core, eth/tracers: move chainconfig to tracers and some refactors

cmd/utils/flags.go

@@ -650,6 +650,7 @@ var (
 	VMTraceConfigFlag = &cli.StringFlag{
 		Name:     "vmtrace.config",
 		Usage:    "Tracer configuration (JSON)",
+		Value:    "{}",
 		Category: flags.VMCategory,
 	}
 	RPCGlobalGasCapFlag = &cli.Uint64Flag{
@@ -2147,13 +2148,8 @@ func SetEthConfig(ctx *cli.Context, stack *node.Node, cfg *ethconfig.Config) {
 	// VM tracing config.
 	if ctx.IsSet(VMTraceFlag.Name) {
 		if name := ctx.String(VMTraceFlag.Name); name != "" {
-			var config string
-			if ctx.IsSet(VMTraceConfigFlag.Name) {
-				config = ctx.String(VMTraceConfigFlag.Name)
-			}
-
 			cfg.VMTrace = name
-			cfg.VMTraceConfig = config
+			cfg.VMTraceConfig = ctx.String(VMTraceConfigFlag.Name)
 		}
 	}
 	if ctx.IsSet(DisableTxBroadcastFromFlag.Name) {
@@ -2428,10 +2424,7 @@ func MakeChain(ctx *cli.Context, stack *node.Node) (chain *core.BlockChain, chai
 	vmcfg := vm.Config{EnablePreimageRecording: ctx.Bool(VMEnableDebugFlag.Name)}
 	if ctx.IsSet(VMTraceFlag.Name) {
 		if name := ctx.String(VMTraceFlag.Name); name != "" {
-			var config json.RawMessage
-			if ctx.IsSet(VMTraceConfigFlag.Name) {
-				config = json.RawMessage(ctx.String(VMTraceConfigFlag.Name))
-			}
+			config := json.RawMessage(ctx.String(VMTraceConfigFlag.Name))
 			t, err := tracers.LiveDirectory.New(name, config)
 			if err != nil {
 				Fatalf("Failed to create tracer %q: %v", name, err)

core/tracing/hooks.go

@@ -62,9 +62,8 @@ type VMContext struct {
 	BlockNumber *big.Int
 	Time        uint64
 	// Effective tx gas price
-	GasPrice    *big.Int
-	ChainConfig *params.ChainConfig
-	StateDB     StateDB
+	GasPrice *big.Int
+	StateDB  StateDB
 }
 
 // BlockEvent is emitted upon tracing an incoming block.

core/vm/evm.go

@@ -695,7 +695,6 @@ func (evm *EVM) GetVMContext() *tracing.VMContext {
 		BlockNumber: evm.Context.BlockNumber,
 		Time:        evm.Context.Time,
 		GasPrice:    evm.TxContext.GasPrice,
-		ChainConfig: evm.ChainConfig(),
 		StateDB:     evm.StateDB,
 	}
 }

core/vm/runtime/runtime_test.go

@@ -330,7 +330,7 @@ func benchmarkNonModifyingCode(gas uint64, code []byte, name string, tracerCode
 	cfg.State, _ = state.New(common.Hash{}, state.NewDatabase(rawdb.NewMemoryDatabase()), nil)
 	cfg.GasLimit = gas
 	if len(tracerCode) > 0 {
-		tracer, err := tracers.DefaultDirectory.New(tracerCode, new(tracers.Context), nil)
+		tracer, err := tracers.DefaultDirectory.New(tracerCode, new(tracers.Context), nil, cfg.ChainConfig)
 		if err != nil {
 			b.Fatal(err)
 		}
@@ -826,7 +826,7 @@ func TestRuntimeJSTracer(t *testing.T) {
 			statedb.SetCode(common.HexToAddress("0xee"), calleeCode)
 			statedb.SetCode(common.HexToAddress("0xff"), depressedCode)
 
-			tracer, err := tracers.DefaultDirectory.New(jsTracer, new(tracers.Context), nil)
+			tracer, err := tracers.DefaultDirectory.New(jsTracer, new(tracers.Context), nil, params.MainnetChainConfig)
 			if err != nil {
 				t.Fatal(err)
 			}
@@ -861,7 +861,7 @@ func TestJSTracerCreateTx(t *testing.T) {
 	code := []byte{byte(vm.PUSH1), 0, byte(vm.PUSH1), 0, byte(vm.RETURN)}
 
 	statedb, _ := state.New(common.Hash{}, state.NewDatabase(rawdb.NewMemoryDatabase()), nil)
-	tracer, err := tracers.DefaultDirectory.New(jsTracer, new(tracers.Context), nil)
+	tracer, err := tracers.DefaultDirectory.New(jsTracer, new(tracers.Context), nil, params.MainnetChainConfig)
 	if err != nil {
 		t.Fatal(err)
 	}

eth/tracers/api.go

@@ -1130,7 +1130,7 @@ func (api *API) traceTx(
 			Stop:      logger.Stop,
 		}
 	} else {
-		tracer, err = DefaultDirectory.New(*config.Tracer, txctx, config.TracerConfig)
+		tracer, err = DefaultDirectory.New(*config.Tracer, txctx, config.TracerConfig, api.backend.ChainConfig())
 		if err != nil {
 			return nil, err
 		}

eth/tracers/dir.go

@@ -22,6 +22,7 @@ import (
 
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core/tracing"
+	"github.com/ethereum/go-ethereum/params"
 )
 
 // Context contains some contextual infos for a transaction execution that is not
@@ -44,8 +45,8 @@ type Tracer struct {
 	Stop func(err error)
 }
 
-type ctorFn func(*Context, json.RawMessage) (*Tracer, error)
-type jsCtorFn func(string, *Context, json.RawMessage) (*Tracer, error)
+type ctorFn func(*Context, json.RawMessage, *params.ChainConfig) (*Tracer, error)
+type jsCtorFn func(string, *Context, json.RawMessage, *params.ChainConfig) (*Tracer, error)
 
 type elem struct {
 	ctor ctorFn
@@ -78,12 +79,15 @@ func (d *directory) RegisterJSEval(f jsCtorFn) {
 // New returns a new instance of a tracer, by iterating through the
 // registered lookups. Name is either name of an existing tracer
 // or an arbitrary JS code.
-func (d *directory) New(name string, ctx *Context, cfg json.RawMessage) (*Tracer, error) {
+func (d *directory) New(name string, ctx *Context, cfg json.RawMessage, chainConfig *params.ChainConfig) (*Tracer, error) {
+	if len(cfg) == 0 {
+		cfg = json.RawMessage("{}")
+	}
 	if elem, ok := d.elems[name]; ok {
-		return elem.ctor(ctx, cfg)
+		return elem.ctor(ctx, cfg, chainConfig)
 	}
 	// Assume JS code
-	return d.jsEval(name, ctx, cfg)
+	return d.jsEval(name, ctx, cfg, chainConfig)
 }
 
 // IsJS will return true if the given tracer will evaluate

eth/tracers/internal/tracetest/calltrace2_test.go

@@ -166,7 +166,7 @@ func testCallTracer2(tracerName string, dirPath string, scheme string, t *testin
 				triedb, _, statedb = tests.MakePreState(rawdb.NewMemoryDatabase(), test.Genesis.Alloc, false, scheme)
 			)
 			defer triedb.Close()
-			tracer, err := tracers.DefaultDirectory.New(tracerName, new(tracers.Context), test.TracerConfig)
+			tracer, err := tracers.DefaultDirectory.New(tracerName, new(tracers.Context), test.TracerConfig, test.Genesis.Config)
 			if err != nil {
 				t.Fatalf("failed to create call tracer: %v", err)
 			}
@@ -276,7 +276,7 @@ func benchTracer2(tracerName string, test *callTracer2Test, b *testing.B) {
 	b.ReportAllocs()
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
-		tracer, err := tracers.DefaultDirectory.New(tracerName, new(tracers.Context), test.TracerConfig)
+		tracer, err := tracers.DefaultDirectory.New(tracerName, new(tracers.Context), test.TracerConfig, test.Genesis.Config)
 		if err != nil {
 			b.Fatalf("failed to create call tracer: %v", err)
 		}
@@ -343,16 +343,17 @@ func TestZeroValueToNotExitCall2(t *testing.T) {
 	}
 	triedb, _, statedb := tests.MakePreState(rawdb.NewMemoryDatabase(), alloc, false, rawdb.HashScheme)
 	defer triedb.Close()
+	chainConfig := params.MainnetChainConfig
 	// Create the tracer, the EVM environment and run it
-	tracer, err := tracers.DefaultDirectory.New("callTracer2", new(tracers.Context), nil)
+	tracer, err := tracers.DefaultDirectory.New("callTracer2", new(tracers.Context), nil, chainConfig)
 	if err != nil {
 		t.Fatalf("failed to create call tracer: %v", err)
 	}
 	msg, err := core.TransactionToMessage(tx, signer, nil)
 	if err != nil {
 		t.Fatalf("failed to prepare transaction for tracing: %v", err)
 	}
-	evm := vm.NewEVM(context, txContext, statedb, params.MainnetChainConfig, vm.Config{Debug: true, Tracer: tracer.Hooks})
+	evm := vm.NewEVM(context, txContext, statedb, chainConfig, vm.Config{Debug: true, Tracer: tracer.Hooks})
 	statedb.SetLogger(tracer.Hooks)
 	tracer.OnTxStart(evm.GetVMContext(), tx, msg.Payer)
 	vmRet, err := core.ApplyMessage(evm, msg, new(core.GasPool).AddGas(tx.Gas()))

eth/tracers/internal/tracetest/calltrace_test.go

@@ -146,7 +146,7 @@ func testCallTracer(tracerName string, dirPath string, scheme string, t *testing
 				triedb, _, statedb = tests.MakePreState(rawdb.NewMemoryDatabase(), test.Genesis.Alloc, false, scheme)
 			)
 			triedb.Close()
-			tracer, err := tracers.DefaultDirectory.New(tracerName, new(tracers.Context), test.TracerConfig)
+			tracer, err := tracers.DefaultDirectory.New(tracerName, new(tracers.Context), test.TracerConfig, test.Genesis.Config)
 			if err != nil {
 				t.Fatalf("failed to create call tracer: %v", err)
 			}
@@ -253,7 +253,7 @@ func benchTracer(tracerName string, test *callTracerTest, b *testing.B) {
 	b.ReportAllocs()
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
-		tracer, err := tracers.DefaultDirectory.New(tracerName, new(tracers.Context), nil)
+		tracer, err := tracers.DefaultDirectory.New(tracerName, new(tracers.Context), nil, test.Genesis.Config)
 		if err != nil {
 			b.Fatalf("failed to create call tracer: %v", err)
 		}
@@ -290,7 +290,7 @@ func TestInternals(t *testing.T) {
 		}
 	)
 	mkTracer := func(name string, cfg json.RawMessage) *tracers.Tracer {
-		tr, err := tracers.DefaultDirectory.New(name, nil, cfg)
+		tr, err := tracers.DefaultDirectory.New(name, nil, cfg, config)
 		if err != nil {
 			t.Fatalf("failed to create call tracer: %v", err)
 		}

eth/tracers/internal/tracetest/flat_calltrace_test.go

@@ -104,7 +104,7 @@ func flatCallTracerTestRunner(tracerName string, filename string, dirPath string
 	defer triedb.Close()
 
 	// Create the tracer, the EVM environment and run it
-	tracer, err := tracers.DefaultDirectory.New(tracerName, new(tracers.Context), test.TracerConfig)
+	tracer, err := tracers.DefaultDirectory.New(tracerName, new(tracers.Context), test.TracerConfig, test.Genesis.Config)
 	if err != nil {
 		return fmt.Errorf("failed to create call tracer: %v", err)
 	}

eth/tracers/internal/tracetest/prestate_test.go

@@ -114,7 +114,7 @@ func testPrestateDiffTracer(tracerName string, dirPath string, scheme string, t
 				triedb, _, statedb = tests.MakePreState(rawdb.NewMemoryDatabase(), test.Genesis.Alloc, false, scheme)
 			)
 			defer triedb.Close()
-			tracer, err := tracers.DefaultDirectory.New(tracerName, new(tracers.Context), test.TracerConfig)
+			tracer, err := tracers.DefaultDirectory.New(tracerName, new(tracers.Context), test.TracerConfig, test.Genesis.Config)
 			if err != nil {
 				t.Fatalf("failed to create call tracer: %v", err)
 			}

eth/tracers/js/goja.go

@@ -27,6 +27,7 @@ import (
 	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/ethereum/go-ethereum/eth/tracers"
 	"github.com/ethereum/go-ethereum/eth/tracers/internal"
+	"github.com/ethereum/go-ethereum/params"
 	"github.com/holiman/uint256"
 
 	"github.com/ethereum/go-ethereum/common"
@@ -45,10 +46,10 @@ func init() {
 	if err != nil {
 		panic(err)
 	}
-	type ctorFn = func(*tracers.Context, json.RawMessage) (*tracers.Tracer, error)
+	type ctorFn = func(*tracers.Context, json.RawMessage, *params.ChainConfig) (*tracers.Tracer, error)
 	lookup := func(code string) ctorFn {
-		return func(ctx *tracers.Context, cfg json.RawMessage) (*tracers.Tracer, error) {
-			return newJsTracer(code, ctx, cfg)
+		return func(ctx *tracers.Context, cfg json.RawMessage, chainConfig *params.ChainConfig) (*tracers.Tracer, error) {
+			return newJsTracer(code, ctx, cfg, chainConfig)
 		}
 	}
 	for name, code := range assetTracers {
@@ -101,6 +102,7 @@ func fromBuf(vm *goja.Runtime, bufType goja.Value, buf goja.Value, allowString b
 type jsTracer struct {
 	vm                *goja.Runtime
 	env               *tracing.VMContext
+	chainConfig       *params.ChainConfig
 	toBig             toBigFn               // Converts a hex string into a JS bigint
 	toBuf             toBufFn               // Converts a []byte into a JS buffer
 	fromBuf           fromBufFn             // Converts an array, hex string or Uint8Array to a []byte
@@ -137,13 +139,14 @@ type jsTracer struct {
 // The methods `result` and `fault` are required to be present.
 // The methods `step`, `enter`, and `exit` are optional, but note that
 // `enter` and `exit` always go together.
-func newJsTracer(code string, ctx *tracers.Context, cfg json.RawMessage) (*tracers.Tracer, error) {
+func newJsTracer(code string, ctx *tracers.Context, cfg json.RawMessage, chainConfig *params.ChainConfig) (*tracers.Tracer, error) {
 	vm := goja.New()
 	// By default field names are exported to JS as is, i.e. capitalized.
 	vm.SetFieldNameMapper(goja.UncapFieldNameMapper())
 	t := &jsTracer{
-		vm:  vm,
-		ctx: make(map[string]goja.Value),
+		vm:          vm,
+		ctx:         make(map[string]goja.Value),
+		chainConfig: chainConfig,
 	}
 
 	t.setTypeConverters()
@@ -243,7 +246,7 @@ func (t *jsTracer) OnTxStart(env *tracing.VMContext, tx *types.Transaction, from
 	db := &dbObj{db: env.StateDB, vm: t.vm, toBig: t.toBig, toBuf: t.toBuf, fromBuf: t.fromBuf}
 	t.dbValue = db.setupObject()
 	// Update list of precompiles based on current block
-	rules := env.ChainConfig.Rules(env.BlockNumber)
+	rules := t.chainConfig.Rules(env.BlockNumber)
 	t.activePrecompiles = vm.ActivePrecompiles(rules)
 	t.ctx["block"] = t.vm.ToValue(t.env.BlockNumber.Uint64())
 	t.ctx["gas"] = t.vm.ToValue(tx.Gas())

eth/tracers/js/tracer_test.go

@@ -90,11 +90,12 @@ func runTrace(tracer *tracers.Tracer, vmctx *vmContext, chaincfg *params.ChainCo
 func TestTracer(t *testing.T) {
 	execTracer := func(code string, contract []byte) ([]byte, string) {
 		t.Helper()
-		tracer, err := newJsTracer(code, nil, nil)
+		chainConfig := params.TestChainConfig
+		tracer, err := newJsTracer(code, nil, nil, chainConfig)
 		if err != nil {
 			t.Fatal(err)
 		}
-		ret, err := runTrace(tracer, testCtx(), params.TestChainConfig, contract)
+		ret, err := runTrace(tracer, testCtx(), chainConfig, contract)
 		if err != nil {
 			return nil, err.Error() // Stringify to allow comparison without nil checks
 		}
@@ -164,7 +165,8 @@ func TestTracer(t *testing.T) {
 
 func TestHalt(t *testing.T) {
 	timeout := errors.New("stahp")
-	tracer, err := newJsTracer("{step: function() { while(1); }, result: function() { return null; }, fault: function(){}}", nil, nil)
+	chainConfig := params.TestChainConfig
+	tracer, err := newJsTracer("{step: function() { while(1); }, result: function() { return null; }, fault: function(){}}", nil, nil, chainConfig)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -178,7 +180,8 @@ func TestHalt(t *testing.T) {
 }
 
 func TestHaltBetweenSteps(t *testing.T) {
-	tracer, err := newJsTracer("{step: function() {}, fault: function() {}, result: function() { return null; }}", nil, nil)
+	chainConfig := params.TestChainConfig
+	tracer, err := newJsTracer("{step: function() {}, fault: function() {}, result: function() { return null; }}", nil, nil, chainConfig)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -203,11 +206,12 @@ func TestHaltBetweenSteps(t *testing.T) {
 func TestNoStepExec(t *testing.T) {
 	execTracer := func(code string) []byte {
 		t.Helper()
-		tracer, err := newJsTracer(code, nil, nil)
+		chainConfig := params.TestChainConfig
+		tracer, err := newJsTracer(code, nil, nil, chainConfig)
 		if err != nil {
 			t.Fatal(err)
 		}
-		env := vm.NewEVM(vm.BlockContext{BlockNumber: big.NewInt(1)}, vm.TxContext{GasPrice: big.NewInt(100)}, &dummyStatedb{}, params.TestChainConfig, vm.Config{Tracer: tracer.Hooks})
+		env := vm.NewEVM(vm.BlockContext{BlockNumber: big.NewInt(1)}, vm.TxContext{GasPrice: big.NewInt(100)}, &dummyStatedb{}, chainConfig, vm.Config{Tracer: tracer.Hooks})
 		tracer.OnTxStart(env.GetVMContext(), types.NewTx(&types.LegacyTx{}), common.Address{})
 		tracer.OnEnter(0, byte(vm.CALL), common.Address{}, common.Address{}, []byte{}, 1000, big.NewInt(0), 0)
 		tracer.OnExit(0, nil, 0, nil, false)
@@ -238,7 +242,7 @@ func TestIsPrecompile(t *testing.T) {
 	chaincfg.IstanbulBlock = big.NewInt(200)
 	chaincfg.BerlinBlock = big.NewInt(300)
 	txCtx := vm.TxContext{GasPrice: big.NewInt(100000)}
-	tracer, err := newJsTracer("{addr: toAddress('0000000000000000000000000000000000000009'), res: null, step: function() { this.res = isPrecompiled(this.addr); }, fault: function() {}, result: function() { return this.res; }}", nil, nil)
+	tracer, err := newJsTracer("{addr: toAddress('0000000000000000000000000000000000000009'), res: null, step: function() { this.res = isPrecompiled(this.addr); }, fault: function() {}, result: function() { return this.res; }}", nil, nil, chaincfg)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -252,7 +256,7 @@ func TestIsPrecompile(t *testing.T) {
 		t.Errorf("tracer should not consider blake2f as precompile in byzantium")
 	}
 
-	tracer, _ = newJsTracer("{addr: toAddress('0000000000000000000000000000000000000009'), res: null, step: function() { this.res = isPrecompiled(this.addr); }, fault: function() {}, result: function() { return this.res; }}", nil, nil)
+	tracer, _ = newJsTracer("{addr: toAddress('0000000000000000000000000000000000000009'), res: null, step: function() { this.res = isPrecompiled(this.addr); }, fault: function() {}, result: function() { return this.res; }}", nil, nil, chaincfg)
 	blockCtx = vm.BlockContext{BlockNumber: big.NewInt(250)}
 	res, err = runTrace(tracer, &vmContext{blockCtx, txCtx}, chaincfg, nil)
 	if err != nil {
@@ -264,15 +268,16 @@ func TestIsPrecompile(t *testing.T) {
 }
 
 func TestEnterExit(t *testing.T) {
+	chainConfig := params.TestChainConfig
 	// test that either both or none of enter() and exit() are defined
-	if _, err := newJsTracer("{step: function() {}, fault: function() {}, result: function() { return null; }, enter: function() {}}", new(tracers.Context), nil); err == nil {
+	if _, err := newJsTracer("{step: function() {}, fault: function() {}, result: function() { return null; }, enter: function() {}}", new(tracers.Context), nil, chainConfig); err == nil {
 		t.Fatal("tracer creation should've failed without exit() definition")
 	}
-	if _, err := newJsTracer("{step: function() {}, fault: function() {}, result: function() { return null; }, enter: function() {}, exit: function() {}}", new(tracers.Context), nil); err != nil {
+	if _, err := newJsTracer("{step: function() {}, fault: function() {}, result: function() { return null; }, enter: function() {}, exit: function() {}}", new(tracers.Context), nil, chainConfig); err != nil {
 		t.Fatal(err)
 	}
 	// test that the enter and exit method are correctly invoked and the values passed
-	tracer, err := newJsTracer("{enters: 0, exits: 0, enterGas: 0, gasUsed: 0, step: function() {}, fault: function() {}, result: function() { return {enters: this.enters, exits: this.exits, enterGas: this.enterGas, gasUsed: this.gasUsed} }, enter: function(frame) { this.enters++; this.enterGas = frame.getGas(); }, exit: function(res) { this.exits++; this.gasUsed = res.getGasUsed(); }}", new(tracers.Context), nil)
+	tracer, err := newJsTracer("{enters: 0, exits: 0, enterGas: 0, gasUsed: 0, step: function() {}, fault: function() {}, result: function() { return {enters: this.enters, exits: this.exits, enterGas: this.enterGas, gasUsed: this.gasUsed} }, enter: function(frame) { this.enters++; this.enterGas = frame.getGas(); }, exit: function(res) { this.exits++; this.gasUsed = res.getGasUsed(); }}", new(tracers.Context), nil, chainConfig)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -293,8 +298,9 @@ func TestEnterExit(t *testing.T) {
 }
 
 func TestSetup(t *testing.T) {
+	chainConfig := params.TestChainConfig
 	// Test empty config
-	_, err := newJsTracer(`{setup: function(cfg) { if (cfg !== "{}") { throw("invalid empty config") } }, fault: function() {}, result: function() {}}`, new(tracers.Context), nil)
+	_, err := newJsTracer(`{setup: function(cfg) { if (cfg !== "{}") { throw("invalid empty config") } }, fault: function() {}, result: function() {}}`, new(tracers.Context), nil, chainConfig)
 	if err != nil {
 		t.Error(err)
 	}
@@ -304,12 +310,12 @@ func TestSetup(t *testing.T) {
 		t.Fatal(err)
 	}
 	// Test no setup func
-	_, err = newJsTracer(`{fault: function() {}, result: function() {}}`, new(tracers.Context), cfg)
+	_, err = newJsTracer(`{fault: function() {}, result: function() {}}`, new(tracers.Context), cfg, chainConfig)
 	if err != nil {
 		t.Fatal(err)
 	}
 	// Test config value
-	tracer, err := newJsTracer("{config: null, setup: function(cfg) { this.config = JSON.parse(cfg) }, step: function() {}, fault: function() {}, result: function() { return this.config.foo }}", new(tracers.Context), cfg)
+	tracer, err := newJsTracer("{config: null, setup: function(cfg) { this.config = JSON.parse(cfg) }, step: function() {}, fault: function() {}, result: function() { return this.config.foo }}", new(tracers.Context), cfg, chainConfig)
 	if err != nil {
 		t.Fatal(err)
 	}

eth/tracers/live.go

@@ -24,6 +24,9 @@ func (d *liveDirectory) Register(name string, f ctorFunc) {
 
 // New instantiates a tracer by name.
 func (d *liveDirectory) New(name string, config json.RawMessage) (*tracing.Hooks, error) {
+	if len(config) == 0 {
+		config = json.RawMessage("{}")
+	}
 	if f, ok := d.elems[name]; ok {
 		return f(config)
 	}