Re: [pbs-devel] [RFC PATCH proxmox-backup 1/2] pbs-config: use trait object for the backup lock guard

[Wolfgang Bumiller <w.bumiller@proxmox.com>]

On Fri, Sep 03, 2021 at 09:17:51AM +0200, Dominik Csapak wrote:
> instead of a fixed type. The old implementation is now
> BackupLockGuardImpl and implements the trait.
> 
> At the same time, introduce a type alias with the same name as the
> previous struct, so that the users of it do not have to change anything.
> 
> This makes it possible for us to have a different lock implementation
> for e.g. tests (where we do not actually want to lock)
> 
> Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
> ---
>  pbs-config/src/lib.rs | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/pbs-config/src/lib.rs b/pbs-config/src/lib.rs
> index 9d8c730d..604fe9d7 100644
> --- a/pbs-config/src/lib.rs
> +++ b/pbs-config/src/lib.rs
> @@ -16,7 +16,14 @@ pub fn backup_group() -> Result<nix::unistd::Group, Error> {
>      pbs_tools::sys::query_group(BACKUP_GROUP_NAME)?
>          .ok_or_else(|| format_err!("Unable to lookup '{}' group.", BACKUP_GROUP_NAME))
>  }
> -pub struct BackupLockGuard(std::fs::File);
> +
> +pub trait BackupLockGuardTrait: Send + Sync + Unpin + std::panic::UnwindSafe + std::panic::RefUnwindSafe { }
> +
> +struct BackupLockGuardImpl(std::fs::File);
> +
> +impl BackupLockGuardTrait for BackupLockGuardImpl {}
> +
> +pub type BackupLockGuard = Box<dyn BackupLockGuardTrait>;

Since the File is abstracted away in a custom type, 2 more possibilities
come to mind:

    pub struct BackupLockGuard(Option<File>);

and cfg(test) would just use `None`

or turn the File into our `Fd` type from the proxmox crate and use
`Fd::from_raw_fd(-1)` when no lock is used

That way we don't even need the trait