zfs send -R -i aborts on identical snapshots

[KamilaBorowska]

System information

Type	Version/Name
Distribution Name	NixOS
Distribution Version	20.09
Linux Kernel	Linux xfix 5.4.72 #1-NixOS SMP Sat Oct 17 08:11:24 UTC 2020 x86_64 GNU/Linux
Architecture	x86_64
ZFS Version	0.8.4-1
SPL Version	0.8.4-1

Describe the problem you're observing

Attempting to zfs send -R -i with the exact same snapshot causes an abort. With ZFS 0.8.4:

0 == nvlist_lookup_nvlist(nvfs, "snapprops", &snapprops)
ASSERT at libzfs_sendrecv.c:1382:dump_snapshot()

With ZFS 0.8.5:

0 == nvlist_lookup_nvlist(nvfs, "snapprops", &snapprops)
ASSERT at libzfs_sendrecv.c:1382:dump_snapshot()

With ZFS 2.0.0-rc4:

0 == nvlist_lookup_nvlist(nvfs, "snapprops", &snapprops)
ASSERT at libzfs_sendrecv.c:1050:dump_snapshot()

Describe how to reproduce the problem

Run the following NixOS test. Alternatively create two snapshots and try to zfs send -R -i the older one (attempting that with newer snapshot won't cause an abort).

import <nixpkgs/nixos/tests/make-test-python.nix> {
  machine = {
    networking.hostId = "deadbeef";
    boot.supportedFilesystems = [ "zfs" ];
  };

  testScript = ''
    machine.succeed(
        "truncate -s 512M part",
        "zpool create test-pool $PWD/part",
        "zfs snapshot test-pool@snap1",
        "zfs snapshot test-pool@snap2",
        "set -o pipefail; zfs send -R -i snap1 test-pool@snap1 | wc -c",
    )
  '';
}

Include any warning/errors/backtraces from the system logs