fix: when the target bucket is empty or low, pull from all other buckets

The neighbors may _also_ be empty or low. This can cause peers with a low number
of peers in their routing table to fail to find _any_ peers in their routing table.

go.mod

@@ -8,3 +8,5 @@ require (
 	github.com/minio/sha256-simd v0.0.0-20190131020904-2d45a736cd16
 	github.com/multiformats/go-multihash v0.0.1
 )
+
+go 1.13

table.go

@@ -243,27 +243,25 @@ func (rt *RoutingTable) NearestPeers(id ID, count int) []peer.ID {
 	// It's assumed that this also protects the buckets.
 	rt.tabLock.RLock()
 
-	// Get bucket at cpl index or last bucket
-	var bucket *Bucket
+	// Get bucket index or last bucket
 	if cpl >= len(rt.Buckets) {
 		cpl = len(rt.Buckets) - 1
 	}
-	bucket = rt.Buckets[cpl]
 
 	pds := peerDistanceSorter{
 		peers:  make([]peerDistance, 0, 3*rt.bucketsize),
 		target: id,
 	}
-	pds.appendPeersFromList(bucket.list)
-	if pds.Len() < count {
-		// In the case of an unusual split, one bucket may be short or empty.
-		// if this happens, search both surrounding buckets for nearby peers
-		if cpl > 0 {
-			pds.appendPeersFromList(rt.Buckets[cpl-1].list)
-		}
-		if cpl < len(rt.Buckets)-1 {
-			pds.appendPeersFromList(rt.Buckets[cpl+1].list)
-		}
+
+	// Add peers from the target bucket and _less_ specific buckets until we
+	// have enough peers.
+	for i := cpl; i < len(rt.Buckets) && pds.Len() < count; i++ {
+		pds.appendPeersFromList(rt.Buckets[i].list)
+	}
+
+	// If we're still short, add more specific buckets (i.e., closer to us).
+	for i := cpl - 1; i >= 0 && pds.Len() < count; i-- {
+		pds.appendPeersFromList(rt.Buckets[i].list)
 	}
 	rt.tabLock.RUnlock()
 

table_test.go

@@ -229,6 +229,26 @@ func TestTableFindMultiple(t *testing.T) {
 	}
 }
 
+func TestTableFindMultipleBuckets(t *testing.T) {
+	local := test.RandPeerIDFatal(t)
+	m := pstore.NewMetrics()
+	rt := NewRoutingTable(5, ConvertPeerID(local), time.Hour, m)
+
+	peers := make([]peer.ID, 100)
+	for i := 0; i < 100; i++ {
+		peers[i] = test.RandPeerIDFatal(t)
+		rt.Update(peers[i])
+	}
+
+	t.Logf("Searching for peer: '%s'", peers[2])
+	// should be able to find at least 30
+	// ~31 (logtwo(100) * 5)
+	found := rt.NearestPeers(ConvertPeerID(peers[2]), 20)
+	if len(found) != 20 {
+		t.Fatalf("asked for 15 peers, got %d", len(found))
+	}
+}
+
 // Looks for race conditions in table operations. For a more 'certain'
 // test, increase the loop counter from 1000 to a much higher number
 // and set GOMAXPROCS above 1