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// Copyright 2018 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.
// Package renameio writes files atomically by renaming temporary files.
package renameio
import (
"bytes"
"io"
"io/fs"
"math/rand"
"os"
"path/filepath"
"strconv"
"cmd/go/internal/robustio"
)
const patternSuffix = ".tmp"
// Pattern returns a glob pattern that matches the unrenamed temporary files
// created when writing to filename.
func Pattern(filename string) string {
return filepath.Join(filepath.Dir(filename), filepath.Base(filename)+patternSuffix)
}
// WriteFile is like ioutil.WriteFile, but first writes data to an arbitrary
// file in the same directory as filename, then renames it atomically to the
// final name.
//
// That ensures that the final location, if it exists, is always a complete file.
func WriteFile(filename string, data []byte, perm fs.FileMode) (err error) {
return WriteToFile(filename, bytes.NewReader(data), perm)
}
// WriteToFile is a variant of WriteFile that accepts the data as an io.Reader
// instead of a slice.
func WriteToFile(filename string, data io.Reader, perm fs.FileMode) (err error) {
f, err := tempFile(filepath.Dir(filename), filepath.Base(filename), perm)
if err != nil {
return err
}
defer func() {
// Only call os.Remove on f.Name() if we failed to rename it: otherwise,
// some other process may have created a new file with the same name after
// that.
if err != nil {
f.Close()
os.Remove(f.Name())
}
}()
if _, err := io.Copy(f, data); err != nil {
return err
}
// Sync the file before renaming it: otherwise, after a crash the reader may
// observe a 0-length file instead of the actual contents.
// See https://golang.org/issue/22397#issuecomment-380831736.
if err := f.Sync(); err != nil {
return err
}
if err := f.Close(); err != nil {
return err
}
return robustio.Rename(f.Name(), filename)
}
// ReadFile is like ioutil.ReadFile, but on Windows retries spurious errors that
// may occur if the file is concurrently replaced.
//
// Errors are classified heuristically and retries are bounded, so even this
// function may occasionally return a spurious error on Windows.
// If so, the error will likely wrap one of:
// - syscall.ERROR_ACCESS_DENIED
// - syscall.ERROR_FILE_NOT_FOUND
// - internal/syscall/windows.ERROR_SHARING_VIOLATION
func ReadFile(filename string) ([]byte, error) {
return robustio.ReadFile(filename)
}
// tempFile creates a new temporary file with given permission bits.
func tempFile(dir, prefix string, perm fs.FileMode) (f *os.File, err error) {
for i := 0; i < 10000; i++ {
name := filepath.Join(dir, prefix+strconv.Itoa(rand.Intn(1000000000))+patternSuffix)
f, err = os.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_EXCL, perm)
if os.IsExist(err) {
continue
}
break
}
return
}
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