Return exit status from Wait of stopped process

This changes Wait() from returning an error whenever you call wait on a
stopped process/task to returning the exit status from the process.

This also adds the exit status to the Status() call on a process/task so
that a user can Wait(), check status, then cancel the wait to avoid
races in event handling.

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

task.go

@@ -28,14 +28,19 @@ import (
 
 const UnknownExitStatus = 255
 
-type Status string
+type Status struct {
+	Status     ProcessStatus
+	ExitStatus uint32
+}
+
+type ProcessStatus string
 
 const (
-	Running Status = "running"
-	Created Status = "created"
-	Stopped Status = "stopped"
-	Paused  Status = "paused"
-	Pausing Status = "pausing"
+	Running ProcessStatus = "running"
+	Created ProcessStatus = "created"
+	Stopped ProcessStatus = "stopped"
+	Paused  ProcessStatus = "paused"
+	Pausing ProcessStatus = "pausing"
 )
 
 type IOCloseInfo struct {
@@ -146,12 +151,14 @@ func (t *task) Status(ctx context.Context) (Status, error) {
 		ContainerID: t.id,
 	})
 	if err != nil {
-		return "", errdefs.FromGRPC(err)
+		return Status{}, errdefs.FromGRPC(err)
 	}
-	return Status(strings.ToLower(r.Process.Status.String())), nil
+	return Status{
+		Status:     ProcessStatus(strings.ToLower(r.Process.Status.String())),
+		ExitStatus: r.Process.ExitStatus,
+	}, nil
 }
 
-// Wait is a blocking call that will wait for the task to exit and return the exit status
 func (t *task) Wait(ctx context.Context) (uint32, error) {
 	eventstream, err := t.client.EventService().Subscribe(ctx, &eventsapi.SubscribeRequest{
 		Filters: []string{"topic==" + runtime.TaskExitEventTopic},
@@ -160,8 +167,12 @@ func (t *task) Wait(ctx context.Context) (uint32, error) {
 		return UnknownExitStatus, errdefs.FromGRPC(err)
 	}
 	// first check if the task has exited
-	if status, _ := t.Status(ctx); status == Stopped {
-		return UnknownExitStatus, errdefs.ErrUnavailable
+	status, err := t.Status(ctx)
+	if err != nil {
+		return UnknownExitStatus, errdefs.FromGRPC(err)
+	}
+	if status.Status == Stopped {
+		return status.ExitStatus, nil
 	}
 	for {
 		evt, err := eventstream.Recv()