fix(l1): don't trigger snap sync if fcu head is already canonical (#4985)

**Motivation**
In hive tests we often receive an fcu with either the genesis or last
imported block as head to notify the node that it is already synced. As
this is not usual in a real-case scenario, we just trigger a snap sync
to the advertised head. This PR solves this by first checking if the fcu
head is already part of our canonic chain before triggering a sync
<!-- Why does this pull request exist? What are its goals? -->

**Description**
* When handling a forkchoice in snap sync mode, check that the head is
not already canonical before triggering a sync, if it is already
canonical, apply the forkchoice
<!-- A clear and concise general description of the changes this PR
introduces -->

<!-- Link to issues: Resolves #111, Resolves #222 -->

Closes #4846

Author: fmoletta

crates/networking/p2p/sync_manager.rs

@@ -96,6 +96,11 @@ impl SyncManager {
         }
     }
 
+    /// Disables snapsync mode
+    pub fn disable_snap(&self) {
+        self.snap_enabled.store(false, Ordering::Relaxed);
+    }
+
     /// Updates the last fcu head. This may be used on the next sync cycle if needed
     fn set_head(&self, fcu_head: H256) {
         if let Ok(mut latest_fcu_head) = self.last_fcu_head.try_lock() {

crates/networking/rpc/engine/fork_choice.rs

@@ -216,10 +216,19 @@ async fn handle_forkchoice(
     }
 
     if context.syncer.sync_mode() == SyncMode::Snap {
-        context
-            .syncer
-            .sync_to_head(fork_choice_state.head_block_hash);
-        return Ok((None, PayloadStatus::syncing().into()));
+        // Don't trigger a sync if the block is already canonical
+        if context
+            .storage
+            .is_canonical_sync(fork_choice_state.head_block_hash)?
+        {
+            // Disable snapsync mode so we can process incoming payloads
+            context.syncer.disable_snap();
+        } else {
+            context
+                .syncer
+                .sync_to_head(fork_choice_state.head_block_hash);
+            return Ok((None, PayloadStatus::syncing().into()));
+        }
     }
 
     match apply_fork_choice(