From 9881fc5483d6298dc969919dddecf2b72ddf8d43 Mon Sep 17 00:00:00 2001
From: Mark Shannon <mark@hotpy.org>
Date: Mon, 17 Mar 2025 17:22:35 +0000
Subject: GH-125174: Make immortality "sticky" (GH-131355)

* Use a higher threshold for immortality for INCREF than DECREF
---
 Include/refcount.h      | 11 +++++++++--
 Lib/test/test_bigmem.py | 24 +++++++++++++++++++++++-
 2 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/Include/refcount.h b/Include/refcount.h
index 3efb7e5..417d91b 100644
--- a/Include/refcount.h
+++ b/Include/refcount.h
@@ -41,6 +41,10 @@ having all the lower 32 bits set, which will avoid the reference count to go
 beyond the refcount limit. Immortality checks for reference count decreases will
 be done by checking the bit sign flag in the lower 32 bits.
 
+To ensure that once an object becomes immortal, it remains immortal, the threshold
+for omitting increfs is much higher than for omitting decrefs. Consequently, once
+the refcount for an object exceeds _Py_IMMORTAL_MINIMUM_REFCNT it will gradually
+increase over time until it reaches _Py_IMMORTAL_INITIAL_REFCNT.
 */
 #define _Py_IMMORTAL_INITIAL_REFCNT (3ULL << 30)
 #define _Py_IMMORTAL_MINIMUM_REFCNT (1ULL << 31)
@@ -288,7 +292,7 @@ static inline Py_ALWAYS_INLINE void Py_INCREF(PyObject *op)
     }
 #elif SIZEOF_VOID_P > 4
     PY_UINT32_T cur_refcnt = op->ob_refcnt;
-    if (((int32_t)cur_refcnt) < 0) {
+    if (cur_refcnt >= _Py_IMMORTAL_INITIAL_REFCNT) {
         // the object is immortal
         _Py_INCREF_IMMORTAL_STAT_INC();
         return;
@@ -303,7 +307,10 @@ static inline Py_ALWAYS_INLINE void Py_INCREF(PyObject *op)
 #endif
     _Py_INCREF_STAT_INC();
 #ifdef Py_REF_DEBUG
-    _Py_INCREF_IncRefTotal();
+    // Don't count the incref if the object is immortal.
+    if (!_Py_IsImmortal(op)) {
+        _Py_INCREF_IncRefTotal();
+    }
 #endif
 #endif
 }
diff --git a/Lib/test/test_bigmem.py b/Lib/test/test_bigmem.py
index c9ab1c1..6d0879a 100644
--- a/Lib/test/test_bigmem.py
+++ b/Lib/test/test_bigmem.py
@@ -9,7 +9,8 @@ high memory limit to regrtest, with the -M option.
 """
 
 from test import support
-from test.support import bigmemtest, _1G, _2G, _4G
+from test.support import bigmemtest, _1G, _2G, _4G, import_helper
+_testcapi = import_helper.import_module('_testcapi')
 
 import unittest
 import operator
@@ -1257,6 +1258,27 @@ class DictTest(unittest.TestCase):
         d[size] = 1
 
 
+class ImmortalityTest(unittest.TestCase):
+
+    @bigmemtest(size=_2G, memuse=pointer_size * 9/8)
+    def test_stickiness(self, size):
+        """Check that immortality is "sticky", so that
+           once an object is immortal it remains so."""
+        if size < _2G:
+            # Not enough memory to cause immortality on overflow
+            return
+        o1 = o2 = o3 = o4 = o5 = o6 = o7 = o8 = object()
+        l = [o1] * (size-20)
+        self.assertFalse(_testcapi.is_immortal(o1))
+        for _ in range(30):
+            l.append(l[0])
+        self.assertTrue(_testcapi.is_immortal(o1))
+        del o2, o3, o4, o5, o6, o7, o8
+        self.assertTrue(_testcapi.is_immortal(o1))
+        del l
+        self.assertTrue(_testcapi.is_immortal(o1))
+
+
 if __name__ == '__main__':
     if len(sys.argv) > 1:
         support.set_memlimit(sys.argv[1])
-- 
cgit v0.12