cmd/tsconnect: stop writing build artifacts into src/

We can't write to src/ when tsconnect is used a dependency in another
repo (see also b763a12331). We therefore
need to switch from writing to src/ to using esbuild plugins to handle
the requests for wasm_exec.js (the Go JS runtime for Wasm) and the
Wasm build of the Go module.

This has the benefit of allowing Go/Wasm changes to be picked up without
restarting the server when in dev mode (Go compilation is fast enough
that we can do this on every request, CSS compilation continues to be
the long pole).

Fixes #5382

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

cmd/tsconnect/.gitignore

@@ -1,4 +1,2 @@
-src/wasm_exec.js
-src/main.wasm
 node_modules/
 dist/

cmd/tsconnect/common.go

@@ -6,6 +6,7 @@ package main
 
 import (
 	"fmt"
+	"io/ioutil"
 	"log"
 	"os"
 	"os/exec"
@@ -32,65 +33,91 @@ func commonSetup(dev bool) (*esbuild.BuildOptions, error) {
 			return nil, fmt.Errorf("Cannot change cwd: %w", err)
 		}
 	}
-	if err := buildDeps(dev); err != nil {
-		return nil, fmt.Errorf("Cannot build deps: %w", err)
+	if err := installJSDeps(); err != nil {
+		return nil, fmt.Errorf("Cannot install JS deps: %w", err)
 	}
 
 	return &esbuild.BuildOptions{
 		EntryPoints: []string{"src/index.ts", "src/index.css"},
-		Loader:      map[string]esbuild.Loader{".wasm": esbuild.LoaderFile},
 		Outdir:      *distDir,
 		Bundle:      true,
 		Sourcemap:   esbuild.SourceMapLinked,
 		LogLevel:    esbuild.LogLevelInfo,
 		Define:      map[string]string{"DEBUG": strconv.FormatBool(dev)},
 		Target:      esbuild.ES2017,
-		Plugins: []esbuild.Plugin{{
-			Name: "tailscale-tailwind",
-			Setup: func(build esbuild.PluginBuild) {
-				setupEsbuildTailwind(build, dev)
+		Plugins: []esbuild.Plugin{
+			{
+				Name: "tailscale-tailwind",
+				Setup: func(build esbuild.PluginBuild) {
+					setupEsbuildTailwind(build, dev)
+				},
+			},
+			{
+				Name:  "tailscale-go-wasm-exec-js",
+				Setup: setupEsbuildWasmExecJS,
 			},
-		}},
+			{
+				Name: "tailscale-wasm",
+				Setup: func(build esbuild.PluginBuild) {
+					setupEsbuildWasm(build, dev)
+				},
+			},
+		},
 		JSXMode: esbuild.JSXModeAutomatic,
 	}, 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
+// setupEsbuildWasmExecJS generates an esbuild plugin that serves the current
+// wasm_exec.js runtime helper library from the Go toolchain.
+func setupEsbuildWasmExecJS(build esbuild.PluginBuild) {
+	wasmExecSrcPath := filepath.Join(runtime.GOROOT(), "misc", "wasm", "wasm_exec.js")
+	build.OnResolve(esbuild.OnResolveOptions{
+		Filter: "./wasm_exec$",
+	}, func(args esbuild.OnResolveArgs) (esbuild.OnResolveResult, error) {
+		return esbuild.OnResolveResult{Path: wasmExecSrcPath}, 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)
+// setupEsbuildWasm generates an esbuild plugin that builds the Tailscale wasm
+// binary and serves it as a file that the JS can load.
+func setupEsbuildWasm(build esbuild.PluginBuild, dev bool) {
+	// Add a resolve hook to convince esbuild that the path exists.
+	build.OnResolve(esbuild.OnResolveOptions{
+		Filter: "./main.wasm$",
+	}, func(args esbuild.OnResolveArgs) (esbuild.OnResolveResult, error) {
+		return esbuild.OnResolveResult{
+			Path:      "./src/main.wasm",
+			Namespace: "generated",
+		}, nil
+	})
+	build.OnLoad(esbuild.OnLoadOptions{
+		Filter: "./src/main.wasm$",
+	}, func(args esbuild.OnLoadArgs) (esbuild.OnLoadResult, error) {
+		contents, err := buildWasm(dev)
+		if err != nil {
+			return esbuild.OnLoadResult{}, fmt.Errorf("Cannot build main.wasm: %w", err)
+		}
+		contentsStr := string(contents)
+		return esbuild.OnLoadResult{
+			Contents: &contentsStr,
+			Loader:   esbuild.LoaderFile,
+		}, nil
+	})
+}
+
+func buildWasm(dev bool) ([]byte, error) {
+	start := time.Now()
+	outputFile, err := ioutil.TempFile("", "main.*.wasm")
 	if err != nil {
-		return err
+		return nil, fmt.Errorf("Cannot create main.wasm output file: %w", err)
 	}
-	return os.WriteFile(wasmExecDstPath, contents, 0600)
-}
+	outputPath := outputFile.Name()
+	defer os.Remove(outputPath)
 
-// 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 {
 		if *devControl != "" {
-			return fmt.Errorf("Development control URL can only be used in dev mode.")
+			return nil, fmt.Errorf("Development control URL can only be used in dev mode.")
 		}
 		// Omit long paths and debug symbols in release builds, to reduce the
 		// generated WASM binary size.
@@ -98,13 +125,19 @@ func buildWasm(dev bool) error {
 	} else if *devControl != "" {
 		args = append(args, "-ldflags", fmt.Sprintf("-X 'main.ControlURL=%v'", *devControl))
 	}
-	args = append(args, "-o", "src/main.wasm", "./wasm")
+
+	args = append(args, "-o", outputPath, "./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()
+	err = cmd.Run()
+	if err != nil {
+		return nil, fmt.Errorf("Cannot build main.wasm: %w", err)
+	}
+	log.Printf("Built wasm in %v\n", time.Since(start))
+	return os.ReadFile(outputPath)
 }
 
 // installJSDeps installs the JavaScript dependencies specified by package.json