Check if mainnet in a function

cmd/geth/main.go

@@ -288,10 +288,9 @@ func main() {
 	}
 }
 
-// prepare manipulates memory cache allowance and setups metric system.
-// This function should be called before launching devp2p stack.
-func prepare(ctx *cli.Context) {
-	// If we're running a known preset, log it for convenience.
+func checkMainnet(ctx *cli.Context) bool {
+	isMainnet := false
+
 	switch {
 	case ctx.GlobalIsSet(utils.LegacyTestnetFlag.Name):
 		log.Info("Starting Geth on Ropsten testnet...")
@@ -333,11 +332,23 @@ func prepare(ctx *cli.Context) {
 
 	case !ctx.GlobalIsSet(utils.NetworkIdFlag.Name):
 		log.Info("Starting Geth on Ethereum mainnet...")
+		isMainnet = true
 	}
+
+	return isMainnet
+}
+
+// prepare manipulates memory cache allowance and setups metric system.
+// This function should be called before launching devp2p stack.
+func prepare(ctx *cli.Context) {
+	// If we're running a known preset, log it for convenience.
+
+	isMainnet := checkMainnet(ctx)
+
 	// If we're a full node on mainnet without --cache specified, bump default cache allowance
 	if ctx.GlobalString(utils.SyncModeFlag.Name) != "light" && !ctx.GlobalIsSet(utils.CacheFlag.Name) && !ctx.GlobalIsSet(utils.NetworkIdFlag.Name) {
 		// Make sure we're not on any supported preconfigured testnet either
-		if !ctx.GlobalIsSet(utils.LegacyTestnetFlag.Name) && !ctx.GlobalIsSet(utils.RopstenFlag.Name) && !ctx.GlobalIsSet(utils.RinkebyFlag.Name) && !ctx.GlobalIsSet(utils.GoerliFlag.Name) && !ctx.GlobalIsSet(utils.DeveloperFlag.Name) {
+		if isMainnet {
 			// Nope, we're really on mainnet. Bump that cache up!
 			log.Info("Bumping default cache on mainnet", "provided", ctx.GlobalInt(utils.CacheFlag.Name), "updated", 4096)
 			ctx.GlobalSet(utils.CacheFlag.Name, strconv.Itoa(4096))