Add zstd to reader

Signed-off-by: Tudor Marcu <tudor.marcu@intel.com>

swupd/archive.go

@@ -29,6 +29,8 @@ var (
 	gzipMagic  = []byte{0x1F, 0x8B}
 	xzMagic    = []byte{0xFD, '7', 'z', 'X', 'Z', 0x00}
 	bzip2Magic = []byte{'B', 'Z', 'h'}
+	// https://github.com/facebook/zstd/blob/dev/lib/zstd.h#L385
+	zstdMagic = []byte{0x28, 0xB5, 0x2F, 0xFD}
 )
 
 // NewCompressedTarReader creates a struct compatible with tar.Reader reading from uncompressed or
@@ -44,7 +46,6 @@ func NewCompressedTarReader(rs io.ReadSeeker) (*CompressedTarReader, error) {
 		return nil, err
 	}
 	result := &CompressedTarReader{}
-
 	switch {
 	case bytes.HasPrefix(h[:], gzipMagic):
 		gr, err := gzip.NewReader(rs)
@@ -63,6 +64,13 @@ func NewCompressedTarReader(rs io.ReadSeeker) (*CompressedTarReader, error) {
 	case bytes.HasPrefix(h[:], bzip2Magic):
 		br := bzip2.NewReader(rs)
 		result.Reader = tar.NewReader(br)
+	case bytes.HasPrefix(h[:], zstdMagic):
+		zr, err := NewExternalReader(rs, "zstd", "-d")
+		if err != nil {
+			return nil, fmt.Errorf("couldn't decompress using zstd: %s", err)
+		}
+		result.CompressionCloser = zr
+		result.Reader = tar.NewReader(zr)
 	default:
 		// Assume uncompressed tar and let it complain if not valid.
 		result.Reader = tar.NewReader(rs)