fix: Do not reexecute all pruned blocks in the prune watcher (#17178)

When deciding whether to slash committee members for an epoch, the epoch
prune watcher tries reexecuting all blocks in the pruned epoch. It also
uses that to decide whether to slash for data withholding, if not all
data is available.

However, the blocks being reexecuted (and the txs being gathered) were
from ALL pruned epochs, which could be more than one if the proof
submission window was long enough. So we ended up slashing committee
members from the first epoch for data withholding offenses perpetrated
by members in future epochs.

Author: PhilWindle

yarn-project/slasher/src/watchers/epoch_prune_watcher.test.ts

@@ -2,6 +2,7 @@ import type { EpochCache } from '@aztec/epoch-cache';
 import { EthAddress } from '@aztec/foundation/eth-address';
 import { sleep } from '@aztec/foundation/sleep';
 import { L2Block, type L2BlockSourceEventEmitter, L2BlockSourceEvents } from '@aztec/stdlib/block';
+import type { L1RollupConstants } from '@aztec/stdlib/epoch-helpers';
 import type {
   BuildBlockResult,
   IFullNodeBlockBuilder,
@@ -28,6 +29,10 @@ describe('EpochPruneWatcher', () => {
   let txProvider: MockProxy<Pick<ITxProvider, 'getAvailableTxs'>>;
   let blockBuilder: MockProxy<IFullNodeBlockBuilder>;
   let fork: MockProxy<MerkleTreeWriteOperations>;
+
+  let ts: bigint;
+  let l1Constants: L1RollupConstants;
+
   const validEpochPrunedPenalty = BigInt(1000000000000000000n);
   const dataWithholdingPenalty = BigInt(2000000000000000000n);
 
@@ -41,6 +46,18 @@ describe('EpochPruneWatcher', () => {
     fork = mock<MerkleTreeWriteOperations>();
     blockBuilder.getFork.mockResolvedValue(fork);
 
+    ts = BigInt(Math.ceil(Date.now() / 1000));
+    l1Constants = {
+      l1StartBlock: 1n,
+      l1GenesisTime: ts,
+      slotDuration: 24,
+      epochDuration: 8,
+      ethereumSlotDuration: 12,
+      proofSubmissionEpochs: 1,
+    };
+
+    epochCache.getL1Constants.mockReturnValue(l1Constants);
+
     watcher = new EpochPruneWatcher(l2BlockSource, l1ToL2MessageSource, epochCache, txProvider, blockBuilder, {
       slashPrunePenalty: validEpochPrunedPenalty,
       slashDataWithholdingPenalty: dataWithholdingPenalty,
@@ -54,9 +71,10 @@ describe('EpochPruneWatcher', () => {
 
   it('should emit WANT_TO_SLASH_EVENT when a validator is in a pruned epoch when data is unavailable', async () => {
     const emitSpy = jest.spyOn(watcher, 'emit');
+    const epochNumber = 1n;
 
     const block = await L2Block.random(
-      1, // block number
+      12, // block number
       4, // txs per block
     );
     txProvider.getAvailableTxs.mockResolvedValue({ txs: [], missingTxs: [block.body.txEffects[0].txHash] });
@@ -68,11 +86,11 @@ describe('EpochPruneWatcher', () => {
     epochCache.getCommitteeForEpoch.mockResolvedValue({
       committee: committee.map(EthAddress.fromString),
       seed: 0n,
-      epoch: 1n,
+      epoch: epochNumber,
     });
 
     l2BlockSource.emit(L2BlockSourceEvents.L2PruneDetected, {
-      epochNumber: 1n,
+      epochNumber,
       blocks: [block],
       type: L2BlockSourceEvents.L2PruneDetected,
     });
@@ -85,13 +103,13 @@ describe('EpochPruneWatcher', () => {
         validator: EthAddress.fromString(committee[0]),
         amount: dataWithholdingPenalty,
         offenseType: OffenseType.DATA_WITHHOLDING,
-        epochOrSlot: 1n,
+        epochOrSlot: epochNumber,
       },
       {
         validator: EthAddress.fromString(committee[1]),
         amount: dataWithholdingPenalty,
         offenseType: OffenseType.DATA_WITHHOLDING,
-        epochOrSlot: 1n,
+        epochOrSlot: epochNumber,
       },
     ] satisfies WantToSlashArgs[]);
   });
@@ -100,7 +118,7 @@ describe('EpochPruneWatcher', () => {
     const emitSpy = jest.spyOn(watcher, 'emit');
 
     const block = await L2Block.random(
-      1, // block number
+      12, // block number
       4, // txs per block
     );
     const tx = Tx.random();
@@ -152,11 +170,11 @@ describe('EpochPruneWatcher', () => {
     const emitSpy = jest.spyOn(watcher, 'emit');
 
     const blockFromL1 = await L2Block.random(
-      1, // block number
+      12, // block number
       1, // txs per block
     );
     const blockFromBuilder = await L2Block.random(
-      2, // block number
+      13, // block number
       1, // txs per block
     );
     const tx = Tx.random();

yarn-project/slasher/src/watchers/epoch_prune_watcher.ts

@@ -9,6 +9,7 @@ import {
   type L2BlockSourceEventEmitter,
   L2BlockSourceEvents,
 } from '@aztec/stdlib/block';
+import { getEpochAtSlot } from '@aztec/stdlib/epoch-helpers';
 import type {
   IFullNodeBlockBuilder,
   ITxProvider,
@@ -78,9 +79,14 @@ export class EpochPruneWatcher extends (EventEmitter as new () => WatcherEmitter
 
   private handlePruneL2Blocks(event: L2BlockPruneEvent): void {
     const { blocks, epochNumber } = event;
-    this.log.info(`Detected chain prune. Validating epoch ${epochNumber}`);
+    const l1Constants = this.epochCache.getL1Constants();
+    const epochBlocks = blocks.filter(b => getEpochAtSlot(b.slot, l1Constants) === epochNumber);
+    this.log.info(
+      `Detected chain prune. Validating epoch ${epochNumber} with blocks ${epochBlocks[0]?.number} to ${epochBlocks[epochBlocks.length - 1]?.number}.`,
+      { blocks: epochBlocks.map(b => b.toBlockInfo()) },
+    );
 
-    this.validateBlocks(blocks)
+    this.validateBlocks(epochBlocks)
       .then(async () => {
         this.log.info(`Pruned epoch ${epochNumber} was valid. Want to slash committee for not having it proven.`);
         const validators = await this.getValidatorsForEpoch(epochNumber);