forked from toolshed/abra
246 lines
6.9 KiB
Go
246 lines
6.9 KiB
Go
// Package atomicwriter provides utilities to perform atomic writes to a
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// file or set of files.
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package atomicwriter
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import (
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"errors"
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"fmt"
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"io"
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"os"
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"path/filepath"
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"syscall"
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"github.com/moby/sys/sequential"
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)
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func validateDestination(fileName string) error {
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if fileName == "" {
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return errors.New("file name is empty")
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}
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if dir := filepath.Dir(fileName); dir != "" && dir != "." && dir != ".." {
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di, err := os.Stat(dir)
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if err != nil {
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return fmt.Errorf("invalid output path: %w", err)
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}
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if !di.IsDir() {
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return fmt.Errorf("invalid output path: %w", &os.PathError{Op: "stat", Path: dir, Err: syscall.ENOTDIR})
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}
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}
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// Deliberately using Lstat here to match the behavior of [os.Rename],
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// which is used when completing the write and does not resolve symlinks.
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fi, err := os.Lstat(fileName)
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if err != nil {
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if os.IsNotExist(err) {
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return nil
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}
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return fmt.Errorf("failed to stat output path: %w", err)
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}
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switch mode := fi.Mode(); {
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case mode.IsRegular():
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return nil // Regular file
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case mode&os.ModeDir != 0:
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return errors.New("cannot write to a directory")
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case mode&os.ModeSymlink != 0:
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return errors.New("cannot write to a symbolic link directly")
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case mode&os.ModeNamedPipe != 0:
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return errors.New("cannot write to a named pipe (FIFO)")
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case mode&os.ModeSocket != 0:
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return errors.New("cannot write to a socket")
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case mode&os.ModeDevice != 0:
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if mode&os.ModeCharDevice != 0 {
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return errors.New("cannot write to a character device file")
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}
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return errors.New("cannot write to a block device file")
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case mode&os.ModeSetuid != 0:
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return errors.New("cannot write to a setuid file")
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case mode&os.ModeSetgid != 0:
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return errors.New("cannot write to a setgid file")
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case mode&os.ModeSticky != 0:
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return errors.New("cannot write to a sticky bit file")
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default:
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return fmt.Errorf("unknown file mode: %[1]s (%#[1]o)", mode)
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}
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}
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// New returns a WriteCloser so that writing to it writes to a
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// temporary file and closing it atomically changes the temporary file to
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// destination path. Writing and closing concurrently is not allowed.
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// NOTE: umask is not considered for the file's permissions.
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//
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// New uses [sequential.CreateTemp] to use sequential file access on Windows,
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// avoiding depleting the standby list un-necessarily. On Linux, this equates to
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// a regular [os.CreateTemp]. Refer to the [Win32 API documentation] for details
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// on sequential file access.
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//
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// [Win32 API documentation]: https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea#FILE_FLAG_SEQUENTIAL_SCAN
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func New(filename string, perm os.FileMode) (io.WriteCloser, error) {
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if err := validateDestination(filename); err != nil {
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return nil, err
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}
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abspath, err := filepath.Abs(filename)
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if err != nil {
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return nil, err
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}
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f, err := sequential.CreateTemp(filepath.Dir(abspath), ".tmp-"+filepath.Base(filename))
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if err != nil {
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return nil, err
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}
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return &atomicFileWriter{
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f: f,
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fn: abspath,
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perm: perm,
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}, nil
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}
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// WriteFile atomically writes data to a file named by filename and with the
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// specified permission bits. The given filename is created if it does not exist,
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// but the destination directory must exist. It can be used as a drop-in replacement
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// for [os.WriteFile], but currently does not allow the destination path to be
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// a symlink. WriteFile is implemented using [New] for its implementation.
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//
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// NOTE: umask is not considered for the file's permissions.
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func WriteFile(filename string, data []byte, perm os.FileMode) error {
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f, err := New(filename, perm)
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if err != nil {
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return err
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}
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n, err := f.Write(data)
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if err == nil && n < len(data) {
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err = io.ErrShortWrite
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f.(*atomicFileWriter).writeErr = err
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}
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if err1 := f.Close(); err == nil {
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err = err1
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}
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return err
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}
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type atomicFileWriter struct {
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f *os.File
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fn string
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writeErr error
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written bool
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perm os.FileMode
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}
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func (w *atomicFileWriter) Write(dt []byte) (int, error) {
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w.written = true
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n, err := w.f.Write(dt)
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if err != nil {
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w.writeErr = err
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}
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return n, err
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}
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func (w *atomicFileWriter) Close() (retErr error) {
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defer func() {
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if err := os.Remove(w.f.Name()); !errors.Is(err, os.ErrNotExist) && retErr == nil {
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retErr = err
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}
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}()
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if err := w.f.Sync(); err != nil {
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_ = w.f.Close()
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return err
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}
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if err := w.f.Close(); err != nil {
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return err
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}
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if err := os.Chmod(w.f.Name(), w.perm); err != nil {
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return err
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}
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if w.writeErr == nil && w.written {
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return os.Rename(w.f.Name(), w.fn)
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}
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return nil
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}
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// WriteSet is used to atomically write a set
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// of files and ensure they are visible at the same time.
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// Must be committed to a new directory.
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type WriteSet struct {
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root string
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}
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// NewWriteSet creates a new atomic write set to
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// atomically create a set of files. The given directory
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// is used as the base directory for storing files before
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// commit. If no temporary directory is given the system
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// default is used.
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func NewWriteSet(tmpDir string) (*WriteSet, error) {
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td, err := os.MkdirTemp(tmpDir, "write-set-")
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if err != nil {
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return nil, err
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}
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return &WriteSet{
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root: td,
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}, nil
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}
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// WriteFile writes a file to the set, guaranteeing the file
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// has been synced.
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func (ws *WriteSet) WriteFile(filename string, data []byte, perm os.FileMode) error {
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f, err := ws.FileWriter(filename, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
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if err != nil {
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return err
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}
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n, err := f.Write(data)
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if err == nil && n < len(data) {
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err = io.ErrShortWrite
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}
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if err1 := f.Close(); err == nil {
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err = err1
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}
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return err
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}
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type syncFileCloser struct {
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*os.File
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}
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func (w syncFileCloser) Close() error {
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err := w.File.Sync()
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if err1 := w.File.Close(); err == nil {
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err = err1
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}
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return err
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}
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// FileWriter opens a file writer inside the set. The file
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// should be synced and closed before calling commit.
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//
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// FileWriter uses [sequential.OpenFile] to use sequential file access on Windows,
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// avoiding depleting the standby list un-necessarily. On Linux, this equates to
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// a regular [os.OpenFile]. Refer to the [Win32 API documentation] for details
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// on sequential file access.
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//
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// [Win32 API documentation]: https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea#FILE_FLAG_SEQUENTIAL_SCAN
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func (ws *WriteSet) FileWriter(name string, flag int, perm os.FileMode) (io.WriteCloser, error) {
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f, err := sequential.OpenFile(filepath.Join(ws.root, name), flag, perm)
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if err != nil {
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return nil, err
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}
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return syncFileCloser{f}, nil
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}
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// Cancel cancels the set and removes all temporary data
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// created in the set.
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func (ws *WriteSet) Cancel() error {
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return os.RemoveAll(ws.root)
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}
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// Commit moves all created files to the target directory. The
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// target directory must not exist and the parent of the target
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// directory must exist.
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func (ws *WriteSet) Commit(target string) error {
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return os.Rename(ws.root, target)
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}
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// String returns the location the set is writing to.
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func (ws *WriteSet) String() string {
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return ws.root
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}
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