don't call dict watcher callback with dead dict

Author: carljm

Include/internal/pycore_dict.h

@@ -164,6 +164,7 @@ _PyDict_NotifyEvent(PyDict_WatchEvent event,
                     PyObject *key,
                     PyObject *value)
 {
+    assert(Py_REFCNT((PyObject*)mp) > 0);
     int watcher_bits = mp->ma_version_tag & DICT_VERSION_MASK;
     if (watcher_bits) {
         _PyDict_SendEvent(watcher_bits, event, mp, key, value);

Lib/test/test_capi/test_watchers.py

@@ -113,6 +113,12 @@ def test_error(self):
                 self.assertEqual(str(cm.unraisable.exc_value), "boom!")
             self.assert_events([])
 
+    def test_dealloc_error(self):
+        d = {}
+        with self.watcher(kind=self.ERROR) as wid:
+            self.watch(wid, d)
+            del d
+
     def test_two_watchers(self):
         d1 = {}
         d2 = {}

Objects/dictobject.c

@@ -2308,7 +2308,10 @@ _PyDict_FromKeys(PyObject *cls, PyObject *iterable, PyObject *value)
 static void
 dict_dealloc(PyDictObject *mp)
 {
+    Py_INCREF(mp);
     _PyDict_NotifyEvent(PyDict_EVENT_DEALLOCATED, mp, NULL, NULL);
+    assert(Py_REFCNT(mp) == 1);
+    Py_SET_REFCNT(mp, 0);
     PyDictValues *values = mp->ma_values;
     PyDictKeysObject *keys = mp->ma_keys;
     Py_ssize_t i, n;