blockchain: Add median time to state snapshot.

This adds a new field to the best chain state snapshot for the
calculated past median time as returned by the calcPastMedianTime
function.  This is useful since it provides fast access to it without
having to acquire the chain lock which is needed to recalculate it.

This will ultimately allow the associated exported function to be
removed since it only exists to be able to calculate this exact value,
however this commit only introduces the new field in order to keep the
changes minimal.

blockchain/chain.go

@@ -128,23 +128,25 @@ func removeChildNode(children []*blockNode, node *blockNode) []*blockNode {
 // However, the returned snapshot must be treated as immutable since it is
 // shared by all callers.
 type BestState struct {
-	Hash      *chainhash.Hash // The hash of the block.
-	Height    int32           // The height of the block.
-	Bits      uint32          // The difficulty bits of the block.
-	BlockSize uint64          // The size of the block.
-	NumTxns   uint64          // The number of txns in the block.
-	TotalTxns uint64          // The total number of txns in the chain.
+	Hash       *chainhash.Hash // The hash of the block.
+	Height     int32           // The height of the block.
+	Bits       uint32          // The difficulty bits of the block.
+	BlockSize  uint64          // The size of the block.
+	NumTxns    uint64          // The number of txns in the block.
+	TotalTxns  uint64          // The total number of txns in the chain.
+	MedianTime time.Time       // Median time as per CalcPastMedianTime.
 }
 
 // newBestState returns a new best stats instance for the given parameters.
-func newBestState(node *blockNode, blockSize, numTxns, totalTxns uint64) *BestState {
+func newBestState(node *blockNode, blockSize, numTxns, totalTxns uint64, medianTime time.Time) *BestState {
 	return &BestState{
-		Hash:      node.hash,
-		Height:    node.height,
-		Bits:      node.bits,
-		BlockSize: blockSize,
-		NumTxns:   numTxns,
-		TotalTxns: totalTxns,
+		Hash:       node.hash,
+		Height:     node.height,
+		Bits:       node.bits,
+		BlockSize:  blockSize,
+		NumTxns:    numTxns,
+		TotalTxns:  totalTxns,
+		MedianTime: medianTime,
 	}
 }
 
@@ -779,6 +781,12 @@ func (b *BlockChain) connectBlock(node *blockNode, block *btcutil.Block, view *U
 			"spent transaction out information")
 	}
 
+	// Calculate the median time for the block.
+	medianTime, err := b.calcPastMedianTime(node)
+	if err != nil {
+		return err
+	}
+
 	// Generate a new best state snapshot that will be used to update the
 	// database and later memory if all database updates are successful.
 	b.stateLock.RLock()
@@ -786,10 +794,11 @@ func (b *BlockChain) connectBlock(node *blockNode, block *btcutil.Block, view *U
 	b.stateLock.RUnlock()
 	numTxns := uint64(len(block.MsgBlock().Transactions))
 	blockSize := uint64(block.MsgBlock().SerializeSize())
-	state := newBestState(node, blockSize, numTxns, curTotalTxns+numTxns)
+	state := newBestState(node, blockSize, numTxns, curTotalTxns+numTxns,
+		medianTime)
 
 	// Atomically insert info into the database.
-	err := b.db.Update(func(dbTx database.Tx) error {
+	err = b.db.Update(func(dbTx database.Tx) error {
 		// Update best block state.
 		err := dbPutBestState(dbTx, state, node.workSum)
 		if err != nil {
@@ -892,6 +901,12 @@ func (b *BlockChain) disconnectBlock(node *blockNode, block *btcutil.Block, view
 		return err
 	}
 
+	// Calculate the median time for the previous block.
+	medianTime, err := b.calcPastMedianTime(prevNode)
+	if err != nil {
+		return err
+	}
+
 	// Load the previous block since some details for it are needed below.
 	var prevBlock *btcutil.Block
 	err = b.db.View(func(dbTx database.Tx) error {
@@ -911,7 +926,8 @@ func (b *BlockChain) disconnectBlock(node *blockNode, block *btcutil.Block, view
 	numTxns := uint64(len(prevBlock.MsgBlock().Transactions))
 	blockSize := uint64(prevBlock.MsgBlock().SerializeSize())
 	newTotalTxns := curTotalTxns - uint64(len(block.MsgBlock().Transactions))
-	state := newBestState(prevNode, blockSize, numTxns, newTotalTxns)
+	state := newBestState(prevNode, blockSize, numTxns, newTotalTxns,
+		medianTime)
 
 	err = b.db.Update(func(dbTx database.Tx) error {
 		// Update best block state.

blockchain/chainio.go

@@ -1058,10 +1058,12 @@ func (b *BlockChain) createChainState() error {
 	// Add the new node to the index which is used for faster lookups.
 	b.index[*node.hash] = node
 
-	// Initialize the state related to the best block.
+	// Initialize the state related to the best block.  Since it is the
+	// genesis block, use its timestamp for the median time.
 	numTxns := uint64(len(genesisBlock.MsgBlock().Transactions))
 	blockSize := uint64(genesisBlock.MsgBlock().SerializeSize())
-	b.stateSnapshot = newBestState(b.bestNode, blockSize, numTxns, numTxns)
+	b.stateSnapshot = newBestState(b.bestNode, blockSize, numTxns, numTxns,
+		b.bestNode.timestamp)
 
 	// Create the initial the database chain state including creating the
 	// necessary index buckets and inserting the genesis block.
@@ -1159,11 +1161,17 @@ func (b *BlockChain) initChainState() error {
 		b.index[*node.hash] = node
 		b.depNodes[*prevHash] = append(b.depNodes[*prevHash], node)
 
+		// Calculate the median time for the block.
+		medianTime, err := b.calcPastMedianTime(node)
+		if err != nil {
+			return err
+		}
+
 		// Initialize the state related to the best block.
 		blockSize := uint64(len(blockBytes))
 		numTxns := uint64(len(block.Transactions))
 		b.stateSnapshot = newBestState(b.bestNode, blockSize, numTxns,
-			state.totalTxns)
+			state.totalTxns, medianTime)
 
 		isStateInitialized = true
 		return nil