Incorrect ordering of fees and tips in Transaction Payment

[xx-labs]

It appears that the ordering of transaction fees and tips is incorrect when passing to the on_unbalanceds function:

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

Lines 122 to 124 in 965be9f

	// Call someone else to handle the imbalance (fee and tip separately)
	let imbalances = adjusted_paid.split(tip);
	OU::on_unbalanceds(Some(imbalances.0).into_iter().chain(Some(imbalances.1)));

The split function returns the split amount first, and the remainder second

substrate/frame/balances/src/lib.rs

Lines 1183 to 1189 in 965be9f

	fn split(self, amount: T::Balance) -> (Self, Self) {
	let first = self.0.min(amount);
	let second = self.0 - first;
	mem::forget(self);
	(Self(first), Self(second))
	}

This means that what is passed to on_unbalanceds is [tip, fees] instead of [fees, tip] as the comments specify.

This implementation of DealWithFees that splits fees between block producer and Treasury illustrates the issue, as the comments clearly specify that the first item is believed to be the fees.

substrate/bin/node/runtime/src/lib.rs

Lines 141 to 155 in 965be9f

	pub struct DealWithFees;
	impl OnUnbalanced<NegativeImbalance> for DealWithFees {
	fn on_unbalanceds<B>(mut fees_then_tips: impl Iterator<Item=NegativeImbalance>) {
	if let Some(fees) = fees_then_tips.next() {
	// for fees, 80% to treasury, 20% to author
	let mut split = fees.ration(80, 20);
	if let Some(tips) = fees_then_tips.next() {
	// for tips, if any, 80% to treasury, 20% to author (though this can be anything)
	tips.ration_merge_into(80, 20, &mut split);
	}
	Treasury::on_unbalanced(split.0);
	Author::on_unbalanced(split.1);
	}
	}
	}

This bug can be found in the Polkadot Mainnet.
Here is an example of a transfer without tips:
https://polkadot.js.org/apps/#/explorer/query/0x0218a64f98f808d654f9113cd1feeb70d66e1fb02742b2ee43df33d3b88922e7
The totality of the transaction fee is deposited in the block producer's account.

Here is an example of a transfer with a tip:
https://polkadot.js.org/apps/#/explorer/query/0xf0d74cc96794b6ef3ff3836ff115bff18fefb9132790ae033d5d5014a24cb102
The totality of the transaction fee + 20% of the tip is deposited in the block producer's account.
80% of the tip is deposited into the Treasury account.

[gui1117]

yes your diagnostic is right. This PR should fix it #8860