#35801: Signals are dispatched to receivers associated with dead senders
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Reporter: bobince | Type: Bug
Status: new | Component: Core (Other)
Version: 5.1 | Severity: Normal
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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django.dispatch.Signal.receivers is keyed on the id() of the receiver and
sender (modulo a thing to make bound methods consistent).
If a sender connected to a signal is destroyed, and a new object is
allocated with the same id() and then also connected as a sender to the
signal, when the signal fires it will match the original sender and call
the receiver that was connected for that sender (as well as the new one).
Signal works around the problem of re-used ids for the receiver by having
a weakref to the receiver (since it needs a reference anyway to be able to
call it), but it doesn't for sender.
In my case this resulted in post-migrate hooks for the wrong apps being
occasionally called in migration tests that mutated INSTALLED_APPS causing
AppConfig senders to be re-created, but it can be more simply provoked
with:
{{{
from django.dispatch import Signal
receivers_called = []
def create_receiver(i):
def receiver(**kwargs):
receivers_called.append(i)
return receiver
n = 100
receivers = [create_receiver(i) for i in range(n)]
signal = Signal()
for i in range(n):
sender = object()
signal.connect(receivers[i], sender=sender)
receivers_called = []
_ = signal.send(sender=sender)
# only the receiver for the new sender object should be called
assert receivers_called == [i], f'Expected [{i}], called
{receivers_called}'
}}}
(how readily this explodes may depend on local memory allocation
differences, but it dies pretty consistently for me on iteratio
n 3.)
Perhaps Signal should be keeping a weakref to each sender as well as
receiver, and detecting when it has gone None? Does this need to
participate in the _dead_receivers mechanism?
--
Ticket URL: <https://code.djangoproject.com/ticket/35801>
Django <https://code.djangoproject.com/>
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