Add missing monitor file

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

reaper/reaper.go

@@ -65,7 +65,6 @@ func (m *Monitor) Start(c *exec.Cmd) error {
 		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 {
@@ -87,7 +86,9 @@ func (m *Monitor) Run(c *exec.Cmd) error {
 }
 
 func (m *Monitor) Wait(c *exec.Cmd) (int, error) {
+	m.mu.Lock()
 	rc, ok := m.cmds[c.Process.Pid]
+	m.mu.Unlock()
 	if !ok {
 		return 255, fmt.Errorf("process does not exist")
 	}

vendor/github.com/crosbymichael/go-runc/monitor.go

@@ -0,0 +1,50 @@
+package runc
+
+import (
+	"os/exec"
+	"syscall"
+)
+
+var Monitor ProcessMonitor = &defaultMonitor{}
+
+// ProcessMonitor is an interface for process monitoring
+//
+// It allows daemons using go-runc to have a SIGCHLD handler
+// to handle exits without introducing races between the handler
+// and go's exec.Cmd
+// These methods should match the methods exposed by exec.Cmd to provide
+// a consistent experience for the caller
+type ProcessMonitor interface {
+	Output(*exec.Cmd) ([]byte, error)
+	CombinedOutput(*exec.Cmd) ([]byte, error)
+	Run(*exec.Cmd) error
+	Start(*exec.Cmd) error
+	Wait(*exec.Cmd) (int, error)
+}
+
+type defaultMonitor struct {
+}
+
+func (m *defaultMonitor) Output(c *exec.Cmd) ([]byte, error) {
+	return c.Output()
+}
+
+func (m *defaultMonitor) CombinedOutput(c *exec.Cmd) ([]byte, error) {
+	return c.CombinedOutput()
+}
+
+func (m *defaultMonitor) Run(c *exec.Cmd) error {
+	return c.Run()
+}
+
+func (m *defaultMonitor) Start(c *exec.Cmd) error {
+	return c.Start()
+}
+
+func (m *defaultMonitor) Wait(c *exec.Cmd) (int, error) {
+	status, err := c.Process.Wait()
+	if err != nil {
+		return -1, err
+	}
+	return status.Sys().(syscall.WaitStatus).ExitStatus(), nil
+}