Update processor to adapt for deadlines set change

- Processor dequeues tasks only when it's available to process
- Processor retries a task when its context's Done channel is closed

context.go

@@ -27,28 +27,14 @@ type ctxKey int
 const metadataCtxKey ctxKey = 0
 
 // createContext returns a context and cancel function for a given task message.
-func createContext(msg *base.TaskMessage) (ctx context.Context, cancel context.CancelFunc) {
+func createContext(msg *base.TaskMessage, deadline time.Time) (ctx context.Context, cancel context.CancelFunc) {
 	metadata := taskMetadata{
 		id:         msg.ID.String(),
 		maxRetry:   msg.Retry,
 		retryCount: msg.Retried,
 	}
 	ctx = context.WithValue(context.Background(), metadataCtxKey, metadata)
-	if msg.Timeout == 0 && msg.Deadline == 0 {
-		panic("asynq: internal error: missing both timeout and deadline")
-	}
-	if msg.Timeout != 0 {
-		timeout := time.Duration(msg.Timeout) * time.Second
-		ctx, cancel = context.WithTimeout(ctx, timeout)
-	}
-	if msg.Deadline != 0 {
-		deadline := time.Unix(int64(msg.Deadline), 0)
-		ctx, cancel = context.WithDeadline(ctx, deadline)
-	}
-	if cancel == nil {
-		ctx, cancel = context.WithCancel(ctx)
-	}
-	return ctx, cancel
+	return context.WithDeadline(ctx, deadline)
 }
 
 // GetTaskID extracts a task ID from a context, if any.