Allow for 0 existential deposit in benchmarks for pallet_staking, pallet_session, and pallet_balances (#4346)

This PR ensures non-zero values are available in benchmarks for
`pallet_staking`, `pallet_session`, and `pallet_balances` where required
for them to run.

This small change makes it possible to run the benchmarks for
`pallet_staking`, `pallet_session`, and `pallet_balances` in a runtime
for which existential deposit is set to 0.

The benchmarks for `pallet_staking` and `pallet_session` will still fail
in runtimes that use `U128CurrencyToVote`, but that is easy to work
around by creating a new `CurrencyToVote` implementation for
benchmarking.

The changes are implemented by checking if existential deposit equals 0
and using 1 if so.

---------

Co-authored-by: command-bot <>
Co-authored-by: Bastian Köcher <[email protected]>

Author: krisbitney

prdoc/pr_4346.prdoc

@@ -0,0 +1,17 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: Allow for 0 existential deposit in benchmarks for pallet_staking, pallet_session, and pallet_balances
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Changes were made to benchmarks for `pallet_staking`, `pallet_session`, and `pallet-balances` to accommodate runtimes with 0 existential deposit. This should not affect the vast majority of runtimes. For runtimes with 0 existential deposit, the benchmarks for `pallet_staking` and `pallet_session` will still fail when using `U128CurrencyToVote` in the `pallet-staking` config; developers can use or write another `CurrencyToVote` implementation for benchmarking to work around this.
+
+crates:
+  - name: pallet-staking
+    bump: patch
+  - name: pallet-session-benchmarking
+    bump: patch
+  - name: pallet-balances
+    bump: patch

substrate/frame/balances/src/benchmarking.rs

@@ -44,7 +44,7 @@ mod benchmarks {
 		let caller = whitelisted_caller();
 
 		// Give some multiple of the existential deposit
-		let balance = existential_deposit.saturating_mul(ED_MULTIPLIER.into());
+		let balance = existential_deposit.saturating_mul(ED_MULTIPLIER.into()).max(1u32.into());
 		let _ = <Balances<T, I> as Currency<_>>::make_free_balance_be(&caller, balance);
 
 		// Transfer `e - 1` existential deposits + 1 unit, which guarantees to create one account,

substrate/frame/staking/src/testing_utils.rs

@@ -77,7 +77,8 @@ pub fn create_stash_controller<T: Config>(
 	destination: RewardDestination<T::AccountId>,
 ) -> Result<(T::AccountId, T::AccountId), &'static str> {
 	let staker = create_funded_user::<T>("stash", n, balance_factor);
-	let amount = T::Currency::minimum_balance() * (balance_factor / 10).max(1).into();
+	let amount =
+		T::Currency::minimum_balance().max(1u64.into()) * (balance_factor / 10).max(1).into();
 	Staking::<T>::bond(RawOrigin::Signed(staker.clone()).into(), amount, destination)?;
 	Ok((staker.clone(), staker))
 }