store/tikv: define KVFilter for customized mutation initialization

store/driver/txn/txn_driver.go

@@ -32,6 +32,7 @@ type tikvTxn struct {
 
 // NewTiKVTxn returns a new Transaction.
 func NewTiKVTxn(txn *tikv.KVTxn) kv.Transaction {
+	txn.SetOption(tikvstore.KVFilter, TiDBKVFilter{})
 	return &tikvTxn{txn, make(map[int64]*model.TableInfo)}
 }
 
@@ -141,3 +142,10 @@ func (txn *tikvTxn) extractKeyExistsErr(key kv.Key) error {
 	}
 	return extractKeyExistsErrFromIndex(key, value, tblInfo, indexID)
 }
+
+type TiDBKVFilter struct{}
+
+// IsUnnecessaryKeyValue defines which kinds of KV pairs from TiDB needn't be committed.
+func (f TiDBKVFilter) IsUnnecessaryKeyValue(key, value []byte, flags tikvstore.KeyFlags) bool {
+	return tablecodec.IsUntouchedIndexKValue(key, value)
+}

store/tikv/2pc.go

@@ -314,6 +314,12 @@ func (c *twoPhaseCommitter) extractKeyExistsErr(err *kv.ErrKeyExist) error {
 	return errors.Trace(err)
 }
 
+// KVFilter is a filter that filters out unnecessary KV pairs.
+type KVFilter interface {
+	// IsUnnecessaryKeyValue returns whether this KV pair should be committed.
+	IsUnnecessaryKeyValue(key, value []byte, flags kv.KeyFlags) bool
+}
+
 func (c *twoPhaseCommitter) initKeysAndMutations() error {
 	var size, putCnt, delCnt, lockCnt, checkCnt int
 
@@ -322,6 +328,7 @@ func (c *twoPhaseCommitter) initKeysAndMutations() error {
 	sizeHint := txn.us.GetMemBuffer().Len()
 	c.mutations = newMemBufferMutations(sizeHint, memBuf)
 	c.isPessimistic = txn.IsPessimistic()
+	filter := txn.getKVFilter()
 
 	var err error
 	for it := memBuf.IterWithFlags(nil, nil); it.Valid(); err = it.Next() {
@@ -340,10 +347,14 @@ func (c *twoPhaseCommitter) initKeysAndMutations() error {
 		} else {
 			value = it.Value()
 			if len(value) > 0 {
-				if tablecodec.IsUntouchedIndexKValue(key, value) {
+				isUnnecessaryKV := filter != nil && filter.IsUnnecessaryKeyValue(key, value, flags)
+				if isUnnecessaryKV {
 					if !flags.HasLocked() {
 						continue
 					}
+					// If the key was locked before, we should prewrite the lock even if
+					// the KV needn't be committed according to the filter. Otherwise, we
+					// were forgetting removing pessimistic locks added before.
 					op = pb.Op_Lock
 					lockCnt++
 				} else {

store/tikv/kv/option.go

@@ -59,6 +59,8 @@ const (
 	IsStalenessReadOnly
 	// MatchStoreLabels indicates the labels the store should be matched
 	MatchStoreLabels
+	// KVFilter filters out the key-value pairs in the memBuf that is unnecessary to be committed
+	KVFilter
 )
 
 // Priority value for transaction priority.

store/tikv/tests/2pc_test.go

@@ -30,6 +30,7 @@ import (
 	"github.com/pingcap/kvproto/pkg/kvrpcpb"
 	pb "github.com/pingcap/kvproto/pkg/kvrpcpb"
 	tidbkv "github.com/pingcap/tidb/kv"
+	drivertxn "github.com/pingcap/tidb/store/driver/txn"
 	"github.com/pingcap/tidb/store/mockstore/mocktikv"
 	"github.com/pingcap/tidb/store/tikv"
 	"github.com/pingcap/tidb/store/tikv/config"
@@ -1032,6 +1033,7 @@ func (s *testCommitterSuite) TestResolvePessimisticLock(c *C) {
 	noValueIndexKey := []byte("t00000001_i000000002")
 	c.Assert(tablecodec.IsUntouchedIndexKValue(untouchedIndexKey, untouchedIndexValue), IsTrue)
 	txn := s.begin(c)
+	txn.SetOption(kv.KVFilter, drivertxn.TiDBKVFilter{})
 	err := txn.Set(untouchedIndexKey, untouchedIndexValue)
 	c.Assert(err, IsNil)
 	lockCtx := &tidbkv.LockCtx{ForUpdateTS: txn.StartTS(), WaitStartTime: time.Now(), LockWaitTime: tidbkv.LockNoWait}

store/tikv/txn.go

@@ -214,6 +214,13 @@ func (txn *KVTxn) IsPessimistic() bool {
 	return txn.us.GetOption(kv.Pessimistic) != nil
 }
 
+func (txn *KVTxn) getKVFilter() KVFilter {
+	if filter := txn.us.GetOption(kv.KVFilter); filter != nil {
+		return filter.(KVFilter)
+	}
+	return nil
+}
+
 // Commit commits the transaction operations to KV store.
 func (txn *KVTxn) Commit(ctx context.Context) error {
 	if span := opentracing.SpanFromContext(ctx); span != nil && span.Tracer() != nil {