illumos gate

The hold ioctl handler returns EINVAL in one of two
cases:

1. the hold tag is empty (new)
2. user wanted to put a hold on something that's not a snapshot (existing)

Sadly, the ioctl return value makes its way to libzfs (zfs_hold_nvl), which does what libzfs does - prints an error message.

http://src.illumos.org/source/xref/illumos-gate/usr/src/lib/libzfs/common/libzfs_dataset.c#4305

It of course assumes that we got an EINVAL because of case #2 and so the user sees:

"operation not applicable to datasets of this type"

My best idea is to add a empty-tag check to libzfs.

A way to test:

$ zfs hold '' storage/upstream@9e835c7628dd0e7764a8341a1774a878dc0b024f
cannot hold: operation not applicable to datasets of this type