aura, babe: don't allow disabled validators to build blocks (#9414)

* frame-support: add trait for checking disabled validators

* pallet-session: implement DisabledValidators trait

* pallet-babe: check for disabled validators

* pallet-babe: add test for disabled validators

* pallet-aura: check for disabled validators

* pallet-aura: add test for disabled validators

* runtime: fix missing DisableValidator

* test-runtime: add missing DisabledValidators

* frame-support: clean up doc

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/frame/aura/src/lib.rs

@@ -39,7 +39,7 @@
 
 use codec::{Decode, Encode};
 use frame_support::{
-	traits::{FindAuthor, Get, OnTimestampSet, OneSessionHandler},
+	traits::{DisabledValidators, FindAuthor, Get, OnTimestampSet, OneSessionHandler},
 	ConsensusEngineId, Parameter,
 };
 use sp_consensus_aura::{AuthorityIndex, ConsensusLog, Slot, AURA_ENGINE_ID};
@@ -70,6 +70,11 @@ pub mod pallet {
 			+ RuntimeAppPublic
 			+ Default
 			+ MaybeSerializeDeserialize;
+
+		/// A way to check whether a given validator is disabled and should not be authoring blocks.
+		/// Blocks authored by a disabled validator will lead to a panic as part of this module's
+		/// initialization.
+		type DisabledValidators: DisabledValidators;
 	}
 
 	#[pallet::pallet]
@@ -84,6 +89,16 @@ pub mod pallet {
 				assert!(current_slot < new_slot, "Slot must increase");
 				CurrentSlot::<T>::put(new_slot);
 
+				if let Some(n_authorities) = <Authorities<T>>::decode_len() {
+					let authority_index = *new_slot % n_authorities as u64;
+					if T::DisabledValidators::is_disabled(authority_index as u32) {
+						panic!(
+							"Validator with index {:?} is disabled and should not be attempting to author blocks.",
+							authority_index,
+						);
+					}
+				}
+
 				// TODO [#3398] Generate offence report for all authorities that skipped their slots.
 
 				T::DbWeight::get().reads_writes(2, 1)