Bug: trying to create reflink on different device results in empty file

When trying to create a clone (reflink) of a file on a different device, you'll get this error:
Invalid cross-device link

However, an empty file is created on the target location. An invalid clone operation should not result in the creation of a file.

Example:

# mount
(...)
/dev/sdb1 on /mnt/diskb type btrfs (rw)
/dev/sdf1 on /mnt/diskf type btrfs (rw)

# ls -l diskb
total 4
-rw-r--r-- 1 root root 11 Dec 11 22:45 testfile.txt

# ls -l diskf
total 0

# cp --reflink diskb/testfile.txt diskf/testcopy.txt
cp: failed to clone `diskf/testcopy.txt' from `diskb/testfile.txt': Invalid cross-device link

# ls -l diskf
total 0
-rw-r--r-- 1 root root 0 Dec 11 22:56 testcopy.txt

The clone operation inside btrfs needs 2 files already open (ie. it works with file descriptors, not with filenames), the empty file exists prior to the ioctl call. So the file should be removed by 'cp' (or whoever calls the clone ioctl) because they know if it makes sense to delete the file or not.

david