use singleflight in retrieval (#338)

pkg/retrieval/retrieval.go

@@ -16,6 +16,7 @@ import (
 	"github.com/ethersphere/bee/pkg/storage"
 	"github.com/ethersphere/bee/pkg/swarm"
 	"github.com/ethersphere/bee/pkg/topology"
+	"golang.org/x/sync/singleflight"
 )
 
 const (
@@ -34,6 +35,7 @@ type Service struct {
 	streamer      p2p.Streamer
 	peerSuggester topology.EachPeerer
 	storer        storage.Storer
+	singleflight  singleflight.Group
 	logger        logging.Logger
 }
 
@@ -72,6 +74,7 @@ const (
 )
 
 func (s *Service) RetrieveChunk(ctx context.Context, addr swarm.Address) (data []byte, err error) {
+	v, err, _ := s.singleflight.Do(addr.String(), func() (v interface{}, err error) {
 		var skipPeers []swarm.Address
 		for i := 0; i < maxPeers; i++ {
 			var peer swarm.Address
@@ -88,6 +91,11 @@ func (s *Service) RetrieveChunk(ctx context.Context, addr swarm.Address) (data [
 			return data, nil
 		}
 		return nil, err
+	})
+	if err != nil {
+		return nil, err
+	}
+	return v.([]byte), nil
 }
 
 func (s *Service) retrieveChunk(ctx context.Context, addr swarm.Address, skipPeers []swarm.Address) (data []byte, peer swarm.Address, err error) {