Although I'm glad that a bug has been uncovered, maybe it's best if I
stick with good old rsync for backups.
It would be kind of ironic if the first data loss that I experienced
in many years of btrfs use would be caused by an ancillary backup
tool.

On Thu, Dec 31, 2020 at 10:36 PM Zygo Blaxell
[off-list ref] wrote:

On Thu, Dec 31, 2020 at 09:48:54PM +0100, john terragon wrote:

quoted

On Thu, Dec 31, 2020 at 8:42 PM Andrei Borzenkov [off-list ref] wrote:

quoted

quoted

How exactly you create subvolume with the same content? There are many
possible interpretations.

Zygo wrote that any subvol could be used with -p. So, out of
curiosity, I did the following

1) btrfs sub create X
2) I unpacked some source (linux kernel) in X
3) btrfs sub create W
4) I unpacked the same source in W (so X and W have the same content
but they are independent)
5) btrfs sub snap -r X X_RO
6) btrfs sub snap -r W W_RO
7) btrfs send W_RO | btrfs receive /mnt/btrfs2
8) btrfs send -p W_RO X_RO | btrfs receive /mnt/btrfs2

And this is the exact output of 8)

At subvol X_RO
At snapshot X_RO
ERROR: chown o257-1648413-0 failed: No such file or directory

Yeah, I only checked that send completed without error and produced a
smaller stream.

I just dumped the send metadata stream from the incremental snapshot now,
and it's more or less garbage at the start:

        # btrfs sub create A
        # btrfs sub create B
        # date > A/date
        # date > B/date
        # mkdir A/t B/u
        # btrfs sub snap -r A A_RO
        # btrfs sub snap -r B B_RO
        # btrfs send A_RO | btrfs receive --dump
        At subvol A_RO
        subvol          ./A_RO                          uuid=995adde4-00ac-5e49-8c6f-f01743def072 transid=7329268
        chown           ./A_RO/                         gid=0 uid=0
        chmod           ./A_RO/                         mode=755
        utimes          ./A_RO/                         atime=2020-12-31T15:51:31-0500 mtime=2020-12-31T15:51:48-0500 ctime=2020-12-31T15:51:48-0500
        mkfile          ./A_RO/o257-7329268-0
        rename          ./A_RO/o257-7329268-0           dest=./A_RO/date
        utimes          ./A_RO/                         atime=2020-12-31T15:51:31-0500 mtime=2020-12-31T15:51:48-0500 ctime=2020-12-31T15:51:48-0500
        write           ./A_RO/date                     offset=0 len=29
        chown           ./A_RO/date                     gid=0 uid=0
        chmod           ./A_RO/date                     mode=644
        utimes          ./A_RO/date                     atime=2020-12-31T15:51:38-0500 mtime=2020-12-31T15:51:38-0500 ctime=2020-12-31T15:51:38-0500
        mkdir           ./A_RO/o258-7329268-0
        rename          ./A_RO/o258-7329268-0           dest=./A_RO/t
        utimes          ./A_RO/                         atime=2020-12-31T15:51:31-0500 mtime=2020-12-31T15:51:48-0500 ctime=2020-12-31T15:51:48-0500
        chown           ./A_RO/t                        gid=0 uid=0
        chmod           ./A_RO/t                        mode=755
        utimes          ./A_RO/t                        atime=2020-12-31T15:51:48-0500 mtime=2020-12-31T15:51:48-0500 ctime=2020-12-31T15:51:48-0500
        # btrfs send B_RO -p A_RO | btrfs receive --dump
        At subvol B_RO
        snapshot        ./B_RO                          uuid=4aa7db26-b219-694e-9b3c-f8f737a46bdb transid=7329268 parent_uuid=995adde4-00ac-5e49-8c6f-f01743def072 parent_transid=7329268
        utimes          ./B_RO/                         atime=2020-12-31T15:51:33-0500 mtime=2020-12-31T15:51:52-0500 ctime=2020-12-31T15:51:52-0500
        link            ./B_RO/date                     dest=date
        unlink          ./B_RO/date
        utimes          ./B_RO/                         atime=2020-12-31T15:51:33-0500 mtime=2020-12-31T15:51:52-0500 ctime=2020-12-31T15:51:52-0500
        write           ./B_RO/date                     offset=0 len=29
        utimes          ./B_RO/date                     atime=2020-12-31T15:51:41-0500 mtime=2020-12-31T15:51:41-0500 ctime=2020-12-31T15:51:41-0500
        rename          ./B_RO/t                        dest=./B_RO/u
        utimes          ./B_RO/                         atime=2020-12-31T15:51:33-0500 mtime=2020-12-31T15:51:52-0500 ctime=2020-12-31T15:51:52-0500
        utimes          ./B_RO/u                        atime=2020-12-31T15:51:52-0500 mtime=2020-12-31T15:51:52-0500 ctime=2020-12-31T15:51:52-0500
        # btrfs send A_RO | btrfs receive -v /tmp/test
        At subvol A_RO
        At subvol A_RO
        receiving subvol A_RO uuid=995adde4-00ac-5e49-8c6f-f01743def072, stransid=7329268
        write date - offset=0 length=29
        BTRFS_IOC_SET_RECEIVED_SUBVOL uuid=995adde4-00ac-5e49-8c6f-f01743def072, stransid=7329268
        # btrfs send B_RO -p A_RO | btrfs receive -v /tmp/test
        At subvol B_RO
        At snapshot B_RO
        receiving snapshot B_RO uuid=4aa7db26-b219-694e-9b3c-f8f737a46bdb, ctransid=7329268 parent_uuid=995adde4-00ac-5e49-8c6f-f01743def072, parent_ctransid=7329268
        ERROR: link date -> date failed: File exists

The btrfs_compare_trees function can handle arbitrary tree differences,
but something happens in one of the support functions and we get a
bogus link command.  The rest of the stream is OK though:  we fill
in the contents of B_RO/date, rename A_RO/t to B_RO/u, and update all
the timestamps.

Oh well, I didn't say send didn't have any bugs.  ;)