core: assign default difficulty to zero for chain without ethash (#31067)

I hit this case while trying something with the simulated backend. The
EVM only enables instruction set forks after the merge when 'Random' is
set. In the simulated backend, the random value will be set via the
engine API for all blocks after genesis. But for the genesis block
itself, the random value will not be assigned in the vm.BlockContext
because the genesis has a non-zero difficulty. For my case, this meant
that estimateGas did not work for the first transaction sent on the
simulated chain, since the contract contained a PUSH0 instruction.

This could also be fixed by explicitly configuring a zero difficulty in
the simulated backend. However, I think that zero difficulty is a better
default these days.

---------

Co-authored-by: lightclient <lightclient@protonmail.com>

core/genesis.go

@@ -464,8 +464,12 @@ func (g *Genesis) toBlockWithRoot(root common.Hash) *types.Block {
 	if g.GasLimit == 0 {
 		head.GasLimit = params.GenesisGasLimit
 	}
-	if g.Difficulty == nil && g.Mixhash == (common.Hash{}) {
-		head.Difficulty = params.GenesisDifficulty
+	if g.Difficulty == nil {
+		if g.Config != nil && g.Config.Ethash == nil {
+			head.Difficulty = big.NewInt(0)
+		} else if g.Mixhash == (common.Hash{}) {
+			head.Difficulty = params.GenesisDifficulty
+		}
 	}
 	if g.Config != nil && g.Config.IsLondon(common.Big0) {
 		if g.BaseFee != nil {

core/genesis_test.go

@@ -44,14 +44,14 @@ func testSetupGenesis(t *testing.T, scheme string) {
 	var (
 		customghash = common.HexToHash("0x89c99d90b79719238d2645c7642f2c9295246e80775b38cfd162b696817fbd50")
 		customg     = Genesis{
-			Config: &params.ChainConfig{HomesteadBlock: big.NewInt(3)},
+			Config: &params.ChainConfig{HomesteadBlock: big.NewInt(3), Ethash: &params.EthashConfig{}},
 			Alloc: types.GenesisAlloc{
 				{1}: {Balance: big.NewInt(1), Storage: map[common.Hash]common.Hash{{1}: {1}}},
 			},
 		}
 		oldcustomg = customg
 	)
-	oldcustomg.Config = &params.ChainConfig{HomesteadBlock: big.NewInt(2)}
+	oldcustomg.Config = &params.ChainConfig{HomesteadBlock: big.NewInt(2), Ethash: &params.EthashConfig{}}
 
 	tests := []struct {
 		name           string