After merging openzfs, our poudriere jail is no longer functional. As part of the jail startup process, a dataset and its children are jailed (zfs set jailed=on ; zfs jail <jail> <dataset>), after which the datasets are mounted inside the jail (zfs mount -a). Under 12.1 and CURRENT prior to the merge, this worked fine.

The datasets relevant to this (seen from jail):
# zfs list -o name,mountpoint
NAME                                        MOUNTPOINT
data                                        /data
data/jails                                  /j
data/jails/poudriere                        /j/poudriere
data/jails/poudriere/workset                /poudriere
data/jails/poudriere/workset/data           /poudriere/data
data/jails/poudriere/workset/data/.m        /poudriere/data/.m
data/jails/poudriere/workset/data/cache     /poudriere/data/cache
data/jails/poudriere/workset/data/images    /poudriere/data/images
data/jails/poudriere/workset/data/logs      /poudriere/data/logs
data/jails/poudriere/workset/data/packages  /poudriere/data/packages
data/jails/poudriere/workset/data/wrkdirs   /poudriere/data/wrkdirs
data/jails/poudriere/workset/jails          /poudriere/jails
data/jails/poudriere/workset/jails/CURRENT  /poudriere/jails/CURRENT

The jailed dataset is 'data/jails/poudriere/workset'.

Now, a number of issues come up:
- 'zfs mount -av' gives 
   # zfs mount -av
   cannot mount 'data': Insufficient privileges
   cannot mount 'data/jails': Insufficient privileges
It would seem it's trying to mount also the parent datasets. However, at this point it seems to stop, not attempting to mount any more datasets and not giving any further errors.

- 'zfs mount -v data/jails/poudriere/workset' gives
  # zfs mount -v data/jails/poudriere/workset
  cannot mount 'data/jails/poudriere/workset': dataset is exported to a local zone
Although the dataset _does_ have jailed=on and _is_ jailed in the current jail. This is confirmed by manually running 'zfs unjail ...' and 'zfs jail ...' from the host.

I believe this might be two different bugs, but I'm not qualified to read the sources to figure out what's going on.