In the following steps[1], if <parent> on receiver side has got
changed via 'btrfs property set', then after doing incremental
updates, receiver gets a different snapshot from what sender has sent.

The reason behind it is that there is no change about file 'foo' in
the send stream, such that receiver simply creates a snapshot of
<parent> on its side with nothing to apply from the send stream.

A possible way to avoid this is to check rtransid and ctranid of
<parent> on receiver side, but I'm not very sure whether the current
behavior is made deliberately, does anyone have an idea? 



$ btrfs sub create /mnt/send/sub
$ touch /mnt/send/sub/foo
$ btrfs sub snap -r /mnt/send/sub /mnt/send/parent

# send parent out
$ btrfs send /mnt/send/parent | btrfs receive /mnt/recv/

# change parent and file under it
$ btrfs property set -t subvol /mnt/recv/parent ro false
$ truncate -s 4096 /mnt/recv/parent/foo

$ btrfs sub snap -r /mnt/send/sub /mnt/send/update
$ btrfs send -p /mnt/send/parent /mnt/send/update | btrfs receive /mnt/recv

$ ls -l /mnt/send/update
total 0
-rw-r--r-- 1 root root 0 Mar  6 11:13 foo

$ ls -l /mnt/recv/update
total 0
-rw-r--r-- 1 root root 4096 Mar  6 11:14 foo

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