Check if qgroup exists before attempting to destroy it (#484)

When a btrfs subvolume is deleted, its associated qgroup is often
automatically destroyed by btrfs. Timeshift would then attempt to
explicitly destroy the qgroup and fail with an error message, even
though the operation was successful.

This change checks if the qgroup still exists before attempting to
destroy it, eliminating the spurious error message. If the qgroup
is already gone (auto-destroyed with the subvolume), we log a debug
message instead. If the qgroup still exists, we attempt to destroy
it and handle any race condition gracefully.

Fixes the "Failed to destroy qgroup" error that occurs during
snapshot deletion when quotas are enabled.

Related: #354, #437

Author: zfogg

src/Core/Subvolume.vala

@@ -193,20 +193,31 @@ public class Subvolume : GLib.Object{
                     sleep(1000);
                 }
 
-				log_msg("%s: 0/%ld".printf(_("Destroying qgroup"), id));
-
-				cmd = "btrfs qgroup destroy 0/%ld '%s'".printf(id, repo.mount_paths[name]);
+				// Check if qgroup still exists before trying to destroy it.
+				// The qgroup may have been auto-destroyed when the subvolume was deleted.
+				string qgroup_id = "0/%ld".printf(id);
+				cmd = "btrfs qgroup show -f '%s'".printf(repo.mount_paths[name]);
 				log_debug(cmd);
 				ret_val = exec_sync(cmd, out std_out, out std_err);
-				if (ret_val != 0){
-					log_error(_("Failed to destroy qgroup") + ": '0/%ld'".printf(id));
 
-					// the subvolume is gone now. So this can be called a success.
-					return true;
+				bool qgroup_exists = (ret_val == 0) && (std_out.contains(qgroup_id));
+
+				if (!qgroup_exists) {
+					log_debug("Qgroup %s already removed (auto-destroyed with subvolume)".printf(qgroup_id));
+				} else {
+					log_msg("%s: %s".printf(_("Destroying qgroup"), qgroup_id));
+
+					cmd = "btrfs qgroup destroy %s '%s'".printf(qgroup_id, repo.mount_paths[name]);
+					log_debug(cmd);
+					ret_val = exec_sync(cmd, out std_out, out std_err);
+					if (ret_val != 0){
+						// Qgroup may have been destroyed between our check and destroy attempt
+						log_debug("Qgroup %s could not be destroyed (likely already removed): %s".printf(qgroup_id, std_err));
+					} else {
+						log_msg("%s: %s\n".printf(_("Destroyed qgroup"), qgroup_id));
+					}
 				}
 
-				log_msg("%s: 0/%ld\n".printf(_("Destroyed qgroup"), id));
-
                 log_debug("Rescanning quotas (post)...");
                 while (exec_sync("btrfs quota rescan %s".printf(mount_path), out std_out, out std_err) != 0) {
                     log_debug("Still rescanning quotas (post)... %s".printf(std_err));