feat(fd): add fault status API (#3476)

Adds a fault status API endpoint so people can easily query if the node
is currently diverged.
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.changeset/dull-lies-melt.md

+---
+'@eth-optimism/fault-detector': patch
+---
+
+Adds a fault status API to the Fault Detector.

packages/fault-detector/src/service.ts

-import { BaseServiceV2, Gauge, validators } from '@eth-optimism/common-ts'
+import {
+  BaseServiceV2,
+  ExpressRouter,
+  Gauge,
+  validators,
+} from '@eth-optimism/common-ts'
 import { getChainId, sleep, toRpcHexString } from '@eth-optimism/core-utils'
 import { CrossChainMessenger } from '@eth-optimism/sdk'
 import { Provider } from '@ethersproject/abstract-provider'
@@ -29,6 +34,7 @@ type State = {
   scc: Contract
   messenger: CrossChainMessenger
   highestCheckedBatchIndex: number
+  diverged: boolean
 }
 
 export class FaultDetector extends BaseServiceV2<Options, Metrics, State> {
@@ -90,6 +96,9 @@ export class FaultDetector extends BaseServiceV2<Options, Metrics, State> {
       l2ChainId: await getChainId(this.options.l2RpcProvider),
     })
 
+    // Not diverged by default.
+    this.state.diverged = false
+
     // We use this a lot, a bit cleaner to pull out to the top level of the state object.
     this.state.scc = this.state.messenger.contracts.l1.StateCommitmentChain
     this.state.fpw = (await this.state.scc.FRAUD_PROOF_WINDOW()).toNumber()
@@ -108,6 +117,14 @@ export class FaultDetector extends BaseServiceV2<Options, Metrics, State> {
     })
   }
 
+  async routes(router: ExpressRouter): Promise<void> {
+    router.get('/status', async (req, res) => {
+      return res.status(200).json({
+        ok: !this.state.diverged,
+      })
+    })
+  }
+
   async main(): Promise<void> {
     let latestBatchIndex: number
     try {
@@ -226,6 +243,7 @@ export class FaultDetector extends BaseServiceV2<Options, Metrics, State> {
 
     for (const [i, stateRoot] of stateRoots.entries()) {
       if (blocks[i].stateRoot !== stateRoot) {
+        this.state.diverged = true
         this.metrics.isCurrentlyMismatched.set(1)
         this.logger.error(`state root mismatch`, {
           blockNumber: blocks[i].number,
@@ -250,6 +268,7 @@ export class FaultDetector extends BaseServiceV2<Options, Metrics, State> {
     )
 
     // If we got through the above without throwing an error, we should be fine to reset.
+    this.state.diverged = false
     this.metrics.isCurrentlyMismatched.set(0)
   }
 }