hetzner: installing grub fails if /dev/sda isn't used

[nh2]

With nixops 1.5.2 I tried this:

    deployment.hetzner.partitions = ''
      clearpart --all --initlabel --drives=sda,sdb,sdc,sdd

      part raid.1 --grow --ondisk=sdc
      part raid.2 --grow --ondisk=sdd

      raid / --level=1 --device=md0 --fstype=ext4 --label=root raid.1 raid.2
    '';

doing nothing with device sda, and got:

machine1> installing the GRUB 2 boot loader on /dev/sda...
machine1> Installing for i386-pc platform.
machine1> /nix/store/67nrc9srnpab6x3zrlprqi4al74pkb0k-grub-2.02/sbin/grub-install: error: unable to identify a filesystem in hostdisk//dev/sda; safety check can't be performed.
machine1> /nix/store/yvg8cplwgj0z8j2x4fdix4bjwgcmwi8h-install-grub.pl: installation of GRUB on /dev/sda failed

CC @aszlig

[nh2]

I think the issue is that nixpart always includes all disks in boot.loader.grub.devices, no matter if they are used or not:

root@rescue ~ # cat kickstart 
clearpart --all --initlabel --drives=sdc,sdd

part raid.1 --grow --ondisk=sdc
part raid.2 --grow --ondisk=sdd

raid / --level=1 --device=md0 --fstype=ext4 --label=root raid.1 raid.2

root@rescue ~ # nixpart -p - < kickstart 
{
  swapDevices = [
  ];
  boot.loader.grub.devices = [
    "/dev/sda"
    "/dev/sdb"
    "/dev/sdc"
    "/dev/sdd"
  ];
  fileSystems = {
    "/" = {
      fsType = "ext4";
      label = "root";
    };
  };
}

[aszlig]

@nh2: This is probably due to https://github.com/NixOS/nixpart/blob/v0.4.1/nixpart#L59, so it needs to check whether it's actually a device that is going to be partitioned.

[aszlig]

@nh2: Can you dump what the repr of device.format is when you have this setup?