Check that validator set is not empty at genesis (#3083)

* Check that the returned validator set is not empty

* Move assert to session module

* Clean up

substrate/node/runtime/src/lib.rs

@@ -411,8 +411,8 @@ construct_runtime!(
 		Authorship: authorship::{Module, Call, Storage},
 		Indices: indices,
 		Balances: balances,
-		Session: session::{Module, Call, Storage, Event, Config<T>},
 		Staking: staking::{default, OfflineWorker},
+		Session: session::{Module, Call, Storage, Event, Config<T>},
 		Democracy: democracy::{Module, Call, Storage, Config, Event<T>},
 		Council: collective::<Instance1>::{Module, Call, Storage, Origin<T>, Event<T>, Config<T>},
 		TechnicalCommittee: collective::<Instance2>::{Module, Call, Storage, Origin<T>, Event<T>, Config<T>},

substrate/srml/session/src/lib.rs

@@ -316,6 +316,8 @@ decl_storage! {
 				let initial_validators = T::SelectInitialValidators::select_initial_validators()
 					.unwrap_or_else(|| config.keys.iter().map(|(ref v, _)| v.clone()).collect());
 
+				assert!(!initial_validators.is_empty(), "Empty validator set in genesis block!");
+
 				let queued_keys: Vec<_> = initial_validators
 					.iter()
 					.cloned()

substrate/srml/staking/src/lib.rs

@@ -599,7 +599,10 @@ decl_storage! {
 		| {
 			with_storage(storage, || {
 				for &(ref stash, ref controller, balance, ref status) in &config.stakers {
-					assert!(T::Currency::free_balance(&stash) >= balance);
+					assert!(
+						T::Currency::free_balance(&stash) >= balance,
+						"Stash does not have enough balance to bond."
+					);
 					let _ = <Module<T>>::bond(
 						T::Origin::from(Some(stash.clone()).into()),
 						T::Lookup::unlookup(controller.clone()),