blockchain: Backport optimize exported header access.

backport of decred/dcrd#1273

Notable difference being that btcd mainline currenlty
doesn't have a blockchain/blockindex_test.go file, so
those changes are omitted.

Great work @davecgh :)

blockchain/chain.go

@@ -1242,25 +1242,17 @@ func (b *BlockChain) BestSnapshot() *BestState {
 	return snapshot
 }
 
-// FetchHeader returns the block header identified by the given hash or an error
-// if it doesn't exist.
-func (b *BlockChain) FetchHeader(hash *chainhash.Hash) (wire.BlockHeader, error) {
-	// Reconstruct the header from the block index if possible.
-	if node := b.index.LookupNode(hash); node != nil {
-		return node.Header(), nil
-	}
-
-	// Fall back to loading it from the database.
-	var header *wire.BlockHeader
-	err := b.db.View(func(dbTx database.Tx) error {
-		var err error
-		header, err = dbFetchHeaderByHash(dbTx, hash)
-		return err
-	})
-	if err != nil {
+// HeaderByHash returns the block header identified by the given hash or an
+// error if it doesn't exist. Note that this will return headers from both the
+// main and side chains.
+func (b *BlockChain) HeaderByHash(hash *chainhash.Hash) (wire.BlockHeader, error) {
+	node := b.index.Lookup(hash)
+	if node == nil {
+		err := fmt.Errorf("block %s is not known", hash)
 		return wire.BlockHeader{}, err
 	}
-	return *header, nil
+
+	return node.Header(), nil
 }
 
 // MainChainHasBlock returns whether or not the block with the given hash is in

rpcserver.go

@@ -1320,7 +1320,7 @@ func handleGetBlockHeader(s *rpcServer, cmd interface{}, closeChan <-chan struct
 	if err != nil {
 		return nil, rpcDecodeHexError(c.Hash)
 	}
-	blockHeader, err := s.cfg.Chain.FetchHeader(hash)
+	blockHeader, err := s.cfg.Chain.HeaderByHash(hash)
 	if err != nil {
 		return nil, &btcjson.RPCError{
 			Code:    btcjson.ErrRPCBlockNotFound,
@@ -2442,7 +2442,7 @@ func handleGetNetworkHashPS(s *rpcServer, cmd interface{}, closeChan <-chan stru
 		}
 
 		// Fetch the header from chain.
-		header, err := s.cfg.Chain.FetchHeader(hash)
+		header, err := s.cfg.Chain.HeaderByHash(hash)
 		if err != nil {
 			context := "Failed to fetch block header"
 			return nil, internalRPCError(err.Error(), context)
@@ -2634,7 +2634,7 @@ func handleGetRawTransaction(s *rpcServer, cmd interface{}, closeChan <-chan str
 	var chainHeight int32
 	if blkHash != nil {
 		// Fetch the header from chain.
-		header, err := s.cfg.Chain.FetchHeader(blkHash)
+		header, err := s.cfg.Chain.HeaderByHash(blkHash)
 		if err != nil {
 			context := "Failed to fetch block header"
 			return nil, internalRPCError(err.Error(), context)
@@ -3262,7 +3262,7 @@ func handleSearchRawTransactions(s *rpcServer, cmd interface{}, closeChan <-chan
 		var blkHeight int32
 		if blkHash := rtx.blkHash; blkHash != nil {
 			// Fetch the header from chain.
-			header, err := s.cfg.Chain.FetchHeader(blkHash)
+			header, err := s.cfg.Chain.HeaderByHash(blkHash)
 			if err != nil {
 				return nil, &btcjson.RPCError{
 					Code:    btcjson.ErrRPCBlockNotFound,