Address review comments - onconflict donothing for inserts

Signed-off-by: Jim Zhang <[email protected]>

go-sdk/internal/sparse-merkle-tree/smt/merkletree.go

@@ -277,10 +277,6 @@ func (mt *sparseMerkleTree) addNode(batch core.Transaction, n core.Node) (core.N
 		return n.Ref(), nil
 	}
 	k := n.Ref()
-	// Check that the node key doesn't already exist
-	if _, err := batch.GetNode(k); err == nil {
-		return nil, ErrNodeIndexAlreadyExists
-	}
 	err := batch.InsertNode(n)
 	return k, err
 }

go-sdk/internal/sparse-merkle-tree/storage/sql.go

@@ -21,6 +21,7 @@ import (
 	"github.com/hyperledger-labs/zeto/go-sdk/internal/sparse-merkle-tree/utils"
 	"github.com/hyperledger-labs/zeto/go-sdk/pkg/sparse-merkle-tree/core"
 	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
 )
 
 type sqlStorage struct {
@@ -164,6 +165,9 @@ func insertNode(batchOrDb *gorm.DB, nodesTableName string, n core.Node) error {
 		dbNode.Index = &idx
 	}
 
-	err := batchOrDb.Table(nodesTableName).Create(dbNode).Error
+	// the merkle tree nodes, whether leaf nodes or branch nodes, are constructed
+	// in such a way that the reference key is the hash of the node's content, so
+	// there's no need to do anything if a node already exists in the DB
+	err := batchOrDb.Table(nodesTableName).Clauses(clause.OnConflict{DoNothing: true}).Create(dbNode).Error
 	return err
 }