fix: enhance nested subvolume deletion verification process

[cyp0633]

The Background

When deleting an old snapshot in BTRFS mode, the current version of Timeshift deletes certain subdirectories as subvolumes recursively (here):

		subpath = path_combine(path, name);
		if (dir_exists(subpath)) { // there is a nested subvol to remove first
			cmd = "btrfs subvolume delete %s '%s'".printf(options, subpath);
			log_debug("Deleting nested subvolume in snapshot");
			log_debug(cmd);
			ret_val = exec_sync(cmd, out std_out, out std_err);
			if (ret_val != 0){
				log_error(std_err);
				log_error(_("Failed to delete snapshot nested subvolume") + ": '%s'".printf(path));
				return false;
			}
		}

In this way, Timeshift correctly handles situations like timeshift-btrfs/snapshots/xxx/@/@. However, btrfs-progs cannot delete an ordinary directory like a subvolume:

$ mkdir test
$ sudo btrfs subvolume delete test
ERROR: Not a Btrfs subvolume: Invalid argument

The Problem

Similarly, if a /@ directory gets a snapshot, then Timeshift would treat it as a subvolume when deleting the snapshot. Steps to reproduce:

$ sudo mkdir /@ 

$ sudo timeshift --create # the snapshot with @
Using system disk as snapshot device for creating snapshots in BTRFS mode
Mounted '/dev/nvme0n1p2' (subvolid=0) at '/run/timeshift/114861/backup'
btrfs: Quotas are not enabled
Creating new backup...(BTRFS)
Saving to device: /dev/nvme0n1p2, mounted at path: /run/timeshift/114861/backup
Created directory: /run/timeshift/114861/backup/timeshift-btrfs/snapshots/2025-10-23_17-05-41
Created subvolume snapshot: /run/timeshift/114861/backup/timeshift-btrfs/snapshots/2025-10-23_17-05-41/@
Created subvolume snapshot: /run/timeshift/114861/backup/timeshift-btrfs/snapshots/2025-10-23_17-05-41/@home
Created control file: /run/timeshift/114861/backup/timeshift-btrfs/snapshots/2025-10-23_17-05-41/info.json
BTRFS Snapshot saved successfully (0s)
Tagged snapshot '2025-10-23_17-05-41': ondemand
------------------------------------------------------------------------------

$ sudo timeshift --delete --snapshot 2025-10-23_17-05-41
Mounted '/dev/nvme0n1p2' (subvolid=0) at '/run/timeshift/114951/backup'
btrfs: Quotas are not enabled
------------------------------------------------------------------------------
Removing snapshot: 2025-10-23_17-05-41
Deleting subvolume: @home (Id:835)
Deleted subvolume: @home (Id:835)

Deleting subvolume: @ (Id:834)
E: ERROR: Not a Btrfs subvolume: Invalid argument # tries to delete subvolumes recursively, but meets a directory

E: Failed to delete snapshot nested subvolume: '/run/timeshift/114951/backup/timeshift-btrfs/snapshots/2025-10-23_17-05-41/@'
E: Failed to remove snapshot: 2025-10-23_17-05-41
------------------------------------------------------------------------------

$ sudo rm -rf /@

$ sudo timeshift --create
Using system disk as snapshot device for creating snapshots in BTRFS mode
Mounted '/dev/nvme0n1p2' (subvolid=0) at '/run/timeshift/115103/backup'
btrfs: Quotas are not enabled
Creating new backup...(BTRFS)
Saving to device: /dev/nvme0n1p2, mounted at path: /run/timeshift/115103/backup
Created directory: /run/timeshift/115103/backup/timeshift-btrfs/snapshots/2025-10-23_17-08-16
Created subvolume snapshot: /run/timeshift/115103/backup/timeshift-btrfs/snapshots/2025-10-23_17-08-16/@
Created subvolume snapshot: /run/timeshift/115103/backup/timeshift-btrfs/snapshots/2025-10-23_17-08-16/@home
Created control file: /run/timeshift/115103/backup/timeshift-btrfs/snapshots/2025-10-23_17-08-16/info.json
BTRFS Snapshot saved successfully (0s)
Tagged snapshot '2025-10-23_17-08-16': ondemand
------------------------------------------------------------------------------

$ sudo timeshift --delete --snapshot 2025-10-23_17-08-16  
Mounted '/dev/nvme0n1p2' (subvolid=0) at '/run/timeshift/115177/backup'
btrfs: Quotas are not enabled
------------------------------------------------------------------------------
Removing snapshot: 2025-10-23_17-08-16
Deleting subvolume: @home (Id:837)
Deleted subvolume: @home (Id:837)

Deleting subvolume: @ (Id:836)
Deleted subvolume: @ (Id:836)

Deleted directory: /run/timeshift/115177/backup/timeshift-btrfs/snapshots/2025-10-23_17-08-16
Deleted directory: /run/timeshift/115177/backup/timeshift-btrfs/snapshots/2025-10-23_17-08-16
Deleted directory: /run/timeshift/115177/backup/timeshift-btrfs/snapshots/2025-10-23_17-08-16
Removed snapshot: 2025-10-23_17-08-16
------------------------------------------------------------------------------

I suspect there have been similar problems around for a while:

• Unable to delete btrfs snapshots teejee2008/timeshift#803
• btrfs and timeshift issues with deleting snapshots (Arch forum)
• Timeshift and subvolume question (r/btrfs)

The Fix

My proposal first checks if there's indeed a subvolume before running btrfs subvolume delete. btrfs subvolume show outputs nothing to the stdout if there's not a subvolume. After applying the fix, the previously failing deletion succeeded:

$ sudo ./timeshift --delete --snapshot 2025-10-23_17-05-41 # using my self-compiled binary
Mounted '/dev/nvme0n1p2' (subvolid=0) at '/run/timeshift/115404/backup'
btrfs: Quotas are not enabled
------------------------------------------------------------------------------
Removing snapshot: 2025-10-23_17-05-41
Deleting subvolume: @ (Id:834)
Deleted subvolume: @ (Id:834)

Deleted directory: /run/timeshift/115404/backup/timeshift-btrfs/snapshots/2025-10-23_17-05-41
Deleted directory: /run/timeshift/115404/backup/timeshift-btrfs/snapshots/2025-10-23_17-05-41
Removed snapshot: 2025-10-23_17-05-41
------------------------------------------------------------------------------

Environment:

• OS: EndeavourOS, Linux 6.17.4-arch2-1
• Timeshift: 25.07.7
• btrfs-progs: 6.17