fix: track last seen nonce in case of stale fallback L1 RPC node

yarn-project/ethereum/src/l1_tx_utils/l1_tx_utils.test.ts

@@ -147,7 +147,7 @@ describe('L1TxUtils', () => {
         address: l1Client.account.address,
       });
 
-      // Next send fails at sendRawTransaction (e.g. network error)
+      // Next send fails at sendRawTransaction (e.g. network error / 429)
       const originalSendRawTransaction = l1Client.sendRawTransaction.bind(l1Client);
       using _sendSpy = jest
         .spyOn(l1Client, 'sendRawTransaction')
@@ -163,6 +163,29 @@ describe('L1TxUtils', () => {
       expect((await l1Client.getTransaction({ hash: txHash })).nonce).toBe(expectedNonce);
     }, 30_000);
 
+    it('bumps nonce when getTransactionCount returns a stale value after a successful send', async () => {
+      // Send a successful tx first to advance the chain nonce
+      await gasUtils.sendAndMonitorTransaction(request);
+
+      const expectedNonce = await l1Client.getTransactionCount({
+        blockTag: 'pending',
+        address: l1Client.account.address,
+      });
+
+      // Simulate a stale fallback RPC node that returns the pre-send nonce
+      const originalGetTransactionCount = l1Client.getTransactionCount.bind(l1Client);
+      using _spy = jest
+        .spyOn(l1Client, 'getTransactionCount')
+        .mockImplementationOnce(() => Promise.resolve(expectedNonce - 1)) // stale: one behind
+        .mockImplementation(originalGetTransactionCount);
+
+      // Despite the stale count, the send should use lastSentNonce+1 = expectedNonce
+      const { txHash, state } = await gasUtils.sendTransaction(request);
+
+      expect(state.nonce).toBe(expectedNonce);
+      expect((await l1Client.getTransaction({ hash: txHash })).nonce).toBe(expectedNonce);
+    }, 30_000);
+
     // Regression for TMNT-312
     it('speed-up of blob tx sets non-zero maxFeePerBlobGas', async () => {
       await cheatCodes.setAutomine(false);

yarn-project/ethereum/src/l1_tx_utils/l1_tx_utils.ts

@@ -45,6 +45,8 @@ const MAX_L1_TX_STATES = 32;
 
 export class L1TxUtils extends ReadOnlyL1TxUtils {
   protected txs: L1TxState[] = [];
+  /** Last nonce successfully sent to the chain. Used as a lower bound when a fallback RPC node returns a stale count. */
+  private lastSentNonce: number | undefined;
   /** Tx delayer for testing. Only set when enableDelayer config is true. */
   public delayer?: Delayer;
   /** KZG instance for blob operations. */
@@ -105,6 +107,11 @@ export class L1TxUtils extends ReadOnlyL1TxUtils {
       this.metrics?.recordMinedTx(l1TxState, new Date(l1Timestamp));
     } else if (newState === TxUtilsState.NOT_MINED) {
       this.metrics?.recordDroppedTx(l1TxState);
+      // The tx was dropped: the chain nonce reverted to l1TxState.nonce, so our lower bound is
+      // no longer valid. Clear it so the next send fetches the real nonce from the chain.
+      if (this.lastSentNonce === l1TxState.nonce) {
+        this.lastSentNonce = undefined;
+      }
     }
 
     // Update state in the store
@@ -246,14 +253,20 @@ export class L1TxUtils extends ReadOnlyL1TxUtils {
         );
       }
 
-      const nonce = await this.client.getTransactionCount({ address: account, blockTag: 'pending' });
+      const chainNonce = await this.client.getTransactionCount({ address: account, blockTag: 'pending' });
+      // If a fallback RPC node returns a stale count (lower than what we last sent), use our
+      // local lower bound to avoid sending a duplicate of an already-pending transaction.
+      const nonce =
+        this.lastSentNonce !== undefined && chainNonce <= this.lastSentNonce ? this.lastSentNonce + 1 : chainNonce;
 
       const baseState = { request, gasLimit, blobInputs, gasPrice, nonce };
       const txData = this.makeTxData(baseState, { isCancelTx: false });
 
       // Send the new tx
       const signedRequest = await this.prepareSignedTransaction(txData);
       const txHash = await this.client.sendRawTransaction({ serializedTransaction: signedRequest });
+      // Update after tx is sent successfully
+      this.lastSentNonce = nonce;
 
       // Create the new state for monitoring
       const l1TxState: L1TxState = {