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84 lines
2.4 KiB
Go
84 lines
2.4 KiB
Go
// Copyright 2018 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package renameio writes files atomically by renaming temporary files.
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package renameio
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import (
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"bytes"
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"io"
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"io/ioutil"
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"os"
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"path/filepath"
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"runtime"
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"strings"
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"time"
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)
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const patternSuffix = "*.tmp"
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// Pattern returns a glob pattern that matches the unrenamed temporary files
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// created when writing to filename.
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func Pattern(filename string) string {
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return filepath.Join(filepath.Dir(filename), filepath.Base(filename)+patternSuffix)
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}
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// WriteFile is like ioutil.WriteFile, but first writes data to an arbitrary
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// file in the same directory as filename, then renames it atomically to the
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// final name.
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//
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// That ensures that the final location, if it exists, is always a complete file.
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func WriteFile(filename string, data []byte) (err error) {
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return WriteToFile(filename, bytes.NewReader(data))
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}
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// WriteToFile is a variant of WriteFile that accepts the data as an io.Reader
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// instead of a slice.
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func WriteToFile(filename string, data io.Reader) (err error) {
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f, err := ioutil.TempFile(filepath.Dir(filename), filepath.Base(filename)+patternSuffix)
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if err != nil {
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return err
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}
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defer func() {
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// Only call os.Remove on f.Name() if we failed to rename it: otherwise,
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// some other process may have created a new file with the same name after
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// that.
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if err != nil {
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f.Close()
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os.Remove(f.Name())
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}
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}()
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if _, err := io.Copy(f, data); err != nil {
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return err
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}
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// Sync the file before renaming it: otherwise, after a crash the reader may
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// observe a 0-length file instead of the actual contents.
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// See https://golang.org/issue/22397#issuecomment-380831736.
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if err := f.Sync(); err != nil {
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return err
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}
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if err := f.Close(); err != nil {
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return err
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}
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var start time.Time
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for {
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err := os.Rename(f.Name(), filename)
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if err == nil || runtime.GOOS != "windows" || !strings.HasSuffix(err.Error(), "Access is denied.") {
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return err
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}
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// Windows seems to occasionally trigger spurious "Access is denied" errors
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// here (see golang.org/issue/31247). We're not sure why. It's probably
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// worth a little extra latency to avoid propagating the spurious errors.
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if start.IsZero() {
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start = time.Now()
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} else if time.Since(start) >= 500*time.Millisecond {
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return err
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}
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time.Sleep(5 * time.Millisecond)
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}
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}
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