trie/sync.go

// Copyright 2015 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 trie

import (
	"errors"
	"fmt"
	"sync"

	"github.com/ethereum/go-ethereum/common"
	"github.com/ethereum/go-ethereum/common/hexutil"
	"github.com/ethereum/go-ethereum/common/prque"
	"github.com/ethereum/go-ethereum/core/rawdb"
	"github.com/ethereum/go-ethereum/core/types"
	"github.com/ethereum/go-ethereum/crypto"
	"github.com/ethereum/go-ethereum/ethdb"
	"github.com/ethereum/go-ethereum/log"
	"github.com/ethereum/go-ethereum/metrics"
)

// ErrNotRequested is returned by the trie sync when it's requested to process a
// node it did not request.
var ErrNotRequested = errors.New("not requested")

// ErrAlreadyProcessed is returned by the trie sync when it's requested to process a
// node it already processed previously.
var ErrAlreadyProcessed = errors.New("already processed")

// maxFetchesPerDepth is the maximum number of pending trie nodes per depth. The
// role of this value is to limit the number of trie nodes that get expanded in
// memory if the node was configured with a significant number of peers.
const maxFetchesPerDepth = 16384

var (
	// deletionGauge is the metric to track how many trie node deletions
	// are performed in total during the sync process.
	deletionGauge = metrics.NewRegisteredGauge("trie/sync/delete", nil)

	// lookupGauge is the metric to track how many trie node lookups are
	// performed to determine if node needs to be deleted.
	lookupGauge = metrics.NewRegisteredGauge("trie/sync/lookup", nil)

	// accountNodeSyncedGauge is the metric to track how many account trie
	// node are written during the sync.
	accountNodeSyncedGauge = metrics.NewRegisteredGauge("trie/sync/nodes/account", nil)

	// storageNodeSyncedGauge is the metric to track how many account trie
	// node are written during the sync.
	storageNodeSyncedGauge = metrics.NewRegisteredGauge("trie/sync/nodes/storage", nil)

	// codeSyncedGauge is the metric to track how many contract codes are
	// written during the sync.
	codeSyncedGauge = metrics.NewRegisteredGauge("trie/sync/codes", nil)
)

// SyncPath is a path tuple identifying a particular trie node either in a single
// trie (account) or a layered trie (account -> storage).
//
// Content wise the tuple either has 1 element if it addresses a node in a single
// trie or 2 elements if it addresses a node in a stacked trie.
//
// To support aiming arbitrary trie nodes, the path needs to support odd nibble
// lengths. To avoid transferring expanded hex form over the network, the last
// part of the tuple (which needs to index into the middle of a trie) is compact
// encoded. In case of a 2-tuple, the first item is always 32 bytes so that is
// simple binary encoded.
//
// Examples:
//   - Path 0x9  -> {0x19}
//   - Path 0x99 -> {0x0099}
//   - Path 0x01234567890123456789012345678901012345678901234567890123456789019  -> {0x0123456789012345678901234567890101234567890123456789012345678901, 0x19}
//   - Path 0x012345678901234567890123456789010123456789012345678901234567890199 -> {0x0123456789012345678901234567890101234567890123456789012345678901, 0x0099}
type SyncPath [][]byte

// NewSyncPath converts an expanded trie path from nibble form into a compact
// version that can be sent over the network.
func NewSyncPath(path []byte) SyncPath {
	// If the hash is from the account trie, append a single item, if it
	// is from a storage trie, append a tuple. Note, the length 64 is
	// clashing between account leaf and storage root. It's fine though
	// because having a trie node at 64 depth means a hash collision was
	// found and we're long dead.
	if len(path) < 64 {
		return SyncPath{hexToCompact(path)}
	}

	return SyncPath{hexToKeybytes(path[:64]), hexToCompact(path[64:])}
}

// LeafCallback is a callback type invoked when a trie operation reaches a leaf
// node.
//
// The keys is a path tuple identifying a particular trie node either in a single
// trie (account) or a layered trie (account -> storage). Each key in the tuple
// is in the raw format(32 bytes).
//
// The path is a composite hexary path identifying the trie node. All the key
// bytes are converted to the hexary nibbles and composited with the parent path
// if the trie node is in a layered trie.
//
// It's used by state sync and commit to allow handling external references
// between account and storage tries. And also it's used in the state healing
// for extracting the raw states(leaf nodes) with corresponding paths.
type LeafCallback func(keys [][]byte, path []byte, leaf []byte, parent common.Hash, parentPath []byte) error

// nodeRequest represents a scheduled or already in-flight trie node retrieval request.
type nodeRequest struct {
	hash common.Hash // Hash of the trie node to retrieve
	path []byte      // Merkle path leading to this node for prioritization
	data []byte      // Data content of the node, cached until all subtrees complete

	parent   *nodeRequest // Parent state node referencing this entry
	deps     int          // Number of dependencies before allowed to commit this node
	callback LeafCallback // Callback to invoke if a leaf node it reached on this branch
}

// codeRequest represents a scheduled or already in-flight bytecode retrieval request.
type codeRequest struct {
	hash    common.Hash    // Hash of the contract bytecode to retrieve
	path    []byte         // Merkle path leading to this node for prioritization
	data    []byte         // Data content of the node, cached until all subtrees complete
	parents []*nodeRequest // Parent state nodes referencing this entry (notify all upon completion)
}

// NodeSyncResult is a response with requested trie node along with its node path.
type NodeSyncResult struct {
	Path string // Path of the originally unknown trie node
	Data []byte // Data content of the retrieved trie node
}

// CodeSyncResult is a response with requested bytecode along with its hash.
type CodeSyncResult struct {
	Hash common.Hash // Hash the originally unknown bytecode
	Data []byte      // Data content of the retrieved bytecode
}

// nodeOp represents an operation upon the trie node. It can either represent a
// deletion to the specific node or a node write for persisting retrieved node.
type nodeOp struct {
	del   bool        // flag if op stands for a delete operation
	owner common.Hash // identifier of the trie (empty for account trie)
	path  []byte      // path from the root to the specified node.
	blob  []byte      // the content of the node (nil for deletion)
	hash  common.Hash // hash of the node content (empty for node deletion)
}

// valid checks whether the node operation is valid.
func (op *nodeOp) valid() bool {
	if op.del && len(op.blob) != 0 {
		return false
	}
	if !op.del && len(op.blob) == 0 {
		return false
	}
	return true
}

// string returns the node operation in string representation.
func (op *nodeOp) string() string {
	var node string
	if op.owner == (common.Hash{}) {
		node = fmt.Sprintf("node: (%v)", op.path)
	} else {
		node = fmt.Sprintf("node: (%x-%v)", op.owner, op.path)
	}
	var blobHex string
	if len(op.blob) == 0 {
		blobHex = "nil"
	} else {
		blobHex = hexutil.Encode(op.blob)
	}
	if op.del {
		return fmt.Sprintf("del %s %s %s", node, blobHex, op.hash.Hex())
	}
	return fmt.Sprintf("write %s %s %s", node, blobHex, op.hash.Hex())
}

// syncMemBatch is an in-memory buffer of successfully downloaded but not yet
// persisted data items.
type syncMemBatch struct {
	scheme string                 // State scheme identifier
	codes  map[common.Hash][]byte // In-memory batch of recently completed codes
	nodes  []nodeOp               // In-memory batch of recently completed/deleted nodes
	size   uint64                 // Estimated batch-size of in-memory data.
}

// newSyncMemBatch allocates a new memory-buffer for not-yet persisted trie nodes.
func newSyncMemBatch(scheme string) *syncMemBatch {
	return &syncMemBatch{
		scheme: scheme,
		codes:  make(map[common.Hash][]byte),
	}
}

// hasCode reports the contract code with specific hash is already cached.
func (batch *syncMemBatch) hasCode(hash common.Hash) bool {
	_, ok := batch.codes[hash]
	return ok
}

// addCode caches a contract code database write operation.
func (batch *syncMemBatch) addCode(hash common.Hash, code []byte) {
	batch.codes[hash] = code
	batch.size += common.HashLength + uint64(len(code))
}

// addNode caches a node database write operation.
func (batch *syncMemBatch) addNode(owner common.Hash, path []byte, blob []byte, hash common.Hash) {
	if batch.scheme == rawdb.PathScheme {
		if owner == (common.Hash{}) {
			batch.size += uint64(len(path) + len(blob))
		} else {
			batch.size += common.HashLength + uint64(len(path)+len(blob))
		}
	} else {
		batch.size += common.HashLength + uint64(len(blob))
	}
	batch.nodes = append(batch.nodes, nodeOp{
		owner: owner,
		path:  path,
		blob:  blob,
		hash:  hash,
	})
}

// delNode caches a node database delete operation.
func (batch *syncMemBatch) delNode(owner common.Hash, path []byte) {
	if batch.scheme != rawdb.PathScheme {
		log.Error("Unexpected node deletion", "owner", owner, "path", path, "scheme", batch.scheme)
		return // deletion is not supported in hash mode.
	}
	if owner == (common.Hash{}) {
		batch.size += uint64(len(path))
	} else {
		batch.size += common.HashLength + uint64(len(path))
	}
	batch.nodes = append(batch.nodes, nodeOp{
		del:   true,
		owner: owner,
		path:  path,
	})
}

// Sync is the main state trie synchronisation scheduler, which provides yet
// unknown trie hashes to retrieve, accepts node data associated with said hashes
// and reconstructs the trie step by step until all is done.
type Sync struct {
	scheme   string                       // Node scheme descriptor used in database.
	database ethdb.KeyValueReader         // Persistent database to check for existing entries
	membatch *syncMemBatch                // Memory buffer to avoid frequent database writes
	nodeReqs map[string]*nodeRequest      // Pending requests pertaining to a trie node path
	codeReqs map[common.Hash]*codeRequest // Pending requests pertaining to a code hash
	queue    *prque.Prque[int64, any]     // Priority queue with the pending requests
	fetches  map[int]int                  // Number of active fetches per trie node depth
}

// NewSync creates a new trie data download scheduler.
func NewSync(root common.Hash, database ethdb.KeyValueReader, callback LeafCallback, scheme string) *Sync {
	ts := &Sync{
		scheme:   scheme,
		database: database,
		membatch: newSyncMemBatch(scheme),
		nodeReqs: make(map[string]*nodeRequest),
		codeReqs: make(map[common.Hash]*codeRequest),
		queue:    prque.New[int64, any](nil), // Ugh, can contain both string and hash, whyyy
		fetches:  make(map[int]int),
	}
	ts.AddSubTrie(root, nil, common.Hash{}, nil, callback)

	return ts
}

// AddSubTrie registers a new trie to the sync code, rooted at the designated
// parent for completion tracking. The given path is a unique node path in
// hex format and contain all the parent path if it's layered trie node.
func (s *Sync) AddSubTrie(root common.Hash, path []byte, parent common.Hash, parentPath []byte, callback LeafCallback) {
	if root == types.EmptyRootHash {
		return
	}
	owner, inner := ResolvePath(path)
	exist, inconsistent := s.hasNode(owner, inner, root)
	if exist {
		// The entire subtrie is already present in the database.
		return
	} else if inconsistent {
		// There is a pre-existing node with the wrong hash in DB, remove it.
		s.membatch.delNode(owner, inner)
	}
	// Assemble the new sub-trie sync request
	req := &nodeRequest{
		hash:     root,
		path:     path,
		callback: callback,
	}
	// If this sub-trie has a designated parent, link them together
	if parent != (common.Hash{}) {
		ancestor := s.nodeReqs[string(parentPath)]
		if ancestor == nil {
			panic(fmt.Sprintf("sub-trie ancestor not found: %x", parent))
		}

		ancestor.deps++
		req.parent = ancestor
	}

	s.scheduleNodeRequest(req)
}

// AddCodeEntry schedules the direct retrieval of a contract code that should not
// be interpreted as a trie node, but rather accepted and stored into the database
// as is.
func (s *Sync) AddCodeEntry(hash common.Hash, path []byte, parent common.Hash, parentPath []byte) {
	// Short circuit if the entry is empty or already known
	if hash == types.EmptyCodeHash {
		return
	}

	if s.membatch.hasCode(hash) {
		return
	}
	// If database says duplicate, the blob is present for sure.
	// Note we only check the existence with new code scheme, snap
	// sync is expected to run with a fresh new node. Even there
	// exists the code with legacy format, fetch and store with
	// new scheme anyway.
	if rawdb.HasCodeWithPrefix(s.database, hash) {
		return
	}
	// Assemble the new sub-trie sync request
	req := &codeRequest{
		path: path,
		hash: hash,
	}
	// If this sub-trie has a designated parent, link them together
	if parent != (common.Hash{}) {
		ancestor := s.nodeReqs[string(parentPath)] // the parent of codereq can ONLY be nodereq
		if ancestor == nil {
			panic(fmt.Sprintf("raw-entry ancestor not found: %x", parent))
		}

		ancestor.deps++
		req.parents = append(req.parents, ancestor)
	}

	s.scheduleCodeRequest(req)
}

// Missing retrieves the known missing nodes from the trie for retrieval. To aid
// both eth/6x style fast sync and snap/1x style state sync, the paths of trie
// nodes are returned too, as well as separate hash list for codes.
func (s *Sync) Missing(max int) ([]string, []common.Hash, []common.Hash) {
	var (
		nodePaths  []string
		nodeHashes []common.Hash
		codeHashes []common.Hash
	)

	for !s.queue.Empty() && (max == 0 || len(nodeHashes)+len(codeHashes) < max) {
		// Retrieve the next item in line
		item, prio := s.queue.Peek()

		// If we have too many already-pending tasks for this depth, throttle
		depth := int(prio >> 56)
		if s.fetches[depth] > maxFetchesPerDepth {
			break
		}
		// Item is allowed to be scheduled, add it to the task list
		s.queue.Pop()

		s.fetches[depth]++

		switch item := item.(type) {
		case common.Hash:
			codeHashes = append(codeHashes, item)
		case string:
			req, ok := s.nodeReqs[item]
			if !ok {
				log.Error("Missing node request", "path", item)
				continue // System very wrong, shouldn't happen
			}

			nodePaths = append(nodePaths, item)
			nodeHashes = append(nodeHashes, req.hash)
		}
	}

	return nodePaths, nodeHashes, codeHashes
}

// ProcessCode injects the received data for requested item. Note it can
// happen that the single response commits two pending requests(e.g.
// there are two requests one for code and one for node but the hash
// is same). In this case the second response for the same hash will
// be treated as "non-requested" item or "already-processed" item but
// there is no downside.
func (s *Sync) ProcessCode(result CodeSyncResult) error {
	// If the code was not requested or it's already processed, bail out
	req := s.codeReqs[result.Hash]
	if req == nil {
		return ErrNotRequested
	}

	if req.data != nil {
		return ErrAlreadyProcessed
	}

	req.data = result.Data

	return s.commitCodeRequest(req)
}

// ProcessNode injects the received data for requested item. Note it can
// happen that the single response commits two pending requests(e.g.
// there are two requests one for code and one for node but the hash
// is same). In this case the second response for the same hash will
// be treated as "non-requested" item or "already-processed" item but
// there is no downside.
func (s *Sync) ProcessNode(result NodeSyncResult) error {
	// If the trie node was not requested or it's already processed, bail out
	req := s.nodeReqs[result.Path]
	if req == nil {
		return ErrNotRequested
	}

	if req.data != nil {
		return ErrAlreadyProcessed
	}
	// Decode the node data content and update the request
	node, err := decodeNode(req.hash.Bytes(), result.Data)
	if err != nil {
		return err
	}

	req.data = result.Data

	// Create and schedule a request for all the children nodes
	requests, err := s.children(req, node)
	if err != nil {
		return err
	}

	if len(requests) == 0 && req.deps == 0 {
		_ = s.commitNodeRequest(req)
	} else {
		req.deps += len(requests)

		for _, child := range requests {
			s.scheduleNodeRequest(child)
		}
	}

	return nil
}

// Commit flushes the data stored in the internal membatch out to persistent
// storage, returning any occurred error. The whole data set will be flushed
// in an atomic database batch.
func (s *Sync) Commit(dbw ethdb.Batch) error {
	// Flush the pending node writes into database batch.
	var (
		account int
		storage int
	)
	for _, op := range s.membatch.nodes {
		if !op.valid() {
			return fmt.Errorf("invalid op, %s", op.string())
		}
		if op.del {
			// node deletion is only supported in path mode.
			if op.owner == (common.Hash{}) {
				rawdb.DeleteAccountTrieNode(dbw, op.path)
			} else {
				rawdb.DeleteStorageTrieNode(dbw, op.owner, op.path)
			}
			deletionGauge.Inc(1)
		} else {
			if op.owner == (common.Hash{}) {
				account += 1
			} else {
				storage += 1
			}
			rawdb.WriteTrieNode(dbw, op.owner, op.path, op.hash, op.blob, s.scheme)
		}
	}
	accountNodeSyncedGauge.Inc(int64(account))
	storageNodeSyncedGauge.Inc(int64(storage))

	// Flush the pending code writes into database batch.
	for hash, value := range s.membatch.codes {
		rawdb.WriteCode(dbw, hash, value)
	}
	codeSyncedGauge.Inc(int64(len(s.membatch.codes)))

	s.membatch = newSyncMemBatch(s.scheme) // reset the batch
	return nil
}

// MemSize returns an estimated size (in bytes) of the data held in the membatch.
func (s *Sync) MemSize() uint64 {
	return s.membatch.size
}

// Pending returns the number of state entries currently pending for download.
func (s *Sync) Pending() int {
	return len(s.nodeReqs) + len(s.codeReqs)
}

// scheduleNodeRequest inserts a new state retrieval request into the fetch queue. If there
// is already a pending request for this node, the new request will be discarded
// and only a parent reference added to the old one.
func (s *Sync) scheduleNodeRequest(req *nodeRequest) {
	s.nodeReqs[string(req.path)] = req

	// Schedule the request for future retrieval. This queue is shared
	// by both node requests and code requests.
	prio := int64(len(req.path)) << 56 // depth >= 128 will never happen, storage leaves will be included in their parents
	for i := 0; i < 14 && i < len(req.path); i++ {
		prio |= int64(15-req.path[i]) << (52 - i*4) // 15-nibble => lexicographic order
	}
	s.queue.Push(string(req.path), prio)
}

// scheduleCodeRequest inserts a new state retrieval request into the fetch queue. If there
// is already a pending request for this node, the new request will be discarded
// and only a parent reference added to the old one.
func (s *Sync) scheduleCodeRequest(req *codeRequest) {
	// If we're already requesting this node, add a new reference and stop
	if old, ok := s.codeReqs[req.hash]; ok {
		old.parents = append(old.parents, req.parents...)
		return
	}

	s.codeReqs[req.hash] = req

	// Schedule the request for future retrieval. This queue is shared
	// by both node requests and code requests.
	prio := int64(len(req.path)) << 56 // depth >= 128 will never happen, storage leaves will be included in their parents
	for i := 0; i < 14 && i < len(req.path); i++ {
		prio |= int64(15-req.path[i]) << (52 - i*4) // 15-nibble => lexicographic order
	}
	s.queue.Push(req.hash, prio)
}

// children retrieves all the missing children of a state trie entry for future
// retrieval scheduling.
// nolint:gocognit
func (s *Sync) children(req *nodeRequest, object node) ([]*nodeRequest, error) {
	// Gather all the children of the node, irrelevant whether known or not
	type childNode struct {
		path []byte
		node node
	}

	var children []childNode

	switch node := (object).(type) {
	case *shortNode:
		key := node.Key
		if hasTerm(key) {
			key = key[:len(key)-1]
		}

		children = []childNode{{
			node: node.Val,
			path: append(append([]byte(nil), req.path...), key...),
		}}
		// Mark all internal nodes between shortNode and its **in disk**
		// child as invalid. This is essential in the case of path mode
		// scheme; otherwise, state healing might overwrite existing child
		// nodes silently while leaving a dangling parent node within the
		// range of this internal path on disk and the persistent state
		// ends up with a very weird situation that nodes on the same path
		// are not inconsistent while they all present in disk. This property
		// would break the guarantee for state healing.
		//
		// While it's possible for this shortNode to overwrite a previously
		// existing full node, the other branches of the fullNode can be
		// retained as they are not accessible with the new shortNode, and
		// also the whole sub-trie is still untouched and complete.
		//
		// This step is only necessary for path mode, as there is no deletion
		// in hash mode at all.
		if _, ok := node.Val.(hashNode); ok && s.scheme == rawdb.PathScheme {
			owner, inner := ResolvePath(req.path)
			for i := 1; i < len(key); i++ {
				// While checking for a non-existent item in Pebble can be less efficient
				// without a bloom filter, the relatively low frequency of lookups makes
				// the performance impact negligible.
				var exists bool
				if owner == (common.Hash{}) {
					exists = rawdb.HasAccountTrieNode(s.database, append(inner, key[:i]...))
				} else {
					exists = rawdb.HasStorageTrieNode(s.database, owner, append(inner, key[:i]...))
				}
				if exists {
					s.membatch.delNode(owner, append(inner, key[:i]...))
					log.Debug("Detected dangling node", "owner", owner, "path", append(inner, key[:i]...))
				}
			}
			lookupGauge.Inc(int64(len(key) - 1))
		}
	case *fullNode:
		for i := 0; i < 17; i++ {
			if node.Children[i] != nil {
				children = append(children, childNode{
					node: node.Children[i],
					path: append(append([]byte(nil), req.path...), byte(i)),
				})
			}
		}
	default:
		panic(fmt.Sprintf("unknown node: %+v", node))
	}
	// Iterate over the children, and request all unknown ones
	var (
		missing = make(chan *nodeRequest, len(children))
		pending sync.WaitGroup
		batchMu sync.Mutex
	)

	for _, child := range children {
		// Notify any external watcher of a new key/value node
		if req.callback != nil {
			if node, ok := (child.node).(valueNode); ok {
				var paths [][]byte
				if len(child.path) == 2*common.HashLength {
					paths = append(paths, hexToKeybytes(child.path))
				} else if len(child.path) == 4*common.HashLength {
					paths = append(paths, hexToKeybytes(child.path[:2*common.HashLength]))
					paths = append(paths, hexToKeybytes(child.path[2*common.HashLength:]))
				}

				if err := req.callback(paths, child.path, node, req.hash, req.path); err != nil {
					return nil, err
				}
			}
		}
		// If the child references another node, resolve or schedule.
		// We check all children concurrently.
		if node, ok := (child.node).(hashNode); ok {
			path := child.path
			hash := common.BytesToHash(node)
			pending.Add(1)
			go func() {
				defer pending.Done()
				owner, inner := ResolvePath(path)
				exist, inconsistent := s.hasNode(owner, inner, hash)
				if exist {
					return
				} else if inconsistent {
					// There is a pre-existing node with the wrong hash in DB, remove it.
					batchMu.Lock()
					s.membatch.delNode(owner, inner)
					batchMu.Unlock()
				}
				// Locally unknown node, schedule for retrieval
				missing <- &nodeRequest{
					path:     path,
					hash:     hash,
					parent:   req,
					callback: req.callback,
				}
			}()
		}
	}

	pending.Wait()

	requests := make([]*nodeRequest, 0, len(children))

	for done := false; !done; {
		select {
		case miss := <-missing:
			requests = append(requests, miss)
		default:
			done = true
		}
	}

	return requests, nil
}

// commitNodeRequest finalizes a retrieval request and stores it into the membatch. If any
// of the referencing parent requests complete due to this commit, they are also
// committed themselves.
func (s *Sync) commitNodeRequest(req *nodeRequest) error {
	// Write the node content to the membatch
	owner, path := ResolvePath(req.path)
	s.membatch.addNode(owner, path, req.data, req.hash)

	// Removed the completed node request
	delete(s.nodeReqs, string(req.path))

	s.fetches[len(req.path)]--

	// Check parent for completion
	if req.parent != nil {
		req.parent.deps--
		if req.parent.deps == 0 {
			if err := s.commitNodeRequest(req.parent); err != nil {
				return err
			}
		}
	}

	return nil
}

// commitCodeRequest finalizes a retrieval request and stores it into the membatch. If any
// of the referencing parent requests complete due to this commit, they are also
// committed themselves.
func (s *Sync) commitCodeRequest(req *codeRequest) error {
	// Write the node content to the membatch
	s.membatch.addCode(req.hash, req.data)

	// Removed the completed code request
	delete(s.codeReqs, req.hash)

	s.fetches[len(req.path)]--

	// Check all parents for completion
	for _, parent := range req.parents {
		parent.deps--
		if parent.deps == 0 {
			if err := s.commitNodeRequest(parent); err != nil {
				return err
			}
		}
	}

	return nil
}

// hasNode reports whether the specified trie node is present in the database.
// 'exists' is true when the node exists in the database and matches the given root
// hash. The 'inconsistent' return value is true when the node exists but does not
// match the expected hash.
func (s *Sync) hasNode(owner common.Hash, path []byte, hash common.Hash) (exists bool, inconsistent bool) {
	// If node is running with hash scheme, check the presence with node hash.
	if s.scheme == rawdb.HashScheme {
		return rawdb.HasLegacyTrieNode(s.database, hash), false
	}
	// If node is running with path scheme, check the presence with node path.
	var blob []byte
	if owner == (common.Hash{}) {
		blob = rawdb.ReadAccountTrieNode(s.database, path)
	} else {
		blob = rawdb.ReadStorageTrieNode(s.database, owner, path)
	}
	h := newBlobHasher()
	defer h.release()
	exists = hash == h.hash(blob)
	inconsistent = !exists && len(blob) != 0
	return exists, inconsistent
}

// ResolvePath resolves the provided composite node path by separating the
// path in account trie if it's existent.
func ResolvePath(path []byte) (common.Hash, []byte) {
	var owner common.Hash
	if len(path) >= 2*common.HashLength {
		owner = common.BytesToHash(hexToKeybytes(path[:2*common.HashLength]))
		path = path[2*common.HashLength:]
	}

	return owner, path
}

// blobHasher is used to compute the sha256 hash of the provided data.
type blobHasher struct{ state crypto.KeccakState }

// blobHasherPool is the pool for reusing pre-allocated hash state.
var blobHasherPool = sync.Pool{
	New: func() interface{} { return &blobHasher{state: crypto.NewKeccakState()} },
}

func newBlobHasher() *blobHasher {
	return blobHasherPool.Get().(*blobHasher)
}

func (h *blobHasher) hash(data []byte) common.Hash {
	return crypto.HashData(h.state, data)
}

func (h *blobHasher) release() {
	blobHasherPool.Put(h)
}