op-e2e: add random withdrawals to fake-proof-of-stake

op-e2e/e2eutils/geth/fakepos.go

 package geth
 
 import (
+	"math/rand"
 	"time"
 
-	"github.com/ethereum-optimism/optimism/op-service/clock"
 	"github.com/ethereum/go-ethereum"
 	"github.com/ethereum/go-ethereum/beacon/engine"
 	"github.com/ethereum/go-ethereum/common"
@@ -12,6 +12,9 @@ import (
 	"github.com/ethereum/go-ethereum/eth/catalyst"
 	"github.com/ethereum/go-ethereum/event"
 	"github.com/ethereum/go-ethereum/log"
+
+	"github.com/ethereum-optimism/optimism/op-service/clock"
+	"github.com/ethereum-optimism/optimism/op-service/testutils"
 )
 
 // fakePoS is a testing-only utility to attach to Geth,
@@ -22,6 +25,8 @@ type fakePoS struct {
 	log       log.Logger
 	blockTime uint64
 
+	withdrawalsIndex uint64
+
 	finalizedDistance uint64
 	safeDistance      uint64
 
@@ -33,6 +38,7 @@ func (f *fakePoS) Start() error {
 	if advancing, ok := f.clock.(*clock.AdvancingClock); ok {
 		advancing.Start()
 	}
+	withdrawalsRNG := rand.New(rand.NewSource(450368975843)) // avoid generating the same address as any test
 	f.sub = event.NewSubscription(func(quit <-chan struct{}) error {
 		// poll every half a second: enough to catch up with any block time when ticks are missed
 		t := f.clock.NewTicker(time.Second / 2)
@@ -64,6 +70,17 @@ func (f *fakePoS) Start() error {
 					// We're a long way behind, let's skip some blocks...
 					newBlockTime = uint64(f.clock.Now().Unix())
 				}
+				// create some random withdrawals
+				withdrawals := make([]*types.Withdrawal, withdrawalsRNG.Intn(4))
+				for i := 0; i < len(withdrawals); i++ {
+					withdrawals[i] = &types.Withdrawal{
+						Index:     f.withdrawalsIndex + uint64(i),
+						Validator: withdrawalsRNG.Uint64() % 100_000_000, // 100 million fake validators
+						Address:   testutils.RandomAddress(withdrawalsRNG),
+						// in gwei, consensus-layer quirk. withdraw non-zero value up to 50 ETH
+						Amount: uint64(withdrawalsRNG.Intn(50_000_000_000) + 1),
+					}
+				}
 				res, err := f.engineAPI.ForkchoiceUpdatedV2(engine.ForkchoiceStateV1{
 					HeadBlockHash:      head.Hash(),
 					SafeBlockHash:      safe.Hash(),
@@ -72,7 +89,7 @@ func (f *fakePoS) Start() error {
 					Timestamp:             newBlockTime,
 					Random:                common.Hash{},
 					SuggestedFeeRecipient: head.Coinbase,
-					Withdrawals:           make([]*types.Withdrawal, 0),
+					Withdrawals:           withdrawals,
 				})
 				if err != nil {
 					f.log.Error("failed to start building L1 block", "err", err)
@@ -109,6 +126,10 @@ func (f *fakePoS) Start() error {
 					f.log.Error("failed to make built L1 block canonical", "err", err)
 					continue
 				}
+				// Increment global withdrawals index in the CL.
+				// The EL doesn't really care about the value,
+				// but it's nice to mock something consistent with the CL specs.
+				f.withdrawalsIndex += uint64(len(withdrawals))
 			case <-quit:
 				return nil
 			}