Subscribe to justifications in Millau->Rialto headers sync (#394)

* maintain MillauHeadersToRialto sync by subscribing to Millau justifications

* more tracing in maintain

* Update relays/substrate/src/headers_maintain.rs

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

* Update relays/substrate/src/headers_maintain.rs

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

* -Please

* -TODO

* revert raise recursion limit

* updated comment

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

bridges/modules/substrate/src/justification.rs

@@ -44,6 +44,15 @@ pub enum Error {
 	InvalidPrecommitAncestries,
 }
 
+/// Decode justification target.
+pub fn decode_justification_target<Header: HeaderT>(
+	raw_justification: &[u8],
+) -> Result<(Header::Hash, Header::Number), Error> {
+	GrandpaJustification::<Header>::decode(&mut &raw_justification[..])
+		.map(|justification| (justification.commit.target_hash, justification.commit.target_number))
+		.map_err(|_| Error::JustificationDecode)
+}
+
 /// Verify that justification, that is generated by given authority set, finalizes given header.
 pub fn verify_justification<Header: HeaderT>(
 	finalized_target: (Header::Hash, Header::Number),

bridges/modules/substrate/src/lib.rs

@@ -41,6 +41,8 @@ use sp_std::{marker::PhantomData, prelude::*};
 // Re-export since the node uses these when configuring genesis
 pub use storage::{AuthoritySet, ScheduledChange};
 
+pub use justification::decode_justification_target;
+
 mod justification;
 mod storage;
 mod storage_proof;