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/common.go

@@ -0,0 +1,105 @@
+// 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 (
+	"fmt"
+	"log"
+	"os"
+	"os/exec"
+	"path"
+	"path/filepath"
+	"runtime"
+	"strconv"
+
+	esbuild "github.com/evanw/esbuild/pkg/api"
+)
+
+const (
+	devMode  = true
+	prodMode = false
+)
+
+// commonSetup performs setup that is common to both dev and build modes.
+func commonSetup(dev bool) (*esbuild.BuildOptions, error) {
+	// Change cwd to to where this file lives -- that's where all inputs for
+	// esbuild and other build steps live.
+	if _, filename, _, ok := runtime.Caller(0); ok {
+		if err := os.Chdir(path.Dir(filename)); err != nil {
+			return nil, fmt.Errorf("Cannot change cwd: %w", err)
+		}
+	}
+	if err := buildDeps(dev); err != nil {
+		return nil, fmt.Errorf("Cannot build deps: %w", err)
+	}
+
+	return &esbuild.BuildOptions{
+		EntryPoints: []string{"src/index.js", "src/index.css"},
+		Loader:      map[string]esbuild.Loader{".wasm": esbuild.LoaderFile},
+		Outdir:      "./dist",
+		Bundle:      true,
+		Sourcemap:   esbuild.SourceMapLinked,
+		LogLevel:    esbuild.LogLevelInfo,
+		Define:      map[string]string{"DEBUG": strconv.FormatBool(dev)},
+		Target:      esbuild.ES2017,
+	}, nil
+}
+
+// buildDeps builds the static assets that are needed for the server (except for
+// JS/CSS bundling, which is  handled by esbuild).
+func buildDeps(dev bool) error {
+	if err := copyWasmExec(); err != nil {
+		return fmt.Errorf("Cannot copy wasm_exec.js: %w", err)
+	}
+	if err := buildWasm(dev); err != nil {
+		return fmt.Errorf("Cannot build main.wasm: %w", err)
+	}
+	if err := installJSDeps(); err != nil {
+		return fmt.Errorf("Cannot install JS deps: %w", err)
+	}
+	return nil
+}
+
+// copyWasmExec grabs the current wasm_exec.js runtime helper library from the
+// Go toolchain.
+func copyWasmExec() error {
+	log.Printf("Copying wasm_exec.js...\n")
+	wasmExecSrcPath := filepath.Join(runtime.GOROOT(), "misc", "wasm", "wasm_exec.js")
+	wasmExecDstPath := filepath.Join("src", "wasm_exec.js")
+	contents, err := os.ReadFile(wasmExecSrcPath)
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(wasmExecDstPath, contents, 0600)
+}
+
+// buildWasm builds the Tailscale wasm binary and places it where the JS can
+// load it.
+func buildWasm(dev bool) error {
+	log.Printf("Building wasm...\n")
+	args := []string{"build", "-tags", "tailscale_go,osusergo,netgo,nethttpomithttp2,omitidna,omitpemdecrypt"}
+	if !dev {
+		// Omit long paths and debug symbols in release builds, to reduce the
+		// generated WASM binary size.
+		args = append(args, "-trimpath", "-ldflags", "-s -w")
+	}
+	args = append(args, "-o", "src/main.wasm", "./wasm")
+	cmd := exec.Command(filepath.Join(runtime.GOROOT(), "bin", "go"), args...)
+	cmd.Env = append(os.Environ(), "GOOS=js", "GOARCH=wasm")
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	return cmd.Run()
+}
+
+// installJSDeps installs the JavaScript dependencies specified by package.json
+func installJSDeps() error {
+	log.Printf("Installing JS deps...\n")
+	stdoutStderr, err := exec.Command("yarn").CombinedOutput()
+	if err != nil {
+		log.Printf("yarn failed: %s", stdoutStderr)
+	}
+	return err
+}