pkg/archive: add function for checksumming a file

This uses sha256 which, after benchmarking, doesn't seem to be any
faster or slower than sha1. md5 was surprisingly slower (on aarch64),
maybe because there are some CPU accelerated things in sha* ?

pkgs/archive/archive.go

@@ -8,8 +8,10 @@ import (
 	"os"
 	"strings"
 	"io"
+	"encoding/hex"
 	"path/filepath"
 	"compress/flate"
+	"crypto/sha256"
 	"github.com/cavaliercoder/go-cpio"
 	"github.com/klauspost/pgzip"
 	"github.com/google/renameio"
@@ -52,6 +54,38 @@ func (archive *Archive) Write(path string, mode os.FileMode) error {
 	return nil
 }
 
+func checksum(path string) (string, error) {
+	var sum string
+
+	buf := make([]byte, 64*1024)
+	sha256 := sha256.New()
+	fd, err := os.Open(path)
+	defer fd.Close()
+
+	if err != nil {
+		log.Print("Unable to checksum: ", path)
+		return sum, err
+	}
+
+	// Read file in chunks
+	for {
+		bytes, err := fd.Read(buf)
+		if bytes > 0 {
+			_, err := sha256.Write(buf[:bytes])
+			if err != nil {
+				log.Print("Unable to checksum: ", path)
+				return sum, err
+			}
+		}
+
+		if err == io.EOF {
+			break
+		}
+	}
+	sum = hex.EncodeToString(sha256.Sum(nil))
+	return sum, nil
+}
+
 func (archive *Archive) AddFile(file string, dest string) error {
 	if err := archive.addDir(filepath.Dir(dest)); err != nil {
 		return err