On Fri, Oct 21, 2016 at 8:32 PM, Nick Coghlan email@example.com wrote:
On 21 October 2016 at 17:09, Nathaniel Smith firstname.lastname@example.org wrote:
But that was 2.4. In the mean time, of course, PEP 442 fixed it so that finalizers and weakrefs mix just fine. In fact, weakref callbacks are now run *before* __del__ methods , so clearly it's now okay for arbitrary code to touch the objects during that phase of the GC -- at least in principle.
So what I'm wondering is, would anything terrible happen if we started passing still-live weakrefs into weakref callbacks, and then clearing them afterwards?
The weakref-before-__del__ ordering change in https://www.python.org/dev/peps/pep-0442/#disposal-of-cyclic-isolates only applies to cyclic garbage collection,so for normal refcount driven object cleanup in CPython, the __del__ still happens first:
>>> class C: ... def __del__(self): ... print("__del__ called") ... >>> c = C() >>> import weakref >>> def cb(): ... print("weakref callback called") ... >>> weakref.finalize(c, cb) <finalize object at 0x7f4300b710a0; for 'C' at 0x7f42f8ae3470> >>> del c __del__ called weakref callback called
Ah, interesting! And in the old days this was of course the right way to do it, because until __del__ has completed it's possible that the object will get resurrected, and you don't want to clear the weakref until you're certain that it's dead.
But PEP 442 already broke all that :-). Now weakref callbacks can happen before __del__, and they can happen on objects that are about to be resurrected. So if we wanted to pursue this then it seems like it would make sense to standardize on the following sequence for object teardown:
0) object becomes collectible (either refcount == 0 or it's part of a cyclic isolate) 1) weakref callbacks fire 2) weakrefs are cleared (unconditionally, so we keep the rule that any given weakref fires at most once, even if the object is resurrected) 3) if _PyGC_REFS_MASK_FINALIZED isn't set, __del__ fires, and then _PyGC_REFS_MASK_FINALIZED is set 4) check for resurrection 5) deallocate the object
On further thought, this does still introduce one new edge case, which is that even if we keep the guarantee that no individual weakref can fire more than once, it's possible for *new* weakrefs to be registered after resurrection, so it becomes possible for an object to be resurrected multiple times. (Currently, resurrection can only happen once, because __del__ is disabled on resurrected objects and weakrefs can't resurrect at all.) I'm not actually sure that this is even a problem, but in any case it's easy to fix by making a rule that you can't take a weakref to an object whose _PyGC_REFS_MASK_FINALIZED flag is already set, plus adjust the teardown sequence to be:
0) object becomes collectible (either refcount == 0 or it's part of a cyclic isolate) 1) if _PyGC_REFS_MASK_FINALIZED is set, then go to step 7. Otherwise: 2) set _PyGC_REFS_MASK_FINALIZED 3) weakref callbacks fire 4) weakrefs are cleared (unconditionally) 5) __del__ fires 6) check for resurrection 7) deallocate the object
There remains one obscure corner case where multiple resurrection is possible, because the resurrection-prevention flag doesn't exist on non-GC objects, so you'd still be able to take new weakrefs to those. But in that case __del__ can already do multiple resurrections, and some fellow named Nick Coghlan seemed to think that was okay back in 2013 , so probably it's not too bad ;-).
This means the main problem with a strong reference being reachable from the weakref callback object remains: if the callback itself is reachable, then the original object is reachable, and you don't have a collectible cycle anymore.
>>> c = C() >>> def cb2(obj): ... print("weakref callback called with object reference") ... >>> weakref.finalize(c, cb2, c) <finalize object at 0x7f4300b710b0; for 'C' at 0x7f42f8ae3470> >>> del c >>>
Changing that to support resurrecting the object so it can be passed into the callback without the callback itself holding a strong reference means losing the main "reasoning about software" benefit that weakref callbacks offer: they currently can't resurrect the object they relate to (since they never receive a strong reference to it), so it nominally doesn't matter if the interpreter calls them before or after that object has been entirely cleaned up.
I guess I'm missing the importance of this -- does the interpreter gain some particular benefit from having flexibility about when to fire weakref callbacks? Obviously it has to pick one in practice.
(The async use case that got me thinking about this is, of course, exactly one where we would want a weakref callback to resurrect the object it refers to. Only once, though.)