Re: [PATCH/RFC] Fix refcount on some Python callbacks

On Mon, Jun 9, 2008 at 11:53 AM, Martin von Gagern
<Martin.vGagern_at_gmx.net> wrote:
> Jelmer Vernooij wrote:
>>
>> The attached patch fixes the reference counting in the Python part of %
>> callback_typemap. At the moment the reference counter of the Python
>> function is not increased when passing it as a baton. This means that if
>> Subversion is the only one still referencing the function it will be
>> garbage collected, causing Subversion to segfault when it tries to call
>> the Python callback.
>> However, this patch doesn't decrement the reference counter once the
>> callback is no longer used since there is no place to free batons as far
>> as I know. That means we'll potentially keep Python methods around in
>> memory that aren't used. Is that a problem or is there perhaps some way
>> to work around this?
>
> Hi!
>
> I'd like to warm up this old thread again, as I have a new suggestion.
> Generally you have some python function, pass it to some interface code that
> builds a wrapper around it to turn it into a c callback type, and wraps that
> internal c thing back into a python object. Correct so far?
>
> Now the only thing needed in order to get clean garbage collection would be
> to attach the python function to the generated python object on the python
> level. I've got a demo for this in one instance:
>
> provider = svn_auth_get_ssl_server_trust_prompt_provider(callback)
> provider.callback_reference = callback
>
> So the subversion bindings see provider as the swig wrapper and can use that
> to access the wrapped callback, just as before. Python, on the other hand,
> sees that the object provider holds a reference to the function callback, so
> it can't garbage collect the latter as long as the provider is live.
>
> This is a simple example and executed in one bzr-svn branch:
> https://code.launchpad.net/~gagern/bzr-svn/bug238529
> related to https://bugs.launchpad.net/bugs/238529
>
> I'm no proficient enough in using swig to formulate this yet, but I would
> assume that this kind of approach should work for all cases of functions
> that take one python object and return another python object that internally
> references the former but doesn't do so in python yet.
>
> So instead of addressing this in each and every application using the python
> bindings, it would be preferable to solve this on the subversion side. Do
> you think this can be done? Is someone willing to do so? Or if not, maybe
> willing to point me or others into the right direction?

This is definitely a real problem.

David James posted a patch to help with something similar (editors,
not callbacks):

http://svn.haxx.se/dev/archive-2008-05/0738.shtml

I didn't really feel qualified to know if it was the right solution
though, and ended up dropping it.

--dave

-- 
David Glasser | glasser@davidglasser.net | http://www.davidglasser.net/
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