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