cmd/geth: make dumpgenesis command load genesis from the datadir if i…

…t exists

cmd/geth/chaincmd.go

@@ -64,7 +64,7 @@ It expects the genesis file as argument.`,
 		Flags:     utils.NetworkFlags,
 		Category:  "BLOCKCHAIN COMMANDS",
 		Description: `
-The dumpgenesis command dumps the genesis block configuration in JSON format to stdout.`,
+The dumpgenesis command prints a genesis block as JSON. What it outputs is determined in order of preference: testnet preset if it is set, preexisting datadir, mainnet.`,
 	}
 	importCommand = cli.Command{
 		Action:    utils.MigrateFlags(importChain),
@@ -207,13 +207,41 @@ func initGenesis(ctx *cli.Context) error {
 }
 
 func dumpGenesis(ctx *cli.Context) error {
-	// TODO(rjl493456442) support loading from the custom datadir
-	genesis := utils.MakeGenesis(ctx)
-	if genesis == nil {
-		genesis = core.DefaultGenesisBlock()
+	// if there is a testnet preset enabled, dump that
+	if utils.IsTestnetPreset(ctx) {
+		genesis := utils.MakeGenesis(ctx)
+		if err := json.NewEncoder(os.Stdout).Encode(genesis); err != nil {
+			utils.Fatalf("could not encode genesis: %s", err)
+		}
+		return nil
+	}
+	// dump whatever already exists in the datadir
+	stack, _ := makeConfigNode(ctx)
+	for _, name := range []string{"chaindata", "lightchaindata"} {
+		db, err := stack.OpenDatabase(name, 0, 0, "", true)
+		if err != nil {
+			if !os.IsNotExist(err) {
+				return err
+			}
+			continue
+		}
+		genesis, err := core.ReadGenesis(db)
+		if err != nil {
+			utils.Fatalf("failed to read genesis: %s", err)
+		}
+		db.Close()
+
+		if err := json.NewEncoder(os.Stdout).Encode(*genesis); err != nil {
+			utils.Fatalf("could not encode stored genesis: %s", err)
+		}
+		return nil
+	}
+	if ctx.GlobalIsSet(utils.DataDirFlag.Name) {
+		utils.Fatalf("no existing datadir at %s", stack.Config().DataDir)
 	}
-	if err := json.NewEncoder(os.Stdout).Encode(genesis); err != nil {
-		utils.Fatalf("could not encode genesis")
+	// no datadir exists or is specified, dump mainnet
+	if err := json.NewEncoder(os.Stdout).Encode(core.DefaultGenesisBlock()); err != nil {
+		utils.Fatalf("could not encode genesis: %s", err)
 	}
 	return nil
 }

cmd/utils/flags.go

@@ -1987,6 +1987,13 @@ func MakeChainDatabase(ctx *cli.Context, stack *node.Node, readonly bool) ethdb.
 	return chainDb
 }
 
+func IsTestnetPreset(ctx *cli.Context) bool {
+	if ctx.GlobalBool(RopstenFlag.Name) || ctx.GlobalBool(SepoliaFlag.Name) || ctx.GlobalBool(RinkebyFlag.Name) || ctx.GlobalBool(GoerliFlag.Name) || ctx.GlobalBool(KilnFlag.Name) || ctx.GlobalBool(DeveloperFlag.Name) {
+		return true
+	}
+	return false
+}
+
 func MakeGenesis(ctx *cli.Context) *core.Genesis {
 	var genesis *core.Genesis
 	switch {

core/genesis.go

@@ -65,6 +65,41 @@ type Genesis struct {
 	BaseFee    *big.Int    `json:"baseFeePerGas"`
 }
 
+func ReadGenesis(db ethdb.Database) (*Genesis, error) {
+	var genesis Genesis
+	stored := rawdb.ReadCanonicalHash(db, 0)
+	if (stored == common.Hash{}) {
+		return nil, fmt.Errorf("invalid genesis hash in database: %x", stored)
+	}
+	blob := rawdb.ReadGenesisState(db, stored)
+	if blob == nil {
+		return nil, fmt.Errorf("genesis state missing from db")
+	}
+	if len(blob) != 0 {
+		if err := genesis.Alloc.UnmarshalJSON(blob); err != nil {
+			return nil, fmt.Errorf("could not unmartial genesis state json: %s", err)
+		}
+	}
+	genesis.Config = rawdb.ReadChainConfig(db, stored)
+	if genesis.Config == nil {
+		return nil, fmt.Errorf("genesis config missing from db")
+	}
+	genesisBlock := rawdb.ReadBlock(db, stored, 0)
+	if genesisBlock == nil {
+		return nil, fmt.Errorf("genesis block missing from db")
+	}
+	genesisHeader := genesisBlock.Header()
+	genesis.Nonce = genesisHeader.Nonce.Uint64()
+	genesis.Timestamp = genesisHeader.Time
+	genesis.ExtraData = genesisHeader.Extra
+	genesis.GasLimit = genesisHeader.GasLimit
+	genesis.Difficulty = genesisHeader.Difficulty
+	genesis.Mixhash = genesisHeader.MixDigest
+	genesis.Coinbase = genesisHeader.Coinbase
+
+	return &genesis, nil
+}
+
 // GenesisAlloc specifies the initial state that is part of the genesis block.
 type GenesisAlloc map[common.Address]GenesisAccount