Enforce isolation between different buckets and topics

Without a limit leveldb will continue looping over
records from different topics and eventually from other buckets as well.

peers/cache.go

@@ -39,7 +39,9 @@ func (d *Cache) RemovePeer(peerID discv5.NodeID, topic discv5.Topic) error {
 // GetPeersRange returns peers for a given topic with a limit.
 func (d *Cache) GetPeersRange(topic discv5.Topic, limit int) (nodes []*discv5.Node) {
 	key := db.Key(db.PeersCache, []byte(topic))
-	iterator := d.db.NewIterator(&util.Range{Start: key}, nil)
+	// it is important to set Limit on the range passed to iterator, so that
+	// we limit reads only to particular topic.
+	iterator := d.db.NewIterator(util.BytesPrefix(key), nil)
 	defer iterator.Release()
 	count := 0
 	for iterator.Next() && count < limit {

peers/cache_test.go

@@ -44,3 +44,25 @@ func TestPeersRange(t *testing.T) {
 	assert.NoError(t, peersDB.RemovePeer(peers[1].ID, topic))
 	require.Len(t, peersDB.GetPeersRange(topic, 3), 2)
 }
+
+func TestMultipleTopics(t *testing.T) {
+	peersDB, err := newInMemoryCache()
+	require.NoError(t, err)
+	topics := []discv5.Topic{discv5.Topic("first"), discv5.Topic("second")}
+	for i := range topics {
+		peers := [3]*discv5.Node{
+			discv5.NewNode(discv5.NodeID{byte(i), 1}, net.IPv4(100, 100, 0, 3), 32311, 32311),
+			discv5.NewNode(discv5.NodeID{byte(i), 2}, net.IPv4(100, 100, 0, 4), 32311, 32311),
+			discv5.NewNode(discv5.NodeID{byte(i), 3}, net.IPv4(100, 100, 0, 2), 32311, 32311)}
+		for _, peer := range peers {
+			assert.NoError(t, peersDB.AddPeer(peer, topics[i]))
+		}
+	}
+	for i := range topics {
+		nodes := peersDB.GetPeersRange(topics[i], 10)
+		assert.Len(t, nodes, 3)
+		for _, n := range nodes {
+			assert.Equal(t, byte(i), n.ID[0])
+		}
+	}
+}