network: add getheaders message processing

This one is essential for the consensus nodes as otherwise they won't give out
the blocks they generate making their generation almost useless. It also makes
our networking part more complete.

pkg/network/server.go

@@ -37,6 +37,7 @@ var (
 	errMaxPeers         = errors.New("max peers reached")
 	errServerShutdown   = errors.New("server shutdown")
 	errInvalidInvType   = errors.New("invalid inventory type")
+	errInvalidHashStart = errors.New("invalid requested HashStart")
 )
 
 type (
@@ -421,6 +422,35 @@ func (s *Server) handleGetDataCmd(p Peer, inv *payload.Inventory) error {
 	return nil
 }
 
+// handleGetHeadersCmd processes the getheaders request.
+func (s *Server) handleGetHeadersCmd(p Peer, gh *payload.GetBlocks) error {
+	if len(gh.HashStart) < 1 {
+		return errInvalidHashStart
+	}
+	startHash := gh.HashStart[0]
+	start, err := s.chain.GetHeader(startHash)
+	if err != nil {
+		return err
+	}
+	resp := payload.Headers{}
+	resp.Hdrs = make([]*core.Header, 0, payload.MaxHeadersAllowed)
+	for i := start.Index + 1; i < start.Index+1+payload.MaxHeadersAllowed; i++ {
+		hash := s.chain.GetHeaderHash(int(i))
+		if hash.Equals(util.Uint256{}) || hash.Equals(gh.HashStop) {
+			break
+		}
+		header, err := s.chain.GetHeader(hash)
+		if err != nil {
+			break
+		}
+		resp.Hdrs = append(resp.Hdrs, header)
+	}
+	if len(resp.Hdrs) == 0 {
+		return nil
+	}
+	return p.WriteMsg(NewMessage(s.Net, CMDHeaders, &resp))
+}
+
 // handleConsensusCmd processes received consensus payload.
 // It never returns an error.
 func (s *Server) handleConsensusCmd(cp *consensus.Payload) error {
@@ -514,6 +544,9 @@ func (s *Server) handleMessage(peer Peer, msg *Message) error {
 		case CMDGetData:
 			inv := msg.Payload.(*payload.Inventory)
 			return s.handleGetDataCmd(peer, inv)
+		case CMDGetHeaders:
+			gh := msg.Payload.(*payload.GetBlocks)
+			return s.handleGetHeadersCmd(peer, gh)
 		case CMDHeaders:
 			headers := msg.Payload.(*payload.Headers)
 			go s.handleHeadersCmd(peer, headers)