Add reaper code for daemon

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

reaper/reaper.go

@@ -0,0 +1,100 @@
+package reaper
+
+import (
+	"bytes"
+	"fmt"
+	"os/exec"
+	"sync"
+
+	"github.com/docker/containerd/utils"
+)
+
+// Reap should be called when the process receives an SIGCHLD.  Reap will reap
+// all exited processes and close their wait channels
+func Reap() error {
+	exits, err := utils.Reap(false)
+	for _, e := range exits {
+		Default.mu.Lock()
+		c, ok := Default.cmds[e.Pid]
+		Default.mu.Unlock()
+		if !ok {
+			continue
+		}
+		// after we get an exit, call wait on the go process to make sure all
+		// pipes are closed and finalizers are run on the process
+		c.c.Wait()
+		c.exitCh <- e.Status
+		Default.mu.Lock()
+		delete(Default.cmds, e.Pid)
+		Default.mu.Unlock()
+	}
+	return err
+}
+
+var Default = &Monitor{
+	cmds: make(map[int]*cmd),
+}
+
+type Monitor struct {
+	mu   sync.Mutex
+	cmds map[int]*cmd
+}
+
+func (m *Monitor) Output(c *exec.Cmd) ([]byte, error) {
+	var b bytes.Buffer
+	c.Stdout = &b
+	if err := m.Run(c); err != nil {
+		return nil, err
+	}
+	return b.Bytes(), nil
+}
+
+func (m *Monitor) CombinedOutput(c *exec.Cmd) ([]byte, error) {
+	var b bytes.Buffer
+	c.Stdout = &b
+	c.Stderr = &b
+	if err := m.Run(c); err != nil {
+		return nil, err
+	}
+	return b.Bytes(), nil
+}
+
+// Start starts the command a registers the process with the reaper
+func (m *Monitor) Start(c *exec.Cmd) error {
+	rc := &cmd{
+		c:      c,
+		exitCh: make(chan int, 1),
+	}
+	// make sure we register the command first before starting the process
+	m.mu.Lock()
+	// start the process
+	if err := rc.c.Start(); err != nil {
+		m.mu.Unlock()
+		return err
+	}
+	m.cmds[rc.c.Process.Pid] = rc
+	m.mu.Unlock()
+	return nil
+}
+
+// Run runs and waits for the command to finish
+func (m *Monitor) Run(c *exec.Cmd) error {
+	if err := m.Start(c); err != nil {
+		return err
+	}
+	_, err := m.Wait(c)
+	return err
+}
+
+func (m *Monitor) Wait(c *exec.Cmd) (int, error) {
+	rc, ok := m.cmds[c.Process.Pid]
+	if !ok {
+		return 255, fmt.Errorf("process does not exist")
+	}
+	return <-rc.exitCh, nil
+}
+
+type cmd struct {
+	c      *exec.Cmd
+	exitCh chan int
+}