Merge pull request #6427 from ethereum-optimism/jm/startblock

feat(chain-mon): Default wd-mon scan from start of FPW

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
@@ -30,7 +31,7 @@ type Metrics = {
 type State = {
   messenger: CrossChainMessenger
   highestUncheckedBlockNumber: number
-  finalizationWindow: number
+  faultProofWindow: number
   forgeryDetected: boolean
 }
 
@@ -109,10 +110,20 @@ export class WithdrawalMonitor extends BaseServiceV2<Options, Metrics, State> {
     // Not detected by default.
     this.state.forgeryDetected = false
 
-    // For now we'll just start take it from the env or the tip of the chain
+    this.state.faultProofWindow =
+      await this.state.messenger.getChallengePeriodSeconds()
+    this.logger.info(
+      `fault proof window is ${this.state.faultProofWindow} seconds`
+    )
+
+    // Set the start block number.
     if (this.options.startBlockNumber === -1) {
-      this.state.highestUncheckedBlockNumber =
-        await this.options.l1RpcProvider.getBlockNumber()
+      // 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
     }