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compaction_iter.go
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compaction_iter.go
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// Copyright 2018 The LevelDB-Go and Pebble Authors. All rights reserved. Use
// of this source code is governed by a BSD-style license that can be found in
// the LICENSE file.
package pebble
import (
"bytes"
"encoding/binary"
"fmt"
"io"
"sort"
"strconv"
"github.com/cockroachdb/errors"
"github.com/cockroachdb/pebble/internal/base"
"github.com/cockroachdb/pebble/internal/bytealloc"
"github.com/cockroachdb/pebble/internal/keyspan"
"github.com/cockroachdb/pebble/internal/rangekey"
"github.com/cockroachdb/redact"
)
// compactionIter provides a forward-only iterator that encapsulates the logic
// for collapsing entries during compaction. It wraps an internal iterator and
// collapses entries that are no longer necessary because they are shadowed by
// newer entries. The simplest example of this is when the internal iterator
// contains two keys: a.PUT.2 and a.PUT.1. Instead of returning both entries,
// compactionIter collapses the second entry because it is no longer
// necessary. The high-level structure for compactionIter is to iterate over
// its internal iterator and output 1 entry for every user-key. There are four
// complications to this story.
//
// 1. Eliding Deletion Tombstones
//
// Consider the entries a.DEL.2 and a.PUT.1. These entries collapse to
// a.DEL.2. Do we have to output the entry a.DEL.2? Only if a.DEL.2 possibly
// shadows an entry at a lower level. If we're compacting to the base-level in
// the LSM tree then a.DEL.2 is definitely not shadowing an entry at a lower
// level and can be elided.
//
// We can do slightly better than only eliding deletion tombstones at the base
// level by observing that we can elide a deletion tombstone if there are no
// sstables that contain the entry's key. This check is performed by
// elideTombstone.
//
// 2. Merges
//
// The MERGE operation merges the value for an entry with the existing value
// for an entry. The logical value of an entry can be composed of a series of
// merge operations. When compactionIter sees a MERGE, it scans forward in its
// internal iterator collapsing MERGE operations for the same key until it
// encounters a SET or DELETE operation. For example, the keys a.MERGE.4,
// a.MERGE.3, a.MERGE.2 will be collapsed to a.MERGE.4 and the values will be
// merged using the specified Merger.
//
// An interesting case here occurs when MERGE is combined with SET. Consider
// the entries a.MERGE.3 and a.SET.2. The collapsed key will be a.SET.3. The
// reason that the kind is changed to SET is because the SET operation acts as
// a barrier preventing further merging. This can be seen better in the
// scenario a.MERGE.3, a.SET.2, a.MERGE.1. The entry a.MERGE.1 may be at lower
// (older) level and not involved in the compaction. If the compaction of
// a.MERGE.3 and a.SET.2 produced a.MERGE.3, a subsequent compaction with
// a.MERGE.1 would merge the values together incorrectly.
//
// 3. Snapshots
//
// Snapshots are lightweight point-in-time views of the DB state. At its core,
// a snapshot is a sequence number along with a guarantee from Pebble that it
// will maintain the view of the database at that sequence number. Part of this
// guarantee is relatively straightforward to achieve. When reading from the
// database Pebble will ignore sequence numbers that are larger than the
// snapshot sequence number. The primary complexity with snapshots occurs
// during compaction: the collapsing of entries that are shadowed by newer
// entries is at odds with the guarantee that Pebble will maintain the view of
// the database at the snapshot sequence number. Rather than collapsing entries
// up to the next user key, compactionIter can only collapse entries up to the
// next snapshot boundary. That is, every snapshot boundary potentially causes
// another entry for the same user-key to be emitted. Another way to view this
// is that snapshots define stripes and entries are collapsed within stripes,
// but not across stripes. Consider the following scenario:
//
// a.PUT.9
// a.DEL.8
// a.PUT.7
// a.DEL.6
// a.PUT.5
//
// In the absence of snapshots these entries would be collapsed to
// a.PUT.9. What if there is a snapshot at sequence number 7? The entries can
// be divided into two stripes and collapsed within the stripes:
//
// a.PUT.9 a.PUT.9
// a.DEL.8 --->
// a.PUT.7
// -- --
// a.DEL.6 ---> a.DEL.6
// a.PUT.5
//
// All of the rules described earlier still apply, but they are confined to
// operate within a snapshot stripe. Snapshots only affect compaction when the
// snapshot sequence number lies within the range of sequence numbers being
// compacted. In the above example, a snapshot at sequence number 10 or at
// sequence number 5 would not have any effect.
//
// 4. Range Deletions
//
// Range deletions provide the ability to delete all of the keys (and values)
// in a contiguous range. Range deletions are stored indexed by their start
// key. The end key of the range is stored in the value. In order to support
// lookup of the range deletions which overlap with a particular key, the range
// deletion tombstones need to be fragmented whenever they overlap. This
// fragmentation is performed by keyspan.Fragmenter. The fragments are then
// subject to the rules for snapshots. For example, consider the two range
// tombstones [a,e)#1 and [c,g)#2:
//
// 2: c-------g
// 1: a-------e
//
// These tombstones will be fragmented into:
//
// 2: c---e---g
// 1: a---c---e
//
// Do we output the fragment [c,e)#1? Since it is covered by [c-e]#2 the answer
// depends on whether it is in a new snapshot stripe.
//
// In addition to the fragmentation of range tombstones, compaction also needs
// to take the range tombstones into consideration when outputting normal
// keys. Just as with point deletions, a range deletion covering an entry can
// cause the entry to be elided.
//
// A note on the stability of keys and values.
//
// The stability guarantees of keys and values returned by the iterator tree
// that backs a compactionIter is nuanced and care must be taken when
// referencing any returned items.
//
// Keys and values returned by exported functions (i.e. First, Next, etc.) have
// lifetimes that fall into two categories:
//
// Lifetime valid for duration of compaction. Range deletion keys and values are
// stable for the duration of the compaction, due to way in which a
// compactionIter is typically constructed (i.e. via (*compaction).newInputIter,
// which wraps the iterator over the range deletion block in a noCloseIter,
// preventing the release of the backing memory until the compaction is
// finished).
//
// Lifetime limited to duration of sstable block liveness. Point keys (SET, DEL,
// etc.) and values must be cloned / copied following the return from the
// exported function, and before a subsequent call to Next advances the iterator
// and mutates the contents of the returned key and value.
type compactionIter struct {
equal Equal
merge Merge
iter internalIterator
err error
// `key.UserKey` is set to `keyBuf` caused by saving `i.iterKey.UserKey`
// and `key.Trailer` is set to `i.iterKey.Trailer`. This is the
// case on return from all public methods -- these methods return `key`.
// Additionally, it is the internal state when the code is moving to the
// next key so it can determine whether the user key has changed from
// the previous key.
key InternalKey
// keyTrailer is updated when `i.key` is updated and holds the key's
// original trailer (eg, before any sequence-number zeroing or changes to
// key kind).
keyTrailer uint64
value []byte
valueCloser io.Closer
// Temporary buffer used for storing the previous user key in order to
// determine when iteration has advanced to a new user key and thus a new
// snapshot stripe.
keyBuf []byte
// Temporary buffer used for storing the previous value, which may be an
// unsafe, i.iter-owned slice that could be altered when the iterator is
// advanced.
valueBuf []byte
// Is the current entry valid?
valid bool
iterKey *InternalKey
iterValue []byte
iterStripeChange stripeChangeType
// `skip` indicates whether the remaining skippable entries in the current
// snapshot stripe should be skipped or processed. An example of a non-
// skippable entry is a range tombstone as we need to return it from the
// `compactionIter`, even if a key covering its start key has already been
// seen in the same stripe. `skip` has no effect when `pos == iterPosNext`.
//
// TODO(jackson): If we use keyspan.InterleavingIter for range deletions,
// like we do for range keys, the only remaining 'non-skippable' key is
// the invalid key. We should be able to simplify this logic and remove this
// field.
skip bool
// `pos` indicates the iterator position at the top of `Next()`. Its type's
// (`iterPos`) values take on the following meanings in the context of
// `compactionIter`.
//
// - `iterPosCur`: the iterator is at the last key returned.
// - `iterPosNext`: the iterator has already been advanced to the next
// candidate key. For example, this happens when processing merge operands,
// where we advance the iterator all the way into the next stripe or next
// user key to ensure we've seen all mergeable operands.
// - `iterPosPrev`: this is invalid as compactionIter is forward-only.
pos iterPos
// `snapshotPinned` indicates whether the last point key returned by the
// compaction iterator was only returned because an open snapshot prevents
// its elision. This field only applies to point keys, and not to range
// deletions or range keys.
//
// For MERGE, it is possible that doing the merge is interrupted even when
// the next point key is in the same stripe. This can happen if the loop in
// mergeNext gets interrupted by sameStripeNonSkippable.
// sameStripeNonSkippable occurs due to RANGEDELs that sort before
// SET/MERGE/DEL with the same seqnum, so the RANGEDEL does not necessarily
// delete the subsequent SET/MERGE/DEL keys.
snapshotPinned bool
// forceObsoleteDueToRangeDel is set to true in a subset of the cases that
// snapshotPinned is true. This value is true when the point is obsolete due
// to a RANGEDEL but could not be deleted due to a snapshot.
//
// NB: it may seem that the additional cases that snapshotPinned captures
// are harmless in that they can also be used to mark a point as obsolete
// (it is merely a duplication of some logic that happens in
// Writer.AddWithForceObsolete), but that is not quite accurate as of this
// writing -- snapshotPinned originated in stats collection and for a
// sequence MERGE, SET, where the MERGE cannot merge with the (older) SET
// due to a snapshot, the snapshotPinned value for the SET is true.
//
// TODO(sumeer,jackson): improve the logic of snapshotPinned and reconsider
// whether we need forceObsoleteDueToRangeDel.
forceObsoleteDueToRangeDel bool
// The index of the snapshot for the current key within the snapshots slice.
curSnapshotIdx int
curSnapshotSeqNum uint64
// The snapshot sequence numbers that need to be maintained. These sequence
// numbers define the snapshot stripes (see the Snapshots description
// above). The sequence numbers are in ascending order.
snapshots []uint64
// frontiers holds a heap of user keys that affect compaction behavior when
// they're exceeded. Before a new key is returned, the compaction iterator
// advances the frontier, notifying any code that subscribed to be notified
// when a key was reached. The primary use today is within the
// implementation of compactionOutputSplitters in compaction.go. Many of
// these splitters wait for the compaction iterator to call Advance(k) when
// it's returning a new key. If the key that they're waiting for is
// surpassed, these splitters update internal state recording that they
// should request a compaction split next time they're asked in
// [shouldSplitBefore].
frontiers frontiers
// Reference to the range deletion tombstone fragmenter (e.g.,
// `compaction.rangeDelFrag`).
rangeDelFrag *keyspan.Fragmenter
rangeKeyFrag *keyspan.Fragmenter
// The fragmented tombstones.
tombstones []keyspan.Span
// The fragmented range keys.
rangeKeys []keyspan.Span
// Byte allocator for the tombstone keys.
alloc bytealloc.A
allowZeroSeqNum bool
elideTombstone func(key []byte) bool
elideRangeTombstone func(start, end []byte) bool
// The on-disk format major version. This informs the types of keys that
// may be written to disk during a compaction.
formatVersion FormatMajorVersion
stats struct {
// count of DELSIZED keys that were missized.
countMissizedDels uint64
}
}
func newCompactionIter(
cmp Compare,
equal Equal,
formatKey base.FormatKey,
merge Merge,
iter internalIterator,
snapshots []uint64,
rangeDelFrag *keyspan.Fragmenter,
rangeKeyFrag *keyspan.Fragmenter,
allowZeroSeqNum bool,
elideTombstone func(key []byte) bool,
elideRangeTombstone func(start, end []byte) bool,
formatVersion FormatMajorVersion,
) *compactionIter {
i := &compactionIter{
equal: equal,
merge: merge,
iter: iter,
snapshots: snapshots,
frontiers: frontiers{cmp: cmp},
rangeDelFrag: rangeDelFrag,
rangeKeyFrag: rangeKeyFrag,
allowZeroSeqNum: allowZeroSeqNum,
elideTombstone: elideTombstone,
elideRangeTombstone: elideRangeTombstone,
formatVersion: formatVersion,
}
i.rangeDelFrag.Cmp = cmp
i.rangeDelFrag.Format = formatKey
i.rangeDelFrag.Emit = i.emitRangeDelChunk
i.rangeKeyFrag.Cmp = cmp
i.rangeKeyFrag.Format = formatKey
i.rangeKeyFrag.Emit = i.emitRangeKeyChunk
return i
}
func (i *compactionIter) First() (*InternalKey, []byte) {
if i.err != nil {
return nil, nil
}
var iterValue LazyValue
i.iterKey, iterValue = i.iter.First()
i.iterValue, _, i.err = iterValue.Value(nil)
if i.err != nil {
return nil, nil
}
if i.iterKey != nil {
i.curSnapshotIdx, i.curSnapshotSeqNum = snapshotIndex(i.iterKey.SeqNum(), i.snapshots)
}
i.pos = iterPosNext
i.iterStripeChange = newStripeNewKey
return i.Next()
}
func (i *compactionIter) Next() (*InternalKey, []byte) {
if i.err != nil {
return nil, nil
}
// Close the closer for the current value if one was open.
if i.closeValueCloser() != nil {
return nil, nil
}
// Prior to this call to `Next()` we are in one of four situations with
// respect to `iterKey` and related state:
//
// - `!skip && pos == iterPosNext`: `iterKey` is already at the next key.
// - `!skip && pos == iterPosCurForward`: We are at the key that has been returned.
// To move forward we advance by one key, even if that lands us in the same
// snapshot stripe.
// - `skip && pos == iterPosCurForward`: We are at the key that has been returned.
// To move forward we skip skippable entries in the stripe.
// - `skip && pos == iterPosNext && i.iterStripeChange == sameStripeNonSkippable`:
// This case may occur when skipping within a snapshot stripe and we
// encounter either:
// a) an invalid key kind; The previous call will have returned
// whatever key it was processing and deferred handling of the
// invalid key to this invocation of Next(). We're responsible for
// ignoring skip=true and falling into the invalid key kind case
// down below.
// b) an interleaved range delete; This is a wart of the current code
// structure. While skipping within a snapshot stripe, a range
// delete interleaved at its start key and sequence number
// interrupts the sequence of point keys. After we return the range
// delete to the caller, we need to pick up skipping at where we
// left off, so we preserve skip=true.
// TODO(jackson): This last case is confusing and can be removed if we
// interleave range deletions at the maximal sequence number using the
// keyspan interleaving iterator. This is the treatment given to range
// keys today.
if i.pos == iterPosCurForward {
if i.skip {
i.skipInStripe()
} else {
i.nextInStripe()
}
} else if i.skip {
if i.iterStripeChange != sameStripeNonSkippable {
panic(errors.AssertionFailedf("compaction iterator has skip=true, but iterator is at iterPosNext"))
}
}
i.pos = iterPosCurForward
i.valid = false
for i.iterKey != nil {
// If we entered a new snapshot stripe with the same key, any key we
// return on this iteration is only returned because the open snapshot
// prevented it from being elided or merged with the key returned for
// the previous stripe. Mark it as pinned so that the compaction loop
// can correctly populate output tables' pinned statistics. We might
// also set snapshotPinned=true down below if we observe that the key is
// deleted by a range deletion in a higher stripe or that this key is a
// tombstone that could be elided if only it were in the last snapshot
// stripe.
i.snapshotPinned = i.iterStripeChange == newStripeSameKey
if i.iterKey.Kind() == InternalKeyKindRangeDelete || rangekey.IsRangeKey(i.iterKey.Kind()) {
// Return the span so the compaction can use it for file truncation and add
// it to the relevant fragmenter. We do not set `skip` to true before
// returning as there may be a forthcoming point key with the same user key
// and sequence number. Such a point key must be visible (i.e., not skipped
// over) since we promise point keys are not deleted by range tombstones at
// the same sequence number.
//
// Although, note that `skip` may already be true before reaching here
// due to an earlier key in the stripe. Then it is fine to leave it set
// to true, as the earlier key must have had a higher sequence number.
//
// NOTE: there is a subtle invariant violation here in that calling
// saveKey and returning a reference to the temporary slice violates
// the stability guarantee for range deletion keys. A potential
// mediation could return the original iterKey and iterValue
// directly, as the backing memory is guaranteed to be stable until
// the compaction completes. The violation here is only minor in
// that the caller immediately clones the range deletion InternalKey
// when passing the key to the deletion fragmenter (see the
// call-site in compaction.go).
// TODO(travers): address this violation by removing the call to
// saveKey and instead return the original iterKey and iterValue.
// This goes against the comment on i.key in the struct, and
// therefore warrants some investigation.
i.saveKey()
// TODO(jackson): Handle tracking pinned statistics for range keys
// and range deletions. This would require updating
// emitRangeDelChunk and rangeKeyCompactionTransform to update
// statistics when they apply their own snapshot striping logic.
i.snapshotPinned = false
i.value = i.iterValue
i.valid = true
return &i.key, i.value
}
if cover := i.rangeDelFrag.Covers(*i.iterKey, i.curSnapshotSeqNum); cover == keyspan.CoversVisibly {
// A pending range deletion deletes this key. Skip it.
i.saveKey()
i.skipInStripe()
continue
} else if cover == keyspan.CoversInvisibly {
// i.iterKey would be deleted by a range deletion if there weren't
// any open snapshots. Mark it as pinned.
//
// NB: there are multiple places in this file where we call
// i.rangeDelFrag.Covers and this is the only one where we are writing
// to i.snapshotPinned. Those other cases occur in mergeNext where the
// caller is deciding whether the value should be merged or not, and the
// key is in the same snapshot stripe. Hence, snapshotPinned is by
// definition false in those cases.
i.snapshotPinned = true
i.forceObsoleteDueToRangeDel = true
} else {
i.forceObsoleteDueToRangeDel = false
}
switch i.iterKey.Kind() {
case InternalKeyKindDelete, InternalKeyKindSingleDelete, InternalKeyKindDeleteSized:
if i.elideTombstone(i.iterKey.UserKey) {
if i.curSnapshotIdx == 0 {
// If we're at the last snapshot stripe and the tombstone
// can be elided skip skippable keys in the same stripe.
i.saveKey()
i.skipInStripe()
if i.iterStripeChange == newStripeSameKey {
panic(errors.AssertionFailedf("pebble: skipInStripe in last stripe found a new stripe within the same key"))
}
if !i.skip && i.iterStripeChange != newStripeNewKey {
panic(errors.AssertionFailedf("pebble: skipInStripe in last stripe disabled skip without advancing to new key"))
}
continue
} else {
// We're not at the last snapshot stripe, so the tombstone
// can NOT yet be elided. Mark it as pinned, so that it's
// included in table statistics appropriately.
i.snapshotPinned = true
}
}
switch i.iterKey.Kind() {
case InternalKeyKindDelete:
i.saveKey()
i.value = i.iterValue
i.valid = true
i.skip = true
return &i.key, i.value
case InternalKeyKindDeleteSized:
// We may skip subsequent keys because of this tombstone. Scan
// ahead to see just how much data this tombstone drops and if
// the tombstone's value should be updated accordingly.
return i.deleteSizedNext()
case InternalKeyKindSingleDelete:
if i.singleDeleteNext() {
return &i.key, i.value
} else if i.err != nil {
return nil, nil
}
continue
default:
panic(errors.AssertionFailedf(
"unexpected kind %s", redact.SafeString(i.iterKey.Kind().String())))
}
case InternalKeyKindSet, InternalKeyKindSetWithDelete:
// The key we emit for this entry is a function of the current key
// kind, and whether this entry is followed by a DEL/SINGLEDEL
// entry. setNext() does the work to move the iterator forward,
// preserving the original value, and potentially mutating the key
// kind.
i.setNext()
if i.err != nil {
return nil, nil
}
return &i.key, i.value
case InternalKeyKindMerge:
// Record the snapshot index before mergeNext as merging
// advances the iterator, adjusting curSnapshotIdx.
origSnapshotIdx := i.curSnapshotIdx
var valueMerger ValueMerger
valueMerger, i.err = i.merge(i.iterKey.UserKey, i.iterValue)
var change stripeChangeType
if i.err == nil {
change = i.mergeNext(valueMerger)
}
var needDelete bool
if i.err == nil {
// includesBase is true whenever we've transformed the MERGE record
// into a SET.
var includesBase bool
switch i.key.Kind() {
case InternalKeyKindSet:
includesBase = true
case InternalKeyKindMerge:
default:
panic(errors.AssertionFailedf(
"unexpected kind %s", redact.SafeString(i.key.Kind().String())))
}
i.value, needDelete, i.valueCloser, i.err = finishValueMerger(valueMerger, includesBase)
}
if i.err == nil {
if needDelete {
i.valid = false
if i.closeValueCloser() != nil {
return nil, nil
}
continue
}
// A non-skippable entry does not necessarily cover later merge
// operands, so we must not zero the current merge result's seqnum.
//
// For example, suppose the forthcoming two keys are a range
// tombstone, `[a, b)#3`, and a merge operand, `a#3`. Recall that
// range tombstones do not cover point keys at the same seqnum, so
// `a#3` is not deleted. The range tombstone will be seen first due
// to its larger value type. Since it is a non-skippable key, the
// current merge will not include `a#3`. If we zeroed the current
// merge result's seqnum, then it would conflict with the upcoming
// merge including `a#3`, whose seqnum will also be zeroed.
if change != sameStripeNonSkippable {
i.maybeZeroSeqnum(origSnapshotIdx)
}
return &i.key, i.value
}
if i.err != nil {
i.valid = false
// TODO(sumeer): why is MarkCorruptionError only being called for
// MERGE?
i.err = base.MarkCorruptionError(i.err)
}
return nil, nil
default:
i.err = base.CorruptionErrorf("invalid internal key kind: %d", errors.Safe(i.iterKey.Kind()))
i.valid = false
return nil, nil
}
}
return nil, nil
}
func (i *compactionIter) closeValueCloser() error {
if i.valueCloser == nil {
return nil
}
i.err = i.valueCloser.Close()
i.valueCloser = nil
if i.err != nil {
i.valid = false
}
return i.err
}
// snapshotIndex returns the index of the first sequence number in snapshots
// which is greater than or equal to seq.
func snapshotIndex(seq uint64, snapshots []uint64) (int, uint64) {
index := sort.Search(len(snapshots), func(i int) bool {
return snapshots[i] > seq
})
if index >= len(snapshots) {
return index, InternalKeySeqNumMax
}
return index, snapshots[index]
}
// skipInStripe skips over skippable keys in the same stripe and user key. It
// may set i.err, in which case i.iterKey will be nil.
func (i *compactionIter) skipInStripe() {
i.skip = true
for i.nextInStripe() == sameStripeSkippable {
if i.err != nil {
panic(i.err)
}
}
// Reset skip if we landed outside the original stripe. Otherwise, we landed
// in the same stripe on a non-skippable key. In that case we should preserve
// `i.skip == true` such that later keys in the stripe will continue to be
// skipped.
if i.iterStripeChange == newStripeNewKey || i.iterStripeChange == newStripeSameKey {
i.skip = false
}
}
func (i *compactionIter) iterNext() bool {
var iterValue LazyValue
i.iterKey, iterValue = i.iter.Next()
i.iterValue, _, i.err = iterValue.Value(nil)
if i.err != nil {
i.iterKey = nil
}
return i.iterKey != nil
}
// stripeChangeType indicates how the snapshot stripe changed relative to the
// previous key. If no change, it also indicates whether the current entry is
// skippable. If the snapshot stripe changed, it also indicates whether the new
// stripe was entered because the iterator progressed onto an entirely new key
// or entered a new stripe within the same key.
type stripeChangeType int
const (
newStripeNewKey stripeChangeType = iota
newStripeSameKey
sameStripeSkippable
sameStripeNonSkippable
)
// nextInStripe advances the iterator and returns one of the above const ints
// indicating how its state changed.
//
// Calls to nextInStripe must be preceded by a call to saveKey to retain a
// temporary reference to the original key, so that forward iteration can
// proceed with a reference to the original key. Care should be taken to avoid
// overwriting or mutating the saved key or value before they have been returned
// to the caller of the exported function (i.e. the caller of Next, First, etc.)
//
// nextInStripe may set i.err, in which case the return value will be
// newStripeNewKey, and i.iterKey will be nil.
func (i *compactionIter) nextInStripe() stripeChangeType {
i.iterStripeChange = i.nextInStripeHelper()
return i.iterStripeChange
}
// nextInStripeHelper is an internal helper for nextInStripe; callers should use
// nextInStripe and not call nextInStripeHelper.
func (i *compactionIter) nextInStripeHelper() stripeChangeType {
if !i.iterNext() {
return newStripeNewKey
}
key := i.iterKey
if !i.equal(i.key.UserKey, key.UserKey) {
i.curSnapshotIdx, i.curSnapshotSeqNum = snapshotIndex(key.SeqNum(), i.snapshots)
return newStripeNewKey
}
// If i.key and key have the same user key, then
// 1. i.key must not have had a zero sequence number (or it would've be the last
// key with its user key).
// 2. i.key must have a strictly larger sequence number
// There's an exception in that either key may be a range delete. Range
// deletes may share a sequence number with a point key if the keys were
// ingested together. Range keys may also share the sequence number if they
// were ingested, but range keys are interleaved into the compaction
// iterator's input iterator at the maximal sequence number so their
// original sequence number will not be observed here.
if prevSeqNum := base.SeqNumFromTrailer(i.keyTrailer); (prevSeqNum == 0 || prevSeqNum <= key.SeqNum()) &&
i.key.Kind() != InternalKeyKindRangeDelete && key.Kind() != InternalKeyKindRangeDelete {
prevKey := i.key
prevKey.Trailer = i.keyTrailer
panic(errors.AssertionFailedf("pebble: invariant violation: %s and %s out of order", prevKey, key))
}
origSnapshotIdx := i.curSnapshotIdx
i.curSnapshotIdx, i.curSnapshotSeqNum = snapshotIndex(key.SeqNum(), i.snapshots)
switch key.Kind() {
case InternalKeyKindRangeDelete:
// Range tombstones need to be exposed by the compactionIter to the upper level
// `compaction` object, so return them regardless of whether they are in the same
// snapshot stripe.
if i.curSnapshotIdx == origSnapshotIdx {
return sameStripeNonSkippable
}
return newStripeSameKey
case InternalKeyKindRangeKeySet, InternalKeyKindRangeKeyUnset, InternalKeyKindRangeKeyDelete:
// Range keys are interleaved at the max sequence number for a given user
// key, so we should not see any more range keys in this stripe.
panic("unreachable")
case InternalKeyKindInvalid:
if i.curSnapshotIdx == origSnapshotIdx {
return sameStripeNonSkippable
}
return newStripeSameKey
case InternalKeyKindDelete, InternalKeyKindSet, InternalKeyKindMerge, InternalKeyKindSingleDelete,
InternalKeyKindSetWithDelete, InternalKeyKindDeleteSized:
// Fall through
default:
i.iterKey = nil
i.err = base.CorruptionErrorf("invalid internal key kind: %d", errors.Safe(i.iterKey.Kind()))
i.valid = false
return newStripeNewKey
}
if i.curSnapshotIdx == origSnapshotIdx {
return sameStripeSkippable
}
return newStripeSameKey
}
func (i *compactionIter) setNext() {
// Save the current key.
i.saveKey()
i.value = i.iterValue
i.valid = true
i.maybeZeroSeqnum(i.curSnapshotIdx)
// There are two cases where we can early return and skip the remaining
// records in the stripe:
// - If the DB does not SETWITHDEL.
// - If this key is already a SETWITHDEL.
if i.formatVersion < FormatSetWithDelete ||
i.iterKey.Kind() == InternalKeyKindSetWithDelete {
i.skip = true
return
}
// We are iterating forward. Save the current value.
i.valueBuf = append(i.valueBuf[:0], i.iterValue...)
i.value = i.valueBuf
// Else, we continue to loop through entries in the stripe looking for a
// DEL. Note that we may stop *before* encountering a DEL, if one exists.
for {
switch i.nextInStripe() {
case newStripeNewKey, newStripeSameKey:
i.pos = iterPosNext
return
case sameStripeNonSkippable:
i.pos = iterPosNext
// We iterated onto a key that we cannot skip. We can
// conservatively transform the original SET into a SETWITHDEL
// as an indication that there *may* still be a DEL/SINGLEDEL
// under this SET, even if we did not actually encounter one.
//
// This is safe to do, as:
//
// - in the case that there *is not* actually a DEL/SINGLEDEL
// under this entry, any SINGLEDEL above this now-transformed
// SETWITHDEL will become a DEL when the two encounter in a
// compaction. The DEL will eventually be elided in a
// subsequent compaction. The cost for ensuring correctness is
// that this entry is kept around for an additional compaction
// cycle(s).
//
// - in the case there *is* indeed a DEL/SINGLEDEL under us
// (but in a different stripe or sstable), then we will have
// already done the work to transform the SET into a
// SETWITHDEL, and we will skip any additional iteration when
// this entry is encountered again in a subsequent compaction.
//
// Ideally, this codepath would be smart enough to handle the
// case of SET <- RANGEDEL <- ... <- DEL/SINGLEDEL <- ....
// This requires preserving any RANGEDEL entries we encounter
// along the way, then emitting the original (possibly
// transformed) key, followed by the RANGEDELs. This requires
// a sizable refactoring of the existing code, as nextInStripe
// currently returns a sameStripeNonSkippable when it
// encounters a RANGEDEL.
// TODO(travers): optimize to handle the RANGEDEL case if it
// turns out to be a performance problem.
i.key.SetKind(InternalKeyKindSetWithDelete)
// By setting i.skip=true, we are saying that after the
// non-skippable key is emitted (which is likely a RANGEDEL),
// the remaining point keys that share the same user key as this
// saved key should be skipped.
i.skip = true
return
case sameStripeSkippable:
// We're still in the same stripe. If this is a
// DEL/SINGLEDEL/DELSIZED, we stop looking and emit a SETWITHDEL.
// Subsequent keys are eligible for skipping.
switch i.iterKey.Kind() {
case InternalKeyKindDelete, InternalKeyKindSingleDelete, InternalKeyKindDeleteSized:
i.key.SetKind(InternalKeyKindSetWithDelete)
i.skip = true
return
case InternalKeyKindSet, InternalKeyKindMerge, InternalKeyKindSetWithDelete:
// Do nothing
default:
i.err = base.CorruptionErrorf("invalid internal key kind: %d", errors.Safe(i.iterKey.Kind()))
i.valid = false
}
default:
panic("pebble: unexpected stripeChangeType: " + strconv.Itoa(int(i.iterStripeChange)))
}
}
}
func (i *compactionIter) mergeNext(valueMerger ValueMerger) stripeChangeType {
// Save the current key.
i.saveKey()
i.valid = true
// Loop looking for older values in the current snapshot stripe and merge
// them.
for {
if i.nextInStripe() != sameStripeSkippable {
i.pos = iterPosNext
return i.iterStripeChange
}
if i.err != nil {
panic(i.err)
}
key := i.iterKey
switch key.Kind() {
case InternalKeyKindDelete, InternalKeyKindSingleDelete, InternalKeyKindDeleteSized:
// We've hit a deletion tombstone. Return everything up to this point and
// then skip entries until the next snapshot stripe. We change the kind
// of the result key to a Set so that it shadows keys in lower
// levels. That is, MERGE+DEL -> SET.
// We do the same for SingleDelete since SingleDelete is only
// permitted (with deterministic behavior) for keys that have been
// set once since the last SingleDelete/Delete, so everything
// older is acceptable to shadow. Note that this is slightly
// different from singleDeleteNext() which implements stricter
// semantics in terms of applying the SingleDelete to the single
// next Set. But those stricter semantics are not observable to
// the end-user since Iterator interprets SingleDelete as Delete.
// We could do something more complicated here and consume only a
// single Set, and then merge in any following Sets, but that is
// complicated wrt code and unnecessary given the narrow permitted
// use of SingleDelete.
i.key.SetKind(InternalKeyKindSet)
i.skip = true
return sameStripeSkippable
case InternalKeyKindSet, InternalKeyKindSetWithDelete:
if i.rangeDelFrag.Covers(*key, i.curSnapshotSeqNum) == keyspan.CoversVisibly {
// We change the kind of the result key to a Set so that it shadows
// keys in lower levels. That is, MERGE+RANGEDEL -> SET. This isn't
// strictly necessary, but provides consistency with the behavior of
// MERGE+DEL.
i.key.SetKind(InternalKeyKindSet)
i.skip = true
return sameStripeSkippable
}
// We've hit a Set or SetWithDel value. Merge with the existing
// value and return. We change the kind of the resulting key to a
// Set so that it shadows keys in lower levels. That is:
// MERGE + (SET*) -> SET.
i.err = valueMerger.MergeOlder(i.iterValue)
if i.err != nil {
i.valid = false
return sameStripeSkippable
}
i.key.SetKind(InternalKeyKindSet)
i.skip = true
return sameStripeSkippable
case InternalKeyKindMerge:
if i.rangeDelFrag.Covers(*key, i.curSnapshotSeqNum) == keyspan.CoversVisibly {
// We change the kind of the result key to a Set so that it shadows
// keys in lower levels. That is, MERGE+RANGEDEL -> SET. This isn't
// strictly necessary, but provides consistency with the behavior of
// MERGE+DEL.
i.key.SetKind(InternalKeyKindSet)
i.skip = true
return sameStripeSkippable
}
// We've hit another Merge value. Merge with the existing value and
// continue looping.
i.err = valueMerger.MergeOlder(i.iterValue)
if i.err != nil {
i.valid = false
return sameStripeSkippable
}
default:
i.err = base.CorruptionErrorf("invalid internal key kind: %d", errors.Safe(i.iterKey.Kind()))
i.valid = false
return sameStripeSkippable
}
}
}
// singleDeleteNext processes a SingleDelete point tombstone. A SingleDelete, or
// SINGLEDEL, is unique in that it deletes exactly 1 internal key. It's a
// performance optimization when the client knows a user key has not been
// overwritten, allowing the elision of the tombstone earlier, avoiding write
// amplification.
//
// singleDeleteNext returns a boolean indicating whether or not the caller
// should yield the SingleDelete key to the consumer of the compactionIter. If
// singleDeleteNext returns false, the caller may consume/elide the
// SingleDelete.
func (i *compactionIter) singleDeleteNext() bool {
// Save the current key.
i.saveKey()
i.value = i.iterValue
i.valid = true
// Loop until finds a key to be passed to the next level.
for {
// If we find a key that can't be skipped, return true so that the
// caller yields the SingleDelete to the caller.
if i.nextInStripe() != sameStripeSkippable {
i.pos = iterPosNext
return i.err == nil
}
if i.err != nil {
panic(i.err)
}
key := i.iterKey
switch key.Kind() {
case InternalKeyKindDelete, InternalKeyKindMerge, InternalKeyKindSetWithDelete, InternalKeyKindDeleteSized:
// We've hit a Delete, DeleteSized, Merge, SetWithDelete, transform
// the SingleDelete into a full Delete.
i.key.SetKind(InternalKeyKindDelete)
i.skip = true
return true
case InternalKeyKindSet:
// This SingleDelete deletes the Set, and we can now elide the
// SingleDel as well. We advance past the Set and return false to
// indicate to the main compaction loop that we should NOT yield the
// current SingleDel key to the compaction loop.
i.nextInStripe()
// TODO(jackson): We could assert that nextInStripe either a)
// stepped onto a new key, or b) stepped on to a Delete, DeleteSized
// or SingleDel key. This would detect improper uses of SingleDel,
// but only when all three internal keys meet in the same compaction
// which is not likely.
i.valid = false
return false
case InternalKeyKindSingleDelete:
// Two single deletes met in a compaction. With proper deterministic
// use of SingleDelete, this should never happen. The expectation is
// that there's exactly 1 set beneath a single delete. Currently, we
// opt to skip it.
// TODO(jackson): Should we make this an error? This would also
// allow us to simplify the code a bit by removing the for loop.
continue
default:
i.err = base.CorruptionErrorf("invalid internal key kind: %d", errors.Safe(i.iterKey.Kind()))
i.valid = false
return false
}
}
}
// deleteSizedNext processes a DELSIZED point tombstone. Unlike ordinary DELs,
// these tombstones carry a value that's a varint indicating the size of the
// entry (len(key)+len(value)) that the tombstone is expected to delete.
//
// When a deleteSizedNext is encountered, we skip ahead to see which keys, if
// any, are elided as a result of the tombstone.
func (i *compactionIter) deleteSizedNext() (*base.InternalKey, []byte) {
i.saveKey()
i.valid = true
i.skip = true
// The DELSIZED tombstone may have no value at all. This happens when the
// tombstone has already deleted the key that the user originally predicted.
// In this case, we still peek forward in case there's another DELSIZED key
// with a lower sequence number, in which case we'll adopt its value.
if len(i.iterValue) == 0 {
i.value = i.valueBuf[:0]
} else {
i.valueBuf = append(i.valueBuf[:0], i.iterValue...)
i.value = i.valueBuf
}
// Loop through all the keys within this stripe that are skippable.
i.pos = iterPosNext
for i.nextInStripe() == sameStripeSkippable {
if i.err != nil {
panic(i.err)
}
switch i.iterKey.Kind() {
case InternalKeyKindDelete, InternalKeyKindDeleteSized, InternalKeyKindSingleDelete:
// We encountered a tombstone (DEL, or DELSIZED) that's deleted by