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<uint256> 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:
CCoinsView
/ \
/ \
CCoinsViewDB CCoinsViewBacked
/ | \
/ | \
/ | \
CCoinsViewErrorCatcher CCoinsViewMemPool CCoinsViewCache
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<uint256> 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<CCoinsViewDB> 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));
类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<uint256> 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<uint256> 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\n", 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(不是加密学安全的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。
本文由 Copernicus团队 喻建写作，转载无需授权
网友评论