Verify messages proofs (#480)

* verify messages proofs

* fmt

* clippy

* grumbles

bridges/bin/millau/runtime/src/rialto_messages.rs

@@ -184,8 +184,8 @@ impl SourceHeaderChain<bp_rialto::Balance> for Rialto {
 	type MessagesProof = FromRialtoMessagesProof;
 
 	fn verify_messages_proof(
-		_proof: Self::MessagesProof,
+		proof: Self::MessagesProof,
 	) -> Result<ProvedMessages<Message<bp_rialto::Balance>>, Self::Error> {
-		unimplemented!("https://github.com/paritytech/parity-bridges-common/issues/397")
+		messages::target::verify_messages_proof::<WithRialtoMessageBridge, Runtime>(proof)
 	}
 }

bridges/bin/rialto/runtime/src/millau_messages.rs

@@ -184,8 +184,8 @@ impl SourceHeaderChain<bp_millau::Balance> for Millau {
 	type MessagesProof = FromMillauMessagesProof;
 
 	fn verify_messages_proof(
-		_proof: Self::MessagesProof,
+		proof: Self::MessagesProof,
 	) -> Result<ProvedMessages<Message<bp_millau::Balance>>, Self::Error> {
-		unimplemented!("https://github.com/paritytech/parity-bridges-common/issues/397")
+		messages::target::verify_messages_proof::<WithMillauMessageBridge, Runtime>(proof)
 	}
 }

bridges/bin/runtime-common/src/messages.rs

@@ -23,8 +23,8 @@
 use bp_message_dispatch::MessageDispatch as _;
 use bp_message_lane::{
 	source_chain::LaneMessageVerifier,
-	target_chain::{DispatchMessage, MessageDispatch},
+	target_chain::{DispatchMessage, MessageDispatch, ProvedLaneMessages, ProvedMessages},
-	InboundLaneData, LaneId, MessageNonce,
+	InboundLaneData, LaneId, Message, MessageData, MessageKey, MessageNonce, OutboundLaneData,
 };
 use bp_runtime::InstanceId;
 use codec::{Compact, Decode, Input};
@@ -90,7 +90,7 @@ pub trait ChainWithMessageLanes {
 	/// different weights.
 	type Weight: From<frame_support::weights::Weight>;
 	/// Type of balances that is used on the chain.
-	type Balance: CheckedAdd + CheckedDiv + CheckedMul + PartialOrd + From<u32> + Copy;
+	type Balance: Decode + CheckedAdd + CheckedDiv + CheckedMul + PartialOrd + From<u32> + Copy;
 
 	/// Instance of the message-lane pallet.
 	type MessageLaneInstance: Instance;
@@ -333,6 +333,76 @@ pub mod target {
 			}
 		}
 	}
+
+	/// Verify proof of Bridged -> This chain messages.
+	pub fn verify_messages_proof<B: MessageBridge, ThisRuntime>(
+		proof: FromBridgedChainMessagesProof<B>,
+	) -> Result<ProvedMessages<Message<BalanceOf<BridgedChain<B>>>>, &'static str>
+	where
+		ThisRuntime: pallet_substrate_bridge::Trait,
+		ThisRuntime: pallet_message_lane::Trait<MessageLaneInstanceOf<BridgedChain<B>>>,
+		HashOf<BridgedChain<B>>:
+			Into<bp_runtime::HashOf<<ThisRuntime as pallet_substrate_bridge::Trait>::BridgedChain>>,
+	{
+		let (bridged_header_hash, bridged_storage_proof, lane_id, begin, end) = proof;
+		pallet_substrate_bridge::Module::<ThisRuntime>::parse_finalized_storage_proof(
+			bridged_header_hash.into(),
+			bridged_storage_proof,
+			|storage| {
+				// Read messages first. All messages that are claimed to be in the proof must
+				// be in the proof. So any error in `read_value`, or even missing value is fatal.
+				//
+				// Mind that we allow proofs with no messages if outbound lane state is proved.
+				let mut messages = Vec::with_capacity(end.saturating_sub(begin) as _);
+				for nonce in begin..=end {
+					let message_key = MessageKey { lane_id, nonce };
+					let storage_message_key = pallet_message_lane::storage_keys::message_key::<
+						ThisRuntime,
+						MessageLaneInstanceOf<BridgedChain<B>>,
+					>(&lane_id, nonce);
+					let raw_message_data = storage
+						.read_value(storage_message_key.0.as_ref())
+						.map_err(|_| "Failed to read message from storage proof")?
+						.ok_or("Message is missing from the messages proof")?;
+					let message_data = MessageData::<BalanceOf<BridgedChain<B>>>::decode(&mut &raw_message_data[..])
+						.map_err(|_| "Failed to decode message from the proof")?;
+					messages.push(Message {
+						key: message_key,
+						data: message_data,
+					});
+				}
+
+				// Now let's check if proof contains outbound lane state proof. It is optional, so we
+				// simply ignore `read_value` errors and missing value.
+				let mut proved_lane_messages = ProvedLaneMessages {
+					lane_state: None,
+					messages,
+				};
+				let storage_outbound_lane_data_key = pallet_message_lane::storage_keys::outbound_lane_data_key::<
+					MessageLaneInstanceOf<BridgedChain<B>>,
+				>(&lane_id);
+				let raw_outbound_lane_data = storage.read_value(storage_outbound_lane_data_key.0.as_ref());
+				if let Ok(Some(raw_outbound_lane_data)) = raw_outbound_lane_data {
+					proved_lane_messages.lane_state = Some(
+						OutboundLaneData::decode(&mut &raw_outbound_lane_data[..])
+							.map_err(|_| "Failed to decode outbound lane data from the proof")?,
+					);
+				}
+
+				// Now we may actually check if the proof is empty or not.
+				if proved_lane_messages.lane_state.is_none() && proved_lane_messages.messages.is_empty() {
+					return Err("Messages proof is empty");
+				}
+
+				// We only support single lane messages in this schema
+				let mut proved_messages = ProvedMessages::new();
+				proved_messages.insert(lane_id, proved_lane_messages);
+
+				Ok(proved_messages)
+			},
+		)
+		.map_err(<&'static str>::from)?
+	}
 }
 
 #[cfg(test)]