Method getPageSizeFromSecondMeta

db.go:382–417

getPageSizeFromSecondMeta reads the pageSize from the second meta page

()

// getPageSizeFromSecondMeta reads the pageSize from the second meta page
func (db *DB) getPageSizeFromSecondMeta() (int, bool, error) {
	var (
		fileSize    int64
		metaCanRead bool
	)

	// get the db file size
	if info, err := db.file.Stat(); err != nil {
		return 0, metaCanRead, err
	} else {
		fileSize = info.Size()
	}

	// We need to read the second meta page, so we should skip the first page;
	// but we don't know the exact page size yet, it's chicken & egg problem.
	// The solution is to try all the possible page sizes, which starts from 1KB
	// and until 16MB (1024<<14) or the end of the db file
	//
	// TODO: should we support larger page size?
	for i := 0; i <= 14; i++ {
		var buf [0x1000]byte
		var pos int64 = 1024 << uint(i)
		if pos >= fileSize-1024 {
			break
		}
		bw, err := db.file.ReadAt(buf[:], pos)
		if (err == nil && bw == len(buf)) || (err == io.EOF && int64(bw) == (fileSize-pos)) {
			metaCanRead = true
			if m := db.pageInBuffer(buf[:], 0).Meta(); m.Validate() == nil {
				return int(m.PageSize()), metaCanRead, nil
			}
		}
	}

	return 0, metaCanRead, berrors.ErrInvalid
}

// loadFreelist reads the freelist if it is synced, or reconstructs it
// by scanning the DB if it is not synced. It assumes there are no