compression: add support for the zstd algorithm

zstd is a compression algorithm that has a very fast decoder, while
providing also good compression ratios.  The fast decoder makes it
suitable for container images, as decompressing the tarballs is a very
expensive operation.

https://github.com/opencontainers/image-spec/pull/788 added support
for zstd to the OCI image specs.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

archive/compression/compression.go

@@ -29,6 +29,7 @@ import (
 	"sync"
 
 	"github.com/containerd/containerd/log"
+	"github.com/klauspost/compress/zstd"
 )
 
 type (
@@ -41,6 +42,8 @@ const (
 	Uncompressed Compression = iota
 	// Gzip is gzip compression algorithm.
 	Gzip
+	// Zstd is zstd compression algorithm.
+	Zstd
 )
 
 const disablePigzEnv = "CONTAINERD_DISABLE_PIGZ"
@@ -126,6 +129,7 @@ func (r *bufferedReader) Peek(n int) ([]byte, error) {
 func DetectCompression(source []byte) Compression {
 	for compression, m := range map[Compression][]byte{
 		Gzip: {0x1F, 0x8B, 0x08},
+		Zstd: {0x28, 0xb5, 0x2f, 0xfd},
 	} {
 		if len(source) < len(m) {
 			// Len too short
@@ -174,6 +178,19 @@ func DecompressStream(archive io.Reader) (DecompressReadCloser, error) {
 				return gzReader.Close()
 			},
 		}, nil
+	case Zstd:
+		zstdReader, err := zstd.NewReader(buf)
+		if err != nil {
+			return nil, err
+		}
+		return &readCloserWrapper{
+			Reader:      zstdReader,
+			compression: compression,
+			closer: func() error {
+				zstdReader.Close()
+				return nil
+			},
+		}, nil
 
 	default:
 		return nil, fmt.Errorf("unsupported compression format %s", (&compression).Extension())
@@ -187,6 +204,8 @@ func CompressStream(dest io.Writer, compression Compression) (io.WriteCloser, er
 		return &writeCloserWrapper{dest, nil}, nil
 	case Gzip:
 		return gzip.NewWriter(dest), nil
+	case Zstd:
+		return zstd.NewWriter(dest)
 	default:
 		return nil, fmt.Errorf("unsupported compression format %s", (&compression).Extension())
 	}
@@ -197,6 +216,8 @@ func (compression *Compression) Extension() string {
 	switch *compression {
 	case Gzip:
 		return "gz"
+	case Zstd:
+		return "zst"
 	}
 	return ""
 }