restore: Don't save (part of) pack in memory

internal/restorer/filerestorer.go

@@ -1,11 +1,12 @@
 package restorer
 
 import (
-	"bytes"
+	"bufio"
 	"context"
 	"io"
 	"math"
 	"path/filepath"
+	"sort"
 	"sync"
 
 	"github.com/restic/restic/internal/crypto"
@@ -179,6 +180,8 @@ func (r *fileRestorer) restoreFiles(ctx context.Context) error {
 	return nil
 }
 
+const maxBufferSize = 4 * 1024 * 1024
+
 func (r *fileRestorer) downloadPack(ctx context.Context, pack *packInfo) {
 
 	// calculate pack byte range and blob->[]files->[]offsets mappings
@@ -226,18 +229,12 @@ func (r *fileRestorer) downloadPack(ctx context.Context, pack *packInfo) {
 		}
 	}
 
-	packData := make([]byte, int(end-start))
-
-	h := restic.Handle{Type: restic.PackFile, Name: pack.id.String()}
-	err := r.packLoader(ctx, h, int(end-start), start, func(rd io.Reader) error {
-		l, err := io.ReadFull(rd, packData)
-		if err != nil {
-			return err
+	sortedBlobs := make([]restic.ID, 0, len(blobs))
+	for blobID := range blobs {
+		sortedBlobs = append(sortedBlobs, blobID)
 	}
-		if l != len(packData) {
-			return errors.Errorf("unexpected pack size: expected %d but got %d", len(packData), l)
-		}
-		return nil
+	sort.Slice(sortedBlobs, func(i, j int) bool {
+		return blobs[sortedBlobs[i]].offset < blobs[sortedBlobs[j]].offset
 	})
 
 	markFileError := func(file *fileInfo, err error) {
@@ -248,23 +245,28 @@ func (r *fileRestorer) downloadPack(ctx context.Context, pack *packInfo) {
 		}
 	}
 
+	h := restic.Handle{Type: restic.PackFile, Name: pack.id.String()}
+	err := r.packLoader(ctx, h, int(end-start), start, func(rd io.Reader) error {
+		bufferSize := int(end - start)
+		if bufferSize > maxBufferSize {
+			bufferSize = maxBufferSize
+		}
+		BufRd := bufio.NewReaderSize(rd, bufferSize)
+		currentBlobEnd := start
+		for _, blobID := range sortedBlobs {
+			blob := blobs[blobID]
+			_, err := BufRd.Discard(int(blob.offset - currentBlobEnd))
 			if err != nil {
-		for file := range pack.files {
-			markFileError(file, err)
+				return err
 			}
-		return
-	}
-
-	rd := bytes.NewReader(packData)
-
-	for blobID, blob := range blobs {
-		blobData, err := r.loadBlob(rd, blobID, blob.offset-start, blob.length)
+			blobData, err := r.loadBlob(BufRd, blobID, blob.length)
 			if err != nil {
 				for file := range blob.files {
 					markFileError(file, err)
 				}
 				continue
 			}
+			currentBlobEnd = blob.offset + int64(blob.length)
 			for file, offsets := range blob.files {
 				for _, offset := range offsets {
 					writeToFile := func() error {
@@ -294,14 +296,25 @@ func (r *fileRestorer) downloadPack(ctx context.Context, pack *packInfo) {
 				}
 			}
 		}
+
+		return nil
+	})
+
+	if err != nil {
+		for file := range pack.files {
+			markFileError(file, err)
+		}
+		return
+	}
+
 }
 
-func (r *fileRestorer) loadBlob(rd io.ReaderAt, blobID restic.ID, offset int64, length int) ([]byte, error) {
+func (r *fileRestorer) loadBlob(rd io.Reader, blobID restic.ID, length int) ([]byte, error) {
 	// TODO reconcile with Repository#loadBlob implementation
 
 	buf := make([]byte, length)
 
-	n, err := rd.ReadAt(buf, offset)
+	n, err := rd.Read(buf)
 	if err != nil {
 		return nil, err
 	}