Add new activation strategy `boot` as equivalent to `nixos-rebuild boot`

This can be useful when e.g. deploying a kernel update to a target host.
You usually plan a reboot (or kexec) after that to activate the new
kernel. However you don't want to wait for services to be restarted
first since these will be "restarted" anyways on the reboot. In cases
like GitLab or the Atlassian stack this actually makes a difference.

This patch changes the following things:

* If `--boot` is provided, `nix-env -p profile-to-activate --set` is
  called for each deployed profile to make sure that it is activated
  automatically after a reboot.

* However, the actual activation (e.g. `switch-to-configuration switch`)
  is skipped. Instead:

  * For NixOS, `switch-to-configuration boot` is called to set the new
    profile as default in the bootloader.

  * For everything else, nothing else is done. The profile is already
    the new default (and thus picked up on the next boot).

1 files changed, 20 insertions, 11 deletions

diff --git a/flake.nix b/flake.nix
index 66ba60b..2d520b6 100644
--- a/flake.nix
+++ b/flake.nix
@@ -63,6 +63,9 @@
                             if [[ "''${DRY_ACTIVATE:-}" == "1" ]]
                             then
                                 ${customSelf.dryActivate or "echo ${final.writeScript "activate" activate}"}
+                            elif [[ "''${BOOT:-}" == "1" ]]
+                            then
+                                ${customSelf.boot or "echo ${final.writeScript "activate" activate}"}
                             else
                                 ${activate}
                             fi
@@ -83,17 +86,23 @@
                   };
               };
 
-            nixos = base: (custom // { dryActivate = "$PROFILE/bin/switch-to-configuration dry-activate"; }) base.config.system.build.toplevel ''
-              # work around https://github.com/NixOS/nixpkgs/issues/73404
-              cd /tmp
-
-              $PROFILE/bin/switch-to-configuration switch
-
-              # https://github.com/serokell/deploy-rs/issues/31
-              ${with base.config.boot.loader;
-              final.lib.optionalString systemd-boot.enable
-              "sed -i '/^default /d' ${efi.efiSysMountPoint}/loader/loader.conf"}
-            '';
+            nixos = base:
+              (custom // {
+                dryActivate = "$PROFILE/bin/switch-to-configuration dry-activate";
+                boot = "$PROFILE/bin/switch-to-configuration boot";
+              })
+              base.config.system.build.toplevel
+              ''
+                # work around https://github.com/NixOS/nixpkgs/issues/73404
+                cd /tmp
+
+                $PROFILE/bin/switch-to-configuration switch
+
+                # https://github.com/serokell/deploy-rs/issues/31
+                ${with base.config.boot.loader;
+                final.lib.optionalString systemd-boot.enable
+                "sed -i '/^default /d' ${efi.efiSysMountPoint}/loader/loader.conf"}
+              '';
 
             home-manager = base: custom base.activationPackage "$PROFILE/activate";