add basic systemd service monitoring (#1153)

Co-authored-by: Shelby Tucker <shelby.tucker@gmail.com>

internal/entities/systemd/systemd.go

@@ -0,0 +1,127 @@
+package systemd
+
+import (
+	"math"
+	"runtime"
+	"time"
+)
+
+// ServiceState represents the status of a systemd service
+type ServiceState uint8
+
+const (
+	StatusActive ServiceState = iota
+	StatusInactive
+	StatusFailed
+	StatusActivating
+	StatusDeactivating
+	StatusReloading
+)
+
+// ServiceSubState represents the sub status of a systemd service
+type ServiceSubState uint8
+
+const (
+	SubStateDead ServiceSubState = iota
+	SubStateRunning
+	SubStateExited
+	SubStateFailed
+	SubStateUnknown
+)
+
+// ParseServiceStatus converts a string status to a ServiceStatus enum value
+func ParseServiceStatus(status string) ServiceState {
+	switch status {
+	case "active":
+		return StatusActive
+	case "inactive":
+		return StatusInactive
+	case "failed":
+		return StatusFailed
+	case "activating":
+		return StatusActivating
+	case "deactivating":
+		return StatusDeactivating
+	case "reloading":
+		return StatusReloading
+	default:
+		return StatusInactive
+	}
+}
+
+// ParseServiceSubState converts a string sub status to a ServiceSubState enum value
+func ParseServiceSubState(subState string) ServiceSubState {
+	switch subState {
+	case "dead":
+		return SubStateDead
+	case "running":
+		return SubStateRunning
+	case "exited":
+		return SubStateExited
+	case "failed":
+		return SubStateFailed
+	default:
+		return SubStateUnknown
+	}
+}
+
+// Service represents a single systemd service with its stats.
+type Service struct {
+	Name         string          `json:"n" cbor:"0,keyasint"`
+	State        ServiceState    `json:"s" cbor:"1,keyasint"`
+	Cpu          float64         `json:"c" cbor:"2,keyasint"`
+	Mem          uint64          `json:"m" cbor:"3,keyasint"`
+	MemPeak      uint64          `json:"mp" cbor:"4,keyasint"`
+	Sub          ServiceSubState `json:"ss" cbor:"5,keyasint"`
+	CpuPeak      float64         `json:"cp" cbor:"6,keyasint"`
+	PrevCpuUsage uint64          `json:"-"`
+	PrevReadTime time.Time       `json:"-"`
+}
+
+// UpdateCPUPercent calculates the CPU usage percentage for the service.
+func (s *Service) UpdateCPUPercent(cpuUsage uint64) {
+	now := time.Now()
+
+	if s.PrevReadTime.IsZero() || cpuUsage < s.PrevCpuUsage {
+		s.Cpu = 0
+		s.PrevCpuUsage = cpuUsage
+		s.PrevReadTime = now
+		return
+	}
+
+	duration := now.Sub(s.PrevReadTime).Nanoseconds()
+	if duration <= 0 {
+		s.PrevCpuUsage = cpuUsage
+		s.PrevReadTime = now
+		return
+	}
+
+	coreCount := int64(runtime.NumCPU())
+	duration *= coreCount
+
+	usageDelta := cpuUsage - s.PrevCpuUsage
+	cpuPercent := float64(usageDelta) / float64(duration)
+	s.Cpu = twoDecimals(cpuPercent * 100)
+
+	if s.Cpu > s.CpuPeak {
+		s.CpuPeak = s.Cpu
+	}
+
+	s.PrevCpuUsage = cpuUsage
+	s.PrevReadTime = now
+}
+
+func twoDecimals(value float64) float64 {
+	return math.Round(value*100) / 100
+}
+
+// ServiceDependency represents a unit that the service depends on.
+type ServiceDependency struct {
+	Name        string `json:"name"`
+	Description string `json:"description,omitempty"`
+	ActiveState string `json:"activeState,omitempty"`
+	SubState    string `json:"subState,omitempty"`
+}
+
+// ServiceDetails contains extended information about a systemd service.
+type ServiceDetails map[string]any