miner: address comment from martin

miner/payload_building.go

@@ -44,17 +44,18 @@ type Payload struct {
 	empty *types.Block
 	full  *types.Block
 	stop  chan struct{}
-	lock  *sync.RWMutex
+	lock  *sync.Mutex
 	cond  *sync.Cond
 }
 
 // newPayload initializes the payload object.
-func newPayload() *Payload {
-	lock := new(sync.RWMutex)
+func newPayload(empty *types.Block) *Payload {
+	lock := new(sync.Mutex)
 	return &Payload{
-		stop: make(chan struct{}),
-		lock: lock,
-		cond: sync.NewCond(lock),
+		empty: empty,
+		stop:  make(chan struct{}),
+		lock:  lock,
+		cond:  sync.NewCond(lock),
 	}
 }
 
@@ -125,8 +126,7 @@ func (w *worker) buildPayload(args *BuildPayloadArgs) (*Payload, error) {
 		return nil, err
 	}
 	// Construct a payload object for return.
-	payload := newPayload()
-	payload.empty = empty
+	payload := newPayload(empty)
 
 	// Spin up a routine for updating the payload in background. This strategy
 	// can maximum the revenue for including transactions with highest fee.