store/tikv: remove use of TaskID transaction option in store/tikv

store/driver/txn/snapshot.go

@@ -73,6 +73,8 @@ func (s *tikvSnapshot) SetOption(opt int, val interface{}) {
 		s.KVSnapshot.SetNotFillCache(val.(bool))
 	case tikvstore.SnapshotTS:
 		s.KVSnapshot.SetSnapshotTS(val.(uint64))
+	case tikvstore.TaskID:
+		s.KVSnapshot.SetTaskID(val.(uint64))
 	default:
 		s.KVSnapshot.SetOption(opt, val)
 	}

store/driver/txn/txn_driver.go

@@ -144,6 +144,8 @@ func (txn *tikvTxn) SetOption(opt int, val interface{}) {
 		txn.SetPessimistic(val.(bool))
 	case tikvstore.SnapshotTS:
 		txn.KVTxn.GetSnapshot().SetSnapshotTS(val.(uint64))
+	case tikvstore.TaskID:
+		txn.KVTxn.GetSnapshot().SetTaskID(val.(uint64))
 	case tikvstore.InfoSchema:
 		txn.SetSchemaVer(val.(tikv.SchemaVer))
 	case tikvstore.CommitHook:

store/tikv/snapshot.go

@@ -569,10 +569,6 @@ func (s *KVSnapshot) SetOption(opt int, val interface{}) {
 		s.mu.Lock()
 		s.mu.replicaRead = val.(kv.ReplicaReadType)
 		s.mu.Unlock()
-	case kv.TaskID:
-		s.mu.Lock()
-		s.mu.taskID = val.(uint64)
-		s.mu.Unlock()
 	case kv.CollectRuntimeStats:
 		s.mu.Lock()
 		s.mu.stats = val.(*SnapshotRuntimeStats)
@@ -625,6 +621,14 @@ func (s *KVSnapshot) SetPriority(pri Priority) {
 	s.priority = pri
 }
 
+// SetTaskID marks current task's unique ID to allow TiKV to schedule
+// tasks more fairly.
+func (s *KVSnapshot) SetTaskID(id uint64) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.mu.taskID = id
+}
+
 // SnapCacheHitCount gets the snapshot cache hit count. Only for test.
 func (s *KVSnapshot) SnapCacheHitCount() int {
 	return int(atomic.LoadInt64(&s.mu.hitCnt))