Add ctr subcommand to print default OCI spec

Signed-off-by: Maksym Pavlenko <pavlenko.maksym@gmail.com>

cmd/ctr/app/main.go

@@ -27,6 +27,7 @@ import (
 	"github.com/containerd/containerd/cmd/ctr/commands/install"
 	"github.com/containerd/containerd/cmd/ctr/commands/leases"
 	namespacesCmd "github.com/containerd/containerd/cmd/ctr/commands/namespaces"
+	ociCmd "github.com/containerd/containerd/cmd/ctr/commands/oci"
 	"github.com/containerd/containerd/cmd/ctr/commands/plugins"
 	"github.com/containerd/containerd/cmd/ctr/commands/pprof"
 	"github.com/containerd/containerd/cmd/ctr/commands/run"
@@ -112,6 +113,7 @@ containerd CLI
 		snapshots.Command,
 		tasks.Command,
 		install.Command,
+		ociCmd.Command,
 	}, extraCmds...)
 	app.Before = func(context *cli.Context) error {
 		if context.GlobalBool("debug") {

cmd/ctr/commands/oci/oci.go

@@ -0,0 +1,51 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package oci
+
+import (
+	"github.com/pkg/errors"
+	"github.com/urfave/cli"
+
+	"github.com/containerd/containerd/cmd/ctr/commands"
+	"github.com/containerd/containerd/containers"
+	"github.com/containerd/containerd/oci"
+)
+
+var Command = cli.Command{
+	Name:  "oci",
+	Usage: "OCI tools",
+	Subcommands: []cli.Command{
+		defaultSpecCommand,
+	},
+}
+
+var defaultSpecCommand = cli.Command{
+	Name:  "spec",
+	Usage: "see the output of the default OCI spec",
+	Action: func(context *cli.Context) error {
+		ctx, cancel := commands.AppContext(context)
+		defer cancel()
+
+		spec, err := oci.GenerateSpec(ctx, nil, &containers.Container{})
+		if err != nil {
+			return errors.Wrap(err, "failed to generate spec")
+		}
+
+		commands.PrintAsJSON(spec)
+		return nil
+	},
+}