Move Go code to src/cmd

This does not get everything where we want it finally, see #1266
nor the optimal way of building, but it gets it out of top level.

Added instructions to build if you have a Go installation.

Not moving `vendor` yet.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

src/initrd/initrd.go

@@ -0,0 +1,136 @@
+package initrd
+
+import (
+	"archive/tar"
+	"bytes"
+	"compress/gzip"
+	"errors"
+	"io"
+
+	"github.com/docker/moby/src/pad4"
+	"github.com/surma/gocpio"
+)
+
+// Writer is an io.WriteCloser that writes to an initrd
+// This is a compressed cpio archive, zero padded to 4 bytes
+type Writer struct {
+	pw *pad4.Writer
+	gw *gzip.Writer
+	cw *cpio.Writer
+}
+
+func typeconv(t byte) int64 {
+	switch t {
+	case tar.TypeReg:
+		return cpio.TYPE_REG
+	case tar.TypeRegA:
+		return cpio.TYPE_REG
+	// Currently hard links not supported
+	case tar.TypeLink:
+		return cpio.TYPE_REG
+	case tar.TypeSymlink:
+		return cpio.TYPE_SYMLINK
+	case tar.TypeChar:
+		return cpio.TYPE_CHAR
+	case tar.TypeBlock:
+		return cpio.TYPE_BLK
+	case tar.TypeDir:
+		return cpio.TYPE_DIR
+	case tar.TypeFifo:
+		return cpio.TYPE_FIFO
+	default:
+		return -1
+	}
+}
+
+// CopyTar copies a tar stream into an initrd
+func CopyTar(w *Writer, r *tar.Reader) (written int64, err error) {
+	for {
+		var thdr *tar.Header
+		thdr, err = r.Next()
+		if err == io.EOF {
+			return written, nil
+		}
+		if err != nil {
+			return
+		}
+		tp := typeconv(thdr.Typeflag)
+		if tp == -1 {
+			return written, errors.New("cannot convert tar file")
+		}
+		size := thdr.Size
+		if tp == cpio.TYPE_SYMLINK {
+			size = int64(len(thdr.Linkname))
+		}
+		chdr := cpio.Header{
+			Mode:     thdr.Mode,
+			Uid:      thdr.Uid,
+			Gid:      thdr.Gid,
+			Mtime:    thdr.ModTime.Unix(),
+			Size:     size,
+			Devmajor: thdr.Devmajor,
+			Devminor: thdr.Devminor,
+			Type:     tp,
+			Name:     thdr.Name,
+		}
+		err = w.WriteHeader(&chdr)
+		if err != nil {
+			return
+		}
+		var n int64
+		if tp == cpio.TYPE_SYMLINK {
+			buffer := bytes.NewBufferString(thdr.Linkname)
+			n, err = io.Copy(w, buffer)
+		} else {
+			n, err = io.Copy(w, r)
+		}
+		written += n
+		if err != nil {
+			return
+		}
+	}
+}
+
+// NewWriter creates a writer that will output an initrd stream
+func NewWriter(w io.Writer) *Writer {
+	initrd := new(Writer)
+	initrd.pw = pad4.NewWriter(w)
+	initrd.gw = gzip.NewWriter(initrd.pw)
+	initrd.cw = cpio.NewWriter(initrd.gw)
+
+	return initrd
+}
+
+// WriteHeader writes a cpio header into an initrd
+func (w *Writer) WriteHeader(hdr *cpio.Header) error {
+	return w.cw.WriteHeader(hdr)
+}
+
+// Write writes a cpio file into an initrd
+func (w *Writer) Write(b []byte) (n int, e error) {
+	return w.cw.Write(b)
+}
+
+// Close closes the writer
+func (w *Writer) Close() error {
+	err1 := w.cw.Close()
+	err2 := w.gw.Close()
+	err3 := w.pw.Close()
+	if err1 != nil {
+		return err1
+	}
+	if err2 != nil {
+		return err2
+	}
+	if err3 != nil {
+		return err3
+	}
+	return nil
+}
+
+// Copy reads a tarball in a stream and outputs a compressed init ram disk
+func Copy(w *Writer, r io.Reader) (int64, error) {
+	tr := tar.NewReader(r)
+
+	return CopyTar(w, tr)
+}