// Open opens the WAL at the given snap.
// The snap SHOULD have been previously saved to the WAL, or the following
// ReadAll will fail.
// The returned WAL is ready to read and the first record will be the one after
// the given snap. The WAL cannot be appended to before reading out all of its
// previous records.
func Open(dirpath string, snap walpb.Snapshot) (*WAL, error) {
w, err := openAtIndex(dirpath, snap, true)
if err != nil {
return nil, err
}
if w.dirFile, err = fileutil.OpenDir(w.dir); err != nil {
return nil, err
}
return w, nil
}
func (w *WAL) renameWal(tmpdirpath string) (*WAL, error) {
// On non-Windows platforms, hold the lock while renaming. Releasing
// the lock and trying to reacquire it quickly can be flaky because
// it's possible the process will fork to spawn a process while this is
// happening. The fds are set up as close-on-exec by the Go runtime,
// but there is a window between the fork and the exec where another
// process holds the lock.
if err := os.RemoveAll(w.dir); err != nil {
return nil, err
}
if err := os.Rename(tmpdirpath, w.dir); err != nil {
return nil, err
}
w.fp = newFilePipeline(w.dir, SegmentSizeBytes)
df, err := fileutil.OpenDir(w.dir)
w.dirFile = df
return w, err
}
// Create creates a WAL ready for appending records. The given metadata is
// recorded at the head of each WAL file, and can be retrieved with ReadAll.
func Create(dirpath string, metadata []byte) (*WAL, error) {
if Exist(dirpath) {
return nil, os.ErrExist
}
// keep temporary wal directory so WAL initialization appears atomic
tmpdirpath := path.Clean(dirpath) + ".tmp"
if fileutil.Exist(tmpdirpath) {
if err := os.RemoveAll(tmpdirpath); err != nil {
return nil, err
}
}
if err := fileutil.CreateDirAll(tmpdirpath); err != nil {
return nil, err
}
p := path.Join(tmpdirpath, walName(0, 0))
f, err := fileutil.LockFile(p, os.O_WRONLY|os.O_CREATE, fileutil.PrivateFileMode)
if err != nil {
return nil, err
}
if _, err = f.Seek(0, os.SEEK_END); err != nil {
return nil, err
}
if err = fileutil.Preallocate(f.File, SegmentSizeBytes, true); err != nil {
return nil, err
}
w := &WAL{
dir: dirpath,
metadata: metadata,
encoder: newEncoder(f, 0),
}
w.locks = append(w.locks, f)
if err = w.saveCrc(0); err != nil {
return nil, err
}
if err = w.encoder.encode(&walpb.Record{Type: metadataType, Data: metadata}); err != nil {
return nil, err
}
if err = w.SaveSnapshot(walpb.Snapshot{}); err != nil {
return nil, err
}
if w, err = w.renameWal(tmpdirpath); err != nil {
return nil, err
}
// directory was renamed; sync parent dir to persist rename
pdir, perr := fileutil.OpenDir(path.Dir(w.dir))
if perr != nil {
return nil, perr
}
if perr = fileutil.Fsync(pdir); perr != nil {
return nil, perr
}
if perr = pdir.Close(); err != nil {
return nil, perr
}
return w, nil
}