Resuming incremental send results in "invalid cross-device link"

[dsnet]

Type	Version/Name
Distribution Name	Ubuntu
Distribution Version	16.04.2
Linux Kernel	4.4.0-62-generic
Architecture	x64
SPL Version	0.7.0-12_g9df9692
ZFS Version	0.7.0-50_g1ea8942

I ran the following:

# Create a new pool.
$ truncate -s 1G ~/dev
$ sudo zpool create tank ~/dev

# Add some data and snapshot.
$ sudo dd if=/dev/urandom of=/tank/foo bs=1M count=50
$ sudo zfs snapshot tank@snap1

# Create a mirror.
$ sudo zfs send tank@snap1 | sudo zfs recv -s tank/mirror

# Add some data and snapshot.
$ sudo dd if=/dev/urandom of=/tank/bar bs=1M count=50
$ sudo zfs snapshot tank@snap2

# Intentionally create a partial send.
$ sudo zfs send -i tank@snap1 tank@snap2 | head -c 20000000 | sudo zfs recv -s tank/mirror
$ TOKEN=$(sudo zfs get -H receive_resume_token tank/mirror | cut -f 3)

# Just try and send the partial snapshot.
$ sudo zfs send -t $TOKEN > /dev/null
warning: cannot send 'tank@snap2': Invalid cross-device link

$ sudo zfs send -nP -t $TOKEN
resume token contents:
nvlist version: 0
	fromguid = 0x7eb9f6bf27d4bfc1
	object = 0x4
	offset = 0x12e0000
	bytes = 0x130cbd8
	toguid = 0xf121966674ca8619
	toname = tank@snap2
incremental	tank/mirror@snap1	tank@snap2

$ echo $?
18

Several strange occurrences:

• The zfs send -t fails with "Invalid cross-device link" warning, which should really be an error.
• The zfs send -nP -t reports that the source is tank/mirror@snap1 instead of tank@snap1. This is probably what causes zfs to think this is a "cross-device link" problem. It is impossible for it to satisfy this token request since that mirror doesn't have the data!
• The zfs send -nP -t prints no error, but the return value is EXDEV