restorer: Optimize empty file restore

don't create fileInfo structs for empty files. this saves memory.
this also avoids extra serial scan of all fileInfo, which should
make restore faster and more consistent.

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>

internal/restorer/restorer_unix_test.go

@@ -0,0 +1,61 @@
+//+build !windows
+
+package restorer
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"syscall"
+	"testing"
+
+	"github.com/restic/restic/internal/repository"
+	"github.com/restic/restic/internal/restic"
+	rtest "github.com/restic/restic/internal/test"
+)
+
+func TestRestorerRestoreEmptyHardlinkedFileds(t *testing.T) {
+	repo, cleanup := repository.TestRepository(t)
+	defer cleanup()
+
+	_, id := saveSnapshot(t, repo, Snapshot{
+		Nodes: map[string]Node{
+			"dirtest": Dir{
+				Nodes: map[string]Node{
+					"file1": File{Links: 2, Inode: 1},
+					"file2": File{Links: 2, Inode: 1},
+				},
+			},
+		},
+	})
+
+	res, err := NewRestorer(repo, id)
+	rtest.OK(t, err)
+
+	res.SelectFilter = func(item string, dstpath string, node *restic.Node) (selectedForRestore bool, childMayBeSelected bool) {
+		return true, true
+	}
+
+	tempdir, cleanup := rtest.TempDir(t)
+	defer cleanup()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	err = res.RestoreTo(ctx, tempdir)
+	rtest.OK(t, err)
+
+	f1, err := os.Stat(filepath.Join(tempdir, "dirtest/file1"))
+	rtest.OK(t, err)
+	rtest.Equals(t, int64(0), f1.Size())
+	s1, ok1 := f1.Sys().(*syscall.Stat_t)
+
+	f2, err := os.Stat(filepath.Join(tempdir, "dirtest/file2"))
+	rtest.OK(t, err)
+	rtest.Equals(t, int64(0), f2.Size())
+	s2, ok2 := f2.Sys().(*syscall.Stat_t)
+
+	if ok1 && ok2 {
+		rtest.Equals(t, s1.Ino, s2.Ino)
+	}
+}