Add exec support to client

This also fixes a deadlock in the shim's reaper where execs would lockup
and/or miss a quick exiting exec process's exit status.

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

linux/shim/service.go

@@ -101,19 +101,18 @@ func (s *Service) Exec(ctx context.Context, r *shimapi.ExecRequest) (*shimapi.Ex
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	s.execID++
-	reaper.Default.Lock()
+
 	process, err := newExecProcess(ctx, s.path, r, s.initProcess, s.execID)
 	if err != nil {
-		reaper.Default.Unlock()
 		return nil, err
 	}
 	pid := process.Pid()
-	s.processes[pid] = process
 	cmd := &reaper.Cmd{
 		ExitCh: make(chan int, 1),
 	}
-	reaper.Default.RegisterNL(pid, cmd)
-	reaper.Default.Unlock()
+	reaper.Default.Register(pid, cmd)
+	s.processes[pid] = process
+
 	s.events <- &task.Event{
 		Type: task.Event_EXEC_ADDED,
 		ID:   s.id,
@@ -290,6 +289,8 @@ func (s *Service) Checkpoint(ctx context.Context, r *shimapi.CheckpointRequest)
 func (s *Service) waitExit(p process, pid int, cmd *reaper.Cmd) {
 	status := <-cmd.ExitCh
 	p.Exited(status)
+
+	reaper.Default.Delete(pid)
 	s.events <- &task.Event{
 		Type:       task.Event_EXIT,
 		ID:         s.id,