core: fix op-mainnet-genesis hash (ethereum-optimism#405)

- fix genesis-spec-from-DB export
- fix OP-Mainnet genesis Commit() of canonical block hash

Author: protolambda

core/genesis.go

@@ -116,9 +116,11 @@ func ReadGenesis(db ethdb.Database) (*Genesis, error) {
 	genesis.BaseFee = genesisHeader.BaseFee
 	genesis.ExcessBlobGas = genesisHeader.ExcessBlobGas
 	genesis.BlobGasUsed = genesisHeader.BlobGasUsed
-	if genesis.Alloc == nil {
-		h := genesisHeader.Hash()
+	// A nil or empty alloc, with a non-matching state-root in the block header, intents to override the state-root.
+	if genesis.Alloc == nil || (len(genesis.Alloc) == 0 && genesisHeader.Root != types.EmptyRootHash) {
+		h := genesisHeader.Root // the genesis block is encoded as RLP in the DB and will contain the state-root
 		genesis.StateHash = &h
+		genesis.Alloc = nil
 	}
 
 	return &genesis, nil
@@ -567,12 +569,23 @@ func (g *Genesis) Commit(db ethdb.Database, triedb *triedb.Database) (*types.Blo
 	if config.Clique != nil && len(g.ExtraData) < 32+crypto.SignatureLength {
 		return nil, errors.New("can't start clique chain without signers")
 	}
-	// flush the data to disk and compute the state root
-	root, err := flushAlloc(&g.Alloc, triedb)
-	if err != nil {
-		return nil, err
+	var stateHash common.Hash
+	if len(g.Alloc) == 0 {
+		if g.StateHash == nil {
+			log.Warn("Empty genesis alloc, and no 'stateHash' override was set")
+			stateHash = types.EmptyRootHash // default to the hash of the empty state. Some unit-tests rely on this.
+		} else {
+			stateHash = *g.StateHash
+		}
+	} else {
+		// flush the data to disk and compute the state root
+		root, err := flushAlloc(&g.Alloc, triedb)
+		if err != nil {
+			return nil, err
+		}
+		stateHash = root
 	}
-	block := g.toBlockWithRoot(root)
+	block := g.toBlockWithRoot(stateHash)
 
 	// Marshal the genesis state specification and persist.
 	blob, err := json.Marshal(g.Alloc)

core/superchain.go

@@ -7,6 +7,7 @@ import (
 	"github.com/ethereum-optimism/superchain-registry/superchain"
 
 	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/ethereum/go-ethereum/params"
 )
 
@@ -35,7 +36,7 @@ func LoadOPStackGenesis(chainID uint64) (*Genesis, error) {
 		Difficulty:    (*big.Int)(gen.Difficulty),
 		Mixhash:       common.Hash(gen.Mixhash),
 		Coinbase:      common.Address(gen.Coinbase),
-		Alloc:         make(GenesisAlloc),
+		Alloc:         make(types.GenesisAlloc),
 		Number:        gen.Number,
 		GasUsed:       gen.GasUsed,
 		ParentHash:    common.Hash(gen.ParentHash),
@@ -76,6 +77,7 @@ func LoadOPStackGenesis(chainID uint64) (*Genesis, error) {
 			return nil, fmt.Errorf("chain definition unexpectedly contains both allocation (%d) and state-hash %s", len(gen.Alloc), *gen.StateHash)
 		}
 		genesis.StateHash = (*common.Hash)(gen.StateHash)
+		genesis.Alloc = nil
 	}
 
 	genesisBlock := genesis.ToBlock()

core/superchain_test.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/ethereum-optimism/superchain-registry/superchain"
 
+	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core/rawdb"
 	"github.com/ethereum/go-ethereum/triedb"
 )
@@ -102,3 +103,23 @@ func TestRegistryChainConfigOverride(t *testing.T) {
 		})
 	}
 }
+
+func TestOPMainnetGenesisDB(t *testing.T) {
+	db := rawdb.NewMemoryDatabase()
+	genesis, err := LoadOPStackGenesis(10)
+	if err != nil {
+		t.Fatal(err)
+	}
+	tdb := triedb.NewDatabase(db, newDbConfig(rawdb.PathScheme))
+	genesis.MustCommit(db, tdb)
+	bl := genesis.ToBlock()
+	expected := common.HexToHash("0x7ca38a1916c42007829c55e69d3e9a73265554b586a499015373241b8a3fa48b")
+	if blockHash := bl.Hash(); blockHash != expected {
+		t.Fatalf("block hash mismatch: %s <> %s", blockHash, expected)
+	}
+	// This is written separately to the DB by Commit() and is thus tested explicitly here
+	canonicalHash := rawdb.ReadCanonicalHash(db, 0)
+	if canonicalHash != expected {
+		t.Fatalf("canonical hash mismatch: %s <> %s", canonicalHash, expected)
+	}
+}