btrfs-progs: add --subvol option to mkfs.btrfs

[maharmstone]

This patch adds a --subvol option, which tells mkfs.btrfs to create the specified directories as subvolumes.

Given a populated directory img, the command

$ mkfs.btrfs --rootdir img --subvol usr --subvol home --subvol home/username /dev/loop0

will create subvolumes usr and home within the FS root, and subvolume username within the home subvolume. It will fail if any of the directories do not yet exist.

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This is version 6 of the patch; I figured it'd be easier doing things here rather on the mailing list, if we're using both.

I've incorporated Qu's suggestion that we can free the subvol list as we go along, and thus speed up iteration slightly. I've also changed it so that the --subvol value is again relative to the --rootdir rather than the working directory, which was this way because of a potential future patch allowing out-of-tree subvolumes.
I've discussed this with @DaanDeMeyer, who wants this for mkosi, and agreed that the best way to do "out-of-tree" subvolumes would be for a future --default-subvol option instead.

[maharmstone]

Whoops, fixed test

[DaanDeMeyer]

I tested this systemd/systemd#33498 and it works perfectly

[maharmstone]

New version including adding --subvol to mkfs.btrfs man page

[adam900710]

Looks good to me.

[adam900710]

I've discussed this with @DaanDeMeyer, who wants this for mkosi, and agreed that the best way to do "out-of-tree" subvolumes would be for a future --default-subvol option instead.

Just a little curious about this future feature. Do you plan to add a new option "--default-subvolume" or something like that, so that you can specify a subvolume inside the rootdir as the default one, after filling the rootdir along with the subvolumes?

If so, that new "--default-subvolume" option should still rely on the current rootdir + subvolume mechanism, just a new offline way to set default subvolue.

And if that's the case, everything looks fine to me, including the future feature.

[maharmstone]

I've discussed this with @DaanDeMeyer, who wants this for mkosi, and agreed that the best way to do "out-of-tree" subvolumes would be for a future --default-subvol option instead.

Just a little curious about this future feature. Do you plan to add a new option "--default-subvolume" or something like that, so that you can specify a subvolume inside the rootdir as the default one, after filling the rootdir along with the subvolumes?

If so, that new "--default-subvolume" option should still rely on the current rootdir + subvolume mechanism, just a new offline way to set default subvolue.

And if that's the case, everything looks fine to me, including the future feature.

Yes, that's right

[Conan-Kudo]

FYI @schaefi, we probably want to use this in kiwi too!