59b1b0e1dc
So, in a funny plot twist, it turns out that WriteFile in Go 1.13 doesn't actually set the read only bit on Windows when called with permissions 0444 so my test was broken. With an improved test it turns out that Rename does not, in fact, overwrite a read-only file on Windows. This adds a fix for that. (Rename might get improved in Go 1.15...)
120 lines
3.1 KiB
Go
120 lines
3.1 KiB
Go
// Copyright (C) 2015 The Syncthing Authors.
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//
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// This Source Code Form is subject to the terms of the Mozilla Public
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// License, v. 2.0. If a copy of the MPL was not distributed with this file,
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// You can obtain one at https://mozilla.org/MPL/2.0/.
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package osutil
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import (
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"errors"
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"path/filepath"
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"runtime"
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"github.com/syncthing/syncthing/lib/fs"
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)
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var (
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ErrClosed = errors.New("write to closed writer")
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TempPrefix = ".syncthing.tmp."
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)
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// An AtomicWriter is an *os.File that writes to a temporary file in the same
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// directory as the final path. On successful Close the file is renamed to
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// its final path. Any error on Write or during Close is accumulated and
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// returned on Close, so a lazy user can ignore errors until Close.
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type AtomicWriter struct {
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path string
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next fs.File
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fs fs.Filesystem
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err error
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}
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// CreateAtomic is like os.Create, except a temporary file name is used
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// instead of the given name. The file is created with secure (0600)
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// permissions.
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func CreateAtomic(path string) (*AtomicWriter, error) {
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fs := fs.NewFilesystem(fs.FilesystemTypeBasic, filepath.Dir(path))
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return CreateAtomicFilesystem(fs, filepath.Base(path))
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}
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// CreateAtomicFilesystem is like os.Create, except a temporary file name is used
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// instead of the given name. The file is created with secure (0600)
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// permissions.
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func CreateAtomicFilesystem(filesystem fs.Filesystem, path string) (*AtomicWriter, error) {
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// The security of this depends on the tempfile having secure
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// permissions, 0600, from the beginning. This is what ioutil.TempFile
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// does. We have a test that verifies that that is the case, should this
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// ever change in the standard library in the future.
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fd, err := TempFile(filesystem, filepath.Dir(path), TempPrefix)
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if err != nil {
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return nil, err
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}
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w := &AtomicWriter{
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path: path,
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next: fd,
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fs: filesystem,
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}
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return w, nil
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}
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// Write is like io.Writer, but is a no-op on an already failed AtomicWriter.
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func (w *AtomicWriter) Write(bs []byte) (int, error) {
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if w.err != nil {
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return 0, w.err
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}
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n, err := w.next.Write(bs)
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if err != nil {
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w.err = err
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w.next.Close()
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}
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return n, err
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}
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// Close closes the temporary file and renames it to the final path. It is
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// invalid to call Write() or Close() after Close().
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func (w *AtomicWriter) Close() error {
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if w.err != nil {
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return w.err
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}
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// Try to not leave temp file around, but ignore error.
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defer w.fs.Remove(w.next.Name())
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if err := w.next.Sync(); err != nil {
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w.err = err
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return err
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}
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if err := w.next.Close(); err != nil {
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w.err = err
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return err
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}
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err := w.fs.Rename(w.next.Name(), w.path)
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if runtime.GOOS == "windows" && fs.IsPermission(err) {
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// On Windows, we might not be allowed to rename over the file
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// because it's read-only. Get us some write permissions and try
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// again.
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_ = w.fs.Chmod(w.path, 0644)
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err = w.fs.Rename(w.next.Name(), w.path)
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}
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if err != nil {
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w.err = err
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return err
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}
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// fsync the directory too
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if fd, err := w.fs.Open(filepath.Dir(w.next.Name())); err == nil {
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fd.Sync()
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fd.Close()
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}
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// Set w.err to return appropriately for any future operations.
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w.err = ErrClosed
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return nil
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}
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