cmd/geth: add flag to set genesis (#32844)

This PR is an alternative to #32556.

Instead of trying to be smart and reuse `geth init`, we can introduce a
new flag `--genesis` that loads the `genesis.json` from file into the
`Genesis` object in the same path that the other network flags currently
work in.

Question: is something like `--genesis` enough to start deprecating
`geth init`?

--

```console
$ geth --datadir data --hoodi
..
INFO [10-06|22:37:11.202]  - BPO2:                        @1762955544
..
$ geth --datadir data --genesis genesis.json
..
INFO [10-06|22:37:27.988]  - BPO2:                       @1862955544
..
```

Pull the genesis [from the
specs](https://raw.githubusercontent.com/eth-clients/hoodi/refs/heads/main/metadata/genesis.json)
and modify one of the BPO timestamps to simulate a shadow fork.

---------

Co-authored-by: rjl493456442 <garyrong0905@gmail.com>

cmd/geth/main.go

@@ -66,6 +66,7 @@ var (
 		utils.OverrideBPO1,
 		utils.OverrideBPO2,
 		utils.OverrideVerkle,
+		utils.OverrideGenesisFlag,
 		utils.EnablePersonal, // deprecated
 		utils.TxPoolLocalsFlag,
 		utils.TxPoolNoLocalsFlag,

cmd/utils/flags.go

@@ -262,6 +262,11 @@ var (
 		Usage:    "Manually specify the Verkle fork timestamp, overriding the bundled setting",
 		Category: flags.EthCategory,
 	}
+	OverrideGenesisFlag = &cli.StringFlag{
+		Name:     "override.genesis",
+		Usage:    "Load genesis block and configuration from file at this path",
+		Category: flags.EthCategory,
+	}
 	SyncModeFlag = &cli.StringFlag{
 		Name:     "syncmode",
 		Usage:    `Blockchain sync mode ("snap" or "full")`,
@@ -1605,7 +1610,7 @@ 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)
+	flags.CheckExclusive(ctx, MainnetFlag, DeveloperFlag, SepoliaFlag, HoleskyFlag, HoodiFlag, NetworkIdFlag, OverrideGenesisFlag)
 	flags.CheckExclusive(ctx, DeveloperFlag, ExternalSignerFlag) // Can't use both ephemeral unlocked and external signer
 
 	// Set configurations from CLI flags
@@ -1891,6 +1896,18 @@ func SetEthConfig(ctx *cli.Context, stack *node.Node, cfg *ethconfig.Config) {
 		if !ctx.IsSet(MinerGasPriceFlag.Name) {
 			cfg.Miner.GasPrice = big.NewInt(1)
 		}
+	case ctx.String(OverrideGenesisFlag.Name) != "":
+		f, err := os.Open(ctx.String(OverrideGenesisFlag.Name))
+		if err != nil {
+			Fatalf("Failed to read genesis file: %v", err)
+		}
+		defer f.Close()
+
+		genesis := new(core.Genesis)
+		if err := json.NewDecoder(f).Decode(genesis); err != nil {
+			Fatalf("Invalid genesis file: %v", err)
+		}
+		cfg.Genesis = genesis
 	default:
 		if cfg.NetworkId == 1 {
 			SetDNSDiscoveryDefaults(cfg, params.MainnetGenesisHash)