sc-consensus-beefy: fix initialization when BEEFY genesis state unavailable (#14752)

When BEEFY voter is initialized from scratch (no aux db persistent data present),
it needs to find BEEFY genesis block and all subsequent Mandatory blocks and
sync justifications for them.

The initialization code was getting active validator sets for these older blocks
from state, but in cases such as 'fast sync', state is unavailable.

This commit adds a fallback initialization mechanism when state is unavailable:
parse header Digests looking for validator set change log deposits.

Signed-off-by: Adrian Catangiu <adrian@parity.io>

substrate/client/consensus/beefy/src/lib.rs

@@ -404,7 +404,7 @@ where
 			let best_beefy = *header.number();
 			// If no session boundaries detected so far, just initialize new rounds here.
 			if sessions.is_empty() {
-				let active_set = expect_validator_set(runtime, header.hash())?;
+				let active_set = expect_validator_set(runtime, backend, &header, beefy_genesis)?;
 				let mut rounds = Rounds::new(best_beefy, active_set);
 				// Mark the round as already finalized.
 				rounds.conclude(best_beefy);
@@ -423,7 +423,7 @@ where
 
 		if *header.number() == beefy_genesis {
 			// We've reached BEEFY genesis, initialize voter here.
-			let genesis_set = expect_validator_set(runtime, header.hash())?;
+			let genesis_set = expect_validator_set(runtime, backend, &header, beefy_genesis)?;
 			info!(
 				target: LOG_TARGET,
 				"🥩 Loading BEEFY voter state from genesis on what appears to be first startup. \
@@ -452,16 +452,8 @@ where
 			sessions.push_front(Rounds::new(*header.number(), active));
 		}
 
-		// Check if state is still available if we move up the chain.
-		let parent_hash = *header.parent_hash();
-		runtime.runtime_api().validator_set(parent_hash).ok().flatten().ok_or_else(|| {
-			let msg = format!("{}. Could not initialize BEEFY voter.", parent_hash);
-			error!(target: LOG_TARGET, "🥩 {}", msg);
-			ClientError::Consensus(sp_consensus::Error::StateUnavailable(msg))
-		})?;
-
 		// Move up the chain.
-		header = blockchain.expect_header(parent_hash)?;
+		header = blockchain.expect_header(*header.parent_hash())?;
 	};
 
 	aux_schema::write_current_version(backend)?;
@@ -512,19 +504,36 @@ where
 	Err(ClientError::Backend(err_msg))
 }
 
-fn expect_validator_set<B, R>(
+fn expect_validator_set<B, BE, R>(
 	runtime: &R,
-	at_hash: B::Hash,
+	backend: &BE,
+	at_header: &B::Header,
+	beefy_genesis: NumberFor<B>,
 ) -> ClientResult<ValidatorSet<AuthorityId>>
 where
 	B: Block,
+	BE: Backend<B>,
 	R: ProvideRuntimeApi<B>,
 	R::Api: BeefyApi<B, AuthorityId>,
 {
 	runtime
 		.runtime_api()
-		.validator_set(at_hash)
+		.validator_set(at_header.hash())
 		.ok()
 		.flatten()
-		.ok_or_else(|| ClientError::Backend("BEEFY pallet expected to be active.".into()))
+		.or_else(|| {
+			// if state unavailable, fallback to walking up the chain looking for the header
+			// Digest emitted when validator set active 'at_header' was enacted.
+			let blockchain = backend.blockchain();
+			let mut header = at_header.clone();
+			while *header.number() >= beefy_genesis {
+				match worker::find_authorities_change::<B>(&header) {
+					Some(active) => return Some(active),
+					// Move up the chain.
+					None => header = blockchain.expect_header(*header.parent_hash()).ok()?,
+				}
+			}
+			None
+		})
+		.ok_or_else(|| ClientError::Backend("Could not find initial validator set".into()))
 }