// NewBlockTemplate returns a new block template that is ready to be solved
// using the transactions from the passed transaction source pool and a coinbase
// that either pays to the passed address if it is not nil, or a coinbase that
// is redeemable by anyone if the passed address is nil. The nil address
// functionality is useful since there are cases such as the getblocktemplate
// RPC where external mining software is responsible for creating their own
// coinbase which will replace the one generated for the block template. Thus
// the need to have configured address can be avoided.
//
// The transactions selected and included are prioritized according to several
// factors. First, each transaction has a priority calculated based on its
// value, age of inputs, and size. Transactions which consist of larger
// amounts, older inputs, and small sizes have the highest priority. Second, a
// fee per kilobyte is calculated for each transaction. Transactions with a
// higher fee per kilobyte are preferred. Finally, the block generation related
// policy settings are all taken into account.
//
// Transactions which only spend outputs from other transactions already in the
// block chain are immediately added to a priority queue which either
// prioritizes based on the priority (then fee per kilobyte) or the fee per
// kilobyte (then priority) depending on whether or not the BlockPrioritySize
// policy setting allots space for high-priority transactions. Transactions
// which spend outputs from other transactions in the source pool are added to a
// dependency map so they can be added to the priority queue once the
// transactions they depend on have been included.
//
// Once the high-priority area (if configured) has been filled with
// transactions, or the priority falls below what is considered high-priority,
// the priority queue is updated to prioritize by fees per kilobyte (then
// priority).
//
// When the fees per kilobyte drop below the TxMinFreeFee policy setting, the
// transaction will be skipped unless the BlockMinSize policy setting is
// nonzero, in which case the block will be filled with the low-fee/free
// transactions until the block size reaches that minimum size.
//
// Any transactions which would cause the block to exceed the BlockMaxSize
// policy setting, exceed the maximum allowed signature operations per block, or
// otherwise cause the block to be invalid are skipped.
//
// Given the above, a block generated by this function is of the following form:
//
// ----------------------------------- -- --
// | Coinbase Transaction | | |
// |-----------------------------------| | |
// | | | | ----- policy.BlockPrioritySize
// | High-priority Transactions | | |
// | | | |
// |-----------------------------------| | --
// | | |
// | | |
// | | |--- policy.BlockMaxSize
// | Transactions prioritized by fee | |
// | until <= policy.TxMinFreeFee | |
// | | |
// | | |
// | | |
// |-----------------------------------| |
// | Low-fee/Non high-priority (free) | |
// | transactions (while block size | |
// | <= policy.BlockMinSize) | |
// ----------------------------------- --
func (g *BlkTmplGenerator) NewBlockTemplate(payToAddress btcutil.Address) (*BlockTemplate, error) {
// Extend the most recently known best block.
best := g.chain.BestSnapshot()
prevHash := best.Hash
nextBlockHeight := best.Height + 1
// Create a standard coinbase transaction paying to the provided
// address. NOTE: The coinbase value will be updated to include the
// fees from the selected transactions later after they have actually
// been selected. It is created here to detect any errors early
// before potentially doing a lot of work below. The extra nonce helps
// ensure the transaction is not a duplicate transaction (paying the
// same value to the same public key address would otherwise be an
// identical transaction for block version 1).
extraNonce := uint64(0)
coinbaseScript, err := standardCoinbaseScript(nextBlockHeight, extraNonce)
if err != nil {
return nil, err
}
// TODO(roasbeef): add witnesss commitment output
coinbaseTx, err := createCoinbaseTx(g.chainParams, coinbaseScript,
nextBlockHeight, payToAddress)
if err != nil {
return nil, err
}
coinbaseSigOpCost := int64(blockchain.CountSigOps(coinbaseTx)) * blockchain.WitnessScaleFactor
// Get the current source transactions and create a priority queue to
// hold the transactions which are ready for inclusion into a block
// along with some priority related and fee metadata. Reserve the same
// number of items that are available for the priority queue. Also,
// choose the initial sort order for the priority queue based on whether
// or not there is an area allocated for high-priority transactions.
sourceTxns := g.txSource.MiningDescs()
sortedByFee := g.policy.BlockPrioritySize == 0
priorityQueue := newTxPriorityQueue(len(sourceTxns), sortedByFee)
// Create a slice to hold the transactions to be included in the
// generated block with reserved space. Also create a utxo view to
// house all of the input transactions so multiple lookups can be
// avoided.
blockTxns := make([]*btcutil.Tx, 0, len(sourceTxns))
blockTxns = append(blockTxns, coinbaseTx)
blockUtxos := blockchain.NewUtxoViewpoint()
// dependers is used to track transactions which depend on another
// transaction in the source pool. This, in conjunction with the
// dependsOn map kept with each dependent transaction helps quickly
// determine which dependent transactions are now eligible for inclusion
// in the block once each transaction has been included.
dependers := make(map[chainhash.Hash]map[chainhash.Hash]*txPrioItem)
// Create slices to hold the fees and number of signature operations
// for each of the selected transactions and add an entry for the
// coinbase. This allows the code below to simply append details about
// a transaction as it is selected for inclusion in the final block.
// However, since the total fees aren't known yet, use a dummy value for
// the coinbase fee which will be updated later.
txFees := make([]int64, 0, len(sourceTxns))
txSigOpCosts := make([]int64, 0, len(sourceTxns))
txFees = append(txFees, -1) // Updated once known
txSigOpCosts = append(txSigOpCosts, coinbaseSigOpCost)
log.Debugf("Considering %d transactions for inclusion to new block",
len(sourceTxns))
mempoolLoop:
for _, txDesc := range sourceTxns {
// A block can't have more than one coinbase or contain
// non-finalized transactions.
tx := txDesc.Tx
if blockchain.IsCoinBase(tx) {
log.Tracef("Skipping coinbase tx %s", tx.Hash())
continue
}
if !blockchain.IsFinalizedTransaction(tx, nextBlockHeight,
g.timeSource.AdjustedTime()) {
log.Tracef("Skipping non-finalized tx %s", tx.Hash())
continue
}
// Fetch all of the utxos referenced by the this transaction.
// NOTE: This intentionally does not fetch inputs from the
// mempool since a transaction which depends on other
// transactions in the mempool must come after those
// dependencies in the final generated block.
utxos, err := g.chain.FetchUtxoView(tx)
if err != nil {
log.Warnf("Unable to fetch utxo view for tx %s: %v",
tx.Hash(), err)
continue
}
// Setup dependencies for any transactions which reference
// other transactions in the mempool so they can be properly
// ordered below.
prioItem := &txPrioItem{tx: tx}
for _, txIn := range tx.MsgTx().TxIn {
originHash := &txIn.PreviousOutPoint.Hash
originIndex := txIn.PreviousOutPoint.Index
utxoEntry := utxos.LookupEntry(originHash)
if utxoEntry == nil || utxoEntry.IsOutputSpent(originIndex) {
if !g.txSource.HaveTransaction(originHash) {
log.Tracef("Skipping tx %s because it "+
"references unspent output %s "+
"which is not available",
tx.Hash(), txIn.PreviousOutPoint)
continue mempoolLoop
}
// The transaction is referencing another
// transaction in the source pool, so setup an
// ordering dependency.
deps, exists := dependers[*originHash]
if !exists {
deps = make(map[chainhash.Hash]*txPrioItem)
dependers[*originHash] = deps
}
deps[*prioItem.tx.Hash()] = prioItem
if prioItem.dependsOn == nil {
prioItem.dependsOn = make(
map[chainhash.Hash]struct{})
}
prioItem.dependsOn[*originHash] = struct{}{}
// Skip the check below. We already know the
// referenced transaction is available.
continue
}
}
// Calculate the final transaction priority using the input
// value age sum as well as the adjusted transaction size. The
// formula is: sum(inputValue * inputAge) / adjustedTxSize
prioItem.priority = CalcPriority(tx.MsgTx(), utxos,
nextBlockHeight)
// Calculate the fee in Satoshi/kB.
// TODO(roasbeef): cost accounting by weight
prioItem.feePerKB = txDesc.FeePerKB
prioItem.fee = txDesc.Fee
// Add the transaction to the priority queue to mark it ready
// for inclusion in the block unless it has dependencies.
if prioItem.dependsOn == nil {
heap.Push(priorityQueue, prioItem)
}
// Merge the referenced outputs from the input transactions to
// this transaction into the block utxo view. This allows the
// code below to avoid a second lookup.
mergeUtxoView(blockUtxos, utxos)
}
log.Tracef("Priority queue len %d, dependers len %d",
priorityQueue.Len(), len(dependers))
// The starting block size is the size of the block header plus the max
// possible transaction count size, plus the size of the coinbase
// transaction.
blockWeight := uint32((blockHeaderOverhead * blockchain.WitnessScaleFactor) +
blockchain.GetTransactionWeight(coinbaseTx))
blockSigOpCost := coinbaseSigOpCost
totalFees := int64(0)
// Query the version bits state to see if segwit has been activated, if
// so then this means that we'll include any transactions with witness
// data in the mempool, and also add the witness commitment as an
// OP_RETURN output in the coinbase transaction.
segwitState, err := g.chain.ThresholdState(chaincfg.DeploymentSegwit)
if err != nil {
return nil, err
}
segwitActive := segwitState == blockchain.ThresholdActive
witnessIncluded := false
// Choose which transactions make it into the block.
for priorityQueue.Len() > 0 {
// Grab the highest priority (or highest fee per kilobyte
// depending on the sort order) transaction.
prioItem := heap.Pop(priorityQueue).(*txPrioItem)
tx := prioItem.tx
switch {
// If segregated witness has not been activated yet, then we
// shouldn't include any witness transactions in the block.
case tx.MsgTx().HasWitness() && !segwitActive:
continue
// Otherwise, Keep track of if we've included a transaction
// with witness data or not. If so, then we'll need to include
// the witness commitment as the last output in the coinbase
// transaction.
case tx.MsgTx().HasWitness() && segwitActive:
// If we're about to include a transaction bearing
// witness data, then we'll also need to include a
// witness commitment in the coinbase transaction.
// Therefore, we account for the additional weight
// within the block.
if !witnessIncluded {
// First we account for the additional witness
// data in the witness nonce of the coinbaes
// transaction: 32-bytes of zeroes.
blockWeight += 2 + 32
// Next we account for the additional flag and
// marker bytes in the transaction
// serialization.
blockWeight += (1 + 1) * blockchain.WitnessScaleFactor
// Finally we account for the weight of the
// additional OP_RETURN output: 8-bytes (value)
// + 1-byte (var-int) + 38-bytes (pkScript),
// scaling up the weight as it's non-witness
// data.
blockWeight += (8 + 1 + 38) * blockchain.WitnessScaleFactor
}
witnessIncluded = true
}
// Grab any transactions which depend on this one.
deps := dependers[*tx.Hash()]
// Enforce maximum block size. Also check for overflow.
txWeight := uint32(blockchain.GetTransactionWeight(tx))
blockPlusTxWeight := uint32(blockWeight + txWeight)
if blockPlusTxWeight < blockWeight ||
blockPlusTxWeight >= g.policy.BlockMaxWeight {
log.Tracef("Skipping tx %s because it would exceed "+
"the max block weight", tx.Hash())
logSkippedDeps(tx, deps)
continue
}
// Enforce maximum signature operation cost per block. Also
// check for overflow.
sigOpCost, err := blockchain.GetSigOpCost(tx, false,
blockUtxos, true, segwitActive)
if err != nil {
log.Tracef("Skipping tx %s due to error in "+
"GetSigOpCost: %v", tx.Hash(), err)
logSkippedDeps(tx, deps)
continue
}
if blockSigOpCost+int64(sigOpCost) < blockSigOpCost ||
blockSigOpCost+int64(sigOpCost) > blockchain.MaxBlockSigOpsCost {
log.Tracef("Skipping tx %s because it would "+
"exceed the maximum sigops per block", tx.Hash())
logSkippedDeps(tx, deps)
continue
}
// Skip free transactions once the block is larger than the
// minimum block size.
if sortedByFee &&
prioItem.feePerKB < int64(g.policy.TxMinFreeFee) &&
blockPlusTxWeight >= g.policy.BlockMinWeight {
log.Tracef("Skipping tx %s with feePerKB %d "+
"< TxMinFreeFee %d and block weight %d >= "+
"minBlockWeight %d", tx.Hash(), prioItem.feePerKB,
g.policy.TxMinFreeFee, blockPlusTxWeight,
g.policy.BlockMinWeight)
logSkippedDeps(tx, deps)
continue
}
// Prioritize by fee per kilobyte once the block is larger than
// the priority size or there are no more high-priority
// transactions.
if !sortedByFee && (blockPlusTxWeight >= g.policy.BlockPrioritySize ||
prioItem.priority <= MinHighPriority) {
log.Tracef("Switching to sort by fees per "+
"kilobyte blockSize %d >= BlockPrioritySize "+
"%d || priority %.2f <= minHighPriority %.2f",
blockPlusTxWeight, g.policy.BlockPrioritySize,
prioItem.priority, MinHighPriority)
sortedByFee = true
priorityQueue.SetLessFunc(txPQByFee)
// Put the transaction back into the priority queue and
// skip it so it is re-priortized by fees if it won't
// fit into the high-priority section or the priority
// is too low. Otherwise this transaction will be the
// final one in the high-priority section, so just fall
// though to the code below so it is added now.
if blockPlusTxWeight > g.policy.BlockPrioritySize ||
prioItem.priority < MinHighPriority {
heap.Push(priorityQueue, prioItem)
continue
}
}
// Ensure the transaction inputs pass all of the necessary
// preconditions before allowing it to be added to the block.
_, err = blockchain.CheckTransactionInputs(tx, nextBlockHeight,
blockUtxos, g.chainParams)
if err != nil {
log.Tracef("Skipping tx %s due to error in "+
"CheckTransactionInputs: %v", tx.Hash(), err)
logSkippedDeps(tx, deps)
continue
}
err = blockchain.ValidateTransactionScripts(tx, blockUtxos,
txscript.StandardVerifyFlags, g.sigCache,
g.hashCache)
if err != nil {
log.Tracef("Skipping tx %s due to error in "+
"ValidateTransactionScripts: %v", tx.Hash(), err)
logSkippedDeps(tx, deps)
continue
}
// Spend the transaction inputs in the block utxo view and add
// an entry for it to ensure any transactions which reference
// this one have it available as an input and can ensure they
// aren't double spending.
spendTransaction(blockUtxos, tx, nextBlockHeight)
// Add the transaction to the block, increment counters, and
// save the fees and signature operation counts to the block
// template.
blockTxns = append(blockTxns, tx)
blockWeight += txWeight
blockSigOpCost += int64(sigOpCost)
totalFees += prioItem.fee
txFees = append(txFees, prioItem.fee)
txSigOpCosts = append(txSigOpCosts, int64(sigOpCost))
log.Tracef("Adding tx %s (priority %.2f, feePerKB %.2f)",
prioItem.tx.Hash(), prioItem.priority, prioItem.feePerKB)
// Add transactions which depend on this one (and also do not
// have any other unsatisified dependencies) to the priority
// queue.
for _, item := range deps {
// Add the transaction to the priority queue if there
// are no more dependencies after this one.
delete(item.dependsOn, *tx.Hash())
if len(item.dependsOn) == 0 {
heap.Push(priorityQueue, item)
}
}
}
// Now that the actual transactions have been selected, update the
// block weight for the real transaction count and coinbase value with
// the total fees accordingly.
blockWeight -= wire.MaxVarIntPayload -
(uint32(wire.VarIntSerializeSize(uint64(len(blockTxns)))) *
blockchain.WitnessScaleFactor)
coinbaseTx.MsgTx().TxOut[0].Value += totalFees
txFees[0] = -totalFees
// If segwit is active and we included transactions with witness data,
// then we'll need to include a commitment to the witness data in an
// OP_RETURN output within the coinbase transaction.
if witnessIncluded {
// The witness of the coinbase transaction MUST be exactly 32-bytes
// of all zeroes.
var witnessNonce [blockchain.CoinbaseWitnessDataLen]byte
coinbaseTx.MsgTx().TxIn[0].Witness = wire.TxWitness{witnessNonce[:]}
// Next, obtain the merkle root of a tree which consists of the
// wtxid of all transactions in the block. The coinbase
// transaction will have a special wtxid of all zeroes.
witnessMerkleTree := blockchain.BuildMerkleTreeStore(blockTxns,
true)
witnessMerkleRoot := witnessMerkleTree[len(witnessMerkleTree)-1]
// The preimage to the witness commitment is:
// witnessRoot || coinbaseWitness
var witnessPreimage [64]byte
copy(witnessPreimage[:32], witnessMerkleRoot[:])
copy(witnessPreimage[32:], witnessNonce[:])
// The witness commitment itself is the double-sha256 of the
// witness preimage generated above. With the commitment
// generated, the witness script for the output is: OP_RETURN
// OP_DATA_36 {0xaa21a9ed || witnessCommitment}. The leading
// prefix is refered to as the "witness magic bytes".
witnessCommitment := chainhash.DoubleHashB(witnessPreimage[:])
witnessScript := append(blockchain.WitnessMagicBytes, witnessCommitment...)
// Finally, create the OP_RETURN carrying witness commitment
// output as an additional output within the coinbase.
commitmentOutput := &wire.TxOut{
Value: 0,
PkScript: witnessScript,
}
coinbaseTx.MsgTx().TxOut = append(coinbaseTx.MsgTx().TxOut,
commitmentOutput)
}
// Calculate the required difficulty for the block. The timestamp
// is potentially adjusted to ensure it comes after the median time of
// the last several blocks per the chain consensus rules.
ts := medianAdjustedTime(best, g.timeSource)
reqDifficulty, err := g.chain.CalcNextRequiredDifficulty(ts)
if err != nil {
return nil, err
}
// Calculate the next expected block version based on the state of the
// rule change deployments.
nextBlockVersion, err := g.chain.CalcNextBlockVersion()
if err != nil {
return nil, err
}
// Create a new block ready to be solved.
merkles := blockchain.BuildMerkleTreeStore(blockTxns, false)
var msgBlock wire.MsgBlock
msgBlock.Header = wire.BlockHeader{
Version: nextBlockVersion,
PrevBlock: *prevHash,
MerkleRoot: *merkles[len(merkles)-1],
Timestamp: ts,
Bits: reqDifficulty,
}
for _, tx := range blockTxns {
if err := msgBlock.AddTransaction(tx.MsgTx()); err != nil {
return nil, err
}
}
// Finally, perform a full check on the created block against the chain
// consensus rules to ensure it properly connects to the current best
// chain with no issues.
block := btcutil.NewBlock(&msgBlock)
block.SetHeight(nextBlockHeight)
if err := g.chain.CheckConnectBlock(block); err != nil {
return nil, err
}
log.Debugf("Created new block template (%d transactions, %d in "+
"fees, %d signature operations cost, %d weight, target difficulty "+
"%064x)", len(msgBlock.Transactions), totalFees, blockSigOpCost,
blockWeight, blockchain.CompactToBig(msgBlock.Header.Bits))
return &BlockTemplate{
Block: &msgBlock,
Fees: txFees,
SigOpCosts: txSigOpCosts,
Height: nextBlockHeight,
ValidPayAddress: payToAddress != nil,
}, nil
}
// checkTransactionStandard performs a series of checks on a transaction to
// ensure it is a "standard" transaction. A standard transaction is one that
// conforms to several additional limiting cases over what is considered a
// "sane" transaction such as having a version in the supported range, being
// finalized, conforming to more stringent size constraints, having scripts
// of recognized forms, and not containing "dust" outputs (those that are
// so small it costs more to process them than they are worth).
func checkTransactionStandard(tx *btcutil.Tx, height int32,
medianTimePast time.Time, minRelayTxFee btcutil.Amount,
maxTxVersion int32) error {
// The transaction must be a currently supported version.
msgTx := tx.MsgTx()
if msgTx.Version > maxTxVersion || msgTx.Version < 1 {
str := fmt.Sprintf("transaction version %d is not in the "+
"valid range of %d-%d", msgTx.Version, 1,
maxTxVersion)
return txRuleError(wire.RejectNonstandard, str)
}
// The transaction must be finalized to be standard and therefore
// considered for inclusion in a block.
if !blockchain.IsFinalizedTransaction(tx, height, medianTimePast) {
return txRuleError(wire.RejectNonstandard,
"transaction is not finalized")
}
// Since extremely large transactions with a lot of inputs can cost
// almost as much to process as the sender fees, limit the maximum
// size of a transaction. This also helps mitigate CPU exhaustion
// attacks.
txWeight := blockchain.GetTransactionWeight(tx)
if txWeight > maxStandardTxWeight {
str := fmt.Sprintf("weight of transaction %v is larger than max "+
"allowed weight of %v", txWeight, maxStandardTxWeight)
return txRuleError(wire.RejectNonstandard, str)
}
for i, txIn := range msgTx.TxIn {
// Each transaction input signature script must not exceed the
// maximum size allowed for a standard transaction. See
// the comment on maxStandardSigScriptSize for more details.
sigScriptLen := len(txIn.SignatureScript)
if sigScriptLen > maxStandardSigScriptSize {
str := fmt.Sprintf("transaction input %d: signature "+
"script size of %d bytes is large than max "+
"allowed size of %d bytes", i, sigScriptLen,
maxStandardSigScriptSize)
return txRuleError(wire.RejectNonstandard, str)
}
// Each transaction input signature script must only contain
// opcodes which push data onto the stack.
if !txscript.IsPushOnlyScript(txIn.SignatureScript) {
str := fmt.Sprintf("transaction input %d: signature "+
"script is not push only", i)
return txRuleError(wire.RejectNonstandard, str)
}
}
// None of the output public key scripts can be a non-standard script or
// be "dust" (except when the script is a null data script).
numNullDataOutputs := 0
for i, txOut := range msgTx.TxOut {
scriptClass := txscript.GetScriptClass(txOut.PkScript)
err := checkPkScriptStandard(txOut.PkScript, scriptClass)
if err != nil {
// Attempt to extract a reject code from the error so
// it can be retained. When not possible, fall back to
// a non standard error.
rejectCode := wire.RejectNonstandard
if rejCode, found := extractRejectCode(err); found {
rejectCode = rejCode
}
str := fmt.Sprintf("transaction output %d: %v", i, err)
return txRuleError(rejectCode, str)
}
// Accumulate the number of outputs which only carry data. For
// all other script types, ensure the output value is not
// "dust".
if scriptClass == txscript.NullDataTy {
numNullDataOutputs++
} else if isDust(txOut, minRelayTxFee) {
str := fmt.Sprintf("transaction output %d: payment "+
"of %d is dust", i, txOut.Value)
return txRuleError(wire.RejectDust, str)
}
}
// A standard transaction must not have more than one output script that
// only carries data.
if numNullDataOutputs > 1 {
str := "more than one transaction output in a nulldata script"
return txRuleError(wire.RejectNonstandard, str)
}
return nil
}