gh-91052: Add C API for watching dictionaries

Doc/c-api/dict.rst

@@ -238,3 +238,54 @@ Dictionary Objects
           for key, value in seq2:
               if override or key not in a:
                   a[key] = value
+
+.. c:function:: int PyDict_AddWatcher(PyDict_WatchCallback callback)
+
+   Register *callback* as a dictionary watcher. Return a non-negative integer
+   id which must be passed to future calls to :c:func:`PyDict_Watch`. In case
+   of error (e.g. no more watcher IDs available), return ``-1`` and set an
+   exception.
+
+.. c:function:: int PyDict_ClearWatcher(int watcher_id)
+
+   Clear watcher identified by *watcher_id* previously returned from
+   :c:func:`PyDict_AddWatcher`. Return ``0`` on success, ``-1`` on error (e.g.
+   if the given *watcher_id* was never registered.)
+
+.. c:function:: int PyDict_Watch(int watcher_id, PyObject *dict)
+
+   Mark dictionary *dict* as watched. The callback granted *watcher_id* by
+   :c:func:`PyDict_AddWatcher` will be called when *dict* is modified or
+   deallocated.
+
+.. c:type:: PyDict_WatchEvent
+
+   Enumeration of possible dictionary watcher events: ``PyDict_EVENT_ADDED``,
+   ``PyDict_EVENT_MODIFIED``, ``PyDict_EVENT_DELETED``, ``PyDict_EVENT_CLONED``,
+   ``PyDict_EVENT_CLEARED``, or ``PyDict_EVENT_DEALLOCATED``.
+
+.. c:type:: int (*PyDict_WatchCallback)(PyDict_WatchEvent event, PyObject *dict, PyObject *key, PyObject *new_value)
+
+   Type of a dict watcher callback function.
+
+   If *event* is ``PyDict_EVENT_CLEARED`` or ``PyDict_EVENT_DEALLOCATED``, both
+   *key* and *new_value* will be ``NULL``. If *event* is ``PyDict_EVENT_ADDED``
+   or ``PyDict_EVENT_MODIFIED``, *new_value* will be the new value for *key*.
+   If *event* is ``PyDict_EVENT_DELETED``, *key* is being deleted from the
+   dictionary and *new_value* will be ``NULL``.
+
+   ``PyDict_EVENT_CLONED`` occurs when *dict* was previously empty and another
+   dict is merged into it. To maintain efficiency of this operation, per-key
+   ``PyDict_EVENT_ADDED`` events are not issued in this case; instead a
+   single ``PyDict_EVENT_CLONED`` is issued, and *key* will be the source
+   dictionary.
+
+   The callback may inspect but must not modify *dict*; doing so could have
+   unpredictable effects, including infinite recursion.
+
+   Callbacks occur before the notified modification to *dict* takes place, so
+   the prior state of *dict* can be inspected.
+
+   If the callback returns with an exception set, it must return ``-1``; this
+   exception will be printed as an unraisable exception using
+   :c:func:`PyErr_WriteUnraisable`. Otherwise it should return ``0``.

Include/cpython/dictobject.h

@@ -83,3 +83,26 @@ typedef struct {
 
 PyAPI_FUNC(PyObject *) _PyDictView_New(PyObject *, PyTypeObject *);
 PyAPI_FUNC(PyObject *) _PyDictView_Intersect(PyObject* self, PyObject *other);
+
+/* Dictionary watchers */
+
+typedef enum {
+    PyDict_EVENT_ADDED,
+    PyDict_EVENT_MODIFIED,
+    PyDict_EVENT_DELETED,
+    PyDict_EVENT_CLONED,
+    PyDict_EVENT_CLEARED,
+    PyDict_EVENT_DEALLOCATED,
+} PyDict_WatchEvent;
+
+// Callback to be invoked when a watched dict is cleared, dealloced, or modified.
+// In clear/dealloc case, key and new_value will be NULL. Otherwise, new_value will be the
+// new value for key, NULL if key is being deleted.
+typedef int(*PyDict_WatchCallback)(PyDict_WatchEvent event, PyObject* dict, PyObject* key, PyObject* new_value);
+
+// Register/unregister a dict-watcher callback
+PyAPI_FUNC(int) PyDict_AddWatcher(PyDict_WatchCallback callback);
+PyAPI_FUNC(int) PyDict_ClearWatcher(int watcher_id);
+
+// Mark given dictionary as "watched" (callback will be called if it is modified)
+PyAPI_FUNC(int) PyDict_Watch(int watcher_id, PyObject* dict);

Include/internal/pycore_dict.h

@@ -154,7 +154,32 @@ struct _dictvalues {
 
 extern uint64_t _pydict_global_version;
 
-#define DICT_NEXT_VERSION() (++_pydict_global_version)
+#define DICT_MAX_WATCHERS 8
+#define DICT_VERSION_INCREMENT (1 << DICT_MAX_WATCHERS)
+#define DICT_VERSION_MASK (DICT_VERSION_INCREMENT - 1)
+
+#define DICT_NEXT_VERSION() (_pydict_global_version += DICT_VERSION_INCREMENT)
+
+void
+_PyDict_SendEvent(int watcher_bits,
+                  PyDict_WatchEvent event,
+                  PyDictObject *mp,
+                  PyObject *key,
+                  PyObject *value);
+
+static inline uint64_t
+_PyDict_NotifyEvent(PyDict_WatchEvent event,
+                    PyDictObject *mp,
+                    PyObject *key,
+                    PyObject *value)
+{
+    int watcher_bits = mp->ma_version_tag & DICT_VERSION_MASK;
+    if (watcher_bits) {
+        _PyDict_SendEvent(watcher_bits, event, mp, key, value);
+        return DICT_NEXT_VERSION() | watcher_bits;
+    }
+    return DICT_NEXT_VERSION();
+}
 
 extern PyObject *_PyObject_MakeDictFromInstanceAttributes(PyObject *obj, PyDictValues *values);
 extern PyObject *_PyDict_FromItems(

Include/internal/pycore_interp.h

@@ -144,6 +144,8 @@ struct _is {
     // Initialized to _PyEval_EvalFrameDefault().
     _PyFrameEvalFunction eval_frame;
 
+    PyDict_WatchCallback dict_watchers[DICT_MAX_WATCHERS];
+
     Py_ssize_t co_extra_user_count;
     freefunc co_extra_freefuncs[MAX_CO_EXTRA_USERS];
 

Lib/test/test_capi.py

@@ -2,6 +2,7 @@
 # these are all functions _testcapi exports whose name begins with 'test_'.
 
 from collections import OrderedDict
+from contextlib import contextmanager
 import _thread
 import importlib.machinery
 import importlib.util
@@ -1393,5 +1394,136 @@ def func2(x=None):
         self.do_test(func2)
 
 
+class TestDictWatchers(unittest.TestCase):
+    # types of watchers testcapimodule can add:
+    EVENTS = 0   # appends dict events as strings to global event list
+    ERROR = 1    # unconditionally sets and signals a RuntimeException
+    SECOND = 2   # always appends "second" to global event list
+
+    def add_watcher(self, kind=EVENTS):
+        return _testcapi.add_dict_watcher(kind)
+
+    def clear_watcher(self, watcher_id):
+        _testcapi.clear_dict_watcher(watcher_id)
+
+    @contextmanager
+    def watcher(self, kind=EVENTS):
+        wid = self.add_watcher(kind)
+        try:
+            yield wid
+        finally:
+            self.clear_watcher(wid)
+
+    def assert_events(self, expected):
+        actual = _testcapi.get_dict_watcher_events()
+        self.assertEqual(actual, expected)
+
+    def watch(self, wid, d):
+        _testcapi.watch_dict(wid, d)
+
+    def test_set_new_item(self):
+        d = {}
+        with self.watcher() as wid:
+            self.watch(wid, d)
+            d["foo"] = "bar"
+            self.assert_events(["new:foo:bar"])
+
+    def test_set_existing_item(self):
+        d = {"foo": "bar"}
+        with self.watcher() as wid:
+            self.watch(wid, d)
+            d["foo"] = "baz"
+            self.assert_events(["mod:foo:baz"])
+
+    def test_clone(self):
+        d = {}
+        d2 = {"foo": "bar"}
+        with self.watcher() as wid:
+            self.watch(wid, d)
+            d.update(d2)
+            self.assert_events(["clone"])
+
+    def test_no_event_if_not_watched(self):
+        d = {}
+        with self.watcher() as wid:
+            d["foo"] = "bar"
+            self.assert_events([])
+
+    def test_del(self):
+        d = {"foo": "bar"}
+        with self.watcher() as wid:
+            self.watch(wid, d)
+            del d["foo"]
+            self.assert_events(["del:foo"])
+
+    def test_pop(self):
+        d = {"foo": "bar"}
+        with self.watcher() as wid:
+            self.watch(wid, d)
+            d.pop("foo")
+            self.assert_events(["del:foo"])
+
+    def test_clear(self):
+        d = {"foo": "bar"}
+        with self.watcher() as wid:
+            self.watch(wid, d)
+            d.clear()
+            self.assert_events(["clear"])
+
+    def test_dealloc(self):
+        d = {"foo": "bar"}
+        with self.watcher() as wid:
+            self.watch(wid, d)
+            del d
+            self.assert_events(["dealloc"])
+
+    def test_error(self):
+        d = {}
+        unraisables = []
+        def unraisable_hook(unraisable):
+            unraisables.append(unraisable)
+        with self.watcher(kind=self.ERROR) as wid:
+            self.watch(wid, d)
+            orig_unraisable_hook = sys.unraisablehook
+            sys.unraisablehook = unraisable_hook
+            try:
+                d["foo"] = "bar"
+            finally:
+                sys.unraisablehook = orig_unraisable_hook
+            self.assert_events([])
+        self.assertEqual(len(unraisables), 1)
+        unraisable = unraisables[0]
+        self.assertIs(unraisable.object, d)
+        self.assertEqual(str(unraisable.exc_value), "boom!")
+
+    def test_two_watchers(self):
+        d1 = {}
+        d2 = {}
+        with self.watcher() as wid1:
+            with self.watcher(kind=self.SECOND) as wid2:
+                self.watch(wid1, d1)
+                self.watch(wid2, d2)
+                d1["foo"] = "bar"
+                d2["hmm"] = "baz"
+                self.assert_events(["new:foo:bar", "second"])
+
+    def test_watch_non_dict(self):
+        with self.watcher() as wid:
+            with self.assertRaisesRegex(ValueError, r"Cannot watch non-dictionary"):
+                self.watch(wid, 1)
+
+    def test_watch_out_of_range_watcher_id(self):
+        d = {}
+        with self.assertRaisesRegex(ValueError, r"Invalid dict watcher ID -1"):
+            self.watch(-1, d)
+        with self.assertRaisesRegex(ValueError, r"Invalid dict watcher ID 8"):
+            self.watch(8, d)  # DICT_MAX_WATCHERS = 8
+
+    def test_unassigned_watcher_id(self):
+        d = {}
+        with self.assertRaisesRegex(ValueError, r"No dict watcher set for ID 1"):
+            self.watch(1, d)
+
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core and Builtins/2022-10-03-16-12-39.gh-issue-91052.MsYL9d.rst

@@ -0,0 +1 @@
+Add API for subscribing to modification events on selected dictionaries.