gh-124296: Remove private dictionary version tag (PEP 699) (#124472)

Author: colesbury

Include/cpython/dictobject.h

@@ -14,16 +14,11 @@ typedef struct {
     /* Number of items in the dictionary */
     Py_ssize_t ma_used;
 
-    /* Dictionary version: globally unique, value change each time
-       the dictionary is modified */
-#ifdef Py_BUILD_CORE
-    /* Bits 0-7 are for dict watchers.
+    /* This is a private field for CPython's internal use.
+     * Bits 0-7 are for dict watchers.
      * Bits 8-11 are for the watched mutation counter (used by tier2 optimization)
-     * The remaining bits (12-63) are the actual version tag. */
-    uint64_t ma_version_tag;
-#else
-    Py_DEPRECATED(3.12) uint64_t ma_version_tag;
-#endif
+     * The remaining bits are not currently used. */
+    uint64_t _ma_watcher_tag;
 
     PyDictKeysObject *ma_keys;
 

Include/internal/pycore_dict.h

@@ -230,31 +230,6 @@ static inline PyDictUnicodeEntry* DK_UNICODE_ENTRIES(PyDictKeysObject *dk) {
 #define DICT_WATCHER_MASK ((1 << DICT_MAX_WATCHERS) - 1)
 #define DICT_WATCHER_AND_MODIFICATION_MASK ((1 << (DICT_MAX_WATCHERS + DICT_WATCHED_MUTATION_BITS)) - 1)
 
-#ifdef Py_GIL_DISABLED
-
-#define THREAD_LOCAL_DICT_VERSION_COUNT 256
-#define THREAD_LOCAL_DICT_VERSION_BATCH THREAD_LOCAL_DICT_VERSION_COUNT * DICT_VERSION_INCREMENT
-
-static inline uint64_t
-dict_next_version(PyInterpreterState *interp)
-{
-    PyThreadState *tstate = PyThreadState_GET();
-    uint64_t cur_progress = (tstate->dict_global_version &
-                            (THREAD_LOCAL_DICT_VERSION_BATCH - 1));
-    if (cur_progress == 0) {
-        uint64_t next = _Py_atomic_add_uint64(&interp->dict_state.global_version,
-                                              THREAD_LOCAL_DICT_VERSION_BATCH);
-        tstate->dict_global_version = next;
-    }
-    return tstate->dict_global_version += DICT_VERSION_INCREMENT;
-}
-
-#define DICT_NEXT_VERSION(INTERP) dict_next_version(INTERP)
-
-#else
-#define DICT_NEXT_VERSION(INTERP) \
-    ((INTERP)->dict_state.global_version += DICT_VERSION_INCREMENT)
-#endif
 
 PyAPI_FUNC(void)
 _PyDict_SendEvent(int watcher_bits,
@@ -263,20 +238,19 @@ _PyDict_SendEvent(int watcher_bits,
                   PyObject *key,
                   PyObject *value);
 
-static inline uint64_t
+static inline void
 _PyDict_NotifyEvent(PyInterpreterState *interp,
                     PyDict_WatchEvent event,
                     PyDictObject *mp,
                     PyObject *key,
                     PyObject *value)
 {
     assert(Py_REFCNT((PyObject*)mp) > 0);
-    int watcher_bits = mp->ma_version_tag & DICT_WATCHER_MASK;
+    int watcher_bits = mp->_ma_watcher_tag & DICT_WATCHER_MASK;
     if (watcher_bits) {
         RARE_EVENT_STAT_INC(watched_dict_modification);
         _PyDict_SendEvent(watcher_bits, event, mp, key, value);
     }
-    return DICT_NEXT_VERSION(interp) | (mp->ma_version_tag & DICT_WATCHER_AND_MODIFICATION_MASK);
 }
 
 extern PyDictObject *_PyObject_MaterializeManagedDict(PyObject *obj);

Include/internal/pycore_dict_state.h

@@ -12,10 +12,6 @@ extern "C" {
 #define DICT_WATCHED_MUTATION_BITS 4
 
 struct _Py_dict_state {
-    /*Global counter used to set ma_version_tag field of dictionary.
-     * It is incremented each time that a dictionary is created and each
-     * time that a dictionary is modified. */
-    uint64_t global_version;
     uint32_t next_keys_version;
     PyDict_WatchCallback watchers[DICT_MAX_WATCHERS];
 };

Lib/test/test_dict_version.py

@@ -142,41 +142,6 @@ def writer_func(l):
             for ref in thread_list:
                 self.assertIsNone(ref())
 
-    @unittest.skipIf(_testcapi is None, 'need _testcapi module')
-    def test_dict_version(self):
-        dict_version = _testcapi.dict_version
-        THREAD_COUNT = 10
-        DICT_COUNT = 10000
-        lists = []
-        writers = []
-
-        def writer_func(thread_list):
-            for i in range(DICT_COUNT):
-                thread_list.append(dict_version({}))
-
-        for x in range(THREAD_COUNT):
-            thread_list = []
-            lists.append(thread_list)
-            writer = Thread(target=partial(writer_func, thread_list))
-            writers.append(writer)
-
-        for writer in writers:
-            writer.start()
-
-        for writer in writers:
-            writer.join()
-
-        total_len = 0
-        values = set()
-        for thread_list in lists:
-            for v in thread_list:
-                if v in values:
-                    print('dup', v, (v/4096)%256)
-                values.add(v)
-            total_len += len(thread_list)
-        versions = set(dict_version for thread_list in lists for dict_version in thread_list)
-        self.assertEqual(len(versions), THREAD_COUNT*DICT_COUNT)
-
 
 if __name__ == "__main__":
     unittest.main()

Lib/test/test_free_threading/test_dict.py

@@ -0,0 +1,3 @@
+:c:type:`PyDictObject` no longer maintains a private version tag field
+``ma_version_tag`` per :pep:`699`.  This field was originally added in
+Python 3.6 (:pep:`509`) and deprecated in Python 3.12.

Misc/NEWS.d/next/C_API/2024-09-24-20-34-21.gh-issue-124296.S4QoS1.rst

@@ -181,19 +181,6 @@ dict_popstring_null(PyObject *self, PyObject *args)
     RETURN_INT(PyDict_PopString(dict, key,  NULL));
 }
 
-static PyObject *
-dict_version(PyObject *self, PyObject *dict)
-{
-    if (!PyDict_Check(dict)) {
-        PyErr_SetString(PyExc_TypeError, "expected dict");
-        return NULL;
-    }
-_Py_COMP_DIAG_PUSH
-_Py_COMP_DIAG_IGNORE_DEPR_DECLS
-    return PyLong_FromUnsignedLongLong(((PyDictObject *)dict)->ma_version_tag);
-_Py_COMP_DIAG_POP
-}
-
 static PyMethodDef test_methods[] = {
     {"dict_containsstring", dict_containsstring, METH_VARARGS},
     {"dict_getitemref", dict_getitemref, METH_VARARGS},
@@ -204,7 +191,6 @@ static PyMethodDef test_methods[] = {
     {"dict_pop_null", dict_pop_null, METH_VARARGS},
     {"dict_popstring", dict_popstring, METH_VARARGS},
     {"dict_popstring_null", dict_popstring_null, METH_VARARGS},
-    {"dict_version", dict_version, METH_O},
     {NULL},
 };
 

Modules/_testcapi/dict.c

@@ -1909,25 +1909,6 @@ getitem_with_error(PyObject *self, PyObject *args)
     return PyObject_GetItem(map, key);
 }
 
-static PyObject *
-dict_get_version(PyObject *self, PyObject *args)
-{
-    PyDictObject *dict;
-    uint64_t version;
-
-    if (!PyArg_ParseTuple(args, "O!", &PyDict_Type, &dict))
-        return NULL;
-
-    _Py_COMP_DIAG_PUSH
-    _Py_COMP_DIAG_IGNORE_DEPR_DECLS
-    version = dict->ma_version_tag;
-    _Py_COMP_DIAG_POP
-
-    static_assert(sizeof(unsigned long long) >= sizeof(version),
-                  "version is larger than unsigned long long");
-    return PyLong_FromUnsignedLongLong((unsigned long long)version);
-}
-
 
 static PyObject *
 raise_SIGINT_then_send_None(PyObject *self, PyObject *args)
@@ -3407,7 +3388,6 @@ static PyMethodDef TestMethods[] = {
     {"return_result_with_error", return_result_with_error, METH_NOARGS},
     {"getitem_with_error", getitem_with_error, METH_VARARGS},
     {"Py_CompileString",     pycompilestring, METH_O},
-    {"dict_get_version", dict_get_version, METH_VARARGS},
     {"raise_SIGINT_then_send_None", raise_SIGINT_then_send_None, METH_VARARGS},
     {"stack_pointer", stack_pointer, METH_NOARGS},
 #ifdef W_STOPCODE