fix(agent): verify systemd on dbus before starting service monitoring (#2573)

agent/systemd.go

@@ -114,6 +114,13 @@ func newSystemdManager() (*systemdManager, error) {
 		return nil, err
 	}
 
+	// dbus may be present without systemd (e.g. runit, OpenRC), so verify systemd is on the bus (#2555)
+	if _, err := conn.GetManagerProperty("Version"); err != nil {
+		conn.Close()
+		slog.Debug("Systemd not available on dbus", "err", err)
+		return nil, nil
+	}
+
 	manager := &systemdManager{
 		serviceStatsMap: make(map[string]*systemd.Service),
 		logsEnabled:     systemdLogsEnabled(),

agent/systemd_test.go

@@ -180,10 +180,13 @@ func TestIsSystemdAvailable(t *testing.T) {
 	// On systems without systemd, it should return false
 	result := isSystemdAvailable()
 
-	// Check if either the /run/systemd/system directory exists or PID 1 is systemd
-	runSystemdExists := false
-	if _, err := os.Stat("/run/systemd/system"); err == nil {
-		runSystemdExists = true
+	// Check if the systemd directory or a dbus socket exists, or PID 1 is systemd
+	pathExists := false
+	for _, path := range []string{"/run/systemd/system", "/run/dbus/system_bus_socket", "/var/run/dbus/system_bus_socket"} {
+		if _, err := os.Stat(path); err == nil {
+			pathExists = true
+			break
+		}
 	}
 
 	pid1IsSystemd := false
@@ -191,7 +194,7 @@ func TestIsSystemdAvailable(t *testing.T) {
 		pid1IsSystemd = strings.TrimSpace(string(data)) == "systemd"
 	}
 
-	expected := runSystemdExists || pid1IsSystemd
+	expected := pathExists || pid1IsSystemd
 
 	assert.Equal(t, expected, result, "isSystemdAvailable should correctly detect systemd presence")