cmd/utils: allow --networkid to override other config options (#32999)

Recently in #31630 we removed support for overriding the network id in
preset networks. While this feature is niche, it is useful for shadow
forks. This PR proposes we add the functionality back, but in a simpler
way.

Instead of checking whether the flag is set in each branch of the
network switch statement, simply apply the network flag after the switch
statement is complete. This retains the following behavior:

1. Auto network id based on chain id still works, because `IsSet` only
returns true if the flag is _actually_ set. Not if it just has a default
set.
2. The preset networks will set their network id directly and only if
the network id flag is set is it overridden. This, combined with the
override genesis flag is what allows the shadow forks.
3. Setting the network id to the same network id that the preset _would
have_ set causes no issues and simply emits the `WARN` that the flag is
being set explicitly. I don't think people explicitly set the network id
flag often.

```
WARN [10-22|09:36:15.052] Setting network id with flag             id=10
```

---------

Co-authored-by: Felix Lange <[email protected]>

Author: lightclient

cmd/geth/consolecmd_test.go

@@ -39,8 +39,9 @@ const (
 // child g gets a temporary data directory.
 func runMinimalGeth(t *testing.T, args ...string) *testgeth {
 	// --holesky to make the 'writing genesis to disk' faster (no accounts)
+	// --networkid=1337 to avoid cache bump
 	// --syncmode=full to avoid allocating fast sync bloom
-	allArgs := []string{"--holesky", "--authrpc.port", "0", "--syncmode=full", "--port", "0",
+	allArgs := []string{"--holesky", "--networkid", "1337", "--authrpc.port", "0", "--syncmode=full", "--port", "0",
 		"--nat", "none", "--nodiscover", "--maxpeers", "0", "--cache", "64",
 		"--datadir.minfreedisk", "0"}
 	return runGeth(t, append(allArgs, args...)...)

cmd/utils/flags.go

@@ -137,7 +137,7 @@ var (
 	}
 	NetworkIdFlag = &cli.Uint64Flag{
 		Name:     "networkid",
-		Usage:    "Explicitly set network id (integer)(For testnets: use --sepolia, --holesky, --hoodi instead)",
+		Usage:    "Explicitly set network ID (integer)(For testnets: use --sepolia, --holesky, --hoodi instead)",
 		Value:    ethconfig.Defaults.NetworkId,
 		Category: flags.EthCategory,
 	}
@@ -1615,8 +1615,8 @@ func setRequiredBlocks(ctx *cli.Context, cfg *ethconfig.Config) {
 
 // SetEthConfig applies eth-related command line flags to the config.
 func SetEthConfig(ctx *cli.Context, stack *node.Node, cfg *ethconfig.Config) {
-	// Avoid conflicting network flags, don't allow network id override on preset networks
-	flags.CheckExclusive(ctx, MainnetFlag, DeveloperFlag, SepoliaFlag, HoleskyFlag, HoodiFlag, NetworkIdFlag, OverrideGenesisFlag)
+	// Avoid conflicting network flags
+	flags.CheckExclusive(ctx, MainnetFlag, DeveloperFlag, SepoliaFlag, HoleskyFlag, HoodiFlag, OverrideGenesisFlag)
 	flags.CheckExclusive(ctx, DeveloperFlag, ExternalSignerFlag) // Can't use both ephemeral unlocked and external signer
 
 	// Set configurations from CLI flags
@@ -1663,9 +1663,6 @@ func SetEthConfig(ctx *cli.Context, stack *node.Node, cfg *ethconfig.Config) {
 		}
 	}
 
-	if ctx.IsSet(NetworkIdFlag.Name) {
-		cfg.NetworkId = ctx.Uint64(NetworkIdFlag.Name)
-	}
 	if ctx.IsSet(CacheFlag.Name) || ctx.IsSet(CacheDatabaseFlag.Name) {
 		cfg.DatabaseCache = ctx.Int(CacheFlag.Name) * ctx.Int(CacheDatabaseFlag.Name) / 100
 	}
@@ -1915,10 +1912,18 @@ func SetEthConfig(ctx *cli.Context, stack *node.Node, cfg *ethconfig.Config) {
 		}
 		cfg.Genesis = genesis
 	default:
-		if cfg.NetworkId == 1 {
+		if ctx.Uint64(NetworkIdFlag.Name) == 1 {
 			SetDNSDiscoveryDefaults(cfg, params.MainnetGenesisHash)
 		}
 	}
+	if ctx.IsSet(NetworkIdFlag.Name) {
+		// Typically it's best to automatically set the network ID to the chainID,
+		// by not passing the --networkid flag at all. Emit a warning when set
+		// explicitly in case overriding the network ID is not the user's intention.
+		id := ctx.Uint64(NetworkIdFlag.Name)
+		log.Warn("Setting network ID with command-line flag", "id", id)
+		cfg.NetworkId = id
+	}
 	// Set any dangling config values
 	if ctx.String(CryptoKZGFlag.Name) != "gokzg" && ctx.String(CryptoKZGFlag.Name) != "ckzg" {
 		Fatalf("--%s flag must be 'gokzg' or 'ckzg'", CryptoKZGFlag.Name)