remove accidental second ServerInit message in bound keypair join flow (#60291)

Author: nklaassen

lib/join/joinclient/join_boundkeypair.go

@@ -144,6 +144,8 @@ func boundKeypairJoin(
 			}); err != nil {
 				return nil, trace.Wrap(err)
 			}
+		default:
+			return nil, trace.Errorf("server sent unexpected message type %T", msg)
 		}
 	}
 }

lib/join/server_boundkeypair.go

@@ -30,8 +30,28 @@ import (
 	"github.com/gravitational/teleport/lib/join/internal/messages"
 )
 
-// handleBoundKeypairJoin takes over the join process after the ClientInit
-// message has been received for the bound keypair join method.
+// handleBoundKeypairJoin handles join attempts for the bound keypair join
+// method.
+//
+// The bound keypair join method involves the following messages:
+//
+// client->server ClientInit
+// client<-server ServerInit
+// client->server BoundKeypairInit
+// client<-server BoundKeypairChallenge
+// client->server BoundKeypairChallengeSolution
+//
+//	(optional additional steps if keypair rotation is required)
+//	client<-server: BoundKeypairRotationRequest
+//	client->server: BoundKeypairRotationResponse
+//	client<-server: BoundKeypairChallenge
+//	client->server: BoundKeypairChallengeSolution
+//
+// client<-server: Result containing BoundKeypairResult
+//
+// At this point the ServerInit message has already been sent, what's left is
+// to receive the BoundKeypairInit message, handle the challenge-response (and
+// rotation if necessary), and send the final result if everything checks out.
 func (s *Server) handleBoundKeypairJoin(
 	stream messages.ServerStream,
 	authCtx *authz.Context,
@@ -45,11 +65,6 @@ func (s *Server) handleBoundKeypairJoin(
 	if clientInit.SystemRole != types.RoleBot.String() {
 		return nil, trace.BadParameter("bound keypair joining is only supported for bots")
 	}
-	if err := stream.Send(&messages.ServerInit{
-		JoinMethod: string(types.JoinMethodBoundKeypair),
-	}); err != nil {
-		return nil, trace.Wrap(err)
-	}
 	boundKeypairInit, err := messages.RecvRequest[*messages.BoundKeypairInit](stream)
 	if err != nil {
 		return nil, trace.Wrap(err)