Geth overrides 0x0 genesis gas limit

[lightclient]

Seems to be due to this code: https://github.com/ethereum/go-ethereum/blob/01e5e9c2c3fa1cf7a9747148dca22d59ff9839b6/core/genesis.go#L370-L372

Unfortunately, other clients don't have this behavior. I think if an invalid gas limit is explicitly specified in the genesis we should fail to initialize.

$ cat genesis.json | jq
{
  "config": {
    "chainId": 1,
    "homesteadBlock": 0,
    "eip150Block": 0,
    "eip150Hash": "0x0000000000000000000000000000000000000000000000000000000000000000",
    "eip155Block": 0,
    "eip158Block": 0,
    "byzantiumBlock": 0,
    "constantinopleBlock": 0,
    "petersburgBlock": 0,
    "istanbulBlock": 0,
    "muirGlacierBlock": 0,
    "berlinBlock": 0,
    "londonBlock": 0,
    "arrowGlacierBlock": 0,
    "grayGlacierBlock": 0,
    "ethash": {}
  },
  "nonce": "0x0",
  "timestamp": "0x0",
  "extraData": "0x",
  "gasLimit": "0x0",
  "difficulty": "0x1",
  "mixHash": "0x0000000000000000000000000000000000000000000000000000000000000000",
  "coinbase": "0x0000000000000000000000000000000000000000",
  "alloc": {
    "658bdf435d810c91414ec09147daa6db62406379": {
      "balance": "0x487a9a304539440000"
    }
  },
  "number": "0x0",
  "gasUsed": "0x0",
  "parentHash": "0x0000000000000000000000000000000000000000000000000000000000000000",
  "baseFeePerGas": "0x3b9aca00"
}

$ geth --datadir=data init genesis.json
INFO [06-19|16:01:41.250] Maximum peer count                       ETH=50 LES=0 total=50
INFO [06-19|16:01:41.250] Smartcard socket not found, disabling    err="stat /run/pcscd/pcscd.comm: no such file or directory"
INFO [06-19|16:01:41.252] Set global gas cap                       cap=50,000,000
INFO [06-19|16:01:41.252] Allocated cache and file handles         database=/home/lc/dev/rpctests/tests/data/geth/chaindata cache=16.00MiB handles=16
INFO [06-19|16:01:41.281] Writing custom genesis block
INFO [06-19|16:01:41.281] Persisted trie from memory database      nodes=1 size=148.00B time="94.299µs" gcnodes=0 gcsize=0.00B gctime=0s livenodes=1 livesize=0.00B
INFO [06-19|16:01:41.282] Successfully wrote genesis state         database=chaindata hash=7ead2c..68bad8
INFO [06-19|16:01:41.282] Allocated cache and file handles         database=/home/lc/dev/rpctests/tests/data/geth/lightchaindata cache=16.00MiB handles=16
INFO [06-19|16:01:41.312] Writing custom genesis block
INFO [06-19|16:01:41.313] Persisted trie from memory database      nodes=1 size=148.00B time="66.308µs" gcnodes=0 gcsize=0.00B gctime=0s livenodes=1 livesize=0.00B
INFO [06-19|16:01:41.313] Successfully wrote genesis state         database=lightchaindata hash=7ead2c..68bad8

$ geth --datadir=data init genesis.json
INFO [06-19|16:05:25.288] Maximum peer count                       ETH=50 LES=0 total=50
INFO [06-19|16:05:25.288] Smartcard socket not found, disabling    err="stat /run/pcscd/pcscd.comm: no such file or directory"
INFO [06-19|16:05:25.289] Set global gas cap                       cap=50,000,000
INFO [06-19|16:05:25.289] Allocated cache and file handles         database=/home/lc/dev/rpctests/tests/data/geth/chaindata cache=16.00MiB handles=16
INFO [06-19|16:05:25.374] Opened ancient database                  database=/home/lc/dev/rpctests/tests/data/geth/chaindata/ancient readonly=false
INFO [06-19|16:05:25.375] Writing custom genesis block
INFO [06-19|16:05:25.375] Persisted trie from memory database      nodes=1 size=148.00B time="91.823µs" gcnodes=0 gcsize=0.00B gctime=0s livenodes=1 livesize=0.00B
INFO [06-19|16:05:25.376] Successfully wrote genesis state         database=chaindata hash=7ead2c..68bad8
INFO [06-19|16:05:25.376] Allocated cache and file handles         database=/home/lc/dev/rpctests/tests/data/geth/lightchaindata cache=16.00MiB handles=16
INFO [06-19|16:05:25.461] Opened ancient database                  database=/home/lc/dev/rpctests/tests/data/geth/lightchaindata/ancient readonly=false
INFO [06-19|16:05:25.461] Writing custom genesis block
INFO [06-19|16:05:25.462] Persisted trie from memory database      nodes=1 size=148.00B time="85.112µs" gcnodes=0 gcsize=0.00B gctime=0s livenodes=1 livesize=0.00B
INFO [06-19|16:05:25.463] Successfully wrote genesis state         database=lightchaindata hash=7ead2c..68bad8

$ geth --datadir=data --nodiscover console --port 30333 2>/dev/null
Welcome to the Geth JavaScript console!

instance: Geth/v1.10.19-stable-23bee162/linux-amd64/go1.17.7
at block: 0 (Thu Jan 01 1970 01:00:00 GMT+0100 (CET))
 datadir: /home/lc/dev/rpctests/tests/data
 modules: admin:1.0 debug:1.0 eth:1.0 ethash:1.0 miner:1.0 net:1.0 personal:1.0 rpc:1.0 txpool:1.0 web3:1.0

To exit, press ctrl-d or type exit
> eth.getBlockByNumber(0)
{
  baseFeePerGas: "0x3b9aca00",
  difficulty: "0x1",
  extraData: "0x",
  gasLimit: "0x47e7c4",
  gasUsed: "0x0",
  hash: "0x7ead2ce6067d325b8e27e55377db7e9acaa63b1de9cc880c174cee690b68bad8",
  logsBloom: "0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000",
  miner: "0x0000000000000000000000000000000000000000",
  mixHash: "0x0000000000000000000000000000000000000000000000000000000000000000",
  nonce: "0x0000000000000000",
  number: "0x0",
  parentHash: "0x0000000000000000000000000000000000000000000000000000000000000000",
  receiptsRoot: "0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421",
  sha3Uncles: "0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347",
  size: "0x1fd",
  stateRoot: "0x21b036e39374a278109134aa04d153e16999e838316561a114a563d6ff9e6707",
  timestamp: "0x0",
  totalDifficulty: "0x1",
  transactions: [],
  transactionsRoot: "0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421",
  uncles: []
}

[holiman]

if g.GasLimit == 0 -- the problem is that GasLimit is a uint64, and if the user didn't specify anything at all, then it's 0. The case where the user explicitly specifies 0 is, I guess, something nobody considered. I don't see any simple way to both "default to params.GenesisLimit if nothing is specified" and "reject explicit zero", but maybe you can figure something out :)