kv: add IsReadOnly method for kv.Transaction.

It can be use for query optimization.

kv/kv.go

@@ -74,6 +74,8 @@ type Transaction interface {
 	SetOption(opt Option, val interface{})
 	// DelOption deletes an option.
 	DelOption(opt Option)
+	// IsReadOnly checks if the transaction has only performed read operations.
+	IsReadOnly() bool
 }
 
 // Snapshot defines the interface for the snapshot fetched from KV store.

store/hbase/txn.go

@@ -36,6 +36,7 @@ type hbaseTxn struct {
 	tid       uint64
 	valid     bool
 	version   kv.Version // commit version
+	dirty     bool
 }
 
 func newHbaseTxn(t themis.Txn, storeName string) *hbaseTxn {
@@ -57,6 +58,7 @@ func (txn *hbaseTxn) Get(k kv.Key) ([]byte, error) {
 
 func (txn *hbaseTxn) Set(k kv.Key, v []byte) error {
 	log.Debugf("[kv] set %q txn:%d", k, txn.tid)
+	txn.dirty = true
 	return txn.us.Set(k, v)
 }
 
@@ -71,6 +73,7 @@ func (txn *hbaseTxn) Seek(k kv.Key) (kv.Iterator, error) {
 
 func (txn *hbaseTxn) Delete(k kv.Key) error {
 	log.Debugf("[kv] delete %q txn:%d", k, txn.tid)
+	txn.dirty = true
 	return txn.us.Delete(k)
 }
 
@@ -154,3 +157,7 @@ func (txn *hbaseTxn) LockKeys(keys ...kv.Key) error {
 	}
 	return nil
 }
+
+func (txn *hbaseTxn) IsReadOnly() bool {
+	return !txn.dirty
+}

store/localstore/txn.go

@@ -33,6 +33,7 @@ type dbTxn struct {
 	valid      bool
 	version    kv.Version          // commit version
 	lockedKeys map[string]struct{} // origin version in snapshot
+	dirty      bool
 }
 
 func newTxn(s *dbStore, ver kv.Version) *dbTxn {
@@ -57,6 +58,7 @@ func (txn *dbTxn) Get(k kv.Key) ([]byte, error) {
 
 func (txn *dbTxn) Set(k kv.Key, data []byte) error {
 	log.Debugf("[kv] set key:%q, txn:%d", k, txn.tid)
+	txn.dirty = true
 	return txn.us.Set(k, data)
 }
 
@@ -71,6 +73,7 @@ func (txn *dbTxn) Seek(k kv.Key) (kv.Iterator, error) {
 
 func (txn *dbTxn) Delete(k kv.Key) error {
 	log.Debugf("[kv] delete key:%q, txn:%d", k, txn.tid)
+	txn.dirty = true
 	return txn.us.Delete(k)
 }
 
@@ -132,3 +135,7 @@ func (txn *dbTxn) LockKeys(keys ...kv.Key) error {
 	}
 	return nil
 }
+
+func (txn *dbTxn) IsReadOnly() bool {
+	return !txn.dirty
+}