refactor(scheduler): makes task exclusive policy more readable

This moves part of the logic to separate method to improve readability
and fixes how PostRun cleans up the resources.

pkg/service/scheduler/policy.go

@@ -63,10 +63,24 @@ func (t *TaskExclusiveLockPolicy) PreRun(clusterID, _, _ uuid.UUID, taskType Tas
 
 	cluster, ok := t.running[clusterID]
 	if !ok {
-		t.running[clusterID] = map[TaskType]struct{}{}
+		cluster = map[TaskType]struct{}{}
+		t.running[clusterID] = cluster
 	}
+
+	if err := t.canRunTaskExclusively(cluster, taskType); err != nil {
+		// cluster is busy
+		return err
+	}
+
+	cluster[taskType] = struct{}{}
+
+	return nil
+}
+
+// canRunTaskExclusively returns nil if taskType can be run in the cluster, otherwise err is returned.
+func (t *TaskExclusiveLockPolicy) canRunTaskExclusively(cluster map[TaskType]struct{}, taskType TaskType) error {
+	// No tasks are running, so we can start the task.
 	if len(cluster) == 0 {
-		t.running[clusterID][taskType] = struct{}{}
 		return nil
 	}
 
@@ -85,14 +99,17 @@ func (t *TaskExclusiveLockPolicy) PreRun(clusterID, _, _ uuid.UUID, taskType Tas
 		return errClusterBusy
 	}
 
-	t.running[clusterID][taskType] = struct{}{}
-
 	return nil
 }
 
 // PostRun releases a lock on a cluster for a provided taskType.
 func (t *TaskExclusiveLockPolicy) PostRun(clusterID, _, _ uuid.UUID, taskType TaskType) {
 	t.mu.Lock()
 	defer t.mu.Unlock()
+	// No need to check if t.running[clusterID] exists, because built-in delete will no-op in case of nil map.
 	delete(t.running[clusterID], taskType)
+	// Cleaning up the map if no more tasks left in the cluster.
+	if len(t.running[clusterID]) == 0 {
+		delete(t.running, clusterID)
+	}
 }

pkg/service/scheduler/policy_test.go

@@ -80,7 +80,6 @@ func TestExclusiveTaskLockPolicy(t *testing.T) {
 		if err != nil {
 			t.Fatalf("PreRun: unexpected err: %v", err)
 		}
-
 	})
 
 	t.Run("when exclusive task is running, other tasks are not allowed", func(t *testing.T) {
@@ -160,9 +159,14 @@ func TestExclusiveTaskLockPolicy(t *testing.T) {
 			t.Fatalf("PreRun: expected errClusterBusy, got: %v", err)
 		}
 
-		// Release a lock.
+		// Release a lock and clean up the underlying map.
 		restoreExclusiveTask.PostRun(clusterID, taskID, runID, RestoreTask)
 
+		if _, ok := restoreExclusiveTask.running[clusterID]; ok {
+			t.Fatalf("t.running[clusterID] should be deleted")
+		}
+
+		// Lock can be acquried again.
 		err = restoreExclusiveTask.PreRun(clusterID, taskID, runID, RestoreTask)
 		if err != nil {
 			t.Fatalf("PreRun: unexpected err: %v", err)