Rework output of `btrfs subvolume list`

[kdave]

The current output of subvolume listing is ancient and the command also available only to the root user (using the search ioctl). We have the user accessible ioctls available and can use them but there's a backward compatibility issue. The command could be (and likely is) used in scripts, that eg. people use for backups, so changing the output will have an impact and potentially a bad one.

Nevertheless, we want to let users to list the subvolumes and also improve the output formatting. The compatibility options:

1. no backward compatibility, rework the output, command available for user and root giving the same output
2. half compatibility, when command is run as user the output will be new, while for root it will be the old one unchanged
3. full compatibility but old output, adding support for user listiging but keeping the output
4. add a new completely new command (eg. 'ls') without any compatibility issues but at some usability cost

Proposed changes:

• drop 'top level' from the output, it's not useful and is confused with parent subvolume
• default to tabular output
• columns: id, generation, flags, path
  • flags are not just readonly, but subvolume (u), snapshot (s), read-only (r), writable (w), send timestamp set (S), received subvolume (R), and maybe something related to qgroups
• additional size information, generations, uuid etc can be optional

Compatibility is a separate problem and the output can be prepared independently in another command available in the experimental mode so we can get some feedback.

Related projects:

• https://github.com/speed47/btrfs-list

[kdave]

Case 4 implemented in branch dev/subvol-list-new, it's the simple enumeration of the subvolumes.

Subvolid Generation Flags Path
-------- ---------- ----- ----------------------------------
    5785    8826658    uw test/test-subvols/source
    5796     408501    sw test/test-subvols/snap1
    5797     408501    sw test/test-subvols/snap2
    5798     408501    sw test/test-subvols/snap3
    5799     408501    sw test/test-subvols/snap4
    5800     408501    sw test/test-subvols/snap5
    5801     408501    sr test/test-subvols/snap6

The example screenshot of btrfs-list shows the tree structure, so that can be added to a separate command eg. btrfs sbuvol tree. The tool itself parses output of btrfs subvol list so that's a case against changing list but rather adding a new command.

[ktpx]

In the current btrfs list there's an option to list only snapshot, but no way to exclude snapshots. Should be the other way around imo (default no snaps included in list, give arg to include).