When doing an incremental send, if we have a file in the parent snapshot
that has prealloc extents beyond EOF and in the send snapshot it got a
hole punch that partially covers the prealloc extents, the send stream,
when replayed by a receiver, can result in a file that has a size bigger
than it should and filled with zeroes past the correct EOF.

For example:

  $ mkfs.btrfs -f /dev/sdb
  $ mount /dev/sdb /mnt

  $ xfs_io -f -c "falloc -k 0 4M" /mnt/foobar
  $ xfs_io -c "pwrite -S 0xea 0 1M" /mnt/foobar

  $ btrfs subvolume snapshot -r /mnt /mnt/snap1
  $ btrfs send -f /tmp/1.send /mnt/snap1

  $ xfs_io -c "fpunch 1M 2M" /mnt/foobar

  $ btrfs subvolume snapshot -r /mnt /mnt/snap2
  $ btrfs send -f /tmp/2.send -p /mnt/snap1 /mnt/snap2

  $ stat --format %s /mnt/snap2/foobar
  1048576
  $ md5sum /mnt/snap2/foobar
  d31659e82e87798acd4669a1e0a19d4f  /mnt/snap2/foobar

  $ umount /mnt
  $ mkfs.btrfs -f /dev/sdc
  $ mount /dev/sdc /mnt

  $ btrfs receive -f /mnt/1.snap /mnt
  $ btrfs receive -f /mnt/2.snap /mnt

  $ stat --format %s /mnt/snap2/foobar
  3145728
  # --> should be 1Mb and not 3Mb (which was the end offset of hole
  #     punch operation)
  $ md5sum /mnt/snap2/foobar
  117baf295297c2a995f92da725b0b651  /mnt/snap2/foobar
  # --> should be d31659e82e87798acd4669a1e0a19d4f as in the original fs

This issue actually happens only since commit ffa7c429 ("Btrfs: send,
do not issue unnecessary truncate operations"), but before that commit we
were issuing a write operation full of zeroes (to "punch" a hole) which
was extending the file size beyond the correct value and then immediately
issue a truncate operation to the correct size and undoing the previous
write operation. Since the send protocol does not support fallocate, for
extent preallocation and hole punching, fix this by not even attempting
to send a "hole" (regular write full of zeroes) if it starts at an offset
greater then or equals to the file's size. This approach, besides being
much more simple then making send issue the truncate operation, adds the
benefit of avoiding the useless pair of write of zeroes and truncate
operations, saving time and IO at the receiver and reducing the size of
the send stream.

A test case for fstests follows soon.

Fixes: ffa7c429

 ("Btrfs: send, do not issue unnecessary truncate operations")
CC: stable@vger.kernel.org # 4.17+
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>