fix(backup): skip loading the backingFile when doing backup or when user requests exporting the volume without backingFile (backport #1187)

pkg/replica/diff_disk.go

@@ -20,7 +20,7 @@ type diffDisk struct {
 	// we don't know the location yet.
 	location []byte
 	// list of files in grandparent, parent, child, etc order.
-	// index 0 is nil or backing file and index n-1 is the active write layer
+	// index 0 is nil, index 1 is backing file, and index n-1 is the active write layer
 	files      []types.DiffDisk
 	sectorSize int64
 	// current size of the head file.
@@ -372,12 +372,17 @@ func (d *diffDisk) initializeSectorLocation(value byte) {
 	}
 }
 
-func (d *diffDisk) preload() error {
+func (d *diffDisk) preload(isThereBackingFile bool) error {
 	for i, f := range d.files {
 		if i == 0 {
 			continue
 		}
 
+		// Skip loading the backingFile if it exists
+		if i == int(backingFileIndex) && isThereBackingFile {
+			continue
+		}
+
 		if err := LoadDiffDiskLocationList(d, f, byte(i)); err != nil {
 			return err
 		}

pkg/replica/extents.go

@@ -11,11 +11,6 @@ const MaxExtentsBuffer = 1024
 func LoadDiffDiskLocationList(diffDisk *diffDisk, disk types.DiffDisk, currentFileIndex byte) error {
 	fd := disk.Fd()
 
-	// The backing file will have a Fd of 0
-	if fd == 0 {
-		return nil
-	}
-
 	start := uint64(0)
 	end := uint64(len(diffDisk.location)) * uint64(diffDisk.sectorSize)
 	for {

pkg/replica/replica.go

@@ -1468,7 +1468,8 @@ func (r *Replica) Preload(includeBackingFileLayer bool) error {
 	} else {
 		r.volume.initializeSectorLocation(nilFileIndex)
 	}
-	return r.volume.preload()
+	isThereBackingFile := r.info.BackingFile != nil
+	return r.volume.preload(isThereBackingFile)
 }
 
 func (r *Replica) GetDataLayout(ctx context.Context) (<-chan sparse.FileInterval, <-chan error, error) {