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Re: Nasty double-replace copy-on-update bug

From: David Glasser <glasser_at_davidglasser.net>
Date: 2007-10-19 16:48:04 CEST

On 10/19/07, Philip Martin <philip@codematters.co.uk> wrote:
> "David Glasser" <glasser@davidglasser.net> writes:
>
> > On 10/19/07, Philip Martin <philip@codematters.co.uk> wrote:
> >> "Ben Collins-Sussman" <sussman@red-bean.com> writes:
> >>
> >> > The question is: why the wc-searching routine
> >> > (update_editor.c:locate_copyfrom()) think that it's found the correct
> >> > version of the file, when it's not? Answer: it seems to be getting
> >> > stale svn_wc_entry_t information. It's reading a couple of entries
> >> > files from disk, rather than from a more recent adm_access set still
> >> > held in memory, and getting an incorrect (stale) last-changed-rev
> >> > value. From poking around in gdb, it seems that the entry_t's which
> >> > are write-cached in memory are correct -- they have the proper
> >> > last-changed-rev values, but simply aren't flushed to disk yet.
> >> >
> >> > I attempted to solve this by passing in the edit_baton->access_set to
> >> > the searching routine, but it's not fixing the problem. I've attached
> >> > my patch-in-progress below. Can we get some more eyes on this
> >> > problem?
> >>
> >> I think there are two bugs here. I believe it is wrong for
> >> locate_copyfrom to use a read-only snapshot of the working copy, your
> >> patch to pass an access baton looks like the right thing to do,
> >> although I think that the _try calls should take write locks. However
> >> that's not what is causing the current failure.
> >
> > Why is that? It sounds like a read-only operation...
>
> Without a write lock some other Subversion process could change or
> delete the file in the interval between the decision to use a file and
> the moment it gets used.

Huh. When is it ever safe to not take the write lock, then?

--dave

-- 
David Glasser | glasser_at_davidglasser.net | http://www.davidglasser.net/
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Received on Fri Oct 19 16:48:18 2007

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