Add validate_xcm_nesting to the ParentAsUmp and ChildParachainRouter (#4236)

This PR:
- moves `validate_xcm_nesting` from `XcmpQueue` into the `VersionedXcm`
- adds `validate_xcm_nesting` to the `ParentAsUmp`
- adds `validate_xcm_nesting` to the `ChildParachainRouter`


Based on discussion
[here](https://github.com/paritytech/polkadot-sdk/pull/4186#discussion_r1571344270)
and/or
[here](https://github.com/paritytech/polkadot-sdk/pull/4186#discussion_r1572076666)
and/or [here]()

## Question/TODO

- [x] To the
[comment](https://github.com/paritytech/polkadot-sdk/pull/4186#discussion_r1572072295)
- Why was `validate_xcm_nesting` added just to the `XcmpQueue` router
and nowhere else? What kind of problem `MAX_XCM_DECODE_DEPTH` is
solving? (see
[comment](https://github.com/paritytech/polkadot-sdk/pull/4236#discussion_r1574605191))

polkadot/runtime/common/src/integration_tests.rs

@@ -39,7 +39,7 @@ use primitives::{
 	MAX_CODE_SIZE,
 };
 use runtime_parachains::{
-	configuration, origin, paras, shared, Origin as ParaOrigin, ParaLifecycle,
+	configuration, dmp, origin, paras, shared, Origin as ParaOrigin, ParaLifecycle,
 };
 use sp_core::H256;
 use sp_io::TestExternalities;
@@ -84,6 +84,7 @@ frame_support::construct_runtime!(
 		Paras: paras,
 		ParasShared: shared,
 		ParachainsOrigin: origin,
+		Dmp: dmp,
 
 		// Para Onboarding Pallets
 		Registrar: paras_registrar,
@@ -201,6 +202,8 @@ impl shared::Config for Test {
 	type DisabledValidators = ();
 }
 
+impl dmp::Config for Test {}
+
 impl origin::Config for Test {}
 
 parameter_types! {

polkadot/runtime/common/src/xcm_sender.rs

@@ -119,7 +119,9 @@ where
 		let config = configuration::ActiveConfig::<T>::get();
 		let para = id.into();
 		let price = P::price_for_delivery(para, &xcm);
-		let blob = W::wrap_version(&d, xcm).map_err(|()| DestinationUnsupported)?.encode();
+		let versioned_xcm = W::wrap_version(&d, xcm).map_err(|()| DestinationUnsupported)?;
+		versioned_xcm.validate_xcm_nesting().map_err(|()| ExceedsMaxMessageSize)?;
+		let blob = versioned_xcm.encode();
 		dmp::Pallet::<T>::can_queue_downward_message(&config, &para, &blob)
 			.map_err(Into::<SendError>::into)?;
 
@@ -236,9 +238,11 @@ impl EnsureForParachain for () {
 #[cfg(test)]
 mod tests {
 	use super::*;
-	use frame_support::parameter_types;
+	use crate::integration_tests::new_test_ext;
+	use frame_support::{assert_ok, parameter_types};
 	use runtime_parachains::FeeTracker;
 	use sp_runtime::FixedU128;
+	use xcm::MAX_XCM_DECODE_DEPTH;
 
 	parameter_types! {
 		pub const BaseDeliveryFee: u128 = 300_000_000;
@@ -297,4 +301,40 @@ mod tests {
 			(FeeAssetId::get(), result).into()
 		);
 	}
+
+	#[test]
+	fn child_parachain_router_validate_nested_xcm_works() {
+		let dest = Parachain(5555);
+
+		type Router = ChildParachainRouter<
+			crate::integration_tests::Test,
+			(),
+			NoPriceForMessageDelivery<ParaId>,
+		>;
+
+		// Message that is not too deeply nested:
+		let mut good = Xcm(vec![ClearOrigin]);
+		for _ in 0..MAX_XCM_DECODE_DEPTH - 1 {
+			good = Xcm(vec![SetAppendix(good)]);
+		}
+
+		new_test_ext().execute_with(|| {
+			configuration::ActiveConfig::<crate::integration_tests::Test>::mutate(|c| {
+				c.max_downward_message_size = u32::MAX;
+			});
+
+			// Check that the good message is validated:
+			assert_ok!(<Router as SendXcm>::validate(
+				&mut Some(dest.into()),
+				&mut Some(good.clone())
+			));
+
+			// Nesting the message one more time should reject it:
+			let bad = Xcm(vec![SetAppendix(good)]);
+			assert_eq!(
+				Err(ExceedsMaxMessageSize),
+				<Router as SendXcm>::validate(&mut Some(dest.into()), &mut Some(bad))
+			);
+		});
+	}
 }