l2geth: rollup client batch api (#516)

* l2geth: add batch querying to rollup client

* l2geth: add patch changeset

* l2geth: complete mock client interface

* l2geth: add comments to rollup client

* l2geth: more idiomatic error handling

.changeset/empty-boxes-repair.md

+---
+"@eth-optimism/l2geth": patch
+---
+
+Add batch API to rollup client

l2geth/rollup/sync_service.go

@@ -249,13 +249,15 @@ func (s *SyncService) initializeLatestL1(ctcDeployHeight *big.Int) error {
 		queueIndex := s.GetLatestEnqueueIndex()
 		if queueIndex == nil {
 			enqueue, err := s.client.GetLastConfirmedEnqueue()
-			if err != nil {
-				return fmt.Errorf("Cannot fetch last confirmed queue tx: %w", err)
-			}
 			// There are no enqueues yet
-			if enqueue == nil {
+			if errors.Is(err, errElementNotFound) {
 				return nil
 			}
+			// Other unexpected error
+			if err != nil {
+				return fmt.Errorf("Cannot fetch last confirmed queue tx: %w", err)
+			}
+			// No error, the queue element was found
 			queueIndex = enqueue.GetMeta().QueueIndex
 		}
 		s.SetLatestEnqueueIndex(queueIndex)
@@ -310,14 +312,13 @@ func (s *SyncService) verify() error {
 	// The verifier polls for ctc transactions.
 	// the ctc transactions are extending the chain.
 	latest, err := s.client.GetLatestTransaction()
-	if err != nil {
-		return err
-	}
-
-	if latest == nil {
+	if errors.Is(err, errElementNotFound) {
 		log.Debug("latest transaction not found")
 		return nil
 	}
+	if err != nil {
+		return err
+	}
 
 	var start uint64
 	if s.GetLatestIndex() == nil {
@@ -372,16 +373,13 @@ func (s *SyncService) sequence() error {
 	// Place as many L1ToL2 transactions in the same context as possible
 	// by executing them one after another.
 	latest, err := s.client.GetLatestEnqueue()
-	if err != nil {
-		return err
-	}
-
-	// This should never happen unless the backend is empty
-	if latest == nil {
+	if errors.Is(err, errElementNotFound) {
 		log.Debug("No enqueue transactions found")
 		return nil
 	}
-
+	if err != nil {
+		return fmt.Errorf("cannot fetch latest enqueue: %w", err)
+	}
 	// Compare the remote latest queue index to the local latest
 	// queue index. If the remote latest queue index is greater
 	// than the local latest queue index, be sure to ingest more
@@ -486,15 +484,15 @@ func (s *SyncService) syncTransactionsToTip() error {
 		// This function must be sure to sync all the way to the tip.
 		// First query the latest transaction
 		latest, err := s.client.GetLatestTransaction()
+		if errors.Is(err, errElementNotFound) {
+			log.Info("No transactions to sync")
+			return nil
+		}
 		if err != nil {
 			log.Error("Cannot get latest transaction", "msg", err)
 			time.Sleep(time.Second * 2)
 			continue
 		}
-		if latest == nil {
-			log.Info("No transactions to sync")
-			return nil
-		}
 		tipHeight := latest.GetMeta().Index
 		index := rawdb.ReadHeadIndex(s.db)
 		start := uint64(0)

l2geth/rollup/sync_service_test.go

@@ -428,6 +428,14 @@ func (m *mockClient) GetLastConfirmedEnqueue() (*types.Transaction, error) {
 	return nil, nil
 }
 
+func (m *mockClient) GetLatestTransactionBatch() (*Batch, []*types.Transaction, error) {
+	return nil, nil, nil
+}
+
+func (m *mockClient) GetTransactionBatch(index uint64) (*Batch, []*types.Transaction, error) {
+	return nil, nil, nil
+}
+
 func (m *mockClient) SyncStatus() (*SyncStatus, error) {
 	return &SyncStatus{
 		Syncing: false,