Optimize tar stream generation

- New writeTo method allows to avoid creating extra pipe.
- Copy with a pooled buffer instead of allocating new buffer for each file.
- Avoid extra object allocations inside the loop.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

tar/asm/assemble.go

@@ -3,8 +3,10 @@ package asm
 import (
 	"bytes"
 	"fmt"
+	"hash"
 	"hash/crc64"
 	"io"
+	"sync"
 
 	"github.com/vbatts/tar-split/tar/storage"
 )
@@ -23,45 +25,106 @@ func NewOutputTarStream(fg storage.FileGetter, up storage.Unpacker) io.ReadClose
 	}
 	pr, pw := io.Pipe()
 	go func() {
-		for {
-			entry, err := up.Next()
-			if err != nil {
-				pw.CloseWithError(err)
-				return
-			}
-			switch entry.Type {
-			case storage.SegmentType:
-				if _, err := pw.Write(entry.Payload); err != nil {
-					pw.CloseWithError(err)
-					return
-				}
-			case storage.FileType:
-				if entry.Size == 0 {
-					continue
-				}
-				fh, err := fg.Get(entry.GetName())
-				if err != nil {
-					pw.CloseWithError(err)
-					return
-				}
-				c := crc64.New(storage.CRCTable)
-				tRdr := io.TeeReader(fh, c)
-				if _, err := io.Copy(pw, tRdr); err != nil {
-					fh.Close()
-					pw.CloseWithError(err)
-					return
-				}
-				if !bytes.Equal(c.Sum(nil), entry.Payload) {
-					// I would rather this be a comparable ErrInvalidChecksum or such,
-					// but since it's coming through the PipeReader, the context of
-					// _which_ file would be lost...
-					fh.Close()
-					pw.CloseWithError(fmt.Errorf("file integrity checksum failed for %q", entry.GetName()))
-					return
-				}
-				fh.Close()
-			}
+		err := WriteOutputTarStream(fg, up, pw)
+		if err != nil {
+			pw.CloseWithError(err)
+		} else {
+			pw.Close()
 		}
 	}()
 	return pr
 }
+
+// WriteOutputTarStream writes assembled tar archive to a writer.
+func WriteOutputTarStream(fg storage.FileGetter, up storage.Unpacker, w io.Writer) error {
+	// ... Since these are interfaces, this is possible, so let's not have a nil pointer
+	if fg == nil || up == nil {
+		return nil
+	}
+	var copyBuffer []byte
+	var crcHash hash.Hash
+	var crcSum []byte
+	var multiWriter io.Writer
+	for {
+		entry, err := up.Next()
+		if err != nil {
+			if err == io.EOF {
+				return nil
+			}
+			return err
+		}
+		switch entry.Type {
+		case storage.SegmentType:
+			if _, err := w.Write(entry.Payload); err != nil {
+				return err
+			}
+		case storage.FileType:
+			if entry.Size == 0 {
+				continue
+			}
+			fh, err := fg.Get(entry.GetName())
+			if err != nil {
+				return err
+			}
+			if crcHash == nil {
+				crcHash = crc64.New(storage.CRCTable)
+				crcSum = make([]byte, 8)
+				multiWriter = io.MultiWriter(w, crcHash)
+				copyBuffer = byteBufferPool.Get().([]byte)
+				defer byteBufferPool.Put(copyBuffer)
+			} else {
+				crcHash.Reset()
+			}
+
+			if _, err := copyWithBuffer(multiWriter, fh, copyBuffer); err != nil {
+				fh.Close()
+				return err
+			}
+
+			if !bytes.Equal(crcHash.Sum(crcSum[:0]), entry.Payload) {
+				// I would rather this be a comparable ErrInvalidChecksum or such,
+				// but since it's coming through the PipeReader, the context of
+				// _which_ file would be lost...
+				fh.Close()
+				return fmt.Errorf("file integrity checksum failed for %q", entry.GetName())
+			}
+			fh.Close()
+		}
+	}
+}
+
+var byteBufferPool = &sync.Pool{
+	New: func() interface{} {
+		return make([]byte, 32*1024)
+	},
+}
+
+// copyWithBuffer is taken from stdlib io.Copy implementation
+// https://github.com/golang/go/blob/go1.5.1/src/io/io.go#L367
+func copyWithBuffer(dst io.Writer, src io.Reader, buf []byte) (written int64, err error) {
+	for {
+		nr, er := src.Read(buf)
+		if nr > 0 {
+			nw, ew := dst.Write(buf[0:nr])
+			if nw > 0 {
+				written += int64(nw)
+			}
+			if ew != nil {
+				err = ew
+				break
+			}
+			if nr != nw {
+				err = io.ErrShortWrite
+				break
+			}
+		}
+		if er == io.EOF {
+			break
+		}
+		if er != nil {
+			err = er
+			break
+		}
+	}
+	return written, err
+}