Inconsistent mount option for TEST_DIR after _test_cycle_mount()

Hi,

Recently I'm testing btrfs with its nodatasum options to verify some 
direct IO bugs.

But it turns out _test_cycle_mount() doesn't respect $MOUNT_OPTIONS, and 
will cause mount options that affect new files to be not applied.

E.g, there is a super simple test case:

  . ./common/preamble
  _begin_fstest auto
  _require_test
  mount | grep "$TEST_DIR"
  _test_cycle_mount
  mount | grep "$TEST_DIR"
  _exit 0

With MOUNT_OPTIONS set to "-o nodatasum", the output looks like the 
following:

/dev/mapper/test-test on /mnt/test type btrfs 
(rw,relatime,nodatasum,discard=async,space_cache=v2,subvolid=5,subvol=/)
/dev/mapper/test-test on /mnt/test type btrfs 
(rw,relatime,discard=async,space_cache=v2,subvolid=5,subvol=/)

Note the missing "nodatasum" mount option after the _test_cycle_mount.

However I failed to find out how the initial _require_test is different 
from the later _test_cycle_mount.

They all call the same _test_mount(), and that function never really 
respect $MOUNT_OPTIONS, but still the first test mount still got the 
correct "nodatasum" option.

Any clue on how to fix this weird problem?

Thanks,
Qu