[1.10.x] Fixed #26686 -- Fixed crash when registering model signals with abstract senders.

Backport of 08014fe75b from master

django/db/models/signals.py

@@ -3,6 +3,7 @@ from functools import partial
 
 from django.db.models.utils import make_model_tuple
 from django.dispatch import Signal
+from django.utils import six
 from django.utils.deprecation import RemovedInDjango20Warning
 
 
@@ -15,15 +16,15 @@ class ModelSignal(Signal):
     of the `app_label.ModelName` form.
     """
     def _lazy_method(self, method, apps, receiver, sender, **kwargs):
+        from django.db.models.options import Options
+
         # This partial takes a single optional argument named "sender".
         partial_method = partial(method, receiver, **kwargs)
-        # import models here to avoid a circular import
-        from django.db import models
-        if isinstance(sender, models.Model) or sender is None:
-            # Skip lazy_model_operation to get a return value for disconnect()
+        if isinstance(sender, six.string_types):
+            apps = apps or Options.default_apps
+            apps.lazy_model_operation(partial_method, make_model_tuple(sender))
+        else:
             return partial_method(sender)
-        apps = apps or models.base.Options.default_apps
-        apps.lazy_model_operation(partial_method, make_model_tuple(sender))
 
     def connect(self, receiver, sender=None, weak=True, dispatch_uid=None, apps=None):
         self._lazy_method(super(ModelSignal, self).connect, apps, receiver, sender, dispatch_uid=dispatch_uid)

tests/signals/tests.py

@@ -1,5 +1,6 @@
 from __future__ import unicode_literals
 
+from django.apps.registry import Apps
 from django.db import models
 from django.db.models import signals
 from django.dispatch import receiver
@@ -317,3 +318,14 @@ class LazyModelRefTest(BaseSignalTest):
 
         Created()
         self.assertEqual(received, [])
+
+    def test_register_model_class_senders_immediately(self):
+        """
+        Model signals registered with model classes as senders don't use the
+        Apps.lazy_model_operation() mechanism.
+        """
+        # Book isn't registered with apps2, so it will linger in
+        # apps2._pending_operations if ModelSignal does the wrong thing.
+        apps2 = Apps()
+        signals.post_init.connect(self.receiver, sender=Book, apps=apps2)
+        self.assertEqual(list(apps2._pending_operations), [])