gh-117657: Make frame clearing thread-safe

Lib/test/test_free_threading/test_frame.py

@@ -0,0 +1,29 @@
+import sys
+import unittest
+import threading
+
+from test.support import threading_helper
+
+NTHREADS = 6
+
+@threading_helper.requires_working_threading()
+class TestFrame(unittest.TestCase):
+    def test_frame_clear_simultaneous(self):
+
+        def getframe():
+            return sys._getframe()
+
+        foo = getframe()
+        def work():
+            for _ in range(4000):
+                foo.clear()
+
+
+        threads = []
+        for i in range(NTHREADS):
+            thread = threading.Thread(target=work)
+            thread.start()
+            threads.append(thread)
+
+        for thread in threads:
+            thread.join()

Objects/frameobject.c

@@ -7,7 +7,7 @@
 #include "pycore_moduleobject.h"  // _PyModule_GetDict()
 #include "pycore_object.h"        // _PyObject_GC_UNTRACK()
 #include "pycore_opcode_metadata.h" // _PyOpcode_Deopt, _PyOpcode_Caches
-
+#include "pycore_critical_section.h" // Py_BEGIN_CRITICAL_SECTION
 
 #include "frameobject.h"          // PyFrameObject
 #include "pycore_frame.h"
@@ -1662,7 +1662,7 @@ frame_tp_clear(PyFrameObject *f)
 }
 
 static PyObject *
-frame_clear(PyFrameObject *f, PyObject *Py_UNUSED(ignored))
+frame_clear_unlocked(PyFrameObject *f, PyObject *Py_UNUSED(ignored))
 {
     if (f->f_frame->owner == FRAME_OWNED_BY_GENERATOR) {
         PyGenObject *gen = _PyFrame_GetGenerator(f->f_frame);
@@ -1692,6 +1692,17 @@ frame_clear(PyFrameObject *f, PyObject *Py_UNUSED(ignored))
     return NULL;
 }
 
+static PyObject *
+frame_clear(PyFrameObject *f, PyObject *Py_UNUSED(ignored))
+{
+    PyObject *res;
+    // Another materialized frame might be racing on clearing the frame.
+    Py_BEGIN_CRITICAL_SECTION(f);
+    res = frame_clear_unlocked(f, NULL);
+    Py_END_CRITICAL_SECTION();
+    return res;
+}
+
 PyDoc_STRVAR(clear__doc__,
 "F.clear(): clear most references held by the frame");