blockchain: Implement new chain view.

This implements a new type in the blockchain package that takes
advantage of the fact that all block nodes are now in memory to provide
a flat view of a specific chain of blocks (a specific branch of the
overall block tree) from a given tip all the way back to the genesis
block along with several convenience functions such as efficiently
comparing two views, quickly finding the fork point (if any) between two
views, and O(1) lookup of the node at a specific height.

The view is not currently used, but the intent is that the code will be
refactored to make use of these views to simplify and optimize several
areas such as best chain selection and reorg logic and finding successor
nodes.  They will also greatly simplify the process of disconnecting the
download logic from the connection logic.

Since the ultimate intent is for there to be a long-lived chain view
instance for the current best chain, this also implements efficient
handling of setting the tip to new values including domain-specific
capacity increase handling which chooses the desired increase amount
more intelligently than the default algorithm, which would way
overshoot, as well as adds some additional space when the view is
initialized.

A comprehensive suite of tests is provided to ensure the chain views
behave correctly.

blockchain/blockindex_test.go

@@ -212,12 +212,9 @@ func TestChainTips(t *testing.T) {
 	bc.index.RUnlock()
 
 	// The expected chain tips are the tips of all of the branches.
-	tip := func(nodes []*blockNode) *blockNode {
-		return nodes[len(nodes)-1]
-	}
 	expectedTips := make(map[*blockNode]struct{})
 	for _, branch := range branches {
-		expectedTips[tip(branch)] = struct{}{}
+		expectedTips[branchTip(branch)] = struct{}{}
 	}
 
 	// Ensure the chain tips are the expected values.

blockchain/chainview.go

@@ -0,0 +1,327 @@
+// Copyright (c) 2017 The btcsuite developers
+// Copyright (c) 2018 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 (
+	"sync"
+)
+
+// approxNodesPerWeek is an approximation of the number of new blocks there are
+// in a week on average.
+const approxNodesPerWeek = 12 * 24 * 7
+
+// 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 int64(cap(c.nodes)) < needed {
+		nodes := make([]*blockNode, needed, needed+approxNodesPerWeek)
+		copy(nodes, c.nodes)
+		c.nodes = nodes
+	} else {
+		prevLen := int64(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() int64 {
+	return int64(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() int64 {
+	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 int64) *blockNode {
+	if height < 0 || height >= int64(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 int64) *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 {
+	if c == other {
+		return true
+	}
+
+	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
+}