cmd, core, eth/tracers: support fancier js tracing (#15516)

* cmd, core, eth/tracers: support fancier js tracing

* eth, internal/web3ext: rework trace API, concurrency, chain tracing

* eth/tracers: add three more JavaScript tracers

* eth/tracers, vendor: swap ottovm to duktape for tracing

* core, eth, internal: finalize call tracer and needed extras

* eth, tests: prestate tracer, call test suite, rewinding

* vendor: fix windows builds for tracer js engine

* vendor: temporary duktape fix

* eth/tracers: fix up 4byte and evmdis tracer

* vendor: pull in latest duktape with my upstream fixes

* eth: fix some review comments

* eth: rename rewind to reexec to make it more obvious

* core/vm: terminate tracing using defers

1 files changed, 727 insertions, 0 deletions

diff --git a/eth/api_tracer.go b/eth/api_tracer.go
new file mode 100644
index 000000000..0d0e2a73c
--- /dev/null
+++ b/eth/api_tracer.go
@@ -0,0 +1,727 @@
+// Copyright 2017 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package eth
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"io/ioutil"
+	"runtime"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/hexutil"
+	"github.com/ethereum/go-ethereum/core"
+	"github.com/ethereum/go-ethereum/core/state"
+	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/core/vm"
+	"github.com/ethereum/go-ethereum/eth/tracers"
+	"github.com/ethereum/go-ethereum/ethdb"
+	"github.com/ethereum/go-ethereum/internal/ethapi"
+	"github.com/ethereum/go-ethereum/log"
+	"github.com/ethereum/go-ethereum/rlp"
+	"github.com/ethereum/go-ethereum/rpc"
+	"github.com/ethereum/go-ethereum/trie"
+)
+
+const (
+	// defaultTraceTimeout is the amount of time a single transaction can execute
+	// by default before being forcefully aborted.
+	defaultTraceTimeout = 5 * time.Second
+
+	// defaultTraceReexec is the number of blocks the tracer is willing to go back
+	// and reexecute to produce missing historical state necessary to run a specific
+	// trace.
+	defaultTraceReexec = uint64(128)
+)
+
+// TraceConfig holds extra parameters to trace functions.
+type TraceConfig struct {
+	*vm.LogConfig
+	Tracer  *string
+	Timeout *string
+	Reexec  *uint64
+}
+
+// txTraceResult is the result of a single transaction trace.
+type txTraceResult struct {
+	Result interface{} `json:"result,omitempty"` // Trace results produced by the tracer
+	Error  string      `json:"error,omitempty"`  // Trace failure produced by the tracer
+}
+
+// blockTraceTask represents a single block trace task when an entire chain is
+// being traced.
+type blockTraceTask struct {
+	statedb *state.StateDB   // Intermediate state prepped for tracing
+	block   *types.Block     // Block to trace the transactions from
+	results []*txTraceResult // Trace results procudes by the task
+}
+
+// blockTraceResult represets the results of tracing a single block when an entire
+// chain is being traced.
+type blockTraceResult struct {
+	Block  hexutil.Uint64   `json:"block"`  // Block number corresponding to this trace
+	Hash   common.Hash      `json:"hash"`   // Block hash corresponding to this trace
+	Traces []*txTraceResult `json:"traces"` // Trace results produced by the task
+}
+
+// txTraceTask represents a single transaction trace task when an entire block
+// is being traced.
+type txTraceTask struct {
+	statedb *state.StateDB // Intermediate state prepped for tracing
+	index   int            // Transaction offset in the block
+}
+
+// ephemeralDatabase is a memory wrapper around a proper database, which acts as
+// an ephemeral write layer. This construct is used by the chain tracer to write
+// state tries for intermediate blocks without serializing to disk, but at the
+// same time to allow disk fallback for reads that do no hit the memory layer.
+type ephemeralDatabase struct {
+	diskdb ethdb.Database     // Persistent disk database to fall back to with reads
+	memdb  *ethdb.MemDatabase // Ephemeral memory database for primary reads and writes
+}
+
+func (db *ephemeralDatabase) Put(key []byte, value []byte) error { return db.memdb.Put(key, value) }
+func (db *ephemeralDatabase) Delete(key []byte) error            { return errors.New("delete not supported") }
+func (db *ephemeralDatabase) Close()                             { db.memdb.Close() }
+func (db *ephemeralDatabase) NewBatch() ethdb.Batch {
+	return db.memdb.NewBatch()
+}
+func (db *ephemeralDatabase) Has(key []byte) (bool, error) {
+	if has, _ := db.memdb.Has(key); has {
+		return has, nil
+	}
+	return db.diskdb.Has(key)
+}
+func (db *ephemeralDatabase) Get(key []byte) ([]byte, error) {
+	if blob, _ := db.memdb.Get(key); blob != nil {
+		return blob, nil
+	}
+	return db.diskdb.Get(key)
+}
+
+// Prune does a state sync into a new memory write layer and replaces the old one.
+// This allows us to discard entries that are no longer referenced from the current
+// state.
+func (db *ephemeralDatabase) Prune(root common.Hash) {
+	// Pull the still relevant state data into memory
+	sync := state.NewStateSync(root, db.diskdb)
+	for sync.Pending() > 0 {
+		hash := sync.Missing(1)[0]
+
+		// Move the next trie node from the memory layer into a sync struct
+		node, err := db.memdb.Get(hash[:])
+		if err != nil {
+			panic(err) // memdb must have the data
+		}
+		if _, _, err := sync.Process([]trie.SyncResult{{Hash: hash, Data: node}}); err != nil {
+			panic(err) // it's not possible to fail processing a node
+		}
+	}
+	// Discard the old memory layer and write a new one
+	db.memdb, _ = ethdb.NewMemDatabaseWithCap(db.memdb.Len())
+	if _, err := sync.Commit(db); err != nil {
+		panic(err) // writing into a memdb cannot fail
+	}
+}
+
+// TraceChain returns the structured logs created during the execution of EVM
+// between two blocks (excluding start) and returns them as a JSON object.
+func (api *PrivateDebugAPI) TraceChain(ctx context.Context, start, end rpc.BlockNumber, config *TraceConfig) (*rpc.Subscription, error) {
+	// Fetch the block interval that we want to trace
+	var from, to *types.Block
+
+	switch start {
+	case rpc.PendingBlockNumber:
+		from = api.eth.miner.PendingBlock()
+	case rpc.LatestBlockNumber:
+		from = api.eth.blockchain.CurrentBlock()
+	default:
+		from = api.eth.blockchain.GetBlockByNumber(uint64(start))
+	}
+	switch end {
+	case rpc.PendingBlockNumber:
+		to = api.eth.miner.PendingBlock()
+	case rpc.LatestBlockNumber:
+		to = api.eth.blockchain.CurrentBlock()
+	default:
+		to = api.eth.blockchain.GetBlockByNumber(uint64(end))
+	}
+	// Trace the chain if we've found all our blocks
+	if from == nil {
+		return nil, fmt.Errorf("starting block #%d not found", start)
+	}
+	if to == nil {
+		return nil, fmt.Errorf("end block #%d not found", end)
+	}
+	return api.traceChain(ctx, from, to, config)
+}
+
+// traceChain configures a new tracer according to the provided configuration, and
+// executes all the transactions contained within. The return value will be one item
+// per transaction, dependent on the requestd tracer.
+func (api *PrivateDebugAPI) traceChain(ctx context.Context, start, end *types.Block, config *TraceConfig) (*rpc.Subscription, error) {
+	// Tracing a chain is a **long** operation, only do with subscriptions
+	notifier, supported := rpc.NotifierFromContext(ctx)
+	if !supported {
+		return &rpc.Subscription{}, rpc.ErrNotificationsUnsupported
+	}
+	sub := notifier.CreateSubscription()
+
+	// Ensure we have a valid starting state before doing any work
+	origin := start.NumberU64()
+
+	memdb, _ := ethdb.NewMemDatabase()
+	db := &ephemeralDatabase{
+		diskdb: api.eth.ChainDb(),
+		memdb:  memdb,
+	}
+	if number := start.NumberU64(); number > 0 {
+		start = api.eth.blockchain.GetBlock(start.ParentHash(), start.NumberU64()-1)
+		if start == nil {
+			return nil, fmt.Errorf("parent block #%d not found", number-1)
+		}
+	}
+	statedb, err := state.New(start.Root(), state.NewDatabase(db))
+	if err != nil {
+		// If the starting state is missing, allow some number of blocks to be reexecuted
+		reexec := defaultTraceReexec
+		if config.Reexec != nil {
+			reexec = *config.Reexec
+		}
+		// Find the most recent block that has the state available
+		for i := uint64(0); i < reexec; i++ {
+			start = api.eth.blockchain.GetBlock(start.ParentHash(), start.NumberU64()-1)
+			if start == nil {
+				break
+			}
+			if statedb, err = state.New(start.Root(), state.NewDatabase(db)); err == nil {
+				break
+			}
+		}
+		// If we still don't have the state available, bail out
+		if err != nil {
+			switch err.(type) {
+			case *trie.MissingNodeError:
+				return nil, errors.New("required historical state unavailable")
+			default:
+				return nil, err
+			}
+		}
+	}
+	// Execute all the transaction contained within the chain concurrently for each block
+	blocks := int(end.NumberU64() - origin)
+
+	threads := runtime.NumCPU()
+	if threads > blocks {
+		threads = blocks
+	}
+	var (
+		pend    = new(sync.WaitGroup)
+		tasks   = make(chan *blockTraceTask, threads)
+		results = make(chan *blockTraceTask, threads)
+	)
+	for th := 0; th < threads; th++ {
+		pend.Add(1)
+		go func() {
+			defer pend.Done()
+
+			// Fetch and execute the next block trace tasks
+			for task := range tasks {
+				signer := types.MakeSigner(api.config, task.block.Number())
+
+				// Trace all the transactions contained within
+				for i, tx := range task.block.Transactions() {
+					msg, _ := tx.AsMessage(signer)
+					vmctx := core.NewEVMContext(msg, task.block.Header(), api.eth.blockchain, nil)
+
+					res, err := api.traceTx(ctx, msg, vmctx, task.statedb, config)
+					if err != nil {
+						task.results[i] = &txTraceResult{Error: err.Error()}
+						log.Warn("Tracing failed", "err", err)
+						break
+					}
+					task.statedb.DeleteSuicides()
+					task.results[i] = &txTraceResult{Result: res}
+				}
+				// Stream the result back to the user or abort on teardown
+				select {
+				case results <- task:
+				case <-notifier.Closed():
+					return
+				}
+			}
+		}()
+	}
+	// Start a goroutine to feed all the blocks into the tracers
+	begin := time.Now()
+	complete := start.NumberU64()
+
+	go func() {
+		var (
+			logged time.Time
+			number uint64
+			traced uint64
+			failed error
+		)
+		// Ensure everything is properly cleaned up on any exit path
+		defer func() {
+			close(tasks)
+			pend.Wait()
+
+			switch {
+			case failed != nil:
+				log.Warn("Chain tracing failed", "start", start.NumberU64(), "end", end.NumberU64(), "transactions", traced, "elapsed", time.Since(begin), "err", failed)
+			case number < end.NumberU64():
+				log.Warn("Chain tracing aborted", "start", start.NumberU64(), "end", end.NumberU64(), "abort", number, "transactions", traced, "elapsed", time.Since(begin))
+			default:
+				log.Info("Chain tracing finished", "start", start.NumberU64(), "end", end.NumberU64(), "transactions", traced, "elapsed", time.Since(begin))
+			}
+			close(results)
+		}()
+		// Feed all the blocks both into the tracer, as well as fast process concurrently
+		for number = start.NumberU64() + 1; number <= end.NumberU64(); number++ {
+			// Stop tracing if interruption was requested
+			select {
+			case <-notifier.Closed():
+				return
+			default:
+			}
+			// Print progress logs if long enough time elapsed
+			if time.Since(logged) > 8*time.Second {
+				if number > origin {
+					log.Info("Tracing chain segment", "start", origin, "end", end.NumberU64(), "current", number, "transactions", traced, "elapsed", time.Since(begin))
+				} else {
+					log.Info("Preparing state for chain trace", "block", number, "start", origin, "elapsed", time.Since(begin))
+				}
+				logged = time.Now()
+			}
+			// Retrieve the next block to trace
+			block := api.eth.blockchain.GetBlockByNumber(number)
+			if block == nil {
+				failed = fmt.Errorf("block #%d not found", number)
+				break
+			}
+			// Send the block over to the concurrent tracers (if not in the fast-forward phase)
+			if number > origin {
+				txs := block.Transactions()
+
+				select {
+				case tasks <- &blockTraceTask{statedb: statedb.Copy(), block: block, results: make([]*txTraceResult, len(txs))}:
+				case <-notifier.Closed():
+					return
+				}
+				traced += uint64(len(txs))
+			} else {
+				atomic.StoreUint64(&complete, number)
+			}
+			// Generate the next state snapshot fast without tracing
+			_, _, _, err := api.eth.blockchain.Processor().Process(block, statedb, vm.Config{})
+			if err != nil {
+				failed = err
+				break
+			}
+			// Finalize the state so any modifications are written to the trie
+			root, err := statedb.CommitTo(db, true)
+			if err != nil {
+				failed = err
+				break
+			}
+			if err := statedb.Reset(root); err != nil {
+				failed = err
+				break
+			}
+			// After every N blocks, prune the database to only retain relevant data
+			if (number-start.NumberU64())%4096 == 0 {
+				// Wait until currently pending trace jobs finish
+				for atomic.LoadUint64(&complete) != number {
+					select {
+					case <-time.After(100 * time.Millisecond):
+					case <-notifier.Closed():
+						return
+					}
+				}
+				// No more concurrent access at this point, prune the database
+				var (
+					nodes = db.memdb.Len()
+					start = time.Now()
+				)
+				db.Prune(root)
+				log.Info("Pruned tracer state entries", "deleted", nodes-db.memdb.Len(), "left", db.memdb.Len(), "elapsed", time.Since(start))
+
+				statedb, _ = state.New(root, state.NewDatabase(db))
+			}
+		}
+	}()
+
+	// Keep reading the trace results and stream the to the user
+	go func() {
+		var (
+			done = make(map[uint64]*blockTraceResult)
+			next = origin + 1
+		)
+		for res := range results {
+			// Queue up next received result
+			result := &blockTraceResult{
+				Block:  hexutil.Uint64(res.block.NumberU64()),
+				Hash:   res.block.Hash(),
+				Traces: res.results,
+			}
+			done[uint64(result.Block)] = result
+
+			// Stream completed traces to the user, aborting on the first error
+			for result, ok := done[next]; ok; result, ok = done[next] {
+				if len(result.Traces) > 0 || next == end.NumberU64() {
+					notifier.Notify(sub.ID, result)
+				}
+				atomic.StoreUint64(&complete, next)
+				delete(done, next)
+				next++
+			}
+		}
+	}()
+	return sub, nil
+}
+
+// TraceBlockByNumber returns the structured logs created during the execution of
+// EVM and returns them as a JSON object.
+func (api *PrivateDebugAPI) TraceBlockByNumber(ctx context.Context, number rpc.BlockNumber, config *TraceConfig) ([]*txTraceResult, error) {
+	// Fetch the block that we want to trace
+	var block *types.Block
+
+	switch number {
+	case rpc.PendingBlockNumber:
+		block = api.eth.miner.PendingBlock()
+	case rpc.LatestBlockNumber:
+		block = api.eth.blockchain.CurrentBlock()
+	default:
+		block = api.eth.blockchain.GetBlockByNumber(uint64(number))
+	}
+	// Trace the block if it was found
+	if block == nil {
+		return nil, fmt.Errorf("block #%d not found", number)
+	}
+	return api.traceBlock(ctx, block, config)
+}
+
+// TraceBlockByHash returns the structured logs created during the execution of
+// EVM and returns them as a JSON object.
+func (api *PrivateDebugAPI) TraceBlockByHash(ctx context.Context, hash common.Hash, config *TraceConfig) ([]*txTraceResult, error) {
+	block := api.eth.blockchain.GetBlockByHash(hash)
+	if block == nil {
+		return nil, fmt.Errorf("block #%x not found", hash)
+	}
+	return api.traceBlock(ctx, block, config)
+}
+
+// TraceBlock returns the structured logs created during the execution of EVM
+// and returns them as a JSON object.
+func (api *PrivateDebugAPI) TraceBlock(ctx context.Context, blob []byte, config *TraceConfig) ([]*txTraceResult, error) {
+	block := new(types.Block)
+	if err := rlp.Decode(bytes.NewReader(blob), block); err != nil {
+		return nil, fmt.Errorf("could not decode block: %v", err)
+	}
+	return api.traceBlock(ctx, block, config)
+}
+
+// TraceBlockFromFile returns the structured logs created during the execution of
+// EVM and returns them as a JSON object.
+func (api *PrivateDebugAPI) TraceBlockFromFile(ctx context.Context, file string, config *TraceConfig) ([]*txTraceResult, error) {
+	blob, err := ioutil.ReadFile(file)
+	if err != nil {
+		return nil, fmt.Errorf("could not read file: %v", err)
+	}
+	return api.TraceBlock(ctx, blob, config)
+}
+
+// traceBlock configures a new tracer according to the provided configuration, and
+// executes all the transactions contained within. The return value will be one item
+// per transaction, dependent on the requestd tracer.
+func (api *PrivateDebugAPI) traceBlock(ctx context.Context, block *types.Block, config *TraceConfig) ([]*txTraceResult, error) {
+	// Create the parent state database
+	if err := api.eth.engine.VerifyHeader(api.eth.blockchain, block.Header(), true); err != nil {
+		return nil, err
+	}
+	parent := api.eth.blockchain.GetBlock(block.ParentHash(), block.NumberU64()-1)
+	if parent == nil {
+		return nil, fmt.Errorf("parent %x not found", block.ParentHash())
+	}
+	reexec := defaultTraceReexec
+	if config.Reexec != nil {
+		reexec = *config.Reexec
+	}
+	statedb, err := api.computeStateDB(parent, reexec)
+	if err != nil {
+		return nil, err
+	}
+	// Execute all the transaction contained within the block concurrently
+	var (
+		signer = types.MakeSigner(api.config, block.Number())
+
+		txs     = block.Transactions()
+		results = make([]*txTraceResult, len(txs))
+
+		pend = new(sync.WaitGroup)
+		jobs = make(chan *txTraceTask, len(txs))
+	)
+	threads := runtime.NumCPU()
+	if threads > len(txs) {
+		threads = len(txs)
+	}
+	for th := 0; th < threads; th++ {
+		pend.Add(1)
+		go func() {
+			defer pend.Done()
+
+			// Fetch and execute the next transaction trace tasks
+			for task := range jobs {
+				msg, _ := txs[task.index].AsMessage(signer)
+				vmctx := core.NewEVMContext(msg, block.Header(), api.eth.blockchain, nil)
+
+				res, err := api.traceTx(ctx, msg, vmctx, task.statedb, config)
+				if err != nil {
+					results[task.index] = &txTraceResult{Error: err.Error()}
+					continue
+				}
+				results[task.index] = &txTraceResult{Result: res}
+			}
+		}()
+	}
+	// Feed the transactions into the tracers and return
+	var failed error
+	for i, tx := range txs {
+		// Send the trace task over for execution
+		jobs <- &txTraceTask{statedb: statedb.Copy(), index: i}
+
+		// Generate the next state snapshot fast without tracing
+		msg, _ := tx.AsMessage(signer)
+		vmctx := core.NewEVMContext(msg, block.Header(), api.eth.blockchain, nil)
+
+		vmenv := vm.NewEVM(vmctx, statedb, api.config, vm.Config{})
+		if _, _, _, err := core.ApplyMessage(vmenv, msg, new(core.GasPool).AddGas(msg.Gas())); err != nil {
+			failed = err
+			break
+		}
+		// Finalize the state so any modifications are written to the trie
+		statedb.Finalise(true)
+	}
+	close(jobs)
+	pend.Wait()
+
+	// If execution failed in between, abort
+	if failed != nil {
+		return nil, failed
+	}
+	return results, nil
+}
+
+// computeStateDB retrieves the state database associated with a certain block.
+// If no state is locally available for the given block, a number of blocks are
+// attempted to be reexecuted to generate the desired state.
+func (api *PrivateDebugAPI) computeStateDB(block *types.Block, reexec uint64) (*state.StateDB, error) {
+	// If we have the state fully available, use that
+	statedb, err := api.eth.blockchain.StateAt(block.Root())
+	if err == nil {
+		return statedb, nil
+	}
+	// Otherwise try to reexec blocks until we find a state or reach our limit
+	origin := block.NumberU64()
+
+	memdb, _ := ethdb.NewMemDatabase()
+	db := &ephemeralDatabase{
+		diskdb: api.eth.ChainDb(),
+		memdb:  memdb,
+	}
+	for i := uint64(0); i < reexec; i++ {
+		block = api.eth.blockchain.GetBlock(block.ParentHash(), block.NumberU64()-1)
+		if block == nil {
+			break
+		}
+		if statedb, err = state.New(block.Root(), state.NewDatabase(db)); err == nil {
+			break
+		}
+	}
+	if err != nil {
+		switch err.(type) {
+		case *trie.MissingNodeError:
+			return nil, errors.New("required historical state unavailable")
+		default:
+			return nil, err
+		}
+	}
+	// State was available at historical point, regenerate
+	var (
+		start  = time.Now()
+		logged time.Time
+	)
+	for block.NumberU64() < origin {
+		// Print progress logs if long enough time elapsed
+		if time.Since(logged) > 8*time.Second {
+			log.Info("Regenerating historical state", "block", block.NumberU64()+1, "target", origin, "elapsed", time.Since(start))
+			logged = time.Now()
+		}
+		// Retrieve the next block to regenerate and process it
+		if block = api.eth.blockchain.GetBlockByNumber(block.NumberU64() + 1); block == nil {
+			return nil, fmt.Errorf("block #%d not found", block.NumberU64()+1)
+		}
+		_, _, _, err := api.eth.blockchain.Processor().Process(block, statedb, vm.Config{})
+		if err != nil {
+			return nil, err
+		}
+		// Finalize the state so any modifications are written to the trie
+		root, err := statedb.CommitTo(db, true)
+		if err != nil {
+			return nil, err
+		}
+		if err := statedb.Reset(root); err != nil {
+			return nil, err
+		}
+		// After every N blocks, prune the database to only retain relevant data
+		if block.NumberU64()%4096 == 0 || block.NumberU64() == origin {
+			var (
+				nodes = db.memdb.Len()
+				begin = time.Now()
+			)
+			db.Prune(root)
+			log.Info("Pruned tracer state entries", "deleted", nodes-db.memdb.Len(), "left", db.memdb.Len(), "elapsed", time.Since(begin))
+
+			statedb, _ = state.New(root, state.NewDatabase(db))
+		}
+	}
+	log.Info("Historical state regenerated", "block", block.NumberU64(), "elapsed", time.Since(start))
+	return statedb, nil
+}
+
+// TraceTransaction returns the structured logs created during the execution of EVM
+// and returns them as a JSON object.
+func (api *PrivateDebugAPI) TraceTransaction(ctx context.Context, hash common.Hash, config *TraceConfig) (interface{}, error) {
+	// Retrieve the transaction and assemble its EVM context
+	tx, blockHash, _, index := core.GetTransaction(api.eth.ChainDb(), hash)
+	if tx == nil {
+		return nil, fmt.Errorf("transaction %x not found", hash)
+	}
+	reexec := defaultTraceReexec
+	if config.Reexec != nil {
+		reexec = *config.Reexec
+	}
+	msg, vmctx, statedb, err := api.computeTxEnv(blockHash, int(index), reexec)
+	if err != nil {
+		return nil, err
+	}
+	// Trace the transaction and return
+	return api.traceTx(ctx, msg, vmctx, statedb, config)
+}
+
+// traceTx configures a new tracer according to the provided configuration, and
+// executes the given message in the provided environment. The return value will
+// be tracer dependent.
+func (api *PrivateDebugAPI) traceTx(ctx context.Context, message core.Message, vmctx vm.Context, statedb *state.StateDB, config *TraceConfig) (interface{}, error) {
+	// Assemble the structured logger or the JavaScript tracer
+	var (
+		tracer vm.Tracer
+		err    error
+	)
+	switch {
+	case config != nil && config.Tracer != nil:
+		// Define a meaningful timeout of a single transaction trace
+		timeout := defaultTraceTimeout
+		if config.Timeout != nil {
+			if timeout, err = time.ParseDuration(*config.Timeout); err != nil {
+				return nil, err
+			}
+		}
+		// Constuct the JavaScript tracer to execute with
+		if tracer, err = tracers.New(*config.Tracer); err != nil {
+			return nil, err
+		}
+		// Handle timeouts and RPC cancellations
+		deadlineCtx, cancel := context.WithTimeout(ctx, timeout)
+		go func() {
+			<-deadlineCtx.Done()
+			tracer.(*tracers.Tracer).Stop(errors.New("execution timeout"))
+		}()
+		defer cancel()
+
+	case config == nil:
+		tracer = vm.NewStructLogger(nil)
+
+	default:
+		tracer = vm.NewStructLogger(config.LogConfig)
+	}
+	// Run the transaction with tracing enabled.
+	vmenv := vm.NewEVM(vmctx, statedb, api.config, vm.Config{Debug: true, Tracer: tracer})
+
+	ret, gas, failed, err := core.ApplyMessage(vmenv, message, new(core.GasPool).AddGas(message.Gas()))
+	if err != nil {
+		return nil, fmt.Errorf("tracing failed: %v", err)
+	}
+	// Depending on the tracer type, format and return the output
+	switch tracer := tracer.(type) {
+	case *vm.StructLogger:
+		return &ethapi.ExecutionResult{
+			Gas:         gas,
+			Failed:      failed,
+			ReturnValue: fmt.Sprintf("%x", ret),
+			StructLogs:  ethapi.FormatLogs(tracer.StructLogs()),
+		}, nil
+
+	case *tracers.Tracer:
+		return tracer.GetResult()
+
+	default:
+		panic(fmt.Sprintf("bad tracer type %T", tracer))
+	}
+}
+
+// computeTxEnv returns the execution environment of a certain transaction.
+func (api *PrivateDebugAPI) computeTxEnv(blockHash common.Hash, txIndex int, reexec uint64) (core.Message, vm.Context, *state.StateDB, error) {
+	// Create the parent state database
+	block := api.eth.blockchain.GetBlockByHash(blockHash)
+	if block == nil {
+		return nil, vm.Context{}, nil, fmt.Errorf("block %x not found", blockHash)
+	}
+	parent := api.eth.blockchain.GetBlock(block.ParentHash(), block.NumberU64()-1)
+	if parent == nil {
+		return nil, vm.Context{}, nil, fmt.Errorf("parent %x not found", block.ParentHash())
+	}
+	statedb, err := api.computeStateDB(parent, reexec)
+	if err != nil {
+		return nil, vm.Context{}, nil, err
+	}
+	// Recompute transactions up to the target index.
+	signer := types.MakeSigner(api.config, block.Number())
+
+	for idx, tx := range block.Transactions() {
+		// Assemble the transaction call message and return if the requested offset
+		msg, _ := tx.AsMessage(signer)
+		context := core.NewEVMContext(msg, block.Header(), api.eth.blockchain, nil)
+		if idx == txIndex {
+			return msg, context, statedb, nil
+		}
+		// Not yet the searched for transaction, execute on top of the current state
+		vmenv := vm.NewEVM(context, statedb, api.config, vm.Config{})
+		if _, _, _, err := core.ApplyMessage(vmenv, msg, new(core.GasPool).AddGas(tx.Gas())); err != nil {
+			return nil, vm.Context{}, nil, fmt.Errorf("tx %x failed: %v", tx.Hash(), err)
+		}
+		statedb.DeleteSuicides()
+	}
+	return nil, vm.Context{}, nil, fmt.Errorf("tx index %d out of range for block %x", txIndex, blockHash)
+}