From 940966e46148b303e246b352339f5ec29319cbae Mon Sep 17 00:00:00 2001
From: "paritytech-cmd-bot-polkadot-sdk[bot]"
 <179002856+paritytech-cmd-bot-polkadot-sdk[bot]@users.noreply.github.com>
Date: Wed, 11 Dec 2024 13:26:09 +0100
Subject: [PATCH] [stable2409] Backport #6506 (#6658)
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Backport #6506 into `stable2409` from Dinonard.

See the
[documentation](https://github.com/paritytech/polkadot-sdk/blob/master/docs/BACKPORT.md)
on how to use this bot.

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---------

Co-authored-by: Dino Pačandi <3002868+Dinonard@users.noreply.github.com>
Co-authored-by: EgorPopelyaev <egor@parity.io>
---
 prdoc/pr_6506.prdoc                           | 10 +++++
 .../frame/transaction-payment/src/payment.rs  | 17 +++++----
 .../frame/transaction-payment/src/tests.rs    | 37 +++++++++++++++++++
 3 files changed, 56 insertions(+), 8 deletions(-)
 create mode 100644 prdoc/pr_6506.prdoc

diff --git a/prdoc/pr_6506.prdoc b/prdoc/pr_6506.prdoc
new file mode 100644
index 00000000000..7c6164a9959
--- /dev/null
+++ b/prdoc/pr_6506.prdoc
@@ -0,0 +1,10 @@
+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
diff --git a/substrate/frame/transaction-payment/src/payment.rs b/substrate/frame/transaction-payment/src/payment.rs
index 0fe61678290..107a995761f 100644
--- a/substrate/frame/transaction-payment/src/payment.rs
+++ b/substrate/frame/transaction-payment/src/payment.rs
@@ -121,14 +121,15 @@ 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)
diff --git a/substrate/frame/transaction-payment/src/tests.rs b/substrate/frame/transaction-payment/src/tests.rs
index 35d5322a6f3..ae4e136ee58 100644
--- a/substrate/frame/transaction-payment/src/tests.rs
+++ b/substrate/frame/transaction-payment/src/tests.rs
@@ -841,3 +841,40 @@ fn genesis_default_works() {
 		assert_eq!(NextFeeMultiplier::<Runtime>::get(), Multiplier::saturating_from_integer(1));
 	});
 }
+
+#[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.",
+		);
+	});
+}
-- 
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