Mock XCM (#876)

* sketch downward messages

* bring in attempt to mock mqc-head from moonbeam

* just patch individual crates

* fing comma

* add some logs

* Holy shit, we actually imported a block!

* Actually mock the message queue chain

* use relay parent number for `sent_at`

* finish moving MQC to primitives

* more complete mock and better config type

* change name

* fix export

* better map types

* fix dependencies after rebase

* try-rejigging branches because this is an override

* try to re-jig for hrmp mcqs

* fix branches

* actually fix branches better

* even better

* Removestray log lines

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Nicer handling of default `ParachainSystem` name

* better docs

* Default MockXcm for people who only who don't care to mock xcm.

* cargo fmt

* trailing commas

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* use the variable for hrmp to

* fix deref

* deduplicate MessageQueueChain

* better docs for MessageQueueChain

* Use `Vec<u8>` instead of `&'static [u8]`

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* cargo fmt

* associated changes for using Vec<u8>

* Unused import

* Fix compilation

Co-authored-by: Joshy Orndorff <admin@joshyorndorff.com>
Co-authored-by: Joshy Orndorff <JoshOrndorff@users.noreply.github.com>

primitives/parachain-inherent/src/lib.rs

@@ -28,6 +28,7 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 
 use cumulus_primitives_core::{
+	relay_chain::{BlakeTwo256, Hash as RelayHash, HashT as _},
 	InboundDownwardMessage, InboundHrmpMessage, ParaId, PersistedValidationData,
 };
 
@@ -42,7 +43,7 @@ pub use client_side::*;
 #[cfg(feature = "std")]
 mod mock;
 #[cfg(feature = "std")]
-pub use mock::MockValidationDataInherentDataProvider;
+pub use mock::{MockValidationDataInherentDataProvider, MockXcmConfig};
 
 /// The identifier for the parachain inherent.
 pub const INHERENT_IDENTIFIER: InherentIdentifier = *b"sysi1337";
@@ -68,3 +69,50 @@ pub struct ParachainInherentData {
 	/// this means `sent_at` is **strictly** greater than the previous one (if any).
 	pub horizontal_messages: BTreeMap<ParaId, Vec<InboundHrmpMessage>>,
 }
+
+/// This struct provides ability to extend a message queue chain (MQC) and compute a new head.
+///
+/// MQC is an instance of a [hash chain] applied to a message queue. Using a hash chain it's
+/// possible to represent a sequence of messages using only a single hash.
+///
+/// A head for an empty chain is agreed to be a zero hash.
+///
+/// An instance is used to track either DMP from the relay chain or HRMP across a channel.
+/// But a given instance is never used to track both. Therefore, you should call either
+/// `extend_downward` or `extend_hrmp`, but not both methods on a single instance.
+///
+/// [hash chain]: https://en.wikipedia.org/wiki/Hash_chain
+#[derive(Default, Clone, codec::Encode, codec::Decode, scale_info::TypeInfo)]
+pub struct MessageQueueChain(RelayHash);
+
+impl MessageQueueChain {
+	/// Extend the hash chain with an HRMP message. This method should be used only when
+	/// this chain is tracking HRMP.
+	pub fn extend_hrmp(&mut self, horizontal_message: &InboundHrmpMessage) -> &mut Self {
+		let prev_head = self.0;
+		self.0 = BlakeTwo256::hash_of(&(
+			prev_head,
+			horizontal_message.sent_at,
+			BlakeTwo256::hash_of(&horizontal_message.data),
+		));
+		self
+	}
+
+	/// Extend the hash chain with a downward message. This method should be used only when
+	/// this chain is tracking DMP.
+	pub fn extend_downward(&mut self, downward_message: &InboundDownwardMessage) -> &mut Self {
+		let prev_head = self.0;
+		self.0 = BlakeTwo256::hash_of(&(
+			prev_head,
+			downward_message.sent_at,
+			BlakeTwo256::hash_of(&downward_message.msg),
+		));
+		self
+	}
+
+	/// Return the current mead of the message queue hash chain.
+	/// This is agreed to be the zero hash for an empty chain.
+	pub fn head(&self) -> RelayHash {
+		self.0
+	}
+}