feat: implement X-RestartIfChanged=false in service activation

Prevents the engine from restarting services like the auto-upgrade service mid-execution when their unit files change.

Author: jfroche

crates/system-manager-engine/src/activate/services.rs

@@ -18,10 +18,16 @@ pub struct ServiceConfig {
     store_path: Option<StorePath>,
     #[serde(default)]
     masked: bool,
+    #[serde(default = "default_restart_if_changed")]
+    restart_if_changed: bool,
 }
 
 pub type Services = HashMap<String, ServiceConfig>;
 
+const fn default_restart_if_changed() -> bool {
+    true
+}
+
 fn print_services(services: &Services) -> String {
     let out = itertools::intersperse(
         services.iter().map(|(name, entry)| {
@@ -134,7 +140,14 @@ fn get_services_to_reload(services: Services, old_services: Services) -> Service
             return false;
         }
         if let Some(old_service) = old_services.get(name) {
-            service.store_path != old_service.store_path
+            if service.store_path == old_service.store_path {
+                return false;
+            }
+            if !service.restart_if_changed {
+                log::info!("Skipping restart of {name}: X-RestartIfChanged=false");
+                return false;
+            }
+            true
         } else {
             // Since we run this on the intersection, this should never happen
             panic!("Something went terribly wrong!");

nix/modules/default.nix

@@ -335,10 +335,15 @@
 
           activeServices = lib.mapAttrs' (
             unitName: unit:
-            lib.nameValuePair unitName {
-              storePath = "${unit.unit}/${unitName}";
-              masked = false;
-            }
+            lib.nameValuePair unitName (
+              {
+                storePath = "${unit.unit}/${unitName}";
+                masked = false;
+              }
+              // lib.optionalAttrs (unit ? restartIfChanged) {
+                inherit (unit) restartIfChanged;
+              }
+            )
           ) enabledUnits;
 
           maskedServices = lib.listToAttrs (

testFlake/container-tests/restart-if-changed.nix

@@ -0,0 +1,84 @@
+{
+  forEachDistro,
+  system-manager,
+  system,
+  ...
+}:
+
+let
+  makeConfig =
+    envValue:
+    system-manager.lib.makeSystemConfig {
+      modules = [
+        (
+          { lib, ... }:
+          {
+            nixpkgs.hostPlatform = system;
+            system-manager.allowAnyDistro = true;
+
+            systemd.services.long-running-task = {
+              description = "Long Running Task";
+              wantedBy = [ "system-manager.target" ];
+              restartIfChanged = false;
+              serviceConfig.Type = "simple";
+              serviceConfig.Environment = "FOO=${envValue}";
+              script = ''
+                sleep infinity
+              '';
+            };
+          }
+        )
+      ];
+    };
+
+  configV1 = makeConfig "v1";
+  configV2 = makeConfig "v2";
+in
+
+forEachDistro "restart-if-changed" {
+  modules = [
+    {
+      systemd.services.long-running-task = {
+        description = "Long Running Task";
+        wantedBy = [ "system-manager.target" ];
+        restartIfChanged = false;
+        serviceConfig.Type = "simple";
+        serviceConfig.Environment = "FOO=v1";
+        script = ''
+          sleep infinity
+        '';
+      };
+    }
+  ];
+  extraPathsToRegister = [ configV2 ];
+  testScriptFunction =
+    { toplevel, hostPkgs, ... }:
+    ''
+      start_all()
+
+      machine.wait_for_unit("multi-user.target")
+
+      with subtest("activate configV1 and verify service is running"):
+          activation_logs = machine.activate()
+          machine.wait_for_unit("system-manager.target")
+          result = machine.succeed("systemctl is-active long-running-task.service").strip()
+          assert result == "active", f"Expected active, got {result}"
+
+      with subtest("record initial PID"):
+          pid_before = machine.succeed("systemctl show -p MainPID --value long-running-task.service").strip()
+          assert pid_before != "0", "Service should have a non-zero PID"
+
+      with subtest("activate configV2 with X-RestartIfChanged=false"):
+          activation_logs = machine.activate(profile="${configV2}")
+          machine.wait_for_unit("system-manager.target")
+
+      with subtest("service was not restarted (same PID)"):
+          pid_after = machine.succeed("systemctl show -p MainPID --value long-running-task.service").strip()
+          assert pid_after == pid_before, \
+              f"Service was restarted: PID changed from {pid_before} to {pid_after}"
+
+      with subtest("activation logs show skip message"):
+          assert "Skipping restart" in activation_logs and "long-running-task" in activation_logs, \
+              f"Expected skip message in logs, got: {activation_logs}"
+    '';
+}