Introduce Task Results

* Added Retention Option to specify retention TTL for tasks
* Added ResultWriter as a client interface to write result data for the associated task

client.go

@@ -46,6 +46,7 @@ const (
 	ProcessAtOpt
 	ProcessInOpt
 	TaskIDOpt
+	RetentionOpt
 )
 
 // Option specifies the task processing behavior.
@@ -70,6 +71,7 @@ type (
 	uniqueOption    time.Duration
 	processAtOption time.Time
 	processInOption time.Duration
+	retentionOption time.Duration
 )
 
 // MaxRetry returns an option to specify the max number of times
@@ -178,6 +180,17 @@ func (d processInOption) String() string     { return fmt.Sprintf("ProcessIn(%v)
 func (d processInOption) Type() OptionType   { return ProcessInOpt }
 func (d processInOption) Value() interface{} { return time.Duration(d) }
 
+// Retention returns an option to specify the duration of retention period for the task.
+// If this option is provided, the task will be stored as a completed task after successful processing.
+// A completed task will be deleted after the specified duration elapses.
+func Retention(d time.Duration) Option {
+	return retentionOption(d)
+}
+
+func (ttl retentionOption) String() string     { return fmt.Sprintf("Retention(%v)", time.Duration(ttl)) }
+func (ttl retentionOption) Type() OptionType   { return RetentionOpt }
+func (ttl retentionOption) Value() interface{} { return time.Duration(ttl) }
+
 // ErrDuplicateTask indicates that the given task could not be enqueued since it's a duplicate of another task.
 //
 // ErrDuplicateTask error only applies to tasks enqueued with a Unique option.
@@ -196,6 +209,7 @@ type option struct {
 	deadline  time.Time
 	uniqueTTL time.Duration
 	processAt time.Time
+	retention time.Duration
 }
 
 // composeOptions merges user provided options into the default options
@@ -237,6 +251,8 @@ func composeOptions(opts ...Option) (option, error) {
 			res.processAt = time.Time(opt)
 		case processInOption:
 			res.processAt = time.Now().Add(time.Duration(opt))
+		case retentionOption:
+			res.retention = time.Duration(opt)
 		default:
 			// ignore unexpected option
 		}
@@ -316,6 +332,7 @@ func (c *Client) Enqueue(task *Task, opts ...Option) (*TaskInfo, error) {
 		Deadline:  deadline.Unix(),
 		Timeout:   int64(timeout.Seconds()),
 		UniqueKey: uniqueKey,
+		Retention: int64(opt.retention.Seconds()),
 	}
 	now := time.Now()
 	var state base.TaskState
@@ -335,7 +352,7 @@ func (c *Client) Enqueue(task *Task, opts ...Option) (*TaskInfo, error) {
 	case err != nil:
 		return nil, err
 	}
-	return newTaskInfo(msg, state, opt.processAt), nil
+	return newTaskInfo(msg, state, opt.processAt, nil), nil
 }
 
 func (c *Client) enqueue(msg *base.TaskMessage, uniqueTTL time.Duration) error {