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Re: Mismatched backwards compat callbacks in libsvn_wc

From: C. Michael Pilato <cmpilato_at_collab.net>
Date: Fri, 26 Jun 2009 15:09:34 -0400

Роман Донченко wrote:
> Greetings,
> While investigating yet another Python testsuite failure, I came upon
> something which looks like a bug in Subversion itself.
> Consider the following scenario. A user creates a diff editor by using
> svn_wc_get_diff_editor4, and passes it a set of callbacks and a callback
> baton. We'll call those real_callbacks and real_baton.
> svn_wc_get_diff_editor4 is a compatibility function, so it encapsulates
> our callbacks and baton in an instance of callbacks2_wrapper_baton, and
> uses callbacks2_wrapper as the new set of callbacks. Then it passes this
> stuff to svn_wc_get_diff_editor5. But that's a compatibility function too,
> and the cycle repeats with callbacks3_wrapper[_baton]. Then the new
> parameters are passed to svn_wc_get_diff_editor6, which is the real
> function.
> Now the editor's real parameters are:
> callbacks = &callbacks3_wrapper (4)
> baton = callback3_wrapper_baton:
> callbacks = &callbacks2_wrapper (3)
> baton = callbacks2_wrapper_baton:
> callbacks = real_callbacks (2)
> real_baton
> Parenthesized numbers indicate the "generation" of the callback structure.
> So let's see what happens when we want to call a callback function,
> callbacks->dir_props_changed. callbacks3_wrapper.dir_props_changed =
> &dir_props_changed2, so dir_props_changed2 is called with a
> (callback3_wrapper_baton *) as the argument. But dir_props_changed2 is
> actually a (3) to (2) wrapper, and casts the baton to
> (callback2_wrapper_baton *)... From there, it just goes downhill (note
> that the structures are not compatible).
> It looks like callback3_wrapper->dir_props_changed isn't the only member
> affected in this way, but I'm yet to determine all the possibilities.
> Cheers,
> Roman.

Roman, I've been down the path you're on right now (briefly, some months
ago), and suspected exactly what you seem to have found. But I kept getting
so lost in wrappers and batons and casts and such that I never fully
convinced myself of the mismatch. If this stuff is fresh in your head and
you've got the time to investigate it, please PLEASE see this through to
completion. I'll try to help where I can and as time allows -- just let me
know what I can do.

C. Michael Pilato <cmpilato_at_collab.net>
CollabNet   <>   www.collab.net   <>   Distributed Development On Demand

Received on 2009-06-26 21:09:59 CEST

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