gh-109496: Detect Py_DECREF() after dealloc in debug mode (#109539)

On a Python built in debug mode, Py_DECREF() now calls
_Py_NegativeRefcount() if the object is a dangling pointer to
deallocated memory: memory filled with 0xDD "dead byte" by the debug
hook on memory allocators. The fix is to check the reference count
*before* checking for _Py_IsImmortal().

Add test_decref_freed_object() to test_capi.test_misc.

1 files changed, 4 insertions, 6 deletions

diff --git a/Include/object.h b/Include/object.h
index 5bcb393..9058558 100644
--- a/Include/object.h
+++ b/Include/object.h
@@ -660,17 +660,15 @@ static inline void Py_DECREF(PyObject *op) {
 #elif defined(Py_REF_DEBUG)
 static inline void Py_DECREF(const char *filename, int lineno, PyObject *op)
 {
+    if (op->ob_refcnt <= 0) {
+        _Py_NegativeRefcount(filename, lineno, op);
+    }
     if (_Py_IsImmortal(op)) {
         return;
     }
     _Py_DECREF_STAT_INC();
     _Py_DECREF_DecRefTotal();
-    if (--op->ob_refcnt != 0) {
-        if (op->ob_refcnt < 0) {
-            _Py_NegativeRefcount(filename, lineno, op);
-        }
-    }
-    else {
+    if (--op->ob_refcnt == 0) {
         _Py_Dealloc(op);
     }
 }