wip constructors

internal/cmd/BUILD

@@ -0,0 +1,8 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["cmd.go"],
+    importpath = "github.com/motiejus/code/undocker/internal/cmd",
+    visibility = ["//src/undocker:__subpackages__"],
+)

internal/cmd/cmd.go

@@ -0,0 +1,20 @@
+package cmd
+
+import (
+	"io"
+	"os"
+)
+
+// BaseCommand provides common fields to all commands.
+type BaseCommand struct {
+	Stdin  io.ReadCloser
+	Stdout io.WriteCloser
+	Stderr io.WriteCloser
+}
+
+// Init initializes BaseCommand with default arguments
+func (b *BaseCommand) Init() {
+	b.Stdin = os.Stdin
+	b.Stdout = os.Stdout
+	b.Stderr = os.Stderr
+}

internal/cmdrootfs/BUILD

@@ -6,6 +6,7 @@ go_library(
     importpath = "github.com/motiejus/code/undocker/internal/cmdrootfs",
     visibility = ["//src/undocker:__subpackages__"],
     deps = [
+        "//src/undocker/internal/cmd:go_default_library",
         "//src/undocker/rootfs:go_default_library",
         "@com_github_jessevdk_go_flags//:go_default_library",
         "@com_github_ulikunitz_xz//:go_default_library",

internal/cmdrootfs/cmdrootfs.go

@@ -6,6 +6,7 @@ import (
 	"os"
 
 	goflags "github.com/jessevdk/go-flags"
+	"github.com/motiejus/code/undocker/internal/cmd"
 	"github.com/motiejus/code/undocker/rootfs"
 	"github.com/ulikunitz/xz"
 	"go.uber.org/multierr"
@@ -13,6 +14,8 @@ import (
 
 // Command is "rootfs" command
 type Command struct {
+	cmd.BaseCommand
+
 	PositionalArgs struct {
 		Infile  goflags.Filename `long:"infile" description:"Input tarball"`
 		Outfile string           `long:"outfile" description:"Output path, stdout is '-'"`
@@ -29,9 +32,7 @@ func (c *Command) Execute(args []string) (err error) {
 		return errors.New("too many args")
 	}
 	if c.rootfsNew == nil {
-		c.rootfsNew = func(r io.ReadSeeker) io.WriterTo {
+		c.init()
-			return rootfs.New(r)
-		}
 	}
 
 	rd, err := os.Open(string(c.PositionalArgs.Infile))
@@ -67,3 +68,14 @@ func (c *Command) Execute(args []string) (err error) {
 	}
 	return nil
 }
+
+// init() initializes Command with the default options.
+//
+// Since constructors for sub-commands requires lots of boilerplate,
+// command will initialize itself.
+func (c *Command) init() {
+	c.BaseCommand.Init()
+	c.rootfsNew = func(r io.ReadSeeker) io.WriterTo {
+		return rootfs.New(r)
+	}
+}