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Update SelectChain documentation

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Luke Parker 2022-10-21 05:53:15 -04:00
parent 0a58d66958
commit 0218db0084
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1 changed files with 19 additions and 15 deletions
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34
substrate/consensus/src/select_chain.rs
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@ -1,3 +1,19 @@
// SelectChain, while provided by Substrate and part of PartialComponents, isn't used by Substrate
// It's common between various block-production/finality crates, yet Substrate as a system doesn't
// rely on it, which is good, because its definition is explicitly incompatible with Tendermint
//
// leaves is supposed to return all leaves of the blockchain. While Tendermint maintains that view,
// an honest node will only build on the most recently finalized block, so it is a 'leaf' despite
// having descendants
//
// best_chain will always be this finalized block, yet Substrate explicitly defines it as one of
// the above leaves, which this finalized block is explicitly not included in. Accordingly, we
// can never provide a compatible decision
//
// Since PartialComponents expects it, an implementation which does its best is provided. It panics
// if leaves is called, yet returns the finalized chain tip for best_chain, as that's intended to
// be the header to build upon
use std::{marker::PhantomData, sync::Arc};
use async_trait::async_trait;
@ -25,21 +41,7 @@ impl<B: Block, Be: Backend<B>> TendermintSelectChain<B, Be> {
#[async_trait]
impl<B: Block, Be: Backend<B>> SelectChain<B> for TendermintSelectChain<B, Be> {
async fn leaves(&self) -> Result<Vec<B::Hash>, Error> {
panic!("should never be called")
// Substrate may call this at some point in the future?
// It doesn't appear to do so now, and we have the question of what to do if/when it does
// Either we return the chain tip, which is the true leaf yet breaks the documented definition,
// or we return the actual leaves, when those don't contribute value
//
// Both become risky as best_chain, which is presumably used for block building, is explicitly
// defined as one of these leaves. If it's returning the best chain, the finalized chain tip,
// then it's wrong. The real comment is that this API does not support the Tendermint model
//
// Since it appears the blockchain operations happen on the Backend's leaves, not the
// SelectChain's, leaving this as a panic for now should be optimal
//
// TODO: Triple check this isn't reachable
panic!("Substrate definition of leaves is incompatible with Tendermint")
}
async fn best_chain(&self) -> Result<B::Header, Error> {
@ -47,7 +49,9 @@ impl<B: Block, Be: Backend<B>> SelectChain<B> for TendermintSelectChain<B, Be> {
self
.0
.blockchain()
// There should always be a finalized block
.header(BlockId::Hash(self.0.blockchain().last_finalized().unwrap()))
// There should not be an error in retrieving it and since it's finalized, it should exist
.unwrap()
.unwrap(),
)