129 lines
3.4 KiB
Go
129 lines
3.4 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 os.CreateTemp
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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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// sync() isn't supported everywhere, our best effort will suffice.
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_ = w.next.Sync()
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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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info, infoErr := w.fs.Lstat(w.path)
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if infoErr != nil && !fs.IsNotExist(infoErr) {
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w.err = infoErr
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return infoErr
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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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if infoErr == nil {
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if err := w.fs.Chmod(w.path, info.Mode()); err != nil {
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w.err = err
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return err
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}
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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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