Add ctr attach for reattaching to running task

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

cmd/ctr/attach.go

@@ -0,0 +1,64 @@
+package main
+
+import (
+	"os"
+
+	"github.com/Sirupsen/logrus"
+	"github.com/containerd/console"
+	"github.com/containerd/containerd"
+	"github.com/urfave/cli"
+)
+
+var attachCommand = cli.Command{
+	Name:      "attach",
+	Usage:     "attach to the IO of a running container",
+	ArgsUsage: "CONTAINER",
+	Action: func(context *cli.Context) error {
+		ctx, cancel := appContext(context)
+		defer cancel()
+		client, err := newClient(context)
+		if err != nil {
+			return err
+		}
+		container, err := client.LoadContainer(ctx, context.Args().First())
+		if err != nil {
+			return err
+		}
+		spec, err := container.Spec()
+		if err != nil {
+			return err
+		}
+		var (
+			con console.Console
+			tty = spec.Process.Terminal
+		)
+		if tty {
+			con = console.Current()
+			defer con.Reset()
+			if err := con.SetRaw(); err != nil {
+				return err
+			}
+		}
+		task, err := container.Task(ctx, containerd.WithAttach(os.Stdin, os.Stdout, os.Stderr))
+		if err != nil {
+			return err
+		}
+		defer task.Delete(ctx)
+		if tty {
+			if err := handleConsoleResize(ctx, task, con); err != nil {
+				logrus.WithError(err).Error("console resize")
+			}
+		} else {
+			sigc := forwardAllSignals(ctx, task)
+			defer stopCatch(sigc)
+		}
+		status, err := task.Wait(ctx)
+		if err != nil {
+			return err
+		}
+		if status != 0 {
+			return cli.NewExitError("", int(status))
+		}
+		return nil
+	},
+}