eth, les: reject light client connection is server is not synced (#19616)

* eth, les: reject light client connection is server is not synced

* eth, les: rename function and variables

* les: format

les/ulc.go

@@ -1,9 +1,24 @@
+// Copyright 2019 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
 package les
 
 import (
-	"fmt"
-
 	"github.com/ethereum/go-ethereum/eth"
+	"github.com/ethereum/go-ethereum/log"
 	"github.com/ethereum/go-ethereum/p2p/enode"
 )
 
@@ -12,24 +27,24 @@ type ulc struct {
 	minTrustedFraction int
 }
 
+// newULC creates and returns a ultra light client instance.
 func newULC(ulcConfig *eth.ULCConfig) *ulc {
 	if ulcConfig == nil {
 		return nil
 	}
-
 	m := make(map[string]struct{}, len(ulcConfig.TrustedServers))
 	for _, id := range ulcConfig.TrustedServers {
 		node, err := enode.ParseV4(id)
 		if err != nil {
-			fmt.Println("node:", id, " err:", err)
+			log.Debug("Failed to parse trusted server", "id", id, "err", err)
 			continue
 		}
 		m[node.ID().String()] = struct{}{}
 	}
-
 	return &ulc{m, ulcConfig.MinTrustedFraction}
 }
 
+// isTrusted return an indicator that whether the specified peer is trusted.
 func (u *ulc) isTrusted(p enode.ID) bool {
 	if u.trustedKeys == nil {
 		return false