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// This file is part of Substrate. // Copyright (C) 2019-2021 Parity Technologies (UK) Ltd. // SPDX-License-Identifier: Apache-2.0 // Licensed under the Apache License, Version 2.0 (the "License"); // you may not use this file except in compliance with the License. // You may obtain a copy of the License at // // http://www.apache.org/licenses/LICENSE-2.0 // // Unless required by applicable law or agreed to in writing, software // distributed under the License is distributed on an "AS IS" BASIS, // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. // See the License for the specific language governing permissions and // limitations under the License. use crate::error::Error; use sp_runtime::traits::{Block as BlockT, NumberFor}; /// The SelectChain trait defines the strategy upon which the head is chosen /// if multiple forks are present for an opaque definition of "best" in the /// specific chain build. /// /// The Strategy can be customized for the two use cases of authoring new blocks /// upon the best chain or which fork to finalize. Unless implemented differently /// by default finalization methods fall back to use authoring, so as a minimum /// `_authoring`-functions must be implemented. /// /// Any particular user must make explicit, however, whether they intend to finalize /// or author through the using the right function call, as these might differ in /// some implementations. /// /// Non-deterministically finalizing chains may only use the `_authoring` functions. #[async_trait::async_trait] pub trait SelectChain<Block: BlockT>: Sync + Send + Clone { /// Get all leaves of the chain, i.e. block hashes that have no children currently. /// Leaves that can never be finalized will not be returned. async fn leaves(&self) -> Result<Vec<<Block as BlockT>::Hash>, Error>; /// Among those `leaves` deterministically pick one chain as the generally /// best chain to author new blocks upon and probably (but not necessarily) /// finalize. async fn best_chain(&self) -> Result<<Block as BlockT>::Header, Error>; /// Get the best descendent of `target_hash` that we should attempt to /// finalize next, if any. It is valid to return the given `target_hash` /// itself if no better descendent exists. async fn finality_target( &self, target_hash: <Block as BlockT>::Hash, _maybe_max_number: Option<NumberFor<Block>>, ) -> Result<Option<<Block as BlockT>::Hash>, Error> { Ok(Some(target_hash)) } }