yonlu
4d64b5dcfe
- Callback API: AtomicWrite(logger, targetPath, func(tmp *os.File) error) - Deterministic .yj-tmp suffix for orphan cleanup - Permission preservation (stat target, chmod temp before rename) - Cross-device rejection via ErrCrossDevice sentinel - Orphan cleanup on startup of each write operation - Sync for durability before rename
102 lines
3.2 KiB
Go
102 lines
3.2 KiB
Go
// Package fileutil provides file system utilities for safe file operations.
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package fileutil
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import (
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"errors"
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"fmt"
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"io/fs"
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"log/slog"
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"os"
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"syscall"
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)
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// ErrCrossDevice is returned when an atomic rename fails because the temp file
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// and target reside on different filesystems. There is no copy-then-delete
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// fallback — callers must ensure both paths share the same mount point.
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var ErrCrossDevice = errors.New("atomic write: cross-device rename not supported")
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// tmpSuffix is the deterministic extension appended to the target path for the
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// temporary file. A fixed suffix (rather than a random one) makes orphan
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// cleanup straightforward.
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const tmpSuffix = ".yj-tmp"
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// AtomicWrite writes to targetPath atomically. It creates a temporary file in
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// the same directory as targetPath (with a ".yj-tmp" suffix), passes it to fn
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// for writing, syncs and closes the file, then renames it over targetPath. If
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// the target already exists its permission bits are preserved; otherwise the
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// new file receives mode 0644.
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//
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// Before writing, any orphaned .yj-tmp file from a previous interrupted
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// operation is removed. If removal fails it is logged at debug level and the
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// write proceeds.
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//
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// On any error after the temp file is created the temp file is removed.
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func AtomicWrite(logger *slog.Logger, targetPath string, fn func(tmp *os.File) error) (err error) {
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tmpPath := targetPath + tmpSuffix
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// --- 1. Clean orphaned temp file from a previous crash ----------------
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if _, statErr := os.Lstat(tmpPath); statErr == nil {
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if rmErr := os.Remove(tmpPath); rmErr != nil {
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logger.Debug("could not remove orphaned temp file",
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slog.String("path", tmpPath),
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slog.String("err", rmErr.Error()),
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)
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}
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}
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// --- 2. Read target permissions (if target exists) --------------------
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mode := fs.FileMode(0o644) //nolint:mnd // default for new files
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if info, statErr := os.Stat(targetPath); statErr == nil {
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mode = info.Mode().Perm()
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}
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// --- 3. Create temp file ---------------------------------------------
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tmp, createErr := os.Create(tmpPath)
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if createErr != nil {
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return fmt.Errorf("atomic write: create temp: %w", createErr)
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}
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// Ensure temp file is removed on any failure path after creation.
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defer func() {
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if err != nil {
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_ = os.Remove(tmpPath)
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}
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}()
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// --- 4. Caller writes data -------------------------------------------
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if err = fn(tmp); err != nil {
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_ = tmp.Close()
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return fmt.Errorf("atomic write: callback: %w", err)
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}
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// --- 5. Sync for durability ------------------------------------------
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if err = tmp.Sync(); err != nil {
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_ = tmp.Close()
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return fmt.Errorf("atomic write: sync: %w", err)
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}
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// --- 6. Close --------------------------------------------------------
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if err = tmp.Close(); err != nil {
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return fmt.Errorf("atomic write: close: %w", err)
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}
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// --- 7. Preserve permissions -----------------------------------------
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if err = os.Chmod(tmpPath, mode); err != nil {
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return fmt.Errorf("atomic write: chmod: %w", err)
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}
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// --- 8. Atomic rename ------------------------------------------------
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if err = os.Rename(tmpPath, targetPath); err != nil {
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if errors.Is(err, syscall.EXDEV) {
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return fmt.Errorf("%w: %w", ErrCrossDevice, err)
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}
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return fmt.Errorf("atomic write: rename: %w", err)
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}
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return nil
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}
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