【区块链 | 智能合约】Ethereum源代码（9）- 以太坊P2P协议接收广播的处理和Fetcher源码分析

上一节主要讲了Ethereum服务和以太坊P2P协议通讯模块 handler.go 的初始化和启动，以及以太坊通讯协议如何广播给其他的网络节点。
这一节讲讲，以太坊通讯协议如何处理接收到的广播消息。以及fetcher怎么工作。

ethereum共拥有两个Fetcher，分别为 BlockFetcher（eth/fetcher/block_fetcher.go），TxFetcher（eth/fetcher/tx_fetcher.go）

一，handler 接收网络节点广播消息
首先看看p2p.Protocol的结构

type Protocol struct {
	// Name should contain the official protocol name,
	// often a three-letter word.
	Name string

	// Version should contain the version number of the protocol.
	Version uint

	// Length should contain the number of message codes used
	// by the protocol.
	Length uint64

	// Run is called in a new goroutine when the protocol has been
	// negotiated with a peer. It should read and write messages from
	// rw. The Payload for each message must be fully consumed.
	//
	// The peer connection is closed when Start returns. It should return
	// any protocol-level error (such as a