[pbs-devel] [PATCH backup] fix #3336: cleanup when deleting last snapshot

[Wolfgang Bumiller]

On Fri, Mar 07, 2025 at 11:37:32AM +0100, Shannon Sterz wrote:
> On Thu Mar 6, 2025 at 1:08 PM CET, Maximiliano Sandoval wrote:
> > When the last snapshot from a group is deleted we clear the entire
> > group, this in turn cleans the owner for the group.
> >
> > Without this change, the user is unable to change the owner of the group
> > after the last snapshot has been deleted. This would prevent a new
> > backups to the same group from a different owner.
> >
> > Signed-off-by: Maximiliano Sandoval <m.sandoval at proxmox.com>
> > ---
> >  src/api2/admin/datastore.rs | 12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> >
> > diff --git a/src/api2/admin/datastore.rs b/src/api2/admin/datastore.rs
> > index dbb7ae47..305673f1 100644
> > --- a/src/api2/admin/datastore.rs
> > +++ b/src/api2/admin/datastore.rs
> > @@ -423,10 +423,20 @@ pub async fn delete_snapshot(
> >              &backup_dir.group,
> >          )?;
> >
> > -        let snapshot = datastore.backup_dir(ns, backup_dir)?;
> > +        let snapshot = datastore.backup_dir(ns.clone(), backup_dir)?;
> >
> >          snapshot.destroy(false)?;
> >
> > +        let group = BackupGroup::from(snapshot);
> > +        if group.list_backups().is_ok_and(|backups| backups.is_empty()) {
> > +            if let Err(err) = datastore.remove_backup_group(&ns, group.as_ref()) {
> > +                log::error!(
> > +                    "error while cleaning group {path:?} - {err}",
> > +                    path = group.full_group_path()
> > +                );
> > +            }
> > +        }
> > +
> 
> this bug... looks so harmless, but comes back to haunt me every time. as
> explained by wobu here [1] it is not really possible to cleanly fix this
> bug without reworking our locking mechanism. i did send some patches for
> that (checks notes) almost 3 years ago ^^', but they are still not
> applied and definitively need a rework at this point [2]. i can pick
> this up again, sorry got focused on other things in the meantime.
> 
> [1]: https://lore.proxmox.com/pbs-devel/20220314093617.n2mc2jv4k6ntzroo@wobu-vie.proxmox.com/
> [2]: https://lore.proxmox.com/pbs-devel/20220824124829.392189-1-s.sterz@proxmox.com/

IIRC we need to figure out a good upgrade strategy so running processes
don't use different locking.

One idea was to have the postinst script create a file in run (eg
`/run/proxmox-backup/old-locking`) which, when present, instructs the
daemons to keep using the old strategy.

The new one would then automatically be used after either a reboot, or
manually removing the file between stop & start of the daemons.