surrender

nix/module.nix

@@ -2,6 +2,14 @@
 
 let
   cfg = config.services.kotsukotsu;
+  launcher = pkgs.writeShellScript "kotsukotsu-launch" ''
+    exec ${cfg.currentPath}/bin/kotsukotsu
+  '';
+  ensureCurrent = pkgs.writeShellScript "kotsukotsu-ensure-current" ''
+    if [ ! -e ${cfg.currentPath} ]; then
+      ln -sfn ${cfg.package} ${cfg.currentPath}
+    fi
+  '';
 in
 {
   options.services.kotsukotsu = {
@@ -38,6 +46,12 @@ in
       description = "Directory for the persistent SQLite database.";
     };
 
+    currentPath = lib.mkOption {
+      type = lib.types.str;
+      default = "/var/lib/kotsukotsu/current";
+      description = "Stable symlink path that points at the active app package.";
+    };
+
     user = lib.mkOption {
       type = lib.types.str;
       default = "kotsukotsu";
@@ -71,6 +85,10 @@ in
         assertion = lib.hasPrefix "/" cfg.dataDir;
         message = "services.kotsukotsu.dataDir must be an absolute path.";
       }
+      {
+        assertion = lib.hasPrefix "/" cfg.currentPath;
+        message = "services.kotsukotsu.currentPath must be an absolute path.";
+      }
     ];
 
     users.groups = lib.mkIf (cfg.group == "kotsukotsu") {
@@ -104,7 +122,8 @@ in
       };
 
       serviceConfig = {
-        ExecStart = lib.getExe cfg.package;
+        ExecStart = launcher;
+        ExecStartPre = ensureCurrent;
         Group = cfg.group;
         Restart = "on-failure";
         User = cfg.user;