// TxPool contains all currently known transactions. Transactions
// enter the pool when they are received from the network or submitted
// locally. They exit the pool when they are included in the blockchain.
// TxPool 包含了当前知的交易， 当前网络接收到交易，或者本地提交的交易会加入到TxPool。
// 当他们已经被添加到区块链的时候被移除。
// The pool separates processable transactions (which can be applied to the
// current state) and future transactions. Transactions move between those
// two states over time as they are received and processed.
// TxPool分为可执行的交易(可以应用到当前的状态)和未来的交易。 交易在这两种状态之间转换，
type TxPool struct {
	config       TxPoolConfig
	chainconfig  *params.ChainConfig
	chain        blockChain
	gasPrice     *big.Int             //最低的GasPrice限制
	txFeed       event.Feed	          //通过txFeed来订阅TxPool的消息
	scope        event.SubscriptionScope
	chainHeadCh  chan ChainHeadEvent  // 订阅了区块头的消息，当有了新的区块头生成的时候会在这里收到通知
	chainHeadSub event.Subscription   // 区块头消息的订阅器。
	signer       types.Signer		  // 封装了事务签名处理。
	mu           sync.RWMutex

	currentState  *state.StateDB      // Current state in the blockchain head
	pendingState  *state.ManagedState // Pending state tracking virtual nonces
	currentMaxGas *big.Int            // Current gas limit for transaction caps 目前交易上限的GasLimit

	locals  *accountSet // Set of local transaction to exepmt from evicion rules  本地交易免除驱逐规则
	journal *txJournal  // Journal of local transaction to back up to disk 本地交易会写入磁盘

	pending map[common.Address]*txList         // All currently processable transactions 所有当前可以处理的交易
	queue   map[common.Address]*txList         // Queued but non-processable transactions 当前还不能处理的交易
	beats   map[common.Address]time.Time       // Last heartbeat from each known account 每一个已知账号的最后一次心跳信息的时间
	all     map[common.Hash]*types.Transaction // All transactions to allow lookups 可以查找到所有交易
	priced  *txPricedList                      // All transactions sorted by price 按照价格排序的交易

	wg sync.WaitGroup // for shutdown sync

	homestead bool  // 家园版本
}

// NewTxPool creates a new transaction pool to gather, sort and filter inbound
// trnsactions from the network.
func NewTxPool(config TxPoolConfig, chainconfig *params.ChainConfig, chain blockChain) *TxPool {
	// Sanitize the input to ensure no vulnerable gas prices are set
	config = (&config).sanitize()

	// Create the transaction pool with its initial settings
	pool := &TxPool{
		config:      config,
		chainconfig: chainconfig,
		chain:       chain,
		signer:      types.NewEIP155Signer(chainconfig.ChainId),
		pending:     make(map[common.Address]*txList),
		queue:       make(map[common.Address]*txList),
		beats:       make(map[common.Address]time.Time),
		all:         make(map[common.Hash]*types.Transaction),
		chainHeadCh: make(chan ChainHeadEvent, chainHeadChanSize),
		gasPrice:    new(big.Int).SetUint64(config.PriceLimit),
	}
	pool.locals = newAccountSet(pool.signer)
	pool.priced = newTxPricedList(&pool.all)
	pool.reset(nil, chain.CurrentBlock().Header())

	// If local transactions and journaling is enabled, load from disk
	// 如果本地交易被允许,而且配置的Journal目录不为空,那么从指定的目录加载日志.
	// 然后rotate交易日志. 因为老的交易可能已经失效了, 所以调用add方法之后再把被接收的交易写入日志.
	// 
	if !config.NoLocals && config.Journal != "" {
		pool.journal = newTxJournal(config.Journal)

		if err := pool.journal.load(pool.AddLocal); err != nil {
			log.Warn("Failed to load transaction journal", "err", err)
		}
		if err := pool.journal.rotate(pool.local()); err != nil {
			log.Warn("Failed to rotate transaction journal", "err", err)
		}
	}
	// Subscribe events from blockchain 从区块链订阅事件。
	pool.chainHeadSub = pool.chain.SubscribeChainHeadEvent(pool.chainHeadCh)

	// Start the event loop and return
	pool.wg.Add(1)
	go pool.loop()

	return pool
}

// reset retrieves the current state of the blockchain and ensures the content
// of the transaction pool is valid with regard to the chain state.
func (pool *TxPool) reset(oldHead, newHead *types.Header) {
	// If we're reorging an old state, reinject all dropped transactions
	var reinject types.Transactions

	if oldHead != nil && oldHead.Hash() != newHead.ParentHash {
		// If the reorg is too deep, avoid doing it (will happen during fast sync)
		oldNum := oldHead.Number.Uint64()
		newNum := newHead.Number.Uint64()

		if depth := uint64(math.Abs(float64(oldNum) - float64(newNum))); depth > 64 { //如果老的头和新的头差距太远, 那么取消重建
			log.Warn("Skipping deep transaction reorg", "depth", depth)
		} else {
			// Reorg seems shallow enough to pull in all transactions into memory
			var discarded, included types.Transactions

			var (
				rem = pool.chain.GetBlock(oldHead.Hash(), oldHead.Number.Uint64())
				add = pool.chain.GetBlock(newHead.Hash(), newHead.Number.Uint64())
			)
			// 如果老的高度大于新的.那么需要把多的全部删除.
			for rem.NumberU64() > add.NumberU64() {
				discarded = append(discarded, rem.Transactions()...)
				if rem = pool.chain.GetBlock(rem.ParentHash(), rem.NumberU64()-1); rem == nil {
					log.Error("Unrooted old chain seen by tx pool", "block", oldHead.Number, "hash", oldHead.Hash())
					return
				}
			}
			// 如果新的高度大于老的, 那么需要增加.
			for add.NumberU64() > rem.NumberU64() {
				included = append(included, add.Transactions()...)
				if add = pool.chain.GetBlock(add.ParentHash(), add.NumberU64()-1); add == nil {
					log.Error("Unrooted new chain seen by tx pool", "block", newHead.Number, "hash", newHead.Hash())
					return
				}
			}
			// 高度相同了.如果hash不同,那么需要往后找,一直找到他们相同hash根的节点.
			for rem.Hash() != add.Hash() {
				discarded = append(discarded, rem.Transactions()...)
				if rem = pool.chain.GetBlock(rem.ParentHash(), rem.NumberU64()-1); rem == nil {
					log.Error("Unrooted old chain seen by tx pool", "block", oldHead.Number, "hash", oldHead.Hash())
					return
				}
				included = append(included, add.Transactions()...)
				if add = pool.chain.GetBlock(add.ParentHash(), add.NumberU64()-1); add == nil {
					log.Error("Unrooted new chain seen by tx pool", "block", newHead.Number, "hash", newHead.Hash())
					return
				}
			}
			// 找出所有存在discard里面,但是不在included里面的值.
			// 需要等下把这些交易重新插入到pool里面。
			reinject = types.TxDifference(discarded, included)
		}
	}
	// Initialize the internal state to the current head
	if newHead == nil {
		newHead = pool.chain.CurrentBlock().Header() // Special case during testing
	}
	statedb, err := pool.chain.StateAt(newHead.Root)
	if err != nil {
		log.Error("Failed to reset txpool state", "err", err)
		return
	}
	pool.currentState = statedb
	pool.pendingState = state.ManageState(statedb)
	pool.currentMaxGas = newHead.GasLimit

	// Inject any transactions discarded due to reorgs
	log.Debug("Reinjecting stale transactions", "count", len(reinject))
	pool.addTxsLocked(reinject, false)

	// validate the pool of pending transactions, this will remove
	// any transactions that have been included in the block or
	// have been invalidated because of another transaction (e.g.
	// higher gas price)
	// 验证pending transaction池里面的交易， 会移除所有已经存在区块链里面的交易，或者是因为其他交易导致不可用的交易(比如有一个更高的gasPrice)
	// demote 降级 将pending中的一些交易降级到queue里面。
	pool.demoteUnexecutables()

	// Update all accounts to the latest known pending nonce
	// 根据pending队列的nonce更新所有账号的nonce
	for addr, list := range pool.pending {
		txs := list.Flatten() // Heavy but will be cached and is needed by the miner anyway
		pool.pendingState.SetNonce(addr, txs[len(txs)-1].Nonce()+1)
	}
	// Check the queue and move transactions over to the pending if possible
	// or remove those that have become invalid
	// 检查队列并尽可能地将事务移到pending，或删除那些已经失效的事务
	// promote 升级 
	pool.promoteExecutables(nil)
}

// addTx enqueues a single transaction into the pool if it is valid.
func (pool *TxPool) addTx(tx *types.Transaction, local bool) error {
	pool.mu.Lock()
	defer pool.mu.Unlock()

	// Try to inject the transaction and update any state
	replace, err := pool.add(tx, local)
	if err != nil {
		return err
	}
	// If we added a new transaction, run promotion checks and return
	if !replace {
		from, _ := types.Sender(pool.signer, tx) // already validated
		pool.promoteExecutables([]common.Address{from})
	}
	return nil
}

// addTxsLocked attempts to queue a batch of transactions if they are valid,
// whilst assuming the transaction pool lock is already held.
// addTxsLocked尝试把有效的交易放入queue队列，调用这个函数的时候假设已经获取到锁
func (pool *TxPool) addTxsLocked(txs []*types.Transaction, local bool) error {
	// Add the batch of transaction, tracking the accepted ones
	dirty := make(map[common.Address]struct{})
	for _, tx := range txs {
		if replace, err := pool.add(tx, local); err == nil {
			if !replace { // replace 是替换的意思， 如果不是替换，那么就说明状态有更新，有可以下一步处理的可能。
				from, _ := types.Sender(pool.signer, tx) // already validated
				dirty[from] = struct{}{}
			}
		}
	}
	// Only reprocess the internal state if something was actually added
	if len(dirty) > 0 {
		addrs := make([]common.Address, 0, len(dirty))
		for addr, _ := range dirty {
			addrs = append(addrs, addr)
		}	
		// 传入了被修改的地址，
		pool.promoteExecutables(addrs)
	}
	return nil
}

// demoteUnexecutables removes invalid and processed transactions from the pools
// executable/pending queue and any subsequent transactions that become unexecutable
// are moved back into the future queue.
func (pool *TxPool) demoteUnexecutables() {
	// Iterate over all accounts and demote any non-executable transactions
	for addr, list := range pool.pending {
		nonce := pool.currentState.GetNonce(addr)

		// Drop all transactions that are deemed too old (low nonce)
		// 删除所有小于当前地址的nonce的交易，并从pool.all删除。
		for _, tx := range list.Forward(nonce) {
			hash := tx.Hash()
			log.Trace("Removed old pending transaction", "hash", hash)
			delete(pool.all, hash)
			pool.priced.Removed()
		}
		// Drop all transactions that are too costly (low balance or out of gas), and queue any invalids back for later
		// 删除所有的太昂贵的交易。 用户的balance可能不够用。或者是out of gas
		drops, invalids := list.Filter(pool.currentState.GetBalance(addr), pool.currentMaxGas)
		for _, tx := range drops {
			hash := tx.Hash()
			log.Trace("Removed unpayable pending transaction", "hash", hash)
			delete(pool.all, hash)
			pool.priced.Removed()
			pendingNofundsCounter.Inc(1)
		}
		for _, tx := range invalids {
			hash := tx.Hash()
			log.Trace("Demoting pending transaction", "hash", hash)
			pool.enqueueTx(hash, tx)
		}
		// If there's a gap in front, warn (should never happen) and postpone all transactions
		// 如果存在一个空洞(nonce空洞)， 那么需要把所有的交易都放入future queue。
		// 这一步确实应该不可能发生，因为Filter已经把 invalids的都处理了。 应该不存在invalids的交易，也就是不存在空洞的。
		if list.Len() > 0 && list.txs.Get(nonce) == nil {
			for _, tx := range list.Cap(0) {
				hash := tx.Hash()
				log.Error("Demoting invalidated transaction", "hash", hash)
				pool.enqueueTx(hash, tx)
			}
		}
		// Delete the entire queue entry if it became empty.
		if list.Empty() { 
			delete(pool.pending, addr)
			delete(pool.beats, addr)
		}
	}
}

// enqueueTx inserts a new transaction into the non-executable transaction queue.
//
// Note, this method assumes the pool lock is held!
func (pool *TxPool) enqueueTx(hash common.Hash, tx *types.Transaction) (bool, error) {
	// Try to insert the transaction into the future queue
	from, _ := types.Sender(pool.signer, tx) // already validated
	if pool.queue[from] == nil {
		pool.queue[from] = newTxList(false)
	}
	inserted, old := pool.queue[from].Add(tx, pool.config.PriceBump)
	if !inserted {
		// An older transaction was better, discard this
		queuedDiscardCounter.Inc(1)
		return false, ErrReplaceUnderpriced
	}
	// Discard any previous transaction and mark this
	if old != nil {
		delete(pool.all, old.Hash())
		pool.priced.Removed()
		queuedReplaceCounter.Inc(1)
	}
	pool.all[hash] = tx
	pool.priced.Put(tx)
	return old != nil, nil
}

// promoteExecutables moves transactions that have become processable from the
// future queue to the set of pending transactions. During this process, all
// invalidated transactions (low nonce, low balance) are deleted.
func (pool *TxPool) promoteExecutables(accounts []common.Address) {
	// Gather all the accounts potentially needing updates
	// accounts存储了所有潜在需要更新的账户。 如果账户传入为nil，代表所有已知的账户。
	if accounts == nil {
		accounts = make([]common.Address, 0, len(pool.queue))
		for addr, _ := range pool.queue {
			accounts = append(accounts, addr)
		}
	}
	// Iterate over all accounts and promote any executable transactions
	for _, addr := range accounts {
		list := pool.queue[addr]
		if list == nil {
			continue // Just in case someone calls with a non existing account
		}
		// Drop all transactions that are deemed too old (low nonce)
		// 删除所有的nonce太低的交易
		for _, tx := range list.Forward(pool.currentState.GetNonce(addr)) {
			hash := tx.Hash()
			log.Trace("Removed old queued transaction", "hash", hash)
			delete(pool.all, hash)
			pool.priced.Removed()
		}
		// Drop all transactions that are too costly (low balance or out of gas)
		// 删除所有余额不足的交易。
		drops, _ := list.Filter(pool.currentState.GetBalance(addr), pool.currentMaxGas)
		for _, tx := range drops {
			hash := tx.Hash()
			log.Trace("Removed unpayable queued transaction", "hash", hash)
			delete(pool.all, hash)
			pool.priced.Removed()
			queuedNofundsCounter.Inc(1)
		}
		// Gather all executable transactions and promote them
		// 得到所有的可以执行的交易，并promoteTx加入pending
		for _, tx := range list.Ready(pool.pendingState.GetNonce(addr)) {
			hash := tx.Hash()
			log.Trace("Promoting queued transaction", "hash", hash)
			pool.promoteTx(addr, hash, tx)
		}
		// Drop all transactions over the allowed limit
		// 删除所有超过限制的交易。
		if !pool.locals.contains(addr) {
			for _, tx := range list.Cap(int(pool.config.AccountQueue)) {
				hash := tx.Hash()
				delete(pool.all, hash)
				pool.priced.Removed()
				queuedRateLimitCounter.Inc(1)
				log.Trace("Removed cap-exceeding queued transaction", "hash", hash)
			}
		}
		// Delete the entire queue entry if it became empty.
		if list.Empty() {
			delete(pool.queue, addr)
		}
	}
	// If the pending limit is overflown, start equalizing allowances
	pending := uint64(0)
	for _, list := range pool.pending {
		pending += uint64(list.Len())
	}
	// 如果pending的总数超过系统的配置。 
	if pending > pool.config.GlobalSlots {
		
		pendingBeforeCap := pending
		// Assemble a spam order to penalize large transactors first
		spammers := prque.New()
		for addr, list := range pool.pending {
			// Only evict transactions from high rollers
			// 首先把所有大于AccountSlots最小值的账户记录下来， 会从这些账户里面剔除一些交易。
			// 注意spammers是一个优先级队列，也就是说是按照交易的多少从大到小排序的。
			if !pool.locals.contains(addr) && uint64(list.Len()) > pool.config.AccountSlots {
				spammers.Push(addr, float32(list.Len()))
			}
		}
		// Gradually drop transactions from offenders
		offenders := []common.Address{}
		for pending > pool.config.GlobalSlots && !spammers.Empty() {
			/*	
			模拟一下offenders队列的账户交易数量的变化情况。 
				第一次循环   [10]    循环结束  [10]
				第二次循环   [10, 9] 循环结束  [9,9]
				第三次循环   [9, 9, 7] 循环结束 [7, 7, 7]
				第四次循环   [7, 7 , 7 ,2] 循环结束 [2, 2 ,2, 2]
			*/
			// Retrieve the next offender if not local address
			offender, _ := spammers.Pop()
			offenders = append(offenders, offender.(common.Address))

			// Equalize balances until all the same or below threshold
			if len(offenders) > 1 { // 第一次进入这个循环的时候， offenders队列里面有交易数量最大的两个账户
				// Calculate the equalization threshold for all current offenders
				// 把最后加入的账户的交易数量当成本次的阈值
				threshold := pool.pending[offender.(common.Address)].Len()

				// Iteratively reduce all offenders until below limit or threshold reached
				// 遍历直到pending有效，或者是倒数第二个的交易数量等于最后一个的交易数量
				for pending > pool.config.GlobalSlots && pool.pending[offenders[len(offenders)-2]].Len() > threshold {
					// 遍历除了最后一个账户以外的所有账户， 把他们的交易数量减去1.
					for i := 0; i < len(offenders)-1; i++ {
						list := pool.pending[offenders[i]]
						for _, tx := range list.Cap(list.Len() - 1) {
							// Drop the transaction from the global pools too
							hash := tx.Hash()
							delete(pool.all, hash)
							pool.priced.Removed()

							// Update the account nonce to the dropped transaction
							if nonce := tx.Nonce(); pool.pendingState.GetNonce(offenders[i]) > nonce {
								pool.pendingState.SetNonce(offenders[i], nonce)
							}
							log.Trace("Removed fairness-exceeding pending transaction", "hash", hash)
						}
						pending--
					}
				}
			}
		}
		// If still above threshold, reduce to limit or min allowance
		// 经过上面的循环，所有的超过AccountSlots的账户的交易数量都变成了之前的最小值。
		// 如果还是超过阈值，那么在继续从offenders里面每次删除一个。
		if pending > pool.config.GlobalSlots && len(offenders) > 0 {
			for pending > pool.config.GlobalSlots && uint64(pool.pending[offenders[len(offenders)-1]].Len()) > pool.config.AccountSlots {
				for _, addr := range offenders {
					list := pool.pending[addr]
					for _, tx := range list.Cap(list.Len() - 1) {
						// Drop the transaction from the global pools too
						hash := tx.Hash()
						delete(pool.all, hash)
						pool.priced.Removed()

						// Update the account nonce to the dropped transaction
						if nonce := tx.Nonce(); pool.pendingState.GetNonce(addr) > nonce {
							pool.pendingState.SetNonce(addr, nonce)
						}
						log.Trace("Removed fairness-exceeding pending transaction", "hash", hash)
					}
					pending--
				}
			}
		}
		pendingRateLimitCounter.Inc(int64(pendingBeforeCap - pending))
	}  //end if pending > pool.config.GlobalSlots {
	// If we've queued more transactions than the hard limit, drop oldest ones
	// 我们处理了pending的限制， 下面需要处理future queue的限制了。
	queued := uint64(0)
	for _, list := range pool.queue {
		queued += uint64(list.Len())
	}
	if queued > pool.config.GlobalQueue {
		// Sort all accounts with queued transactions by heartbeat
		addresses := make(addresssByHeartbeat, 0, len(pool.queue))
		for addr := range pool.queue {
			if !pool.locals.contains(addr) { // don't drop locals
				addresses = append(addresses, addressByHeartbeat{addr, pool.beats[addr]})
			}
		}
		sort.Sort(addresses)

		// Drop transactions until the total is below the limit or only locals remain
		// 从后往前，也就是心跳越新的就越会被删除。
		for drop := queued - pool.config.GlobalQueue; drop > 0 && len(addresses) > 0; {
			addr := addresses[len(addresses)-1]
			list := pool.queue[addr.address]

			addresses = addresses[:len(addresses)-1]

			// Drop all transactions if they are less than the overflow
			if size := uint64(list.Len()); size <= drop {
				for _, tx := range list.Flatten() {
					pool.removeTx(tx.Hash())
				}
				drop -= size
				queuedRateLimitCounter.Inc(int64(size))
				continue
			}
			// Otherwise drop only last few transactions
			txs := list.Flatten()
			for i := len(txs) - 1; i >= 0 && drop > 0; i-- {
				pool.removeTx(txs[i].Hash())
				drop--
				queuedRateLimitCounter.Inc(1)
			}
		}
	}
}

// promoteTx adds a transaction to the pending (processable) list of transactions.
//
// Note, this method assumes the pool lock is held!
func (pool *TxPool) promoteTx(addr common.Address, hash common.Hash, tx *types.Transaction) {
	// Try to insert the transaction into the pending queue
	if pool.pending[addr] == nil {
		pool.pending[addr] = newTxList(true)
	}
	list := pool.pending[addr]

	inserted, old := list.Add(tx, pool.config.PriceBump)
	if !inserted { // 如果不能替换, 已经存在一个老的交易了. 删除.
		// An older transaction was better, discard this
		delete(pool.all, hash)
		pool.priced.Removed()

		pendingDiscardCounter.Inc(1)
		return
	}
	// Otherwise discard any previous transaction and mark this
	if old != nil { 
		delete(pool.all, old.Hash())
		pool.priced.Removed()

		pendingReplaceCounter.Inc(1)
	}
	// Failsafe to work around direct pending inserts (tests)
	if pool.all[hash] == nil {
		pool.all[hash] = tx
		pool.priced.Put(tx)
	}
	// Set the potentially new pending nonce and notify any subsystems of the new tx
	// 把交易加入到队列,并发送消息告诉所有的订阅者, 这个订阅者在eth协议内部. 会接收这个消息并把这个消息通过网路广播出去.
	pool.beats[addr] = time.Now()
	pool.pendingState.SetNonce(addr, tx.Nonce()+1)
	go pool.txFeed.Send(TxPreEvent{tx})
}

// removeTx removes a single transaction from the queue, moving all subsequent
// transactions back to the future queue.
func (pool *TxPool) removeTx(hash common.Hash) {
	// Fetch the transaction we wish to delete
	tx, ok := pool.all[hash]
	if !ok {
		return
	}
	addr, _ := types.Sender(pool.signer, tx) // already validated during insertion

	// Remove it from the list of known transactions
	delete(pool.all, hash)
	pool.priced.Removed()

	// Remove the transaction from the pending lists and reset the account nonce
	// 把交易从pending删除， 并把因为这个交易的删除而变得无效的交易放到future queue
	// 然后更新pendingState的状态
	if pending := pool.pending[addr]; pending != nil {
		if removed, invalids := pending.Remove(tx); removed {
			// If no more transactions are left, remove the list
			if pending.Empty() {
				delete(pool.pending, addr)
				delete(pool.beats, addr)
			} else {
				// Otherwise postpone any invalidated transactions
				for _, tx := range invalids {
					pool.enqueueTx(tx.Hash(), tx)
				}
			}
			// Update the account nonce if needed
			if nonce := tx.Nonce(); pool.pendingState.GetNonce(addr) > nonce {
				pool.pendingState.SetNonce(addr, nonce)
			}
			return
		}
	}
	// Transaction is in the future queue
	// 把交易从future queue删除.
	if future := pool.queue[addr]; future != nil {
		future.Remove(tx)
		if future.Empty() {
			delete(pool.queue, addr)
		}
	}
}

// loop is the transaction pool's main event loop, waiting for and reacting to
// outside blockchain events as well as for various reporting and transaction
// eviction events.
func (pool *TxPool) loop() {
	defer pool.wg.Done()

	// Start the stats reporting and transaction eviction tickers
	var prevPending, prevQueued, prevStales int

	report := time.NewTicker(statsReportInterval)
	defer report.Stop()

	evict := time.NewTicker(evictionInterval)
	defer evict.Stop()

	journal := time.NewTicker(pool.config.Rejournal)
	defer journal.Stop()

	// Track the previous head headers for transaction reorgs
	head := pool.chain.CurrentBlock()

	// Keep waiting for and reacting to the various events
	for {
		select {
		// Handle ChainHeadEvent
		// 监听到区块头的事件, 获取到新的区块头.
		// 调用reset方法
		case ev := <-pool.chainHeadCh:
			if ev.Block != nil {
				pool.mu.Lock()
				if pool.chainconfig.IsHomestead(ev.Block.Number()) {
					pool.homestead = true
				}
				pool.reset(head.Header(), ev.Block.Header())
				head = ev.Block

				pool.mu.Unlock()
			}
		// Be unsubscribed due to system stopped
		case <-pool.chainHeadSub.Err():
			return

		// Handle stats reporting ticks 报告就是打印了一些日志
		case <-report.C:
			pool.mu.RLock()
			pending, queued := pool.stats()
			stales := pool.priced.stales
			pool.mu.RUnlock()

			if pending != prevPending || queued != prevQueued || stales != prevStales {
				log.Debug("Transaction pool status report", "executable", pending, "queued", queued, "stales", stales)
				prevPending, prevQueued, prevStales = pending, queued, stales
			}

		// Handle inactive account transaction eviction
		// 处理超时的交易信息,
		case <-evict.C:
			pool.mu.Lock()
			for addr := range pool.queue {
				// Skip local transactions from the eviction mechanism
				if pool.locals.contains(addr) {
					continue
				}
				// Any non-locals old enough should be removed
				if time.Since(pool.beats[addr]) > pool.config.Lifetime {
					for _, tx := range pool.queue[addr].Flatten() {
						pool.removeTx(tx.Hash())
					}
				}
			}
			pool.mu.Unlock()

		// Handle local transaction journal rotation 处理定时写交易日志的信息.
		case <-journal.C:
			if pool.journal != nil {
				pool.mu.Lock()
				if err := pool.journal.rotate(pool.local()); err != nil {
					log.Warn("Failed to rotate local tx journal", "err", err)
				}
				pool.mu.Unlock()
			}
		}
	}
}

// add validates a transaction and inserts it into the non-executable queue for
// later pending promotion and execution. If the transaction is a replacement for
// an already pending or queued one, it overwrites the previous and returns this
// so outer code doesn't uselessly call promote.
//
// If a newly added transaction is marked as local, its sending account will be
// whitelisted, preventing any associated transaction from being dropped out of
// the pool due to pricing constraints.
func (pool *TxPool) add(tx *types.Transaction, local bool) (bool, error) {
	// If the transaction is already known, discard it
	hash := tx.Hash()
	if pool.all[hash] != nil {
		log.Trace("Discarding already known transaction", "hash", hash)
		return false, fmt.Errorf("known transaction: %x", hash)
	}
	// If the transaction fails basic validation, discard it
	// 如果交易不能通过基本的验证,那么丢弃它
	if err := pool.validateTx(tx, local); err != nil {
		log.Trace("Discarding invalid transaction", "hash", hash, "err", err)
		invalidTxCounter.Inc(1)
		return false, err
	}
	// If the transaction pool is full, discard underpriced transactions
	// 如果交易池满了. 那么删除一些低价的交易.
	if uint64(len(pool.all)) >= pool.config.GlobalSlots+pool.config.GlobalQueue {
		// If the new transaction is underpriced, don't accept it
		// 如果新交易本身就是低价的.那么不接收它
		if pool.priced.Underpriced(tx, pool.locals) {
			log.Trace("Discarding underpriced transaction", "hash", hash, "price", tx.GasPrice())
			underpricedTxCounter.Inc(1)
			return false, ErrUnderpriced
		}
		// New transaction is better than our worse ones, make room for it
		// 否则删除低价值的给他腾空间.
		drop := pool.priced.Discard(len(pool.all)-int(pool.config.GlobalSlots+pool.config.GlobalQueue-1), pool.locals)
		for _, tx := range drop {
			log.Trace("Discarding freshly underpriced transaction", "hash", tx.Hash(), "price", tx.GasPrice())
			underpricedTxCounter.Inc(1)
			pool.removeTx(tx.Hash())
		}
	}
	// If the transaction is replacing an already pending one, do directly
	from, _ := types.Sender(pool.signer, tx) // already validated
	if list := pool.pending[from]; list != nil && list.Overlaps(tx) {
		// Nonce already pending, check if required price bump is met
		// 如果交易对应的Nonce已经在pending队列了,那么产看是否能够替换.
		inserted, old := list.Add(tx, pool.config.PriceBump)
		if !inserted {
			pendingDiscardCounter.Inc(1)
			return false, ErrReplaceUnderpriced
		}
		// New transaction is better, replace old one
		if old != nil {
			delete(pool.all, old.Hash())
			pool.priced.Removed()
			pendingReplaceCounter.Inc(1)
		}
		pool.all[tx.Hash()] = tx
		pool.priced.Put(tx)
		pool.journalTx(from, tx)

		log.Trace("Pooled new executable transaction", "hash", hash, "from", from, "to", tx.To())
		return old != nil, nil
	}
	// New transaction isn't replacing a pending one, push into queue
	// 新交易不能替换pending里面的任意一个交易,那么把他push到futuren 队列里面.
	replace, err := pool.enqueueTx(hash, tx)
	if err != nil {
		return false, err
	}
	// Mark local addresses and journal local transactions
	if local {
		pool.locals.add(from)
	}
	// 如果是本地的交易,会被记录进入journalTx
	pool.journalTx(from, tx)

	log.Trace("Pooled new future transaction", "hash", hash, "from", from, "to", tx.To())
	return replace, nil
}

// validateTx checks whether a transaction is valid according to the consensus
// rules and adheres to some heuristic limits of the local node (price and size).
func (pool *TxPool) validateTx(tx *types.Transaction, local bool) error {
	// Heuristic limit, reject transactions over 32KB to prevent DOS attacks
	if tx.Size() > 32*1024 {
		return ErrOversizedData
	}
	// Transactions can't be negative. This may never happen using RLP decoded
	// transactions but may occur if you create a transaction using the RPC.
	if tx.Value().Sign() < 0 {
		return ErrNegativeValue
	}
	// Ensure the transaction doesn't exceed the current block limit gas.
	if pool.currentMaxGas.Cmp(tx.Gas()) < 0 {
		return ErrGasLimit
	}
	// Make sure the transaction is signed properly
	// 确保交易被正确签名.
	from, err := types.Sender(pool.signer, tx)
	if err != nil {
		return ErrInvalidSender
	}
	// Drop non-local transactions under our own minimal accepted gas price
	local = local || pool.locals.contains(from) // account may be local even if the transaction arrived from the network
	// 如果不是本地的交易,并且GasPrice低于我们的设置,那么也不会接收.
	if !local && pool.gasPrice.Cmp(tx.GasPrice()) > 0 {
		return ErrUnderpriced
	}
	// Ensure the transaction adheres to nonce ordering
	// 确保交易遵守了Nonce的顺序
	if pool.currentState.GetNonce(from) > tx.Nonce() {
		return ErrNonceTooLow
	}
	// Transactor should have enough funds to cover the costs
	// cost == V + GP * GL
	// 确保用户有足够的余额来支付.
	if pool.currentState.GetBalance(from).Cmp(tx.Cost()) < 0 {
		return ErrInsufficientFunds
	}
	intrGas := IntrinsicGas(tx.Data(), tx.To() == nil, pool.homestead)
	// 如果交易是一个合约创建或者调用. 那么看看是否有足够的 初始Gas.
	if tx.Gas().Cmp(intrGas) < 0 {
		return ErrIntrinsicGas
	}
	return nil
}