fix optimistic sync fork choice update (#465)

Author: frisitano

crates/chain-orchestrator/src/lib.rs

@@ -1166,14 +1166,12 @@ impl<
         }
 
         // Update the FCS to the new head.
-        let result = self
-            .engine
-            .update_fcs(
-                Some(BlockInfo { number: chain_head_number, hash: chain_head_hash }),
-                None,
-                None,
-            )
-            .await?;
+        let head = BlockInfo { number: chain_head_number, hash: chain_head_hash };
+        let result = if self.sync_state.l2().is_syncing() {
+            self.engine.optimistic_sync(head).await?
+        } else {
+            self.engine.update_fcs(Some(head), None, None).await?
+        };
 
         // If the FCS update resulted in an invalid state, we return an error.
         if result.is_invalid() {

crates/chain-orchestrator/src/sync.rs

@@ -39,6 +39,11 @@ impl SyncState {
     pub const fn is_synced(&self) -> bool {
         self.l1.is_synced() && self.l2.is_synced()
     }
+
+    /// Returns true if either L1 or L2 is syncing.
+    pub const fn is_syncing(&self) -> bool {
+        self.l1.is_syncing() || self.l2.is_syncing()
+    }
 }
 
 /// The sync mode of the chain orchestrator.