systemd: avoid iteration into unit list using UnitState

To avoid an additional iteration of unit states for each unit, set
UnitState.ActiveEnterTimestamp accordingly, by calling
m.systemd.GetUnitProperty(). Note that we need to retrieve the property
not only in the normal loop for the current systemd (>=230), but also
in the fallback handling for the older version of systemd (<=229).

Author: Dongsu Park

systemd/manager.go

@@ -174,9 +174,10 @@ func (m *systemdUnitManager) getUnitState(name string) (*unit.UnitState, error)
 		return nil, err
 	}
 	us := unit.UnitState{
-		LoadState:   info["LoadState"].(string),
-		ActiveState: info["ActiveState"].(string),
-		SubState:    info["SubState"].(string),
+		LoadState:            info["LoadState"].(string),
+		ActiveState:          info["ActiveState"].(string),
+		SubState:             info["SubState"].(string),
+		ActiveEnterTimestamp: info["ActiveEnterTimestamp"].(uint64),
 	}
 	return &us, nil
 }
@@ -238,6 +239,9 @@ func (m *systemdUnitManager) GetUnitStates(filter pkg.Set) (map[string]*unit.Uni
 		if h, ok := m.hashes[dus.Name]; ok {
 			us.UnitHash = h.String()
 		}
+
+		us.ActiveEnterTimestamp = m.getActiveEnterTimestamp(dus.Name)
+
 		states[dus.Name] = us
 	}
 
@@ -256,19 +260,11 @@ func (m *systemdUnitManager) GetUnitStates(filter pkg.Set) (map[string]*unit.Uni
 			if h, ok := m.hashes[name]; ok {
 				us.UnitHash = h.String()
 			}
-			states[name] = us
-		}
-	}
 
-	// add Active enter time to UnitState
-	for name, us := range states {
-		prop, err := m.systemd.GetUnitProperty(name, "ActiveEnterTimestamp")
-		if err != nil {
-			return nil, err
-		}
+			us.ActiveEnterTimestamp = m.getActiveEnterTimestamp(name)
 
-		us.ActiveEnterTimestamp = prop.Value.Value().(uint64)
-		states[name] = us
+			states[name] = us
+		}
 	}
 
 	return states, nil
@@ -328,6 +324,14 @@ func (m *systemdUnitManager) getUnitFilePath(name string) string {
 	return path.Join(m.unitsDir, name)
 }
 
+func (m *systemdUnitManager) getActiveEnterTimestamp(name string) (aTimestamp uint64) {
+	prop, err := m.systemd.GetUnitProperty(name, "ActiveEnterTimestamp")
+	if err != nil {
+		return 0
+	}
+	return prop.Value.Value().(uint64)
+}
+
 func lsUnitsDir(dir string) ([]string, error) {
 	filterFunc := func(name string) bool {
 		if !unit.RecognizedUnitType(name) {