path: root/swarm/storage/ldbstore.go

// Copyright 2016 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/>.

// disk storage layer for the package bzz
// DbStore implements the ChunkStore interface and is used by the FileStore as
// persistent storage of chunks
// it implements purging based on access count allowing for external control of
// max capacity

package storage

import (
    "archive/tar"
    "bytes"
    "context"
    "encoding/binary"
    "encoding/hex"
    "fmt"
    "io"
    "io/ioutil"
    "sort"
    "sync"

    "github.com/ethereum/go-ethereum/metrics"
    "github.com/ethereum/go-ethereum/rlp"
    "github.com/ethereum/go-ethereum/swarm/chunk"
    "github.com/ethereum/go-ethereum/swarm/log"
    "github.com/ethereum/go-ethereum/swarm/storage/mock"
    "github.com/syndtr/goleveldb/leveldb"
    "github.com/syndtr/goleveldb/leveldb/opt"
)

const (
    gcArrayFreeRatio = 0.1
    maxGCitems       = 5000 // max number of items to be gc'd per call to collectGarbage()
)

var (
    dbEntryCount = metrics.NewRegisteredCounter("ldbstore.entryCnt", nil)
)

var (
    keyIndex       = byte(0)
    keyOldData     = byte(1)
    keyAccessCnt   = []byte{2}
    keyEntryCnt    = []byte{3}
    keyDataIdx     = []byte{4}
    keyData        = byte(6)
    keyDistanceCnt = byte(7)
)

type gcItem struct {
    idx    uint64
    value  uint64
    idxKey []byte
    po     uint8
}

type LDBStoreParams struct {
    *StoreParams
    Path string
    Po   func(Address) uint8
}

// NewLDBStoreParams constructs LDBStoreParams with the specified values.
func NewLDBStoreParams(storeparams *StoreParams, path string) *LDBStoreParams {
    return &LDBStoreParams{
        StoreParams: storeparams,
        Path:        path,
        Po:          func(k Address) (ret uint8) { return uint8(Proximity(storeparams.BaseKey[:], k[:])) },
    }
}

type LDBStore struct {
    db *LDBDatabase

    // this should be stored in db, accessed transactionally
    entryCnt  uint64 // number of items in the LevelDB
    accessCnt uint64 // ever-accumulating number increased every time we read/access an entry
    dataIdx   uint64 // similar to entryCnt, but we only increment it
    capacity  uint64
    bucketCnt []uint64

    hashfunc SwarmHasher
    po       func(Address) uint8

    batchC   chan bool
    batchesC chan struct{}
    batch    *leveldb.Batch
    lock     sync.RWMutex
    quit     chan struct{}

    // Functions encodeDataFunc is used to bypass
    // the default functionality of DbStore with
    // mock.NodeStore for testing purposes.
    encodeDataFunc func(chunk *Chunk) []byte
    // If getDataFunc is defined, it will be used for
    // retrieving the chunk data instead from the local
    // LevelDB database.
    getDataFunc func(addr Address) (data []byte, err error)
}

// TODO: Instead of passing the distance function, just pass the address from which distances are calculated
// to avoid the appearance of a pluggable distance metric and opportunities of bugs associated with providing
// a function different from the one that is actually used.
func NewLDBStore(params *LDBStoreParams) (s *LDBStore, err error) {
    s = new(LDBStore)
    s.hashfunc = params.Hash
    s.quit = make(chan struct{})

    s.batchC = make(chan bool)
    s.batchesC = make(chan struct{}, 1)
    go s.writeBatches()
    s.batch = new(leveldb.Batch)
    // associate encodeData with default functionality
    s.encodeDataFunc = encodeData

    s.db, err = NewLDBDatabase(params.Path)
    if err != nil {
        return nil, err
    }

    s.po = params.Po
    s.setCapacity(params.DbCapacity)

    s.bucketCnt = make([]uint64, 0x100)
    for i := 0; i < 0x100; i++ {
        k := make([]byte, 2)
        k[0] = keyDistanceCnt
        k[1] = uint8(i)
        cnt, _ := s.db.Get(k)
        s.bucketCnt[i] = BytesToU64(cnt)
        s.bucketCnt[i]++
    }
    data, _ := s.db.Get(keyEntryCnt)
    s.entryCnt = BytesToU64(data)
    s.entryCnt++
    data, _ = s.db.Get(keyAccessCnt)
    s.accessCnt = BytesToU64(data)
    s.accessCnt++
    data, _ = s.db.Get(keyDataIdx)
    s.dataIdx = BytesToU64(data)
    s.dataIdx++

    return s, nil
}

// NewMockDbStore creates a new instance of DbStore with
// mockStore set to a provided value. If mockStore argument is nil,
// this function behaves exactly as NewDbStore.
func NewMockDbStore(params *LDBStoreParams, mockStore *mock.NodeStore) (s *LDBStore, err error) {
    s, err = NewLDBStore(params)
    if err != nil {
        return nil, err
    }

    // replace put and get with mock store functionality
    if mockStore != nil {
        s.encodeDataFunc = newMockEncodeDataFunc(mockStore)
        s.getDataFunc = newMockGetDataFunc(mockStore)
    }
    return
}

type dpaDBIndex struct {
    Idx    uint64
    Access uint64
}

func BytesToU64(data []byte) uint64 {
    if len(data) < 8 {
        return 0
    }
    return binary.BigEndian.Uint64(data)
}

func U64ToBytes(val uint64) []byte {
    data := make([]byte, 8)
    binary.BigEndian.PutUint64(data, val)
    return data
}

func (s *LDBStore) updateIndexAccess(index *dpaDBIndex) {
    index.Access = s.accessCnt
}

func getIndexKey(hash Address) []byte {
    hashSize := len(hash)
    key := make([]byte, hashSize+1)
    key[0] = keyIndex
    copy(key[1:], hash[:])
    return key
}

func getOldDataKey(idx uint64) []byte {
    key := make([]byte, 9)
    key[0] = keyOldData
    binary.BigEndian.PutUint64(key[1:9], idx)

    return key
}

func getDataKey(idx uint64, po uint8) []byte {
    key := make([]byte, 10)
    key[0] = keyData
    key[1] = po
    binary.BigEndian.PutUint64(key[2:], idx)

    return key
}

func encodeIndex(index *dpaDBIndex) []byte {
    data, _ := rlp.EncodeToBytes(index)
    return data
}

func encodeData(chunk *Chunk) []byte {
    // Always create a new underlying array for the returned byte slice.
    // The chunk.Key array may be used in the returned slice which
    // may be changed later in the code or by the LevelDB, resulting
    // that the Key is changed as well.
    return append(append([]byte{}, chunk.Addr[:]...), chunk.SData...)
}

func decodeIndex(data []byte, index *dpaDBIndex) error {
    dec := rlp.NewStream(bytes.NewReader(data), 0)
    return dec.Decode(index)
}

func decodeData(data []byte, chunk *Chunk) {
    chunk.SData = data[32:]
    chunk.Size = int64(binary.BigEndian.Uint64(data[32:40]))
}

func decodeOldData(data []byte, chunk *Chunk) {
    chunk.SData = data
    chunk.Size = int64(binary.BigEndian.Uint64(data[0:8]))
}

func (s *LDBStore) collectGarbage(ratio float32) {
    metrics.GetOrRegisterCounter("ldbstore.collectgarbage", nil).Inc(1)

    it := s.db.NewIterator()
    defer it.Release()

    garbage := []*gcItem{}
    gcnt := 0

    for ok := it.Seek([]byte{keyIndex}); ok && (gcnt < maxGCitems) && (uint64(gcnt) < s.entryCnt); ok = it.Next() {
        itkey := it.Key()

        if (itkey == nil) || (itkey[0] != keyIndex) {
            break
        }

        // it.Key() contents change on next call to it.Next(), so we must copy it
        key := make([]byte, len(it.Key()))
        copy(key, it.Key())

        val := it.Value()

        var index dpaDBIndex

        hash := key[1:]
        decodeIndex(val, &index)
        po := s.po(hash)

        gci := &gcItem{
            idxKey: key,
            idx:    index.Idx,
            value:  index.Access, // the smaller, the more likely to be gc'd. see sort comparator below.
            po:     po,
        }

        garbage = append(garbage, gci)
        gcnt++
    }

    sort.Slice(garbage[:gcnt], func(i, j int) bool { return garbage[i].value < garbage[j].value })

    cutoff := int(float32(gcnt) * ratio)
    metrics.GetOrRegisterCounter("ldbstore.collectgarbage.delete", nil).Inc(int64(cutoff))

    for i := 0; i < cutoff; i++ {
        s.delete(garbage[i].idx, garbage[i].idxKey, garbage[i].po)
    }
}

// Export writes all chunks from the store to a tar archive, returning the
// number of chunks written.
func (s *LDBStore) Export(out io.Writer) (int64, error) {
    tw := tar.NewWriter(out)
    defer tw.Close()

    it := s.db.NewIterator()
    defer it.Release()
    var count int64
    for ok := it.Seek([]byte{keyIndex}); ok; ok = it.Next() {
        key := it.Key()
        if (key == nil) || (key[0] != keyIndex) {
            break
        }

        var index dpaDBIndex

        hash := key[1:]
        decodeIndex(it.Value(), &index)
        po := s.po(hash)
        datakey := getDataKey(index.Idx, po)
        log.Trace("store.export", "dkey", fmt.Sprintf("%x", datakey), "dataidx", index.Idx, "po", po)
        data, err := s.db.Get(datakey)
        if err != nil {
            log.Warn(fmt.Sprintf("Chunk %x found but could not be accessed: %v", key[:], err))
            continue
        }

        hdr := &tar.Header{
            Name: hex.EncodeToString(hash),
            Mode: 0644,
            Size: int64(len(data)),
        }
        if err := tw.WriteHeader(hdr); err != nil {
            return count, err
        }
        if _, err := tw.Write(data); err != nil {
            return count, err
        }
        count++
    }

    return count, nil
}

// of chunks read.
func (s *LDBStore) Import(in io.Reader) (int64, error) {
    tr := tar.NewReader(in)

    var count int64
    var wg sync.WaitGroup
    for {
        hdr, err := tr.Next()
        if err == io.EOF {
            break
        } else if err != nil {
            return count, err
        }

        if len(hdr.Name) != 64 {
            log.Warn("ignoring non-chunk file", "name", hdr.Name)
            continue
        }

        keybytes, err := hex.DecodeString(hdr.Name)
        if err != nil {
            log.Warn("ignoring invalid chunk file", "name", hdr.Name, "err", err)
            continue
        }

        data, err := ioutil.ReadAll(tr)
        if err != nil {
            return count, err
        }
        key := Address(keybytes)
        chunk := NewChunk(key, nil)
        chunk.SData = data[32:]
        s.Put(context.TODO(), chunk)
        wg.Add(1)
        go func() {
            defer wg.Done()
            <-chunk.dbStoredC
        }()
        count++
    }
    wg.Wait()
    return count, nil
}

func (s *LDBStore) Cleanup() {
    //Iterates over the database and checks that there are no chunks bigger than 4kb
    var errorsFound, removed, total int

    it := s.db.NewIterator()
    defer it.Release()
    for ok := it.Seek([]byte{keyIndex}); ok; ok = it.Next() {
        key := it.Key()
        if (key == nil) || (key[0] != keyIndex) {
            break
        }
        total++
        var index dpaDBIndex
        err := decodeIndex(it.Value(), &index)
        if err != nil {
            log.Warn("Cannot decode")
            errorsFound++
            continue
        }
        hash := key[1:]
        po := s.po(hash)
        datakey := getDataKey(index.Idx, po)
        data, err := s.db.Get(datakey)
        if err != nil {
            found := false

            // highest possible proximity is 255
            for po = 1; po <= 255; po++ {
                datakey = getDataKey(index.Idx, po)
                data, err = s.db.Get(datakey)
                if err == nil {
                    found = true
                    break
                }
            }

            if !found {
                log.Warn(fmt.Sprintf("Chunk %x found but count not be accessed with any po", key[:]))
                errorsFound++
                continue
            }
        }

        c := &Chunk{}
        ck := data[:32]
        decodeData(data, c)

        cs := int64(binary.LittleEndian.Uint64(c.SData[:8]))
        log.Trace("chunk", "key", fmt.Sprintf("%x", key[:]), "ck", fmt.Sprintf("%x", ck), "dkey", fmt.Sprintf("%x", datakey), "dataidx", index.Idx, "po", po, "len data", len(data), "len sdata", len(c.SData), "size", cs)

        if len(c.SData) > chunk.DefaultSize+8 {
            log.Warn("chunk for cleanup", "key", fmt.Sprintf("%x", key[:]), "ck", fmt.Sprintf("%x", ck), "dkey", fmt.Sprintf("%x", datakey), "dataidx", index.Idx, "po", po, "len data", len(data), "len sdata", len(c.SData), "size", cs)
            s.delete(index.Idx, getIndexKey(key[1:]), po)
            removed++
            errorsFound++
        }
    }

    log.Warn(fmt.Sprintf("Found %v errors out of %v entries. Removed %v chunks.", errorsFound, total, removed))
}

func (s *LDBStore) ReIndex() {
    //Iterates over the database and checks that there are no faulty chunks
    it := s.db.NewIterator()
    startPosition := []byte{keyOldData}
    it.Seek(startPosition)
    var key []byte
    var errorsFound, total int
    for it.Valid() {
        key = it.Key()
        if (key == nil) || (key[0] != keyOldData) {
            break
        }
        data := it.Value()
        hasher := s.hashfunc()
        hasher.Write(data)
        hash := hasher.Sum(nil)

        newKey := make([]byte, 10)
        oldCntKey := make([]byte, 2)
        newCntKey := make([]byte, 2)
        oldCntKey[0] = keyDistanceCnt
        newCntKey[0] = keyDistanceCnt
        key[0] = keyData
        key[1] = s.po(Address(key[1:]))
        oldCntKey[1] = key[1]
        newCntKey[1] = s.po(Address(newKey[1:]))
        copy(newKey[2:], key[1:])
        newValue := append(hash, data...)

        batch := new(leveldb.Batch)
        batch.Delete(key)
        s.bucketCnt[oldCntKey[1]]--
        batch.Put(oldCntKey, U64ToBytes(s.bucketCnt[oldCntKey[1]]))
        batch.Put(newKey, newValue)
        s.bucketCnt[newCntKey[1]]++
        batch.Put(newCntKey, U64ToBytes(s.bucketCnt[newCntKey[1]]))
        s.db.Write(batch)
        it.Next()
    }
    it.Release()
    log.Warn(fmt.Sprintf("Found %v errors out of %v entries", errorsFound, total))
}

func (s *LDBStore) delete(idx uint64, idxKey []byte, po uint8) {
    metrics.GetOrRegisterCounter("ldbstore.delete", nil).Inc(1)

    batch := new(leveldb.Batch)
    batch.Delete(idxKey)
    batch.Delete(getDataKey(idx, po))
    s.entryCnt--
    dbEntryCount.Dec(1)
    s.bucketCnt[po]--
    cntKey := make([]byte, 2)
    cntKey[0] = keyDistanceCnt
    cntKey[1] = po
    batch.Put(keyEntryCnt, U64ToBytes(s.entryCnt))
    batch.Put(cntKey, U64ToBytes(s.bucketCnt[po]))
    s.db.Write(batch)
}

func (s *LDBStore) CurrentBucketStorageIndex(po uint8) uint64 {
    s.lock.RLock()
    defer s.lock.RUnlock()

    return s.bucketCnt[po]
}

func (s *LDBStore) Size() uint64 {
    s.lock.Lock()
    defer s.lock.Unlock()
    return s.entryCnt
}

func (s *LDBStore) CurrentStorageIndex() uint64 {
    s.lock.RLock()
    defer s.lock.RUnlock()
    return s.dataIdx
}

func (s *LDBStore) Put(ctx context.Context, chunk *Chunk) {
    metrics.GetOrRegisterCounter("ldbstore.put", nil).Inc(1)
    log.Trace("ldbstore.put", "key", chunk.Addr)

    ikey := getIndexKey(chunk.Addr)
    var index dpaDBIndex

    po := s.po(chunk.Addr)
    s.lock.Lock()
    defer s.lock.Unlock()

    log.Trace("ldbstore.put: s.db.Get", "key", chunk.Addr, "ikey", fmt.Sprintf("%x", ikey))
    idata, err := s.db.Get(ikey)
    if err != nil {
        s.doPut(chunk, &index, po)
        batchC := s.batchC
        go func() {
            <-batchC
            chunk.markAsStored()
        }()
    } else {
        log.Trace("ldbstore.put: chunk already exists, only update access", "key", chunk.Addr)
        decodeIndex(idata, &index)
        chunk.markAsStored()
    }
    index.Access = s.accessCnt
    s.accessCnt++
    idata = encodeIndex(&index)
    s.batch.Put(ikey, idata)
    select {
    case s.batchesC <- struct{}{}:
    default:
    }
}

// force putting into db, does not check access index
func (s *LDBStore) doPut(chunk *Chunk, index *dpaDBIndex, po uint8) {
    data := s.encodeDataFunc(chunk)
    dkey := getDataKey(s.dataIdx, po)
    s.batch.Put(dkey, data)
    index.Idx = s.dataIdx
    s.bucketCnt[po] = s.dataIdx
    s.entryCnt++
    dbEntryCount.Inc(1)
    s.dataIdx++

    cntKey := make([]byte, 2)
    cntKey[0] = keyDistanceCnt
    cntKey[1] = po
    s.batch.Put(cntKey, U64ToBytes(s.bucketCnt[po]))
}

func (s *LDBStore) writeBatches() {
mainLoop:
    for {
        select {
        case <-s.quit:
            break mainLoop
        case <-s.batchesC:
            s.lock.Lock()
            b := s.batch
            e := s.entryCnt
            d := s.dataIdx
            a := s.accessCnt
            c := s.batchC
            s.batchC = make(chan bool)
            s.batch = new(leveldb.Batch)
            err := s.writeBatch(b, e, d, a)
            // TODO: set this error on the batch, then tell the chunk
            if err != nil {
                log.Error(fmt.Sprintf("spawn batch write (%d entries): %v", b.Len(), err))
            }
            close(c)
            for e > s.capacity {
                // Collect garbage in a separate goroutine
                // to be able to interrupt this loop by s.quit.
                done := make(chan struct{})
                go func() {
                    s.collectGarbage(gcArrayFreeRatio)
                    close(done)
                }()

                e = s.entryCnt
                select {
                case <-s.quit:
                    s.lock.Unlock()
                    break mainLoop
                case <-done:
                }
            }
            s.lock.Unlock()
        }
    }
    log.Trace(fmt.Sprintf("DbStore: quit batch write loop"))
}

// must be called non concurrently
func (s *LDBStore) writeBatch(b *leveldb.Batch, entryCnt, dataIdx, accessCnt uint64) error {
    b.Put(keyEntryCnt, U64ToBytes(entryCnt))
    b.Put(keyDataIdx, U64ToBytes(dataIdx))
    b.Put(keyAccessCnt, U64ToBytes(accessCnt))
    l := b.Len()
    if err := s.db.Write(b); err != nil {
        return fmt.Errorf("unable to write batch: %v", err)
    }
    log.Trace(fmt.Sprintf("batch write (%d entries)", l))
    return nil
}

// newMockEncodeDataFunc returns a function that stores the chunk data
// to a mock store to bypass the default functionality encodeData.
// The constructed function always returns the nil data, as DbStore does
// not need to store the data, but still need to create the index.
func newMockEncodeDataFunc(mockStore *mock.NodeStore) func(chunk *Chunk) []byte {
    return func(chunk *Chunk) []byte {
        if err := mockStore.Put(chunk.Addr, encodeData(chunk)); err != nil {
            log.Error(fmt.Sprintf("%T: Chunk %v put: %v", mockStore, chunk.Addr.Log(), err))
        }
        return chunk.Addr[:]
    }
}

// try to find index; if found, update access cnt and return true
func (s *LDBStore) tryAccessIdx(ikey []byte, index *dpaDBIndex) bool {
    idata, err := s.db.Get(ikey)
    if err != nil {
        return false
    }
    decodeIndex(idata, index)
    s.batch.Put(keyAccessCnt, U64ToBytes(s.accessCnt))
    s.accessCnt++
    index.Access = s.accessCnt
    idata = encodeIndex(index)
    s.batch.Put(ikey, idata)
    select {
    case s.batchesC <- struct{}{}:
    default:
    }
    return true
}

func (s *LDBStore) Get(ctx context.Context, addr Address) (chunk *Chunk, err error) {
    metrics.GetOrRegisterCounter("ldbstore.get", nil).Inc(1)
    log.Trace("ldbstore.get", "key", addr)

    s.lock.Lock()
    defer s.lock.Unlock()
    return s.get(addr)
}

func (s *LDBStore) get(addr Address) (chunk *Chunk, err error) {
    var indx dpaDBIndex

    if s.tryAccessIdx(getIndexKey(addr), &indx) {
        var data []byte
        if s.getDataFunc != nil {
            // if getDataFunc is defined, use it to retrieve the chunk data
            log.Trace("ldbstore.get retrieve with getDataFunc", "key", addr)
            data, err = s.getDataFunc(addr)
            if err != nil {
                return
            }
        } else {
            // default DbStore functionality to retrieve chunk data
            proximity := s.po(addr)
            datakey := getDataKey(indx.Idx, proximity)
            data, err = s.db.Get(datakey)
            log.Trace("ldbstore.get retrieve", "key", addr, "indexkey", indx.Idx, "datakey", fmt.Sprintf("%x", datakey), "proximity", proximity)
            if err != nil {
                log.Trace("ldbstore.get chunk found but could not be accessed", "key", addr, "err", err)
                s.delete(indx.Idx, getIndexKey(addr), s.po(addr))
                return
            }
        }

        chunk = NewChunk(addr, nil)
        chunk.markAsStored()
        decodeData(data, chunk)
    } else {
        err = ErrChunkNotFound
    }

    return
}

// newMockGetFunc returns a function that reads chunk data from
// the mock database, which is used as the value for DbStore.getFunc
// to bypass the default functionality of DbStore with a mock store.
func newMockGetDataFunc(mockStore *mock.NodeStore) func(addr Address) (data []byte, err error) {
    return func(addr Address) (data []byte, err error) {
        data, err = mockStore.Get(addr)
        if err == mock.ErrNotFound {
            // preserve ErrChunkNotFound error
            err = ErrChunkNotFound
        }
        return data, err
    }
}

func (s *LDBStore) updateAccessCnt(addr Address) {

    s.lock.Lock()
    defer s.lock.Unlock()

    var index dpaDBIndex
    s.tryAccessIdx(getIndexKey(addr), &index) // result_chn == nil, only update access cnt

}

func (s *LDBStore) setCapacity(c uint64) {
    s.lock.Lock()
    defer s.lock.Unlock()

    s.capacity = c

    if s.entryCnt > c {
        ratio := float32(1.01) - float32(c)/float32(s.entryCnt)
        if ratio < gcArrayFreeRatio {
            ratio = gcArrayFreeRatio
        }
        if ratio > 1 {
            ratio = 1
        }
        for s.entryCnt > c {
            s.collectGarbage(ratio)
        }
    }
}

func (s *LDBStore) Close() {
    close(s.quit)
    s.db.Close()
}

// SyncIterator(start, stop, po, f) calls f on each hash of a bin po from start to stop
func (s *LDBStore) SyncIterator(since uint64, until uint64, po uint8, f func(Address, uint64) bool) error {
    metrics.GetOrRegisterCounter("ldbstore.synciterator", nil).Inc(1)

    sincekey := getDataKey(since, po)
    untilkey := getDataKey(until, po)
    it := s.db.NewIterator()
    defer it.Release()

    for ok := it.Seek(sincekey); ok; ok = it.Next() {
        metrics.GetOrRegisterCounter("ldbstore.synciterator.seek", nil).Inc(1)

        dbkey := it.Key()
        if dbkey[0] != keyData || dbkey[1] != po || bytes.Compare(untilkey, dbkey) < 0 {
            break
        }
        key := make([]byte, 32)
        val := it.Value()
        copy(key, val[:32])
        if !f(Address(key), binary.BigEndian.Uint64(dbkey[2:])) {
            break
        }
    }
    return it.Error()
}

func databaseExists(path string) bool {
    o := &opt.Options{
        ErrorIfMissing: true,
    }
    tdb, err := leveldb.OpenFile(path, o)
    if err != nil {
        return false
    }
    defer tdb.Close()
    return true
}