Add hydra-auto-upgrade module, still wip

hosts/common/global/auto-upgrade.nix

@@ -0,0 +1,16 @@
+{
+  inputs,
+  config,
+  ...
+}: {
+  system.hydraAutoUpgrade = {
+    # Only enable if not dirty
+    enable = inputs.self ? rev;
+    dates = "*:0/10"; # Every 10 minutes
+    instance = "http://hydra.julian-mutter.de";
+    project = "dotfiles";
+    jobset = "main";
+    job = "hosts.${config.networking.hostName}";
+    oldFlakeRef = "self";
+  };
+}

modules/nixos/hydra-auto-upgrade.nix

@@ -0,0 +1,129 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}: let
+  cfg = config.system.hydraAutoUpgrade;
+in {
+  # Taken from Misterio
+  options = {
+    system.hydraAutoUpgrade = {
+      enable = lib.mkEnableOption "periodic hydra-based auto upgrade";
+      operation = lib.mkOption {
+        type = lib.types.enum ["switch" "boot"];
+        default = "switch";
+      };
+      dates = lib.mkOption {
+        type = lib.types.str;
+        default = "04:40";
+        example = "daily";
+      };
+
+      instance = lib.mkOption {
+        type = lib.types.str;
+        example = "http://hydra.julian-mutter.de";
+      };
+      project = lib.mkOption {
+        type = lib.types.str;
+        example = "dotfiles";
+      };
+      jobset = lib.mkOption {
+        type = lib.types.str;
+        example = "main";
+      };
+      job = lib.mkOption {
+        type = lib.types.str;
+        default = config.networking.hostName;
+      };
+
+      oldFlakeRef = lib.mkOption {
+        type = lib.types.nullOr lib.types.str;
+        default = null;
+        description = ''
+          Current system's flake reference
+
+          If non-null, the service will only upgrade if the new config is newer
+          than this one's.
+        '';
+      };
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    assertions = [
+      {
+        assertion = cfg.enable -> !config.system.autoUpgrade.enable;
+        message = ''
+          hydraAutoUpgrade and autoUpgrade are mutually exclusive.
+        '';
+      }
+    ];
+    systemd.services.nixos-upgrade = {
+      description = "NixOS Upgrade";
+      restartIfChanged = false;
+      unitConfig.X-StopOnRemoval = false;
+      serviceConfig.Type = "oneshot";
+
+      path = with pkgs; [
+        config.nix.package.out
+        config.programs.ssh.package
+        coreutils
+        curl
+        gitMinimal
+        gnutar
+        gzip
+        jq
+        nvd
+      ];
+
+      script = let
+        buildUrl = "${cfg.instance}/job/${cfg.project}/${cfg.jobset}/${cfg.job}/latest";
+      in
+        (lib.optionalString (cfg.oldFlakeRef != null) ''
+          eval="$(curl -sLH 'accept: application/json' "${buildUrl}" | jq -r '.jobsetevals[0]')"
+          flake="$(curl -sLH 'accept: application/json' "${cfg.instance}/eval/$eval" | jq -r '.flake')"
+          echo "New flake: $flake" >&2
+          new="$(nix flake metadata "$flake" --json | jq -r '.lastModified')"
+          echo "Modified at: $(date -d @$new)" >&2
+
+          echo "Current flake: ${cfg.oldFlakeRef}" >&2
+          current="$(nix flake metadata "${cfg.oldFlakeRef}" --json | jq -r '.lastModified')"
+          echo "Modified at: $(date -d @$current)" >&2
+
+          if [ "$new" -le "$current" ]; then
+            echo "Skipping upgrade, not newer" >&2
+            exit 0
+          fi
+        '')
+        + ''
+          profile="/nix/var/nix/profiles/system"
+          path="$(curl -sLH 'accept: application/json' ${buildUrl} | jq -r '.buildoutputs.out.path')"
+
+          if [ "$(readlink -f "$profile")" = "$path" ]; then
+            echo "Already up to date" >&2
+            exit 0
+          fi
+
+          echo "Building $path" >&2
+          nix build --no-link "$path"
+
+          echo "Comparing changes" >&2
+          nvd --color=always diff "$profile" "$path"
+
+          echo "Activating configuration" >&2
+          "$path/bin/switch-to-configuration" test
+
+          echo "Setting profile" >&2
+          nix build --no-link --profile "$profile" "$path"
+
+          echo "Adding to bootloader" >&2
+          "$path/bin/switch-to-configuration" boot
+        '';
+
+      startAt = cfg.dates;
+      after = ["network-online.target"];
+      wants = ["network-online.target"];
+    };
+  };
+}