ssize_t result = 0;
 
     // gc_collect_main() must not be called before _PyGC_Init
     // or after _PyGC_Fini()
@@ -1455,9 +1456,7 @@ gc_collect_main(PyThreadState *tstate, int generation, _PyGC_Reason reason)
         // objects from that generation and all generations younger than it.
         generation = gc_select_generation(gcstate);
         if (generation < 0) {
-            // No generation needs to be collected.
-            _Py_atomic_store_int(&gcstate->collecting, 0);
-            return 0;
+            goto exit;
         }
     }
 
@@ -1664,10 +1663,13 @@ gc_collect_main(PyThreadState *tstate, int generation, _PyGC_Reason reason)
         invoke_gc_callback(tstate, "stop", generation, &stats);
     }
 
+    result = stats.uncollectable + stats.collected;
+
+exit:
     assert(!_PyErr_Occurred(tstate));
     gcstate->frame = NULL;
     _Py_atomic_store_int(&gcstate->collecting, 0);
-    return stats.uncollectable + stats.collected;
+    return result;
 }
 
 static int