Introduce fallibleCommit which can error

This allows the inner loop in the case of "on-demand" commits to bail out if the
txPool has been terminated before the doCommit channel is closed.

This has been tested with a known-flaky test which, before this commit was
spinlooping and logging unfathomable reams of warning messages. Now, when the
race occurs, only a single instance of the warning is logged.

eth/catalyst/simulated_beacon.go

@@ -302,11 +302,18 @@ func (c *SimulatedBeacon) setCurrentState(headHash, finalizedHash common.Hash) {
 
 // Commit seals a block on demand.
 func (c *SimulatedBeacon) Commit() common.Hash {
+	hash, _ := c.fallibleCommit()
+	return hash
+}
+
+// fallibleCommit attempts to seal a block on demand, but may return an error.
+func (c *SimulatedBeacon) fallibleCommit() (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)
+		return common.Hash{}, err
 	}
-	return c.eth.BlockChain().CurrentBlock().Hash()
+	return c.eth.BlockChain().CurrentBlock().Hash(), nil
 }
 
 // Rollback un-sends previously added transactions.

eth/catalyst/simulated_beacon_api.go

@@ -70,11 +70,15 @@ func (a *simulatedBeaconAPI) loop() {
 				if executable, _ := a.sim.eth.TxPool().Stats(); executable == 0 {
 					break
 				}
-				a.sim.Commit()
-				// Avoid a race condition where tx pool is terminated during Commit.
-				if err := a.sim.eth.TxPool().Sync(); err != nil {
-					break
+				// Avoid a race condition where doCommit is closed while in this loop.
+				select {
+				case _, ok := <-doCommit:
+					if !ok {
+						break
+					}
+				default:
 				}
+				a.sim.Commit()
 			}
 		}
 	}()