add support for symlinks in tarballs

This PR extends the downloader to support symlinks in tarball archives.

Fixes kubernetes-sigs#359

pkg/download/downloader.go

@@ -125,6 +125,10 @@ func extractTARGZ(targetDir string, at io.ReaderAt, size int64) error {
 			if err := os.MkdirAll(path, os.FileMode(hdr.Mode)); err != nil {
 				return errors.Wrap(err, "failed to create directory from tar")
 			}
+		case tar.TypeSymlink:
+			if err := os.Symlink(hdr.Linkname, path); err != nil {
+				return errors.Wrap(err, "failed to create symlink from tar")
+			}
 		case tar.TypeReg:
 			dir := filepath.Dir(path)
 			glog.V(4).Infof("tar: ensuring parent dirs exist for regular file, dir=%s", dir)

pkg/download/downloader_test.go

@@ -89,6 +89,10 @@ func Test_extractTARGZ(t *testing.T) {
 			in:    "test-without-directory.tar.gz",
 			files: []string{"/foo"},
 		},
+		{
+			in:    "test-with-symlinks.tar.gz",
+			files: []string{"/symlinks/", "/symlinks/message", "/symlinks/msg"},
+		},
 		{
 			in: "test-with-nesting-with-directory-entries.tar.gz",
 			files: []string{