Signal receivers cache should weakly reference senders.

[Simon Charette]

The signal receivers caching introduced in 704ee33f503c96b96c2682b946a11b3b42318ba7 (#16679) to speed up model initialization keeps strong reference to sender thus preventing them from being garbage collected.

In [1]: import gc

In [2]: import weakref

In [3]: from django.dispatch import Signal

In [4]: signal = Signal(use_caching=True)

In [5]: def receiver(**kwargs): pass

In [6]: signal.connect(receiver)

In [7]: class cls: pass

In [8]: wref = weakref.ref(cls)

In [9]: signal.send(cls)
Out[9]: [(<function __main__.receiver>, None)]

In [10]: del cls

In [11]: gc.collect()
Out[11]: 33

In [12]: wref() is None
Out[12]: False

Since the cache is only enabled on signals with BaseModel ((pre|post)_init, (pre|post)_save, (pre|post)_delete and m2m_changed) or module (post_syncdb) instances senders this shouldn't be an issue in usual cases since those objects are not meant to be garbage collected during the life of the application.

However it might cause issues to people dynamically creating model classes and modules since those objects won't be garbage collected as of Django 1.6.

I suggest we use a WeakKeyDictionary to cache sender receivers in order to prevent regressions in 1.6 and avoid hard to traceback memory leaks.

Running djangobench on the soon to be attached patch revealed no significant slowdowns.

[Simon Charette]

I guess this shouldn't be marked as Release Blocker since dynamic model creation is not usual among Django users.

[Anssi Kääriäinen]

The patch looks good to me, and I can confirm that there isn't any performance regression in query_all test, which is dependant on model initialization signal speeds.

EDIT: just noticed, caching was removed from post_syncdb. Any reason for that?

[Simon Charette <charette.s@…>]

In e55ca60903adcfd525938335b1ad9dbb6fd96c3e:

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

[Simon Charette <charette.s@…>]

In f0bc2865ff9d85c952fa86ae19aee062a5e883cd:

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

Backport of e55ca60903 from master.

[Andrew Godwin <andrew@…>]

In 63378163f9c5f3f9f9b42a6f260f798aa7e4b1f6:

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