Deliver dict added event only after it's guaranteed to succeed

Summary:
Back port of: python/cpython#122207 fixing python/cpython#122208

The current dictionary watchers implementation delivers the added event before it checks to see if we need to re-size the dictionary. This resize can fail so the value isn't added, and then the tracker is out of sync with the true state of the dictionary.

This moves the delivery of the event to after any necessary allocations have happened.

Reviewed By: jbower-fb

Differential Revision: D60182094

fbshipit-source-id: f34940e98ce1caadeee364f9d126d35839661961

Objects/dictobject.c

@@ -1349,15 +1349,15 @@ insertdict(PyDictObject *mp, PyObject *key, Py_hash_t hash, PyObject *value)
     }
 
     if (ix == DKIX_EMPTY) {
-        uint64_t new_version = _PyDict_NotifyEvent(
-            PyDict_EVENT_ADDED, mp, key, value);
         /* Insert into new slot. */
         assert(old_value == NULL);
         if (mp->ma_keys->dk_usable <= 0) {
             /* Need to resize. */
             if (insertion_resize(mp) < 0)
                 goto Fail;
         }
+        uint64_t new_version = _PyDict_NotifyEvent(
+            PyDict_EVENT_ADDED, mp, key, value);
         if (!PyUnicode_CheckExact(key)) {
             if (mp->ma_keys->dk_lookup == lookdict_with_lazy_imports_unicode) {
                 mp->ma_keys->dk_lookup = lookdict_with_lazy_imports;
@@ -1428,13 +1428,13 @@ insert_to_emptydict(PyDictObject *mp, PyObject *key, Py_hash_t hash,
 {
     assert(mp->ma_keys == Py_EMPTY_KEYS);
 
-    uint64_t new_version = _PyDict_NotifyEvent(
-        PyDict_EVENT_ADDED, mp, key, value);
-
     PyDictKeysObject *newkeys = new_keys_object(PyDict_MINSIZE);
     if (newkeys == NULL) {
         return -1;
     }
+    uint64_t new_version = _PyDict_NotifyEvent(
+        PyDict_EVENT_ADDED, mp, key, value);
+
     dictkeys_decref(Py_EMPTY_KEYS);
     mp->ma_keys = newkeys;
     mp->ma_values = NULL;
@@ -3517,14 +3517,14 @@ PyDict_SetDefault(PyObject *d, PyObject *key, PyObject *defaultobj)
 
     if (ix == DKIX_EMPTY) {
         PyDictKeyEntry *ep, *ep0;
-        uint64_t new_version = _PyDict_NotifyEvent(
-            PyDict_EVENT_ADDED, mp, key, defaultobj);
         value = defaultobj;
         if (mp->ma_keys->dk_usable <= 0) {
             if (insertion_resize(mp) < 0) {
                 return NULL;
             }
         }
+        uint64_t new_version = _PyDict_NotifyEvent(
+            PyDict_EVENT_ADDED, mp, key, defaultobj);
         if (!PyUnicode_CheckExact(key) && mp->ma_keys->dk_lookup != lookdict) {
             mp->ma_keys->dk_lookup = lookdict;
         }