fix: ignore bad peers when fetching the latest value

They may not support the protocol. Or they may just not give us what we're
looking for. We might as well keep asking till someone answers.

Author: Stebalien

pubsub.go

@@ -478,7 +478,7 @@ func (p *PubsubValueStore) handleSubscription(ctx context.Context, ti *topicInfo
 				case <-ctx.Done():
 					return
 				default:
-					log.Errorf("PubsubPeerJoin: error interacting with new peer", err)
+					log.Errorf("PubsubPeerJoin: error interacting with new peer: %s", err)
 				}
 			}
 		}
@@ -524,29 +524,29 @@ func (p *PubsubValueStore) handleNewMsgs(ctx context.Context, sub *pubsub.Subscr
 }
 
 func (p *PubsubValueStore) handleNewPeer(ctx context.Context, peerEvtHandler *pubsub.TopicEventHandler, key string) ([]byte, error) {
-	select {
-	case <-ctx.Done():
-		return nil, ctx.Err()
-	default:
-	}
-
-	var pid peer.ID
-
-	for {
+	tried := make(map[peer.ID]bool)
+	for ctx.Err() == nil {
 		peerEvt, err := peerEvtHandler.NextPeerEvent(ctx)
 		if err != nil {
 			if err != context.Canceled {
 				log.Warningf("PubsubNewPeer: subscription error in %s: %s", key, err.Error())
 			}
 			return nil, err
 		}
-		if peerEvt.Type == pubsub.PeerJoin {
-			pid = peerEvt.Peer
-			break
+
+		pid := peerEvt.Peer
+		if peerEvt.Type != pubsub.PeerJoin || tried[pid] {
+			continue
 		}
-	}
+		tried[pid] = true
 
-	return p.fetch.Fetch(ctx, pid, key)
+		value, err := p.fetch.Fetch(ctx, pid, key)
+		if err == nil {
+			return value, nil
+		}
+		log.Debugf("failed to fetch latest pubsub value for key '%s' from peer '%s': %s", key, pid, err)
+	}
+	return nil, ctx.Err()
 }
 
 func (p *PubsubValueStore) notifyWatchers(key string, data []byte) {