Opened 11 years ago
Closed 11 years ago
#21085 closed Bug (invalid)
Race condition in BoundMethodWeakref used by signal mechanism.
Reported by: | Graham Dumpleton | Owned by: | nobody |
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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 )
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 , 11 years ago
Component: | Uncategorized → Core (Other) |
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Description: | modified (diff) |
Triage Stage: | Unreviewed → Accepted |
Type: | Uncategorized → Bug |
comment:2 by , 11 years ago
comment:3 by , 11 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 , 11 years ago
Resolution: | → invalid |
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Status: | new → closed |
Our implementation of weakrefs has now gone in favour of (backported) code from python 3.4. This should hopefully no longer be an issue.
See also #19511.