func NewHandler()

in cmd/golangorg/server.go [134:242]
71 lines of code
12 McCabe index (conditional complexity)

func NewHandler(contentDir, goroot string) http.Handler {
	mux := http.NewServeMux()

	// Serve files from _content, falling back to GOROOT.

	// Use explicit contentDir if specified, otherwise embedded copy.
	var contentFS fs.FS
	if contentDir != "" {
		contentFS = os.DirFS(contentDir)
	} else {
		contentFS = website.Content()
	}

	var gorootFS fs.FS
	if strings.HasSuffix(goroot, ".zip") {
		z, err := zip.OpenReader(goroot)
		if err != nil {
			log.Fatal(err)
		}
		gorootFS = &seekableFS{z}
	} else {
		gorootFS = os.DirFS(goroot)
	}

	// tip.golang.org serves content from the very latest Git commit
	// of the main Go repo, instead of the one the app is bundled with.
	var tipGoroot atomicFS
	if _, err := newSite(mux, "tip.golang.org", contentFS, &tipGoroot); err != nil {
		log.Fatalf("loading tip site: %v", err)
	}
	if *tipFlag {
		go watchTip(&tipGoroot)
	}

	// beta.golang.org is an old name for tip.
	mux.Handle("beta.golang.org/", redirectPrefix("https://tip.golang.org/"))

	// By default, golang.org/foo redirects to go.dev/foo.
	// All the user-facing golang.org subdomains have moved to go.dev subdirectories.
	// There are some exceptions below, like for golang.org/x.
	mux.Handle("golang.org/", redirectPrefix("https://go.dev/"))
	mux.Handle("blog.golang.org/", redirectPrefix("https://go.dev/blog/"))
	mux.Handle("learn.go.dev/", redirectPrefix("https://go.dev/learn/"))
	mux.Handle("talks.golang.org/", redirectPrefix("https://go.dev/talks/"))
	mux.Handle("tour.golang.org/", redirectPrefix("https://go.dev/tour/"))

	// Redirect subdirectory-like domains to the actual subdirectories,
	// for people whose fingers learn to type go.dev instead of golang.org
	// but not the rest of the URL schema change.
	// Note that these domains have to be listed in knownHosts below as well.
	mux.Handle("blog.go.dev/", redirectPrefix("https://go.dev/blog/"))
	mux.Handle("play.go.dev/", redirectPrefix("https://go.dev/play/"))
	mux.Handle("talks.go.dev/", redirectPrefix("https://go.dev/talks/"))
	mux.Handle("tour.go.dev/", redirectPrefix("https://go.dev/tour/"))

	// m.golang.org is an old shortcut for golang.org mail.
	// Gmail itself can serve this redirect, but only on HTTP (not HTTPS).
	// Golang.org's HSTS header tells browsers to use HTTPS for all subdomains,
	// which broke the redirect.
	mux.Handle("m.golang.org/", http.RedirectHandler("https://mail.google.com/a/golang.org/", http.StatusMovedPermanently))

	// Register a redirect handler for tip.golang.org/dl/ to the golang.org download page.
	// (golang.org/dl and golang.google.cn/dl are registered separately.)
	mux.Handle("tip.golang.org/dl/", http.RedirectHandler("https://go.dev/dl/", http.StatusFound))

	// TODO(rsc): The unionFS is a hack until we move the files in a followup CL.
	siteMux := http.NewServeMux()
	godevSite, err := newSite(siteMux, "", contentFS, gorootFS)
	if err != nil {
		log.Fatalf("newSite go.dev: %v", err)
	}
	chinaSite, err := newSite(siteMux, "golang.google.cn", contentFS, gorootFS)
	if err != nil {
		log.Fatalf("newSite golang.google.cn: %v", err)
	}
	if runningOnAppEngine {
		appEngineSetup(mux)
	}
	dl.RegisterHandlers(siteMux, godevSite, "", datastoreClient, memcacheClient)
	dl.RegisterHandlers(siteMux, chinaSite, "golang.google.cn", datastoreClient, memcacheClient)
	mux.Handle("/", siteMux)

	play.RegisterHandlers(mux, godevSite, chinaSite)

	mux.Handle("/explore/", http.StripPrefix("/explore/", redirectPrefix("https://pkg.go.dev/")))
	if err := blog.RegisterFeeds(mux, "", godevSite); err != nil {
		log.Fatalf("blog: %v", err)
	}

	// Note: Only golang.org/x/, no go.dev/x/.
	mux.Handle("golang.org/x/", http.HandlerFunc(xHandler))

	redirect.Register(mux)

	// Note: Using godevSite (non-China) for global mux registration because there's no sharing in talks.
	// Don't need the hassle of two separate registrations for different domains in siteMux.
	if err := talks.RegisterHandlers(mux, godevSite, contentFS); err != nil {
		log.Fatalf("talks: %v", err)
	}
	if err := tour.RegisterHandlers(mux); err != nil {
		log.Fatalf("tour: %v", err)
	}

	var h http.Handler = mux
	h = addCSP(mux)
	h = hostEnforcerHandler(h)
	h = hostPathHandler(h)
	return h
}