mpp task may get the same ID and lead to query failed

[wjhuang2016]

Bug Report

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1. Minimal reproduce step (Required)

I run the TPC-DS query56.

mysql> source /Users/xxx/pc/tidb-bench/tpcds/queries/query_56.sql;
ERROR 1105 (HY000): DB::Exception: The task [427626637160349697,9] has been registered

I added some logs to TiDB, and got.

[2021/09/10 17:29:23.280 +08:00] [WARN] [session.go:972] [AllocMPPTaskID] [id=2]
[2021/09/10 17:29:23.280 +08:00] [WARN] [session.go:972] [AllocMPPTaskID] [id=3]
[2021/09/10 17:29:23.280 +08:00] [WARN] [fragment.go:381] [constructMPPTasksForSinglePartitionTable] [id=3] [tableID=112]
[2021/09/10 17:29:23.280 +08:00] [WARN] [fragment.go:381] [constructMPPTasksForSinglePartitionTable] [id=3] [tableID=112]
...
[2021/09/10 17:29:23.280 +08:00] [INFO] [mpp_gather.go:76] ["Dispatch mpp task"] [timestamp=427626637160349697] [ID=3] [address=192.168.197.180:3930] [plan="Table(date_dim)->Sel([eq(tpcds.date_dim.d_year, 2000) eq(tpcds.date_dim.d_moy, 1)])->Send(4, )"] [pf=474]
...
[2021/09/10 17:29:23.280 +08:00] [INFO] [mpp_gather.go:76] ["Dispatch mpp task"] [timestamp=427626637160349697] [ID=3] [address=192.168.197.180:3930] [plan="Table(date_dim)->Sel([eq(tpcds.date_dim.d_year, 2000) eq(tpcds.date_dim.d_moy, 1)])->Send(6, )"] [pf=278]
...

After I added a mutex to protect taskID, the query successed.

func (s *SessionVars) AllocMPPTaskID(startTS uint64) int64 {
	s.mppTaskIDAllocator.mu.Lock()
	defer s.mppTaskIDAllocator.mu.Unlock()
	if s.mppTaskIDAllocator.lastTS == startTS {
		s.mppTaskIDAllocator.taskID++
		log.Warn("AllocMPPTaskID", zap.Int64("id", s.mppTaskIDAllocator.taskID))
		return s.mppTaskIDAllocator.taskID
	}
	s.mppTaskIDAllocator.lastTS = startTS
	s.mppTaskIDAllocator.taskID = 1
	return 1
}

#23747 Tried to fix it but I think it doesn't resolve the problem completely.

4. What is your TiDB version? (Required)

master

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