Try using a custom error type

This way, we only break the spinloop when it is definitely because the
transaction pool has already been terminated.

eth/catalyst/simulated_beacon.go

@@ -80,6 +80,17 @@ func (w *withdrawalQueue) subscribe(ch chan<- newWithdrawalsEvent) event.Subscri
 	return w.subs.Track(sub)
 }
 
+// txPoolTerminatedError is returned when the txpool is already terminated when
+// trying to seal a block.
+type txPoolTerminatedError struct {
+	error
+}
+
+// Error returns the message from the wrapped error.
+func (t *txPoolTerminatedError) Error() string {
+	return t.error.Error()
+}
+
 // SimulatedBeacon drives an Ethereum instance as if it were a real beacon
 // client. It can run in period mode where it mines a new block every period
 // (seconds) or on every transaction via Commit, Fork and AdjustTime.
@@ -181,7 +192,7 @@ func (c *SimulatedBeacon) sealBlock(withdrawals []*types.Withdrawal, timestamp u
 	// behavior, the pool will be explicitly blocked on its reset before
 	// continuing to the block production below.
 	if err := c.eth.APIBackend.TxPool().Sync(); err != nil {
-		return fmt.Errorf("failed to sync txpool: %w", err)
+		return &txPoolTerminatedError{fmt.Errorf("failed to sync txpool: %w", err)}
 	}
 
 	version := payloadVersion(c.eth.BlockChain().Config(), timestamp)
@@ -302,12 +313,12 @@ func (c *SimulatedBeacon) setCurrentState(headHash, finalizedHash common.Hash) {
 
 // Commit seals a block on demand.
 func (c *SimulatedBeacon) Commit() common.Hash {
-	hash, _ := c.fallibleCommit()
+	hash, _ := c.commit()
 	return hash
 }
 
 // fallibleCommit attempts to seal a block on demand, but may return an error.
-func (c *SimulatedBeacon) fallibleCommit() (common.Hash, error) {
+func (c *SimulatedBeacon) commit() (common.Hash, error) {
 	withdrawals := c.withdrawals.pop(10)
 	if err := c.sealBlock(withdrawals, uint64(time.Now().Unix())); err != nil {
 		log.Warn("Error performing sealing work", "err", err)

eth/catalyst/simulated_beacon_api.go

@@ -18,6 +18,7 @@ package catalyst
 
 import (
 	"context"
+	"errors"
 
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core"
@@ -71,11 +72,14 @@ func (a *simulatedBeaconAPI) loop() {
 					break
 				}
 				// Avoids spinlooping if the txPool is alredy terminated.
-				if _, err := a.sim.fallibleCommit(); err != nil {
+				if _, err := a.sim.commit(); err != nil {
+					var txpTermErr *txPoolTerminatedError
+					if errors.As(err, &txpTermErr) {
 						break
 					}
 				}
 			}
+		}
 	}()
 
 	for {