上一节中留了一个悬念，那就是在Session类中我们没有找到有意义的处理代码，这部分代码隐藏在哪里呢？
答案其实在上上节的末尾中
那就是EthereumHost::newPeerCapability()实现中！
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 shared_ptr <Capability> EthereumHost::newPeerCapability(shared_ptr <SessionFace> const & _s, unsigned _idOffset, p2p::CapDesc const & _cap){ auto ret = HostCapability<EthereumPeer>::newPeerCapability(_s, _idOffset, _cap); auto cap = capabilityFromSession<EthereumPeer>(*_s, _cap.second); assert(cap); cap->init( protocolVersion(), m_networkId, m_chain.details().totalDifficulty, m_chain.currentHash(), m_chain.genesisHash(), m_hostData, m_peerObserver ); return ret; }
这个函数上来先调用了父类的实现，然后调用capabilityFromSession<EthereumPeer>()函数返回EthereumPeer对象的指针，最后调用了EthereumPeer::init()。看来有必要深入这个函数里看看。
1 2 3 4 5 6 7 void EthereumPeer::init(unsigned _hostProtocolVersion, u256 _hostNetworkId, u256 _chainTotalDifficulty, h256 _chainCurrentHash, h256 _chainGenesisHash, shared_ptr <EthereumHostDataFace> _hostData, shared_ptr <EthereumPeerObserverFace> _observer){ m_hostData = _hostData; m_observer = _observer; m_hostProtocolVersion = _hostProtocolVersion; requestStatus(_hostNetworkId, _chainTotalDifficulty, _chainCurrentHash, _chainGenesisHash); }
可以看到这里有我们感兴趣的requestStatus()这个函数了！EthereumPeer::requestStatus()函数实现也比较简单，就是向对方发送一个StatusPacket包：
1 2 3 4 5 6 7 8 9 10 void EthereumPeer::requestStatus(u256 _hostNetworkId, u256 _chainTotalDifficulty, h256 _chainCurrentHash, h256 _chainGenesisHash){ assert(m_asking == Asking::Nothing); setAsking(Asking::State); m_requireTransactions = true ; RLPStream s; prep(s, StatusPacket, 5 ) << m_hostProtocolVersion << _hostNetworkId << _chainTotalDifficulty << _chainCurrentHash << _chainGenesisHash; sealAndSend(s); }
这个数据包真正开始了与peer有意义的通讯！
可以看到发送这个包的同时告知了对方我自己的版本号 ，网络id ，区块链难度 ，当前区块链最新块hash值 ，当前区块链创世区块的hash值 ，这些信息非常重要！StatusPacket包以后怎么应答呢？前面讲过EthereumPeer类是Session类的消息处理器，那么我们到EthereumPeer::interpret()中去寻找：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 case StatusPacket: { cout << "StatusPacket" << endl ; m_protocolVersion = _r[0 ].toInt<unsigned >(); m_networkId = _r[1 ].toInt<u256>(); m_totalDifficulty = _r[2 ].toInt<u256>(); m_latestHash = _r[3 ].toHash<h256>(); m_genesisHash = _r[4 ].toHash<h256>(); if (m_peerCapabilityVersion == m_hostProtocolVersion) m_protocolVersion = m_hostProtocolVersion; LOG(m_logger) << "Status: " << m_protocolVersion << " / " << m_networkId << " / " << m_genesisHash.hex().c_str() << ", TD: " << m_totalDifficulty << " = " << m_latestHash.hex().c_str(); setIdle(); observer->onPeerStatus(dynamic_pointer_cast<EthereumPeer>(dynamic_pointer_cast<EthereumPeer>(shared_from_this()))); break ; }
可以看到这里将对方的协议版本号 ，网络id 等信息取出来，然后交给observer去处理。observer也是从EthereumPeer::init()函数中由EthereumHost类传过来的，所以还是要去EthereumHost那找，在EthereumHost类的构造函数中有这么一句话：
1 m_peerObserver = make_shared<EthereumPeerObserver>(m_sync, m_tq);
EthereumPeerObserver这个类其实是一个过渡类，我们可以看看它的实现：
1 2 3 4 5 6 7 8 9 10 11 12 13 void onPeerStatus (std ::shared_ptr <EthereumPeer> _peer) override try { m_sync->onPeerStatus(_peer); } catch (FailedInvariant const &) { cwarn << "Failed invariant during sync, restarting sync" ; m_sync->restartSync(); } }
这个类里有许多这样的函数，都是过渡用的，其实真正调用的是m_sync里对应的函数。而m_sync从上面代码中可以看到其实是EthereumHost类里的m_sync，这个是BlockChainSync类对象。
因此我们能够得出结论，所有数据包都是由BlockChainSync类对象来处理的！
我们再来看看BlockChainSync::onPeerStatus()函数的实现：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 void BlockChainSync::onPeerStatus(std ::shared_ptr <EthereumPeer> _peer){ char const * disconnectReason = nullptr ; if (_peer->m_genesisHash != host().chain().genesisHash()) disconnectReason = "Invalid genesis hash." ; else if (_peer->m_protocolVersion != host().protocolVersion()) disconnectReason = "Invalid protocol version." ; else if (_peer->m_networkId != host().networkId()) disconnectReason = "Invalid network identifier." ; else if (session->info().clientVersion.find("/v0.7.0/" ) != string ::npos) disconnectReason = "Blacklisted client version." ; else if (host().isBanned(session->id())) disconnectReason = "Peer banned for previous bad behaviour." ; else if (_peer->m_asking != Asking::State && _peer->m_asking != Asking::Nothing) disconnectReason = "Peer banned for unexpected status message." ; if (disconnectReason) { LOG(m_logger) << "Peer not suitable for sync: " << disconnectReason; cout << "Peer not suitable for sync: " << disconnectReason << endl ; _peer->disconnect(); return ; } if (!requestDaoForkBlockHeader(_peer)) { cout << "onPeerStatus syncPeer" << endl ; syncPeer(_peer, false ); } }
这个函数先是做了一些检测，判断peer的一些参数和自己参数不一致就关闭连接。syncPeer()来对peer进行区块链同步！
