[resend] btrfs-send -c fails: reproduction case

[Zachary Vance <vanceza@gmail.com>]

Apologies for the repeat send for anyone that gets this twice; I was
informed there was a problem with my mail setup and many ISPs would
block the first message. I have also simplified the reproduction case.

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Reproduction case after running into the same problem as Paride
Legovini: http://article.gmane.org/gmane.comp.file-systems.btrfs/48706/match=send

To recap, the basic problem is that you can get into a state where
"btrfs-send -c" fails for a subvolume, with "ERROR: parent
determination failed for 9622". It appears the problem happens when
the 'parent_uuid' does not exist.

I'm pretty sure this isn't the intended behavior, and it's certainly
preventing me from copying my backups right now.

burn% sudo btrfs subvol create a
Create subvolume './a'
burn% sudo btrfs subvol snapshot a -r a-r
Create a readonly snapshot of 'a' in './a-r'
burn% sudo btrfs subvol create b1
Create subvolume './b1'
burn% sudo btrfs subvol snapshot b1 -r b2
Create a readonly snapshot of 'b1' in './b2'
burn% sudo btrfs subvol del b1
Delete subvolume (no-commit): '/data/b1'

burn% sudo btrfs send -c b2 a-r >/dev/null
At subvol a-r
ERROR: parent determination failed for 9637

/data/a-r
        Name:                   a-r
        UUID:                   819415e9-ea57-4a4d-9745-78a65f19b614
        Parent UUID:            e63b985a-3463-be4c-8ad5-900cd147ef7d
        Received UUID:          -
        Creation time:          2016-02-20 10:19:59 -0800
        Subvolume ID:           9638
        Generation:             18524
        Gen at creation:        18524
        Parent ID:              5
        Top level ID:           5
        Flags:                  readonly
        Snapshot(s):
burn% sudo btrfs subvolume show b2
/data/b2
        Name:                   b2
        UUID:                   e1d0314d-9c2a-5240-9f4c-1716b7e87741
        Parent UUID:            00920504-c78b-ee42-af9f-05f4600321bb
        Received UUID:          -
        Creation time:          2016-02-20 10:19:01 -0800
        Subvolume ID:           9637
        Generation:             18522
        Gen at creation:        18522
        Parent ID:              5
        Top level ID:           5
        Flags:                  readonly
        Snapshot(s):

[root@burn ~]# btrfs --version
btrfs-progs v4.4.1

[root@burn ~]# uname -a
Linux burn 4.5.0-1-ARCH #1 SMP PREEMPT Tue Mar 15 09:41:03 CET 2016
x86_64 GNU/Linux

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Zachary Vance