feat(statesync): introduce Finalizer interface for syncer cleanup

graft/coreth/plugin/evm/atomic/sync/syncer.go

@@ -33,6 +33,7 @@ const (
 var (
 	_ sync.Syncer             = (*Syncer)(nil)
 	_ syncclient.LeafSyncTask = (*syncerLeafTask)(nil)
+	_ sync.Finalizer          = (*Syncer)(nil)
 
 	errTargetHeightRequired = errors.New("target height must be > 0")
 )
@@ -125,7 +126,6 @@ func NewSyncer(client syncclient.LeafClient, db *versiondb.Database, atomicTrie
 	syncer.syncer = syncclient.NewCallbackLeafSyncer(client, tasks, &syncclient.LeafSyncerConfig{
 		RequestSize: cfg.requestSize,
 		NumWorkers:  cfg.numWorkers,
-		OnFailure:   func() {}, // No-op since we flush progress to disk at the regular commit interval.
 	})
 
 	return syncer, nil
@@ -146,6 +146,13 @@ func (s *Syncer) Sync(ctx context.Context) error {
 	return s.syncer.Sync(ctx)
 }
 
+// Finalize commits any pending database changes to disk.
+// This ensures that even if the sync is cancelled or fails, we preserve
+// the progress up to the last fully synced height.
+func (s *Syncer) Finalize() error {
+	return s.db.Commit()
+}
+
 // addZeroes returns the big-endian representation of `height`, prefixed with [common.HashLength] zeroes.
 func addZeroes(height uint64) []byte {
 	// Key format is [height(8 bytes)][blockchainID(32 bytes)]. Start should be the

graft/coreth/plugin/evm/vmsync/registry.go

@@ -53,6 +53,10 @@ func (r *SyncerRegistry) Register(syncer syncpkg.Syncer) error {
 
 // RunSyncerTasks executes all registered syncers synchronously.
 func (r *SyncerRegistry) RunSyncerTasks(ctx context.Context, summary message.Syncable) error {
+	// Ensure finalization runs regardless of how this function exits.
+	// This guarantees cleanup even on early returns or panics.
+	defer r.FinalizeAll(summary)
+
 	// Early return if context is already canceled (e.g., during shutdown).
 	if err := ctx.Err(); err != nil {
 		return err
@@ -102,3 +106,15 @@ func (r *SyncerRegistry) StartAsync(ctx context.Context, summary message.Syncabl
 
 	return g
 }
+
+// FinalizeAll iterates over all registered syncers and calls Finalize on those that implement the Finalizer interface.
+// Errors are logged but not returned to ensure best-effort cleanup of all syncers.
+func (r *SyncerRegistry) FinalizeAll(summary message.Syncable) {
+	for _, task := range r.syncers {
+		if f, ok := task.syncer.(syncpkg.Finalizer); ok {
+			if err := f.Finalize(); err != nil {
+				log.Error("failed to finalize syncer", "syncer", task.name, "err", err, "summary", summary.GetBlockHash().Hex(), "height", summary.Height())
+			}
+		}
+	}
+}

graft/coreth/sync/client/leaf_syncer.go

@@ -37,7 +37,6 @@ type LeafSyncTask interface {
 type LeafSyncerConfig struct {
 	RequestSize uint16 // Number of leafs to request from a peer at a time
 	NumWorkers  int    // Number of workers to process leaf sync tasks
-	OnFailure   func() // Callback for handling errors during sync
 }
 
 type CallbackLeafSyncer struct {
@@ -159,9 +158,9 @@ func (c *CallbackLeafSyncer) Sync(ctx context.Context) error {
 		})
 	}
 
-	err := eg.Wait()
-	if err != nil {
-		c.config.OnFailure()
+	if err := eg.Wait(); err != nil {
+		return err
 	}
-	return err
+
+	return nil
 }

graft/coreth/sync/statesync/code_queue.go

@@ -15,11 +15,15 @@ import (
 	"github.com/ava-labs/libevm/libevm/options"
 
 	"github.com/ava-labs/avalanchego/graft/coreth/plugin/evm/customrawdb"
+
+	syncpkg "github.com/ava-labs/avalanchego/graft/coreth/sync"
 )
 
 const defaultQueueCapacity = 5000
 
 var (
+	_ syncpkg.Finalizer = (*CodeQueue)(nil)
+
 	errFailedToAddCodeHashesToQueue = errors.New("failed to add code hashes to queue")
 	errFailedToFinalizeCodeQueue    = errors.New("failed to finalize code queue")
 )

graft/coreth/sync/statesync/state_syncer.go

@@ -106,7 +106,6 @@ func NewSyncer(client syncclient.Client, db ethdb.Database, root common.Hash, co
 	ss.syncer = syncclient.NewCallbackLeafSyncer(client, ss.segments, &syncclient.LeafSyncerConfig{
 		RequestSize: leafsRequestSize,
 		NumWorkers:  defaultNumWorkers,
-		OnFailure:   ss.onSyncFailure,
 	})
 
 	if codeQueue == nil {
@@ -301,19 +300,27 @@ func (t *stateSync) removeTrieInProgress(root common.Hash) (int, error) {
 	return len(t.triesInProgress), nil
 }
 
-// onSyncFailure is called if the sync fails, this writes all
-// batches of in-progress trie segments to disk to have maximum
-// progress to restore.
-func (t *stateSync) onSyncFailure() {
+// Finalize checks if there are any in-progress tries and flushes their batches to disk
+// to preserve progress. On successful sync completion, triesInProgress will be empty
+// and this is effectively a no-op. This is called by the syncer registry regardless
+// of sync success or failure.
+func (t *stateSync) Finalize() error {
 	t.lock.RLock()
 	defer t.lock.RUnlock()
 
+	// Nothing to flush if no tries are in progress.
+	// This is the case after a successful sync.
+	if len(t.triesInProgress) == 0 {
+		return nil
+	}
+
 	for _, trie := range t.triesInProgress {
 		for _, segment := range trie.segments {
 			if err := segment.batch.Write(); err != nil {
-				log.Error("failed to write segment batch on sync failure", "err", err)
-				return
+				log.Error("failed to write segment batch on finalize", "err", err)
+				return err
 			}
 		}
 	}
+	return nil
 }

graft/coreth/sync/types.go

@@ -31,6 +31,13 @@ type Syncer interface {
 	ID() string
 }
 
+// Finalizer provides a mechanism to perform cleanup operations after a sync operation.
+// This is useful for handling inflight requests, flushing to disk, or other cleanup tasks.
+type Finalizer interface {
+	// Finalize performs any necessary cleanup operations.
+	Finalize() error
+}
+
 // SummaryProvider is an interface for providing state summaries.
 type SummaryProvider interface {
 	StateSummaryAtBlock(ethBlock *types.Block) (block.StateSummary, error)