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To: Proxmox Backup Server development discussion
<pbs-devel@lists.proxmox.com>, Stefan Sterz <s.sterz@proxmox.com>
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Subject: Re: [pbs-devel] [PATCH proxmox-backup] fix #3336: api: remove
backup group if the last snapshot is removed
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On 09.03.22 14:50, Stefan Sterz wrote:
> Signed-off-by: Stefan Sterz <s.sterz@proxmox.com>
> ---
> pbs-datastore/src/datastore.rs | 22 ++++++++++++++++++++++
> 1 file changed, 22 insertions(+)
>
> diff --git a/pbs-datastore/src/datastore.rs b/pbs-datastore/src/datastore.rs
> index d416c8d8..623b7688 100644
> --- a/pbs-datastore/src/datastore.rs
> +++ b/pbs-datastore/src/datastore.rs
> @@ -346,6 +346,28 @@ impl DataStore {
> )
> })?;
>
> + // check if this was the last snapshot and if so remove the group
> + if backup_dir
> + .group()
> + .list_backups(&self.base_path())?
> + .is_empty()
> + {
a log::info could be appropriate in the "success" (i.e., delete dir) case.
I'd factor the this block below out into a non-pub (or pub(crate)) remove_empty_group_dir fn.
> + let group_path = self.group_path(backup_dir.group());
> + let _guard = proxmox_sys::fs::lock_dir_noblock(
> + &group_path,
> + "backup group",
> + "possible running backup",
> + )?;
> +
> + std::fs::remove_dir_all(&group_path).map_err(|err| {
this is still unsafe as there's a TOCTOU race, the lock does not protects you from the
following sequence with two threads/async-excutions t1 and t1
t1.1 snapshot deleted
t1.2 empty dir check holds up, entering "delete group dir" code branch
t2.1 create new snapshot in group -> lock group dir
t2.2 finish new snapshot in group -> unlock group dir
t1.3 lock group dir
t1.4 delete all files, including the new snapshot made in-between.
Rather, just use the safer "remove_dir" variant, that way the TOCTOU race doesn't matters,
the check merely becomes a short cut; if we'd explicitly check for
`err.kind() != ErrorKind::DirectoryNotEmpty
and silent it we could even do away with the check, should result in the same amount of
syscalls in the best-case (one rmdir vs. one readir) and can be better on success
(readdir + rmdir vs. rmdir only), not that perfromance matters much in this case.
fyi, "remove_backup_group", the place where I think you copied this part, can use the
remove_dir_all safely because there's no check to made there, so no TOCTOU.
> + format_err!(
> + "removing backup group directory {:?} failed - {}",
> + group_path,
> + err,
> + )
> + })?;
> + }
> +
> // the manifest does not exists anymore, we do not need to keep the lock
> if let Ok(path) = self.manifest_lock_path(backup_dir) {
> // ignore errors