From 9b82e64a112295794ff1173bff4feb803b4182b2 Mon Sep 17 00:00:00 2001
From: Harshavardhana <harsha@minio.io>
Date: Wed, 22 Jun 2016 12:55:23 -0700
Subject: [PATCH] XL/erasure-read: Avoid memory copy, write to writer directly
 all the dataBlocks.

---
 erasure-readfile.go | 19 +++++------
 erasure-utils.go    | 81 +++++++++++++++++++++++++++++++++++----------
 server_xl_test.go   |  8 +++--
 3 files changed, 77 insertions(+), 31 deletions(-)

diff --git a/erasure-readfile.go b/erasure-readfile.go
index 7f9a0389c..91f494db8 100644
--- a/erasure-readfile.go
+++ b/erasure-readfile.go
@@ -189,6 +189,7 @@ func erasureReadFile(writer io.Writer, disks []StorageAPI, volume string, path s
 			}
 		}
 
+		var outSize, outOffset int64
 		// enBlocks data can have 0-padding hence we need to figure the exact number
 		// of bytes we want to read from enBlocks.
 		blockSize := eInfo.BlockSize
@@ -196,26 +197,24 @@ func erasureReadFile(writer io.Writer, disks []StorageAPI, volume string, path s
 			// For the last block, the block size can be less than BlockSize.
 			blockSize = totalLength % eInfo.BlockSize
 		}
-		data, err := getDataBlocks(enBlocks, eInfo.DataBlocks, int(blockSize))
-		if err != nil {
-			return bytesWritten, err
-		}
 
 		// If this is start block, skip unwanted bytes.
 		if block == startBlock {
-			data = data[bytesToSkip:]
+			outOffset = bytesToSkip
 		}
 
-		if len(data) > int(length-bytesWritten) {
+		// Total data to be read.
+		outSize = blockSize
+		if length-bytesWritten < blockSize {
 			// We should not send more data than what was requested.
-			data = data[:length-bytesWritten]
+			outSize = length - bytesWritten
 		}
-
-		_, err = writer.Write(data)
+		// Write data blocks.
+		n, err := writeDataBlocks(writer, enBlocks, eInfo.DataBlocks, outOffset, outSize)
 		if err != nil {
 			return bytesWritten, err
 		}
-		bytesWritten += int64(len(data))
+		bytesWritten += n
 	}
 
 	return bytesWritten, nil
diff --git a/erasure-utils.go b/erasure-utils.go
index 521bc7b21..9daf39325 100644
--- a/erasure-utils.go
+++ b/erasure-utils.go
@@ -17,6 +17,7 @@
 package main
 
 import (
+	"bytes"
 	"crypto/sha512"
 	"hash"
 	"io"
@@ -62,30 +63,74 @@ func hashSum(disk StorageAPI, volume, path string, writer hash.Hash) ([]byte, er
 	return writer.Sum(nil), nil
 }
 
-// getDataBlocks - fetches the data block only part of the input encoded blocks.
-func getDataBlocks(enBlocks [][]byte, dataBlocks int, curBlockSize int) (data []byte, err error) {
-	if len(enBlocks) < dataBlocks {
-		return nil, reedsolomon.ErrTooFewShards
-	}
+// getDataBlockLen - get length of data blocks from encoded blocks.
+func getDataBlockLen(enBlocks [][]byte, dataBlocks int) int {
 	size := 0
-	blocks := enBlocks[:dataBlocks]
-	for _, block := range blocks {
+	// Figure out the data block length.
+	for _, block := range enBlocks[:dataBlocks] {
 		size += len(block)
 	}
-	if size < curBlockSize {
-		return nil, reedsolomon.ErrShortData
+	return size
+}
+
+// Writes all the data blocks from encoded blocks until requested
+// outSize length. Provides a way to skip bytes until the offset.
+func writeDataBlocks(dst io.Writer, enBlocks [][]byte, dataBlocks int, outOffset int64, outSize int64) (int64, error) {
+	// Do we have enough blocks?
+	if len(enBlocks) < dataBlocks {
+		return 0, reedsolomon.ErrTooFewShards
 	}
 
-	write := curBlockSize
-	for _, block := range blocks {
-		if write < len(block) {
-			data = append(data, block[:write]...)
-			return data, nil
-		}
-		data = append(data, block...)
-		write -= len(block)
+	// Do we have enough data?
+	if int64(getDataBlockLen(enBlocks, dataBlocks)) < outSize {
+		return 0, reedsolomon.ErrShortData
 	}
-	return data, nil
+
+	// Counter to decrement total left to write.
+	write := outSize
+
+	// Counter to increment total written.
+	totalWritten := int64(0)
+
+	// Write all data blocks to dst.
+	for _, block := range enBlocks[:dataBlocks] {
+		// Skip blocks until we have reached our offset.
+		if outOffset >= int64(len(block)) {
+			// Decrement offset.
+			outOffset -= int64(len(block))
+			continue
+		} else {
+			// Skip until offset.
+			block = block[outOffset:]
+
+			// Reset the offset for next iteration to read everything
+			// from subsequent blocks.
+			outOffset = 0
+		}
+		// We have written all the blocks, write the last remaining block.
+		if write < int64(len(block)) {
+			n, err := io.Copy(dst, bytes.NewReader(block[:write]))
+			if err != nil {
+				return 0, err
+			}
+			totalWritten += n
+			break
+		}
+		// Copy the block.
+		n, err := io.Copy(dst, bytes.NewReader(block))
+		if err != nil {
+			return 0, err
+		}
+
+		// Decrement output size.
+		write -= n
+
+		// Increment written.
+		totalWritten += n
+	}
+
+	// Success.
+	return totalWritten, nil
 }
 
 // getBlockInfo - find start/end block and bytes to skip for given offset, length and block size.
diff --git a/server_xl_test.go b/server_xl_test.go
index 4814cdc47..6e6be7de3 100644
--- a/server_xl_test.go
+++ b/server_xl_test.go
@@ -840,15 +840,17 @@ func (s *MyAPIXLSuite) TestPartialContent(c *C) {
 	c.Assert(response.StatusCode, Equals, http.StatusOK)
 
 	// Prepare request
-	var table = []struct {
+	var testCases = []struct {
 		byteRange      string
 		expectedString string
 	}{
-		{"6-7", "Wo"},
+		{"4-7", "o Wo"},
+		{"1-", "ello World"},
 		{"6-", "World"},
+		{"-2", "ld"},
 		{"-7", "o World"},
 	}
-	for _, t := range table {
+	for _, t := range testCases {
 		request, err = newTestRequest("GET", s.testServer.Server.URL+"/partial-content/bar",
 			0, nil, s.testServer.AccessKey, s.testServer.SecretKey)
 		c.Assert(err, IsNil)