Implement hooks in libcontainer

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

libcontainer/configs/config.go

@@ -1,5 +1,11 @@
 package configs
 
+import (
+	"bytes"
+	"encoding/json"
+	"os/exec"
+)
+
 type Rlimit struct {
 	Type int    `json:"type"`
 	Hard uint64 `json:"hard"`
@@ -159,4 +165,71 @@ type Config struct {
 	// A number of rules are given, each having an action to be taken if a syscall matches it.
 	// A default action to be taken if no rules match is also given.
 	Seccomp *Seccomp `json:"seccomp"`
+
+	// Hooks are a collection of actions to perform at various container lifecycle events.
+	// Hooks are not able to be marshaled to json but they are also not needed to.
+	Hooks *Hooks `json:"-"`
+}
+
+type Hooks struct {
+	// Prestart commands are executed after the container namespaces are created,
+	// but before the user supplied command is executed from init.
+	Prestart []Hook
+
+	// PostStop commands are executed after the container init process exits.
+	Poststop []Hook
+}
+
+// HookState is the payload provided to a hook on execution.
+type HookState struct {
+	ID  string `json:"id"`
+	Pid int    `json:"pid"`
+}
+
+type Hook interface {
+	// Run executes the hook with the provided state.
+	Run(*HookState) error
+}
+
+// NewFunctionHooks will call the provided function when the hook is run.
+func NewFunctionHook(f func(*HookState) error) *FuncHook {
+	return &FuncHook{
+		Run: f,
+	}
+}
+
+type FuncHook struct {
+	Run func(*HookState) error
+}
+
+type Command struct {
+	Path string   `json:"path"`
+	Args []string `json:"args"`
+	Env  []string `json:"env"`
+	Dir  string   `json:"dir"`
+}
+
+// NewCommandHooks will execute the provided command when the hook is run.
+func NewCommandHook(cmd Command) *CommandHook {
+	return &CommandHook{
+		Command: cmd,
+	}
+}
+
+type CommandHook struct {
+	Command
+}
+
+func (c *Command) Run(s *HookState) error {
+	b, err := json.Marshal(s)
+	if err != nil {
+		return err
+	}
+	cmd := exec.Cmd{
+		Path:  c.Path,
+		Args:  c.Args,
+		Env:   c.Env,
+		Stdin: bytes.NewReader(b),
+	}
+	return cmd.Run()
 }