Tendermint SelectChain

This is incompatible with Substrate's expectations, yet should be valid 
for ours

substrate/consensus/src/lib.rs

@@ -16,6 +16,9 @@ mod weights;
 mod import_queue;
 use import_queue::TendermintImportQueue;
 
+mod select_chain;
+pub use select_chain::TendermintSelectChain;
+
 pub struct ExecutorDispatch;
 impl NativeExecutionDispatch for ExecutorDispatch {
   #[cfg(feature = "runtime-benchmarks")]

substrate/consensus/src/select_chain.rs

@@ -0,0 +1,55 @@
+use std::{marker::PhantomData, sync::Arc};
+
+use async_trait::async_trait;
+
+use sp_api::BlockId;
+use sp_runtime::traits::Block;
+use sp_blockchain::{HeaderBackend, Backend as BlockchainBackend};
+use sc_client_api::Backend;
+use sp_consensus::{Error, SelectChain};
+
+pub struct TendermintSelectChain<B: Block, Be: Backend<B>>(Arc<Be>, PhantomData<B>);
+
+impl<B: Block, Be: Backend<B>> Clone for TendermintSelectChain<B, Be> {
+  fn clone(&self) -> Self {
+    TendermintSelectChain(self.0.clone(), PhantomData)
+  }
+}
+
+impl<B: Block, Be: Backend<B>> TendermintSelectChain<B, Be> {
+  pub fn new(backend: Arc<Be>) -> TendermintSelectChain<B, Be> {
+    TendermintSelectChain(backend, PhantomData)
+  }
+}
+
+#[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
+  }
+
+  async fn best_chain(&self) -> Result<B::Header, Error> {
+    Ok(
+      self
+        .0
+        .blockchain()
+        .header(BlockId::Hash(self.0.blockchain().last_finalized().unwrap()))
+        .unwrap()
+        .unwrap(),
+    )
+  }
+}