peer: Optimize merkle block handling.

This commit updates the merkle block handling to for the latest changes
to the btcutil API and optimizes it along the way.

Previously, the code was inefficiently reloading the transactions for
the matched hashes from the database instead of simply pulling them from
the full block that was used to create the merkle block.

peer.go

@@ -604,28 +604,8 @@ func (p *peer) pushMerkleBlockMsg(sha *wire.ShaHash, doneChan, waitChan chan str
 	}
 
 	// Generate a merkle block by filtering the requested block according
-	// to the filter for the peer and fetch any matched transactions from
-	// the database.
-	merkle, matchedHashes := bloom.NewMerkleBlock(blk, p.filter)
-	txList := p.server.db.FetchTxByShaList(matchedHashes)
-
-	// Warn on any missing transactions which should not happen since the
-	// matched transactions come from an existing block.  Also, find the
-	// final valid transaction index for later.
-	finalValidTxIndex := -1
-	for i, txR := range txList {
-		if txR.Err != nil || txR.Tx == nil {
-			warnMsg := fmt.Sprintf("Failed to fetch transaction "+
-				"%v which was matched by merkle block %v",
-				txR.Sha, sha)
-			if txR.Err != nil {
-				warnMsg += ": " + err.Error()
-			}
-			peerLog.Warnf(warnMsg)
-			continue
-		}
-		finalValidTxIndex = i
-	}
+	// to the filter for the peer.
+	merkle, matchedTxIndices := bloom.NewMerkleBlock(blk, p.filter)
 
 	// Once we have fetched data wait for any previous operation to finish.
 	if waitChan != nil {
@@ -635,20 +615,21 @@ func (p *peer) pushMerkleBlockMsg(sha *wire.ShaHash, doneChan, waitChan chan str
 	// Send the merkleblock.  Only send the done channel with this message
 	// if no transactions will be sent afterwards.
 	var dc chan struct{}
-	if finalValidTxIndex == -1 {
+	if len(matchedTxIndices) == 0 {
 		dc = doneChan
 	}
 	p.QueueMessage(merkle, dc)
 
 	// Finally, send any matched transactions.
-	for i, txR := range txList {
+	blkTransactions := blk.MsgBlock().Transactions
+	for i, txIndex := range matchedTxIndices {
 		// Only send the done channel on the final transaction.
 		var dc chan struct{}
-		if i == finalValidTxIndex {
+		if i == len(matchedTxIndices)-1 {
 			dc = doneChan
 		}
-		if txR.Err == nil && txR.Tx != nil {
-			p.QueueMessage(txR.Tx, dc)
+		if txIndex < uint32(len(blkTransactions)) {
+			p.QueueMessage(blkTransactions[txIndex], dc)
 		}
 	}