code: func ReadHeadFastBlockHash(db ethdb.Reader) common.Hash {
  	data, _ := db.Get(headFastBlockKey)
  	if len(data) == 0 {
  		return common.Hash{}
  	}
  	return common.BytesToHash(data)
  }
docs: // ReadHeadFastBlockHash retrieves the hash of the current fast-sync head block.