Add --only-mandatory-headers mode (#1004)

bridges/relays/finality/src/finality_loop.rs

@@ -58,6 +58,8 @@ pub struct FinalitySyncParams {
 	pub recent_finality_proofs_limit: usize,
 	/// Timeout before we treat our transactions as lost and restart the whole sync process.
 	pub stall_timeout: Duration,
+	/// If true, only mandatory headers are relayed.
+	pub only_mandatory_headers: bool,
 }
 
 /// Source client used in finality synchronization loop.
@@ -364,7 +366,7 @@ where
 	}
 }
 
-async fn select_header_to_submit<P, SC, TC>(
+pub(crate) async fn select_header_to_submit<P, SC, TC>(
 	source_client: &SC,
 	target_client: &TC,
 	finality_proofs_stream: &mut RestartableFinalityProofsStream<SC::FinalityProofsStream>,
@@ -397,6 +399,11 @@ where
 	.await?;
 	let (mut unjustified_headers, mut selected_finality_proof) = match selected_finality_proof {
 		SelectedFinalityProof::Mandatory(header, finality_proof) => return Ok(Some((header, finality_proof))),
+		_ if sync_params.only_mandatory_headers => {
+			// we are not reading finality proofs from the stream, so eventually it'll break
+			// but we don't care about transient proofs at all, so it is acceptable
+			return Ok(None);
+		}
 		SelectedFinalityProof::Regular(unjustified_headers, header, finality_proof) => {
 			(unjustified_headers, Some((header, finality_proof)))
 		}