lbcd/blockchain/chainview.go

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// Copyright (c) 2017 The btcsuite developers
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.
package blockchain
import (
"sync"
)
// approxNodesPerWeek is an approximation of the number of new blocks there are
// in a week on average.
const approxNodesPerWeek = 6 * 24 * 7
blockchain: Refactor to use new chain view. - Remove inMainChain from block nodes since that can now be efficiently determined by using the chain view - Track the best chain via a chain view instead of a single block node - Use the tip of the best chain view everywhere bestNode was used - Update chain view tip instead of updating best node - Change reorg logic to use more efficient chain view fork finding logic - Change block locator code over to use more efficient chain view logic - Remove now unused block-index-based block locator code - Move BlockLocator definition to chain.go - Move BlockLocatorFromHash and LatestBlockLocator to chain.go - Update both to use more efficient chain view logic - Rework IsCheckpointCandidate to use block index and chain view - Optimize MainChainHasBlock to use chain view instead of hitting db - Move to chain.go since it no longer involves database I/O - Removed error return since it can no longer fail - Optimize BlockHeightByHash to use chain view instead of hitting db - Move to chain.go since it no longer involves database I/O - Removed error return since it can no longer fail - Optimize BlockHashByHeight to use chain view instead of hitting db - Move to chain.go since it no longer involves database I/O - Removed error return since it can no longer fail - Optimize HeightRange to use chain view instead of hitting db - Move to chain.go since it no longer involves database I/O - Optimize BlockByHeight to use chain view for main chain check - Optimize BlockByHash to use chain view for main chain check
2017-08-18 14:25:54 +02:00
// log2FloorMasks defines the masks to use when quickly calculating
// floor(log2(x)) in a constant log2(32) = 5 steps, where x is a uint32, using
// shifts. They are derived from (2^(2^x) - 1) * (2^(2^x)), for x in 4..0.
var log2FloorMasks = []uint32{0xffff0000, 0xff00, 0xf0, 0xc, 0x2}
// fastLog2Floor calculates and returns floor(log2(x)) in a constant 5 steps.
func fastLog2Floor(n uint32) uint8 {
rv := uint8(0)
exponent := uint8(16)
for i := 0; i < 5; i++ {
if n&log2FloorMasks[i] != 0 {
rv += exponent
n >>= exponent
}
exponent >>= 1
}
return rv
}
// chainView provides a flat view of a specific branch of the block chain from
// its tip back to the genesis block and provides various convenience functions
// for comparing chains.
//
// For example, assume a block chain with a side chain as depicted below:
// genesis -> 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8
// \-> 4a -> 5a -> 6a
//
// The chain view for the branch ending in 6a consists of:
// genesis -> 1 -> 2 -> 3 -> 4a -> 5a -> 6a
type chainView struct {
mtx sync.Mutex
nodes []*blockNode
}
// newChainView returns a new chain view for the given tip block node. Passing
// nil as the tip will result in a chain view that is not initialized. The tip
// can be updated at any time via the setTip function.
func newChainView(tip *blockNode) *chainView {
// The mutex is intentionally not held since this is a constructor.
var c chainView
c.setTip(tip)
return &c
}
// genesis returns the genesis block for the chain view. This only differs from
// the exported version in that it is up to the caller to ensure the lock is
// held.
//
// This function MUST be called with the view mutex locked (for reads).
func (c *chainView) genesis() *blockNode {
if len(c.nodes) == 0 {
return nil
}
return c.nodes[0]
}
// Genesis returns the genesis block for the chain view.
//
// This function is safe for concurrent access.
func (c *chainView) Genesis() *blockNode {
c.mtx.Lock()
genesis := c.genesis()
c.mtx.Unlock()
return genesis
}
// tip returns the current tip block node for the chain view. It will return
// nil if there is no tip. This only differs from the exported version in that
// it is up to the caller to ensure the lock is held.
//
// This function MUST be called with the view mutex locked (for reads).
func (c *chainView) tip() *blockNode {
if len(c.nodes) == 0 {
return nil
}
return c.nodes[len(c.nodes)-1]
}
// Tip returns the current tip block node for the chain view. It will return
// nil if there is no tip.
//
// This function is safe for concurrent access.
func (c *chainView) Tip() *blockNode {
c.mtx.Lock()
tip := c.tip()
c.mtx.Unlock()
return tip
}
// setTip sets the chain view to use the provided block node as the current tip
// and ensures the view is consistent by populating it with the nodes obtained
// by walking backwards all the way to genesis block as necessary. Further
// calls will only perform the minimum work needed, so switching between chain
// tips is efficient. This only differs from the exported version in that it is
// up to the caller to ensure the lock is held.
//
// This function MUST be called with the view mutex locked (for writes).
func (c *chainView) setTip(node *blockNode) {
if node == nil {
// Keep the backing array around for potential future use.
c.nodes = c.nodes[:0]
return
}
// Create or resize the slice that will hold the block nodes to the
// provided tip height. When creating the slice, it is created with
// some additional capacity for the underlying array as append would do
// in order to reduce overhead when extending the chain later. As long
// as the underlying array already has enough capacity, simply expand or
// contract the slice accordingly. The additional capacity is chosen
// such that the array should only have to be extended about once a
// week.
needed := node.height + 1
if int32(cap(c.nodes)) < needed {
nodes := make([]*blockNode, needed, needed+approxNodesPerWeek)
copy(nodes, c.nodes)
c.nodes = nodes
} else {
prevLen := int32(len(c.nodes))
c.nodes = c.nodes[0:needed]
for i := prevLen; i < needed; i++ {
c.nodes[i] = nil
}
}
for node != nil && c.nodes[node.height] != node {
c.nodes[node.height] = node
node = node.parent
}
}
// SetTip sets the chain view to use the provided block node as the current tip
// and ensures the view is consistent by populating it with the nodes obtained
// by walking backwards all the way to genesis block as necessary. Further
// calls will only perform the minimum work needed, so switching between chain
// tips is efficient.
//
// This function is safe for concurrent access.
func (c *chainView) SetTip(node *blockNode) {
c.mtx.Lock()
c.setTip(node)
c.mtx.Unlock()
}
// height returns the height of the tip of the chain view. It will return -1 if
// there is no tip (which only happens if the chain view has not been
// initialized). This only differs from the exported version in that it is up
// to the caller to ensure the lock is held.
//
// This function MUST be called with the view mutex locked (for reads).
func (c *chainView) height() int32 {
return int32(len(c.nodes) - 1)
}
// Height returns the height of the tip of the chain view. It will return -1 if
// there is no tip (which only happens if the chain view has not been
// initialized).
//
// This function is safe for concurrent access.
func (c *chainView) Height() int32 {
c.mtx.Lock()
height := c.height()
c.mtx.Unlock()
return height
}
// nodeByHeight returns the block node at the specified height. Nil will be
// returned if the height does not exist. This only differs from the exported
// version in that it is up to the caller to ensure the lock is held.
//
// This function MUST be called with the view mutex locked (for reads).
func (c *chainView) nodeByHeight(height int32) *blockNode {
if height < 0 || height >= int32(len(c.nodes)) {
return nil
}
return c.nodes[height]
}
// NodeByHeight returns the block node at the specified height. Nil will be
// returned if the height does not exist.
//
// This function is safe for concurrent access.
func (c *chainView) NodeByHeight(height int32) *blockNode {
c.mtx.Lock()
node := c.nodeByHeight(height)
c.mtx.Unlock()
return node
}
// Equals returns whether or not two chain views are the same. Uninitialized
// views (tip set to nil) are considered equal.
//
// This function is safe for concurrent access.
func (c *chainView) Equals(other *chainView) bool {
c.mtx.Lock()
other.mtx.Lock()
equals := len(c.nodes) == len(other.nodes) && c.tip() == other.tip()
other.mtx.Unlock()
c.mtx.Unlock()
return equals
}
// contains returns whether or not the chain view contains the passed block
// node. This only differs from the exported version in that it is up to the
// caller to ensure the lock is held.
//
// This function MUST be called with the view mutex locked (for reads).
func (c *chainView) contains(node *blockNode) bool {
return c.nodeByHeight(node.height) == node
}
// Contains returns whether or not the chain view contains the passed block
// node.
//
// This function is safe for concurrent access.
func (c *chainView) Contains(node *blockNode) bool {
c.mtx.Lock()
contains := c.contains(node)
c.mtx.Unlock()
return contains
}
// next returns the successor to the provided node for the chain view. It will
// return nil if there is no successor or the provided node is not part of the
// view. This only differs from the exported version in that it is up to the
// caller to ensure the lock is held.
//
// See the comment on the exported function for more details.
//
// This function MUST be called with the view mutex locked (for reads).
func (c *chainView) next(node *blockNode) *blockNode {
if node == nil || !c.contains(node) {
return nil
}
return c.nodeByHeight(node.height + 1)
}
// Next returns the successor to the provided node for the chain view. It will
// return nil if there is no successfor or the provided node is not part of the
// view.
//
// For example, assume a block chain with a side chain as depicted below:
// genesis -> 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8
// \-> 4a -> 5a -> 6a
//
// Further, assume the view is for the longer chain depicted above. That is to
// say it consists of:
// genesis -> 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8
//
// Invoking this function with block node 5 would return block node 6 while
// invoking it with block node 5a would return nil since that node is not part
// of the view.
//
// This function is safe for concurrent access.
func (c *chainView) Next(node *blockNode) *blockNode {
c.mtx.Lock()
next := c.next(node)
c.mtx.Unlock()
return next
}
// findFork returns the final common block between the provided node and the
// the chain view. It will return nil if there is no common block. This only
// differs from the exported version in that it is up to the caller to ensure
// the lock is held.
//
// See the exported FindFork comments for more details.
//
// This function MUST be called with the view mutex locked (for reads).
func (c *chainView) findFork(node *blockNode) *blockNode {
// No fork point for node that doesn't exist.
if node == nil {
return nil
}
// When the height of the passed node is higher than the height of the
// tip of the current chain view, walk backwards through the nodes of
// the other chain until the heights match (or there or no more nodes in
// which case there is no common node between the two).
//
// NOTE: This isn't strictly necessary as the following section will
// find the node as well, however, it is more efficient to avoid the
// contains check since it is already known that the common node can't
// possibly be past the end of the current chain view. It also allows
// this code to take advantage of any potential future optimizations to
// the Ancestor function such as using an O(log n) skip list.
chainHeight := c.height()
if node.height > chainHeight {
node = node.Ancestor(chainHeight)
}
// Walk the other chain backwards as long as the current one does not
// contain the node or there are no more nodes in which case there is no
// common node between the two.
for node != nil && !c.contains(node) {
node = node.parent
}
return node
}
// FindFork returns the final common block between the provided node and the
// the chain view. It will return nil if there is no common block.
//
// For example, assume a block chain with a side chain as depicted below:
// genesis -> 1 -> 2 -> ... -> 5 -> 6 -> 7 -> 8
// \-> 6a -> 7a
//
// Further, assume the view is for the longer chain depicted above. That is to
// say it consists of:
// genesis -> 1 -> 2 -> ... -> 5 -> 6 -> 7 -> 8.
//
// Invoking this function with block node 7a would return block node 5 while
// invoking it with block node 7 would return itself since it is already part of
// the branch formed by the view.
//
// This function is safe for concurrent access.
func (c *chainView) FindFork(node *blockNode) *blockNode {
c.mtx.Lock()
fork := c.findFork(node)
c.mtx.Unlock()
return fork
}
// blockLocator returns a block locator for the passed block node. The passed
// node can be nil in which case the block locator for the current tip
// associated with the view will be returned. This only differs from the
// exported version in that it is up to the caller to ensure the lock is held.
//
// See the exported BlockLocator function comments for more details.
//
// This function MUST be called with the view mutex locked (for reads).
func (c *chainView) blockLocator(node *blockNode) BlockLocator {
// Use the current tip if requested.
if node == nil {
node = c.tip()
}
if node == nil {
return nil
}
// Calculate the max number of entries that will ultimately be in the
// block locator. See the description of the algorithm for how these
// numbers are derived.
var maxEntries uint8
if node.height <= 12 {
maxEntries = uint8(node.height) + 1
} else {
// Requested hash itself + previous 10 entries + genesis block.
// Then floor(log2(height-10)) entries for the skip portion.
adjustedHeight := uint32(node.height) - 10
maxEntries = 12 + fastLog2Floor(adjustedHeight)
}
locator := make(BlockLocator, 0, maxEntries)
step := int32(1)
for node != nil {
locator = append(locator, &node.hash)
// Nothing more to add once the genesis block has been added.
if node.height == 0 {
break
}
// Calculate height of previous node to include ensuring the
// final node is the genesis block.
height := node.height - step
if height < 0 {
height = 0
}
// When the node is in the current chain view, all of its
// ancestors must be too, so use a much faster O(1) lookup in
// that case. Otherwise, fall back to walking backwards through
// the nodes of the other chain to the correct ancestor.
if c.contains(node) {
node = c.nodes[height]
} else {
node = node.Ancestor(height)
}
// Once 11 entries have been included, start doubling the
// distance between included hashes.
if len(locator) > 10 {
step *= 2
}
}
return locator
}
// BlockLocator returns a block locator for the passed block node. The passed
// node can be nil in which case the block locator for the current tip
// associated with the view will be returned.
//
// See the BlockLocator type for details on the algorithm used to create a block
// locator.
//
// This function is safe for concurrent access.
func (c *chainView) BlockLocator(node *blockNode) BlockLocator {
c.mtx.Lock()
locator := c.blockLocator(node)
c.mtx.Unlock()
return locator
}