code: func (f *freelist) reindex() {
  	f.cache = make(map[pgid]bool, len(f.ids))
  	for _, id := range f.ids {
  		f.cache[id] = true
  	}
  	for _, pendingIDs := range f.pending {
  		for _, pendingID := range pendingIDs {
  			f.cache[pendingID] = true
  		}
  	}
  }
docs: // reindex rebuilds the free cache based on available and pending free lists.