Fixed #20943 -- Weakly reference senders when caching their associated receivers

Backport of e55ca60903 from master.

3 files changed, 30 insertions, 5 deletions

diff --git a/django/db/models/signals.py b/django/db/models/signals.py
index 3e321893c1..07824421d8 100644
--- a/django/db/models/signals.py
+++ b/django/db/models/signals.py
@@ -13,6 +13,6 @@ pre_delete = Signal(providing_args=["instance", "using"], use_caching=True)
 post_delete = Signal(providing_args=["instance", "using"], use_caching=True)
 
 pre_syncdb = Signal(providing_args=["app", "create_models", "verbosity", "interactive", "db"])
-post_syncdb = Signal(providing_args=["class", "app", "created_models", "verbosity", "interactive", "db"], use_caching=True)
+post_syncdb = Signal(providing_args=["class", "app", "created_models", "verbosity", "interactive", "db"])
 
 m2m_changed = Signal(providing_args=["action", "instance", "reverse", "model", "pk_set", "using"], use_caching=True)
diff --git a/django/dispatch/dispatcher.py b/django/dispatch/dispatcher.py
index 65c5c408ff..a8cdc93b21 100644
--- a/django/dispatch/dispatcher.py
+++ b/django/dispatch/dispatcher.py
@@ -4,8 +4,10 @@ import threading
 from django.dispatch import saferef
 from django.utils.six.moves import xrange
 
+
 WEAKREF_TYPES = (weakref.ReferenceType, saferef.BoundMethodWeakref)
 
+
 def _make_id(target):
     if hasattr(target, '__func__'):
         return (id(target.__self__), id(target.__func__))
@@ -15,6 +17,7 @@ NONE_ID = _make_id(None)
 # A marker for caching
 NO_RECEIVERS = object()
 
+
 class Signal(object):
     """
     Base class for all signals
@@ -42,7 +45,7 @@ class Signal(object):
         # distinct sender we cache the receivers that sender has in
         # 'sender_receivers_cache'. The cache is cleaned when .connect() or
         # .disconnect() is called and populated on send().
-        self.sender_receivers_cache = {}
+        self.sender_receivers_cache = weakref.WeakKeyDictionary() if use_caching else {}
 
     def connect(self, receiver, sender=None, weak=True, dispatch_uid=None):
         """
@@ -116,7 +119,7 @@ class Signal(object):
                     break
             else:
                 self.receivers.append((lookup_key, receiver))
-            self.sender_receivers_cache = {}
+            self.sender_receivers_cache.clear()
 
     def disconnect(self, receiver=None, sender=None, weak=True, dispatch_uid=None):
         """
@@ -151,7 +154,7 @@ class Signal(object):
                 if r_key == lookup_key:
                     del self.receivers[index]
                     break
-            self.sender_receivers_cache = {}
+            self.sender_receivers_cache.clear()
 
     def has_listeners(self, sender=None):
         return bool(self._live_receivers(sender))
@@ -276,7 +279,8 @@ class Signal(object):
                 for idx, (r_key, _) in enumerate(reversed(self.receivers)):
                     if r_key == key:
                         del self.receivers[last_idx - idx]
-            self.sender_receivers_cache = {}
+            self.sender_receivers_cache.clear()
+
 
 def receiver(signal, **kwargs):
     """
diff --git a/tests/dispatch/tests/test_dispatcher.py b/tests/dispatch/tests/test_dispatcher.py
index a1d4c7e176..2586aeee62 100644
--- a/tests/dispatch/tests/test_dispatcher.py
+++ b/tests/dispatch/tests/test_dispatcher.py
@@ -1,6 +1,7 @@
 import gc
 import sys
 import time
+import weakref
 
 from django.dispatch import Signal, receiver
 from django.utils import unittest
@@ -35,6 +36,8 @@ class Callable(object):
 a_signal = Signal(providing_args=["val"])
 b_signal = Signal(providing_args=["val"])
 c_signal = Signal(providing_args=["val"])
+d_signal = Signal(providing_args=["val"], use_caching=True)
+
 
 class DispatcherTests(unittest.TestCase):
     """Test suite for dispatcher (barely started)"""
@@ -72,6 +75,24 @@ class DispatcherTests(unittest.TestCase):
         self.assertEqual(result, expected)
         self._testIsClean(a_signal)
 
+    def testCachedGarbagedCollected(self):
+        """
+        Make sure signal caching sender receivers don't prevent garbage
+        collection of senders.
+        """
+        class sender:
+            pass
+        wref = weakref.ref(sender)
+        d_signal.connect(receiver_1_arg)
+        d_signal.send(sender, val='garbage')
+        del sender
+        garbage_collect()
+        try:
+            self.assertIsNone(wref())
+        finally:
+            # Disconnect after reference check since it flushes the tested cache.
+            d_signal.disconnect(receiver_1_arg)
+
     def testMultipleRegistration(self):
         a = Callable()
         a_signal.connect(a)