rpcserver: Update getblockchaininfo best header.

This modifies the getblockchaininfo RPC handler to use the newly-exposed
best header that is not known to be invalid from chain versus the height
of the current best chain tip.

internal/rpcserver/interface.go

@@ -236,6 +236,10 @@ type Chain interface {
 	// treated as immutable since it is shared by all callers.
 	BestSnapshot() *blockchain.BestState
 
+	// BestHeader returns the header with the most cumulative work that is NOT
+	// known to be invalid.
+	BestHeader() (chainhash.Hash, int64)
+
 	// BlockByHash returns the block for the given hash, regardless of whether the
 	// block is part of the main chain or not.
 	BlockByHash(hash *chainhash.Hash) (*dcrutil.Block, error)

internal/rpcserver/rpcserver.go

@@ -2018,6 +2018,7 @@ func handleGetBlock(_ context.Context, s *Server, cmd interface{}) (interface{},
 func handleGetBlockchainInfo(_ context.Context, s *Server, cmd interface{}) (interface{}, error) {
 	chain := s.cfg.Chain
 	best := chain.BestSnapshot()
+	_, bestHeaderHeight := chain.BestHeader()
 
 	// Fetch the current chain work using the the best block hash.
 	chainWork, err := chain.ChainWork(&best.Hash)
@@ -2087,7 +2088,7 @@ func handleGetBlockchainInfo(_ context.Context, s *Server, cmd interface{}) (int
 	response := types.GetBlockChainInfoResult{
 		Chain:                params.Name,
 		Blocks:               best.Height,
-		Headers:              best.Height,
+		Headers:              bestHeaderHeight,
 		SyncHeight:           syncHeight,
 		ChainWork:            fmt.Sprintf("%064x", chainWork),
 		InitialBlockDownload: !chain.IsCurrent(),

internal/rpcserver/rpcserverhandlers_test.go

@@ -127,6 +127,8 @@ type tspendVotes struct {
 // testRPCChain provides a mock block chain by implementing the Chain interface.
 type testRPCChain struct {
 	bestSnapshot                  *blockchain.BestState
+	bestHeaderHash                chainhash.Hash
+	bestHeaderHeight              int64
 	blockByHash                   *dcrutil.Block
 	blockByHashErr                error
 	blockByHeight                 *dcrutil.Block
@@ -193,6 +195,11 @@ func (c *testRPCChain) BestSnapshot() *blockchain.BestState {
 	return c.bestSnapshot
 }
 
+// BestHeader returns a mocked best header hash and height.
+func (c *testRPCChain) BestHeader() (chainhash.Hash, int64) {
+	return c.bestHeaderHash, c.bestHeaderHeight
+}
+
 // BlockByHash returns a mocked block for the given hash.
 func (c *testRPCChain) BlockByHash(hash *chainhash.Hash) (*dcrutil.Block, error) {
 	return c.blockByHash, c.blockByHashErr
@@ -3495,6 +3502,8 @@ func TestHandleGetBlockchainInfo(t *testing.T) {
 				Hash:     *hash,
 				PrevHash: *prevHash,
 			}
+			chain.bestHeaderHash = *hash
+			chain.bestHeaderHeight = 463073
 			chain.chainWork = big.NewInt(0).SetBytes([]byte{0x11, 0x5d, 0x28, 0x33, 0x84,
 				0x90, 0x90, 0xb0, 0x02, 0x65, 0x06})
 			chain.isCurrent = false