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Bitcoin UTXO代码分析(一):UTXO的相关表示

在 Bitcoin 代码中,使用 Coin 类来表示单个交易对象中某个输出的币:

class Coin { public: //! unspent transaction output CTxOut out; //! whether containing transaction was a coinbase unsigned int fCoinBase : 1; //! at which height this containing transaction was included in the active block chain uint32_t nHeight : 31; ..... ..... }

数据元素除了CTxOut中的币值(nValue)、花费条件(scriptPubKey)之外, 还附带了一些元信息:是否是coinbase, 所在交易在哪个高度被打包进入 Blockchain。

再使用CCoinsView抽象类表达整个 blockchain 上的币的集合:

class CCoinsView { public: /** Retrieve the Coin (unspent transaction output) for a given outpoint. * Returns true only when an unspent coin was found, which is returned in coin. * When false is returned, coin's value is unspecified. */ virtual bool GetCoin(const COutPoint &outpoint, Coin &coin) const; //! Just check whether a given outpoint is unspent. virtual bool HaveCoin(const COutPoint &outpoint) const; //! Retrieve the block hash whose state this CCoinsView currently represents virtual uint256 GetBestBlock() const; //! Retrieve the range of blocks that may have been only partially written. //! If the database is in a consistent state, the result is the empty vector. //! Otherwise, a two-element vector is returned consisting of the new and //! the old block hash, in that order. virtual std::vector GetHeadBlocks() const; //! Do a bulk modification (multiple Coin changes + BestBlock change). //! The passed mapCoins can be modified. virtual bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock); //! Get a cursor to iterate over the whole state virtual CCoinsViewCursor *Cursor() const; //! As we use CCoinsViews polymorphically, have a virtual destructor virtual ~CCoinsView() {} //! Estimate database size (0 if not implemented) virtual size_t EstimateSize() const { return 0; } };

Cinsview 类作为接口类,有很多具体实现子类:

Bitcoin UTXO代码分析(一):UTXO的相关表示

CoinsViewDB类主要服务于从 Bitcoin 数据目录下的 chainstate 子目录下保存和读取存盘的 UTXO 集合:

class CCoinsViewDB final : public CCoinsView { protected: CDBWrapper db; public: explicit CCoinsViewDB(size_t nCacheSize, bool fMemory = false, bool fWipe = false); bool GetCoin(const COutPoint &outpoint, Coin &coin) const override; bool HaveCoin(const COutPoint &outpoint) const override; uint256 GetBestBlock() const override; std::vector GetHeadBlocks() const override; bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) override; CCoinsViewCursor *Cursor() const override; //! Attempt to update from an older database format. Returns whether an error occurred. bool Upgrade(); size_t EstimateSize() const override; };

此类只有一个全局实例,在validation.cpp中定义:

std::unique_ptr pcoinsdbview;

在init.cpp中进程启动时, 会对改对象进行初始化:

pcoinsdbview.reset(new CCoinsViewDB(nCoinDBCache, false, fReset || fReindexChainState));

CCoinsViewBacked本身没什么实际用处, 主要是作为多个Coinview层级之间的转接层, 它的数据成员 CCoinView *base 指向的就是后端即parent view , 如果某个coinsviewBacked的子类没有覆盖接口类CCoinsView 中的方法, 就会调用base指向的后端相应的方法。

class CCoinsViewBacked : public CCoinsView { protected: CCoinsView *base;public: CCoinsViewBacked(CCoinsView *viewIn); bool GetCoin(const COutPoint &outpoint, Coin &coin) const override; bool HaveCoin(const COutPoint &outpoint) const override; uint256 GetBestBlock() const override; std::vector GetHeadBlocks() const override; void SetBackend(CCoinsView &viewIn); bool BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) override; CCoinsViewCursor *Cursor() const override; size_t EstimateSize() const override; };
CCoinsViewBacked::CCoinsViewBacked(CCoinsView *viewIn) : base(viewIn) { } bool CCoinsViewBacked::GetCoin(const COutPoint &outpoint, Coin &coin) const { return base->GetCoin(outpoint, coin); } bool CCoinsViewBacked::HaveCoin(const COutPoint &outpoint) const { return base->HaveCoin(outpoint); } uint256 CCoinsViewBacked::GetBestBlock() const { return base->GetBestBlock(); } std::vector CCoinsViewBacked::GetHeadBlocks() const { return base->GetHeadBlocks(); } void CCoinsViewBacked::SetBackend(CCoinsView &viewIn) { base = &viewIn; } bool CCoinsViewBacked::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) { return base->BatchWrite(mapCoins, hashBlock); } CCoinsViewCursor *CCoinsViewBacked::Cursor() const { return base->Cursor(); } size_t CCoinsViewBacked::EstimateSize() const { return base->EstimateSize(); }

CCoinsViewErrorCatcher , CCoinsViewMemPool , CCoinsViewCache 三个定制实现在初始化时需要指定parent view,所以要继承于CCoinsViewBacked类。

coinsviewErrorCatcher 主要用途是包装对数据库读取做错误处理,后端是全局的磁盘实现pcoinsdbview。

class CCoinsViewErrorCatcher final : public CCoinsViewBacked { public: explicit CCoinsViewErrorCatcher(CCoinsView* view) : CCoinsViewBacked(view) {} bool GetCoin(const COutPoint &outpoint, Coin &coin) const override { try { return CCoinsViewBacked::GetCoin(outpoint, coin); } catch(const std::runtime_error& e) { uiInterface.ThreadSafeMessageBox(_("Error reading from database, shutting down."), "", CClientUIInterface::MSG_ERROR); LogPrintf("Error reading from database: %s
", e.what()); abort(); } } };

启动时的初始化代码:

//init.cpp AppInitMain() pcoinscatcher.reset(new CCoinsViewErrorCatcher(pcoinsdbview.get()));

CCoinsViewCache 类是一个内存缓存的实现,内部使用hashmap 存储了某个outpoint 到Coin对象的映射,有一个全局实例pcoinsTip , 指向atctiveChain 的utxo,后端是磁盘实现CCoinsViewDB对象pcoinsdbview。

class CCoinsViewCache : public CCoinsViewBacked { protected: /** * Make mutable so that we can "fill the cache" even from Get-methods * declared as "const". */ mutable uint256 hashBlock; mutable CCoinsMap cacheCoins; /* Cached dynamic memory usage for the inner Coin objects. */ mutable size_t cachedCoinsUsage; ... ... }

启动时的初始化代码:

//init.cpp AppInitMain() pcoinsTip.reset(new CCoinsViewCache(pcoinscatcher.get()));

它的内部hashmap使用了定制的hash 方法siphash, 没有使用默认的std::hash方法(不是加密安全的hash), 估计是防止hash的key冲突,:

typedef std::unordered_map<COutPoint, CCoinsCacheEntry, SaltedOutpointHasher> CCoinsMap;class SaltedOutpointHasher { private: /** Salt */ const uint64_t k0, k1;public: SaltedOutpointHasher();
size_t operator()(const COutPoint& id) const { return SipHashUint256Extra(k0, k1, id.hash, id.n); } };

这篇文章介绍了表示UTXO的相关表示的数据结构,下一篇文章将会UTXO的标记以及保存。

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