GH-125174: Make immortality "sticky" (GH-131184)

diff --git a/Include/refcount.h b/Include/refcount.h
index ba14bc6965ce3ef049bad9c3b1eb5b83f7a2c199..b8ea5c6281408fd45ca0858b4946261de48d535b 100644 (file)
--- 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;

diff --git a/Lib/test/test_bigmem.py b/Lib/test/test_bigmem.py
index c9ab1c1de9e1865ac17ca55b510b36dc55f78565..6d0879ad65d53c61ddae5accea0491e046c135b7 100644 (file)
--- 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])