Magic Boot Rollback

[MichailiK]

Related to #32.

For systems where you may not have easy access a system that fails to boot (like a VPS or remote server) it would be neat if wire could apply a new configuration for the next boot only.

This would require telling the bootloader to boot the new boot entry once this for us, and depends on the bootloader:

• systemd-boot users can use bootctl set-oneshot <bootloader entry> as well as systemctl reboot --boot-loader-entry=<bootloader entry>
• GRUB users can use grub-reboot <index of bootlooader entry>
• For other bootloaders, users could specify their own script to run.

Some implementation details would need to be considered, e.g. how do we determine which bootloader the system uses? how do we determine the correct bootloader entry? how do we default to the previous generation? what should happen if setting the boot entry fails? perhaps specializations could be useful for this feature?

[itslychee]

For systemd-boot we can use a simple command to determine that.
bootctl list --json=pretty

[
        {
                "type" : "type1",
                "id" : "nixos-generation-39.conf",
                "path" : "/boot/loader/entries/nixos-generation-39.conf",
                "root" : "/boot",
                "title" : "NixOS",
                "showTitle" : "NixOS (Generation 39 NixOS Vicuna 24.11pre-git (Linux 6.6.84), built on 2025-03-28)",
                "sortKey" : "nixos",
                "version" : "Generation 39 NixOS Vicuna 24.11pre-git (Linux 6.6.84), built on 2025-03-28",
                "machineId" : "85c7da83411b4809b2fef5c60e17ca8c",
                "options" : "init=/nix/store/gd0apisybhgq8x57q3lp1dmskpm1x3qc-nixos-system-hearth-24.11pre-git/init root=fstab loglevel=4",
                "linux" : "/EFI/nixos/2h4mdp6ql8rzdq7k4a5a1kism0ha3a3x-linux-6.6.84-bzImage.efi",
                "initrd" : [
                        "/EFI/nixos/djmhc69n1xcriqjjmj9zdl56zk70g2jw-initrd-linux-6.6.84-initrd.efi"
                ],
                "isReported" : true,
                "isDefault" : true,
                "isSelected" : true,
                "addons" : null,
                "cmdline" : "init=/nix/store/gd0apisybhgq8x57q3lp1dmskpm1x3qc-nixos-system-hearth-24.11pre-git/init root=fstab loglevel=4"
        },
        {
                "type" : "type1",
                "id" : "nixos-generation-38.conf",
                "path" : "/boot/loader/entries/nixos-generation-38.conf",
                "root" : "/boot",
                "title" : "NixOS",
                "showTitle" : "NixOS (Generation 38 NixOS Vicuna 24.11pre-git (Linux 6.6.83), built on 2025-03-23)",
                "sortKey" : "nixos",
                "version" : "Generation 38 NixOS Vicuna 24.11pre-git (Linux 6.6.83), built on 2025-03-23",
                "machineId" : "85c7da83411b4809b2fef5c60e17ca8c",
                "options" : "init=/nix/store/x6kl09nz7w6898khla0y8c2fvgy6dadq-nixos-system-hearth-24.11pre-git/init root=fstab loglevel=4",
                "linux" : "/EFI/nixos/23kpj1bc2zqifnyjal8rllm9z49zffyq-linux-6.6.83-bzImage.efi",
                "initrd" : [
                        "/EFI/nixos/fazak12cjyr8v60a29j42d6wcagihvk2-initrd-linux-6.6.83-initrd.efi"
                ],
                "isReported" : true,
                "isDefault" : false,
                "isSelected" : false,
                "addons" : null,
                "cmdline" : "init=/nix/store/x6kl09nz7w6898khla0y8c2fvgy6dadq-nixos-system-hearth-24.11pre-git/init root=fstab loglevel=4"
        },
        {
                "type" : "type1",
                "id" : "nixos-generation-37.conf",
                "path" : "/boot/loader/entries/nixos-generation-37.conf",
                "root" : "/boot",
                "title" : "NixOS",
                "showTitle" : "NixOS (Generation 37 NixOS Vicuna 24.11pre-git (Linux 6.6.83), built on 2025-03-23)",
                "sortKey" : "nixos",
                "version" : "Generation 37 NixOS Vicuna 24.11pre-git (Linux 6.6.83), built on 2025-03-23",
                "machineId" : "85c7da83411b4809b2fef5c60e17ca8c",
                "options" : "init=/nix/store/r6kp2gzksr6phhw8z9lmk0sm0qnjm448-nixos-system-hearth-24.11pre-git/init root=fstab loglevel=4",
                "linux" : "/EFI/nixos/23kpj1bc2zqifnyjal8rllm9z49zffyq-linux-6.6.83-bzImage.efi",
                "initrd" : [
                        "/EFI/nixos/fazak12cjyr8v60a29j42d6wcagihvk2-initrd-linux-6.6.83-initrd.efi"
                ],
                "isReported" : true,
                "isDefault" : false,
lines 1-60

Unfortunately GRUB doesn't provide a good interface like systemd-boot does

[mrshmllow]

How is this different from wire apply boot?

[itslychee]

I imagine systemd boot will revert to the previous boot if a gen fails to boot

[MichailiK]

How is this different from wire apply boot?

If the system fails to boot (e.g. updated kernel causes panic on boot,) reverting to the previous generation can be difficult if you dont have direct access to the machine (think VPS or a remote server,) thus locking yourself out of that system. Making the bootloader boot into the new generation only once & defaulting to the previous generation should prevent this scenario.

[itslychee]

@mrshmllow did we decide if we're going to support this or not?

[mrshmllow]

I don't know if it's entirely possible