lbcd/chain.go

// Copyright (c) 2013 Conformal Systems LLC.
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.

package btcchain

import (
	"container/list"
	"fmt"
	"github.com/conformal/btcdb"
	"github.com/conformal/btcutil"
	"github.com/conformal/btcwire"
	"math/big"
	"sort"
	"time"
)

// maxOrphanBlocks is the maximum number of orphan blocks that can be queued.
const maxOrphanBlocks = 100

// blockNode represents a block within the block chain and is primarily used to
// aid in selecting the best chain to be the main chain.  The main chain is
// stored into the block database.
type blockNode struct {
	// parent is the parent block for this node.
	parent *blockNode

	// children contains the child nodes for this node.  Typically there
	// will only be one, but sometimes there can be more than one and that
	// is when the best chain selection algorithm is used.
	children []*blockNode

	// hash is the double sha 256 of the block.
	hash *btcwire.ShaHash

	// height is the position in the block chain.
	height int64

	// workSum is the total amount of work in the chain up to and including
	// this node.
	workSum *big.Rat

	// inMainChain denotes whether the block node is currently on the
	// the main chain or not.  This is used to help find the common
	// ancestor when switching chains.
	inMainChain bool

	// Some fields from block headers to aid in best chain selection.
	version   uint32
	bits      uint32
	timestamp time.Time
}

// newBlockNode returns a new block node for the given block.  It is completely
// disconnected from the chain and the workSum value is just the work for the
// passed block.  The work sum is updated accordingly when the node is inserted
// into a chain.
func newBlockNode(block *btcutil.Block) *blockNode {
	// Get the block sha.  It's ok to ignore the error here since
	// sha has already been called and an error there would have caused
	// an exit before this function is called.
	blockSha, _ := block.Sha()

	blockHeader := block.MsgBlock().Header
	node := blockNode{
		hash:      blockSha,
		workSum:   calcWork(blockHeader.Bits),
		height:    block.Height(),
		version:   blockHeader.Version,
		bits:      blockHeader.Bits,
		timestamp: blockHeader.Timestamp,
	}
	return &node
}

// orphanBlock represents a block that we don't yet have the parent for.  It
// is a normal block plus an expiration time to prevent caching the orphan
// forever.
type orphanBlock struct {
	block      *btcutil.Block
	expiration time.Time
}

// addChildrenWork adds the passed work amount to all children all the way
// down the chain.  It is used primarily to allow a new node to be dynamically
// inserted from the database into the memory chain prior to nodes we already
// have and update their work values accordingly.
func addChildrenWork(node *blockNode, work *big.Rat) {
	for _, childNode := range node.children {
		childNode.workSum.Add(childNode.workSum, work)
		addChildrenWork(childNode, work)
	}
}

// removeChildNode deletes node from the provided slice of child block
// nodes.  It ensures the final pointer reference is set to nil to prevent
// potential memory leaks.  The original slice is returned unmodified if node
// is invalid or not in the slice.
func removeChildNode(children []*blockNode, node *blockNode) []*blockNode {
	if node == nil {
		return children
	}
	for i, n := range children {
		if n.hash.IsEqual(node.hash) {
			copy(children[i:], children[i+1:])
			children[len(children)-1] = nil
			return children[:len(children)-1]
		}
	}
	return children
}

// BlockChain provides functions for working with the bitcoin block chain.
// It includes functionality such as rejecting duplicate blocks, ensuring blocks
// follow all rules, orphan handling, checkpoint handling, and best chain
// selection with reorganization.
type BlockChain struct {
	db            btcdb.Db
	btcnet        btcwire.BitcoinNet
	notifications chan *Notification
	root          *blockNode
	bestChain     *blockNode
	index         map[btcwire.ShaHash]*blockNode
	depNodes      map[btcwire.ShaHash][]*blockNode
	orphans       map[btcwire.ShaHash]*orphanBlock
	prevOrphans   map[btcwire.ShaHash][]*orphanBlock
	oldestOrphan  *orphanBlock
	blockCache    map[btcwire.ShaHash]*btcutil.Block
	noVerify      bool
	noCheckpoints bool
}

// DisableVerify provides a mechanism to disable transaction script validation
// which you DO NOT want to do in production as it could allow double spends
// and othe undesirable things.  It is provided only for debug purposes since
// script validation is extremely intensive and when debugging it is sometimes
// nice to quickly get the chain.
func (b *BlockChain) DisableVerify(disable bool) {
	b.noVerify = disable
}

// getOrphanRoot returns the head of the chain for the provided hash from the
// map of orphan blocks.
func (b *BlockChain) getOrphanRoot(sha *btcwire.ShaHash) *btcwire.ShaHash {
	// Keep looping while the parent of each orphaned block is
	// known and is an orphan itself.
	prevHash := sha
	for {
		orphan, exists := b.orphans[*prevHash]
		if !exists {
			break
		}
		prevHash = &orphan.block.MsgBlock().Header.PrevBlock
	}

	return prevHash
}

// removeOrphanBlock removes the passed orphan block from the orphan pool and
// previous orphan index.
func (b *BlockChain) removeOrphanBlock(orphan *orphanBlock) {
	// Remove the orphan block from the orphan pool.  It's safe to ignore
	// the error on Sha since it's cached.
	orphanHash, _ := orphan.block.Sha()
	delete(b.orphans, *orphanHash)

	// Remove the reference from the previous orphan index too.
	prevHash := &orphan.block.MsgBlock().Header.PrevBlock
	orphans := b.prevOrphans[*prevHash]
	for i, ob := range orphans {
		hash, _ := ob.block.Sha()
		if hash.IsEqual(orphanHash) {
			copy(orphans[i:], orphans[i+1:])
			orphans[len(orphans)-1] = nil
			b.prevOrphans[*prevHash] = orphans[:len(orphans)-1]
		}
	}

	// Remove the map entry altogether if there are no longer any orphans
	// which depend on the parent hash.
	if len(b.prevOrphans[*prevHash]) == 0 {
		delete(b.prevOrphans, *prevHash)
	}
}

// addOrphanBlock adds the passed block (which is already determined to be
// an orphan prior calling this function) to the orphan pool.  It lazily cleans
// up any expired blocks so a separate cleanup poller doesn't need to be run.
// It also imposes a maximum limit on the number of outstanding orphan
// blocks and will remove the oldest received orphan block if the limit is
// exceeded.
func (b *BlockChain) addOrphanBlock(block *btcutil.Block) {
	// Remove expired orphan blocks.
	for _, oBlock := range b.orphans {
		if time.Now().After(oBlock.expiration) {
			b.removeOrphanBlock(oBlock)
			continue
		}

		// Update the oldest orphan block pointer so it can be discarded
		// in case the orphan pool fills up.
		if b.oldestOrphan == nil || oBlock.expiration.Before(b.oldestOrphan.expiration) {
			b.oldestOrphan = oBlock
		}
	}

	// Limit orphan blocks to prevent memory exhaustion.
	if len(b.orphans)+1 > maxOrphanBlocks {
		// Remove the oldest orphan to make room for the new one.
		b.removeOrphanBlock(b.oldestOrphan)
		b.oldestOrphan = nil
	}

	// Get the block sha.  It is safe to ignore the error here since any
	// errors would've been caught prior to calling this function.
	blockSha, _ := block.Sha()

	// Insert the block into the orphan map with an expiration time
	// 1 hour from now.
	expiration := time.Now().Add(time.Hour)
	oBlock := &orphanBlock{
		block:      block,
		expiration: expiration,
	}
	b.orphans[*blockSha] = oBlock

	// Add to previous hash lookup index for faster dependency lookups.
	prevHash := &block.MsgBlock().Header.PrevBlock
	b.prevOrphans[*prevHash] = append(b.prevOrphans[*prevHash], oBlock)

	return
}

// loadBlockNode loads the block identified by hash from the block database,
// creates a block node from it, and updates the memory block chain accordingly.
// It is used mainly to dynamically load previous blocks from database as they
// are needed to avoid needing to put the entire block chain in memory.
func (b *BlockChain) loadBlockNode(hash *btcwire.ShaHash) (*blockNode, error) {
	// Load the block from the db.
	block, err := b.db.FetchBlockBySha(hash)
	if err != nil {
		return nil, err
	}

	// Create the new block node for the block and set the work.
	node := newBlockNode(block)
	node.inMainChain = true

	// Add the node to the chain.
	// There are several possibilities here:
	//  1) This node is a child of an existing block node
	//  2) This node is the parent of one or more nodes
	//  3) Neither 1 or 2 is true, and this is not the first node being
	//     added to the tree which implies it's an orphan block and
	//     therefore is an error to insert into the chain
	//  4) Neither 1 or 2 is true, but this is the first node being added
	//     to the tree, so it's the root.
	prevHash := &block.MsgBlock().Header.PrevBlock
	if parentNode, ok := b.index[*prevHash]; ok {
		// Case 1 -- This node is a child of an existing block node.
		// Update the node's work sum with the sum of the parent node's
		// work sum and this node's work, append the node as a child of
		// the parent node and set this node's parent to the parent
		// node.
		node.workSum = node.workSum.Add(parentNode.workSum, node.workSum)
		parentNode.children = append(parentNode.children, node)
		node.parent = parentNode

	} else if childNodes, ok := b.depNodes[*hash]; ok {
		// Case 2 -- This node is the parent of one or more nodes.
		// Connect this block node to all of its children and update
		// all of the children (and their children) with the new work
		// sums.
		for _, childNode := range childNodes {
			childNode.parent = node
			node.children = append(node.children, childNode)
			addChildrenWork(childNode, node.workSum)
			b.root = node
		}

	} else {
		// Case 3 -- The node does't have a parent and is not the parent
		// of another node.  This is only acceptable for the first node
		// inserted into the chain.  Otherwise it means an arbitrary
		// orphan block is trying to be loaded which is not allowed.
		if b.root != nil {
			str := "loadBlockNode: attempt to insert orphan block %v"
			return nil, fmt.Errorf(str, hash)
		}

		// Case 4 -- This is the root since it's the first and only node.
		b.root = node
	}

	// Add the new node to the indices for faster lookups.
	b.index[*hash] = node
	b.depNodes[*prevHash] = append(b.depNodes[*prevHash], node)

	return node, nil
}

// getPrevNodeFromBlock returns a block node for the block previous to the
// passed block (the passed block's parent).  When it is already in the memory
// block chain, it simply returns it.  Otherwise, it loads the previous block
// from the block database, creates a new block node from it, and returns it.
// The returned node will be nil if the genesis block is passed.
func (b *BlockChain) getPrevNodeFromBlock(block *btcutil.Block) (*blockNode, error) {
	// Genesis block.
	prevHash := &block.MsgBlock().Header.PrevBlock
	if prevHash.IsEqual(zeroHash) {
		return nil, nil
	}

	// Return the existing previous block node if it's already there.
	if bn, ok := b.index[*prevHash]; ok {
		return bn, nil
	}

	// Dynamically load the previous block from the block database, create
	// a new block node for it, and update the memory chain accordingly.
	prevBlockNode, err := b.loadBlockNode(prevHash)
	if err != nil {
		return nil, err
	}
	return prevBlockNode, nil
}

// getPrevNodeFromNode returns a block node for the block previous to the
// passed block node (the passed block node's parent).  When the node is already
// connected to a parent, it simply returns it.  Otherwise, it loads the
// associated block from the database to obtain the previous hash and uses that
// to dynamically create a new block node and return it.  The memory block
// chain is updated accordingly.  The returned node will be nil if the genesis
// block is passed.
func (b *BlockChain) getPrevNodeFromNode(node *blockNode) (*blockNode, error) {
	// Return the existing previous block node if it's already there.
	if node.parent != nil {
		return node.parent, nil
	}

	// Genesis block.
	if node.hash.IsEqual(b.chainParams().GenesisHash) {
		return nil, nil
	}

	// Load the actual block for this block node from the db to ascertain
	// the previous hash.
	block, err := b.db.FetchBlockBySha(node.hash)
	if err != nil {
		return nil, err
	}

	// Dynamically load the previous block from the block database, create
	// a new block node for it, and update the memory chain accordingly.
	prevHash := &block.MsgBlock().Header.PrevBlock
	prevBlockNode, err := b.loadBlockNode(prevHash)
	if err != nil {
		return nil, err
	}

	return prevBlockNode, nil
}

// isMajorityVersion determines if a previous number of blocks in the chain
// starting with startNode are at least the minimum passed version.
func (b *BlockChain) isMajorityVersion(minVer uint32, startNode *blockNode, numRequired, numToCheck uint64) bool {
	numFound := uint64(0)
	iterNode := startNode
	for i := uint64(0); i < numToCheck && iterNode != nil; i++ {
		// This node has a version that is at least the minimum version.
		if iterNode.version >= minVer {
			numFound++
		}

		// Get the previous block node.  This function is used over
		// simply accessing iterNode.parent directly as it will
		// dynamically create previous block nodes as needed.  This
		// helps allow only the pieces of the chain that are needed
		// to remain in memory.
		var err error
		iterNode, err = b.getPrevNodeFromNode(iterNode)
		if err != nil {
			break
		}
	}

	return numFound >= numRequired
}

// calcPastMedianTime calculates the median time of the previous few blocks
// prior to, and including, the passed block node.  It is primarily used to
// validate new blocks have sane timestamps.
func (b *BlockChain) calcPastMedianTime(startNode *blockNode) (time.Time, error) {
	// Genesis block.
	if startNode == nil {
		return b.chainParams().GenesisBlock.Header.Timestamp, nil
	}

	// Create a slice of the previous few block timestamps used to calculate
	// the median per the number defined by the constant medianTimeBlocks.
	timestamps := make([]time.Time, medianTimeBlocks)
	numNodes := 0
	iterNode := startNode
	for i := 0; i < medianTimeBlocks && iterNode != nil; i++ {
		timestamps[i] = iterNode.timestamp
		numNodes++

		// Get the previous block node.  This function is used over
		// simply accessing iterNode.parent directly as it will
		// dynamically create previous block nodes as needed.  This
		// helps allow only the pieces of the chain that are needed
		// to remain in memory.
		var err error
		iterNode, err = b.getPrevNodeFromNode(iterNode)
		if err != nil {
			log.Errorf("getPrevNodeFromNode: %v", err)
			return time.Time{}, err
		}
	}

	// Prune the slice to the actual number of available timestamps which
	// will be fewer than desired near the beginning of the block chain
	// and sort them.
	timestamps = timestamps[:numNodes]
	sort.Sort(timeSorter(timestamps))

	// NOTE: bitcoind incorrectly calculates the median for even numbers of
	// blocks.  A true median averages the middle two elements for a set
	// with an even number of elements in it.   Since the constant for the
	// previous number of blocks to be used is odd, this is only an issue
	// for a few blocks near the beginning of the chain.  I suspect this is
	// an optimization even though the result is slightly wrong for a few
	// of the first blocks since after the first few blocks, there will
	// always be an odd number of blocks in the set per the constant.
	//
	// This code follows suit to ensure the same rules are used as bitcoind
	// however, be aware that should the medianTimeBlocks constant ever be
	// changed to an even number, this code will be wrong.
	medianTimestamp := timestamps[numNodes/2]
	return medianTimestamp, nil
}

// getReorganizeNodes finds the fork point between the main chain and the passed
// node and returns a list of block nodes that would need to be detached from
// the main chain and a list of block nodes that would need to be attached to
// the fork point (which will be the end of the main chain after detaching the
// returned list of block nodes) in order to reorganize the chain such that the
// passed node is the new end of the main chain.  The lists will be empty if the
// passed node is not on a side chain.
func (b *BlockChain) getReorganizeNodes(node *blockNode) (*list.List, *list.List) {
	// Nothing to detach or attach if there is no node.
	attachNodes := list.New()
	detachNodes := list.New()
	if node == nil {
		return detachNodes, attachNodes
	}

	// Find the fork point (if any) adding each block to the list of nodes
	// to attach to the main tree.  Push them onto the list in reverse order
	// so they are attached in the appropriate order when iterating the list
	// later.
	ancestor := node
	for ; ancestor.parent != nil; ancestor = ancestor.parent {
		if ancestor.inMainChain {
			break
		}
		attachNodes.PushFront(ancestor)
	}

	// TODO(davec): Use prevNodeFromNode function in case the requested
	// node is further back than the what is in memory.  This shouldn't
	// happen in the normal course of operation, but the ability to fetch
	// input transactions of arbitrary blocks will likely to be exposed at
	// some point and that could lead to an issue here.

	// Start from the end of the main chain and work backwards until the
	// common ancestor adding each block to the list of nodes to detach from
	// the main chain.
	for n := b.bestChain; n != nil && n.parent != nil; n = n.parent {
		if n.hash.IsEqual(ancestor.hash) {
			break
		}
		detachNodes.PushBack(n)
	}

	return detachNodes, attachNodes
}

// connectBlock handles connecting the passed node/block to the end of the main
// (best) chain.
func (b *BlockChain) connectBlock(node *blockNode, block *btcutil.Block) error {
	// Make sure it's extending the end of the best chain.
	prevHash := &block.MsgBlock().Header.PrevBlock
	if b.bestChain != nil && !prevHash.IsEqual(b.bestChain.hash) {
		return fmt.Errorf("connectBlock must be called with a block " +
			"that extends the main chain")
	}

	// Insert the block into the database which houses the main chain.
	_, err := b.db.InsertBlock(block)
	if err != nil {
		return err
	}

	// TODO(davec): Remove transactions from memory transaction pool.

	// Add the new node to the memory main chain indices for faster
	// lookups.
	node.inMainChain = true
	b.index[*node.hash] = node
	b.depNodes[*prevHash] = append(b.depNodes[*prevHash], node)

	// This node is now the end of the best chain.
	b.bestChain = node

	// Notify the caller that the block was connected to the main chain.
	// The caller would typically want to react with actions such as
	// updating wallets.
	b.sendNotification(NTBlockConnected, block)

	return nil
}

// disconnectBlock handles disconnecting the passed node/block from the end of
// the main (best) chain.
func (b *BlockChain) disconnectBlock(node *blockNode, block *btcutil.Block) error {
	// Make sure the node being disconnected is the end of the best chain.
	if b.bestChain == nil || !node.hash.IsEqual(b.bestChain.hash) {
		return fmt.Errorf("disconnectBlock must be called with the " +
			"block at the end of the main chain")
	}

	// Remove the block from the database which houses the main chain.
	prevNode, err := b.getPrevNodeFromNode(node)
	if err != nil {
		return err
	}
	err = b.db.DropAfterBlockBySha(prevNode.hash)
	if err != nil {
		return err
	}

	// TODO(davec): Put transactions back in memory transaction pool.

	// Put block in the side chain cache.
	node.inMainChain = false
	b.blockCache[*node.hash] = block

	// This node's parent is now the end of the best chain.
	b.bestChain = node.parent

	// Notify the caller that the block was disconnect from the main chain.
	// The caller would typically want to react with actions such as
	// updating wallets.
	b.sendNotification(NTBlockDisconnected, block)

	return nil
}

// reorganizeChain reorganizes the block chain by disconnecting the nodes in the
// detachNodes list and connecting the nodes in the attach list.  It expects
// that the lists are already in the correct order and are in sync with the
// end of the current best chain.  Specifically, nodes that are being
// disconnected must be in reverse order (think of popping them off
// the end of the chain) and nodes the are being attached must be in forwards
// order (think pushing them onto the end of the chain).
func (b *BlockChain) reorganizeChain(detachNodes, attachNodes *list.List) error {
	// Ensure all of the needed side chain blocks are in the cache.
	for e := attachNodes.Front(); e != nil; e = e.Next() {
		n := e.Value.(*blockNode)
		if _, exists := b.blockCache[*n.hash]; !exists {
			return fmt.Errorf("block %v is missing from the side "+
				"chain block cache", n.hash)
		}
	}

	// Perform several checks to verify each block that needs to be attached
	// to the main chain can be connected without violating any rules and
	// without actually connecting the block.
	//
	// NOTE: bitcoind does these checks directly when it connects a block.
	// The downside to that approach is that if any of these checks fail
	// after disconneting some blocks or attaching others, all of the
	// operations have to be rolled back to get the chain back into the
	// state it was before the rule violation (or other failure).  There are
	// at least a couple of ways accomplish that rollback, but both involve
	// tweaking the chain.  This approach catches these issues before ever
	// modifying the chain.
	for e := attachNodes.Front(); e != nil; e = e.Next() {
		n := e.Value.(*blockNode)
		block := b.blockCache[*n.hash]
		err := b.checkConnectBlock(n, block)
		if err != nil {
			return err
		}

	}

	// Disconnect blocks from the main chain.
	for e := detachNodes.Front(); e != nil; e = e.Next() {
		n := e.Value.(*blockNode)
		block, err := b.db.FetchBlockBySha(n.hash)
		if err != nil {
			return err
		}
		err = b.disconnectBlock(n, block)
		if err != nil {
			return err
		}
	}

	// Connect the new best chain blocks.
	for e := attachNodes.Front(); e != nil; e = e.Next() {
		n := e.Value.(*blockNode)
		block := b.blockCache[*n.hash]
		err := b.connectBlock(n, block)
		if err != nil {
			return err
		}
		delete(b.blockCache, *n.hash)
	}

	// Log the point where the chain forked.
	firstAttachNode := attachNodes.Front().Value.(*blockNode)
	forkNode, err := b.getPrevNodeFromNode(firstAttachNode)
	if err == nil {
		log.Infof("REORGANIZE: Chain forks at %v", forkNode.hash)
	}

	// Log the old and new best chain heads.
	firstDetachNode := detachNodes.Front().Value.(*blockNode)
	lastAttachNode := attachNodes.Back().Value.(*blockNode)
	log.Infof("REORGANIZE: Old best chain head was %v", firstDetachNode.hash)
	log.Infof("REORGANIZE: New best chain head is %v", lastAttachNode.hash)

	return nil
}

// connectBestChain handles connecting the passed block to the chain while
// respecting proper chain selection according to the chain with the most
// proof of work.  In the typical case, the new block simply extends the main
// chain.  However, it may also be extending (or creating) a side chain (fork)
// which may or may not end up becoming the main chain depending on which fork
// cumulatively has the most proof of work.
func (b *BlockChain) connectBestChain(node *blockNode, block *btcutil.Block) error {
	// We haven't selected a best chain yet or we are extending the main
	// (best) chain with a new block.  This is the most common case.
	if b.bestChain == nil || node.parent.hash.IsEqual(b.bestChain.hash) {
		// Perform several checks to verify the block can be connected
		// to the main chain (including whatever reorganization might
		// be necessary to get this node to the main chain) without
		// violating any rules and without actually connecting the
		// block.
		err := b.checkConnectBlock(node, block)
		if err != nil {
			return err
		}

		// Connect the block to the main chain.
		err = b.connectBlock(node, block)
		if err != nil {
			return err
		}

		// Connect the parent node to this node.
		if node.parent != nil {
			node.parent.children = append(node.parent.children, node)
		}

		return nil
	}

	// We're extending (or creating) a side chain which may or may not
	// become the main chain, but in either case we need the block stored
	// for future processing, so add the block to the side chain holding
	// cache.
	log.Debugf("Adding block %v to side chain cache", node.hash)
	b.blockCache[*node.hash] = block
	b.index[*node.hash] = node

	// We're extending (or creating) a side chain, but the cumulative
	// work for this new side chain is not enough to make it the new chain.
	if node.workSum.Cmp(b.bestChain.workSum) <= 0 {
		// Connect the parent node to this node.
		node.inMainChain = false
		node.parent.children = append(node.parent.children, node)

		// Find the fork point.
		fork := node
		for ; fork.parent != nil; fork = fork.parent {
			if fork.inMainChain {
				break
			}
		}

		// Log information about how the block is forking the chain.
		if fork.hash.IsEqual(node.parent.hash) {
			log.Infof("FORK: Block %v forks the chain at height %d"+
				"/block %v, but does not cause a reorganize",
				node.hash, fork.height, fork.hash)
		} else {
			log.Infof("EXTEND FORK: Block %v extends a side chain "+
				"which forks the chain at height %d/block %v",
				node.hash, fork.height, fork.hash)
		}
		return nil
	}

	// We're extending (or creating) a side chain and the cumulative work
	// for this new side chain is more than the old best chain, so this side
	// chain needs to become the main chain.  In order to accomplish that,
	// find the common ancestor of both sides of the fork, disconnect the
	// blocks that form the (now) old fork from the main chain, and attach
	// the blocks that form the new chain to the main chain starting at the
	// common ancenstor (the point where the chain forked).
	detachNodes, attachNodes := b.getReorganizeNodes(node)

	// Reorganize the chain.
	log.Infof("REORGANIZE: Block %v is causing a reorganize.", node.hash)
	err := b.reorganizeChain(detachNodes, attachNodes)
	if err != nil {
		return err
	}

	return nil
}

// New returns a BlockChain instance for the passed bitcoin network using the
// provided backing database.  It accepts a channel on which asynchronous
// notifications will be sent when various events take place.  See the
// documentation for Notification and NotificationType for details on the
// types and contents of notifications.  The provided channel can be nil if the
// caller is not interested in receiving notifications.
func New(db btcdb.Db, btcnet btcwire.BitcoinNet, c chan *Notification) *BlockChain {
	b := BlockChain{
		db:            db,
		btcnet:        btcnet,
		notifications: c,
		root:          nil,
		bestChain:     nil,
		index:         make(map[btcwire.ShaHash]*blockNode),
		depNodes:      make(map[btcwire.ShaHash][]*blockNode),
		orphans:       make(map[btcwire.ShaHash]*orphanBlock),
		prevOrphans:   make(map[btcwire.ShaHash][]*orphanBlock),
		blockCache:    make(map[btcwire.ShaHash]*btcutil.Block),
	}
	return &b
}