accept.go

// Copyright (c) 2013-2016 The btcsuite developers
// Copyright (c) 2015-2016 The Decred developers
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.

package blockchain

import (
	"encoding/binary"
	"fmt"
	"math"
	"time"

	"github.com/decred/dcrd/blockchain/stake"
	"github.com/decred/dcrd/chaincfg/chainhash"
	"github.com/decred/dcrd/dcrutil"
	"github.com/decred/dcrd/txscript"
)

// checkCoinbaseUniqueHeight checks to ensure that for all blocks height > 1
// that the coinbase contains the height encoding to make coinbase hash collisions
// impossible.
func checkCoinbaseUniqueHeight(blockHeight int64, block *dcrutil.Block) error {
	// Coinbase TxOut[0] is always tax, TxOut[1] is always
	// height + extranonce, so at least two outputs must
	// exist.
	if len(block.MsgBlock().Transactions[0].TxOut) < 2 {
		str := fmt.Sprintf("block %v is missing necessary coinbase "+
			"outputs", block.Hash())
		return ruleError(ErrFirstTxNotCoinbase, str)
	}

	// The first 4 bytes of the NullData output must be the
	// encoded height of the block, so that every coinbase
	// created has a unique transaction hash.
	nullData, err := txscript.GetNullDataContent(
		block.MsgBlock().Transactions[0].TxOut[1].Version,
		block.MsgBlock().Transactions[0].TxOut[1].PkScript)
	if err != nil {
		str := fmt.Sprintf("block %v txOut 1 has wrong pkScript "+
			"type", block.Hash())
		return ruleError(ErrFirstTxNotCoinbase, str)
	}

	if len(nullData) < 4 {
		str := fmt.Sprintf("block %v txOut 1 has too short nullData "+
			"push to contain height", block.Hash())
		return ruleError(ErrFirstTxNotCoinbase, str)
	}

	// Check the height and ensure it is correct.
	cbHeight := binary.LittleEndian.Uint32(nullData[0:4])
	if cbHeight != uint32(blockHeight) {
		prevBlock := block.MsgBlock().Header.PrevBlock
		str := fmt.Sprintf("block %v txOut 1 has wrong height in "+
			"coinbase; want %v, got %v; prevBlock %v, header height %v",
			block.Hash(), blockHeight, cbHeight, prevBlock,
			block.MsgBlock().Header.Height)
		return ruleError(ErrCoinbaseHeight, str)
	}

	return nil
}

// IsFinalizedTransaction determines whether or not a transaction is finalized.
func IsFinalizedTransaction(tx *dcrutil.Tx, blockHeight int64, blockTime time.Time) bool {
	// Lock time of zero means the transaction is finalized.
	msgTx := tx.MsgTx()
	lockTime := msgTx.LockTime
	if lockTime == 0 {
		return true
	}

	// The lock time field of a transaction is either a block height at
	// which the transaction is finalized or a timestamp depending on if the
	// value is before the txscript.LockTimeThreshold.  When it is under the
	// threshold it is a block height.
	var blockTimeOrHeight int64
	if lockTime < txscript.LockTimeThreshold {
		blockTimeOrHeight = blockHeight
	} else {
		blockTimeOrHeight = blockTime.Unix()
	}
	if int64(lockTime) < blockTimeOrHeight {
		return true
	}

	// At this point, the transaction's lock time hasn't occurred yet, but
	// the transaction might still be finalized if the sequence number
	// for all transaction inputs is maxed out.
	for _, txIn := range msgTx.TxIn {
		if txIn.Sequence != math.MaxUint32 {
			return false
		}
	}
	return true
}

// checkBlockContext peforms several validation checks on the block which depend
// on its position within the block chain.
//
// The flags modify the behavior of this function as follows:
//  - BFFastAdd: The transaction are not checked to see if they are finalized
//    and the somewhat expensive duplication transaction check is not performed.
//
// The flags are also passed to checkBlockHeaderContext.  See its documentation
// for how the flags modify its behavior.
func (b *BlockChain) checkBlockContext(block *dcrutil.Block, prevNode *blockNode, flags BehaviorFlags) error {
	// The genesis block is valid by definition.
	if prevNode == nil {
		return nil
	}

	// Perform all block header related validation checks.
	header := &block.MsgBlock().Header
	err := b.checkBlockHeaderContext(header, prevNode, flags)
	if err != nil {
		return err
	}

	fastAdd := flags&BFFastAdd == BFFastAdd
	if !fastAdd {
		// A block must not exceed the maximum allowed size as defined
		// by the network parameters and the current status of any hard
		// fork votes to change it when serialized.
		maxBlockSize, err := b.maxBlockSize(prevNode)
		if err != nil {
			return err
		}
		serializedSize := int64(block.MsgBlock().Header.Size)
		if serializedSize > maxBlockSize {
			str := fmt.Sprintf("serialized block is too big - "+
				"got %d, max %d", serializedSize,
				maxBlockSize)
			return ruleError(ErrBlockTooBig, str)
		}

		// Switch to using the past median time of the block prior to
		// the block being checked for all checks related to lock times
		// once the stake vote for the agenda is active.
		blockTime := header.Timestamp
		lnFeaturesActive, err := b.isLNFeaturesAgendaActive(prevNode)
		if err != nil {
			return err
		}
		if lnFeaturesActive {
			medianTime, err := b.calcPastMedianTime(prevNode)
			if err != nil {
				return err
			}

			blockTime = medianTime
		}

		// The height of this block is one more than the referenced
		// previous block.
		blockHeight := prevNode.height + 1

		// Ensure all transactions in the block are finalized.
		for _, tx := range block.Transactions() {
			if !IsFinalizedTransaction(tx, blockHeight, blockTime) {
				str := fmt.Sprintf("block contains unfinalized regular "+
					"transaction %v", tx.Hash())
				return ruleError(ErrUnfinalizedTx, str)
			}
		}
		for _, stx := range block.STransactions() {
			if !IsFinalizedTransaction(stx, blockHeight, blockTime) {
				str := fmt.Sprintf("block contains unfinalized stake "+
					"transaction %v", stx.Hash())
				return ruleError(ErrUnfinalizedTx, str)
			}
		}

		// Check that the node is at the correct height in the blockchain,
		// as specified in the block header.
		if blockHeight != int64(block.MsgBlock().Header.Height) {
			errStr := fmt.Sprintf("Block header height invalid; expected %v"+
				" but %v was found", blockHeight, header.Height)
			return ruleError(ErrBadBlockHeight, errStr)
		}

		// Check that the coinbase contains at minimum the block
		// height in output 1.
		if blockHeight > 1 {
			err := checkCoinbaseUniqueHeight(blockHeight, block)
			if err != nil {
				return err
			}
		}
	}

	return nil
}

// ticketsSpentInBlock fetches a list of tickets that were spent in the
// block.
func ticketsSpentInBlock(bl *dcrutil.Block) []chainhash.Hash {
	var tickets []chainhash.Hash
	for _, stx := range bl.MsgBlock().STransactions {
		if stake.DetermineTxType(stx) == stake.TxTypeSSGen {
			tickets = append(tickets, stx.TxIn[1].PreviousOutPoint.Hash)
		}
	}

	return tickets
}

// ticketsRevokedInBlock fetches a list of tickets that were revoked in the
// block.
func ticketsRevokedInBlock(bl *dcrutil.Block) []chainhash.Hash {
	var tickets []chainhash.Hash
	for _, stx := range bl.MsgBlock().STransactions {
		if stake.DetermineTxType(stx) == stake.TxTypeSSRtx {
			tickets = append(tickets, stx.TxIn[0].PreviousOutPoint.Hash)
		}
	}

	return tickets
}

// voteBitsInBlock returns a list of vote bits for the voters in this block.
func voteBitsInBlock(bl *dcrutil.Block) []VoteVersionTuple {
	var voteBits []VoteVersionTuple
	for _, stx := range bl.MsgBlock().STransactions {
		if is, _ := stake.IsSSGen(stx); !is {
			continue
		}

		voteBits = append(voteBits, VoteVersionTuple{
			Version: stake.SSGenVersion(stx),
			Bits:    stake.SSGenVoteBits(stx),
		})
	}

	return voteBits
}

// maybeAcceptBlock potentially accepts a block into the block chain and, if
// accepted, returns whether or not it is on the main chain.  It performs
// several validation checks which depend on its position within the block chain
// before adding it.  The block is expected to have already gone through
// ProcessBlock before calling this function with it.
//
// The flags modify the behavior of this function as follows:
//  - BFDryRun: The memory chain index will not be pruned and no accept
//    notification will be sent since the block is not being accepted.
//
// This function MUST be called with the chain state lock held (for writes).
func (b *BlockChain) maybeAcceptBlock(block *dcrutil.Block, flags BehaviorFlags) (bool, error) {
	dryRun := flags&BFDryRun == BFDryRun

	// Get a block node for the block previous to this one.  Will be nil
	// if this is the genesis block.
	prevNode, err := b.getPrevNodeFromBlock(block)
	if err != nil {
		log.Debugf("getPrevNodeFromBlock: %v", err)
		return false, err
	}

	blockHeight := block.Height()

	// The block must pass all of the validation rules which depend on the
	// position of the block within the block chain.
	err = b.checkBlockContext(block, prevNode, flags)
	if err != nil {
		return false, err
	}

	// Prune stake nodes which are no longer needed before creating a new
	// node.
	if !dryRun {
		b.pruner.pruneChainIfNeeded()
	}

	// Create a new block node for the block and add it to the in-memory
	// block chain (could be either a side chain or the main chain).
	blockHeader := &block.MsgBlock().Header
	newNode := newBlockNode(blockHeader, ticketsSpentInBlock(block),
		ticketsRevokedInBlock(block), voteBitsInBlock(block))
	if prevNode != nil {
		newNode.parent = prevNode
		newNode.height = blockHeight
		newNode.workSum.Add(prevNode.workSum, newNode.workSum)
	}

	// Fetching a stake node could enable a new DoS vector, so restrict
	// this only to blocks that are recent in history.
	if newNode.height < b.bestNode.height-minMemoryNodes {
		newNode.stakeNode, err = b.fetchStakeNode(newNode)
		if err != nil {
			return false, err
		}
		newNode.stakeUndoData = newNode.stakeNode.UndoData()
	}

	// Connect the passed block to the chain while respecting proper chain
	// selection according to the chain with the most proof of work.  This
	// also handles validation of the transaction scripts.
	isMainChain, err := b.connectBestChain(newNode, block, flags)
	if err != nil {
		return false, err
	}

	// Notify the caller that the new block was accepted into the block
	// chain.  The caller would typically want to react by relaying the
	// inventory to other peers.
	if !dryRun {
		b.chainLock.Unlock()
		b.sendNotification(NTBlockAccepted,
			&BlockAcceptedNtfnsData{isMainChain, block})
		b.chainLock.Lock()
	}

	return isMainChain, nil
}