client/web: add barebones vite dev setup

Currently just serving a "Hello world" page when running the web
cli in --dev mode.

Updates tailscale/corp#13775

Co-authored-by: Will Norris <will@tailscale.com>
Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

client/web/dev.go

@@ -0,0 +1,75 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package web
+
+import (
+	"log"
+	"net/http"
+	"net/http/httputil"
+	"net/url"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+)
+
+// startDevServer starts the JS dev server that does on-demand rebuilding
+// and serving of web client JS and CSS resources.
+func (s *Server) startDevServer() (cleanup func()) {
+	root := gitRootDir()
+	webClientPath := filepath.Join(root, "client", "web")
+
+	yarn := filepath.Join(root, "tool", "yarn")
+	node := filepath.Join(root, "tool", "node")
+	vite := filepath.Join(webClientPath, "node_modules", ".bin", "vite")
+
+	log.Printf("installing JavaScript deps using %s... (might take ~30s)", yarn)
+	out, err := exec.Command(yarn, "--non-interactive", "-s", "--cwd", webClientPath, "install").CombinedOutput()
+	if err != nil {
+		log.Fatalf("error running tailscale web's yarn install: %v, %s", err, out)
+	}
+	log.Printf("starting JavaScript dev server...")
+	cmd := exec.Command(node, vite)
+	cmd.Dir = webClientPath
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Start(); err != nil {
+		log.Fatalf("Starting JS dev server: %v", err)
+	}
+	log.Printf("JavaScript dev server running as pid %d", cmd.Process.Pid)
+	return func() {
+		cmd.Process.Signal(os.Interrupt)
+		err := cmd.Wait()
+		log.Printf("JavaScript dev server exited: %v", err)
+	}
+}
+
+func (s *Server) addProxyToDevServer() {
+	if !s.devMode {
+		return // only using Vite proxy in dev mode
+	}
+	// We use Vite to develop on the web client.
+	// Vite starts up its own local server for development,
+	// which we proxy requests to from Server.ServeHTTP.
+	// Here we set up the proxy to Vite's server.
+	handleErr := func(w http.ResponseWriter, r *http.Request, err error) {
+		w.Header().Set("Content-Type", "text/plain")
+		w.WriteHeader(http.StatusBadGateway)
+		w.Write([]byte("The web client development server isn't running. " +
+			"Run `./tool/yarn --cwd client/web start` from " +
+			"the repo root to start the development server."))
+		w.Write([]byte("\n\nError: " + err.Error()))
+	}
+	viteTarget, _ := url.Parse("http://127.0.0.1:4000")
+	s.devProxy = httputil.NewSingleHostReverseProxy(viteTarget)
+	s.devProxy.ErrorHandler = handleErr
+}
+
+func gitRootDir() string {
+	top, err := exec.Command("git", "rev-parse", "--show-toplevel").Output()
+	if err != nil {
+		log.Fatalf("failed to find git top level (not in corp git?): %v", err)
+	}
+	return strings.TrimSpace(string(top))
+}