eth/tracers: add onlyTopCall option to callTracer (#25430)

This PR allows users to pass in a config object directly to the tracers. Previously only the struct logger was configurable.

It also adds an option to the call tracer which if enabled makes it ignore any subcall and collect only information about the top-level call. See #25419 for discussion.

The tracers will silently ignore if they are passed a config they don't care about.
This commit is contained in:
Sina Mahmoodi 2022-08-09 11:04:57 +02:00 committed by GitHub
parent 759d795c56
commit 86de2e516e
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23
13 changed files with 174 additions and 41 deletions

View File

@ -333,7 +333,7 @@ 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.New(tracerCode, new(tracers.Context))
tracer, err := tracers.New(tracerCode, new(tracers.Context), nil)
if err != nil {
b.Fatal(err)
}
@ -832,7 +832,7 @@ func TestRuntimeJSTracer(t *testing.T) {
statedb.SetCode(common.HexToAddress("0xee"), calleeCode)
statedb.SetCode(common.HexToAddress("0xff"), depressedCode)
tracer, err := tracers.New(jsTracer, new(tracers.Context))
tracer, err := tracers.New(jsTracer, new(tracers.Context), nil)
if err != nil {
t.Fatal(err)
}
@ -868,7 +868,7 @@ 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.New(jsTracer, new(tracers.Context))
tracer, err := tracers.New(jsTracer, new(tracers.Context), nil)
if err != nil {
t.Fatal(err)
}

View File

@ -20,6 +20,7 @@ import (
"bufio"
"bytes"
"context"
"encoding/json"
"errors"
"fmt"
"os"
@ -169,15 +170,15 @@ type TraceConfig struct {
Tracer *string
Timeout *string
Reexec *uint64
// Config specific to given tracer. Note struct logger
// config are historically embedded in main object.
TracerConfig json.RawMessage
}
// TraceCallConfig is the config for traceCall API. It holds one more
// field to override the state for tracing.
type TraceCallConfig struct {
*logger.Config
Tracer *string
Timeout *string
Reexec *uint64
TraceConfig
StateOverrides *ethapi.StateOverride
BlockOverrides *ethapi.BlockOverrides
}
@ -882,7 +883,7 @@ func (api *API) traceTx(ctx context.Context, message core.Message, txctx *Contex
// Default tracer is the struct logger
tracer = logger.NewStructLogger(config.Config)
if config.Tracer != nil {
tracer, err = New(*config.Tracer, txctx)
tracer, err = New(*config.Tracer, txctx, config.TracerConfig)
if err != nil {
return nil, err
}

View File

@ -121,6 +121,7 @@ type callTracerTest struct {
Genesis *core.Genesis `json:"genesis"`
Context *callContext `json:"context"`
Input string `json:"input"`
TracerConfig json.RawMessage `json:"tracerConfig"`
Result *callTrace `json:"result"`
}
@ -179,7 +180,7 @@ func testCallTracer(tracerName string, dirPath string, t *testing.T) {
}
_, statedb = tests.MakePreState(rawdb.NewMemoryDatabase(), test.Genesis.Alloc, false)
)
tracer, err := tracers.New(tracerName, new(tracers.Context))
tracer, err := tracers.New(tracerName, new(tracers.Context), test.TracerConfig)
if err != nil {
t.Fatalf("failed to create call tracer: %v", err)
}
@ -293,7 +294,7 @@ func benchTracer(tracerName string, test *callTracerTest, b *testing.B) {
b.ReportAllocs()
b.ResetTimer()
for i := 0; i < b.N; i++ {
tracer, err := tracers.New(tracerName, new(tracers.Context))
tracer, err := tracers.New(tracerName, new(tracers.Context), nil)
if err != nil {
b.Fatalf("failed to create call tracer: %v", err)
}
@ -359,7 +360,7 @@ func TestZeroValueToNotExitCall(t *testing.T) {
}
_, statedb := tests.MakePreState(rawdb.NewMemoryDatabase(), alloc, false)
// Create the tracer, the EVM environment and run it
tracer, err := tracers.New("callTracer", nil)
tracer, err := tracers.New("callTracer", nil, nil)
if err != nil {
t.Fatalf("failed to create call tracer: %v", err)
}

View File

@ -0,0 +1,72 @@
{
"context": {
"difficulty": "3502894804",
"gasLimit": "4722976",
"miner": "0x1585936b53834b021f68cc13eeefdec2efc8e724",
"number": "2289806",
"timestamp": "1513601314"
},
"genesis": {
"alloc": {
"0x0024f658a46fbb89d8ac105e98d7ac7cbbaf27c5": {
"balance": "0x0",
"code": "0x",
"nonce": "22",
"storage": {}
},
"0x3b873a919aa0512d5a0f09e6dcceaa4a6727fafe": {
"balance": "0x4d87094125a369d9bd5",
"code": "0x606060405236156100935763ffffffff60e060020a60003504166311ee8382811461009c57806313af4035146100be5780631f5e8f4c146100ee57806324daddc5146101125780634921a91a1461013b57806363e4bff414610157578063764978f91461017f578063893d20e8146101a1578063ba40aaa1146101cd578063cebc9a82146101f4578063e177246e14610216575b61009a5b5b565b005b34156100a457fe5b6100ac61023d565b60408051918252519081900360200190f35b34156100c657fe5b6100da600160a060020a0360043516610244565b604080519115158252519081900360200190f35b34156100f657fe5b6100da610307565b604080519115158252519081900360200190f35b341561011a57fe5b6100da6004351515610318565b604080519115158252519081900360200190f35b6100da6103d6565b604080519115158252519081900360200190f35b6100da600160a060020a0360043516610420565b604080519115158252519081900360200190f35b341561018757fe5b6100ac61046c565b60408051918252519081900360200190f35b34156101a957fe5b6101b1610473565b60408051600160a060020a039092168252519081900360200190f35b34156101d557fe5b6100da600435610483565b604080519115158252519081900360200190f35b34156101fc57fe5b6100ac61050d565b60408051918252519081900360200190f35b341561021e57fe5b6100da600435610514565b604080519115158252519081900360200190f35b6003545b90565b60006000610250610473565b600160a060020a031633600160a060020a03161415156102705760006000fd5b600160a060020a03831615156102865760006000fd5b50600054600160a060020a0390811690831681146102fb57604051600160a060020a0380851691908316907ffcf23a92150d56e85e3a3d33b357493246e55783095eb6a733eb8439ffc752c890600090a360008054600160a060020a031916600160a060020a03851617905560019150610300565b600091505b5b50919050565b60005460a060020a900460ff165b90565b60006000610324610473565b600160a060020a031633600160a060020a03161415156103445760006000fd5b5060005460a060020a900460ff16801515831515146102fb576000546040805160a060020a90920460ff1615158252841515602083015280517fe6cd46a119083b86efc6884b970bfa30c1708f53ba57b86716f15b2f4551a9539281900390910190a16000805460a060020a60ff02191660a060020a8515150217905560019150610300565b600091505b5b50919050565b60006103e0610307565b801561040557506103ef610473565b600160a060020a031633600160a060020a031614155b156104105760006000fd5b610419336105a0565b90505b5b90565b600061042a610307565b801561044f5750610439610473565b600160a060020a031633600160a060020a031614155b1561045a5760006000fd5b610463826105a0565b90505b5b919050565b6001545b90565b600054600160a060020a03165b90565b6000600061048f610473565b600160a060020a031633600160a060020a03161415156104af5760006000fd5b506001548281146102fb57604080518281526020810185905281517f79a3746dde45672c9e8ab3644b8bb9c399a103da2dc94b56ba09777330a83509929181900390910190a160018381559150610300565b600091505b5b50919050565b6002545b90565b60006000610520610473565b600160a060020a031633600160a060020a03161415156105405760006000fd5b506002548281146102fb57604080518281526020810185905281517ff6991a728965fedd6e927fdf16bdad42d8995970b4b31b8a2bf88767516e2494929181900390910190a1600283905560019150610300565b600091505b5b50919050565b60006000426105ad61023d565b116102fb576105c46105bd61050d565b4201610652565b6105cc61046c565b604051909150600160a060020a038416908290600081818185876187965a03f1925050501561063d57604080518281529051600160a060020a038516917f9bca65ce52fdef8a470977b51f247a2295123a4807dfa9e502edf0d30722da3b919081900360200190a260019150610300565b6102fb42610652565b5b600091505b50919050565b60038190555b505600a165627a7a72305820f3c973c8b7ed1f62000b6701bd5b708469e19d0f1d73fde378a56c07fd0b19090029",
"nonce": "1",
"storage": {
"0x0000000000000000000000000000000000000000000000000000000000000000": "0x000000000000000000000001b436ba50d378d4bbc8660d312a13df6af6e89dfb",
"0x0000000000000000000000000000000000000000000000000000000000000001": "0x00000000000000000000000000000000000000000000000006f05b59d3b20000",
"0x0000000000000000000000000000000000000000000000000000000000000002": "0x000000000000000000000000000000000000000000000000000000000000003c",
"0x0000000000000000000000000000000000000000000000000000000000000003": "0x000000000000000000000000000000000000000000000000000000005a37b834"
}
},
"0xb436ba50d378d4bbc8660d312a13df6af6e89dfb": {
"balance": "0x1780d77678137ac1b775",
"code": "0x",
"nonce": "29072",
"storage": {}
}
},
"config": {
"byzantiumBlock": 1700000,
"chainId": 3,
"daoForkSupport": true,
"eip150Block": 0,
"eip150Hash": "0x41941023680923e0fe4d74a34bdac8141f2540e3ae90623718e47d66d1ca4a2d",
"eip155Block": 10,
"eip158Block": 10,
"ethash": {},
"homesteadBlock": 0
},
"difficulty": "3509749784",
"extraData": "0x4554482e45544846414e532e4f52472d4641313738394444",
"gasLimit": "4727564",
"hash": "0x609948ac3bd3c00b7736b933248891d6c901ee28f066241bddb28f4e00a9f440",
"miner": "0xbbf5029fd710d227630c8b7d338051b8e76d50b3",
"mixHash": "0xb131e4507c93c7377de00e7c271bf409ec7492767142ff0f45c882f8068c2ada",
"nonce": "0x4eb12e19c16d43da",
"number": "2289805",
"stateRoot": "0xc7f10f352bff82fac3c2999d3085093d12652e19c7fd32591de49dc5d91b4f1f",
"timestamp": "1513601261",
"totalDifficulty": "7143276353481064"
},
"input": "0xf88b8271908506fc23ac0083015f90943b873a919aa0512d5a0f09e6dcceaa4a6727fafe80a463e4bff40000000000000000000000000024f658a46fbb89d8ac105e98d7ac7cbbaf27c52aa0bdce0b59e8761854e857fe64015f06dd08a4fbb7624f6094893a79a72e6ad6bea01d9dde033cff7bb235a3163f348a6d7ab8d6b52bc0963a95b91612e40ca766a4",
"tracerConfig": {
"onlyTopCall": true
},
"result": {
"from": "0xb436ba50d378d4bbc8660d312a13df6af6e89dfb",
"gas": "0x10738",
"gasUsed": "0x3ef9",
"input": "0x63e4bff40000000000000000000000000024f658a46fbb89d8ac105e98d7ac7cbbaf27c5",
"output": "0x0000000000000000000000000000000000000000000000000000000000000001",
"to": "0x3b873a919aa0512d5a0f09e6dcceaa4a6727fafe",
"type": "CALL",
"value": "0x0"
}
}

View File

@ -125,7 +125,7 @@ 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) (tracers.Tracer, error) {
func newJsTracer(code string, ctx *tracers.Context, cfg json.RawMessage) (tracers.Tracer, error) {
if c, ok := assetTracers[code]; ok {
code = c
}
@ -177,6 +177,17 @@ func newJsTracer(code string, ctx *tracers.Context) (tracers.Tracer, error) {
t.exit = exit
t.result = result
t.fault = fault
// Pass in config
if setup, ok := goja.AssertFunction(obj.Get("setup")); ok {
cfgStr := "{}"
if cfg != nil {
cfgStr = string(cfg)
}
if _, err := setup(obj, vm.ToValue(cfgStr)); err != nil {
return nil, err
}
}
// Setup objects carrying data to JS. These are created once and re-used.
t.log = &steplog{
vm: vm,

View File

@ -85,7 +85,7 @@ func runTrace(tracer tracers.Tracer, vmctx *vmContext, chaincfg *params.ChainCon
func TestTracer(t *testing.T) {
execTracer := func(code string) ([]byte, string) {
t.Helper()
tracer, err := newJsTracer(code, nil)
tracer, err := newJsTracer(code, nil, nil)
if err != nil {
t.Fatal(err)
}
@ -149,7 +149,7 @@ 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)
tracer, err := newJsTracer("{step: function() { while(1); }, result: function() { return null; }, fault: function(){}}", nil, nil)
if err != nil {
t.Fatal(err)
}
@ -163,7 +163,7 @@ func TestHalt(t *testing.T) {
}
func TestHaltBetweenSteps(t *testing.T) {
tracer, err := newJsTracer("{step: function() {}, fault: function() {}, result: function() { return null; }}", nil)
tracer, err := newJsTracer("{step: function() {}, fault: function() {}, result: function() { return null; }}", nil, nil)
if err != nil {
t.Fatal(err)
}
@ -187,7 +187,7 @@ func TestHaltBetweenSteps(t *testing.T) {
func TestNoStepExec(t *testing.T) {
execTracer := func(code string) []byte {
t.Helper()
tracer, err := newJsTracer(code, nil)
tracer, err := newJsTracer(code, nil, nil)
if err != nil {
t.Fatal(err)
}
@ -221,7 +221,7 @@ 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)
tracer, err := newJsTracer("{addr: toAddress('0000000000000000000000000000000000000009'), res: null, step: function() { this.res = isPrecompiled(this.addr); }, fault: function() {}, result: function() { return this.res; }}", nil, nil)
if err != nil {
t.Fatal(err)
}
@ -235,7 +235,7 @@ 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)
tracer, _ = newJsTracer("{addr: toAddress('0000000000000000000000000000000000000009'), res: null, step: function() { this.res = isPrecompiled(this.addr); }, fault: function() {}, result: function() { return this.res; }}", nil, nil)
blockCtx = vm.BlockContext{BlockNumber: big.NewInt(250)}
res, err = runTrace(tracer, &vmContext{blockCtx, txCtx}, chaincfg)
if err != nil {
@ -248,14 +248,14 @@ func TestIsPrecompile(t *testing.T) {
func TestEnterExit(t *testing.T) {
// 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)); err == nil {
if _, err := newJsTracer("{step: function() {}, fault: function() {}, result: function() { return null; }, enter: function() {}}", new(tracers.Context), nil); 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)); err != nil {
if _, err := newJsTracer("{step: function() {}, fault: function() {}, result: function() { return null; }, enter: function() {}, exit: function() {}}", new(tracers.Context), nil); 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))
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)
if err != nil {
t.Fatal(err)
}
@ -274,3 +274,33 @@ func TestEnterExit(t *testing.T) {
t.Errorf("Number of invocations of enter() and exit() is wrong. Have %s, want %s\n", have, want)
}
}
func TestSetup(t *testing.T) {
// Test empty config
_, err := newJsTracer(`{setup: function(cfg) { if (cfg !== "{}") { throw("invalid empty config") } }, fault: function() {}, result: function() {}}`, new(tracers.Context), nil)
if err != nil {
t.Error(err)
}
cfg, err := json.Marshal(map[string]string{"foo": "bar"})
if err != nil {
t.Fatal(err)
}
// Test no setup func
_, err = newJsTracer(`{fault: function() {}, result: function() {}}`, new(tracers.Context), cfg)
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)
if err != nil {
t.Fatal(err)
}
have, err := tracer.GetResult()
if err != nil {
t.Fatal(err)
}
if string(have) != `"bar"` {
t.Errorf("tracer returned wrong result. have: %s, want: \"bar\"\n", string(have))
}
}

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@ -55,11 +55,11 @@ type fourByteTracer struct {
// newFourByteTracer returns a native go tracer which collects
// 4 byte-identifiers of a tx, and implements vm.EVMLogger.
func newFourByteTracer(ctx *tracers.Context) tracers.Tracer {
func newFourByteTracer(ctx *tracers.Context, _ json.RawMessage) (tracers.Tracer, error) {
t := &fourByteTracer{
ids: make(map[string]int),
}
return t
return t, nil
}
// isPrecompiled returns whether the addr is a precompile. Logic borrowed from newJsTracer in eth/tracers/js/tracer.go

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@ -50,16 +50,27 @@ type callFrame struct {
type callTracer struct {
env *vm.EVM
callstack []callFrame
config callTracerConfig
interrupt uint32 // Atomic flag to signal execution interruption
reason error // Textual reason for the interruption
}
type callTracerConfig struct {
OnlyTopCall bool `json:"onlyTopCall"` // If true, call tracer won't collect any subcalls
}
// newCallTracer returns a native go tracer which tracks
// call frames of a tx, and implements vm.EVMLogger.
func newCallTracer(ctx *tracers.Context) tracers.Tracer {
func newCallTracer(ctx *tracers.Context, cfg json.RawMessage) (tracers.Tracer, error) {
var config callTracerConfig
if cfg != nil {
if err := json.Unmarshal(cfg, &config); err != nil {
return nil, err
}
}
// First callframe contains tx context info
// and is populated on start and end.
return &callTracer{callstack: make([]callFrame, 1)}
return &callTracer{callstack: make([]callFrame, 1), config: config}, nil
}
// CaptureStart implements the EVMLogger interface to initialize the tracing operation.
@ -101,6 +112,9 @@ func (t *callTracer) CaptureFault(pc uint64, op vm.OpCode, gas, cost uint64, _ *
// CaptureEnter is called when EVM enters a new scope (via call, create or selfdestruct).
func (t *callTracer) CaptureEnter(typ vm.OpCode, from common.Address, to common.Address, input []byte, gas uint64, value *big.Int) {
if t.config.OnlyTopCall {
return
}
// Skip if tracing was interrupted
if atomic.LoadUint32(&t.interrupt) > 0 {
t.env.Cancel()
@ -121,6 +135,9 @@ func (t *callTracer) CaptureEnter(typ vm.OpCode, from common.Address, to common.
// CaptureExit is called when EVM exits a scope, even if the scope didn't
// execute any code.
func (t *callTracer) CaptureExit(output []byte, gasUsed uint64, err error) {
if t.config.OnlyTopCall {
return
}
size := len(t.callstack)
if size <= 1 {
return

View File

@ -35,8 +35,8 @@ func init() {
type noopTracer struct{}
// newNoopTracer returns a new noop tracer.
func newNoopTracer(ctx *tracers.Context) tracers.Tracer {
return &noopTracer{}
func newNoopTracer(ctx *tracers.Context, _ json.RawMessage) (tracers.Tracer, error) {
return &noopTracer{}, nil
}
// CaptureStart implements the EVMLogger interface to initialize the tracing operation.

View File

@ -51,10 +51,10 @@ type prestateTracer struct {
reason error // Textual reason for the interruption
}
func newPrestateTracer(ctx *tracers.Context) tracers.Tracer {
func newPrestateTracer(ctx *tracers.Context, _ json.RawMessage) (tracers.Tracer, error) {
// First callframe contains tx context info
// and is populated on start and end.
return &prestateTracer{prestate: prestate{}}
return &prestateTracer{prestate: prestate{}}, nil
}
// CaptureStart implements the EVMLogger interface to initialize the tracing operation.

View File

@ -46,8 +46,8 @@ type revertReasonTracer struct {
}
// newRevertReasonTracer returns a new revert reason tracer.
func newRevertReasonTracer(_ *tracers.Context) tracers.Tracer {
return &revertReasonTracer{}
func newRevertReasonTracer(_ *tracers.Context, _ json.RawMessage) (tracers.Tracer, error) {
return &revertReasonTracer{}, nil
}
// CaptureStart implements the EVMLogger interface to initialize the tracing operation.

View File

@ -35,6 +35,7 @@ func init() {
package native
import (
"encoding/json"
"errors"
"github.com/ethereum/go-ethereum/eth/tracers"
@ -46,7 +47,7 @@ func init() {
}
// ctorFn is the constructor signature of a native tracer.
type ctorFn = func(*tracers.Context) tracers.Tracer
type ctorFn = func(*tracers.Context, json.RawMessage) (tracers.Tracer, error)
/*
ctors is a map of package-local tracer constructors.
@ -71,12 +72,12 @@ func register(name string, ctor ctorFn) {
}
// lookup returns a tracer, if one can be matched to the given name.
func lookup(name string, ctx *tracers.Context) (tracers.Tracer, error) {
func lookup(name string, ctx *tracers.Context, cfg json.RawMessage) (tracers.Tracer, error) {
if ctors == nil {
ctors = make(map[string]ctorFn)
}
if ctor, ok := ctors[name]; ok {
return ctor(ctx), nil
return ctor(ctx, cfg)
}
return nil, errors.New("no tracer found")
}

View File

@ -42,7 +42,7 @@ type Tracer interface {
Stop(err error)
}
type lookupFunc func(string, *Context) (Tracer, error)
type lookupFunc func(string, *Context, json.RawMessage) (Tracer, error)
var (
lookups []lookupFunc
@ -62,9 +62,9 @@ func RegisterLookup(wildcard bool, lookup lookupFunc) {
// New returns a new instance of a tracer, by iterating through the
// registered lookups.
func New(code string, ctx *Context) (Tracer, error) {
func New(code string, ctx *Context, cfg json.RawMessage) (Tracer, error) {
for _, lookup := range lookups {
if tracer, err := lookup(code, ctx); err == nil {
if tracer, err := lookup(code, ctx, cfg); err == nil {
return tracer, nil
}
}