If we a find that an extent is shared but its end offset is not sector
size aligned, then we don't clone it and issue write operations instead.
This is because the reflink (remap_file_range) operation does not allow
to clone unaligned ranges, except if the end offset of the range matches
the i_size of the source and destination files (and the start offset is
sector size aligned).

While this is not incorrect because send can only guarantee that a file
has the same data in the source and destination snapshots, it's not
optimal and generates confusion and surprising behaviour for users.

For example, running this test:

  $ cat test.sh
  #!/bin/bash

  DEV=/dev/sdi
  MNT=/mnt/sdi

  mkfs.btrfs -f $DEV
  mount $DEV $MNT

  # Use a file size not aligned to any possible sector size.
  file_size=$((1 * 1024 * 1024 + 5)) # 1MB + 5 bytes
  dd if=/dev/random of=$MNT/foo bs=$file_size count=1
  cp --reflink=always $MNT/foo $MNT/bar

  btrfs subvolume snapshot -r $MNT/ $MNT/snap
  rm -f /tmp/send-test
  btrfs send -f /tmp/send-test $MNT/snap

  umount $MNT
  mkfs.btrfs -f $DEV
  mount $DEV $MNT

  btrfs receive -vv -f /tmp/send-test $MNT

  xfs_io -r -c "fiemap -v" $MNT/snap/bar

  umount $MNT

Gives the following result:

  (...)
  mkfile o258-7-0
  rename o258-7-0 -> bar
  write bar - offset=0 length=49152
  write bar - offset=49152 length=49152
  write bar - offset=98304 length=49152
  write bar - offset=147456 length=49152
  write bar - offset=196608 length=49152
  write bar - offset=245760 length=49152
  write bar - offset=294912 length=49152
  write bar - offset=344064 length=49152
  write bar - offset=393216 length=49152
  write bar - offset=442368 length=49152
  write bar - offset=491520 length=49152
  write bar - offset=540672 length=49152
  write bar - offset=589824 length=49152
  write bar - offset=638976 length=49152
  write bar - offset=688128 length=49152
  write bar - offset=737280 length=49152
  write bar - offset=786432 length=49152
  write bar - offset=835584 length=49152
  write bar - offset=884736 length=49152
  write bar - offset=933888 length=49152
  write bar - offset=983040 length=49152
  write bar - offset=1032192 length=16389
  chown bar - uid=0, gid=0
  chmod bar - mode=0644
  utimes bar
  utimes
  BTRFS_IOC_SET_RECEIVED_SUBVOL uuid=06d640da-9ca1-604c-b87c-3375175a8eb3, stransid=7
  /mnt/sdi/snap/bar:
   EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
     0: [0..2055]:       26624..28679      2056   0x1

There's no clone operation to clone extents from the file foo into file
bar and fiemap confirms there's no shared flag (0x2000).

So update send_write_or_clone() so that it proceeds with cloning if the
source and destination ranges end at the i_size of the respective files.

After this changes the result of the test is:

  (...)
  mkfile o258-7-0
  rename o258-7-0 -> bar
  clone bar - source=foo source offset=0 offset=0 length=1048581
  chown bar - uid=0, gid=0
  chmod bar - mode=0644
  utimes bar
  utimes
  BTRFS_IOC_SET_RECEIVED_SUBVOL uuid=582420f3-ea7d-564e-bbe5-ce440d622190, stransid=7
  /mnt/sdi/snap/bar:
   EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
     0: [0..2055]:       26624..28679      2056 0x2001

A test case for fstests will also follow up soon.

Link: https://github.com/kdave/btrfs-progs/issues/572#issuecomment-2282841416
CC: stable@vger.kernel.org # 5.10+
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>