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Re: the last M4 (M5) bugs

From: Greg Stein <gstein_at_lyra.org>
Date: 2001-10-18 21:59:36 CEST

On Wed, Oct 17, 2001 at 03:19:20PM -0500, Ben Collins-Sussman wrote:
>...
> 1. to replace a file, the user must {delete, add, commit}. If the
> user instead attempts to {delete, commit, add, commit}, the second
> commit will fail.
>...
> mod_dav_svn should be trapping and ignoring the "item already
> missing" error that it's getting from the filesystem. Why?
> Because mod_dav_svn is *merging* on the fly, and in the world of
> merging, deletion is an idempotent operation. (svn_fs_merge() has
> the same rule: if you need to delete something, and it's already
> gone, it just moves on.) This is a pretty easy fix, and I'm
> almost done with it.

I don't understand how this can be the correct fix. What if a DELETE is
attempted and the file *truly* isn't there? I see no way to distinguish
between "deleting *again*" and "deleting *originally*"

We cannot simply toss out an error message if it accidentally captures the
second issue.

IMO, we just mark this as an inconvenience until the WC is fixed. If you
want to "delete, commit, add, commit", then the user must instead do
"delete, commit, *update*, add, commit". Just tell them to stick an extra
update in there for the odd chance that they committed between the delete
and add.

Basically: why the dubious act of ignoring an error, which was originally
caused by erroneous behavior on the client. And if we have a low-pain
workaround, then let's simply use that one.

> 2. ra_dav is unable to prevent users from committing propchanges on
> out-of-date dirs. Thus users could end up with working copies
> containing dirs whose revision numbers are incorrect.
>
> ANALYSIS
>
> Well, duh. When ra_dav sends PROPPATCH on a directory,
> mod_dav_svn is always applying it to the "latest" version of the
> directory. There's a good chance that the change will apply
> cleanly, and then ra_dav goes and bumps the directory revision --
> but the whole thing should have failed an out-of-dateness check to
> begin with.
>
> SOLUTION
>
> ra_dav can't know if the directory is out-of-date or not; only
> someone looking at the filesystem can see that. So the change
> needs to happen in mod_dav_svn.
>
> Specifically, the "working" revision of the directory needs to be
> sent in the PROPPATCH request body. mod_dav_svn can then look at
> the node-rev-id of the latest version of the directory, see what
> revision it was *created* in (it's part of the skel), and compare
> it to the "working" revision from the client. It's an easy
> out-of-dateness check that requires no undeltifying, and is a
> foreshadowing of the commit-system of the future. :-)

No. You cannot send the working revision in the body. Blech.

A CHECKOUT is performed before doing the PROPPATCH. That checkout uses the
version url from when the directory was initially checked out. The system
should have punted on that initial CHECKOUT. If it didn't, then the question
is "why?"

Two answers that I can see:

1) the client had the wrong version url stored
2) the server does the comparison wrong

Karl did some work on #1 recently, maybe that fixed this problem? In any
case, tracking that URL is a precondition to fixing this issue. We need to
know if it is getting bumped improperly (e.g. the version url must remain
unchanged w.r.t revision; if the revision doesn't change, neither does the
url).

For the second, I'd be a bit surprised. It does the check for other
resources. There is a bit of special casing in the code, with a comment
about it (although rereading the comment, it isn't perfectly clear why, so
maybe the comment needs some more tweaking).

So on the problems, I'd say the first isn't an issue right now, and the
second needs a bit of investigation before any whacking on it.

Cheers,
-g

-- 
Greg Stein, http://www.lyra.org/
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Received on Sat Oct 21 14:36:45 2006

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