fix: don't round down average gas per epoch

Also corrects the targetGps constant in the test harness. The constant
was chosen to be the expected Gps of the initial burst, causing the Gps
to stablize in the second part of the test at the floor.

Adding a check that the average measured gas price matches our desired
constant prevents this from silently passing.

.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

@@ -11,10 +11,11 @@ import (
 type GetTargetGasPerSecond func() float64
 
 type GasPricer struct {
-	curPrice              uint64
-	floorPrice            uint64
-	getTargetGasPerSecond GetTargetGasPerSecond
-	maxChangePerEpoch     float64
+	curPrice                 uint64
+	avgGasPerSecondLastEpoch float64
+	floorPrice               uint64
+	getTargetGasPerSecond    GetTargetGasPerSecond
+	maxChangePerEpoch        float64
 }
 
 // LinearInterpolation can be used to dynamically update target gas per second
@@ -80,6 +81,7 @@ func (p *GasPricer) CompleteEpoch(avgGasPerSecondLastEpoch float64) (uint64, err
 		return gp, err
 	}
 	p.curPrice = gp
+	p.avgGasPerSecondLastEpoch = avgGasPerSecondLastEpoch
 	return gp, nil
 }