archive: support using zstd (MR 25)

This external module was chosen because it's a native Go implementation
of zstd, and not a wrapper around some external utility or some CGO
thing.

go.mod

@@ -4,5 +4,6 @@ go 1.16
 
 require (
 	github.com/cavaliercoder/go-cpio v0.0.0-20180626203310-925f9528c45e
+	github.com/klauspost/compress v1.15.12
 	golang.org/x/sys v0.0.0-20210630005230-0f9fa26af87c
 )

go.sum

@@ -1,4 +1,6 @@
 github.com/cavaliercoder/go-cpio v0.0.0-20180626203310-925f9528c45e h1:hHg27A0RSSp2Om9lubZpiMgVbvn39bsUmW9U5h0twqc=
 github.com/cavaliercoder/go-cpio v0.0.0-20180626203310-925f9528c45e/go.mod h1:oDpT4efm8tSYHXV5tHSdRvBet/b/QzxZ+XyyPehvm3A=
+github.com/klauspost/compress v1.15.12 h1:YClS/PImqYbn+UILDnqxQCZ3RehC9N318SU3kElDUEM=
+github.com/klauspost/compress v1.15.12/go.mod h1:QPwzmACJjUTFsnSHH934V6woptycfrDDJnH7hvFVbGM=
 golang.org/x/sys v0.0.0-20210630005230-0f9fa26af87c h1:F1jZWGFhYfh0Ci55sIpILtKKK8p3i2/krTr0H1rg74I=
 golang.org/x/sys v0.0.0-20210630005230-0f9fa26af87c/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=

internal/archive/archive.go

@@ -17,6 +17,7 @@ import (
 	"syscall"
 
 	"github.com/cavaliercoder/go-cpio"
+	"github.com/klauspost/compress/zstd"
 	"gitlab.com/postmarketOS/postmarketos-mkinitfs/internal/filelist"
 	"gitlab.com/postmarketOS/postmarketos-mkinitfs/internal/osutil"
 )
@@ -25,6 +26,7 @@ type CompressFormat string
 
 const (
 	FormatGzip CompressFormat = "gzip"
+	FormatZstd CompressFormat = "zstd"
 )
 
 type Archive struct {
@@ -233,6 +235,11 @@ func (archive *Archive) writeCompressed(path string, mode os.FileMode) error {
 	switch archive.compress_format {
 	case FormatGzip:
 		compressor = gzip.NewWriter(fd)
+	case FormatZstd:
+		compressor, err = zstd.NewWriter(fd)
+		if err != nil {
+			return err
+		}
 	default:
 		log.Print("Unknown or no compression format set, using gzip")
 		compressor = gzip.NewWriter(fd)