syncthing/lib/osutil/atomic.go

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// Copyright (C) 2015 The Syncthing Authors.
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this file,
// You can obtain one at http://mozilla.org/MPL/2.0/.
package osutil
import (
"errors"
"io/ioutil"
"os"
"path/filepath"
"runtime"
)
var (
ErrClosed = errors.New("write to closed writer")
TempPrefix = ".syncthing.tmp."
)
// An AtomicWriter is an *os.File that writes to a temporary file in the same
// directory as the final path. On successful Close the file is renamed to
// it's final path. Any error on Write or during Close is accumulated and
// returned on Close, so a lazy user can ignore errors until Close.
type AtomicWriter struct {
path string
next *os.File
err error
}
// CreateAtomic is like os.Create, except a temporary file name is used
// instead of the given name. The file is created with secure (0600)
// permissions.
func CreateAtomic(path string) (*AtomicWriter, error) {
// The security of this depends on the tempfile having secure
// permissions, 0600, from the beginning. This is what ioutil.TempFile
// does. We have a test that verifies that that is the case, should this
// ever change in the standard library in the future.
fd, err := ioutil.TempFile(filepath.Dir(path), TempPrefix)
if err != nil {
return nil, err
}
w := &AtomicWriter{
path: path,
next: fd,
}
return w, nil
}
// Write is like io.Writer, but is a no-op on an already failed AtomicWriter.
func (w *AtomicWriter) Write(bs []byte) (int, error) {
if w.err != nil {
return 0, w.err
}
n, err := w.next.Write(bs)
if err != nil {
w.err = err
w.next.Close()
}
return n, err
}
// Close closes the temporary file and renames it to the final path. It is
// invalid to call Write() or Close() after Close().
func (w *AtomicWriter) Close() error {
if w.err != nil {
return w.err
}
// Try to not leave temp file around, but ignore error.
defer os.Remove(w.next.Name())
if err := w.next.Sync(); err != nil {
w.err = err
return err
}
if err := w.next.Close(); err != nil {
w.err = err
return err
}
// Remove the destination file, on Windows only. If it fails, and not due
// to the file not existing, we won't be able to complete the rename
// either. Return this error because it may be more informative. On non-
// Windows we want the atomic rename behavior so we don't attempt remove.
if runtime.GOOS == "windows" {
if err := os.Remove(w.path); err != nil && !os.IsNotExist(err) {
return err
}
}
if err := os.Rename(w.next.Name(), w.path); err != nil {
w.err = err
return err
}
SyncDir(filepath.Dir(w.next.Name()))
// Set w.err to return appropriately for any future operations.
w.err = ErrClosed
return nil
}