FEMT: Basic frontend-service implementation (#104229)

* create the most basic frontend-server module

* expose prom metrics??

* add todo list

* move frontend-service to its own folder in services

* check error from writer.Write

* reword comment, add launch config

.github/CODEOWNERS

@@ -138,6 +138,7 @@
 /pkg/services/dashboardversion/ @grafana/grafana-backend-group
 /pkg/services/encryption/ @grafana/grafana-operator-experience-squad
 /pkg/services/folder/ @grafana/grafana-search-and-storage
+/pkg/services/frontend/ @grafana/grafana-frontend-platform
 /pkg/services/apiserver @grafana/grafana-app-platform-squad
 /pkg/services/hooks/ @grafana/grafana-backend-group
 /pkg/services/kmsproviders/ @grafana/grafana-operator-experience-squad

.vscode/launch.json

@@ -82,6 +82,16 @@
       "cwd": "${workspaceFolder}",
       "args": ["server", "target", "--homepath", "${workspaceFolder}", "--packaging", "dev"]
     },
+    {
+      "name": "Run Frontend Server",
+      "type": "go",
+      "request": "launch",
+      "mode": "auto",
+      "program": "${workspaceFolder}/pkg/cmd/grafana/",
+      "cwd": "${workspaceFolder}",
+      "env": { "GF_DEFAULT_TARGET": "frontend-server", "GF_SERVER_HTTP_PORT": "3003" },
+      "args": ["server", "target", "--homepath", "${workspaceFolder}", "--packaging", "dev"]
+    },
     {
       "name": "Attach to Chrome",
       "port": 9222,

pkg/modules/dependencies.go

@@ -9,6 +9,7 @@ const (
 	StorageServer         string = "storage-server"
 	ZanzanaServer         string = "zanzana-server"
 	InstrumentationServer string = "instrumentation-server"
+	FrontendServer        string = "frontend-server"
 )
 
 var dependencyMap = map[string][]string{
@@ -17,4 +18,5 @@ var dependencyMap = map[string][]string{
 	ZanzanaServer:    {InstrumentationServer},
 	Core:             {},
 	All:              {Core},
+	FrontendServer:   {},
 }

pkg/server/module_server.go

@@ -15,6 +15,7 @@ import (
 	"github.com/grafana/grafana/pkg/modules"
 	"github.com/grafana/grafana/pkg/services/authz"
 	"github.com/grafana/grafana/pkg/services/featuremgmt"
+	"github.com/grafana/grafana/pkg/services/frontend"
 	"github.com/grafana/grafana/pkg/setting"
 	"github.com/grafana/grafana/pkg/storage/unified/resource"
 	"github.com/grafana/grafana/pkg/storage/unified/sql"
@@ -120,7 +121,7 @@ func (s *ModuleServer) Run() error {
 
 	// only run the instrumentation server module if were not running a module that already contains an http server
 	m.RegisterInvisibleModule(modules.InstrumentationServer, func() (services.Service, error) {
-		if m.IsModuleEnabled(modules.All) || m.IsModuleEnabled(modules.Core) {
+		if m.IsModuleEnabled(modules.All) || m.IsModuleEnabled(modules.Core) || m.IsModuleEnabled(modules.FrontendServer) {
 			return services.NewBasicService(nil, nil, nil).WithName(modules.InstrumentationServer), nil
 		}
 		return NewInstrumentationService(s.log, s.cfg, s.promGatherer)
@@ -151,6 +152,10 @@ func (s *ModuleServer) Run() error {
 		return authz.ProvideZanzanaService(s.cfg, s.features)
 	})
 
+	m.RegisterModule(modules.FrontendServer, func() (services.Service, error) {
+		return frontend.ProvideFrontendService(s.cfg, s.promGatherer)
+	})
+
 	m.RegisterModule(modules.All, nil)
 
 	return m.Run(s.context)

pkg/services/frontend/frontend_service.go

@@ -0,0 +1,109 @@
+package frontend
+
+import (
+	"context"
+	"net"
+	"net/http"
+	"time"
+
+	"github.com/grafana/dskit/services"
+	"github.com/grafana/grafana/pkg/infra/log"
+	"github.com/grafana/grafana/pkg/setting"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+type frontendService struct {
+	*services.BasicService
+	cfg          *setting.Cfg
+	httpServ     *http.Server
+	log          log.Logger
+	errChan      chan error
+	promGatherer prometheus.Gatherer
+}
+
+func ProvideFrontendService(cfg *setting.Cfg, promGatherer prometheus.Gatherer) (*frontendService, error) {
+	s := &frontendService{
+		cfg:          cfg,
+		log:          log.New("frontend-server"),
+		promGatherer: promGatherer,
+	}
+	s.BasicService = services.NewBasicService(s.start, s.running, s.stop)
+	return s, nil
+}
+
+func (s *frontendService) start(ctx context.Context) error {
+	s.httpServ = s.newFrontendServer(ctx)
+	s.errChan = make(chan error)
+	go func() {
+		s.errChan <- s.httpServ.ListenAndServe()
+	}()
+	return nil
+}
+
+func (s *frontendService) running(ctx context.Context) error {
+	select {
+	case <-ctx.Done():
+		return nil
+	case err := <-s.errChan:
+		return err
+	}
+}
+
+func (s *frontendService) stop(failureReason error) error {
+	s.log.Info("stopping frontend server", "reason", failureReason)
+	if err := s.httpServ.Shutdown(context.Background()); err != nil {
+		s.log.Error("failed to shutdown frontend server", "error", err)
+		return err
+	}
+	return nil
+}
+
+func (s *frontendService) newFrontendServer(ctx context.Context) *http.Server {
+	s.log.Info("starting frontend server", "addr", ":"+s.cfg.HTTPPort)
+
+	router := http.NewServeMux()
+	router.Handle("/metrics", promhttp.HandlerFor(s.promGatherer, promhttp.HandlerOpts{EnableOpenMetrics: true}))
+	router.HandleFunc("/", s.handleRequest)
+
+	server := &http.Server{
+		// 5s timeout for header reads to avoid Slowloris attacks (https://thetooth.io/blog/slowloris-attack/)
+		ReadHeaderTimeout: 5 * time.Second,
+		Addr:              ":" + s.cfg.HTTPPort,
+		Handler:           router,
+		BaseContext:       func(_ net.Listener) context.Context { return ctx },
+	}
+
+	return server
+}
+
+func (s *frontendService) handleRequest(writer http.ResponseWriter, request *http.Request) {
+	// This should:
+	// - get correct asset urls from fs or cdn
+	// - generate a nonce
+	// - render them into the index.html
+	// - and return it to the user!
+
+	s.log.Info("handling request", "method", request.Method, "url", request.URL.String())
+	htmlContent := `<!DOCTYPE html>
+<html>
+<head>
+    <title>Grafana Frontend Server</title>
+    <style>
+        body {
+            font-family: sans-serif;
+        }
+    </style>
+</head>
+<body>
+    <h1>Grafana Frontend Server</h1>
+    <p>This is a simple static HTML page served by the Grafana frontend server module.</p>
+</body>
+</html>`
+
+	writer.Header().Set("Content-Type", "text/html; charset=utf-8")
+	_, err := writer.Write([]byte(htmlContent))
+	if err != nil {
+		s.log.Error("could not write to response", "err", err)
+	}
+}