From c20e4871cde75e8d85f62ed21ac7b0d5de50f71c Mon Sep 17 00:00:00 2001
From: Jonathan Harvey-Buschel <j.s.harbus@gmail.com>
Date: Thu, 26 Oct 2023 13:00:17 -0400
Subject: [PATCH] chainrpc+lncli: expose GetBlockHeader + add to CLI

---
 cmd/lncli/chainrpc_active.go   | 40 ++++++++++++++++++++++++++++++++++
 cmd/lncli/neutrino_active.go   |  8 +++----
 lnrpc/chainrpc/chain_server.go | 29 ++++++++++++++++++++++++
 3 files changed, 73 insertions(+), 4 deletions(-)

diff --git a/cmd/lncli/chainrpc_active.go b/cmd/lncli/chainrpc_active.go
index 0d8b891ac..48946e0d5 100644
--- a/cmd/lncli/chainrpc_active.go
+++ b/cmd/lncli/chainrpc_active.go
@@ -25,6 +25,7 @@ func chainCommands() []cli.Command {
 				getBlockCommand,
 				getBestBlockCommand,
 				getBlockHashCommand,
+				getBlockHeaderCommand,
 			},
 		},
 	}
@@ -113,6 +114,45 @@ func getBlock(ctx *cli.Context) error {
 	return nil
 }
 
+var getBlockHeaderCommand = cli.Command{
+	Name:        "getblockheader",
+	Usage:       "Get a block header.",
+	Category:    "On-chain",
+	Description: "Returns a block header with a particular block hash.",
+	ArgsUsage:   "hash",
+	Action:      actionDecorator(getBlockHeader),
+}
+
+func getBlockHeader(ctx *cli.Context) error {
+	ctxc := getContext()
+	args := ctx.Args()
+
+	// Display the command's help message if we do not have the expected
+	// number of arguments/flags.
+	if !args.Present() {
+		return cli.ShowCommandHelp(ctx, "getblockheader")
+	}
+
+	blockHash, err := chainhash.NewHashFromStr(args.First())
+	if err != nil {
+		return err
+	}
+
+	req := &chainrpc.GetBlockHeaderRequest{BlockHash: blockHash[:]}
+
+	client, cleanUp := getChainClient(ctx)
+	defer cleanUp()
+
+	resp, err := client.GetBlockHeader(ctxc, req)
+	if err != nil {
+		return err
+	}
+
+	printRespJSON(resp)
+
+	return nil
+}
+
 var getBestBlockCommand = cli.Command{
 	Name:     "getbestblock",
 	Category: "On-chain",
diff --git a/cmd/lncli/neutrino_active.go b/cmd/lncli/neutrino_active.go
index 4267738e1..099da46c6 100644
--- a/cmd/lncli/neutrino_active.go
+++ b/cmd/lncli/neutrino_active.go
@@ -155,16 +155,16 @@ func isBanned(ctx *cli.Context) error {
 	return nil
 }
 
-var getBlockHeaderCommand = cli.Command{
+var getBlockHeaderNeutrinoCommand = cli.Command{
 	Name:        "getblockheader",
 	Usage:       "Get a block header.",
 	Category:    "Neutrino",
 	Description: "Returns a block header with a particular block hash.",
 	ArgsUsage:   "hash",
-	Action:      actionDecorator(getBlockHeader),
+	Action:      actionDecorator(getBlockHeaderNeutrino),
 }
 
-func getBlockHeader(ctx *cli.Context) error {
+func getBlockHeaderNeutrino(ctx *cli.Context) error {
 	ctxc := getContext()
 	args := ctx.Args()
 
@@ -239,7 +239,7 @@ func neutrinoCommands() []cli.Command {
 				addPeerCommand,
 				disconnectPeerCommand,
 				isBannedCommand,
-				getBlockHeaderCommand,
+				getBlockHeaderNeutrinoCommand,
 				getCFilterCommand,
 			},
 		},
diff --git a/lnrpc/chainrpc/chain_server.go b/lnrpc/chainrpc/chain_server.go
index 89f480b8b..00a30c07c 100644
--- a/lnrpc/chainrpc/chain_server.go
+++ b/lnrpc/chainrpc/chain_server.go
@@ -46,6 +46,10 @@ var (
 			Entity: "onchain",
 			Action: "read",
 		}},
+		"/chainrpc.ChainKit/GetBlockHeader": {{
+			Entity: "onchain",
+			Action: "read",
+		}},
 		"/chainrpc.ChainKit/GetBestBlock": {{
 			Entity: "onchain",
 			Action: "read",
@@ -293,6 +297,31 @@ func (s *Server) GetBlock(_ context.Context,
 	return &GetBlockResponse{RawBlock: rawBlock}, nil
 }
 
+// GetBlockHeader returns a block header given the corresponding block hash.
+func (s *Server) GetBlockHeader(_ context.Context,
+	in *GetBlockHeaderRequest) (*GetBlockHeaderResponse, error) {
+
+	// We'll start by reconstructing the RPC request into what the
+	// underlying chain functionality expects.
+	var blockHash chainhash.Hash
+	copy(blockHash[:], in.BlockHash)
+
+	blockHeader, err := s.cfg.Chain.GetBlockHeader(&blockHash)
+	if err != nil {
+		return nil, err
+	}
+
+	// Serialize block header for RPC response.
+	var headerBuf bytes.Buffer
+	err = blockHeader.Serialize(&headerBuf)
+	if err != nil {
+		return nil, err
+	}
+	rawHeader := headerBuf.Bytes()
+
+	return &GetBlockHeaderResponse{RawBlockHeader: rawHeader}, nil
+}
+
 // GetBestBlock returns the latest block hash and current height of the valid
 // most-work chain.
 func (s *Server) GetBestBlock(_ context.Context,