[Feature] Basic systemd service monitoring (#1153)

* basic systemd service monitoring

* update to work after /internal rename

* monitor systemd service cpu and memory usage

---------

Co-authored-by: henrygd <hank@henrygd.me>

agent/systemd.go

@@ -0,0 +1,107 @@
+//go:build linux
+
+package agent
+
+import (
+	"context"
+	"log/slog"
+	"math"
+	"strings"
+	"sync"
+
+	"github.com/coreos/go-systemd/v22/dbus"
+	"github.com/henrygd/beszel/internal/entities/systemd"
+)
+
+// systemdManager manages the collection of systemd service statistics.
+type systemdManager struct {
+	conn            *dbus.Conn
+	serviceStatsMap map[string]*systemd.Service
+	mu              sync.Mutex
+}
+
+// newSystemdManager creates a new systemdManager.
+func newSystemdManager() (*systemdManager, error) {
+	conn, err := dbus.New()
+	if err != nil {
+		if strings.Contains(err.Error(), "permission denied") {
+			slog.Error("Permission denied when connecting to systemd. Run as root or with appropriate user permissions.", "err", err)
+			return nil, err
+		}
+		slog.Error("Error connecting to systemd", "err", err)
+		return nil, err
+	}
+
+	return &systemdManager{
+		conn:            conn,
+		serviceStatsMap: make(map[string]*systemd.Service),
+	}, nil
+}
+
+// getServiceStats collects statistics for all running systemd services.
+func (sm *systemdManager) getServiceStats() []*systemd.Service {
+	units, err := sm.conn.ListUnitsContext(context.Background())
+	if err != nil {
+		slog.Error("Error listing systemd units", "err", err)
+		return nil
+	}
+
+	var services []*systemd.Service
+	for _, unit := range units {
+		if strings.HasSuffix(unit.Name, ".service") {
+			service := sm.updateServiceStats(unit)
+			services = append(services, service)
+		}
+	}
+	return services
+}
+
+// updateServiceStats updates the statistics for a single systemd service.
+func (sm *systemdManager) updateServiceStats(unit dbus.UnitStatus) *systemd.Service {
+	sm.mu.Lock()
+	defer sm.mu.Unlock()
+
+	props, err := sm.conn.GetUnitTypeProperties(unit.Name, "Service")
+	if err != nil {
+		slog.Debug("could not get unit type properties", "unit", unit.Name, "err", err)
+		return &systemd.Service{
+			Name:   unit.Name,
+			Status: unit.ActiveState,
+		}
+	}
+
+	var cpuUsage uint64
+	if val, ok := props["CPUUsageNSec"]; ok {
+		if v, ok := val.(uint64); ok {
+			cpuUsage = v
+		}
+	}
+
+	var memUsage uint64
+	if val, ok := props["MemoryCurrent"]; ok {
+		if v, ok := val.(uint64); ok {
+			memUsage = v
+		}
+	}
+
+	service, exists := sm.serviceStatsMap[unit.Name]
+	if !exists {
+		service = &systemd.Service{
+			Name:   unit.Name,
+			Status: unit.ActiveState,
+		}
+		sm.serviceStatsMap[unit.Name] = service
+	}
+
+	service.Status = unit.ActiveState
+
+	// If memUsage is MaxUint64 the api is saying it's not available, return 0
+	if memUsage == math.MaxUint64 {
+		memUsage = 0
+	}
+
+	service.Mem = float64(memUsage) / (1024 * 1024) // Convert to MB
+	service.CalculateCPUPercent(cpuUsage)
+
+	return service
+}