Add _summary attribute as a human-readable description of Disko configs

[Zocker1999NET]

For context: I am creating a NixOS installer UI, allowing the user to select which from all configurations a flake declares, to install on any computer. In some cases, those configurations differ in their disko configuration. So I would like to show a summary of a selected NixOS / disko configuration so users can differentiate the configurations provided in their or others flakes. But I think such a summary could be helpful to others as well.

What I was thinking was a new option, which is built recursively (comparable to the current _create option), but which provides a human-readable description of what will be installed on the disk. To give some examples based on disko’s examples:

• simple-efi.nix:

  disks:
    "main" (/dev/disk/by-id/some-disk-id): GUID Partition Table
      500M "ESP": vfat filesystem for /boot
      100% "root": ext4 filesystem for /
  

• swap.nix

  disks:
    "main" (/dev/vdb): GUID Partition Table
      500M "ESP": vfat filesystem for /boot
      ends -1G "root": ext4 filesystem for /
      10M "encryptedSwap": swap, encrypted, priority=100
      100% "plainSwap": swap, discardPolicy=both, resumeDevice
  

• lvm-raid.nix

  disks:
    "one" (/dev/sda): GUID Partition Table
      500M "boot": assigned to mdraid "boot"
      100% "primary": LVM PV assigned to VG "pool"
    "two" (/dev/sdb): GUID Partition Table
      500M "boot": assigned to mdraid "boot"
      100% "primary": LVM PV assigned to VG "pool"
  mdadm:
    "boot", RAID 1, metadata 1.0: vfat filesystem for /boot
  LVM VGs:
    "pool":
      100M "root", mirror: ext4 filesystem for /
      10M "home", raid0: ext4 filesystem for /home
  

While coming up with my examples, I decided for the structure to be <container info>: <content info>, where content info can provide further lines for subcontainers. This structure can be discussed through. In general, I think important requirements on such a summary are:

• be concise
  • avoid technical details while providing all important info
  • use less lines that the (formatted) Nix code
• represent structural division and technologies used
  • how many disks are required? how many partitions are created?
  • how much storage will be made available to each partition/filesystem?
  • are LVM / mdadm / ZFS used (or possibly even combined)?
  • which filesystems are used for what mountpoints?

I created this issue because I think this could be helpful in other use cases (or installer realizations) as well, so I think something like that could be integrated into disko itself. And also because I currently have not enough experience with disko’s code base to built something like that myself (in the hope that someone else is faster than me).

[Mic92]

We are in fact also creating an installer UI for NixOS in https://git.clan.lol/clan/clan-core
However we are at the moment only using pre-defined templates where users just provide the device to install to, but we are interested what pattern you come up with.