store/mockstore/unistore: fix several issues of coprocessor paging in unistore

store/mockstore/unistore/cophandler/cop_handler.go

@@ -179,6 +179,7 @@ func buildAndRunMPPExecutor(dagCtx *dagContext, dagReq *tipb.DAGRequest, pagingS
 	if pagingSize > 0 {
 		lastRange = &coprocessor.KeyRange{}
 		builder.paging = lastRange
+		builder.pagingSize = pagingSize
 	}
 	exec, err := builder.buildMPPExecutor(rootExec)
 	if err != nil {
@@ -221,7 +222,7 @@ func mppExecute(exec mppExec, dagCtx *dagContext, dagReq *tipb.DAGRequest, pagin
 			if pagingSize > 0 {
 				totalRows += uint64(chk.NumRows())
 				if totalRows > pagingSize {
-					break
+					return
 				}
 			}
 		default:

store/mockstore/unistore/cophandler/mpp.go

@@ -51,14 +51,15 @@ const (
 )
 
 type mppExecBuilder struct {
-	sc       *stmtctx.StatementContext
-	dbReader *dbreader.DBReader
-	mppCtx   *MPPCtx
-	dagReq   *tipb.DAGRequest
-	dagCtx   *dagContext
-	counts   []int64
-	ndvs     []int64
-	paging   *coprocessor.KeyRange
+	sc         *stmtctx.StatementContext
+	dbReader   *dbreader.DBReader
+	mppCtx     *MPPCtx
+	dagReq     *tipb.DAGRequest
+	dagCtx     *dagContext
+	counts     []int64
+	ndvs       []int64
+	paging     *coprocessor.KeyRange
+	pagingSize uint64
 }
 
 func (b *mppExecBuilder) buildMPPTableScan(pb *tipb.TableScan) (*tableScanExec, error) {
@@ -199,7 +200,7 @@ func (b *mppExecBuilder) buildLimit(pb *tipb.Limit) (*limitExec, error) {
 	return exec, nil
 }
 
-func (b *mppExecBuilder) buildTopN(pb *tipb.TopN) (*topNExec, error) {
+func (b *mppExecBuilder) buildTopN(pb *tipb.TopN) (mppExec, error) {
 	child, err := b.buildMPPExecutor(pb.Child)
 	if err != nil {
 		return nil, err
@@ -227,6 +228,12 @@ func (b *mppExecBuilder) buildTopN(pb *tipb.TopN) (*topNExec, error) {
 		row:         newTopNSortRow(len(conds)),
 		topn:        pb.Limit,
 	}
+
+	// When using paging protocol, if paging size < topN limit, the topN exec degenerate to do nothing.
+	if b.paging != nil && b.pagingSize < pb.Limit {
+		exec.dummy = true
+	}
+
 	return exec, nil
 }
 

store/mockstore/unistore/cophandler/mpp_exec.go

@@ -319,7 +319,8 @@ func (e *indexScanExec) Process(key, value []byte) error {
 	if e.chk.IsFull() {
 		e.chunks = append(e.chunks, e.chk)
 		if e.paging != nil {
-			e.chunkLastProcessedKeys = append(e.chunkLastProcessedKeys, key)
+			lastProcessed := kv.Key(append([]byte{}, key...)) // need a deep copy to store the key
+			e.chunkLastProcessedKeys = append(e.chunkLastProcessedKeys, lastProcessed)
 		}
 		e.chk = chunk.NewChunkWithCapacity(e.fieldTypes, DefaultBatchSize)
 	}
@@ -423,6 +424,9 @@ type topNExec struct {
 	conds []expression.Expression
 	row   *sortRow
 	recv  []*chunk.Chunk
+
+	// When dummy is true, topNExec just copy what it read from children to its parent.
+	dummy bool
 }
 
 func (e *topNExec) open() error {
@@ -432,6 +436,11 @@ func (e *topNExec) open() error {
 	if err != nil {
 		return err
 	}
+
+	if e.dummy {
+		return nil
+	}
+
 	for {
 		chk, err = e.children[0].next()
 		if err != nil {
@@ -466,6 +475,10 @@ func (e *topNExec) open() error {
 }
 
 func (e *topNExec) next() (*chunk.Chunk, error) {
+	if e.dummy {
+		return e.children[0].next()
+	}
+
 	chk := chunk.NewChunkWithCapacity(e.getFieldTypes(), DefaultBatchSize)
 	for ; !chk.IsFull() && e.idx < e.topn && e.idx < uint64(e.heap.heapSize); e.idx++ {
 		row := e.heap.rows[e.idx]