// Copyright (c) Tailscale Inc & contributors
// SPDX-License-Identifier: BSD-3-Clause

package web

import (
	"errors"
	"io/fs"
	"log"
	"net/http"
	"net/http/httputil"
	"net/url"
	"os"
	"os/exec"
	"path/filepath"
	"strings"

	prebuilt "github.com/tailscale/web-client-prebuilt"
	"tailscale.com/tsweb/compserve"
	"tailscale.com/tsweb/vcstime"
)

func assetsHandler(devMode bool) (_ http.Handler, cleanup func()) {
	if devMode {
		// When in dev mode, proxy asset requests to the Vite dev server.
		cleanup := startDevServer()
		return devServerProxy(), cleanup
	}

	// vcstime supplies the VCS commit time as mod time for the embedded
	// assets, enabling conditional requests.
	fsys := vcstime.FS(prebuilt.FS())
	opts := compserve.Options{}
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		path := strings.TrimPrefix(r.URL.Path, "/")
		if path == "" {
			// The root path is the SPA entrypoint.
			path = "index.html"
		}
		if strings.HasPrefix(path, "assets/") {
			// Aggressively cache static assets, since we cache-bust our assets with
			// hashed filenames.
			w.Header().Set("Cache-Control", "public, max-age=31535996")
		}
		err := compserve.ServeFile(w, r, fsys, path, opts)
		if errors.Is(err, fs.ErrNotExist) {
			// Rewrite request to just fetch index.html and let
			// the frontend router handle it. The fallback must not inherit
			// cache headers of the original path; nothing has been written
			// yet, so they can still be adjusted.
			w.Header().Del("Cache-Control")
			err = compserve.ServeFile(w, r, fsys, "index.html", opts)
		}
		if err != nil {
			if errors.Is(err, fs.ErrNotExist) {
				http.NotFound(w, r)
				return
			}
			// Errors may embed the request path; log rather than reflect it.
			log.Printf("web client: serving %q: %v", path, err)
			http.Error(w, "internal error", http.StatusInternalServerError)
		}
	}), nil
}

// startDevServer starts the JS dev server that does on-demand rebuilding
// and serving of web client JS and CSS resources.
func 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...", 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)
	}
}

// devServerProxy returns a reverse proxy to the vite dev server.
func devServerProxy() *httputil.ReverseProxy {
	// 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")
	devProxy := httputil.NewSingleHostReverseProxy(viteTarget)
	devProxy.ErrorHandler = handleErr
	return devProxy
}

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))
}