/usr/share/gocode/src/github.com/couchbase/moss/segment.go is in golang-github-couchbase-moss-dev 0.0~git20170914.0.07c86e8-4.
This file is owned by root:root, with mode 0o644.
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// 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 moss
import (
"bytes"
"fmt"
"sort"
)
// SegmentKindBasic is the code for a basic, persistable segment
// implementation, which represents a segment as two arrays: an array
// of contiguous key-val bytes [key0, val0, key1, val1, ... keyN,
// valN], and an array of offsets plus lengths into the first array.
var SegmentKindBasic = "a"
func init() {
SegmentLoaders[SegmentKindBasic] = loadBasicSegment
SegmentPersisters[SegmentKindBasic] = persistBasicSegment
}
// A SegmentCursor represents a handle for iterating through consecutive
// op/key/value tuples.
type SegmentCursor interface {
// Current returns the operation/key/value pointed to by the cursor.
Current() (operation uint64, key []byte, val []byte)
// Seek advances current to point to specified key.
// If the seek key is less than the original startKeyInclusive
// used to create this cursor, it will seek to that startKeyInclusive
// instead.
// If the cursor is not pointing at a valid entry ErrIteratorDone
// is returned.
Seek(startKeyInclusive []byte) error
// Next moves the cursor to the next entry. If there is no Next
// entry, ErrIteratorDone is returned.
Next() error
}
// A Segment represents the read-oriented interface for a segment.
type Segment interface {
// Returns the kind of segment, used for persistence.
Kind() string
// Len returns the number of ops in the segment.
Len() int
// NumKeyValBytes returns the number of bytes used for key-val data.
NumKeyValBytes() (uint64, uint64)
// Get returns the operation and value associated with the given key.
// If the key does not exist, the operation is 0, and the val is nil.
// If an error occurs it is returned instead of the operation and value.
Get(key []byte) (operation uint64, val []byte, err error)
// Cursor returns an SegmentCursor that will iterate over entries
// from the given (inclusive) start key, through the given (exclusive)
// end key.
Cursor(startKeyInclusive []byte, endKeyExclusive []byte) (SegmentCursor,
error)
// Returns true if the segment is already sorted, and returns
// false if the sorting is only asynchronously scheduled.
RequestSort(synchronous bool) bool
}
// SegmentValidater is an optional interface that can be implemented by
// any Segment to allow additional validation in test cases. The
// method of this interface is NOT invoked during the normal
// runtime usage of a Segment.
type SegmentValidater interface {
// Valid examines the state of the segment, any problem is returned
// as an error.
Valid() error
}
// A SegmentMutator represents the mutation methods of a segment.
type SegmentMutator interface {
Mutate(operation uint64, key, val []byte) error
}
// A SegmentPersister represents a segment that can be persisted.
type SegmentPersister interface {
Persist(file File, options *StoreOptions) (SegmentLoc, error)
}
// A segment is a basic implementation of the segment related
// interfaces and represents a sequence of key-val entries or
// operations. A segment's kvs will be sorted by key when the segment
// is pushed into the collection. A segment implements the Batch
// interface.
type segment struct {
// Each key-val operation is encoded as 2 uint64's...
// - operation (see: maskOperation) |
// key length (see: maskKeyLength) |
// val length (see: maskValLength).
// - start index into buf for key-val bytes.
kvs []uint64
// Contiguous backing memory for the keys and vals of the segment.
buf []byte
// If this segment needs sorting, then needSorterCh will be
// non-nil and also the first goroutine that reads successfully
// from needSorterCh becomes the sorter of this segment. All
// other goroutines must instead wait on the waitSortedCh.
needSorterCh chan bool
// Once the sorter of this segment is done sorting the kvs, it
// close()'s the waitSortedCh, treating waitSortedCh like a
// one-way latch. The needSorterCh and waitSortedCh will either
// be nil or non-nil together. A segment that was "born
// sorted" will have needSorterCh and waitSortedCh as both nil.
waitSortedCh chan struct{}
totOperationSet uint64
totOperationDel uint64
totOperationMerge uint64
totKeyByte uint64
totValByte uint64
rootCollection *collection // Non-nil when segment is from a batch.
// In-memory index, immutable after segment initialization.
index *segmentKeysIndex
}
// See the OperationXxx consts.
const maskOperation = uint64(0x0F00000000000000)
// Max key length is 2^24, from 24 bits key length.
const maskKeyLength = uint64(0x00FFFFFF00000000)
const maxKeyLength = 1<<24 - 1
// Max val length is 2^28, from 28 bits val length.
const maskValLength = uint64(0x000000000FFFFFFF)
const maxValLength = 1<<28 - 1
const maskRESERVED = uint64(0xF0000000F0000000)
// newSegment() allocates a segment with hinted amount of resources.
func newSegment(totalOps, totalKeyValBytes int) (*segment, error) {
return &segment{
kvs: make([]uint64, 0, totalOps*2),
buf: make([]byte, 0, totalKeyValBytes),
}, nil
}
func (a *segment) Kind() string { return SegmentKindBasic }
// Close releases resources associated with the segment.
func (a *segment) Close() error {
return nil
}
// Set copies the key and val bytes into the segment as a "set"
// mutation. The key must be unique (not repeated) within the
// segment.
func (a *segment) Set(key, val []byte) error {
return a.mutate(OperationSet, key, val)
}
// Del copies the key bytes into the segment as a "deletion" mutation.
// The key must be unique (not repeated) within the segment.
func (a *segment) Del(key []byte) error {
return a.mutate(OperationDel, key, nil)
}
// Merge creates or updates a key-val entry in the Collection via the
// MergeOperator defined in the CollectionOptions. The key must be
// unique (not repeated) within the segment.
func (a *segment) Merge(key, val []byte) error {
return a.mutate(OperationMerge, key, val)
}
// ------------------------------------------------------
// Alloc provides a slice of bytes "owned" by the segment, to reduce
// extra copying of memory. See the Collection.NewBatch() method.
func (a *segment) Alloc(numBytes int) ([]byte, error) {
bufLen := len(a.buf)
bufCap := cap(a.buf)
if numBytes > bufCap-bufLen {
return nil, ErrAllocTooLarge
}
rv := a.buf[bufLen : bufLen+numBytes]
a.buf = a.buf[0 : bufLen+numBytes]
return rv, nil
}
// AllocSet is like Set(), but the caller must provide []byte
// parameters that came from Alloc(), for less buffer copying.
func (a *segment) AllocSet(keyFromAlloc, valFromAlloc []byte) error {
bufCap := cap(a.buf)
keyStart := bufCap - cap(keyFromAlloc)
return a.mutateEx(OperationSet,
keyStart, len(keyFromAlloc), len(valFromAlloc))
}
// AllocDel is like Del(), but the caller must provide []byte
// parameters that came from Alloc(), for less buffer copying.
func (a *segment) AllocDel(keyFromAlloc []byte) error {
bufCap := cap(a.buf)
keyStart := bufCap - cap(keyFromAlloc)
return a.mutateEx(OperationDel,
keyStart, len(keyFromAlloc), 0)
}
// AllocMerge is like Merge(), but the caller must provide []byte
// parameters that came from Alloc(), for less buffer copying.
func (a *segment) AllocMerge(keyFromAlloc, valFromAlloc []byte) error {
bufCap := cap(a.buf)
keyStart := bufCap - cap(keyFromAlloc)
return a.mutateEx(OperationMerge,
keyStart, len(keyFromAlloc), len(valFromAlloc))
}
// ------------------------------------------------------
func (a *segment) Mutate(operation uint64, key, val []byte) error {
return a.mutate(operation, key, val)
}
func (a *segment) mutate(operation uint64, key, val []byte) error {
keyStart := len(a.buf)
a.buf = append(a.buf, key...)
keyLength := len(a.buf) - keyStart
valStart := len(a.buf)
a.buf = append(a.buf, val...)
valLength := len(a.buf) - valStart
return a.mutateEx(operation, keyStart, keyLength, valLength)
}
func (a *segment) mutateEx(operation uint64,
keyStart, keyLength, valLength int) error {
if keyLength > maxKeyLength {
return ErrKeyTooLarge
}
if valLength > maxValLength {
return ErrValueTooLarge
}
if keyLength <= 0 && valLength <= 0 {
keyStart = 0
}
opKlVl := encodeOpKeyLenValLen(operation, keyLength, valLength)
a.kvs = append(a.kvs, opKlVl, uint64(keyStart))
switch operation {
case OperationSet:
a.totOperationSet++
case OperationDel:
a.totOperationDel++
case OperationMerge:
a.totOperationMerge++
default:
}
a.totKeyByte += uint64(keyLength)
a.totValByte += uint64(valLength)
return nil
}
// ------------------------------------------------------
// NumKeyValBytes returns the number of bytes used for key-val data.
func (a *segment) NumKeyValBytes() (uint64, uint64) {
return a.totKeyByte, a.totValByte
}
// ------------------------------------------------------
// Len returns the number of ops in the segment.
func (a *segment) Len() int {
return len(a.kvs) / 2
}
func (a *segment) Swap(i, j int) {
x := i * 2
y := j * 2
// Operation + key length + val length.
a.kvs[x], a.kvs[y] = a.kvs[y], a.kvs[x]
x++
y++
a.kvs[x], a.kvs[y] = a.kvs[y], a.kvs[x] // Buf index.
}
func (a *segment) Less(i, j int) bool {
x := i * 2
y := j * 2
kxLength := int((maskKeyLength & a.kvs[x]) >> 32)
kxStart := int(a.kvs[x+1])
kx := a.buf[kxStart : kxStart+kxLength]
kyLength := int((maskKeyLength & a.kvs[y]) >> 32)
kyStart := int(a.kvs[y+1])
ky := a.buf[kyStart : kyStart+kyLength]
return bytes.Compare(kx, ky) < 0
}
// ------------------------------------------------------
type segmentCursor struct {
s *segment
start int
end int
curr int
}
func (c *segmentCursor) Current() (operation uint64, key []byte, val []byte) {
if c.curr >= c.start && c.curr < c.end {
operation, key, val = c.s.getOperationKeyVal(c.curr)
}
return
}
func (c *segmentCursor) Seek(startKeyInclusive []byte) error {
c.curr = c.s.findStartKeyInclusivePos(startKeyInclusive)
if c.curr < c.start {
c.curr = c.start
}
if c.curr >= c.end {
return ErrIteratorDone
}
return nil
}
func (c *segmentCursor) Next() error {
c.curr++
if c.curr >= c.end {
return ErrIteratorDone
}
return nil
}
// nextDelta advances the cursor position by 'delta' steps.
func (c *segmentCursor) nextDelta(delta int) error {
c.curr += delta
if c.curr >= c.end {
return ErrIteratorDone
}
return nil
}
// currentKey returns the array position and the key pointed to by the cursor.
func (c *segmentCursor) currentKey() (idx int, key []byte) {
if c.curr >= c.start && c.curr < c.end {
idx = c.curr
_, key, _ = c.s.getOperationKeyVal(c.curr)
}
return
}
func (a *segment) Cursor(startKeyInclusive []byte, endKeyExclusive []byte) (
SegmentCursor, error) {
rv := &segmentCursor{
s: a,
end: a.Len(),
}
rv.start = a.findStartKeyInclusivePos(startKeyInclusive)
if endKeyExclusive != nil {
rv.end = a.findStartKeyInclusivePos(endKeyExclusive)
}
rv.curr = rv.start
return rv, nil
}
func (a *segment) Get(key []byte) (operation uint64, val []byte, err error) {
pos := a.findKeyPos(key)
if pos >= 0 {
operation, _, val = a.getOperationKeyVal(pos)
}
return
}
// Searches for the key within the in-memory index of the segment
// if available. Returns left and right positions between which
// the key likely exists.
func (a *segment) searchIndex(key []byte) (int, int) {
if a.index != nil {
// Check the in-memory index for a more accurate window.
return a.index.lookup(key)
}
return 0, a.Len()
}
func (a *segment) findKeyPos(key []byte) int {
kvs := a.kvs
buf := a.buf
i, j := a.searchIndex(key)
if i == j {
return -1
}
// If key smaller than smallest key, return early.
startKeyLen := int((maskKeyLength & kvs[0]) >> 32)
startKeyBeg := int(kvs[1])
startCmp := bytes.Compare(key, buf[startKeyBeg:startKeyBeg+startKeyLen])
if startCmp < 0 {
return -1
}
for i < j {
h := i + (j-i)/2 // Keep i <= h < j.
x := h * 2
klen := int((maskKeyLength & kvs[x]) >> 32)
kbeg := int(kvs[x+1])
cmp := bytes.Compare(buf[kbeg:kbeg+klen], key)
if cmp == 0 {
return h
} else if cmp < 0 {
i = h + 1
} else {
j = h
}
}
return -1
}
// FindStartKeyInclusivePos() returns the logical entry position for
// the given (inclusive) start key. With segment keys of [b, d, f],
// looking for 'c' will return 1. Looking for 'd' will return 1.
// Looking for 'g' will return 3. Looking for 'a' will return 0.
func (a *segment) findStartKeyInclusivePos(startKeyInclusive []byte) int {
kvs := a.kvs
buf := a.buf
i, j := a.searchIndex(startKeyInclusive)
if i == j {
return i
}
startKeyLen := int((maskKeyLength & kvs[0]) >> 32)
startKeyBeg := int(kvs[1])
startCmp := bytes.Compare(startKeyInclusive,
buf[startKeyBeg:startKeyBeg+startKeyLen])
if startCmp < 0 { // If key smaller than smallest key, return early.
return i
}
for i < j {
h := i + (j-i)/2 // Keep i <= h < j.
x := h * 2
klen := int((maskKeyLength & kvs[x]) >> 32)
kbeg := int(kvs[x+1])
cmp := bytes.Compare(buf[kbeg:kbeg+klen], startKeyInclusive)
if cmp == 0 {
return h
} else if cmp < 0 {
i = h + 1
} else {
j = h
}
}
return i
// TODO: Do better than binary search?
// TODO: Consider a perfectly balanced btree?
}
// GetOperationKeyVal() returns the operation, key, val for a given
// logical entry position in the segment.
func (a *segment) getOperationKeyVal(pos int) (uint64, []byte, []byte) {
x := pos * 2
if x < len(a.kvs) {
opklvl := a.kvs[x]
kstart := int(a.kvs[x+1])
operation, keyLen, valLen := decodeOpKeyLenValLen(opklvl)
vstart := kstart + keyLen
return operation, a.buf[kstart:vstart], a.buf[vstart : vstart+valLen]
}
return 0, nil, nil
}
// ------------------------------------------------------
func encodeOpKeyLenValLen(operation uint64, keyLen, valLen int) uint64 {
return (maskOperation & operation) |
(maskKeyLength & (uint64(keyLen) << 32)) |
(maskValLength & (uint64(valLen)))
}
func decodeOpKeyLenValLen(opklvl uint64) (uint64, int, int) {
operation := maskOperation & opklvl
keyLen := int((maskKeyLength & opklvl) >> 32)
valLen := int(maskValLength & opklvl)
return operation, keyLen, valLen
}
// ------------------------------------------------------
// readyDeferredSort() will create a ticket for the future sorter and
// a channel to wait for its completion
func (a *segment) readyDeferredSort() {
a.needSorterCh = make(chan bool, 1)
a.needSorterCh <- true // A ticket for the future sorter.
close(a.needSorterCh)
a.waitSortedCh = make(chan struct{})
}
// RequestSort() will either perform the previously deferred sorting,
// if the goroutine can acquire the 1 ticket from the needSorterCh.
// Or, requestSort() will ensure that a sorter is working on this
// segment. Returns true if the segment is sorted, and returns false
// if the sorting is only asynchronously scheduled.
func (a *segment) RequestSort(synchronous bool) bool {
if a.needSorterCh == nil {
return true
}
iAmTheSorter := <-a.needSorterCh
if iAmTheSorter {
a.doSort()
close(a.waitSortedCh) // Signal any waiters.
return true
}
if synchronous {
<-a.waitSortedCh // Wait for the sorter to be done.
return true
}
return false
}
// doSort() will immediately sort this segment.
func (a *segment) doSort() {
// After sorting, the segment is immutable and then safe for
// concurrent reads.
sort.Sort(a)
go a.rootCollection.updateStats(a)
}
// ------------------------------------------------------
// Persist persists a basic segment, and allows a segment to meet the
// SegmentPersister interface.
func (a *segment) Persist(file File, options *StoreOptions) (rv SegmentLoc, err error) {
finfo, err := file.Stat()
if err != nil {
return rv, err
}
persistKind := DefaultPersistKind
if options.PersistKind != "" {
persistKind = options.PersistKind
}
segmentPersister, exists := SegmentPersisters[persistKind]
if !exists || segmentPersister == nil {
return rv, fmt.Errorf("store: unknown PersistKind: %+v", persistKind)
}
return segmentPersister(a, file, finfo.Size(), nil)
}
// ------------------------------------------------------
// loadBasicSegment loads a basic segment.
func loadBasicSegment(sloc *SegmentLoc) (Segment, error) {
var kvs []uint64
var buf []byte
var err error
if sloc.KvsBytes > 0 {
if sloc.KvsBytes > uint64(len(sloc.mref.buf)) {
return nil, fmt.Errorf("store: load basic segment KvsOffset/KvsBytes too big,"+
" len(mref.buf): %d, sloc: %+v", len(sloc.mref.buf), sloc)
}
kvsBytes := sloc.mref.buf[0:sloc.KvsBytes]
kvs, err = ByteSliceToUint64Slice(kvsBytes)
if err != nil {
return nil, err
}
}
if sloc.BufBytes > 0 {
bufStart := sloc.BufOffset - sloc.KvsOffset
if bufStart+sloc.BufBytes > uint64(len(sloc.mref.buf)) {
return nil, fmt.Errorf("store: load basic segment BufOffset/BufBytes too big,"+
" len(mref.buf): %d, sloc: %+v", len(sloc.mref.buf), sloc)
}
buf = sloc.mref.buf[bufStart : bufStart+sloc.BufBytes]
}
return &segment{
kvs: kvs,
buf: buf,
totOperationSet: sloc.TotOpsSet,
totOperationDel: sloc.TotOpsDel,
totKeyByte: sloc.TotKeyByte,
totValByte: sloc.TotValByte,
}, nil
}
// ------------------------------------------------------
func persistBasicSegment(
s Segment, file File, pos int64, options *StoreOptions) (rv SegmentLoc, err error) {
seg, ok := s.(*segment)
if !ok {
return rv, fmt.Errorf("wrong segment type")
}
kvsBuf, err := Uint64SliceToByteSlice(seg.kvs)
if err != nil {
return rv, err
}
kvsPos := pageAlignCeil(pos)
bufPos := pageAlignCeil(kvsPos + int64(len(kvsBuf)))
ioCh := make(chan ioResult)
go func() {
kvsWritten, err := file.WriteAt(kvsBuf, kvsPos)
ioCh <- ioResult{kind: "kvs", want: len(kvsBuf), got: kvsWritten, err: err}
}()
go func() {
bufWritten, err := file.WriteAt(seg.buf, bufPos)
ioCh <- ioResult{kind: "buf", want: len(seg.buf), got: bufWritten, err: err}
}()
resMap := map[string]ioResult{}
for len(resMap) < 2 {
res := <-ioCh
if res.err != nil {
return rv, res.err
}
if res.want != res.got {
return rv, fmt.Errorf("store: persistSegment error writing,"+
" res: %+v, err: %v", res, res.err)
}
resMap[res.kind] = res
}
close(ioCh)
return SegmentLoc{
Kind: seg.Kind(),
KvsOffset: uint64(kvsPos),
KvsBytes: uint64(resMap["kvs"].got),
BufOffset: uint64(bufPos),
BufBytes: uint64(resMap["buf"].got),
TotOpsSet: seg.totOperationSet,
TotOpsDel: seg.totOperationDel,
TotKeyByte: seg.totKeyByte,
TotValByte: seg.totValByte,
}, nil
}
func (a *segment) Valid() error {
if a.kvs == nil || len(a.kvs) <= 0 {
return fmt.Errorf("expected kvs")
}
if a.buf == nil || len(a.buf) <= 0 {
return fmt.Errorf("expected buf")
}
for pos := 0; pos < a.Len(); pos++ {
x := pos * 2
if x < 0 || x >= len(a.kvs) {
return fmt.Errorf("pos to x error")
}
opklvl := a.kvs[x]
operation, keyLen, valLen := decodeOpKeyLenValLen(opklvl)
if operation == 0 {
return fmt.Errorf("should have some nonzero op")
}
kstart := int(a.kvs[x+1])
vstart := kstart + keyLen
if kstart+keyLen > len(a.buf) {
return fmt.Errorf("key larger than buf, pos: %d, kstart: %d, keyLen: %d, len(buf): %d, op: %x",
pos, kstart, keyLen, len(a.buf), operation)
}
if vstart+valLen > len(a.buf) {
return fmt.Errorf("val larger than buf, pos: %d, vstart: %d, valLen: %d, len(buf): %d, op: %x",
pos, vstart, valLen, len(a.buf), operation)
}
}
return nil
}
// ------------------------------------------------------
// Builds and initializes the in-memory index for the segment.
func (a *segment) buildIndex(quota int, minKeyBytes int) {
if int(a.totKeyByte) < minKeyBytes {
// Build the index only if the total key bytes is greater
// than or equal to the SegmentKeysIndexMinKeyBytes.
return
}
keyCount := a.Len()
if keyCount == 0 {
// No keys to index.
return
}
keyAvgSize := int(a.totKeyByte) / keyCount
// Initialize the index.
sindex := newSegmentKeysIndex(quota, keyCount, keyAvgSize)
if sindex == nil {
return
}
scursor := &segmentCursor{
s: a,
end: a.Len(),
}
// Build the index for the segment's data.
for {
keyIdx, key := scursor.currentKey()
if key == nil {
break
}
if !sindex.add(keyIdx, key) {
// Out of space.
break
}
err := scursor.nextDelta(sindex.hop)
if err != nil {
break
}
}
// Update segment's index.
a.index = sindex
}
// ------------------------------------------------------
type batch struct {
// Compose batch as a type of segment extending it with childCollections
*segment
// childBatches track the segments of child collections indexed by their
// unique collection names.
childBatches map[string]*batch
}
// deletedChildBatchMarker is used as a conduit to convey the delete
// request from DelChildCollection() to ExecuteBatch()
var deletedChildBatchMarker = &batch{}
// newBatch() allocates a segment with hinted amount of resources.
func newBatch(rootCollection *collection, options BatchOptions) (
*batch, error) {
return &batch{
segment: &segment{
kvs: make([]uint64, 0, options.TotalOps*2),
buf: make([]byte, 0, options.TotalKeyValBytes),
rootCollection: rootCollection,
},
childBatches: nil, // Created later on demand.
}, nil
}
func (b *batch) NewChildCollectionBatch(collectionName string,
options BatchOptions) (Batch, error) {
if len(collectionName) == 0 {
return nil, ErrBadCollectionName
}
childBatch, err := newBatch(b.rootCollection, options)
if b.childBatches == nil { // First creation of child batch.
b.childBatches = make(map[string]*batch)
}
b.childBatches[collectionName] = childBatch
return childBatch, err
}
func (b *batch) DelChildCollection(collectionName string) error {
if len(collectionName) == 0 {
return ErrNoSuchCollection
}
if b.childBatches == nil { // No previous child batches seen.
b.childBatches = make(map[string]*batch)
}
// Load the parent batch with this batch having special signature.
b.childBatches[collectionName] = deletedChildBatchMarker
return nil
}
func (b *batch) readyDeferredSort() {
if b == deletedChildBatchMarker {
return
}
for _, childBatch := range b.childBatches {
childBatch.readyDeferredSort()
}
b.segment.readyDeferredSort()
}
// RequestSort() returns true if all child batches are sorted and
// false if sorting has been asynchronously scheduled.
func (b *batch) RequestSort() bool {
if b == deletedChildBatchMarker {
return true
}
// false because we must never wait for sorter else it can deadlock.
sorted := b.segment.RequestSort(false)
for _, childBatch := range b.childBatches {
sorted = childBatch.RequestSort() && sorted
}
return sorted
}
func (b *batch) doSort() {
if b == deletedChildBatchMarker {
return
}
b.segment.doSort()
for _, childBatch := range b.childBatches {
childBatch.doSort()
}
}
func (b *batch) isEmpty() bool {
if len(b.childBatches) != 0 {
// Very presence of child batches indicates a non-empty batch
// even if the child batches themselves are empty. This is so that
// collection creation/deletions can still work.
return false
}
return b.Len() <= 0
}
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