Fix re-entrancy problem while initialization interpreter frame

Summary:
Fixes S544229

There is some surprise re-entrancy below `_PyFrame_InitializeExternalFrame`. The allocation can presumably release the GIL or cause GC and therefore the function can be initialized before we try calling it a 2nd time. This causes us to only perform the initialization if the object is still a non-function.

D78124674 would presumably fix this as well in that we're not replacing the function object anymore, so we'd just update it. But w/ D78124674 someone else's reifier could say they don't want to update the function in which case this is the right fix.

Reviewed By: diansheng92

Differential Revision: D78930696

fbshipit-source-id: bbc37bee98744c1e04939baea84b3108d046b8cf

Author: DinoV

Python/frame.c

@@ -167,6 +167,10 @@ _PyFrame_InitializeExternalFrame(_PyInterpreterFrame *frame) {
     if (frame_addr == NULL) {
         return -1;
     }
+    if (PyFunction_Check(frame->f_funcobj)) {
+        // re-entrancy during the allocation caused the frame to be initialized
+        return 0;
+    }
     PyObject *tmp = PyObject_Vectorcall(frame->f_funcobj, &frame_addr, 1, NULL);
     Py_DECREF(frame_addr);
     if (tmp == NULL) {