Vanishing GPT/UFS labels

Hi Hackers,

I have some code at hand that rely on GPT and UFS labels to abstract
things a bit. But a few observations puzzle me.

My initial observations were made on an nda(4) disk but I managed to
reproduce with an md(4) one on a15.0-RELEASE-p3 (5cf7232732d5) machine:

    dd if=/dev/zero of=zero.img bs=100M count=1
    md=`mdconfig -f zero.img`
    gpart create -s gpt $md
    gpart add -i 1 -l foo -t freebsd-swap $md
    gpart add -i 2 -l bar -t freebsd-ufs  $md
    newfs -L bar /dev/${md}p2

From here the following devices are available to match the labels:

    /dev/gpt/foo
    /dev/gpt/bar
    /dev/ufs/bar

When I mount using the underlying device, /dev/${md}p2, both
/dev/gpt/bar and /dev/ufs/bar vanish. And when I umount (using the mount
point) both are back.

When I mount using the UFS label, /dev/ufs/bar, only /dev/gpt/bar
vanish. And when I umount the GPT label don't come back.

GEOM don't seems to agree with itself: `gpart show -l $md` do confirm both
partitions kept their GPT label but `glabel status` don't:

    $ gpart show -l $md
    =>    40  204720  md0  GPT  (100M)
          40   20480    1  foo  (10M)
       20520  184240    2  bar  (90M)
    $ glabel status | grep -e foo -e bar
         gpt/foo     N/A  md0p1
         ufs/bar     N/A  md0p2

I would be happy to hunt what looks like bugs to me but I need some
insight first: what is the rationale behind the idea to remove a device
on mount and, possibly, restore it on umount?

Regards,

Stéphane