Introduce XcmFeesToAccount fee manager (#1234)

Combination of paritytech/polkadot#7005, its addon PR
paritytech/polkadot#7585 and its companion paritytech/cumulus#2433.

This PR introduces a new XcmFeesToAccount struct which implements the
`FeeManager` trait, and assigns this struct as the `FeeManager` in the
XCM config for all runtimes.

The struct simply deposits all fees handled by the XCM executor to a
specified account. In all runtimes, the specified account is configured
as the treasury account.

XCM __delivery__ fees are now being introduced (unless the root origin
is sending a message to a system parachain on behalf of the originating
chain).

# Note for reviewers

Most file changes are tests that had to be modified to account for the
new fees.
Main changes are in:
- cumulus/pallets/xcmp-queue/src/lib.rs <- To make it track the delivery
fees exponential factor
- polkadot/xcm/xcm-builder/src/fee_handling.rs <- Added. Has the
FeeManager implementation
- All runtime xcm_config files <- To add the FeeManager to the XCM
configuration

# Important note

After this change, instructions that create and send a new XCM (Query*,
Report*, ExportMessage, InitiateReserveWithdraw, InitiateTeleport,
DepositReserveAsset, TransferReserveAsset, LockAsset and RequestUnlock)
will require the corresponding origin account in the origin register to
pay for transport delivery fees, and the onward message will fail to be
sent if the origin account does not have the required amount. This
delivery fee is on top of what we already collect as tx fees in
pallet-xcm and XCM BuyExecution fees!

Wallet UIs that want to expose the new delivery fee can do so using the
formula:

```
delivery_fee_factor * (base_fee + encoded_msg_len * per_byte_fee)
```

where the delivery fee factor can be obtained from the corresponding
pallet based on which transport you are using (UMP, HRMP or bridges),
the base fee is a constant, the encoded message length from the message
itself and the per byte fee is the same as the configured per byte fee
for txs (i.e. `TransactionByteFee`).

---------

Co-authored-by: Branislav Kontur <bkontur@gmail.com>
Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>
Co-authored-by: Giles Cope <gilescope@gmail.com>
Co-authored-by: command-bot <>
Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

cumulus/pallets/xcmp-queue/src/tests.rs

@@ -324,13 +324,14 @@ fn xcmp_queue_consumes_dest_and_msg_on_ok_validate() {
 	let dest = (Parent, X1(Parachain(5555)));
 	let mut dest_wrapper = Some(dest.into());
 	let mut msg_wrapper = Some(message.clone());
-	assert!(<XcmpQueue as SendXcm>::validate(&mut dest_wrapper, &mut msg_wrapper).is_ok());
-
-	// check wrapper were consumed
-	assert_eq!(None, dest_wrapper.take());
-	assert_eq!(None, msg_wrapper.take());
 
 	new_test_ext().execute_with(|| {
+		assert!(<XcmpQueue as SendXcm>::validate(&mut dest_wrapper, &mut msg_wrapper).is_ok());
+
+		// check wrapper were consumed
+		assert_eq!(None, dest_wrapper.take());
+		assert_eq!(None, msg_wrapper.take());
+
 		// another try with router chain with asserting sender
 		assert_eq!(
 			Err(SendError::Transport("NoChannel")),
@@ -370,3 +371,74 @@ fn xcmp_queue_send_xcm_works() {
 			.any(|(para_id, _)| para_id == &sibling_para_id));
 	})
 }
+
+#[test]
+fn verify_fee_factor_increase_and_decrease() {
+	use cumulus_primitives_core::AbridgedHrmpChannel;
+	use sp_runtime::FixedU128;
+
+	let sibling_para_id = ParaId::from(12345);
+	let destination = (Parent, Parachain(sibling_para_id.into())).into();
+	let xcm = Xcm(vec![ClearOrigin; 100]);
+	let versioned_xcm = VersionedXcm::from(xcm.clone());
+	let mut xcmp_message = XcmpMessageFormat::ConcatenatedVersionedXcm.encode();
+	xcmp_message.extend(versioned_xcm.encode());
+
+	new_test_ext().execute_with(|| {
+		let initial = InitialFactor::get();
+		assert_eq!(DeliveryFeeFactor::<Test>::get(sibling_para_id), initial);
+
+		// Open channel so messages can actually be sent
+		ParachainSystem::open_custom_outbound_hrmp_channel_for_benchmarks_or_tests(
+			sibling_para_id,
+			AbridgedHrmpChannel {
+				max_capacity: 10,
+				max_total_size: 1000,
+				max_message_size: 104,
+				msg_count: 0,
+				total_size: 0,
+				mqc_head: None,
+			},
+		);
+
+		// Fee factor is only increased in `send_fragment`, which is called by `send_xcm`.
+		// When queue is not congested, fee factor doesn't change.
+		assert_ok!(send_xcm::<XcmpQueue>(destination, xcm.clone())); // Size 104
+		assert_ok!(send_xcm::<XcmpQueue>(destination, xcm.clone())); // Size 208
+		assert_ok!(send_xcm::<XcmpQueue>(destination, xcm.clone())); // Size 312
+		assert_ok!(send_xcm::<XcmpQueue>(destination, xcm.clone())); // Size 416
+		assert_eq!(DeliveryFeeFactor::<Test>::get(sibling_para_id), initial);
+
+		// Sending the message right now is cheap
+		let (_, delivery_fees) = validate_send::<XcmpQueue>(destination, xcm.clone())
+			.expect("message can be sent; qed");
+		let Fungible(delivery_fee_amount) = delivery_fees.inner()[0].fun else { unreachable!("asset is fungible; qed"); };
+		assert_eq!(delivery_fee_amount, 402_000_000);
+
+		let smaller_xcm = Xcm(vec![ClearOrigin; 30]);
+
+		// When we get to half of `max_total_size`, because `THRESHOLD_FACTOR` is 2,
+		// then the fee factor starts to increase.
+		assert_ok!(send_xcm::<XcmpQueue>(destination, xcm.clone())); // Size 520
+		assert_eq!(DeliveryFeeFactor::<Test>::get(sibling_para_id), FixedU128::from_float(1.05));
+
+		for _ in 0..12 { // We finish at size 929
+			assert_ok!(send_xcm::<XcmpQueue>(destination, smaller_xcm.clone()));
+		}
+		assert!(DeliveryFeeFactor::<Test>::get(sibling_para_id) > FixedU128::from_float(1.88));
+
+		// Sending the message right now is expensive
+		let (_, delivery_fees) = validate_send::<XcmpQueue>(destination, xcm.clone())
+			.expect("message can be sent; qed");
+		let Fungible(delivery_fee_amount) = delivery_fees.inner()[0].fun else { unreachable!("asset is fungible; qed"); };
+		assert_eq!(delivery_fee_amount, 758_030_955);
+
+		// Fee factor only decreases in `take_outbound_messages`
+		for _ in 0..5 { // We take 5 100 byte pages
+			XcmpQueue::take_outbound_messages(1);
+		}
+		assert!(DeliveryFeeFactor::<Test>::get(sibling_para_id) < FixedU128::from_float(1.72));
+		XcmpQueue::take_outbound_messages(1);
+		assert!(DeliveryFeeFactor::<Test>::get(sibling_para_id) < FixedU128::from_float(1.63));
+	});
+}