了解了区块和交易的数据结构，接下来就是介于这两者之间的一个重要的数据结构：交易池。
当比特币网络把某个时刻产生的交易广播到网络时，矿工接收到交易后并不是立即打包到备选区块。而是将接收到的交易放到类似缓冲区的一个交易池里，然后会根据一定的优先顺序来选择交易打包，以此来保障自己能获得尽可能多的交易费。
所以了解交易池的数据结构，对理解矿工打包交易会有很大的裨益。
首先我猜测有关交易池的类可能叫transactionPool，于是我试着全局搜这个词：
于是，我扩大搜索范围，搜索pool。搜到的结果很多，我大概地往下翻，试图寻找是交易池的那个。找到很多处mempool，于是我点进去试着搜索mempool，找到了txmempool.h这个头文件，交易内存池，应该就是这个了。
- 代码路径: bitcoin/src/txmempool.h
/** Fake height value used in Coin to signify they are only in the memory pool (since 0.8) */
//一个"假"的高度值，用来标识它们只存在于交易池中
static const uint32_t MEMPOOL_HEIGHT = 0x7FFFFFFF;
//交易锁定点,交易最后的区块高度和打包时间
struct LockPoints
{
// Will be set to the blockchain height and median time past
// values that would be necessary to satisfy all relative locktime
// constraints (BIP68) of this tx given our view of block chain history
/**
* 将设置为区块链高度和中值时间过去值,
* 这些值对于满足tx相对时间锁是至关重要的(BIP68)
*
*/
int height;
int64_t time;
// As long as the current chain descends from the highest height block
// containing one of the inputs used in the calculation, then the cached
// values are still valid even after a reorg.
/**
* 只要当前链包含计算中使用的某个输入的最高快高度，
* 则即使在链重新构建后缓存的值依然有效
*/
CBlockIndex* maxInputBlock;
LockPoints() : height(0), time(0), maxInputBlock(nullptr) { }
};class CTxMemPool;
/** \class CTxMemPoolEntry
*
* CTxMemPoolEntry stores data about the corresponding transaction, as well
* as data about all in-mempool transactions that depend on the transaction
* ("descendant" transactions).
*
* When a new entry is added to the mempool, we update the descendant state
* (nCountWithDescendants, nSizeWithDescendants, and nModFeesWithDescendants) for
* all ancestors of the newly added transaction.
*
**CTxMemPoolEntry 存储相应的交易
* 以及该交易对应的所有子孙交易
*
* 当一个新的CTxMemPoolEntry被添加到交易池，我们会更新新添加交易的所有子孙交易的状态
* (包括子孙交易数量，大小，和交易费用）和祖父交易状态
*/
//交易池基本构成元素
class CTxMemPoolEntry
{
private:
CTransactionRef tx; //交易引用
CAmount nFee; //交易费用 //!< Cached to avoid expensive parent-transaction lookups
size_t nTxWeight; // //!< ... and avoid recomputing tx weight (also used for GetTxSize())
size_t nUsageSize; //大小 //!< ... and total memory usage
int64_t nTime; //交易时间戳 //!< Local time when entering the mempool
unsigned int entryHeight; //区块高度 //!< Chain height when entering the mempool
bool spendsCoinbase; //上个交易是否是创币交易 //!< keep track of transactions that spend a coinbase
int64_t sigOpCost; //？？？ !< Total sigop cost
int64_t feeDelta; //交易优先级的一个标量 //!< Used for determining the priority of the transaction for mining in a block
LockPoints lockPoints; //锁定点，交易最后的区块高度和打包时间 //!< Track the height and time at which tx was final
// Information about descendants of this transaction that are in the
// mempool; if we remove this transaction we must remove all of these
// descendants as well.
/*
** 子孙交易信息
* 如果我们移除一个交易，我们也必须同时移除它所有的子孙交易
*/
uint64_t nCountWithDescendants; //子孙交易数量 //!< number of descendant transactions
uint64_t nSizeWithDescendants; //大小 //!< ... and size
CAmount nModFeesWithDescendants; //费用总和，包括当前交易 //!< ... and total fees (all including us)
// Analogous statistics for ancestor transactions
//祖先交易信息
uint64_t nCountWithAncestors; //祖先交易数量
uint64_t nSizeWithAncestors; //大小
CAmount nModFeesWithAncestors; //费用总和
int64_t nSigOpCostWithAncestors; //？？？
public:
CTxMemPoolEntry(const CTransactionRef& _tx, const CAmount& _nFee,
int64_t _nTime, unsigned int _entryHeight,
bool spendsCoinbase,
int64_t nSigOpsCost, LockPoints lp);
const CTransaction& GetTx() const { return *this->tx; }
CTransactionRef GetSharedTx() const { return this->tx; }
const CAmount& GetFee() const { return nFee; }
size_t GetTxSize() const;
size_t GetTxWeight() const { return nTxWeight; }
int64_t GetTime() const { return nTime; }
unsigned int GetHeight() const { return entryHeight; }
int64_t GetSigOpCost() const { return sigOpCost; }
int64_t GetModifiedFee() const { return nFee + feeDelta; }
size_t DynamicMemoryUsage() const { return nUsageSize; }
const LockPoints& GetLockPoints() const { return lockPoints; }
// Adjusts the descendant state.
// 更新子孙交易状态
void UpdateDescendantState(int64_t modifySize, CAmount modifyFee, int64_t modifyCount);
// Adjusts the ancestor state
// 更新祖先交易状态
void UpdateAncestorState(int64_t modifySize, CAmount modifyFee, int64_t modifyCount, int64_t modifySigOps);
// Updates the fee delta used for mining priority score, and the
// modified fees with descendants.
// 更新交易优先级
void UpdateFeeDelta(int64_t feeDelta);
// Update the LockPoints after a reorg
// 更新锁定点
void UpdateLockPoints(const LockPoints& lp);
//获取子孙交易信息
uint64_t GetCountWithDescendants() const { return nCountWithDescendants; }
uint64_t GetSizeWithDescendants() const { return nSizeWithDescendants; }
CAmount GetModFeesWithDescendants() const { return nModFeesWithDescendants; }
bool GetSpendsCoinbase() const { return spendsCoinbase; }
//获取祖先交易信息
uint64_t GetCountWithAncestors() const { return nCountWithAncestors; }
uint64_t GetSizeWithAncestors() const { return nSizeWithAncestors; }
CAmount GetModFeesWithAncestors() const { return nModFeesWithAncestors; }
int64_t GetSigOpCostWithAncestors() const { return nSigOpCostWithAncestors; }
mutable size_t vTxHashesIdx; //交易池哈希的下标 //!< Index in mempool's vTxHashes
};/** \class CompareTxMemPoolEntryByDescendantScore
*
* Sort an entry by max(score/size of entry's tx, score/size with all descendants).
*
** 按score/size原则对CTxMemPoolEntry排序
*/
class CompareTxMemPoolEntryByDescendantScore
{
public:
bool operator()(const CTxMemPoolEntry& a, const CTxMemPoolEntry& b) const
{
double a_mod_fee, a_size, b_mod_fee, b_size;
GetModFeeAndSize(a, a_mod_fee, a_size);
GetModFeeAndSize(b, b_mod_fee, b_size);
// Avoid division by rewriting (a/b > c/d) as (a*d > c*b).
double f1 = a_mod_fee * b_size;
double f2 = a_size * b_mod_fee;
if (f1 == f2) {
return a.GetTime() >= b.GetTime();
}
return f1 < f2;
}
// Return the fee/size we're using for sorting this entry.
void GetModFeeAndSize(const CTxMemPoolEntry &a, double &mod_fee, double &size) const
{
// Compare feerate with descendants to feerate of the transaction, and
// return the fee/size for the max.
double f1 = (double)a.GetModifiedFee() * a.GetSizeWithDescendants();
double f2 = (double)a.GetModFeesWithDescendants() * a.GetTxSize();
if (f2 > f1) {
mod_fee = a.GetModFeesWithDescendants();
size = a.GetSizeWithDescendants();
} else {
mod_fee = a.GetModifiedFee();
size = a.GetTxSize();
}
}
};
/** \class CompareTxMemPoolEntryByScore
*
* Sort by score of entry ((fee+delta)/size) in descending order
**
* 按(fee+delta)/size原则对CTxMemPoolEntry排序
*/
class CompareTxMemPoolEntryByScore
{
public:
bool operator()(const CTxMemPoolEntry& a, const CTxMemPoolEntry& b) const
{
double f1 = (double)a.GetModifiedFee() * b.GetTxSize();
double f2 = (double)b.GetModifiedFee() * a.GetTxSize();
if (f1 == f2) {
return b.GetTx().GetHash() < a.GetTx().GetHash();
}
return f1 > f2;
}
};
//按时间CTxMemPoolEntry对排序
class CompareTxMemPoolEntryByEntryTime
{
public:
bool operator()(const CTxMemPoolEntry& a, const CTxMemPoolEntry& b) const
{
return a.GetTime() < b.GetTime();
}
};
/** \class CompareTxMemPoolEntryByAncestorScore
*
* Sort an entry by min(score/size of entry's tx, score/size with all ancestors).
*/
class CompareTxMemPoolEntryByAncestorFee
{
public:
template<typename T>
bool operator()(const T& a, const T& b) const
{
double a_mod_fee, a_size, b_mod_fee, b_size;
GetModFeeAndSize(a, a_mod_fee, a_size);
GetModFeeAndSize(b, b_mod_fee, b_size);
// Avoid division by rewriting (a/b > c/d) as (a*d > c*b).
double f1 = a_mod_fee * b_size;
double f2 = a_size * b_mod_fee;
if (f1 == f2) {
return a.GetTx().GetHash() < b.GetTx().GetHash();
}
return f1 > f2;
}
// Return the fee/size we're using for sorting this entry.
template <typename T>
void GetModFeeAndSize(const T &a, double &mod_fee, double &size) const
{
// Compare feerate with ancestors to feerate of the transaction, and
// return the fee/size for the min.
double f1 = (double)a.GetModifiedFee() * a.GetSizeWithAncestors();
double f2 = (double)a.GetModFeesWithAncestors() * a.GetTxSize();
if (f1 > f2) {
mod_fee = a.GetModFeesWithAncestors();
size = a.GetSizeWithAncestors();
} else {
mod_fee = a.GetModifiedFee();
size = a.GetTxSize();
}
}
};/**
* Information about a mempool transaction.
* 交易进入内存池的信息
*/
struct TxMempoolInfo
{
/** The transaction itself */
CTransactionRef tx; //交易引用
/** Time the transaction entered the mempool. */
int64_t nTime; //交易进入内存池时间
/** Feerate of the transaction. */
CFeeRate feeRate; //交易费率
/** The fee delta. */
int64_t nFeeDelta; //交易优先级
};/** Reason why a transaction was removed from the mempool,
* this is passed to the notification signal.
*
* 交易被移出内存池的原因
*/
enum class MemPoolRemovalReason {
UNKNOWN = 0, //未知原因 //! Manually removed or unknown reason
EXPIRY, //过期 //! Expired from mempool
SIZELIMIT, //大小限制 //! Removed in size limiting
REORG, //被重组 //! Removed for reorganization
BLOCK, //因为区块 //! Removed for block
CONFLICT, //区块内交易冲突//! Removed for conflict with in-block transaction
REPLACED //被替代 //! Removed for replacement
};/**
* CTxMemPool stores valid-according-to-the-current-best-chain transactions
* that may be included in the next block.
*
* CTxMemPool 保存当前主链所有的交易。这些交易有可能被加入到下一个有效区块中
*
* Transactions are added when they are seen on the network (or created by the
* local node), but not all transactions seen are added to the pool. For
* example, the following new transactions will not be added to the mempool:
* - a transaction which doesn't meet the minimum fee requirements.
* - a new transaction that double-spends an input of a transaction already in
* the pool where the new transaction does not meet the Replace-By-Fee
* requirements as defined in BIP 125.
* - a non-standard transaction.
*
**当交易在比特币网络上广播时会被加入到交易池。
* 比如以下新的交易将不会被加入到交易池中：
* - 1.没有满足最低交易费的交易
* - 2."双花"交易
* - 3.一个非标准交易
*
* CTxMemPool::mapTx, and CTxMemPoolEntry bookkeeping:
*
* mapTx is a boost::multi_index that sorts the mempool on 4 criteria:
* - transaction hash //交易hash
* - //交易费率(包括所有子孙交易)
* - feerate [we use max(feerate of tx, feerate of tx with all descendants)]
* - time in mempool //加入交易池的时间
*
* Note: the term "descendant" refers to in-mempool transactions that depend on
* this one, while "ancestor" refers to in-mempool transactions that a given
* transaction depends on.
*
* In order for the feerate sort to remain correct, we must update transactions
* in the mempool when new descendants arrive. To facilitate this, we track
* the set of in-mempool direct parents and direct children in mapLinks. Within
* each CTxMemPoolEntry, we track the size and fees of all descendants.
*
** 为了保障交易费的正确性，当新交易被加入到交易池时，我们必须更新该交易的所有祖先交易和子孙交易。
*
* Usually when a new transaction is added to the mempool, it has no in-mempool
* children (because any such children would be an orphan). So in
* addUnchecked(), we:
* - update a new entry's setMemPoolParents to include all in-mempool parents
* - update the new entry's direct parents to include the new tx as a child
* - update all ancestors of the transaction to include the new tx's size/fee
*
* When a transaction is removed from the mempool, we must:
* - update all in-mempool parents to not track the tx in setMemPoolChildren
* - update all ancestors to not include the tx's size/fees in descendant state
* - update all in-mempool children to not include it as a parent
*
* These happen in UpdateForRemoveFromMempool(). (Note that when removing a
* transaction along with its descendants, we must calculate that set of
* transactions to be removed before doing the removal, or else the mempool can
* be in an inconsistent state where it's impossible to walk the ancestors of
* a transaction.)
*
* In the event of a reorg, the assumption that a newly added tx has no
* in-mempool children is false. In particular, the mempool is in an
* inconsistent state while new transactions are being added, because there may
* be descendant transactions of a tx coming from a disconnected block that are
* unreachable from just looking at transactions in the mempool (the linking
* transactions may also be in the disconnected block, waiting to be added).
* Because of this, there's not much benefit in trying to search for in-mempool
* children in addUnchecked(). Instead, in the special case of transactions
* being added from a disconnected block, we require the caller to clean up the
* state, to account for in-mempool, out-of-block descendants for all the
* in-block transactions by calling UpdateTransactionsFromBlock(). Note that
* until this is called, the mempool state is not consistent, and in particular
* mapLinks may not be correct (and therefore functions like
* CalculateMemPoolAncestors() and CalculateDescendants() that rely
* on them to walk the mempool are not generally safe to use).
*
* Computational limits:
*
* Updating all in-mempool ancestors of a newly added transaction can be slow,
* if no bound exists on how many in-mempool ancestors there may be.
* CalculateMemPoolAncestors() takes configurable limits that are designed to
* prevent these calculations from being too CPU intensive.
*
*/
class CTxMemPool
{
private:
uint32_t nCheckFrequency; //2^32时间检查的次数 //!< Value n means that n times in 2^32 we check.
unsigned int nTransactionsUpdated; //!< Used by getblocktemplate to trigger CreateNewBlock() invocation
CBlockPolicyEstimator* minerPolicyEstimator;
uint64_t totalTxSize; //交易池虚拟大小，不包括见证数据 //!< sum of all mempool tx's virtual sizes. Differs from serialized tx size since witness data is discounted. Defined in BIP 141.
uint64_t cachedInnerUsage; //map使用的动态内存大小 //!< sum of dynamic memory usage of all the map elements (NOT the maps themselves)
mutable int64_t lastRollingFeeUpdate;
mutable bool blockSinceLastRollingFeeBump;
mutable double rollingMinimumFeeRate; //进入交易池需要满足的最小费用 //!< minimum fee to get into the pool, decreases exponentially
void trackPackageRemoved(const CFeeRate& rate);
public:
static const int ROLLING_FEE_HALFLIFE = 60 * 60 * 12; // public only for testing
typedef boost::multi_index_container<
CTxMemPoolEntry,
boost::multi_index::indexed_by<
// sorted by txid 根据交易哈希排序
boost::multi_index::hashed_unique<mempoolentry_txid, SaltedTxidHasher>,
// sorted by fee rate 交易费
boost::multi_index::ordered_non_unique<
boost::multi_index::tag<descendant_score>,
boost::multi_index::identity<CTxMemPoolEntry>,
CompareTxMemPoolEntryByDescendantScore
>,
// sorted by entry time 进入交易池的时间
boost::multi_index::ordered_non_unique<
boost::multi_index::tag<entry_time>,
boost::multi_index::identity<CTxMemPoolEntry>,
CompareTxMemPoolEntryByEntryTime
>,
// sorted by fee rate with ancestors 祖父交易交易费
boost::multi_index::ordered_non_unique<
boost::multi_index::tag<ancestor_score>,
boost::multi_index::identity<CTxMemPoolEntry>,
CompareTxMemPoolEntryByAncestorFee
>
>
> indexed_transaction_set;
mutable CCriticalSection cs;
indexed_transaction_set mapTx;
typedef indexed_transaction_set::nth_index<0>::type::iterator txiter;
std::vector<std::pair<uint256, txiter> > vTxHashes; //见证数据的哈希 //!< All tx witness hashes/entries in mapTx, in random order
struct CompareIteratorByHash {
bool operator()(const txiter &a, const txiter &b) const {
return a->GetTx().GetHash() < b->GetTx().GetHash();
}
};
typedef std::set<txiter, CompareIteratorByHash> setEntries;
const setEntries & GetMemPoolParents(txiter entry) const;
const setEntries & GetMemPoolChildren(txiter entry) const;
private:
typedef std::map<txiter, setEntries, CompareIteratorByHash> cacheMap;
struct TxLinks {
setEntries parents;
setEntries children;
};
typedef std::map<txiter, TxLinks, CompareIteratorByHash> txlinksMap;
txlinksMap mapLinks;
void UpdateParent(txiter entry, txiter parent, bool add);
void UpdateChild(txiter entry, txiter child, bool add);
std::vector<indexed_transaction_set::const_iterator> GetSortedDepthAndScore() const;
public:
indirectmap<COutPoint, const CTransaction*> mapNextTx;
std::map<uint256, CAmount> mapDeltas;
/** Create a new CTxMemPool.
* 创建一个新的交易池
*/
explicit CTxMemPool(CBlockPolicyEstimator* estimator = nullptr);
/**
* If sanity-checking is turned on, check makes sure the pool is
* consistent (does not contain two transactions that spend the same inputs,
* all inputs are in the mapNextTx array). If sanity-checking is turned off,
* check does nothing.
*
**如果开启了sanity-check，check函数将保证pool的一致性(不包含两个在同一个输入中的交易)
* 所有的输入都在mapNextTx数组里；sanity-check关闭，check函数无效
*
*/
void check(const CCoinsViewCache *pcoins) const;
void setSanityCheck(double dFrequency = 1.0) { nCheckFrequency = static_cast<uint32_t>(dFrequency * 4294967295.0); }
// addUnchecked must updated state for all ancestors of a given transaction,
// to track size/count of descendant transactions. First version of
// addUnchecked can be used to have it call CalculateMemPoolAncestors(), and
// then invoke the second version.
// Note that addUnchecked is ONLY called from ATMP outside of tests
// and any other callers may break wallet's in-mempool tracking (due to
// lack of CValidationInterface::TransactionAddedToMempool callbacks).
/**
* addUnchecked函数必先更新祖先交易的状态
* 第一个addUnchecked函数可以用来调用CalculateMemPoolAncestors
* 然后再调用第二个addUnchecked
*/
bool addUnchecked(const uint256& hash, const CTxMemPoolEntry &entry, bool validFeeEstimate = true);
bool addUnchecked(const uint256& hash, const CTxMemPoolEntry &entry, setEntries &setAncestors, bool validFeeEstimate = true);
void removeRecursive(const CTransaction &tx, MemPoolRemovalReason reason = MemPoolRemovalReason::UNKNOWN);
void removeForReorg(const CCoinsViewCache *pcoins, unsigned int nMemPoolHeight, int flags);
void removeConflicts(const CTransaction &tx);
void removeForBlock(const std::vector<CTransactionRef>& vtx, unsigned int nBlockHeight);
void clear();
void _clear(); //lock free
bool CompareDepthAndScore(const uint256& hasha, const uint256& hashb);
void queryHashes(std::vector<uint256>& vtxid);
bool isSpent(const COutPoint& outpoint);
unsigned int GetTransactionsUpdated() const;
void AddTransactionsUpdated(unsigned int n);
/**
* Check that none of this transactions inputs are in the mempool, and thus
* the tx is not dependent on other mempool transactions to be included in a block.
**
* 检查交易的输入是否在当前交易池中
*/
bool HasNoInputsOf(const CTransaction& tx) const;
/** Affect CreateNewBlock prioritisation of transactions */
//调整CreateNewBlock时的交易优先级
void PrioritiseTransaction(const uint256& hash, const CAmount& nFeeDelta);
void ApplyDelta(const uint256 hash, CAmount &nFeeDelta) const;
void ClearPrioritisation(const uint256 hash);
public:
/** Remove a set of transactions from the mempool.
* If a transaction is in this set, then all in-mempool descendants must
* also be in the set, unless this transaction is being removed for being
* in a block.
* Set updateDescendants to true when removing a tx that was in a block, so
* that any in-mempool descendants have their ancestor state updated.
**
* 从mempool中移除一个交易集合，
* 如果一个交易在这个集合中，那么它的所有子孙交易都必须在集合中，
* 除非该交易已经被打包到区块中。
* 如果要移除一个已经被打包到区块中的交易，
* 那么要把updateDescendants设为true，
* 从而更新mempool中所有子孙节点的祖先信息
*/
void RemoveStaged(setEntries &stage, bool updateDescendants, MemPoolRemovalReason reason = MemPoolRemovalReason::UNKNOWN);
/** When adding transactions from a disconnected block back to the mempool,
* new mempool entries may have children in the mempool (which is generally
* not the case when otherwise adding transactions).
* UpdateTransactionsFromBlock() will find child transactions and update the
* descendant state for each transaction in vHashesToUpdate (excluding any
* child transactions present in vHashesToUpdate, which are already accounted
* for). Note: vHashesToUpdate should be the set of transactions from the
* disconnected block that have been accepted back into the mempool.
**
* 更新每一个交易的所有子孙交易状态
*
*/
void UpdateTransactionsFromBlock(const std::vector<uint256> &vHashesToUpdate);
/** Try to calculate all in-mempool ancestors of entry.
* (these are all calculated including the tx itself)
* limitAncestorCount = max number of ancestors
* limitAncestorSize = max size of ancestors
* limitDescendantCount = max number of descendants any ancestor can have
* limitDescendantSize = max size of descendants any ancestor can have
* errString = populated with error reason if any limits are hit
* fSearchForParents = whether to search a tx's vin for in-mempool parents, or
* look up parents from mapLinks. Must be true for entries not in the mempool
**
* 计算mempool中所有entry的祖先
* limitAncestorCount = 最大祖先数量
* limitAncestorSize = 最大祖先交易大小
* limitDescendantCount = 任意祖先的最大子孙数量
* limitDescendantSize = 任意祖先的最大子孙大小
* errString = 超过了任何limit限制的错误提示
* fSearchForParents = 是否在mempool中搜索交易的输入，
* 或者从mapLinks中查找，对于不在mempool中的entry必须设为true
*/
bool CalculateMemPoolAncestors(const CTxMemPoolEntry &entry, setEntries &setAncestors, uint64_t limitAncestorCount, uint64_t limitAncestorSize, uint64_t limitDescendantCount, uint64_t limitDescendantSize, std::string &errString, bool fSearchForParents = true) const;
/** Populate setDescendants with all in-mempool descendants of hash.
* Assumes that setDescendants includes all in-mempool descendants of anything
* already in it. */
//计算所有子孙交易
void CalculateDescendants(txiter it, setEntries &setDescendants);
/** The minimum fee to get into the mempool, which may itself not be enough
* for larger-sized transactions.
* The incrementalRelayFee policy variable is used to bound the time it
* takes the fee rate to go back down all the way to 0. When the feerate
* would otherwise be half of this, it is set to 0 instead.
**
* 获取进入交易池需要满足的最小交易费，本身可能不够适用于大型交易
* incrementalRelayFee变量用来限制feerate降到0所需的时间
* 当交易费是它的一半时，它被设置为0
*/
CFeeRate GetMinFee(size_t sizelimit) const;
/** Remove transactions from the mempool until its dynamic size is <= sizelimit.
* pvNoSpendsRemaining, if set, will be populated with the list of outpoints
* which are not in mempool which no longer have any spends in this mempool.
**
* 移除所有动态大小超过sizelimit的交易，
* 如果传入了pvNoSpendsRemaining，那么将返回不在mempool中并且也没有
* 任何输出在mempool的交易列表
*/
void TrimToSize(size_t sizelimit, std::vector<COutPoint>* pvNoSpendsRemaining=nullptr);
/** Expire all transaction (and their dependencies) in the mempool older than time. Return the number of removed transactions. */
/*
** 移除所有在time之前的交易和它的子孙交易，
* 并返回被移除交易的数量
int Expire(int64_t time);
/** Returns false if the transaction is in the mempool and not within the chain limit specified. */
//如果交易不满足chain limit，返回false
bool TransactionWithinChainLimit(const uint256& txid, size_t chainLimit) const;
unsigned long size()
{
LOCK(cs);
return mapTx.size();
}
uint64_t GetTotalTxSize() const
{
LOCK(cs);
return totalTxSize;
}
bool exists(uint256 hash) const
{
LOCK(cs);
return (mapTx.count(hash) != 0);
}
CTransactionRef get(const uint256& hash) const;
TxMempoolInfo info(const uint256& hash) const;
std::vector<TxMempoolInfo> infoAll() const;
size_t DynamicMemoryUsage() const;
boost::signals2::signal<void (CTransactionRef)> NotifyEntryAdded;
boost::signals2::signal<void (CTransactionRef, MemPoolRemovalReason)> NotifyEntryRemoved;
private:
/** UpdateForDescendants is used by UpdateTransactionsFromBlock to update
* the descendants for a single transaction that has been added to the
* mempool but may have child transactions in the mempool, eg during a
* chain reorg. setExclude is the set of descendant transactions in the
* mempool that must not be accounted for (because any descendants in
* setExclude were added to the mempool after the transaction being
* updated and hence their state is already reflected in the parent
* state).
*
* cachedDescendants will be updated with the descendants of the transaction
* being updated, so that future invocations don't need to walk the
* same transaction again, if encountered in another transaction chain.
**
* UpdateForDescendants 是被 UpdateTransactionsFromBlock 调用，
* 用来更新被加入pool中的单个交易的子孙节节点；
* setExclude 是内存池中不用更新的子孙交易集合 (because any descendants in
* setExclude were added to the mempool after the transaction being
* updated and hence their state is already reflected in the parent
* state).
*
* 当子孙交易被更新时，cachedDescendants也同时更新
*/
void UpdateForDescendants(txiter updateIt,
cacheMap &cachedDescendants,
const std::set<uint256> &setExclude);
/** Update ancestors of hash to add/remove it as a descendant transaction. */
//更新一个祖先交易去添加或移除 为一个子孙交易
void UpdateAncestorsOf(bool add, txiter hash, setEntries &setAncestors);
/** Set ancestor state for an entry */
//设置一个祖先交易
void UpdateEntryForAncestors(txiter it, const setEntries &setAncestors);
/** For each transaction being removed, update ancestors and any direct children.
* If updateDescendants is true, then also update in-mempool descendants'
* ancestor state. */
/** 对于每一个要移除的交易，更新它的祖先和直接的儿子。
* 如果updateDescendants 设为 true, 那么还同时更新mempool中子孙的祖先状态
*/
void UpdateForRemoveFromMempool(const setEntries &entriesToRemove, bool updateDescendants);
/** Sever link between specified transaction and direct children. */
//切断指定交易与直接子女之间的链接
void UpdateChildrenForRemoval(txiter entry);
/** Before calling removeUnchecked for a given transaction,
* UpdateForRemoveFromMempool must be called on the entire (dependent) set
* of transactions being removed at the same time. We use each
* CTxMemPoolEntry's setMemPoolParents in order to walk ancestors of a
* given transaction that is removed, so we can't remove intermediate
* transactions in a chain before we've updated all the state for the
* removal.
**
* 对于一个特定的交易，调用 removeUnchecked 之前，
* 必须为同时为要移除的交易集合调用UpdateForRemoveFromMempool。
* 我们使用每个CTxMemPoolEntry中的setMemPoolParents来遍历
* 要移除交易的祖先，这样能保证我们更新的正确性。
*/
void removeUnchecked(txiter entry, MemPoolRemovalReason reason = MemPoolRemovalReason::UNKNOWN);
};有关交易池的概念，光头文件就900多行。我们先大概了解下一个交易池(TxMemPool)是由若干个CTxMemPoolEntry构成。然后对交易池某些关键函数知道其意思，以后具体遇到了再回过头来查看。