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Re: revprop changes and hooks

From: Daniel Shahaf <d.s_at_daniel.shahaf.name>
Date: Thu, 17 Jun 2010 02:29:14 +0300 (Jerusalem Daylight Time)

Philip Martin wrote on Tue, 15 Jun 2010 at 11:16 -0000:
> Philip Martin <philip.martin_at_wandisco.com> writes:
>
> > Multiple clients. It arises from my idea to solve issue 3546
> >
> > http://subversion.tigris.org/issues/show_bug.cgi?id=3546
> >
> > In svn_repos_fs_change_rev_prop3 the code first gets the old property
> > value which it uses to calculate the action: 'A', 'M' or 'D'. Then it
> > passes the action to the pre-revprop-change hook, then it changes the
> > property and finally it runs the post-revprop-change hook. Some other
> > process can change the revprop at any time so although the
> > pre-revprop-change hook might get passed an 'A' say, when the change
> > is made it could be effectively an 'M'. The action passed to the hook
> > is not a reliable indication of the change to be made.
> >
> > Putting the get, pre hook and set into a transaction would allow the
> > action in the hook to accurately reflect the change made (or not made
> > if the transaction fails).
>
> We don't need a new transaction to fix this, we can rev the
> svn_fs_change_rev_prop interface instead:
>
> svn_error_t *
> svn_fs_change_rev_prop(svn_fs_t *fs,
> svn_revnum_t rev,
> const char *name,
> const svn_string_t *value,
> apr_pool_t *pool);
>
> to include the current value of the revprop, and then reject the
> change if the current value does not match. That should be simple
> because the FSFS implementation already takes a write lock and the BDB
> implementation already uses a transaction.
>

(this is now done)

> Then the repos layer can loop (in practice only if the
> use_pre_revprop_change_hook flag is set):
>
> do
> svn_fs_revision_prop(&current_value)
> action = ...
> svn_repos__hooks_pre_revprop_change(action)
> error = svn_fs_change_rev_prop2(current_value, new_value)
> while error is current value doesn't match
>
> This doesn't alter the fact that the revprop can change at any time
> during the loop but that doesn't matter. The revprop is unversioned
> so only the current state matters and the above will guarantee that
> the current state when the change is made is equal to the state
> validated by the pre-revprop-change hook.
>

If we just upgrade the svn_fs_change_rev_prop() call in
svn_repos_fs_change_rev_prop3() to svn_fs_change_rev_prop2() (a
single-line change), then we will be guaranteed that the 'action'
parameter (A/D/M) will be accurate. (However, the pre- hook script doesn't
know the old property value.)

This is sufficient for the svnsync use case (issue 3546), where "allow
adds/deletes only" will provide the necessary mutual exclusion.

But I wonder if, while here, we could go further and obtain the
"expected old property value" from the RA layer (and pass it to the pre-
hook). (This probably means revving svn_ra_change_rev_prop() the same
way svn_fs_change_rev_prop() was revved.) That will allow "svn propset
k v --if-old-value-is=vprime" to work...

>
Received on 2010-06-17 01:28:54 CEST

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