daemon/job/active: fix race in updateTasks

If concurrent updates strictly modify *different* members of the tasks
struct, the copying + lock-drop still constitutes a race condition:
The last updater always wins and sets tasks to its copy + changes.
This eliminates the other updater's changes.

daemon/job/active.go

@@ -43,16 +43,14 @@ type activeSideTasks struct {
 
 func (a *ActiveSide) updateTasks(u func(*activeSideTasks)) activeSideTasks {
 	a.tasksMtx.Lock()
+	defer a.tasksMtx.Unlock()
 	var copy activeSideTasks
 	copy = a.tasks
-	a.tasksMtx.Unlock()
 	if u == nil {
 		return copy
 	}
 	u(&copy)
-	a.tasksMtx.Lock()
 	a.tasks = copy
-	a.tasksMtx.Unlock()
 	return copy
 }