don't count connections in the grace period against the limit

fixes #46

connmgr.go

@@ -220,6 +220,7 @@ func (cm *BasicConnMgr) getConnsToClose(ctx context.Context) []inet.Conn {
 
 	npeers := cm.segments.countPeers()
 	candidates := make([]*peerInfo, 0, npeers)
+	ncandidates := 0
 	cm.plk.RLock()
 	for _, s := range cm.segments {
 		s.Lock()
@@ -228,12 +229,25 @@ func (cm *BasicConnMgr) getConnsToClose(ctx context.Context) []inet.Conn {
 				// skip over protected peer.
 				continue
 			}
+			if inf.firstSeen.Add(cm.gracePeriod).After(now) {
+				// skip peers in the grace period.
+				continue
+			}
 			candidates = append(candidates, inf)
+			ncandidates += len(inf.conns)
 		}
 		s.Unlock()
 	}
 	cm.plk.RUnlock()
 
+	if ncandidates < cm.lowWater {
+		// We have too many connections but fewer than lowWater
+		// connections out of the grace period.
+		//
+		// If we trimmed now, we'd kill potentially useful connections.
+		return nil
+	}
+
 	// Sort peers according to their value.
 	sort.Slice(candidates, func(i, j int) bool {
 		left, right := candidates[i], candidates[j]
@@ -245,7 +259,7 @@ func (cm *BasicConnMgr) getConnsToClose(ctx context.Context) []inet.Conn {
 		return left.value < right.value
 	})
 
-	target := nconns - cm.lowWater
+	target := ncandidates - cm.lowWater
 
 	// slightly overallocate because we may have more than one conns per peer
 	selected := make([]inet.Conn, 0, target+10)