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// Copyright 2013 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.
//go:build go1.16
// +build go1.16
package godoc
import (
// An rwValue wraps a value and permits mutually exclusive
// access to it and records the time the value was last set.
type rwValue struct {
mutex sync.RWMutex
value interface{}
timestamp time.Time // time of last set()
func (v *rwValue) Set(value interface{}) {
v.value = value
v.timestamp = time.Now()
func (v *rwValue) Get() (interface{}, time.Time) {
defer v.mutex.RUnlock()
return v.value, v.timestamp
// IsText reports whether a significant prefix of s looks like correct UTF-8;
// that is, if it is likely that s is human-readable text.
func IsText(s []byte) bool {
const max = 1024 // at least utf8.UTFMax
if len(s) > max {
s = s[0:max]
for i, c := range string(s) {
if i+utf8.UTFMax > len(s) {
// last char may be incomplete - ignore
if c == 0xFFFD || c < ' ' && c != '\n' && c != '\t' && c != '\f' {
// decoding error or control character - not a text file
return false
return true
// textExt[x] is true if the extension x indicates a text file, and false otherwise.
var textExt = map[string]bool{
".css": false, // must be served raw
".js": false, // must be served raw
".svg": false, // must be served raw
// isTextFile reports whether the file has a known extension indicating
// a text file, or if a significant chunk of the specified file looks like
// correct UTF-8; that is, if it is likely that the file contains human-
// readable text.
func isTextFile(fsys fs.FS, filename string) bool {
// if the extension is known, use it for decision making
if isText, found := textExt[path.Ext(filename)]; found {
return isText
// the extension is not known; read an initial chunk
// of the file and check if it looks like text
f, err := fsys.Open(filename)
if err != nil {
return false
defer f.Close()
var buf [1024]byte
n, err := f.Read(buf[0:])
if err != nil {
return false
return IsText(buf[0:n])