l2geth: replica gas estimation fix (#519)

* l2geth: replica must also keep track of gas

* l2geth: refactor sequencer loop

* l2geth: updateL1GasPrice comment

* l2geth: explicitly test gas updating

* l2geth: add changeset

.changeset/tough-bugs-invent.md

+---
+"@eth-optimism/l2geth": patch
+---
+
+Allow gas estimation for replicas

l2geth/rollup/sync_service.go

@@ -296,6 +296,9 @@ func (s *SyncService) Stop() error {
 func (s *SyncService) VerifierLoop() {
 	log.Info("Starting Verifier Loop", "poll-interval", s.pollInterval, "timestamp-refresh-threshold", s.timestampRefreshThreshold)
 	for {
+		if err := s.updateL1GasPrice(); err != nil {
+			log.Error("Cannot update L1 gas price", "msg", err)
+		}
 		if err := s.verify(); err != nil {
 			log.Error("Could not verify", "error", err)
 		}
@@ -344,6 +347,9 @@ func (s *SyncService) verify() error {
 func (s *SyncService) SequencerLoop() {
 	log.Info("Starting Sequencer Loop", "poll-interval", s.pollInterval, "timestamp-refresh-threshold", s.timestampRefreshThreshold)
 	for {
+		if err := s.updateL1GasPrice(); err != nil {
+			log.Error("Cannot update L1 gas price", "msg", err)
+		}
 		s.txLock.Lock()
 		err := s.sequence()
 		if err != nil {
@@ -360,14 +366,6 @@ func (s *SyncService) SequencerLoop() {
 }
 
 func (s *SyncService) sequence() error {
-	// Update to the latest L1 gas price
-	l1GasPrice, err := s.client.GetL1GasPrice()
-	if err != nil {
-		return err
-	}
-	s.L1gpo.SetL1GasPrice(l1GasPrice)
-	log.Info("Adjusted L1 Gas Price", "gasprice", l1GasPrice)
-
 	// Only the sequencer needs to poll for enqueue transactions
 	// and then can choose when to apply them. We choose to apply
 	// transactions such that it makes for efficient batch submitting.
@@ -445,6 +443,19 @@ func (s *SyncService) sequence() error {
 	return nil
 }
 
+// updateL1GasPrice queries for the current L1 gas price and then stores it
+// in the L1 Gas Price Oracle. This must be called over time to properly
+// estimate the transaction fees that the sequencer should charge.
+func (s *SyncService) updateL1GasPrice() error {
+	l1GasPrice, err := s.client.GetL1GasPrice()
+	if err != nil {
+		return err
+	}
+	s.L1gpo.SetL1GasPrice(l1GasPrice)
+	log.Info("Adjusted L1 Gas Price", "gasprice", l1GasPrice)
+	return nil
+}
+
 /// Update the execution context's timestamp and blocknumber
 /// over time. This is only necessary for the sequencer.
 func (s *SyncService) updateContext() error {

l2geth/rollup/sync_service_test.go

@@ -169,8 +169,8 @@ func TestSyncServiceL1GasPrice(t *testing.T) {
 		t.Fatal("expected 0 gas price, got", gasBefore)
 	}
 
-	// run 1 iteration of the eloop
-	service.sequence()
+	// Update the gas price
+	service.updateL1GasPrice()
 
 	gasAfter, err := service.L1gpo.SuggestDataPrice(context.Background())
 	if err != nil {