fix(engine): apply finalized state after syncing FCU head import

crates/engine/primitives/src/forkchoice.rs

@@ -21,7 +21,8 @@ impl ForkchoiceStateTracker {
     /// `sync_target` to `None`, since we're now fully synced.
     pub const fn set_latest(&mut self, state: ForkchoiceState, status: ForkchoiceStatus) {
         if status.is_valid() {
-            self.set_valid(state);
+            self.last_syncing = None;
+            self.last_valid = Some(state);
         } else if status.is_syncing() {
             self.last_syncing = Some(state);
         }
@@ -30,11 +31,24 @@ impl ForkchoiceStateTracker {
         self.latest = Some(received);
     }
 
-    const fn set_valid(&mut self, state: ForkchoiceState) {
-        // we no longer need to sync to this state.
+    /// Promotes a previously tracked syncing forkchoice state to valid, without overwriting a
+    /// newer `latest` state.
+    ///
+    /// This is used when a `Syncing` FCU's head finally becomes canonical via the downloaded-block
+    /// flow, so the safe/finalized anchors of that FCU can be applied. Unlike
+    /// [`Self::set_latest`], this preserves a newer `latest` (e.g. an `Invalid` FCU received
+    /// after the syncing one) and only flips `latest` to `Valid` when it still refers to the same
+    /// syncing FCU being promoted.
+    pub fn promote_sync_target_to_valid(&mut self, state: ForkchoiceState) {
         self.last_syncing = None;
-
         self.last_valid = Some(state);
+
+        if let Some(received) = self.latest.as_mut() &&
+            received.state == state &&
+            received.status.is_syncing()
+        {
+            received.status = ForkchoiceStatus::Valid;
+        }
     }
 
     /// Returns the [`ForkchoiceStatus`] of the latest received FCU.

crates/engine/tree/src/tree/mod.rs

@@ -1917,9 +1917,37 @@ where
             self.on_canonical_chain_update(chain_update);
         }
 
+        self.on_canonicalized_sync_target(target);
+
         Ok(())
     }
 
+    /// Applies the tracked forkchoice state once its sync target head becomes canonical.
+    fn on_canonicalized_sync_target(&mut self, target: B256) {
+        let Some(sync_target_state) = self
+            .state
+            .forkchoice_state_tracker
+            .sync_target_state()
+            .filter(|state| state.head_block_hash == target)
+        else {
+            return;
+        };
+
+        if let Err(outcome) = self.ensure_consistent_forkchoice_state(sync_target_state) {
+            debug!(
+                target: "engine::tree",
+                head = %sync_target_state.head_block_hash,
+                safe = %sync_target_state.safe_block_hash,
+                finalized = %sync_target_state.finalized_block_hash,
+                ?outcome,
+                "Canonicalized sync target head before safe/finalized could be applied"
+            );
+            return;
+        }
+
+        self.state.forkchoice_state_tracker.promote_sync_target_to_valid(sync_target_state);
+    }
+
     /// Convenience function to handle an optional tree event.
     fn on_maybe_tree_event(&mut self, event: Option<TreeEvent>) -> ProviderResult<()> {
         if let Some(event) = event {

crates/engine/tree/src/tree/tests.rs

@@ -2254,3 +2254,65 @@ fn test_on_valid_downloaded_head_sync_target_returns_make_canonical() {
         other => panic!("Expected MakeCanonical for head block, got: {other:?}"),
     }
 }
+
+/// Tests that canonicalizing a downloaded sync target head also applies the tracked finalized
+/// block from the original `SYNCING` forkchoice state.
+#[test]
+fn test_canonicalizing_downloaded_sync_target_head_updates_finalized() {
+    reth_tracing::init_test_tracing();
+
+    let chain_spec = MAINNET.clone();
+    let mut test_harness = TestHarness::new(chain_spec);
+
+    let blocks: Vec<_> = test_harness.block_builder.get_executed_blocks(0..3).collect();
+    let genesis = &blocks[0];
+    let finalized_block = &blocks[1];
+    let head_block = &blocks[2];
+
+    test_harness = test_harness.with_blocks(vec![
+        genesis.clone(),
+        finalized_block.clone(),
+        head_block.clone(),
+    ]);
+
+    let finalized_num_hash = finalized_block.recovered_block().num_hash();
+    let head_num_hash = head_block.recovered_block().num_hash();
+
+    test_harness.tree.state.tree_state.set_canonical_head(genesis.recovered_block().num_hash());
+
+    let fcu_state = ForkchoiceState {
+        head_block_hash: head_num_hash.hash,
+        safe_block_hash: head_num_hash.hash,
+        finalized_block_hash: finalized_num_hash.hash,
+    };
+    test_harness
+        .tree
+        .state
+        .forkchoice_state_tracker
+        .set_latest(fcu_state, ForkchoiceStatus::Syncing);
+
+    let event = test_harness
+        .tree
+        .on_valid_downloaded_block(head_num_hash)
+        .unwrap()
+        .expect("expected canonicalization event for sync target head");
+
+    test_harness.tree.on_tree_event(event).unwrap();
+
+    assert_eq!(test_harness.tree.state.tree_state.canonical_block_hash(), head_num_hash.hash);
+    assert_eq!(
+        test_harness.tree.canonical_in_memory_state.get_finalized_num_hash(),
+        Some(finalized_num_hash),
+        "Finalized block from the syncing FCU should be applied once the head becomes canonical"
+    );
+    assert_eq!(
+        test_harness.tree.canonical_in_memory_state.get_safe_num_hash(),
+        Some(head_num_hash),
+        "Safe block from the syncing FCU should be applied once the head becomes canonical"
+    );
+    assert_eq!(
+        test_harness.tree.state.forkchoice_state_tracker.last_valid_state(),
+        Some(fcu_state)
+    );
+    assert!(test_harness.tree.state.forkchoice_state_tracker.sync_target_state().is_none());
+}