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/container.go

@@ -478,6 +478,7 @@ func (c *container) Exec(pid string, pspec specs.ProcessSpec, s Stdio) (pp Proce
 }
 
 func (c *container) startCmd(pid string, cmd *exec.Cmd, p *process) error {
+	p.cmd = cmd
 	if err := cmd.Start(); err != nil {
 		if exErr, ok := err.(*exec.Error); ok {
 			if exErr.Err == exec.ErrNotFound || exErr.Err == os.ErrNotExist {
@@ -699,6 +700,9 @@ func (c *container) waitForStart(p *process, cmd *exec.Cmd) error {
 
 // isAlive checks if the shim that launched the container is still alive
 func isAlive(cmd *exec.Cmd) (bool, error) {
+	if _, err := syscall.Wait4(cmd.Process.Pid, nil, syscall.WNOHANG, nil); err == nil {
+		return true, nil
+	}
 	if err := syscall.Kill(cmd.Process.Pid, 0); err != nil {
 		if err == syscall.ESRCH {
 			return false, nil