比特币源码分析-boost::signal 的使用
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bitcoin 代码中大量使用 boost::signal, boost::signal 实现了信号与槽的事件通知机制，或者说是一种消息的发布与订阅机制， signal 类型是一个可调用类型，slot 就是callback 对象，或者说事件的订阅者，signal 实例是一个可调用对象，调用signal 对象，就相当于发布了相应的事件, signal 的connect， disconnect 方法分别相当于对事件的订阅，取消。
#include <boost/signals2.hpp>#include void print_args(float x, float y){std::cout << "The arguments are " << x << " and " << y << std::endl;}void print_sum(float x, float y){std::cout << "The sum is " << x + y << std::endl;}void print_product(float x, float y){std::cout << "The product is " << x * y << std::endl;}void print_difference(float x, float y){std::cout << "The difference is " << x - y << std::endl;}void print_quotient(float x, float y){std::cout << "The quotient is " << x / y << std::endl;}int main() {boost::signals2::signal<void(float, float)> sig;sig.connect(print_args);sig.connect(print_sum);sig.connect(print_product);sig.connect(print_difference);sig.connect(print_quotient);sig(5., 3.);return 0;
上面这个例子, 有五个函数订阅sig 事件，sig(5. , 3.) 的调用触发事件，参数5，3，相当于事件携带的消息paylaod, 传给了五个事件订阅者。
bitcoin 中定义了类型CMainSignals 来统一管理各个功能模块的事件通知，CMainSignal 是一个资源管理类型， 主要工作代理给由unique_ptr 管理内存的成员 m_internals , 它的类型是MainSignalsInstance，内部定义十个boost signal 变量， 分别表达十种要通知的事件。
class CMainSignals {private:std::unique_ptr m_internals;friend void ::RegisterValidationInterface(CValidationInterface*);friend void ::UnregisterValidationInterface(CValidationInterface*);friend void ::UnregisterAllValidationInterfaces();friend void ::CallFunctionInValidationInterfaceQueue(std::function func); void MempoolEntryRemoved(CTransactionRef tx, MemPoolRemovalReason reason);public: /** Register a CScheduler to give callbacks which should run in the background (may only be called once) */void RegisterBackgroundSignalScheduler(CScheduler& scheduler);/** Unregister a CScheduler to give callbacks which should run in the background - these callbacks will now be dropped! */void UnregisterBackgroundSignalScheduler();/** Call any remaining callbacks on the calling thread */void FlushBackgroundCallbacks();size_t CallbacksPending();/** Register with mempool to call TransactionRemovedFromMempool callbacks */void RegisterWithMempoolSignals(CTxMemPool& pool);/** Unregister with mempool */void UnregisterWithMempoolSignals(CTxMemPool& pool);void UpdatedBlockTip(const CBlockIndex *, const CBlockIndex *, bool fInitialDownload);void TransactionAddedToMempool(const CTransactionRef &);void BlockConnected(const std::shared_ptr &, const CBlockIndex *pindex, const std::shared_ptr> &);void BlockDisconnected(const std::shared_ptr &);void SetBestChain(const CBlockLocator &);void Inventory(const uint256 &);void Broadcast(int64_t nBestBlockTime, CConnman* connman);void BlockChecked(const CBlock&, const CValidationState&);void NewPoWValidBlock(const CBlockIndex *, const std::shared_ptr&);};struct MainSignalsInstance {boost::signals2::signal UpdatedBlockTip;boost::signals2::signal TransactionAddedToMempool;boost::signals2::signal &, const CBlockIndex *pindex, const std::vector&)> BlockConnected;boost::signals2::signal &)> BlockDisconnected;boost::signals2::signal TransactionRemovedFromMempool;boost::signals2::signal SetBestChain;boost::signals2::signal Inventory;boost::signals2::signal Broadcast;boost::signals2::signal BlockChecked;boost::signals2::signal&)> NewPoWValidBlock;// We are not allowed to assume the scheduler only runs in one thread,// but must ensure all callbacks happen in-order, so we end up creating// our own queue here :(SingleThreadedSchedulerClient m_schedulerClient;explicit MainSignalsInstance(CScheduler *pscheduler) : m_schedulerClient(pscheduler) {}};
类型CValidationInterface 主要是统一表示某些对MainSignalsInstance 中定义的十个事件感兴趣的对象， 即这些事件的订阅者， 所有对这些事件感兴趣代码继承CValidationInterface类型，提供自己版本的这些虚成员函数的实现，覆盖baseCValidationInterface 中对应的空方法， 表达对相应的事件感兴趣， 不感兴趣的事件的回调方法继续是那些继承自base class 的空方法。
class CValidationInterface {protected:virtual void UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) {}virtual void TransactionAddedToMempool(const CTransactionRef &ptxn) {}virtual void TransactionRemovedFromMempool(const CTransactionRef &ptx) {}virtual void BlockConnected(const std::shared_ptr &block, const CBlockIndex *pindex, const std::vector &txnConflicted) {}virtual void BlockDisconnected(const std::shared_ptr &block) {}virtual void SetBestChain(const CBlockLocator &locator) {}virtual void Inventory(const uint256 &hash) {}virtual void ResendWalletTransactions(int64_t nBestBlockTime, CConnman* connman) {}virtual void BlockChecked(const CBlock&, const CValidationState&) {}virtual void NewPoWValidBlock(const CBlockIndex *pindex, const std::shared_ptr& block) {};friend void ::RegisterValidationInterface(CValidationInterface*);friend void ::UnregisterValidationInterface(CValidationInterface*);friend void ::UnregisterAllValidationInterfaces();};
CValidationInterface 有四个子类， CWallet , CZMQNotificationInterface, submitblock_StateCatcher, PeerLogicValidation 分别对应四个对MainSignalsInstance 中的事件感兴趣的订阅者。
class CWallet final : public CCryptoKeyStore, public CValidationInterface{...............void TransactionAddedToMempool(const CTransactionRef& tx) override;void BlockConnected(const std::shared_ptr& pblock, const CBlockIndex *pindex, const std::vector& vtxConflicted) override;void BlockDisconnected(const std::shared_ptr& pblock) override;void TransactionRemovedFromMempool(const CTransactionRef &ptx) override;void ResendWalletTransactions(int64_t nBestBlockTime, CConnman* connman) override;void SetBestChain(const CBlockLocator& loc) override;void Inventory(const uint256 &hash) override{{LOCK(cs_wallet);std::map<uint256, int>::iterator mi = mapRequestCount.find(hash);if (mi != mapRequestCount.end())(*mi).second++;}}...............};class CZMQNotificationInterface final : public CValidationInterface{................// CValidationInterfacevoid TransactionAddedToMempool(const CTransactionRef& tx) override;void BlockConnected(const std::shared_ptr& pblock, const CBlockIndex* pindexConnected, const std::vector& vtxConflicted) override;void BlockDisconnected(const std::shared_ptr& pblock) override;void UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) override;.................};class submitblock_StateCatcher : public CValidationInterface{public:uint256 hash;bool found;CValidationState state;explicit submitblock_StateCatcher(const uint256 &hashIn) : hash(hashIn), found(false), state() {}protected:void BlockChecked(const CBlock& block, const CValidationState& stateIn) override {if (block.GetHash() != hash)return;found = true;state = stateIn;}};class PeerLogicValidation : public CValidationInterface, public NetEventsInterface{private:CConnman* const connman;public:explicit PeerLogicValidation(CConnman* connman, CScheduler &scheduler);void BlockConnected(const std::shared_ptr& pblock, const CBlockIndex* pindexConnected, const std::vector& vtxConflicted) override;void UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) override;void BlockChecked(const CBlock& block, const CValidationState& state) override;void NewPoWValidBlock(const CBlockIndex *pindex, const std::shared_ptr& pblock) override;........};
这四个订阅者通过调用RegisterValidationInterface， UnregisterValidationInterface 订阅，取消事件通知，函数接受参数是指向订阅者的指针。
void RegisterValidationInterface(CValidationInterface* pwalletIn) {g_signals.m_internals->UpdatedBlockTip.connect(boost::bind(&CValidationInterface::UpdatedBlockTip, pwalletIn, _1, _2, _3));g_signals.m_internals->TransactionAddedToMempool.connect(boost::bind(&CValidationInterface::TransactionAddedToMempool, pwalletIn, _1));g_signals.m_internals->BlockConnected.connect(boost::bind(&CValidationInterface::BlockConnected, pwalletIn, _1, _2, _3));g_signals.m_internals->BlockDisconnected.connect(boost::bind(&CValidationInterface::BlockDisconnected, pwalletIn, _1));g_signals.m_internals->TransactionRemovedFromMempool.connect(boost::bind(&CValidationInterface::TransactionRemovedFromMempool, pwalletIn, _1));g_signals.m_internals->SetBestChain.connect(boost::bind(&CValidationInterface::SetBestChain, pwalletIn, _1));g_signals.m_internals->Inventory.connect(boost::bind(&CValidationInterface::Inventory, pwalletIn, _1));g_signals.m_internals->Broadcast.connect(boost::bind(&CValidationInterface::ResendWalletTransactions, pwalletIn, _1, _2));g_signals.m_internals->BlockChecked.connect(boost::bind(&CValidationInterface::BlockChecked, pwalletIn, _1, _2));g_signals.m_internals->NewPoWValidBlock.connect(boost::bind(&CValidationInterface::NewPoWValidBlock, pwalletIn, _1, _2));}void UnregisterValidationInterface(CValidationInterface* pwalletIn) {g_signals.m_internals->BlockChecked.disconnect(boost::bind(&CValidationInterface::BlockChecked, pwalletIn, _1, _2));g_signals.m_internals->Broadcast.disconnect(boost::bind(&CValidationInterface::ResendWalletTransactions, pwalletIn, _1, _2));g_signals.m_internals->Inventory.disconnect(boost::bind(&CValidationInterface::Inventory, pwalletIn, _1));g_signals.m_internals->SetBestChain.disconnect(boost::bind(&CValidationInterface::SetBestChain, pwalletIn, _1));g_signals.m_internals->TransactionAddedToMempool.disconnect(boost::bind(&CValidationInterface::TransactionAddedToMempool, pwalletIn, _1));g_signals.m_internals->BlockConnected.disconnect(boost::bind(&CValidationInterface::BlockConnected, pwalletIn, _1, _2, _3));g_signals.m_internals->BlockDisconnected.disconnect(boost::bind(&CValidationInterface::BlockDisconnected, pwalletIn, _1));g_signals.m_internals->TransactionRemovedFromMempool.disconnect(boost::bind(&CValidationInterface::TransactionRemovedFromMempool, pwalletIn, _1));g_signals.m_internals->UpdatedBlockTip.disconnect(boost::bind(&CValidationInterface::UpdatedBlockTip, pwalletIn, _1, _2, _3));g_signals.m_internals->NewPoWValidBlock.disconnect(boost::bind(&CValidationInterface::NewPoWValidBlock, pwalletIn, _1, _2));}
在程序启动，初始化阶段，启动调度器线程
bool AppInitMain(){.................CScheduler::Function serviceLoop = boost::bind(&CScheduler::serviceQueue, &scheduler);threadGroup.create_thread(boost::bind(&TraceThread, "scheduler", serviceLoop));.................}
调用RegisterBackgroundSignalScheduler(), 初始化全局MainSignalsInstance 实例; 调用RegisterWithMempoolSignals(), 订阅全局内存池对象的NotifyEntryRemoved 事件通知, MainSignalsInstance 只对由于超时，大小限制， blockchian 重组，替换等原因发生的离开内存池事件感兴趣， 收到后MainSignalsInstance 再作为事件发布者， 转发给其他订阅者， 如CWallet。
bool AppInitMain(){......GetMainSignals().RegisterBackgroundSignalScheduler(scheduler);GetMainSignals().RegisterWithMempoolSignals(mempool);........}
初始化全局的连接管理对象connman 后， 初始化全局的peerLogic 对象， 然后调用RegisterValidationInterface(), peerLogic 成为MainSignalsInstance 对象的订阅者, 如果用户编译了zeromq支持模块，调用RegisterValidationInterface(), pzmqNotificationInterface 订阅MainSignalsInstance 。
bool AppInitMain(){.............assert(!g_connman);g_connman = std::unique_ptr(new CConnman(GetRand(std::numeric_limits<uint64_t>::max()), GetRand(std::numeric_limits<uint64_t>::max())));CConnman& connman = *g_connman;
peerLogic.reset(new PeerLogicValidation(&connman, scheduler));RegisterValidationInterface(peerLogic.get());.............#if ENABLE_ZMQpzmqNotificationInterface = CZMQNotificationInterface::Create();if (pzmqNotificationInterface) {RegisterValidationInterface(pzmqNotificationInterface);}#endif...............}
如果钱包功能开启， 启动后打开钱包过程，会把钱包注册成为MainSignalsInstance的订阅者。
bool AppInitMain(){........#ifdef ENABLE_WALLETif (!OpenWallets())return false;#elseLogPrintf("No wallet support compiled in!\n");#endif........}bool OpenWallets(){if (gArgs.GetBoolArg("-disablewallet", DEFAULT_DISABLE_WALLET)) {LogPrintf("Wallet disabled!\n");return true;}for (const std::string& walletFile : gArgs.GetArgs("-wallet")) {CWallet * const pwallet = CWallet::CreateWalletFromFile(walletFile);if (!pwallet) {return false;}vpwallets.push_back(pwallet);}return true;}
CWallet* CWallet::CreateWalletFromFile(const std::string walletFile){......CWallet *walletInstance = new CWallet(std::move(dbw));RegisterValidationInterface(walletInstance);......}
在submitblock rpc调用中， 用户提交hex编码的原始block， 解析后， 调用ProcessNewBlock()检查处理，使用类型submitblock_StateCatcher 的对象sc 作为MainSignalsInstance 的订阅者，
对提交过去的block 的验证结果，作为事件通知返回给rpc 调用。
UniValue submitblock(const JSONRPCRequest& request){...........submitblock_StateCatcher sc(block.GetHash());RegisterValidationInterface(&sc);bool fAccepted = ProcessNewBlock(Params(), blockptr, true, nullptr);UnregisterValidationInterface(&sc);...........}
CMainSignals 类型上面定义了一堆触发事件的方法, 别的代码模块调用这些方法， 触发相应的事件，把事件通知发给相关的订阅者。
void CMainSignals::UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) {m_internals->m_schedulerClient.AddToProcessQueue([pindexNew, pindexFork, fInitialDownload, this] {m_internals->UpdatedBlockTip(pindexNew, pindexFork, fInitialDownload);});}void CMainSignals::TransactionAddedToMempool(const CTransactionRef &ptx) {m_internals->m_schedulerClient.AddToProcessQueue([ptx, this] {m_internals->TransactionAddedToMempool(ptx);});}void CMainSignals::BlockConnected(const std::shared_ptr &pblock, const CBlockIndex *pindex, const std::shared_ptr>& pvtxConflicted) {m_internals->m_schedulerClient.AddToProcessQueue([pblock, pindex, pvtxConflicted, this] {m_internals->BlockConnected(pblock, pindex, *pvtxConflicted);});}void CMainSignals::BlockDisconnected(const std::shared_ptr &pblock) {m_internals->m_schedulerClient.AddToProcessQueue([pblock, this] {m_internals->BlockDisconnected(pblock);});}void CMainSignals::SetBestChain(const CBlockLocator &locator) {m_internals->m_schedulerClient.AddToProcessQueue([locator, this] {m_internals->SetBestChain(locator);});}void CMainSignals::Inventory(const uint256 &hash) {m_internals->m_schedulerClient.AddToProcessQueue([hash, this] {m_internals->Inventory(hash);});}void CMainSignals::Broadcast(int64_t nBestBlockTime, CConnman* connman) {m_internals->Broadcast(nBestBlockTime, connman);}void CMainSignals::BlockChecked(const CBlock& block, const CValidationState& state) {m_internals->BlockChecked(block, state);}void CMainSignals::NewPoWValidBlock(const CBlockIndex *pindex, const std::shared_ptr &block) {m_internals->NewPoWValidBlock(pindex, block);}
CChainState 的ActivateBestChain方法里， 发布BlockConnected， UpdatedBlockTip 事件:
bool CChainState::ActivateBestChain(CValidationState &state, const CChainParams& chainparams, std::shared_ptr pblock) {..............................for (const PerBlockConnectTrace& trace : connectTrace.GetBlocksConnected()) {assert(trace.pblock && trace.pindex);GetMainSignals().BlockConnected(trace.pblock, trace.pindex, trace.conflictedTxs);}..............................GetMainSignals().UpdatedBlockTip(pindexNewTip, pindexFork, fInitialDownload);...............................}
CChainState 的AcceptBlock 方法里，发布NewPoWValidBlock 事件:
bool CChainState::AcceptBlock(const std::shared_ptr& pblock, CValidationState& state, const CChainParams& chainparams, CBlockIndex** ppindex, bool fRequested, const CDiskBlockPos* dbp, bool* fNewBlock){.......................if (!IsInitialBlockDownload() && chainActive.Tip() == pindex->pprev)GetMainSignals().NewPoWValidBlock(pindex, pblock);......................}
CChainState 的DisconnectTip 方法里，发布 BlockDisconnected 事件:
bool CChainState::DisconnectTip(CValidationState& state, const CChainParams& chainparams, DisconnectedBlockTransactions *disconnectpool){...................GetMainSignals().BlockDisconnected(pblock);..............}
CChainState 的ConnectTip 方法里，发布 BlockChecked 事件:
bool CChainState::ConnectTip(CValidationState& state, const CChainParams& chainparams, CBlockIndex* pindexNew, const std::shared_ptr& pblock, ConnectTrace& connectTrace, DisconnectedBlockTransactions &disconnectpool){................CCoinsViewCache view(pcoinsTip.get());bool rv = ConnectBlock(blockConnecting, state, pindexNew, view, chainparams);GetMainSignals().BlockChecked(blockConnecting, state);................}
PeerLogicValidation 的 SendMessage 方法里， 发布Broadcast事件, 通知钱包重新发送未确认的交易:
bool PeerLogicValidation::SendMessages(CNode* pto, std::atomic& interruptMsgProc){.............if (!fReindex && !fImporting && !IsInitialBlockDownload()){GetMainSignals().Broadcast(nTimeBestReceived, connman);}.............}
从网络上收到INV消息后，通知给钱包，更新内部状态。
bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStream& vRecv, int64_t nTimeReceived, const CChainParams& chainparams, CConnman* connman, const std::atomic& interruptMsgProc){.................for (CInv &inv : vInv){............GetMainSignals().Inventory(inv.hash);}................}
validation.cpp 里面的 ProcessNewBlock在内部调用 CheckBlock 后，检查失败后，发布 BlockCheck s事件， 通告给相关订阅者。
bool ProcessNewBlock(const CChainParams& chainparams, const std::shared_ptr pblock, bool fForceProcessing, bool *fNewBlock){............bool ret = CheckBlock(*pblock, state, chainparams.GetConsensus());if (!ret) {GetMainSignals().BlockChecked(*pblock, state);return error("%s: AcceptBlock FAILED (%s)", __func__, state.GetDebugMessage());}...........}
FlushStateToDisk, 发布SetBestChain 事件， 通知钱包
bool static FlushStateToDisk(const CChainParams& chainparams, CValidationState &state, FlushStateMode mode, int nManualPruneHeight) {...............if (fDoFullFlush || ((mode == FLUSH_STATE_ALWAYS || mode == FLUSH_STATE_PERIODIC) && nNow > nLastSetChain + (int64_t)DATABASE_WRITE_INTERVAL * 1000000)) {// Update best block in wallet (so we can detect restored wallets).GetMainSignals().SetBestChain(chainActive.GetLocator());nLastSetChain = nNow;}...............}
validation.cpp 里面的AcceptToMemoryPoolWorker, 在结束前， 发布TransactionAddedToMempool 事件。
static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool& pool, CValidationState& state, const CTransactionRef& ptx,bool* pfMissingInputs, int64_t nAcceptTime, std::list* plTxnReplaced,bool bypass_limits, const CAmount& nAbsurdFee, std::vector& coins_to_uncache){................GetMainSignals().TransactionAddedToMempool(ptx);}