feat: implement X-RestartIfChanged=false in service activation

We parse the directive from the unit file text and skip restarting the service if X-RestartIfChanged=false is set

required for #436

Author: jfroche

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

@@ -22,6 +22,12 @@ pub struct ServiceConfig {
 
 pub type Services = HashMap<String, ServiceConfig>;
 
+fn unit_has_directive(store_path: &StorePath, directive: &str) -> bool {
+    fs::read_to_string(&store_path.store_path)
+        .map(|content| content.contains(directive))
+        .unwrap_or(false)
+}
+
 fn print_services(services: &Services) -> String {
     let out = itertools::intersperse(
         services.iter().map(|(name, entry)| {
@@ -134,7 +140,16 @@ 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 let Some(ref path) = service.store_path {
+                if unit_has_directive(path, "X-RestartIfChanged=false") {
+                    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!");

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}"
+    '';
+}