Zero refund check for FungibleAdapter (#6506)

`FungibleAdapter` will now check if the _refund amount_ is zero before
calling deposit & emitting an event.

Fixes https://github.com/paritytech/polkadot-sdk/issues/6469.

---------

Co-authored-by: GitHub Action <action@github.com>

prdoc/pr_6506.prdoc

+title: Zero refund check for FungibleAdapter
+doc:
+- audience: Runtime User
+  description: |-
+    `FungibleAdapter` will now check if the _refund amount_ is zero before calling deposit & emitting an event.
+
+    Fixes https://github.com/paritytech/polkadot-sdk/issues/6469.
+crates:
+- name: pallet-transaction-payment
+  bump: patch

substrate/frame/transaction-payment/src/payment.rs

@@ -155,14 +155,15 @@ where
 		if let Some(paid) = already_withdrawn {
 			// Calculate how much refund we should return
 			let refund_amount = paid.peek().saturating_sub(corrected_fee);
-			// refund to the the account that paid the fees if it exists. otherwise, don't refind
-			// anything.
-			let refund_imbalance = if F::total_balance(who) > F::Balance::zero() {
-				F::deposit(who, refund_amount, Precision::BestEffort)
-					.unwrap_or_else(|_| Debt::<T::AccountId, F>::zero())
-			} else {
-				Debt::<T::AccountId, F>::zero()
-			};
+			// Refund to the the account that paid the fees if it exists & refund is non-zero.
+			// Otherwise, don't refund anything.
+			let refund_imbalance =
+				if refund_amount > Zero::zero() && F::total_balance(who) > F::Balance::zero() {
+					F::deposit(who, refund_amount, Precision::BestEffort)
+						.unwrap_or_else(|_| Debt::<T::AccountId, F>::zero())
+				} else {
+					Debt::<T::AccountId, F>::zero()
+				};
 			// merge the imbalance caused by paying the fees and refunding parts of it again.
 			let adjusted_paid: Credit<T::AccountId, F> = paid
 				.offset(refund_imbalance)

substrate/frame/transaction-payment/src/tests.rs

@@ -877,3 +877,40 @@ fn no_fee_and_no_weight_for_other_origins() {
 		assert_eq!(post_info.actual_weight, Some(info.call_weight));
 	})
 }
+
+#[test]
+fn fungible_adapter_no_zero_refund_action() {
+	type FungibleAdapterT = payment::FungibleAdapter<Balances, DealWithFees>;
+
+	ExtBuilder::default().balance_factor(10).build().execute_with(|| {
+		System::set_block_number(10);
+
+		let dummy_acc = 1;
+		let (actual_fee, no_tip) = (10, 0);
+		let already_paid = <FungibleAdapterT as OnChargeTransaction<Runtime>>::withdraw_fee(
+			&dummy_acc,
+			CALL,
+			&CALL.get_dispatch_info(),
+			actual_fee,
+			no_tip,
+		).expect("Account must have enough funds.");
+
+		// Correction action with no expected side effect.
+		assert!(<FungibleAdapterT as OnChargeTransaction<Runtime>>::correct_and_deposit_fee(
+			&dummy_acc,
+			&CALL.get_dispatch_info(),
+			&default_post_info(),
+			actual_fee,
+			no_tip,
+			already_paid,
+		).is_ok());
+
+		// Ensure no zero amount deposit event is emitted.
+		let events = System::events();
+		assert!(!events
+			.iter()
+			.any(|record| matches!(record.event, RuntimeEvent::Balances(pallet_balances::Event::Deposit { amount, .. }) if amount.is_zero())),
+    		"No zero amount deposit amount event should be emitted.",
+		);
+	});
+}