feat(chain-mon): Add backwards walk to find last finalized block

packages/chain-mon/src/wd-mon/helpers.ts

+import { Provider } from '@ethersproject/abstract-provider'
+import { Logger } from '@eth-optimism/common-ts'
+
+/**
+ * Finds
+ *
+ * @param
+ * @param
+ * @param
+ * @returns
+ */
+export const getLastFinalizedBlock = async (
+  l1RpcProvider: Provider,
+  faultProofWindow: number,
+  logger: Logger
+): Promise<number> => {
+  let guessWindowStartBlock
+  try {
+    const l1Block = await l1RpcProvider.getBlock('latest')
+
+    // The time corresponding to the start of the FPW, based on the current block.
+    const windowStartTime = l1Block.timestamp - faultProofWindow
+
+    // Use the FPW to find the block number that is the start of the FPW.
+    guessWindowStartBlock = l1Block.number - faultProofWindow / 12
+
+    let block = await l1RpcProvider.getBlock(guessWindowStartBlock)
+    while (block.timestamp > windowStartTime) {
+      guessWindowStartBlock--
+      block = await l1RpcProvider.getBlock(guessWindowStartBlock)
+    }
+    return block.number
+  } catch (err) {
+    logger.fatal('error when calling querying for block', {
+      errors: err,
+    })
+    throw new Error(
+      `unable to find block number ${guessWindowStartBlock || 'latest'}`
+    )
+  }
+}

packages/chain-mon/src/wd-mon/service.ts

@@ -13,6 +13,7 @@ import { Event } from 'ethers'
 import dateformat from 'dateformat'
 
 import { version } from '../../package.json'
+import { getLastFinalizedBlock as getLastFinalizedBlock } from './helpers'
 
 type Options = {
   l1RpcProvider: Provider
@@ -117,10 +118,12 @@ export class WithdrawalMonitor extends BaseServiceV2<Options, Metrics, State> {
 
     // Set the start block number.
     if (this.options.startBlockNumber === -1) {
-      // We default to starting from the earliest block still in the fault proof window.
-      const l1BlockNumber = await this.options.l1RpcProvider.getBlockNumber()
-      this.state.highestUncheckedBlockNumber =
-        l1BlockNumber - this.state.faultProofWindow / 12
+      // We default to starting from the last finalized block.
+      this.state.highestUncheckedBlockNumber = await getLastFinalizedBlock(
+        this.options.l1RpcProvider,
+        this.state.faultProofWindow,
+        this.logger
+      )
     } else {
       this.state.highestUncheckedBlockNumber = this.options.startBlockNumber
     }