From 607b838deddeab29501abb56167eee8bea41fe85 Mon Sep 17 00:00:00 2001
From: stasatdaglabs <39559713+stasatdaglabs@users.noreply.github.com>
Date: Tue, 28 Jan 2020 12:24:09 +0200
Subject: [PATCH] [NOD-702] Fix netsync slowing down significantly due to
 excessive allocs in serializeUTXO (#605)

* [NOD-702] Fix netsync slowing down significantly due to excessive allocs in serializeUTXO.

* [NOD-702] Fix bad make statement.

* [NOD-702] Move writeBuffer to flushToDB.
---
 blockdag/utxodiffstore.go | 22 +++++++++++++++++++---
 blockdag/utxoio.go        | 11 +++++------
 2 files changed, 24 insertions(+), 9 deletions(-)

diff --git a/blockdag/utxodiffstore.go b/blockdag/utxodiffstore.go
index 6162a91a7..ec7c2ac87 100644
--- a/blockdag/utxodiffstore.go
+++ b/blockdag/utxodiffstore.go
@@ -1,6 +1,7 @@
 package blockdag
 
 import (
+	"bytes"
 	"github.com/kaspanet/kaspad/database"
 	"github.com/kaspanet/kaspad/util/daghash"
 	"github.com/kaspanet/kaspad/util/locks"
@@ -161,9 +162,13 @@ func (diffStore *utxoDiffStore) flushToDB(dbTx database.Tx) error {
 		return nil
 	}
 
+	// Allocate a buffer here to avoid needless allocations/grows
+	// while writing each entry.
+	buffer := &bytes.Buffer{}
 	for hash := range diffStore.dirty {
+		buffer.Reset()
 		diffData := diffStore.loaded[hash]
-		err := dbStoreDiffData(dbTx, &hash, diffData)
+		err := dbStoreDiffData(dbTx, buffer, &hash, diffData)
 		if err != nil {
 			return err
 		}
@@ -177,12 +182,23 @@ func (diffStore *utxoDiffStore) clearDirtyEntries() {
 
 // dbStoreDiffData stores the UTXO diff data to the database.
 // This overwrites the current entry if there exists one.
-func dbStoreDiffData(dbTx database.Tx, hash *daghash.Hash, diffData *blockUTXODiffData) error {
-	serializedDiffData, err := serializeBlockUTXODiffData(diffData)
+func dbStoreDiffData(dbTx database.Tx, writeBuffer *bytes.Buffer, hash *daghash.Hash, diffData *blockUTXODiffData) error {
+	// To avoid a ton of allocs, use the given writeBuffer
+	// instead of allocating one. We expect the buffer to
+	// already be initalized and, in most cases, to already
+	// be large enough to accommodate the serialized data
+	// without growing.
+	err := serializeBlockUTXODiffData(writeBuffer, diffData)
 	if err != nil {
 		return err
 	}
 
+	// Bucket.Put doesn't copy on its own, so we manually
+	// copy here. We do so because we expect the buffer
+	// to be reused once we're done with it.
+	serializedDiffData := make([]byte, writeBuffer.Len())
+	copy(serializedDiffData, writeBuffer.Bytes())
+
 	return dbTx.Metadata().Bucket(utxoDiffsBucketName).Put(hash[:], serializedDiffData)
 }
 
diff --git a/blockdag/utxoio.go b/blockdag/utxoio.go
index 7e62c23e5..753f3af3f 100644
--- a/blockdag/utxoio.go
+++ b/blockdag/utxoio.go
@@ -19,26 +19,25 @@ import (
 // 	hasDiffChild | Boolean   | Indicates if a diff child exist
 //  diffChild    | Hash      | The diffChild's hash. Empty if hasDiffChild is true.
 //  diff		 | UTXODiff  | The diff data's diff
-func serializeBlockUTXODiffData(diffData *blockUTXODiffData) ([]byte, error) {
-	w := &bytes.Buffer{}
+func serializeBlockUTXODiffData(w io.Writer, diffData *blockUTXODiffData) error {
 	hasDiffChild := diffData.diffChild != nil
 	err := wire.WriteElement(w, hasDiffChild)
 	if err != nil {
-		return nil, err
+		return err
 	}
 	if hasDiffChild {
 		err := wire.WriteElement(w, diffData.diffChild.hash)
 		if err != nil {
-			return nil, err
+			return err
 		}
 	}
 
 	err = serializeUTXODiff(w, diffData.diff)
 	if err != nil {
-		return nil, err
+		return err
 	}
 
-	return w.Bytes(), nil
+	return nil
 }
 
 // utxoEntryHeaderCode returns the calculated header code to be used when