Fixed #22967 -- Made Model._do_update consistent

Made _do_update behave more strictly according to its docs,
including a corner case when specific concurent updates are
executed and select_on_save is set.

django/db/models/base.py

@@ -757,8 +757,14 @@ class Model(six.with_metaclass(ModelBase)):
             return update_fields is not None or filtered.exists()
         if self._meta.select_on_save and not forced_update:
             if filtered.exists():
-                filtered._update(values)
-                return True
+                # It may happen that the object is deleted from the DB right after
+                # this check, causing the subsequent UPDATE to return zero matching
+                # rows. The same result can occur in some rare cases when the
+                # database returns zero despite the UPDATE being executed
+                # successfully (a row is matched and updated). In order to
+                # distinguish these two cases, the object's existence in the
+                # database is again checked for if the UPDATE query returns 0.
+                return filtered._update(values) > 0 or filtered.exists()
             else:
                 return False
         return filtered._update(values) > 0

tests/basic/tests.py

@@ -701,7 +701,7 @@ class SelectOnSaveTests(TestCase):
         try:
             Article._base_manager.__class__ = FakeManager
             asos = ArticleSelectOnSave.objects.create(pub_date=datetime.now())
-            with self.assertNumQueries(2):
+            with self.assertNumQueries(3):
                 asos.save()
                 self.assertTrue(FakeQuerySet.called)
             # This is not wanted behavior, but this is how Django has always