Remove sigchld reaper from containerd process

Because we are launching alot of different runc commands to do
operations there is a race between doing a `cmd.Wait()` and getting the
sigchld and reaping it.  We can remove the sigchild reaper from
containerd as long as we make sure we reap the shim process if we are
the parent, i.e. not restored.

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

runtime/process.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"io/ioutil"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"strconv"
 	"syscall"
@@ -39,6 +40,8 @@ type Process interface {
 	SystemPid() int
 	// State returns if the process is running or not
 	State() State
+	// Wait reaps the shim process if avaliable
+	Wait()
 }
 
 type processConfig struct {
@@ -139,6 +142,7 @@ type process struct {
 	container   *container
 	spec        specs.ProcessSpec
 	stdio       Stdio
+	cmd         *exec.Cmd
 }
 
 func (p *process) ID() string {
@@ -219,6 +223,13 @@ func (p *process) getPidFromFile() (int, error) {
 	return i, nil
 }
 
+// Wait will reap the shim process
+func (p *process) Wait() {
+	if p.cmd != nil {
+		p.cmd.Wait()
+	}
+}
+
 func getExitPipe(path string) (*os.File, error) {
 	if err := syscall.Mkfifo(path, 0755); err != nil && !os.IsExist(err) {
 		return nil, err