lib.modules.systemd: generate timer unit when startAt is set

When the startAt option is used, NixOS generates a corresponding .timer
unit. Include it in the outputs alongside the .service file using
symlinkJoin.

Author: Lassulus

checks/systemd.nix

@@ -142,8 +142,31 @@ let
     }
   );
 
+  # Test 7: startAt generates a timer
+  withTimer = self.lib.wrapModule (
+    {
+      config,
+      lib,
+      wlib,
+      ...
+    }:
+    {
+      imports = [ wlib.modules.systemd ];
+      config = {
+        pkgs = pkgs;
+        package = pkgs.hello;
+        systemd = {
+          serviceConfig.Type = "oneshot";
+          startAt = "hourly";
+        };
+      };
+    }
+  );
+
   readUserService = drv: name: builtins.readFile "${drv}/systemd/user/${name}.service";
   readSystemService = drv: name: builtins.readFile "${drv}/systemd/system/${name}.service";
+  readUserTimer = drv: name: builtins.readFile "${drv}/systemd/user/${name}.timer";
+  readSystemTimer = drv: name: builtins.readFile "${drv}/systemd/system/${name}.timer";
 in
 pkgs.runCommand "systemd-test" { } ''
   echo "Testing systemd module..."
@@ -213,6 +236,17 @@ pkgs.runCommand "systemd-test" { } ''
   echo "$hooks" | grep -q 'ExecStopPost=.*hello-post-stop' || { echo "FAIL: postHook not mapped to ExecStopPost"; echo "$hooks"; exit 1; }
   echo "PASS: exePath, extraPackages, preHook, postHook"
 
+  # Test 7: startAt generates a timer
+  echo "Test 7: startAt generates a timer"
+  timerSvc='${readUserService withTimer.outputs.systemd-user "hello"}'
+  echo "$timerSvc" | grep -q 'ExecStart=.*/bin/hello' || { echo "FAIL: service missing ExecStart"; echo "$timerSvc"; exit 1; }
+  timer='${readUserTimer withTimer.outputs.systemd-user "hello"}'
+  echo "$timer" | grep -q 'OnCalendar=hourly' || { echo "FAIL: timer missing OnCalendar"; echo "$timer"; exit 1; }
+  echo "$timer" | grep -q 'WantedBy=timers.target' || { echo "FAIL: timer missing WantedBy"; echo "$timer"; exit 1; }
+  systemTimer='${readSystemTimer withTimer.outputs.systemd-system "hello"}'
+  echo "$systemTimer" | grep -q 'OnCalendar=hourly' || { echo "FAIL: system timer missing OnCalendar"; echo "$systemTimer"; exit 1; }
+  echo "PASS: startAt generates a timer"
+
   echo "SUCCESS: All systemd tests passed"
   touch $out
 ''

lib/modules/systemd.nix

@@ -8,7 +8,7 @@ let
 
   serviceName = builtins.unsafeDiscardStringContext (config.binName or "unknown");
 
-  mkService =
+  mkNixosEval =
     type:
     let
       nixosPath =
@@ -23,29 +23,77 @@ let
             "systemd"
             "services"
           ];
+    in
+    import (config.pkgs.path + "/nixos/lib/eval-config.nix") {
+      inherit lib;
+      system = null;
+      modules = [
+        { config.nixpkgs.hostPlatform = config.pkgs.stdenv.hostPlatform.system; }
+        { config.systemd.globalEnvironment = lib.mkForce { }; }
+        { config = lib.setAttrByPath nixosPath { ${serviceName} = cfg; }; }
+      ];
+    };
 
-      nixos = import (config.pkgs.path + "/nixos/lib/eval-config.nix") {
-        inherit lib;
-        system = null;
-        modules = [
-          { config.nixpkgs.hostPlatform = config.pkgs.stdenv.hostPlatform.system; }
-          { config.systemd.globalEnvironment = lib.mkForce { }; }
-          { config = lib.setAttrByPath nixosPath { ${serviceName} = cfg; }; }
-        ];
-      };
-
-      utils = import (config.pkgs.path + "/nixos/lib/utils.nix") {
-        inherit lib;
-        inherit (config) pkgs;
-        inherit (nixos) config;
-      };
-
-      evaluated = lib.getAttrFromPath nixosPath nixos.config;
-      unit = utils.systemdUtils.lib.serviceToUnit evaluated.${serviceName};
+  mkUtils =
+    nixos:
+    import (config.pkgs.path + "/nixos/lib/utils.nix") {
+      inherit lib;
+      inherit (config) pkgs;
+      inherit (nixos) config;
+    };
+
+  mkOutputs =
+    type:
+    let
+      nixos = mkNixosEval type;
+      utils = mkUtils nixos;
+
+      servicePath =
+        if type == "user" then
+          [
+            "systemd"
+            "user"
+            "services"
+          ]
+        else
+          [
+            "systemd"
+            "services"
+          ];
+
+      timerPath =
+        if type == "user" then
+          [
+            "systemd"
+            "user"
+            "timers"
+          ]
+        else
+          [
+            "systemd"
+            "timers"
+          ];
 
       unitDir = if type == "user" then "systemd/user" else "systemd/system";
+
+      evaluatedService = (lib.getAttrFromPath servicePath nixos.config).${serviceName};
+      serviceUnit = utils.systemdUtils.lib.serviceToUnit evaluatedService;
+
+      evaluatedTimers = lib.getAttrFromPath timerPath nixos.config;
+      hasTimer = evaluatedTimers ? ${serviceName};
+      timerUnit = utils.systemdUtils.lib.timerToUnit evaluatedTimers.${serviceName};
     in
-    config.pkgs.writeTextDir "${unitDir}/${serviceName}.service" unit.text;
+    {
+      service = config.pkgs.writeTextDir "${unitDir}/${serviceName}.service" serviceUnit.text;
+      timer =
+        if hasTimer then
+          config.pkgs.writeTextDir "${unitDir}/${serviceName}.timer" timerUnit.text
+        else
+          null;
+    };
+
+  userOutputs = mkOutputs "user";
+  systemOutputs = mkOutputs "system";
 in
 {
   _file = "lib/modules/systemd.nix";
@@ -61,6 +109,9 @@ in
       ExecStart, Environment, PATH, preStart and postStop are set from the
       wrapper by default.
 
+      If startAt is set, a corresponding .timer unit is generated alongside
+      the service.
+
       The generated unit files are available at outputs.systemd-user and
       outputs.systemd-system.
     '';
@@ -80,14 +131,34 @@ in
   options.outputs.systemd-user = lib.mkOption {
     type = lib.types.package;
     readOnly = true;
-    description = "The generated systemd user service file.";
-    default = mkService "user";
+    description = "The generated systemd user service file (and timer if startAt is set).";
+    default =
+      if userOutputs.timer != null then
+        config.pkgs.symlinkJoin {
+          name = "${serviceName}-user-units";
+          paths = [
+            userOutputs.service
+            userOutputs.timer
+          ];
+        }
+      else
+        userOutputs.service;
   };
 
   options.outputs.systemd-system = lib.mkOption {
     type = lib.types.package;
     readOnly = true;
-    description = "The generated systemd system service file.";
-    default = mkService "system";
+    description = "The generated systemd system service file (and timer if startAt is set).";
+    default =
+      if systemOutputs.timer != null then
+        config.pkgs.symlinkJoin {
+          name = "${serviceName}-system-units";
+          paths = [
+            systemOutputs.service
+            systemOutputs.timer
+          ];
+        }
+      else
+        systemOutputs.service;
   };
 }