l2geth: don't block read rpc when doing initial sync

Allow for read requests during initial sync but
don't allow for write requests during initial sync.
This is done by spawning a goroutine so that the
initial sync logic is non-blocking.

.changeset/nasty-boats-type.md

+---
+'@eth-optimism/l2geth': patch
+---
+
+Don't block read rpc requests when syncing

l2geth/internal/ethapi/api.go

@@ -53,6 +53,7 @@ import (
 var (
 	errOVMUnsupported = errors.New("OVM: Unsupported RPC Method")
 	errNoSequencerURL = errors.New("sequencer transaction forwarding not configured")
+	errStillSyncing   = errors.New("sequencer still syncing, cannot accept transactions")
 )
 
 const (
@@ -1597,6 +1598,12 @@ func (args *SendTxArgs) toTransaction() *types.Transaction {
 
 // SubmitTransaction is a helper function that submits tx to txPool and logs a message.
 func SubmitTransaction(ctx context.Context, b Backend, tx *types.Transaction) (common.Hash, error) {
+	// Do not accept transactions if running as the sequencer and
+	// the node is still syncing.
+	if !b.IsVerifier() && b.IsSyncing() {
+		return common.Hash{}, errStillSyncing
+	}
+
 	if err := b.SendTx(ctx, tx); err != nil {
 		return common.Hash{}, err
 	}

l2geth/rollup/sync_service.go

@@ -245,17 +245,16 @@ func (s *SyncService) Start() error {
 	if s.verifier {
 		go s.VerifierLoop()
 	} else {
-		// The sequencer must sync the transactions to the tip and the
-		// pending queue transactions on start before setting sync status
-		// to false and opening up the RPC to accept transactions.
-		if err := s.syncTransactionsToTip(); err != nil {
-			return fmt.Errorf("Sequencer cannot sync transactions to tip: %w", err)
-		}
-		if err := s.syncQueueToTip(); err != nil {
-			return fmt.Errorf("Sequencer cannot sync queue to tip: %w", err)
-		}
-		s.setSyncStatus(false)
-		go s.SequencerLoop()
+		go func() {
+			if err := s.syncTransactionsToTip(); err != nil {
+				log.Crit("Sequencer cannot sync transactions to tip: %w", err)
+			}
+			if err := s.syncQueueToTip(); err != nil {
+				log.Crit("Sequencer cannot sync queue to tip: %w", err)
+			}
+			s.setSyncStatus(false)
+			go s.SequencerLoop()
+		}()
 	}
 	return nil
 }