add health check

backend/runner/runner.go

@@ -126,6 +126,7 @@ func Start(ctx context.Context, config Config) error {
 	return rpc.Serve(ctx, config.Bind,
 		rpc.GRPC(ftlv1connect.NewVerbServiceHandler, svc),
 		rpc.HTTP("/", svc),
+		rpc.HealthCheck(svc.healthCheck),
 	)
 }
 
@@ -484,3 +485,11 @@ func (s *Service) getDeploymentLogger(ctx context.Context, deploymentKey model.D
 	sink := newDeploymentLogsSink(s.deploymentLogQueue)
 	return log.FromContext(ctx).AddSink(sink).Attrs(attrs)
 }
+
+func (s *Service) healthCheck(writer http.ResponseWriter, request *http.Request) {
+	if s.state.Load() == ftlv1.RunnerState_RUNNER_ASSIGNED {
+		writer.WriteHeader(http.StatusOK)
+		return
+	}
+	writer.WriteHeader(http.StatusServiceUnavailable)
+}

internal/rpc/server.go

@@ -24,6 +24,7 @@ const ShutdownGracePeriod = time.Second * 5
 type serverOptions struct {
 	mux             *http.ServeMux
 	reflectionPaths []string
+	healthCheck     http.HandlerFunc
 }
 
 type Option func(*serverOptions)
@@ -49,6 +50,12 @@ func PProf() Option {
 	}
 }
 
+func HealthCheck(check http.HandlerFunc) Option {
+	return func(options *serverOptions) {
+		options.healthCheck = check
+	}
+}
+
 // RawGRPC is a convenience function for registering a GRPC server with default options without Pingable.
 func RawGRPC[Iface, Impl any](constructor RawGRPCServerConstructor[Iface], impl Impl, options ...connect.HandlerOption) Option {
 	return func(o *serverOptions) {
@@ -75,16 +82,17 @@ type Server struct {
 func NewServer(ctx context.Context, listen *url.URL, options ...Option) (*Server, error) {
 	opts := &serverOptions{
 		mux: http.NewServeMux(),
+		healthCheck: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusOK)
+		}),
 	}
 
-	opts.mux.Handle("/healthz", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		w.WriteHeader(http.StatusOK)
-	}))
-
 	for _, option := range options {
 		option(opts)
 	}
 
+	opts.mux.Handle("/healthz", opts.healthCheck)
+
 	// Register reflection services.
 	reflector := grpcreflect.NewStaticReflector(opts.reflectionPaths...)
 	opts.mux.Handle(grpcreflect.NewHandlerV1(reflector))