gitea/vendor/github.com/blevesearch/zap/v13/merge.go
Lunny Xiao d17efaa114
Upgrade bleve to v1.0.10 (#12737)
* Fix bug on migration 111

* Upgrade bleve to 1.0.10

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
2020-09-06 18:51:14 -04:00

848 lines
22 KiB
Go
Vendored

// Copyright (c) 2017 Couchbase, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package zap
import (
"bufio"
"bytes"
"encoding/binary"
"fmt"
"math"
"os"
"sort"
"github.com/RoaringBitmap/roaring"
seg "github.com/blevesearch/bleve/index/scorch/segment"
"github.com/couchbase/vellum"
"github.com/golang/snappy"
)
var DefaultFileMergerBufferSize = 1024 * 1024
const docDropped = math.MaxUint64 // sentinel docNum to represent a deleted doc
// Merge takes a slice of segments and bit masks describing which
// documents may be dropped, and creates a new segment containing the
// remaining data. This new segment is built at the specified path.
func (*ZapPlugin) Merge(segments []seg.Segment, drops []*roaring.Bitmap, path string,
closeCh chan struct{}, s seg.StatsReporter) (
[][]uint64, uint64, error) {
segmentBases := make([]*SegmentBase, len(segments))
for segmenti, segment := range segments {
switch segmentx := segment.(type) {
case *Segment:
segmentBases[segmenti] = &segmentx.SegmentBase
case *SegmentBase:
segmentBases[segmenti] = segmentx
default:
panic(fmt.Sprintf("oops, unexpected segment type: %T", segment))
}
}
return mergeSegmentBases(segmentBases, drops, path, DefaultChunkMode, closeCh, s)
}
func mergeSegmentBases(segmentBases []*SegmentBase, drops []*roaring.Bitmap, path string,
chunkMode uint32, closeCh chan struct{}, s seg.StatsReporter) (
[][]uint64, uint64, error) {
flag := os.O_RDWR | os.O_CREATE
f, err := os.OpenFile(path, flag, 0600)
if err != nil {
return nil, 0, err
}
cleanup := func() {
_ = f.Close()
_ = os.Remove(path)
}
// buffer the output
br := bufio.NewWriterSize(f, DefaultFileMergerBufferSize)
// wrap it for counting (tracking offsets)
cr := NewCountHashWriterWithStatsReporter(br, s)
newDocNums, numDocs, storedIndexOffset, fieldsIndexOffset, docValueOffset, _, _, _, err :=
MergeToWriter(segmentBases, drops, chunkMode, cr, closeCh)
if err != nil {
cleanup()
return nil, 0, err
}
err = persistFooter(numDocs, storedIndexOffset, fieldsIndexOffset,
docValueOffset, chunkMode, cr.Sum32(), cr)
if err != nil {
cleanup()
return nil, 0, err
}
err = br.Flush()
if err != nil {
cleanup()
return nil, 0, err
}
err = f.Sync()
if err != nil {
cleanup()
return nil, 0, err
}
err = f.Close()
if err != nil {
cleanup()
return nil, 0, err
}
return newDocNums, uint64(cr.Count()), nil
}
func MergeToWriter(segments []*SegmentBase, drops []*roaring.Bitmap,
chunkMode uint32, cr *CountHashWriter, closeCh chan struct{}) (
newDocNums [][]uint64,
numDocs, storedIndexOffset, fieldsIndexOffset, docValueOffset uint64,
dictLocs []uint64, fieldsInv []string, fieldsMap map[string]uint16,
err error) {
docValueOffset = uint64(fieldNotUninverted)
var fieldsSame bool
fieldsSame, fieldsInv = mergeFields(segments)
fieldsMap = mapFields(fieldsInv)
numDocs = computeNewDocCount(segments, drops)
if isClosed(closeCh) {
return nil, 0, 0, 0, 0, nil, nil, nil, seg.ErrClosed
}
if numDocs > 0 {
storedIndexOffset, newDocNums, err = mergeStoredAndRemap(segments, drops,
fieldsMap, fieldsInv, fieldsSame, numDocs, cr, closeCh)
if err != nil {
return nil, 0, 0, 0, 0, nil, nil, nil, err
}
dictLocs, docValueOffset, err = persistMergedRest(segments, drops,
fieldsInv, fieldsMap, fieldsSame,
newDocNums, numDocs, chunkMode, cr, closeCh)
if err != nil {
return nil, 0, 0, 0, 0, nil, nil, nil, err
}
} else {
dictLocs = make([]uint64, len(fieldsInv))
}
fieldsIndexOffset, err = persistFields(fieldsInv, cr, dictLocs)
if err != nil {
return nil, 0, 0, 0, 0, nil, nil, nil, err
}
return newDocNums, numDocs, storedIndexOffset, fieldsIndexOffset, docValueOffset, dictLocs, fieldsInv, fieldsMap, nil
}
// mapFields takes the fieldsInv list and returns a map of fieldName
// to fieldID+1
func mapFields(fields []string) map[string]uint16 {
rv := make(map[string]uint16, len(fields))
for i, fieldName := range fields {
rv[fieldName] = uint16(i) + 1
}
return rv
}
// computeNewDocCount determines how many documents will be in the newly
// merged segment when obsoleted docs are dropped
func computeNewDocCount(segments []*SegmentBase, drops []*roaring.Bitmap) uint64 {
var newDocCount uint64
for segI, segment := range segments {
newDocCount += segment.numDocs
if drops[segI] != nil {
newDocCount -= drops[segI].GetCardinality()
}
}
return newDocCount
}
func persistMergedRest(segments []*SegmentBase, dropsIn []*roaring.Bitmap,
fieldsInv []string, fieldsMap map[string]uint16, fieldsSame bool,
newDocNumsIn [][]uint64, newSegDocCount uint64, chunkMode uint32,
w *CountHashWriter, closeCh chan struct{}) ([]uint64, uint64, error) {
var bufMaxVarintLen64 []byte = make([]byte, binary.MaxVarintLen64)
var bufLoc []uint64
var postings *PostingsList
var postItr *PostingsIterator
rv := make([]uint64, len(fieldsInv))
fieldDvLocsStart := make([]uint64, len(fieldsInv))
fieldDvLocsEnd := make([]uint64, len(fieldsInv))
// these int coders are initialized with chunk size 1024
// however this will be reset to the correct chunk size
// while processing each individual field-term section
tfEncoder := newChunkedIntCoder(1024, newSegDocCount-1)
locEncoder := newChunkedIntCoder(1024, newSegDocCount-1)
var vellumBuf bytes.Buffer
newVellum, err := vellum.New(&vellumBuf, nil)
if err != nil {
return nil, 0, err
}
newRoaring := roaring.NewBitmap()
// for each field
for fieldID, fieldName := range fieldsInv {
// collect FST iterators from all active segments for this field
var newDocNums [][]uint64
var drops []*roaring.Bitmap
var dicts []*Dictionary
var itrs []vellum.Iterator
var segmentsInFocus []*SegmentBase
for segmentI, segment := range segments {
// check for the closure in meantime
if isClosed(closeCh) {
return nil, 0, seg.ErrClosed
}
dict, err2 := segment.dictionary(fieldName)
if err2 != nil {
return nil, 0, err2
}
if dict != nil && dict.fst != nil {
itr, err2 := dict.fst.Iterator(nil, nil)
if err2 != nil && err2 != vellum.ErrIteratorDone {
return nil, 0, err2
}
if itr != nil {
newDocNums = append(newDocNums, newDocNumsIn[segmentI])
if dropsIn[segmentI] != nil && !dropsIn[segmentI].IsEmpty() {
drops = append(drops, dropsIn[segmentI])
} else {
drops = append(drops, nil)
}
dicts = append(dicts, dict)
itrs = append(itrs, itr)
segmentsInFocus = append(segmentsInFocus, segment)
}
}
}
var prevTerm []byte
newRoaring.Clear()
var lastDocNum, lastFreq, lastNorm uint64
// determines whether to use "1-hit" encoding optimization
// when a term appears in only 1 doc, with no loc info,
// has freq of 1, and the docNum fits into 31-bits
use1HitEncoding := func(termCardinality uint64) (bool, uint64, uint64) {
if termCardinality == uint64(1) && locEncoder.FinalSize() <= 0 {
docNum := uint64(newRoaring.Minimum())
if under32Bits(docNum) && docNum == lastDocNum && lastFreq == 1 {
return true, docNum, lastNorm
}
}
return false, 0, 0
}
finishTerm := func(term []byte) error {
tfEncoder.Close()
locEncoder.Close()
postingsOffset, err := writePostings(newRoaring,
tfEncoder, locEncoder, use1HitEncoding, w, bufMaxVarintLen64)
if err != nil {
return err
}
if postingsOffset > 0 {
err = newVellum.Insert(term, postingsOffset)
if err != nil {
return err
}
}
newRoaring.Clear()
tfEncoder.Reset()
locEncoder.Reset()
lastDocNum = 0
lastFreq = 0
lastNorm = 0
return nil
}
enumerator, err := newEnumerator(itrs)
for err == nil {
term, itrI, postingsOffset := enumerator.Current()
if !bytes.Equal(prevTerm, term) {
// check for the closure in meantime
if isClosed(closeCh) {
return nil, 0, seg.ErrClosed
}
// if the term changed, write out the info collected
// for the previous term
err = finishTerm(prevTerm)
if err != nil {
return nil, 0, err
}
}
if !bytes.Equal(prevTerm, term) || prevTerm == nil {
// compute cardinality of field-term in new seg
var newCard uint64
lowItrIdxs, lowItrVals := enumerator.GetLowIdxsAndValues()
for i, idx := range lowItrIdxs {
pl, err := dicts[idx].postingsListFromOffset(lowItrVals[i], drops[idx], nil)
if err != nil {
return nil, 0, err
}
newCard += pl.Count()
}
// compute correct chunk size with this
chunkSize, err := getChunkSize(chunkMode, newCard, newSegDocCount)
if err != nil {
return nil, 0, err
}
// update encoders chunk
tfEncoder.SetChunkSize(chunkSize, newSegDocCount-1)
locEncoder.SetChunkSize(chunkSize, newSegDocCount-1)
}
postings, err = dicts[itrI].postingsListFromOffset(
postingsOffset, drops[itrI], postings)
if err != nil {
return nil, 0, err
}
postItr = postings.iterator(true, true, true, postItr)
// can no longer optimize by copying, since chunk factor could have changed
lastDocNum, lastFreq, lastNorm, bufLoc, err = mergeTermFreqNormLocs(
fieldsMap, term, postItr, newDocNums[itrI], newRoaring,
tfEncoder, locEncoder, bufLoc)
if err != nil {
return nil, 0, err
}
prevTerm = prevTerm[:0] // copy to prevTerm in case Next() reuses term mem
prevTerm = append(prevTerm, term...)
err = enumerator.Next()
}
if err != vellum.ErrIteratorDone {
return nil, 0, err
}
err = finishTerm(prevTerm)
if err != nil {
return nil, 0, err
}
dictOffset := uint64(w.Count())
err = newVellum.Close()
if err != nil {
return nil, 0, err
}
vellumData := vellumBuf.Bytes()
// write out the length of the vellum data
n := binary.PutUvarint(bufMaxVarintLen64, uint64(len(vellumData)))
_, err = w.Write(bufMaxVarintLen64[:n])
if err != nil {
return nil, 0, err
}
// write this vellum to disk
_, err = w.Write(vellumData)
if err != nil {
return nil, 0, err
}
rv[fieldID] = dictOffset
// get the field doc value offset (start)
fieldDvLocsStart[fieldID] = uint64(w.Count())
// update the field doc values
// NOTE: doc values continue to use legacy chunk mode
chunkSize, err := getChunkSize(LegacyChunkMode, 0, 0)
if err != nil {
return nil, 0, err
}
fdvEncoder := newChunkedContentCoder(chunkSize, newSegDocCount-1, w, true)
fdvReadersAvailable := false
var dvIterClone *docValueReader
for segmentI, segment := range segmentsInFocus {
// check for the closure in meantime
if isClosed(closeCh) {
return nil, 0, seg.ErrClosed
}
fieldIDPlus1 := uint16(segment.fieldsMap[fieldName])
if dvIter, exists := segment.fieldDvReaders[fieldIDPlus1-1]; exists &&
dvIter != nil {
fdvReadersAvailable = true
dvIterClone = dvIter.cloneInto(dvIterClone)
err = dvIterClone.iterateAllDocValues(segment, func(docNum uint64, terms []byte) error {
if newDocNums[segmentI][docNum] == docDropped {
return nil
}
err := fdvEncoder.Add(newDocNums[segmentI][docNum], terms)
if err != nil {
return err
}
return nil
})
if err != nil {
return nil, 0, err
}
}
}
if fdvReadersAvailable {
err = fdvEncoder.Close()
if err != nil {
return nil, 0, err
}
// persist the doc value details for this field
_, err = fdvEncoder.Write()
if err != nil {
return nil, 0, err
}
// get the field doc value offset (end)
fieldDvLocsEnd[fieldID] = uint64(w.Count())
} else {
fieldDvLocsStart[fieldID] = fieldNotUninverted
fieldDvLocsEnd[fieldID] = fieldNotUninverted
}
// reset vellum buffer and vellum builder
vellumBuf.Reset()
err = newVellum.Reset(&vellumBuf)
if err != nil {
return nil, 0, err
}
}
fieldDvLocsOffset := uint64(w.Count())
buf := bufMaxVarintLen64
for i := 0; i < len(fieldDvLocsStart); i++ {
n := binary.PutUvarint(buf, fieldDvLocsStart[i])
_, err := w.Write(buf[:n])
if err != nil {
return nil, 0, err
}
n = binary.PutUvarint(buf, fieldDvLocsEnd[i])
_, err = w.Write(buf[:n])
if err != nil {
return nil, 0, err
}
}
return rv, fieldDvLocsOffset, nil
}
func mergeTermFreqNormLocs(fieldsMap map[string]uint16, term []byte, postItr *PostingsIterator,
newDocNums []uint64, newRoaring *roaring.Bitmap,
tfEncoder *chunkedIntCoder, locEncoder *chunkedIntCoder, bufLoc []uint64) (
lastDocNum uint64, lastFreq uint64, lastNorm uint64, bufLocOut []uint64, err error) {
next, err := postItr.Next()
for next != nil && err == nil {
hitNewDocNum := newDocNums[next.Number()]
if hitNewDocNum == docDropped {
return 0, 0, 0, nil, fmt.Errorf("see hit with dropped docNum")
}
newRoaring.Add(uint32(hitNewDocNum))
nextFreq := next.Frequency()
nextNorm := uint64(math.Float32bits(float32(next.Norm())))
locs := next.Locations()
err = tfEncoder.Add(hitNewDocNum,
encodeFreqHasLocs(nextFreq, len(locs) > 0), nextNorm)
if err != nil {
return 0, 0, 0, nil, err
}
if len(locs) > 0 {
numBytesLocs := 0
for _, loc := range locs {
ap := loc.ArrayPositions()
numBytesLocs += totalUvarintBytes(uint64(fieldsMap[loc.Field()]-1),
loc.Pos(), loc.Start(), loc.End(), uint64(len(ap)), ap)
}
err = locEncoder.Add(hitNewDocNum, uint64(numBytesLocs))
if err != nil {
return 0, 0, 0, nil, err
}
for _, loc := range locs {
ap := loc.ArrayPositions()
if cap(bufLoc) < 5+len(ap) {
bufLoc = make([]uint64, 0, 5+len(ap))
}
args := bufLoc[0:5]
args[0] = uint64(fieldsMap[loc.Field()] - 1)
args[1] = loc.Pos()
args[2] = loc.Start()
args[3] = loc.End()
args[4] = uint64(len(ap))
args = append(args, ap...)
err = locEncoder.Add(hitNewDocNum, args...)
if err != nil {
return 0, 0, 0, nil, err
}
}
}
lastDocNum = hitNewDocNum
lastFreq = nextFreq
lastNorm = nextNorm
next, err = postItr.Next()
}
return lastDocNum, lastFreq, lastNorm, bufLoc, err
}
func writePostings(postings *roaring.Bitmap, tfEncoder, locEncoder *chunkedIntCoder,
use1HitEncoding func(uint64) (bool, uint64, uint64),
w *CountHashWriter, bufMaxVarintLen64 []byte) (
offset uint64, err error) {
termCardinality := postings.GetCardinality()
if termCardinality <= 0 {
return 0, nil
}
if use1HitEncoding != nil {
encodeAs1Hit, docNum1Hit, normBits1Hit := use1HitEncoding(termCardinality)
if encodeAs1Hit {
return FSTValEncode1Hit(docNum1Hit, normBits1Hit), nil
}
}
var tfOffset uint64
tfOffset, _, err = tfEncoder.writeAt(w)
if err != nil {
return 0, err
}
var locOffset uint64
locOffset, _, err = locEncoder.writeAt(w)
if err != nil {
return 0, err
}
postingsOffset := uint64(w.Count())
n := binary.PutUvarint(bufMaxVarintLen64, tfOffset)
_, err = w.Write(bufMaxVarintLen64[:n])
if err != nil {
return 0, err
}
n = binary.PutUvarint(bufMaxVarintLen64, locOffset)
_, err = w.Write(bufMaxVarintLen64[:n])
if err != nil {
return 0, err
}
_, err = writeRoaringWithLen(postings, w, bufMaxVarintLen64)
if err != nil {
return 0, err
}
return postingsOffset, nil
}
type varintEncoder func(uint64) (int, error)
func mergeStoredAndRemap(segments []*SegmentBase, drops []*roaring.Bitmap,
fieldsMap map[string]uint16, fieldsInv []string, fieldsSame bool, newSegDocCount uint64,
w *CountHashWriter, closeCh chan struct{}) (uint64, [][]uint64, error) {
var rv [][]uint64 // The remapped or newDocNums for each segment.
var newDocNum uint64
var curr int
var data, compressed []byte
var metaBuf bytes.Buffer
varBuf := make([]byte, binary.MaxVarintLen64)
metaEncode := func(val uint64) (int, error) {
wb := binary.PutUvarint(varBuf, val)
return metaBuf.Write(varBuf[:wb])
}
vals := make([][][]byte, len(fieldsInv))
typs := make([][]byte, len(fieldsInv))
poss := make([][][]uint64, len(fieldsInv))
var posBuf []uint64
docNumOffsets := make([]uint64, newSegDocCount)
vdc := visitDocumentCtxPool.Get().(*visitDocumentCtx)
defer visitDocumentCtxPool.Put(vdc)
// for each segment
for segI, segment := range segments {
// check for the closure in meantime
if isClosed(closeCh) {
return 0, nil, seg.ErrClosed
}
segNewDocNums := make([]uint64, segment.numDocs)
dropsI := drops[segI]
// optimize when the field mapping is the same across all
// segments and there are no deletions, via byte-copying
// of stored docs bytes directly to the writer
if fieldsSame && (dropsI == nil || dropsI.GetCardinality() == 0) {
err := segment.copyStoredDocs(newDocNum, docNumOffsets, w)
if err != nil {
return 0, nil, err
}
for i := uint64(0); i < segment.numDocs; i++ {
segNewDocNums[i] = newDocNum
newDocNum++
}
rv = append(rv, segNewDocNums)
continue
}
// for each doc num
for docNum := uint64(0); docNum < segment.numDocs; docNum++ {
// TODO: roaring's API limits docNums to 32-bits?
if dropsI != nil && dropsI.Contains(uint32(docNum)) {
segNewDocNums[docNum] = docDropped
continue
}
segNewDocNums[docNum] = newDocNum
curr = 0
metaBuf.Reset()
data = data[:0]
posTemp := posBuf
// collect all the data
for i := 0; i < len(fieldsInv); i++ {
vals[i] = vals[i][:0]
typs[i] = typs[i][:0]
poss[i] = poss[i][:0]
}
err := segment.visitDocument(vdc, docNum, func(field string, typ byte, value []byte, pos []uint64) bool {
fieldID := int(fieldsMap[field]) - 1
vals[fieldID] = append(vals[fieldID], value)
typs[fieldID] = append(typs[fieldID], typ)
// copy array positions to preserve them beyond the scope of this callback
var curPos []uint64
if len(pos) > 0 {
if cap(posTemp) < len(pos) {
posBuf = make([]uint64, len(pos)*len(fieldsInv))
posTemp = posBuf
}
curPos = posTemp[0:len(pos)]
copy(curPos, pos)
posTemp = posTemp[len(pos):]
}
poss[fieldID] = append(poss[fieldID], curPos)
return true
})
if err != nil {
return 0, nil, err
}
// _id field special case optimizes ExternalID() lookups
idFieldVal := vals[uint16(0)][0]
_, err = metaEncode(uint64(len(idFieldVal)))
if err != nil {
return 0, nil, err
}
// now walk the non-"_id" fields in order
for fieldID := 1; fieldID < len(fieldsInv); fieldID++ {
storedFieldValues := vals[fieldID]
stf := typs[fieldID]
spf := poss[fieldID]
var err2 error
curr, data, err2 = persistStoredFieldValues(fieldID,
storedFieldValues, stf, spf, curr, metaEncode, data)
if err2 != nil {
return 0, nil, err2
}
}
metaBytes := metaBuf.Bytes()
compressed = snappy.Encode(compressed[:cap(compressed)], data)
// record where we're about to start writing
docNumOffsets[newDocNum] = uint64(w.Count())
// write out the meta len and compressed data len
_, err = writeUvarints(w,
uint64(len(metaBytes)),
uint64(len(idFieldVal)+len(compressed)))
if err != nil {
return 0, nil, err
}
// now write the meta
_, err = w.Write(metaBytes)
if err != nil {
return 0, nil, err
}
// now write the _id field val (counted as part of the 'compressed' data)
_, err = w.Write(idFieldVal)
if err != nil {
return 0, nil, err
}
// now write the compressed data
_, err = w.Write(compressed)
if err != nil {
return 0, nil, err
}
newDocNum++
}
rv = append(rv, segNewDocNums)
}
// return value is the start of the stored index
storedIndexOffset := uint64(w.Count())
// now write out the stored doc index
for _, docNumOffset := range docNumOffsets {
err := binary.Write(w, binary.BigEndian, docNumOffset)
if err != nil {
return 0, nil, err
}
}
return storedIndexOffset, rv, nil
}
// copyStoredDocs writes out a segment's stored doc info, optimized by
// using a single Write() call for the entire set of bytes. The
// newDocNumOffsets is filled with the new offsets for each doc.
func (s *SegmentBase) copyStoredDocs(newDocNum uint64, newDocNumOffsets []uint64,
w *CountHashWriter) error {
if s.numDocs <= 0 {
return nil
}
indexOffset0, storedOffset0, _, _, _ :=
s.getDocStoredOffsets(0) // the segment's first doc
indexOffsetN, storedOffsetN, readN, metaLenN, dataLenN :=
s.getDocStoredOffsets(s.numDocs - 1) // the segment's last doc
storedOffset0New := uint64(w.Count())
storedBytes := s.mem[storedOffset0 : storedOffsetN+readN+metaLenN+dataLenN]
_, err := w.Write(storedBytes)
if err != nil {
return err
}
// remap the storedOffset's for the docs into new offsets relative
// to storedOffset0New, filling the given docNumOffsetsOut array
for indexOffset := indexOffset0; indexOffset <= indexOffsetN; indexOffset += 8 {
storedOffset := binary.BigEndian.Uint64(s.mem[indexOffset : indexOffset+8])
storedOffsetNew := storedOffset - storedOffset0 + storedOffset0New
newDocNumOffsets[newDocNum] = storedOffsetNew
newDocNum += 1
}
return nil
}
// mergeFields builds a unified list of fields used across all the
// input segments, and computes whether the fields are the same across
// segments (which depends on fields to be sorted in the same way
// across segments)
func mergeFields(segments []*SegmentBase) (bool, []string) {
fieldsSame := true
var segment0Fields []string
if len(segments) > 0 {
segment0Fields = segments[0].Fields()
}
fieldsExist := map[string]struct{}{}
for _, segment := range segments {
fields := segment.Fields()
for fieldi, field := range fields {
fieldsExist[field] = struct{}{}
if len(segment0Fields) != len(fields) || segment0Fields[fieldi] != field {
fieldsSame = false
}
}
}
rv := make([]string, 0, len(fieldsExist))
// ensure _id stays first
rv = append(rv, "_id")
for k := range fieldsExist {
if k != "_id" {
rv = append(rv, k)
}
}
sort.Strings(rv[1:]) // leave _id as first
return fieldsSame, rv
}
func isClosed(closeCh chan struct{}) bool {
select {
case <-closeCh:
return true
default:
return false
}
}