illumos gate

How to reproduce:
1. Create pool over one file vdev:

root@danbuild:/root# mkfile 64m test
root@danbuild:/root# zpool create test /root/test
root@danbuild:/root# echo -n "" > test
root@danbuild:/root# reboot -p

root@danbuild:/root# zpool list
NAME    SIZE  ALLOC   FREE  EXPANDSZ   FRAG    CAP  DEDUP  HEALTH  ALTROOT
rpool  15.9G  14.1G  1.79G         -    59%    88%  1.00x  ONLINE  -
test       -      -      -         -      -      -      -  FAULTED  -
root@danbuild:/root# zpool status test
  pool: test
 state: UNAVAIL
status: One or more devices could not be used because the label is missing
        or invalid.  There are insufficient replicas for the pool to continue
        functioning.
action: Destroy and re-create the pool from
        a backup source.
   see: http://illumos.org/msg/ZFS-8000-5E
  scan: none requested
config:

        NAME          STATE     READ WRITE CKSUM
        test          UNAVAIL      0     0     0  insufficient replicas
          /root/test  UNAVAIL      0     0     0  corrupted data

# zpool get all tank 
NAME  PROPERTY                       VALUE                          SOURCE
test  size                           -                              -
test  capacity                       -                              -
test  altroot                        -                              default
test  health                         FAULTED                        -
test  guid                           3563445091455472388            -
test  version                        -                              -
test  bootfs                         -                              -
test  delegation                     -                              -
test  autoreplace                    -                              -
test  cachefile                      -                              default
test  failmode                       -                              -
test  listsnapshots                  -                              -
test  autoexpand                     -                              -
test  dedupditto                     -                              -
test  dedupratio                     -                              -
test  free                           -                              -
test  allocated                      -                              -
test  readonly                       -                              -
test  comment                        -                              default
test  expandsize                     -                              -
test  freeing                        -                              -
test  fragmentation                  -                              -
test  leaked                         -                              -
Assertion failed: nvlist_lookup_nvlist(config, "feature_stats", &features) == 0, file ../common/libzfs_config.c, line 250, function zpool_get_features
Abort (core dumped)

The problem is that we are asserting that we must have at least some features enabled. But that is not the case when the pool is faulted and we still have it in /etc/zfs/zpool.cache.
When SPA loads it from the cache, it initialises the features structure to NULL, but will never get a change to actually write anything (all vdevs are unavailable), so it leads to this situation.
The same issue appears when we run zpool upgrade related commands. I believe this is also the root cause for issue #5484.