Opened 11 years ago

Closed 10 years ago

#21085 closed Bug (invalid)

Race condition in BoundMethodWeakref used by signal mechanism.

Reported by: Graham Dumpleton Owned by: nobody
Component: Core (Other) Version: 1.5
Severity: Normal Keywords:
Cc: Triage Stage: Accepted
Has patch: no Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no

Description (last modified by Anssi Kääriäinen)

In the __new__ method of BoundMethodWeakref at:

https://github.com/django/django/blob/master/django/dispatch/saferef.py#L73

there is a potential threading race condition.

The code is:

    def __new__( cls, target, onDelete=None, *arguments,**named ):
        """Create new instance or return current instance

        Basically this method of construction allows us to
        short-circuit creation of references to already-
        referenced instance methods.  The key corresponding
        to the target is calculated, and if there is already
        an existing reference, that is returned, with its
        deletionMethods attribute updated.  Otherwise the
        new instance is created and registered in the table
        of already-referenced methods.
        """
        key = cls.calculateKey(target)
        current =cls._allInstances.get(key)
        if current is not None:
            current.deletionMethods.append( onDelete)
            return current
        else:
            base = super( BoundMethodWeakref, cls).__new__( cls )
            cls._allInstances[key] = base
            base.__init__( target, onDelete, *arguments,**named)
            return base

The problem is that multiple threads could at the same time invoke cls._allInstances.get(key) and get None.

They will both then create instances of BoundMethodWeakref but only one will win as far as updating cls._allInstances.

Because all the onDelete callbacks are stored in the BoundMethodWeakref instance which is added to the cache. the thread who lost as far as getting their instance in the cache, will have its onDelete lost.

Code should in the else clause instead do something like:

            base = super( BoundMethodWeakref, cls).__new__( cls )
            base.__init__( target, onDelete, *arguments,**named)

            instance = cls._allInstances.setdefault(key, base)

            if instance is not base:
                instance.deletionMethods.append(onDelete)

            return instance

Change History (4)

comment:1 by Anssi Kääriäinen, 11 years ago

Component: UncategorizedCore (Other)
Description: modified (diff)
Triage Stage: UnreviewedAccepted
Type: UncategorizedBug

comment:2 by Anssi Kääriäinen, 11 years ago

See also #19511.

comment:3 by Vajrasky Kok, 10 years ago

Is this bug related with this issue? With python 3.4:

$ python runtests.py null_fk
Creating test database for alias 'default'...
Creating test database for alias 'other'...
..
----------------------------------------------------------------------
Ran 2 tests in 0.063s

OK
Destroying test database for alias 'default'...
Destroying test database for alias 'other'...
Exception ignored in: <bound method Signal._remove_receiver of <django.dispatch.dispatcher.Signal object at 0x7f662b75b330>>
Traceback (most recent call last):
  File "/home/sky/Code/python/django/django/dispatch/dispatcher.py", line 276, in _remove_receiver
TypeError: 'NoneType' object is not callable
Exception ignored in: <bound method Signal._remove_receiver of <django.dispatch.dispatcher.Signal object at 0x7f662b1be6d8>>
Traceback (most recent call last):
  File "/home/sky/Code/python/django/django/dispatch/dispatcher.py", line 276, in _remove_receiver
TypeError: 'NoneType' object is not callable
Exception ignored in: <bound method Signal._remove_receiver of <django.dispatch.dispatcher.Signal object at 0x7f662b1be6d8>>
Traceback (most recent call last):
...omitted...

comment:4 by Marc Tamlyn, 10 years ago

Resolution: invalid
Status: newclosed

Our implementation of weakrefs has now gone in favour of (backported) code from python 3.4. This should hopefully no longer be an issue.

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