1 files changed, 31 insertions, 2 deletions
diff --git a/nixos/modules/services/continuous-integration/hydra/default.nix b/nixos/modules/services/continuous-integration/hydra/default.nix
index 8b56207590a..0c335f14f78 100644
--- a/nixos/modules/services/continuous-integration/hydra/default.nix
+++ b/nixos/modules/services/continuous-integration/hydra/default.nix
@@ -37,6 +37,8 @@ let
 
   haveLocalDB = cfg.dbi == localDB;
 
+  inherit (config.system) stateVersion;
+
 in
 
 {
@@ -63,8 +65,7 @@ in
       };
 
       package = mkOption {
-        type = types.path;
-        default = pkgs.hydra;
+        type = types.package;
         defaultText = "pkgs.hydra";
         description = "The Hydra package.";
       };
@@ -194,6 +195,34 @@ in
 
   config = mkIf cfg.enable {
 
+    warnings = optional (cfg.package.migration or false) ''
+      You're currently deploying an older version of Hydra which is needed to
+      make some required database changes[1]. As soon as this is done, it's recommended
+      to run `hydra-backfill-ids` and set `services.hydra.package` to either `pkgs.hydra-unstable`
+      or `pkgs.hydra-flakes` after that.
+
+      [1] https://github.com/NixOS/hydra/pull/711
+    '';
+
+    services.hydra.package = with pkgs;
+      mkDefault (
+        if pkgs ? hydra
+          then throw ''
+            The Hydra package doesn't exist anymore in `nixpkgs`! It probably exists
+            due to an overlay. To upgrade Hydra, you need to take two steps as some
+            bigger changes in the database schema were implemented recently[1]. You first
+            need to deploy `pkgs.hydra-migration`, run `hydra-backfill-ids` on the server
+            and then deploy either `pkgs.hydra-unstable` or `pkgs.hydra-flakes`.
+
+            If you want to use `pkgs.hydra` from your overlay, please set `services.hydra.package`
+            explicitly to `pkgs.hydra` and make sure you know what you're doing.
+
+            [1] https://github.com/NixOS/hydra/pull/711
+          ''
+        else if versionOlder stateVersion "20.03" then hydra-migration
+        else hydra-unstable
+      );
+
     users.groups.hydra = {
       gid = config.ids.gids.hydra;
     };