zfs errors with receive -x prevent robust backups

[jgoerzen]

System information

Type	Version/Name
Distribution Name	Debian
Distribution Version	buster
Linux Kernel	4.19.0-13-amd64
Architecture	amd64
ZFS Version	0.8.6
SPL Version	0.8.6

Describe the problem you're observing

I have a system that is designed to receive backups. In order to protect against the zfs send side being compromised and malicious, it is strongly desirable to use -x mountpoint with zfs receive, and also nearly as desirable to use -o volmode=none for zvols.

The problem arises that the receiving end cannot know whether it is receiving a filesystem or a zvol. With -x mountpoint, as noted in #6371, I get:

cannot receive incremental stream: property 'mountpoint' does not apply to datasets of this type

Similar would apply to -o volmode=none for non-zvols.

It should be noted that, security-wise, a zfs receive command cannot have control over whether they are receiving a replication stream.

Describe how to reproduce the problem

Receive a zfs send from a zvol with `-x mountpoint

Include any warning/errors/backtraces from the system logs

Possible workaround

Importing with zpool -R mitigates the immediate security concern (a malicious mounting over, say, /etc) but raises other complications.

[IvanVolosyuk]

Another possible workaround - spawn two 'zfs recv' - one which will work for zvol another for filesystem. Send the stream to both, like:
https://unix.stackexchange.com/questions/40277/is-there-a-way-to-pipe-the-output-of-one-program-into-two-other-programs
One of the 'zfs recv' will fail instantly because of the wrong arguments.
It is kind of a dirty hack.