Re: [pve-devel] [PATCH pve-guest-common 1/1] add pre/post-snapshot hooks

[Fiona Ebner <f.ebner@proxmox.com>]

Am 22.09.22 um 13:54 schrieb Stefan Hanreich:
> Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>
> ---
> 

Should there be a third hook that's called when the snapshot fails? That
would allow doing cleanup in all cases. Could still be added later when
actually requested by users of course.

What are the common use cases you have in mind for these hooks?

> I have opted to include the snapshot hooks in the abstract class, since this
> seemed like the more straightforward way.
> The other option would have been duplicating the calls of the hooks into the
> respective Backends, but I couldn't see any upsides to this.
> 
> This hook runs before the preparation steps, since otherwise in case of a
> failing pre-snapshot hook the VM/CT is left in a locked state, which I would
> need to clean up, which would add unnecessary complexity imo.
> 
> Otoh, there is a case to be made that the hook should only run after we checked
> every precondition and are as certain as we can be that the snapshot will be
> successfully created. This would be more convenient from a user's pov.
> This can be particularly convenient as it would avoid errors with user scripts
> that are not idempotent. Although those would still fail in case of a failure
> during the snapshotting process.

Calling the hook script only after setting the lock in the config would
add protection against other modifications happening at the same time.
Stupid example: if we add another such hook outside a lock, and both
that and the 'pre-snapshot' hook would modify the config, they could
interfere when happening right after another. But it doesn't need to be
another hook script, the config modification could also come from
somewhere else.

Although this approach does requires users to pass the --skiplock option
to modify the config if using our API/CLI. And we need to repeat the
checks after calling the hook script, because they might not apply any
more ;)

> 
> What do you think would be the better solution?
> 
>  src/PVE/AbstractConfig.pm | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/src/PVE/AbstractConfig.pm b/src/PVE/AbstractConfig.pm
> index a0c0bc6..8052fde 100644
> --- a/src/PVE/AbstractConfig.pm
> +++ b/src/PVE/AbstractConfig.pm
> @@ -799,11 +799,15 @@ sub __snapshot_activate_storages {
>  sub snapshot_create {
>      my ($class, $vmid, $snapname, $save_vmstate, $comment) = @_;
>  
> +    my $conf = $class->load_config($vmid);
> +    PVE::GuestHelpers::exec_hookscript($conf, $vmid, "pre-snapshot", 1);
> +
>      my $snap = $class->__snapshot_prepare($vmid, $snapname, $save_vmstate, $comment);
>  
>      $save_vmstate = 0 if !$snap->{vmstate};
>  
> -    my $conf = $class->load_config($vmid);
> +    # reload config after changes in snapshot preparation step

I think there should be a cfs_update() call?

> +    $conf = $class->load_config($vmid);
>  
>      my ($running, $freezefs) = $class->__snapshot_check_freeze_needed($vmid, $conf, $snap->{vmstate});
>  
> @@ -843,6 +847,8 @@ sub snapshot_create {
>      }
>  
>      $class->__snapshot_commit($vmid, $snapname);
> +
> +    PVE::GuestHelpers::exec_hookscript($conf, $vmid, "post-snapshot");
>  }
>  
>  # Check if the snapshot might still be needed by a replication job.