Tim Peters wrote:
[Mike C. Fletcher]Well, as of now it seems to have eliminated the last leaks in TwistedSNMP, and that's likely going to eliminate the last of our leaks in our products, so yes, I suppose it was :) .
I'm looking at rewriting parts of Twisted and TwistedSNMP to eliminateA worthy goal!
__del__ methods (and the memory leaks they create).
A callback is strongly referenced from the weakref(s) containing it.
Thanks.
It would really help to give a minimal example of self-containedSorry about that. I was expecting a theoretical problem, not a practical one, so didn't think to provide a practical example.
executable code. I'm not sure how you're using this code, and
guessing takes too long.
Alex already scooped you on this "smack yourself on the head, Mikey". I was assuming the weakref object was a proxy for a reference in an internal structure; I was expecting a list of weak references that was nothing but a series of functions to call after finalising the object. My bad.The "2" there is important, just because this is an interactive session. The point of it was to stop `_` from holding a reference to the created weakref. In your code
weakref.ref( self, self.close )
the weakref itself becomes trash immediately after it's created, so
the weakref is thrown away entirely (including its strong reference to
self.close, and its weak reference to self) right after that line
ends. Of course callbacks aren't *supposed* to be called when the
weakref itself goes away, they're supposed to be called when the thing
being weakly referenced goes away. So this all looks vanilla and
inevitable to me -- the callback is never invoked because the weakref
itself goes away long before self goes away.
Yes, so it requires an external storage mechanism and code to clean that up... ick. __del__ looks cleaner and cleaner by comparison. You'd think I would have noticed the huge tables of weakref objects in PyDispatcher as I was working in there...I'm pretty sure it's just because there's nothing here to keep the weakref itself alive after it's created. You could try, e.g.,
self.wr = weakref.ref( self, self.close )
to keep it alive, but things get fuzzy then if self becomes part of
cyclic trash.
Sigh, guess I should stop listening to rumours just because they are saying something about that for which I yearn.I can work around it in this particular case by defining a __del__ on
the Closer, but that just fixes this particular instance (and leaves
just as many __del__'s hanging around). I'm wondering if there's a
ready recipe that can *always* replace a __del__'s operation?
In the presence of cycles it gets tricky; there's a lot of
more-or-less recent words (less than a year old <wink>) about that on
python-dev. It gets complicated quickly, and it seems nobody can make
more time to think about it.
Well, we darn well better solve it by then! Don't want memory leaks when we're hard-wired into someone's brain.I know I heard a rumour somewhere about Uncle Timmy wanting to eliminate
__del__ in 2.5 or thereabouts,
Python 3 at the earliest. That's the earliest everything nobody can
make time for lands <0.5 wink>.
Yes, this was the "work around" I'd implemented (well, with __call__ instead and del just calling it). Of course, that didn't actually eliminate any __del__ methods, but oh well, if we're not losing those until 3.x it doesn't really matter :) .There are unresolved issues about how to get all this stuff to work sanely. The driving principle behind cyclic-trash weakref endcases right now is "do anything defensible that won't segfault".
I'll note that one fairly obvious pattern works very well for weakrefs and __del__ methods (mutatis mutandis): don't put the __del__ method in self, put it in a dead-simple object hanging *off* of self. Like the simple:
class BTreeCloser: def __init__(self, btree): self.btree = btree
def __del__(self):
if self.btree:
self.btree.close()
self.btree = None
The fix for the Deferred in Twisted got a little hideous (there the __del__ is poking around into lots of different attributes of the Deferred. To hack that I created a descriptor class that forwards a variable to the held object and wrapped each of the needed variables in one of those... I'm going to have to find something a little less baroque if I'm going to get it into Twisted... really, it doesn't look like any of it's necessary, it's just logging an error if the Deferred ended on a Failure. Something should be done though, leaking memory from a core object in the framework is just a PITA.
When a weakref and its weak referent are both in cyclic trash, PythonSo external storage of a weakref is basically a requirement to make it useful. Hmm.
currently says "the order in which they die is undefined, so we'll
pretend the weakref dies first", and then, as at the start of this
msg, the callback is never invoked.
The problem this avoids is that
the callback may itself be part of cyclic trash too, in which case
it's possible for the callback to resurrect anything else in cyclic
trash, including the weakly-referenced object associated with the
callback. But by hypothesis in this case, that object is also in
cyclic trash, and horrible things can happen if a partially destructed
object gets resurrected.
Reasonable I suppose.
So the real rule right now is that, if you want a guarantee that aGuess I'll go back to the sub-object approach for all things and merely dream of a far off weakref utopia free of all __del__ methods.
callback on a weakly referenced object gets invoked, you have to write
your code to guarantee that the referent becomes trash before its
weakref becomes trash. (And note that if you do, the callback
necessarily will be alive too, because a weakref does in fact hold a
strong reference to its callback.)
Thanks, Tim, Mike
________________________________________________ Mike C. Fletcher Designer, VR Plumber, Coder http://www.vrplumber.com http://blog.vrplumber.com
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