gh-161226: Copy the tuple in _PyTuple_Resize() if the hash is cached

* Add tuple_resize_inplace() helper function.
* Add compute_hash optional parameter to _testcapi._tuple_resize().

Author: vstinner

Doc/c-api/tuple.rst

@@ -144,6 +144,11 @@ Tuple Objects
    ``*p`` is destroyed.  On failure, returns ``-1`` and sets ``*p`` to ``NULL``, and
    raises :exc:`MemoryError` or :exc:`SystemError`.
 
+   While tuple objects are usually immutable in Python, this special C API
+   allows mutating a tuple object in-place. The returned tuple object can still
+   be mutated using :c:func:`PyTuple_SetItem`; except if *newsize* is zero in
+   which case it returns the immutable empty tuple.
+
 
 .. _struct-sequence-objects:
 

Lib/test/test_capi/test_tuple.py

@@ -1,6 +1,6 @@
-import unittest
 import gc
-from sys import getrefcount
+import sys
+import unittest
 from test.support import import_helper
 
 _testcapi = import_helper.import_module('_testcapi')
@@ -131,9 +131,9 @@ def check_tuple_from_pair(self, from_pair):
         class Temp:
             pass
         temp = Temp()
-        temp_rc = getrefcount(temp)
+        temp_rc = sys.getrefcount(temp)
         self.assertEqual(from_pair(temp, temp), (temp, temp))
-        self.assertEqual(getrefcount(temp), temp_rc)
+        self.assertEqual(sys.getrefcount(temp), temp_rc)
 
         self._not_tracked(from_pair(1, 2))
         self._not_tracked(from_pair(None, None))
@@ -299,29 +299,64 @@ def test_tuple_set_item(self):
 
     def test__tuple_resize(self):
         # Test _PyTuple_Resize()
-        resize = _testcapi._tuple_resize
+        _resize = _testcapi._tuple_resize
         checknull = _testcapi._check_tuple_item_is_NULL
 
-        a = ()
-        b = resize(a, 0, False)
-        self.assertEqual(len(a), 0)
-        self.assertEqual(len(b), 0)
-        b = resize(a, 2, False)
-        self.assertEqual(len(a), 0)
-        self.assertEqual(len(b), 2)
-        self.assertTrue(checknull(b, 0))
-        self.assertTrue(checknull(b, 1))
-
-        a = ([1], [2], [3])
-        b = resize(a, 3)
-        self.assertEqual(b, a)
-        b = resize(a, 2)
-        self.assertEqual(b, a[:2])
-        b = resize(a, 5)
-        self.assertEqual(len(b), 5)
-        self.assertEqual(b[:3], a)
-        self.assertTrue(checknull(b, 3))
-        self.assertTrue(checknull(b, 4))
+        def assert_is_fresh_copy(result, refcnt, is_new_obj):
+            self.assertEqual(refcnt, 1)
+            self.assertTrue(is_new_obj)
+            self.assertFalse(sys._is_immortal(result))
+
+        def resize(tup, new_size, new=True, compute_hash=False):
+            if isinstance(tup, tuple):
+                old_size = len(tup)
+            else:
+                old_size = 0
+            result, refcnt, is_new_obj = _resize(tup, new_size, new,
+                                                 compute_hash=compute_hash)
+            if new_size == old_size:
+                # Return the same object unchanged
+                self.assertFalse(is_new_obj)
+            elif old_size == 0:
+                assert_is_fresh_copy(result, refcnt, is_new_obj)
+            elif new_size == 0:
+                # check that the result is the empty bytes string singleton
+                self.assertEqual(result, ())
+                self.assertTrue(sys._is_immortal(result))
+                self.assertTrue(is_new_obj)
+            elif (not new) or compute_hash:
+                # Make sure that the result is a fresh object.
+                assert_is_fresh_copy(result, refcnt, is_new_obj)
+            else:
+                # An in-place resize can return the same memory address, or
+                # not. 'is_new_obj' cannot be tested.
+                self.assertEqual(refcnt, 1)
+                self.assertFalse(sys._is_immortal(result))
+
+            return result
+
+        for compute_hash in True, False:
+            with self.subTest(compute_hash=compute_hash):
+                a = ()
+                b = resize(a, 0, False, compute_hash)
+                self.assertEqual(len(a), 0)
+                self.assertEqual(len(b), 0)
+                b = resize(a, 2, False, compute_hash)
+                self.assertEqual(len(a), 0)
+                self.assertEqual(len(b), 2)
+                self.assertTrue(checknull(b, 0))
+                self.assertTrue(checknull(b, 1))
+
+                a = tuple(object() for _ in range(3))
+                b = resize(a, 3, True, compute_hash)
+                self.assertEqual(b, a)
+                b = resize(a, 2, True, compute_hash)
+                self.assertEqual(b, a[:2])
+                b = resize(a, 5, True, compute_hash)
+                self.assertEqual(len(b), 5)
+                self.assertEqual(b[:3], a)
+                self.assertTrue(checknull(b, 3))
+                self.assertTrue(checknull(b, 4))
 
         a = ()
         self.assertRaises(MemoryError, resize, a, PY_SSIZE_T_MAX)

Misc/NEWS.d/next/C_API/2026-11-03-01-24-01.gh-issue-161226.MPrJKp.rst

@@ -0,0 +1,2 @@
+:c:func:`_PyTuple_Resize` now returns a copy if the hash value was already
+computed. Patch by Victor Stinner.

Modules/_testcapi/tuple.c

@@ -66,12 +66,17 @@ tuple_set_item(PyObject *Py_UNUSED(module), PyObject *args)
 }
 
 static PyObject *
-_tuple_resize(PyObject *Py_UNUSED(module), PyObject *args)
+_tuple_resize(PyObject *Py_UNUSED(module), PyObject *args, PyObject *kwargs)
 {
+    static char *kwlist[] = {"obj", "newsize", "new", "compute_hash", NULL};
     PyObject *tup;
     Py_ssize_t newsize;
     int new = 1;
-    if (!PyArg_ParseTuple(args, "On|p", &tup, &newsize, &new)) {
+    int compute_hash = 0;
+
+    if (!PyArg_ParseTupleAndKeywords(args, kwargs,
+                                     "On|pi", kwlist,
+                                     &tup, &newsize, &new, &compute_hash)) {
         return NULL;
     }
     if (new) {
@@ -84,12 +89,24 @@ _tuple_resize(PyObject *Py_UNUSED(module), PyObject *args)
         NULLABLE(tup);
         Py_XINCREF(tup);
     }
+
+    if (compute_hash) {
+        if (PyObject_Hash(tup) == -1) {
+            Py_DECREF(tup);
+            return NULL;
+        }
+    }
+
+    PyObject *old_tup = tup;
     int r = _PyTuple_Resize(&tup, newsize);
     if (r == -1) {
         assert(tup == NULL);
         return NULL;
     }
-    return tup;
+    assert(tup != NULL);
+
+    Py_ssize_t refcnt = Py_REFCNT(tup);
+    return Py_BuildValue("Nnp", tup, refcnt, tup != old_tup);
 }
 
 static PyObject *
@@ -135,7 +152,7 @@ static PyMethodDef test_methods[] = {
     {"tuple_get_size", tuple_get_size, METH_O},
     {"tuple_get_item", tuple_get_item, METH_VARARGS},
     {"tuple_set_item", tuple_set_item, METH_VARARGS},
-    {"_tuple_resize", _tuple_resize, METH_VARARGS},
+    {"_tuple_resize", _PyCFunction_CAST(_tuple_resize), METH_VARARGS | METH_KEYWORDS},
     {"_check_tuple_item_is_NULL", _check_tuple_item_is_NULL, METH_VARARGS},
     {"tuple_fromarray", tuple_fromarray, METH_VARARGS},
     {NULL},

Objects/tupleobject.c

@@ -1015,49 +1015,12 @@ PyTypeObject PyTuple_Type = {
     ._tp_iteritem = tuple_iteritem,
 };
 
-/* The following function breaks the notion that tuples are immutable:
-   it changes the size of a tuple.  We get away with this only if there
-   is only one module referencing the object.  You can also think of it
-   as creating a new tuple object and destroying the old one, only more
-   efficiently.  In any case, don't use this if the tuple may already be
-   known to some other part of the code. */
 
-int
-_PyTuple_Resize(PyObject **pv, Py_ssize_t newsize)
+static int
+tuple_resize_inplace(PyObject **pv, Py_ssize_t newsize)
 {
-    PyTupleObject *v;
-    PyTupleObject *sv;
-    Py_ssize_t i;
-    Py_ssize_t oldsize;
+    PyTupleObject *v = (PyTupleObject *) *pv;
 
-    v = (PyTupleObject *) *pv;
-    if (v == NULL || !Py_IS_TYPE(v, &PyTuple_Type) || newsize < 0 ||
-        (Py_SIZE(v) != 0 && !_PyObject_IsUniquelyReferenced(*pv))) {
-        *pv = 0;
-        Py_XDECREF(v);
-        PyErr_BadInternalCall();
-        return -1;
-    }
-
-    oldsize = Py_SIZE(v);
-    if (oldsize == newsize) {
-        return 0;
-    }
-    if (newsize == 0) {
-        Py_DECREF(v);
-        *pv = tuple_get_empty();
-        return 0;
-    }
-    if (oldsize == 0) {
-#ifdef Py_DEBUG
-        assert(v == &_Py_SINGLETON(tuple_empty));
-#endif
-        /* The empty tuple is statically allocated so we never
-           resize it in-place. */
-        Py_DECREF(v);
-        *pv = PyTuple_New(newsize);
-        return *pv == NULL ? -1 : 0;
-    }
     /* Check for overflow */
     if ((size_t)newsize > ((size_t)PY_SSIZE_T_MAX - (sizeof(PyTupleObject) -
                 sizeof(PyObject *))) / sizeof(PyObject *)) {
@@ -1074,34 +1037,109 @@ _PyTuple_Resize(PyObject **pv, Py_ssize_t newsize)
     _Py_ForgetReference((PyObject *) v);
 #endif
     /* DECREF items deleted by shrinkage */
-    for (i = newsize; i < oldsize; i++) {
+    Py_ssize_t oldsize = Py_SIZE(v);
+    for (Py_ssize_t i = newsize; i < oldsize; i++) {
         Py_CLEAR(v->ob_item[i]);
     }
     _PyReftracerTrack((PyObject *)v, PyRefTracer_DESTROY);
-    sv = PyObject_GC_Resize(PyTupleObject, v, newsize);
+
+    PyTupleObject *sv = PyObject_GC_Resize(PyTupleObject, v, newsize);
     if (sv == NULL) {
         *pv = NULL;
 #ifdef Py_REF_DEBUG
         _Py_DecRefTotal(_PyThreadState_GET());
 #endif
         /* The failed reallocation leaves the original items owned by v. */
-        i = oldsize;
+        Py_ssize_t i = oldsize;
         while (--i >= 0) {
             Py_XDECREF(v->ob_item[i]);
         }
         PyObject_GC_Del(v);
         return -1;
     }
+
     _Py_NewReferenceNoTotal((PyObject *) sv);
     /* Zero out items added by growing */
-    if (newsize > oldsize)
+    if (newsize > oldsize) {
         memset(&sv->ob_item[oldsize], 0,
-               sizeof(*sv->ob_item) * (newsize - oldsize));
+               sizeof(sv->ob_item[0]) * (newsize - oldsize));
+    }
+
     *pv = (PyObject *) sv;
     _PyObject_GC_TRACK(sv);
     return 0;
 }
 
+
+/* The following function breaks the notion that tuples are immutable:
+   it changes the size of a tuple.  We get away with this only if there
+   is only one module referencing the object.  You can also think of it
+   as creating a new tuple object and destroying the old one, only more
+   efficiently.  In any case, don't use this if the tuple may already be
+   known to some other part of the code. */
+
+int
+_PyTuple_Resize(PyObject **pv, Py_ssize_t newsize)
+{
+    PyTupleObject *v = (PyTupleObject *) *pv;
+    if (v == NULL || !Py_IS_TYPE(v, &PyTuple_Type) || newsize < 0 ||
+        (Py_SIZE(v) != 0 && !_PyObject_IsUniquelyReferenced(*pv))) {
+        *pv = 0;
+        Py_XDECREF(v);
+        PyErr_BadInternalCall();
+        return -1;
+    }
+
+    Py_ssize_t oldsize = Py_SIZE(v);
+    if (oldsize == newsize) {
+        return 0;
+    }
+
+    if (newsize == 0) {
+        *pv = tuple_get_empty();
+        Py_DECREF(v);
+        return 0;
+    }
+
+    if (oldsize == 0) {
+#ifdef Py_DEBUG
+        assert(v == &_Py_SINGLETON(tuple_empty));
+#endif
+        /* The empty tuple is statically allocated so we never
+           resize it in-place. */
+        *pv = PyTuple_New(newsize);
+        Py_DECREF(v);
+        return *pv == NULL ? -1 : 0;
+    }
+
+    // gh-161226: Return a copy if the hash value was already computed
+    if (v->ob_hash != -1) {
+        PyTupleObject *new_tuple = tuple_alloc(newsize);
+        if (new_tuple == NULL) {
+            *pv = NULL;
+            Py_DECREF(v);
+            return -1;
+        }
+        // Copy items
+        Py_ssize_t copy = Py_MIN(oldsize, newsize);
+        for (Py_ssize_t i = 0; i < copy; i++) {
+            new_tuple->ob_item[i] = Py_NewRef(v->ob_item[i]);
+        }
+        if (newsize > oldsize) {
+            // Set new items to NULL
+            memset(&new_tuple->ob_item[oldsize], 0,
+                   (newsize - oldsize) * sizeof(new_tuple->ob_item[0]));
+        }
+        _PyObject_GC_TRACK(new_tuple);
+
+        *pv = _PyObject_CAST(new_tuple);
+        Py_DECREF(v);
+        return 0;
+    }
+
+    return tuple_resize_inplace(pv, newsize);
+}
+
 /*********************** Tuple Iterator **************************/
 
 #define _PyTupleIterObject_CAST(op) ((_PyTupleIterObject *)(op))