FreeBSD zfs_ioc_rename() has an option, not present upstream, that
allows to rename snapshots without unmounting them first. I am not sure
what is a rationale for that option, but its actual behavior was the
opposite of the intended behavior. That is, by default the snapshots
were not unmounted.
The option was introduced as part of a large update from upstream in
r248498.
One of the consequences was a havoc under .zfs/snapshot after the rename.
The snapshots got new names but were mounted on top of directories with
old names, so readdir would list the new names, but lookup would still
find the old mounts.
PR: 209093
Reported by: Frédéric VANNIÈRE <f.vanniere@planet-work.com>
MFC after: 5 days
a288444e8c