Broken DIR_ITEMs on snapshot

[Jan Schmidt <list.btrfs@jan-o-sch.net>]

Hi,

while still busy with btrfs send, I came across some strange DIR_ITEMs.
I looked into that briefly, but I'd rather return to implementing btrfs
send, hoping someone is willing to make up his mind on this one :-)

To reproduce, do the following:

# mkfs.btrfs /dev/sdv2
# mount /dev/sdv2 /mnt
# btrfs subvol snap /mnt /mnt/snap1

You've a freshly created snapshot. However, file tree 256 (the
snap1-tree) will contain two strange items:

        item 2 key (256 DIR_ITEM 3645318598) itemoff 3788 itemsize 35
                location key (256 ROOT_ITEM 18446744073709551615) type 2
                namelen 5 datalen 0 name: snap1
        item 3 key (256 DIR_INDEX 2) itemoff 3753 itemsize 35
                location key (256 ROOT_ITEM 18446744073709551615) type 2
                namelen 5 datalen 0 name: snap1

These items are needed in tree 5 (fs tree) to reference snap1. However,
within snap1, I'd not expect the entries. A brief look into
create_pending_snapshot reveals

   ...
   btrfs_insert_dir_item()
   ...
   /* some delayed stuff with scary comments */
   ...
   btrfs_cow_block()
   ...

I'm not sure whether cowing earlier would help, I'm particularly
uncertain because of the run_delayed_* code in between. So I haven't
tried to fix this, I'm convinced it should be fixed, though.

These items lead to some strange effects:

# cd /mnt/snap1
# ls -l
dr-xr-xr-x 1 root root 10 Jan  1  1970 .
dr-xr-xr-x 1 root root 16 Oct 18 15:56 ..
# mkdir snap1
mkdir: cannot create directory `snap1': File exists
# stat snap1
  File: `snap1'
  Size: 0               Blocks: 0          IO Block: 4096   directory
Device: 11h/17d Inode: 2           Links: 1
# rmdir snap1
# stat snap1
stat: cannot stat `snap1': No such file or directory

Inode number 2 seems to be BTRFS_EMPTY_SUBVOL_DIR_OBJECTID, the pseudo
object is created by btrfs_lookup_dentry() in inode.c when ENOENT is
encountered.

As a side note: the timestamp of the snap-dir item could be prettier.

No such pseudo items are created when the snapshot is placed outside of
the subvolume to be snapshotted, obviously. In the above example, do ...

# btrfs subvol snap /mnt/snap1 /mnt/snap2

... and no such items will be created, which makes me quite certain the
existence of above mentioned DIR_ITEMs is a bug, isn't it?

-Jan