Better Handle Dead Accounts in Balances (#7843)

* Don't mutate storage when account is dead and should stay dead

* cargo run --release --features=runtime-benchmarks --manifest-path=bin/node/cli/Cargo.toml -- benchmark --chain=dev --steps=50 --repeat=20 --pallet=pallet_balances --extrinsic=* --execution=wasm --wasm-execution=compiled --heap-pages=4096 --output=./frame/balances/src/weights.rs --template=./.maintain/frame-weight-template.hbs

* more concrete storage noop

Co-authored-by: Parity Benchmarking Bot <admin@parity.io>

substrate/frame/balances/src/tests.rs

@@ -40,7 +40,7 @@ macro_rules! decl_tests {
 		use crate::*;
 		use sp_runtime::{FixedPointNumber, traits::{SignedExtension, BadOrigin}};
 		use frame_support::{
-			assert_noop, assert_ok, assert_err,
+			assert_noop, assert_storage_noop, assert_ok, assert_err,
 			traits::{
 				LockableCurrency, LockIdentifier, WithdrawReasons,
 				Currency, ReservableCurrency, ExistenceRequirement::AllowDeath, StoredMap
@@ -796,5 +796,28 @@ macro_rules! decl_tests {
 					);
 				});
 		}
+
+		#[test]
+		fn operations_on_dead_account_should_not_change_state() {
+			// These functions all use `mutate_account` which may introduce a storage change when
+			// the account never existed to begin with, and shouldn't exist in the end.
+			<$ext_builder>::default()
+				.existential_deposit(0)
+				.build()
+				.execute_with(|| {
+					assert!(!<Test as Config>::AccountStore::is_explicit(&1337));
+
+					// Unreserve
+					assert_storage_noop!(assert_eq!(Balances::unreserve(&1337, 42), 42));
+					// Reserve
+					assert_noop!(Balances::reserve(&1337, 42), Error::<Test, _>::InsufficientBalance);
+					// Slash Reserve
+					assert_storage_noop!(assert_eq!(Balances::slash_reserved(&1337, 42).1, 42));
+					// Repatriate Reserve
+					assert_noop!(Balances::repatriate_reserved(&1337, &1338, 42, Status::Free), Error::<Test, _>::DeadAccount);
+					// Slash
+					assert_storage_noop!(assert_eq!(Balances::slash(&1337, 42).1, 42));
+				});
+		}
 	}
 }