pkg/boot: Use a library for xz files

Fixes u-root#2750

At the moment, we shell out for decompressing xz files. This change
will change that to using a library instead.

Signed-off-by: Navid Paya <navidpaya@google.com>

pkg/boot/bzimage/bzimage.go

@@ -54,9 +54,7 @@ var (
 		// GZIP
 		{[]byte{0x1F, 0x8B}, gunzip},
 		// XZ
-		// It would be nice to use a Go package instead of shelling out to 'unxz'.
-		// https://github.com/ulikunitz/xz fails to decompress the payloads and returns an error: "unsupported filter count"
-		{[]byte{0xFD, 0x37, 0x7A, 0x58, 0x5A, 0x00}, stripSize(execer("unxz"))},
+		{[]byte{0xFD, 0x37, 0x7A, 0x58, 0x5A, 0x00}, stripSize(unxz)},
 		// LZMA
 		{[]byte{0x5D, 0x00, 0x00}, stripSize(unlzma)},
 		// LZO
@@ -83,6 +81,7 @@ var (
 func findDecompressor(b []byte) (decompressor, error) {
 	for _, m := range magics {
 		if bytes.Index(b, m.signature) == 0 {
+			fmt.Println("found decompressor for signature: ", m.signature)
 			return m.decompressor, nil
 		}
 	}

pkg/boot/bzimage/bzimage_decompress.go

@@ -12,6 +12,7 @@ import (
 	"io"
 	"os/exec"
 
+	"github.com/therootcompany/xz/v/v1/xz"
 	"github.com/klauspost/compress/zstd"
 	"github.com/pierrec/lz4/v4"
 	"github.com/ulikunitz/xz/lzma"
@@ -127,3 +128,17 @@ func unzstd(w io.Writer, r io.Reader) error {
 	}
 	return nil
 }
+
+// unxz reads compressed bytes from the io.Reader and writes the uncompressed bytes to the
+// writer. unxz satisfies the decompressor interface.
+func unxz(w io.Writer, r io.Reader) error {
+	unxzReader, err := xz.NewReader(r, 0)
+	if err != nil {
+		return fmt.Errorf("error creating unxz reader: %v", err)
+	}
+
+	if _, err := io.Copy(w, unxzReader); err != nil {
+		return fmt.Errorf("failed writing decompressed bytes to writer: %v", err)
+	}
+	return nil
+}