cmd/tsconnect: initial scaffolding for Tailscale Connect browser client

Runs a Tailscale client in the browser (via a WebAssembly build of the
wasm package) and allows SSH access to machines. The wasm package exports
a newIPN function, which returns a simple JS object with methods like
start(), login(), logout() and ssh(). The golang.org/x/crypto/ssh
package is used for the SSH client.

Terminal emulation and QR code renedring is done via NPM packages (xterm
and qrcode respectively), thus we also need a JS toolchain that can
install and bundle them. Yarn is used for installation, and esbuild
handles loading them and bundling for production serving.

Updates #3157

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

cmd/tsconnect/serve.go

@@ -0,0 +1,134 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"bytes"
+	"embed"
+	"encoding/json"
+	"fmt"
+	"io"
+	"io/fs"
+	"log"
+	"net/http"
+	"path"
+	"time"
+
+	"tailscale.com/tsweb"
+)
+
+//go:embed dist/* index.html
+var embeddedFS embed.FS
+
+var serveStartTime = time.Now()
+
+func runServe() {
+	mux := http.NewServeMux()
+
+	indexBytes, err := generateServeIndex()
+	if err != nil {
+		log.Fatalf("Could not generate index.html: %v", err)
+	}
+	mux.Handle("/", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.ServeContent(w, r, "index.html", serveStartTime, bytes.NewReader(indexBytes))
+	}))
+	mux.Handle("/dist/", http.HandlerFunc(handleServeDist))
+	tsweb.Debugger(mux)
+
+	log.Printf("Listening on %s", *addr)
+	err = http.ListenAndServe(*addr, mux)
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+func generateServeIndex() ([]byte, error) {
+	log.Printf("Generating index.html...\n")
+	rawIndexBytes, err := embeddedFS.ReadFile("index.html")
+	if err != nil {
+		return nil, fmt.Errorf("Could not read index.html: %w", err)
+	}
+
+	esbuildMetadataBytes, err := embeddedFS.ReadFile("dist/esbuild-metadata.json")
+	if err != nil {
+		return nil, fmt.Errorf("Could not read esbuild-metadata.json: %w", err)
+	}
+	var esbuildMetadata EsbuildMetadata
+	if err := json.Unmarshal(esbuildMetadataBytes, &esbuildMetadata); err != nil {
+		return nil, fmt.Errorf("Could not parse esbuild-metadata.json: %w", err)
+	}
+	entryPointsToHashedDistPaths := make(map[string]string)
+	for outputPath, output := range esbuildMetadata.Outputs {
+		if output.EntryPoint != "" {
+			entryPointsToHashedDistPaths[output.EntryPoint] = outputPath
+		}
+	}
+
+	indexBytes := rawIndexBytes
+	for entryPointPath, defaultDistPath := range entryPointsToDefaultDistPaths {
+		hashedDistPath := entryPointsToHashedDistPaths[entryPointPath]
+		if hashedDistPath != "" {
+			indexBytes = bytes.ReplaceAll(indexBytes, []byte(defaultDistPath), []byte(hashedDistPath))
+		}
+	}
+
+	return indexBytes, nil
+}
+
+// EsbuildMetadata is the subset of metadata struct (described by
+// https://esbuild.github.io/api/#metafile) that we care about for mapping
+// from entry points to hashed file names.
+type EsbuildMetadata = struct {
+	Outputs map[string]struct {
+		EntryPoint string `json:"entryPoint,omitempty"`
+	} `json:"outputs,omitempty"`
+}
+
+var entryPointsToDefaultDistPaths = map[string]string{
+	"src/index.css": "dist/index.css",
+	"src/index.js":  "dist/index.js",
+}
+
+func handleServeDist(w http.ResponseWriter, r *http.Request) {
+	p := r.URL.Path[1:]
+	var f fs.File
+	// Prefer pre-compressed versions generated during the build step.
+	if tsweb.AcceptsEncoding(r, "br") {
+		if brotliFile, err := embeddedFS.Open(p + ".br"); err == nil {
+			f = brotliFile
+			w.Header().Set("Content-Encoding", "br")
+		}
+	}
+	if f == nil && tsweb.AcceptsEncoding(r, "gzip") {
+		if gzipFile, err := embeddedFS.Open(p + ".gz"); err == nil {
+			f = gzipFile
+			w.Header().Set("Content-Encoding", "gzip")
+		}
+	}
+
+	if f == nil {
+		if rawFile, err := embeddedFS.Open(r.URL.Path[1:]); err == nil {
+			f = rawFile
+		} else {
+			http.Error(w, err.Error(), http.StatusNotFound)
+			return
+		}
+	}
+	defer f.Close()
+
+	// fs.File does not claim to implement Seeker, but in practice it does.
+	fSeeker, ok := f.(io.ReadSeeker)
+	if !ok {
+		http.Error(w, "Not seekable", http.StatusInternalServerError)
+		return
+	}
+
+	// Aggressively cache static assets, since we cache-bust our assets with
+	// hashed filenames.
+	w.Header().Set("Cache-Control", "public, max-age=31535996")
+	w.Header().Set("Vary", "Accept-Encoding")
+
+	http.ServeContent(w, r, path.Base(r.URL.Path), serveStartTime, fSeeker)
+}