Add oci-hook command to containerd

This allows many different commands to be used as OCI hooks.  It allows
these commands to template out different args and env vars so that
normal commands can accept the OCI spec State payload over stdin.

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

cmd/containerd/command/main.go

@@ -89,6 +89,7 @@ func App() *cli.App {
 	app.Commands = []cli.Command{
 		configCommand,
 		publishCommand,
+		ociHook,
 	}
 	app.Action = func(context *cli.Context) error {
 		var (

cmd/containerd/command/oci-hook.go

@@ -0,0 +1,131 @@
+/*
+   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 command
+
+import (
+	"bytes"
+	"encoding/json"
+	"io"
+	"os"
+	"path/filepath"
+	"syscall"
+	"text/template"
+
+	specs "github.com/opencontainers/runtime-spec/specs-go"
+	"github.com/urfave/cli"
+)
+
+var ociHook = cli.Command{
+	Name:  "oci-hook",
+	Usage: "provides a base for OCI runtime hooks that allow arguements to be templated.",
+	Action: func(context *cli.Context) error {
+		state, err := loadHookState(os.Stdin)
+		if err != nil {
+			return err
+		}
+		var (
+			ctx  = newTemplateContext(state)
+			args = []string(context.Args())
+			env  = os.Environ()
+		)
+		if err := newList(&args).render(ctx); err != nil {
+			return err
+		}
+		if err := newList(&env).render(ctx); err != nil {
+			return err
+		}
+		return syscall.Exec(args[0], args, env)
+	},
+}
+
+func loadHookState(r io.Reader) (*specs.State, error) {
+	var s specs.State
+	if err := json.NewDecoder(r).Decode(&s); err != nil {
+		return nil, err
+	}
+	return &s, nil
+}
+
+func newTemplateContext(state *specs.State) *templateContext {
+	t := &templateContext{
+		state: state,
+	}
+	t.funcs = template.FuncMap{
+		"id":         t.id,
+		"bundle":     t.bundle,
+		"rootfs":     t.rootfs,
+		"pid":        t.pid,
+		"annotation": t.annotation,
+	}
+	return t
+}
+
+type templateContext struct {
+	state *specs.State
+	funcs template.FuncMap
+}
+
+func (t *templateContext) id() string {
+	return t.state.ID
+}
+
+func (t *templateContext) bundle() string {
+	return t.state.Bundle
+}
+
+func (t *templateContext) rootfs() string {
+	return filepath.Join(t.state.Bundle, "rootfs")
+}
+
+func (t *templateContext) pid() int {
+	return t.state.Pid
+}
+
+func (t *templateContext) annotation(k string) string {
+	return t.state.Annotations[k]
+}
+
+func render(ctx *templateContext, source string, out io.Writer) error {
+	t, err := template.New("oci-hook").Funcs(ctx.funcs).Parse(source)
+	if err != nil {
+		return err
+	}
+	return t.Execute(out, ctx)
+}
+
+func newList(l *[]string) *templateList {
+	return &templateList{
+		l: l,
+	}
+}
+
+type templateList struct {
+	l *[]string
+}
+
+func (l *templateList) render(ctx *templateContext) error {
+	buf := bytes.NewBuffer(nil)
+	for i, s := range *l.l {
+		buf.Reset()
+		if err := render(ctx, s, buf); err != nil {
+			return err
+		}
+		(*l.l)[i] = buf.String()
+	}
+	buf.Reset()
+	return nil
+}

container_test.go

@@ -22,6 +22,7 @@ import (
 	"io"
 	"io/ioutil"
 	"os"
+	"os/exec"
 	"runtime"
 	"strings"
 	"syscall"
@@ -34,6 +35,7 @@ import (
 	"github.com/containerd/containerd/oci"
 	_ "github.com/containerd/containerd/runtime"
 	"github.com/containerd/typeurl"
+	specs "github.com/opencontainers/runtime-spec/specs-go"
 
 	"github.com/containerd/containerd/errdefs"
 	"github.com/containerd/containerd/windows/hcsshimtypes"
@@ -1469,3 +1471,61 @@ func TestContainerLabels(t *testing.T) {
 		t.Fatalf("expected label \"test\" to be \"no\"")
 	}
 }
+
+func TestContainerHook(t *testing.T) {
+	t.Parallel()
+
+	client, err := newClient(t, address)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer client.Close()
+
+	var (
+		image       Image
+		ctx, cancel = testContext()
+		id          = t.Name()
+	)
+	defer cancel()
+
+	image, err = client.GetImage(ctx, testImage)
+	if err != nil {
+		t.Fatal(err)
+	}
+	hook := func(_ context.Context, _ oci.Client, _ *containers.Container, s *specs.Spec) error {
+		if s.Hooks == nil {
+			s.Hooks = &specs.Hooks{}
+		}
+		path, err := exec.LookPath("containerd")
+		if err != nil {
+			return err
+		}
+		psPath, err := exec.LookPath("ps")
+		if err != nil {
+			return err
+		}
+		s.Hooks.Prestart = []specs.Hook{
+			{
+				Path: path,
+				Args: []string{
+					"containerd",
+					"oci-hook", "--",
+					psPath, "--pid", "{{pid}}",
+				},
+				Env: os.Environ(),
+			},
+		}
+		return nil
+	}
+	container, err := client.NewContainer(ctx, id, WithNewSpec(oci.WithImageConfig(image), hook), WithNewSnapshot(id, image))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer container.Delete(ctx, WithSnapshotCleanup)
+
+	task, err := container.NewTask(ctx, empty())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer task.Delete(ctx, WithProcessKill)
+}