fix(dtl): consistency checks for L1 sync

.changeset/many-lizards-drop.md

@@ -0,0 +1,5 @@
+---
+'@eth-optimism/data-transport-layer': patch
+---
+
+Adds consistency checks for transaction entries in L1 syncing nodes

packages/data-transport-layer/src/services/l1-ingestion/handlers/errors.ts

@@ -2,6 +2,7 @@ export type EventName =
   | 'TransactionEnqueued'
   | 'SequencerBatchAppended'
   | 'StateBatchAppended'
+  | 'SequencerBatchAppendedTransaction'
 
 export class MissingElementError extends Error {
   constructor(public name: EventName) {

packages/data-transport-layer/src/services/l1-ingestion/handlers/sequencer-batch-appended.ts

@@ -206,6 +206,21 @@ export const handleEventsSequencerBatchAppended: EventHandlerSet<
       }
     }
 
+    // Same consistency checks but for transaction entries.
+    if (
+      entry.transactionEntries.length > 0 &&
+      entry.transactionEntries[0].index > 0
+    ) {
+      const prevTransactionEntry = await db.getTransactionByIndex(
+        entry.transactionEntries[0].index - 1
+      )
+
+      // We should *always* have a previous transaction here.
+      if (prevTransactionEntry === null) {
+        throw new MissingElementError('SequencerBatchAppendedTransaction')
+      }
+    }
+
     await db.putTransactionEntries(entry.transactionEntries)
 
     // Add an additional field to the enqueued transactions in the database

packages/data-transport-layer/src/services/l1-ingestion/service.ts

@@ -277,6 +277,8 @@ export class L1IngestionService extends BaseService<L1IngestionServiceOptions> {
           const handlers = {
             SequencerBatchAppended:
               this.state.db.getLatestTransactionBatch.bind(this.state.db),
+            SequencerBatchAppendedTransaction:
+              this.state.db.getLatestTransaction.bind(this.state.db),
             StateBatchAppended: this.state.db.getLatestStateRootBatch.bind(
               this.state.db
             ),