blockchain: add exports methods based on block header tip

The added exported methods on BlockChain provide access to the block
header tip like fetching block hashes and heights from the headers
chain.

blockchain/chain.go

@@ -1344,6 +1344,15 @@ func (b *BlockChain) BestSnapshot() *BestState {
 	return snapshot
 }
 
+// BestHeader returns the hash and the height of the best header.
+func (b *BlockChain) BestHeader() (chainhash.Hash, int32) {
+	b.chainLock.RLock()
+	defer b.chainLock.RUnlock()
+
+	best := b.bestHeader.Tip()
+	return best.hash, best.height
+}
+
 // TipStatus is the status of a chain tip.
 type TipStatus byte
 
@@ -1535,6 +1544,54 @@ func (b *BlockChain) BlockHashByHeight(blockHeight int32) (*chainhash.Hash, erro
 	return &node.hash, nil
 }
 
+// IsValidHeader checks that we've already checked that this header connects to the
+// chain of best headers and did not receive an invalid state.
+func (b *BlockChain) IsValidHeader(blockHash *chainhash.Hash) bool {
+	node := b.index.LookupNode(blockHash)
+	if node == nil || !b.bestHeader.Contains(node) {
+		return false
+	}
+
+	return !b.index.NodeStatus(node).KnownInvalid()
+}
+
+// LatestBlockLocatorByHeader returns a block locator for the latest known tip of the
+// header chain.
+//
+// This function is safe for concurrent access.
+func (b *BlockChain) LatestBlockLocatorByHeader() (BlockLocator, error) {
+	b.chainLock.RLock()
+	locator := b.bestHeader.BlockLocator(nil)
+	b.chainLock.RUnlock()
+	return locator, nil
+}
+
+// HeaderHashByHeight returns the block header's hash given its height.
+//
+// NOTE: If the blockNode at the given blockHeight is not included in the
+// bestHeader chain, the function will return an error indicating that the
+// blockHeight wasn't found.
+func (b *BlockChain) HeaderHashByHeight(blockHeight int32) (
+	*chainhash.Hash, error) {
+
+	node := b.bestHeader.NodeByHeight(blockHeight)
+	if node == nil || !b.bestHeader.Contains(node) {
+		return nil, fmt.Errorf("blockheight %v not found", blockHeight)
+	}
+
+	return &node.hash, nil
+}
+
+// HeaderHeightByHash returns the height of the header given its hash.
+func (b *BlockChain) HeaderHeightByHash(blockHash chainhash.Hash) (int32, error) {
+	node := b.index.LookupNode(&blockHash)
+	if node == nil || !b.bestHeader.Contains(node) {
+		return -1, fmt.Errorf("blockhash %v not found", blockHash)
+	}
+
+	return node.height, nil
+}
+
 // HeightRange returns a range of block hashes for the given start and end
 // heights.  It is inclusive of the start height and exclusive of the end
 // height.  The end height will be limited to the current main chain height.