Merge pull request #1847 from cfromknecht/gas-oracle-rounding

fix: don't round down average gas per epoch

.changeset/silly-bugs-sit.md

+---
+'@eth-optimism/gas-oracle': patch
+---
+
+fix rounding error in average gas/epoch calculation

go/gas-oracle/gasprices/gas_price_updater.go

@@ -78,7 +78,7 @@ func (g *GasPriceUpdater) UpdateGasPrice() error {
 		totalGasUsed += gasUsed
 	}
 
-	averageGasPerSecond := float64(totalGasUsed / g.epochLengthSeconds)
+	averageGasPerSecond := float64(totalGasUsed) / float64(g.epochLengthSeconds)
 
 	log.Debug("UpdateGasPrice", "average-gas-per-second", averageGasPerSecond, "current-price", g.gasPricer.curPrice)
 	_, err = g.gasPricer.CompleteEpoch(averageGasPerSecond)

go/gas-oracle/gasprices/gas_price_updater_test.go

@@ -13,7 +13,7 @@ type MockEpoch struct {
 
 // Return a gas pricer that targets 3 blocks per epoch & 10% max change per epoch.
 func makeTestGasPricerAndUpdater(curPrice uint64) (*GasPricer, *GasPriceUpdater, func(uint64), error) {
-	gpsTarget := 3300000.0
+	gpsTarget := 990000.3
 	getGasTarget := func() float64 { return gpsTarget }
 	epochLengthSeconds := uint64(10)
 	averageBlockGasLimit := uint64(11000000)
@@ -136,6 +136,13 @@ func TestUsageOfGasPriceUpdater(t *testing.T) {
 				if prevGasPrice != curPrice {
 					t.Fatalf("Expected gas price to stablize. Got %d, was %d", curPrice, prevGasPrice)
 				}
+
+				targetGps := gasPriceUpdater.gasPricer.getTargetGasPerSecond()
+				averageGps := gasPriceUpdater.gasPricer.avgGasPerSecondLastEpoch
+				if targetGps != averageGps {
+					t.Fatalf("Average gas/second (%f) did not converge to target (%f)",
+						averageGps, targetGps)
+				}
 			},
 		},
 		// Then reduce the demand to show the fee goes back down to the floor

go/gas-oracle/gasprices/l2_gas_pricer.go

@@ -12,6 +12,7 @@ type GetTargetGasPerSecond func() float64
 
 type GasPricer struct {
 	curPrice                 uint64
+	avgGasPerSecondLastEpoch float64
 	floorPrice               uint64
 	getTargetGasPerSecond    GetTargetGasPerSecond
 	maxChangePerEpoch        float64
@@ -80,6 +81,7 @@ func (p *GasPricer) CompleteEpoch(avgGasPerSecondLastEpoch float64) (uint64, err
 		return gp, err
 	}
 	p.curPrice = gp
+	p.avgGasPerSecondLastEpoch = avgGasPerSecondLastEpoch
 	return gp, nil
 }