cmd/godoc/main.go - tools - Git at Google

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // godoc: Go Documentation Server

 // Web server tree:
 //
 //	http://godoc/		redirect to /pkg/
 //	http://godoc/src/	serve files from $GOROOT/src; .go gets pretty-printed
 //	http://godoc/cmd/	serve documentation about commands
 //	http://godoc/pkg/	serve documentation about packages
 //				(idea is if you say import "compress/zlib", you go to
 //				http://godoc/pkg/compress/zlib)
 //

 package main

 import (
 	"archive/zip"
 	"bytes"
 	"context"
 	"encoding/json"
 	"errors"
 	_ "expvar" // to serve /debug/vars
 	"flag"
 	"fmt"
 	"go/build"
 	"io"
 	"log"
 	"net/http"
 	_ "net/http/pprof" // to serve /debug/pprof/*
 	"net/url"
 	"os"
 	"os/exec"
 	"path"
 	"path/filepath"
 	"regexp"
 	"runtime"
 	"strings"

 	"golang.org/x/tools/godoc"
 	"golang.org/x/tools/godoc/static"
 	"golang.org/x/tools/godoc/vfs"
 	"golang.org/x/tools/godoc/vfs/gatefs"
 	"golang.org/x/tools/godoc/vfs/mapfs"
 	"golang.org/x/tools/godoc/vfs/zipfs"
 	"golang.org/x/tools/internal/gocommand"
 )

 const defaultAddr = "localhost:6060" // default webserver address

 var (
 	// file system to serve
 	// (with e.g.: zip -r go.zip $GOROOT -i \*.go -i \*.html -i \*.css -i \*.js -i \*.txt -i \*.c -i \*.h -i \*.s -i \*.png -i \*.jpg -i \*.sh -i favicon.ico)
 	zipfile = flag.String("zip", "", "zip file providing the file system to serve; disabled if empty")

 	// file-based index
 	writeIndex = flag.Bool("write_index", false, "write index to a file; the file name must be specified with -index_files")

 	// network
 	httpAddr = flag.String("http", defaultAddr, "HTTP service address")

 	// layout control
 	urlFlag = flag.String("url", "", "print HTML for named URL")

 	verbose = flag.Bool("v", false, "verbose mode")

 	// file system roots
 	// TODO(gri) consider the invariant that goroot always end in '/'
 	goroot = flag.String("goroot", findGOROOT(), "Go root directory")

 	// layout control
 	showTimestamps = flag.Bool("timestamps", false, "show timestamps with directory listings")
 	templateDir    = flag.String("templates", "", "load templates/JS/CSS from disk in this directory")
 	showPlayground = flag.Bool("play", false, "enable playground")
 	declLinks      = flag.Bool("links", true, "link identifiers to their declarations")

 	// search index
 	indexEnabled  = flag.Bool("index", false, "enable search index")
 	indexFiles    = flag.String("index_files", "", "glob pattern specifying index files; if not empty, the index is read from these files in sorted order")
 	indexInterval = flag.Duration("index_interval", 0, "interval of indexing; 0 for default (5m), negative to only index once at startup")
 	maxResults    = flag.Int("maxresults", 10000, "maximum number of full text search results shown")
 	indexThrottle = flag.Float64("index_throttle", 0.75, "index throttle value; 0.0 = no time allocated, 1.0 = full throttle")

 	// source code notes
 	notesRx = flag.String("notes", "BUG", "regular expression matching note markers to show")
 )

 // An httpResponseRecorder is an http.ResponseWriter
 type httpResponseRecorder struct {
 	body   *bytes.Buffer
 	header http.Header
 	code   int
 }

 func (w *httpResponseRecorder) Header() http.Header         { return w.header }
 func (w *httpResponseRecorder) Write(b []byte) (int, error) { return w.body.Write(b) }
 func (w *httpResponseRecorder) WriteHeader(code int)        { w.code = code }

 func usage() {
 	fmt.Fprintf(os.Stderr, "usage: godoc -http="+defaultAddr+"\n")
 	flag.PrintDefaults()
 	os.Exit(2)
 }

 func loggingHandler(h http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
 		log.Printf("%s\t%s", req.RemoteAddr, req.URL)
 		h.ServeHTTP(w, req)
 	})
 }

 func handleURLFlag() {
 	// Try up to 10 fetches, following redirects.
 	urlstr := *urlFlag
 	for i := 0; i < 10; i++ {
 		// Prepare request.
 		u, err := url.Parse(urlstr)
 		if err != nil {
 			log.Fatal(err)
 		}
 		req := &http.Request{
 			URL: u,
 		}

 		// Invoke default HTTP handler to serve request
 		// to our buffering httpWriter.
 		w := &httpResponseRecorder{code: 200, header: make(http.Header), body: new(bytes.Buffer)}
 		http.DefaultServeMux.ServeHTTP(w, req)

 		// Return data, error, or follow redirect.
 		switch w.code {
 		case 200: // ok
 			os.Stdout.Write(w.body.Bytes())
 			return
 		case 301, 302, 303, 307: // redirect
 			redirect := w.header.Get("Location")
 			if redirect == "" {
 				log.Fatalf("HTTP %d without Location header", w.code)
 			}
 			urlstr = redirect
 		default:
 			log.Fatalf("HTTP error %d", w.code)
 		}
 	}
 	log.Fatalf("too many redirects")
 }

 func initCorpus(corpus *godoc.Corpus) {
 	err := corpus.Init()
 	if err != nil {
 		log.Fatal(err)
 	}
 }

 func main() {
 	flag.Usage = usage
 	flag.Parse()

 	// Check usage.
 	if flag.NArg() > 0 {
 		fmt.Fprintln(os.Stderr, `Unexpected arguments. Use "go doc" for command-line help output instead. For example, "go doc fmt.Printf".`)
 		usage()
 	}
 	if *httpAddr == "" && *urlFlag == "" && !*writeIndex {
 		fmt.Fprintln(os.Stderr, "At least one of -http, -url, or -write_index must be set to a non-zero value.")
 		usage()
 	}

 	// Set the resolved goroot.
 	vfs.GOROOT = *goroot

 	fsGate := make(chan bool, 20)

 	// Determine file system to use.
 	if *zipfile == "" {
 		// use file system of underlying OS
 		rootfs := gatefs.New(vfs.OS(*goroot), fsGate)
 		fs.Bind("/", rootfs, "/", vfs.BindReplace)
 	} else {
 		// use file system specified via .zip file (path separator must be '/')
 		rc, err := zip.OpenReader(*zipfile)
 		if err != nil {
 			log.Fatalf("%s: %s\n", *zipfile, err)
 		}
 		defer rc.Close() // be nice (e.g., -writeIndex mode)
 		fs.Bind("/", zipfs.New(rc, *zipfile), *goroot, vfs.BindReplace)
 	}
 	if *templateDir != "" {
 		fs.Bind("/lib/godoc", vfs.OS(*templateDir), "/", vfs.BindBefore)
 		fs.Bind("/favicon.ico", vfs.OS(*templateDir), "/favicon.ico", vfs.BindReplace)
 	} else {
 		fs.Bind("/lib/godoc", mapfs.New(static.Files), "/", vfs.BindReplace)
 		fs.Bind("/favicon.ico", mapfs.New(static.Files), "/favicon.ico", vfs.BindReplace)
 	}

 	// Get the GOMOD value, use it to determine if godoc is being invoked in module mode.
 	goModFile, err := goMod()
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "failed to determine go env GOMOD value: %v", err)
 		goModFile = "" // Fall back to GOPATH mode.
 	}

 	if goModFile != "" {
 		fmt.Printf("using module mode; GOMOD=%s\n", goModFile)

 		// Detect whether to use vendor mode or not.
 		vendorEnabled, mainModVendor, err := gocommand.VendorEnabled(context.Background(), gocommand.Invocation{}, &gocommand.Runner{})
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "failed to determine if vendoring is enabled: %v", err)
 			os.Exit(1)
 		}
 		if vendorEnabled {
 			// Bind the root directory of the main module.
 			fs.Bind(path.Join("/src", mainModVendor.Path), gatefs.New(vfs.OS(mainModVendor.Dir), fsGate), "/", vfs.BindAfter)

 			// Bind the vendor directory.
 			//
 			// Note that in module mode, vendor directories in locations
 			// other than the main module's root directory are ignored.
 			// See https://golang.org/ref/mod#vendoring.
 			vendorDir := filepath.Join(mainModVendor.Dir, "vendor")
 			fs.Bind("/src", gatefs.New(vfs.OS(vendorDir), fsGate), "/", vfs.BindAfter)

 		} else {
 			// Try to download dependencies that are not in the module cache in order to
 			// show their documentation.
 			// This may fail if module downloading is disallowed (GOPROXY=off) or due to
 			// limited connectivity, in which case we print errors to stderr and show
 			// documentation only for packages that are available.
 			fillModuleCache(os.Stderr, goModFile)

 			// Determine modules in the build list.
 			mods, err := buildList(goModFile)
 			if err != nil {
 				fmt.Fprintf(os.Stderr, "failed to determine the build list of the main module: %v", err)
 				os.Exit(1)
 			}

 			// Bind module trees into Go root.
 			for _, m := range mods {
 				if m.Dir == "" {
 					// Module is not available in the module cache, skip it.
 					continue
 				}
 				dst := path.Join("/src", m.Path)
 				fs.Bind(dst, gatefs.New(vfs.OS(m.Dir), fsGate), "/", vfs.BindAfter)
 			}
 		}
 	} else {
 		fmt.Println("using GOPATH mode")

 		// Bind $GOPATH trees into Go root.
 		for _, p := range filepath.SplitList(build.Default.GOPATH) {
 			fs.Bind("/src", gatefs.New(vfs.OS(p), fsGate), "/src", vfs.BindAfter)
 		}
 	}

 	var corpus *godoc.Corpus
 	if goModFile != "" {
 		corpus = godoc.NewCorpus(moduleFS{fs})
 	} else {
 		corpus = godoc.NewCorpus(fs)
 	}
 	corpus.Verbose = *verbose
 	corpus.MaxResults = *maxResults
 	corpus.IndexEnabled = *indexEnabled
 	if *maxResults == 0 {
 		corpus.IndexFullText = false
 	}
 	corpus.IndexFiles = *indexFiles
 	corpus.IndexDirectory = func(dir string) bool {
 		return dir != "/pkg" && !strings.HasPrefix(dir, "/pkg/")
 	}
 	corpus.IndexThrottle = *indexThrottle
 	corpus.IndexInterval = *indexInterval
 	if *writeIndex || *urlFlag != "" {
 		corpus.IndexThrottle = 1.0
 		corpus.IndexEnabled = true
 		initCorpus(corpus)
 	} else {
 		go initCorpus(corpus)
 	}

 	// Initialize the version info before readTemplates, which saves
 	// the map value in a method value.
 	corpus.InitVersionInfo()

 	pres = godoc.NewPresentation(corpus)
 	pres.ShowTimestamps = *showTimestamps
 	pres.ShowPlayground = *showPlayground
 	pres.DeclLinks = *declLinks
 	if *notesRx != "" {
 		pres.NotesRx = regexp.MustCompile(*notesRx)
 	}

 	readTemplates(pres)
 	registerHandlers(pres)

 	if *writeIndex {
 		// Write search index and exit.
 		if *indexFiles == "" {
 			log.Fatal("no index file specified")
 		}

 		log.Println("initialize file systems")
 		*verbose = true // want to see what happens

 		corpus.UpdateIndex()

 		log.Println("writing index file", *indexFiles)
 		f, err := os.Create(*indexFiles)
 		if err != nil {
 			log.Fatal(err)
 		}
 		index, _ := corpus.CurrentIndex()
 		_, err = index.WriteTo(f)
 		if err != nil {
 			log.Fatal(err)
 		}

 		log.Println("done")
 		return
 	}

 	// Print content that would be served at the URL *urlFlag.
 	if *urlFlag != "" {
 		handleURLFlag()
 		return
 	}

 	var handler http.Handler = http.DefaultServeMux
 	if *verbose {
 		log.Printf("Go Documentation Server")
 		log.Printf("version = %s", runtime.Version())
 		log.Printf("address = %s", *httpAddr)
 		log.Printf("goroot = %s", *goroot)
 		switch {
 		case !*indexEnabled:
 			log.Print("search index disabled")
 		case *maxResults > 0:
 			log.Printf("full text index enabled (maxresults = %d)", *maxResults)
 		default:
 			log.Print("identifier search index enabled")
 		}
 		fs.Fprint(os.Stderr)
 		handler = loggingHandler(handler)
 	}

 	// Initialize search index.
 	if *indexEnabled {
 		go corpus.RunIndexer()
 	}

 	// Start http server.
 	if *verbose {
 		log.Println("starting HTTP server")
 	}
 	if err := http.ListenAndServe(*httpAddr, handler); err != nil {
 		log.Fatalf("ListenAndServe %s: %v", *httpAddr, err)
 	}
 }

 // goMod returns the go env GOMOD value in the current directory
 // by invoking the go command.
 //
 // GOMOD is documented at https://golang.org/cmd/go/#hdr-Environment_variables:
 //
 //	The absolute path to the go.mod of the main module,
 //	or the empty string if not using modules.
 func goMod() (string, error) {
 	out, err := exec.Command("go", "env", "-json", "GOMOD").Output()
 	if ee := (*exec.ExitError)(nil); errors.As(err, &ee) {
 		return "", fmt.Errorf("go command exited unsuccessfully: %v\n%s", ee.ProcessState.String(), ee.Stderr)
 	} else if err != nil {
 		return "", err
 	}
 	var env struct {
 		GoMod string
 	}
 	err = json.Unmarshal(out, &env)
 	if err != nil {
 		return "", err
 	}
 	return env.GoMod, nil
 }

 // fillModuleCache does a best-effort attempt to fill the module cache
 // with all dependencies of the main module in the current directory
 // by invoking the go command. Module download logs are streamed to w.
 // If there are any problems encountered, they are also written to w.
 // It should only be used in module mode, when vendor mode isn't on.
 //
 // See https://golang.org/cmd/go/#hdr-Download_modules_to_local_cache.
 func fillModuleCache(w io.Writer, goMod string) {
 	if goMod == os.DevNull {
 		// No module requirements, nothing to do.
 		return
 	}

 	cmd := exec.Command("go", "mod", "download", "-json")
 	var out bytes.Buffer
 	cmd.Stdout = &out
 	cmd.Stderr = w
 	err := cmd.Run()
 	if ee := (*exec.ExitError)(nil); errors.As(err, &ee) && ee.ExitCode() == 1 {
 		// Exit code 1 from this command means there were some
 		// non-empty Error values in the output. Print them to w.
 		fmt.Fprintf(w, "documentation for some packages is not shown:\n")
 		for dec := json.NewDecoder(&out); ; {
 			var m struct {
 				Path    string // Module path.
 				Version string // Module version.
 				Error   string // Error loading module.
 			}
 			err := dec.Decode(&m)
 			if err == io.EOF {
 				break
 			} else if err != nil {
 				fmt.Fprintf(w, "error decoding JSON object from go mod download -json: %v\n", err)
 				continue
 			}
 			if m.Error == "" {
 				continue
 			}
 			fmt.Fprintf(w, "\tmodule %s@%s is not in the module cache and there was a problem downloading it: %s\n", m.Path, m.Version, m.Error)
 		}
 	} else if err != nil {
 		fmt.Fprintf(w, "there was a problem filling module cache: %v\n", err)
 	}
 }

 type mod struct {
 	Path string // Module path.
 	Dir  string // Directory holding files for this module, if any.
 }

 // buildList determines the build list in the current directory
 // by invoking the go command. It should only be used in module mode,
 // when vendor mode isn't on.
 //
 // See https://golang.org/cmd/go/#hdr-The_main_module_and_the_build_list.
 func buildList(goMod string) ([]mod, error) {
 	if goMod == os.DevNull {
 		// Empty build list.
 		return nil, nil
 	}

 	out, err := exec.Command("go", "list", "-m", "-json", "all").Output()
 	if ee := (*exec.ExitError)(nil); errors.As(err, &ee) {
 		return nil, fmt.Errorf("go command exited unsuccessfully: %v\n%s", ee.ProcessState.String(), ee.Stderr)
 	} else if err != nil {
 		return nil, err
 	}
 	var mods []mod
 	for dec := json.NewDecoder(bytes.NewReader(out)); ; {
 		var m mod
 		err := dec.Decode(&m)
 		if err == io.EOF {
 			break
 		} else if err != nil {
 			return nil, err
 		}
 		mods = append(mods, m)
 	}
 	return mods, nil
 }

 // moduleFS is a vfs.FileSystem wrapper used when godoc is running
 // in module mode. It's needed so that packages inside modules are
 // considered to be third party.
 //
 // It overrides the RootType method of the underlying filesystem
 // and implements it using a heuristic based on the import path.
 // If the first element of the import path does not contain a dot,
 // that package is considered to be inside GOROOT. If it contains
 // a dot, then that package is considered to be third party.
 //
 // TODO(dmitshur): The RootType abstraction works well when GOPATH
 // workspaces are bound at their roots, but scales poorly in the
 // general case. It should be replaced by a more direct solution
 // for determining whether a package is third party or not.
 type moduleFS struct{ vfs.FileSystem }

 func (moduleFS) RootType(path string) vfs.RootType {
 	if !strings.HasPrefix(path, "/src/") {
 		return ""
 	}
 	domain := path[len("/src/"):]
 	if i := strings.Index(domain, "/"); i >= 0 {
 		domain = domain[:i]
 	}
 	if !strings.Contains(domain, ".") {
 		// No dot in the first element of import path
 		// suggests this is a package in GOROOT.
 		return vfs.RootTypeGoRoot
 	} else {
 		// A dot in the first element of import path
 		// suggests this is a third party package.
 		return vfs.RootTypeGoPath
 	}
 }
 func (fs moduleFS) String() string { return "module(" + fs.FileSystem.String() + ")" }
	// Copyright 2009 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// godoc: Go Documentation Server

	// Web server tree:
	//
	// http://godoc/ redirect to /pkg/
	// http://godoc/src/ serve files from $GOROOT/src; .go gets pretty-printed
	// http://godoc/cmd/ serve documentation about commands
	// http://godoc/pkg/ serve documentation about packages
	// (idea is if you say import "compress/zlib", you go to
	// http://godoc/pkg/compress/zlib)
	//

	package main

	import (
	"archive/zip"
	"bytes"
	"context"
	"encoding/json"
	"errors"
	_ "expvar" // to serve /debug/vars
	"flag"
	"fmt"
	"go/build"
	"io"
	"log"
	"net/http"
	_ "net/http/pprof" // to serve /debug/pprof/*
	"net/url"
	"os"
	"os/exec"
	"path"
	"path/filepath"
	"regexp"
	"runtime"
	"strings"

	"golang.org/x/tools/godoc"
	"golang.org/x/tools/godoc/static"
	"golang.org/x/tools/godoc/vfs"
	"golang.org/x/tools/godoc/vfs/gatefs"
	"golang.org/x/tools/godoc/vfs/mapfs"
	"golang.org/x/tools/godoc/vfs/zipfs"
	"golang.org/x/tools/internal/gocommand"
	)

	const defaultAddr = "localhost:6060" // default webserver address

	var (
	// file system to serve
	// (with e.g.: zip -r go.zip $GOROOT -i \.go -i \.html -i \.css -i \.js -i \.txt -i \.c -i \.h -i \.s -i \.png -i \.jpg -i \*.sh -i favicon.ico)
	zipfile = flag.String("zip", "", "zip file providing the file system to serve; disabled if empty")

	// file-based index
	writeIndex = flag.Bool("write_index", false, "write index to a file; the file name must be specified with -index_files")

	// network
	httpAddr = flag.String("http", defaultAddr, "HTTP service address")

	// layout control
	urlFlag = flag.String("url", "", "print HTML for named URL")

	verbose = flag.Bool("v", false, "verbose mode")

	// file system roots
	// TODO(gri) consider the invariant that goroot always end in '/'
	goroot = flag.String("goroot", findGOROOT(), "Go root directory")

	// layout control
	showTimestamps = flag.Bool("timestamps", false, "show timestamps with directory listings")
	templateDir = flag.String("templates", "", "load templates/JS/CSS from disk in this directory")
	showPlayground = flag.Bool("play", false, "enable playground")
	declLinks = flag.Bool("links", true, "link identifiers to their declarations")

	// search index
	indexEnabled = flag.Bool("index", false, "enable search index")
	indexFiles = flag.String("index_files", "", "glob pattern specifying index files; if not empty, the index is read from these files in sorted order")
	indexInterval = flag.Duration("index_interval", 0, "interval of indexing; 0 for default (5m), negative to only index once at startup")
	maxResults = flag.Int("maxresults", 10000, "maximum number of full text search results shown")
	indexThrottle = flag.Float64("index_throttle", 0.75, "index throttle value; 0.0 = no time allocated, 1.0 = full throttle")

	// source code notes
	notesRx = flag.String("notes", "BUG", "regular expression matching note markers to show")
	)

	// An httpResponseRecorder is an http.ResponseWriter
	type httpResponseRecorder struct {
	body *bytes.Buffer
	header http.Header
	code int
	}

	func (w *httpResponseRecorder) Header() http.Header { return w.header }
	func (w *httpResponseRecorder) Write(b []byte) (int, error) { return w.body.Write(b) }
	func (w *httpResponseRecorder) WriteHeader(code int) { w.code = code }

	func usage() {
	fmt.Fprintf(os.Stderr, "usage: godoc -http="+defaultAddr+"\n")
	flag.PrintDefaults()
	os.Exit(2)
	}

	func loggingHandler(h http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
	log.Printf("%s\t%s", req.RemoteAddr, req.URL)
	h.ServeHTTP(w, req)
	})
	}

	func handleURLFlag() {
	// Try up to 10 fetches, following redirects.
	urlstr := *urlFlag
	for i := 0; i < 10; i++ {
	// Prepare request.
	u, err := url.Parse(urlstr)
	if err != nil {
	log.Fatal(err)
	}
	req := &http.Request{
	URL: u,
	}

	// Invoke default HTTP handler to serve request
	// to our buffering httpWriter.
	w := &httpResponseRecorder{code: 200, header: make(http.Header), body: new(bytes.Buffer)}
	http.DefaultServeMux.ServeHTTP(w, req)

	// Return data, error, or follow redirect.
	switch w.code {
	case 200: // ok
	os.Stdout.Write(w.body.Bytes())
	return
	case 301, 302, 303, 307: // redirect
	redirect := w.header.Get("Location")
	if redirect == "" {
	log.Fatalf("HTTP %d without Location header", w.code)
	}
	urlstr = redirect
	default:
	log.Fatalf("HTTP error %d", w.code)
	}
	}
	log.Fatalf("too many redirects")
	}

	func initCorpus(corpus *godoc.Corpus) {
	err := corpus.Init()
	if err != nil {
	log.Fatal(err)
	}
	}

	func main() {
	flag.Usage = usage
	flag.Parse()

	// Check usage.
	if flag.NArg() > 0 {
	fmt.Fprintln(os.Stderr, `Unexpected arguments. Use "go doc" for command-line help output instead. For example, "go doc fmt.Printf".`)
	usage()
	}
	if httpAddr == "" && urlFlag == "" && !*writeIndex {
	fmt.Fprintln(os.Stderr, "At least one of -http, -url, or -write_index must be set to a non-zero value.")
	usage()
	}

	// Set the resolved goroot.
	vfs.GOROOT = *goroot

	fsGate := make(chan bool, 20)

	// Determine file system to use.
	if *zipfile == "" {
	// use file system of underlying OS
	rootfs := gatefs.New(vfs.OS(*goroot), fsGate)
	fs.Bind("/", rootfs, "/", vfs.BindReplace)
	} else {
	// use file system specified via .zip file (path separator must be '/')
	rc, err := zip.OpenReader(*zipfile)
	if err != nil {
	log.Fatalf("%s: %s\n", *zipfile, err)
	}
	defer rc.Close() // be nice (e.g., -writeIndex mode)
	fs.Bind("/", zipfs.New(rc, zipfile), goroot, vfs.BindReplace)
	}
	if *templateDir != "" {
	fs.Bind("/lib/godoc", vfs.OS(*templateDir), "/", vfs.BindBefore)
	fs.Bind("/favicon.ico", vfs.OS(*templateDir), "/favicon.ico", vfs.BindReplace)
	} else {
	fs.Bind("/lib/godoc", mapfs.New(static.Files), "/", vfs.BindReplace)
	fs.Bind("/favicon.ico", mapfs.New(static.Files), "/favicon.ico", vfs.BindReplace)
	}

	// Get the GOMOD value, use it to determine if godoc is being invoked in module mode.
	goModFile, err := goMod()
	if err != nil {
	fmt.Fprintf(os.Stderr, "failed to determine go env GOMOD value: %v", err)
	goModFile = "" // Fall back to GOPATH mode.
	}

	if goModFile != "" {
	fmt.Printf("using module mode; GOMOD=%s\n", goModFile)

	// Detect whether to use vendor mode or not.
	vendorEnabled, mainModVendor, err := gocommand.VendorEnabled(context.Background(), gocommand.Invocation{}, &gocommand.Runner{})
	if err != nil {
	fmt.Fprintf(os.Stderr, "failed to determine if vendoring is enabled: %v", err)
	os.Exit(1)
	}
	if vendorEnabled {
	// Bind the root directory of the main module.
	fs.Bind(path.Join("/src", mainModVendor.Path), gatefs.New(vfs.OS(mainModVendor.Dir), fsGate), "/", vfs.BindAfter)

	// Bind the vendor directory.
	//
	// Note that in module mode, vendor directories in locations
	// other than the main module's root directory are ignored.
	// See https://golang.org/ref/mod#vendoring.
	vendorDir := filepath.Join(mainModVendor.Dir, "vendor")
	fs.Bind("/src", gatefs.New(vfs.OS(vendorDir), fsGate), "/", vfs.BindAfter)

	} else {
	// Try to download dependencies that are not in the module cache in order to
	// show their documentation.
	// This may fail if module downloading is disallowed (GOPROXY=off) or due to
	// limited connectivity, in which case we print errors to stderr and show
	// documentation only for packages that are available.
	fillModuleCache(os.Stderr, goModFile)

	// Determine modules in the build list.
	mods, err := buildList(goModFile)
	if err != nil {
	fmt.Fprintf(os.Stderr, "failed to determine the build list of the main module: %v", err)
	os.Exit(1)
	}

	// Bind module trees into Go root.
	for _, m := range mods {
	if m.Dir == "" {
	// Module is not available in the module cache, skip it.
	continue
	}
	dst := path.Join("/src", m.Path)
	fs.Bind(dst, gatefs.New(vfs.OS(m.Dir), fsGate), "/", vfs.BindAfter)
	}
	}
	} else {
	fmt.Println("using GOPATH mode")

	// Bind $GOPATH trees into Go root.
	for _, p := range filepath.SplitList(build.Default.GOPATH) {
	fs.Bind("/src", gatefs.New(vfs.OS(p), fsGate), "/src", vfs.BindAfter)
	}
	}

	var corpus *godoc.Corpus
	if goModFile != "" {
	corpus = godoc.NewCorpus(moduleFS{fs})
	} else {
	corpus = godoc.NewCorpus(fs)
	}
	corpus.Verbose = *verbose
	corpus.MaxResults = *maxResults
	corpus.IndexEnabled = *indexEnabled
	if *maxResults == 0 {
	corpus.IndexFullText = false
	}
	corpus.IndexFiles = *indexFiles
	corpus.IndexDirectory = func(dir string) bool {
	return dir != "/pkg" && !strings.HasPrefix(dir, "/pkg/")
	}
	corpus.IndexThrottle = *indexThrottle
	corpus.IndexInterval = *indexInterval
	if writeIndex \|\| urlFlag != "" {
	corpus.IndexThrottle = 1.0
	corpus.IndexEnabled = true
	initCorpus(corpus)
	} else {
	go initCorpus(corpus)
	}

	// Initialize the version info before readTemplates, which saves
	// the map value in a method value.
	corpus.InitVersionInfo()

	pres = godoc.NewPresentation(corpus)
	pres.ShowTimestamps = *showTimestamps
	pres.ShowPlayground = *showPlayground
	pres.DeclLinks = *declLinks
	if *notesRx != "" {
	pres.NotesRx = regexp.MustCompile(*notesRx)
	}

	readTemplates(pres)
	registerHandlers(pres)

	if *writeIndex {
	// Write search index and exit.
	if *indexFiles == "" {
	log.Fatal("no index file specified")
	}

	log.Println("initialize file systems")
	*verbose = true // want to see what happens

	corpus.UpdateIndex()

	log.Println("writing index file", *indexFiles)
	f, err := os.Create(*indexFiles)
	if err != nil {
	log.Fatal(err)
	}
	index, _ := corpus.CurrentIndex()
	_, err = index.WriteTo(f)
	if err != nil {
	log.Fatal(err)
	}

	log.Println("done")
	return
	}

	// Print content that would be served at the URL *urlFlag.
	if *urlFlag != "" {
	handleURLFlag()
	return
	}

	var handler http.Handler = http.DefaultServeMux
	if *verbose {
	log.Printf("Go Documentation Server")
	log.Printf("version = %s", runtime.Version())
	log.Printf("address = %s", *httpAddr)
	log.Printf("goroot = %s", *goroot)
	switch {
	case !*indexEnabled:
	log.Print("search index disabled")
	case *maxResults > 0:
	log.Printf("full text index enabled (maxresults = %d)", *maxResults)
	default:
	log.Print("identifier search index enabled")
	}
	fs.Fprint(os.Stderr)
	handler = loggingHandler(handler)
	}

	// Initialize search index.
	if *indexEnabled {
	go corpus.RunIndexer()
	}

	// Start http server.
	if *verbose {
	log.Println("starting HTTP server")
	}
	if err := http.ListenAndServe(*httpAddr, handler); err != nil {
	log.Fatalf("ListenAndServe %s: %v", *httpAddr, err)
	}
	}

	// goMod returns the go env GOMOD value in the current directory
	// by invoking the go command.
	//
	// GOMOD is documented at https://golang.org/cmd/go/#hdr-Environment_variables:
	//
	// The absolute path to the go.mod of the main module,
	// or the empty string if not using modules.
	func goMod() (string, error) {
	out, err := exec.Command("go", "env", "-json", "GOMOD").Output()
	if ee := (*exec.ExitError)(nil); errors.As(err, &ee) {
	return "", fmt.Errorf("go command exited unsuccessfully: %v\n%s", ee.ProcessState.String(), ee.Stderr)
	} else if err != nil {
	return "", err
	}
	var env struct {
	GoMod string
	}
	err = json.Unmarshal(out, &env)
	if err != nil {
	return "", err
	}
	return env.GoMod, nil
	}

	// fillModuleCache does a best-effort attempt to fill the module cache
	// with all dependencies of the main module in the current directory
	// by invoking the go command. Module download logs are streamed to w.
	// If there are any problems encountered, they are also written to w.
	// It should only be used in module mode, when vendor mode isn't on.
	//
	// See https://golang.org/cmd/go/#hdr-Download_modules_to_local_cache.
	func fillModuleCache(w io.Writer, goMod string) {
	if goMod == os.DevNull {
	// No module requirements, nothing to do.
	return
	}

	cmd := exec.Command("go", "mod", "download", "-json")
	var out bytes.Buffer
	cmd.Stdout = &out
	cmd.Stderr = w
	err := cmd.Run()
	if ee := (*exec.ExitError)(nil); errors.As(err, &ee) && ee.ExitCode() == 1 {
	// Exit code 1 from this command means there were some
	// non-empty Error values in the output. Print them to w.
	fmt.Fprintf(w, "documentation for some packages is not shown:\n")
	for dec := json.NewDecoder(&out); ; {
	var m struct {
	Path string // Module path.
	Version string // Module version.
	Error string // Error loading module.
	}
	err := dec.Decode(&m)
	if err == io.EOF {
	break
	} else if err != nil {
	fmt.Fprintf(w, "error decoding JSON object from go mod download -json: %v\n", err)
	continue
	}
	if m.Error == "" {
	continue
	}
	fmt.Fprintf(w, "\tmodule %s@%s is not in the module cache and there was a problem downloading it: %s\n", m.Path, m.Version, m.Error)
	}
	} else if err != nil {
	fmt.Fprintf(w, "there was a problem filling module cache: %v\n", err)
	}
	}

	type mod struct {
	Path string // Module path.
	Dir string // Directory holding files for this module, if any.
	}

	// buildList determines the build list in the current directory
	// by invoking the go command. It should only be used in module mode,
	// when vendor mode isn't on.
	//
	// See https://golang.org/cmd/go/#hdr-The_main_module_and_the_build_list.
	func buildList(goMod string) ([]mod, error) {
	if goMod == os.DevNull {
	// Empty build list.
	return nil, nil
	}

	out, err := exec.Command("go", "list", "-m", "-json", "all").Output()
	if ee := (*exec.ExitError)(nil); errors.As(err, &ee) {
	return nil, fmt.Errorf("go command exited unsuccessfully: %v\n%s", ee.ProcessState.String(), ee.Stderr)
	} else if err != nil {
	return nil, err
	}
	var mods []mod
	for dec := json.NewDecoder(bytes.NewReader(out)); ; {
	var m mod
	err := dec.Decode(&m)
	if err == io.EOF {
	break
	} else if err != nil {
	return nil, err
	}
	mods = append(mods, m)
	}
	return mods, nil
	}

	// moduleFS is a vfs.FileSystem wrapper used when godoc is running
	// in module mode. It's needed so that packages inside modules are
	// considered to be third party.
	//
	// It overrides the RootType method of the underlying filesystem
	// and implements it using a heuristic based on the import path.
	// If the first element of the import path does not contain a dot,
	// that package is considered to be inside GOROOT. If it contains
	// a dot, then that package is considered to be third party.
	//
	// TODO(dmitshur): The RootType abstraction works well when GOPATH
	// workspaces are bound at their roots, but scales poorly in the
	// general case. It should be replaced by a more direct solution
	// for determining whether a package is third party or not.
	type moduleFS struct{ vfs.FileSystem }

	func (moduleFS) RootType(path string) vfs.RootType {
	if !strings.HasPrefix(path, "/src/") {
	return ""
	}
	domain := path[len("/src/"):]
	if i := strings.Index(domain, "/"); i >= 0 {
	domain = domain[:i]
	}
	if !strings.Contains(domain, ".") {
	// No dot in the first element of import path
	// suggests this is a package in GOROOT.
	return vfs.RootTypeGoRoot
	} else {
	// A dot in the first element of import path
	// suggests this is a third party package.
	return vfs.RootTypeGoPath
	}
	}
	func (fs moduleFS) String() string { return "module(" + fs.FileSystem.String() + ")" }