feat: avoid ioutil.ReadAll on ApplyDelta

Signed-off-by: Nao YONASHIRO <owan.orisano@gmail.com>

Author: orisano

plumbing/format/packfile/patch_delta.go

@@ -1,8 +1,9 @@
 package packfile
 
 import (
+	"bytes"
 	"errors"
-	"io/ioutil"
+	"sync"
 
 	"gopkg.in/src-d/go-git.v4/plumbing"
 )
@@ -14,6 +15,12 @@ import (
 
 const deltaSizeMin = 4
 
+var bytesBufferPool = sync.Pool{
+	New: func() interface{} {
+		return &bytes.Buffer{}
+	},
+}
+
 // ApplyDelta writes to target the result of applying the modification deltas in delta to base.
 func ApplyDelta(target, base plumbing.EncodedObject, delta []byte) error {
 	r, err := base.Reader()
@@ -26,10 +33,16 @@ func ApplyDelta(target, base plumbing.EncodedObject, delta []byte) error {
 		return err
 	}
 
-	src, err := ioutil.ReadAll(r)
+	buf := bytesBufferPool.Get().(*bytes.Buffer)
+	defer func() {
+		buf.Reset()
+		bytesBufferPool.Put(buf)
+	} ()
+	_, err = buf.ReadFrom(r)
 	if err != nil {
 		return err
 	}
+	src := buf.Bytes()
 
 	dst, err := PatchDelta(src, delta)
 	if err != nil {