code: func dbFetchBlockHashBySerializedID(dbTx database.Tx, serializedID []byte) (*chainhash.Hash, error) {
  	idIndex := dbTx.Metadata().Bucket(hashByIDIndexBucketName)
  	hashBytes := idIndex.Get(serializedID)
  	if hashBytes == nil {
  		return nil, errNoBlockIDEntry
  	}
  
  	var hash chainhash.Hash
  	copy(hash[:], hashBytes)
  	return &hash, nil
  }
docs: // dbFetchBlockHashBySerializedID uses an existing database transaction to
  // retrieve the hash for the provided serialized block id from the index.