compaction.go

// Copyright 2013 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"
	"context"
	"fmt"
	"math"
	"runtime/pprof"
	"slices"
	"sync"
	"sync/atomic"
	"time"

	"github.com/cockroachdb/errors"
	"github.com/cockroachdb/pebble/internal/base"
	"github.com/cockroachdb/pebble/internal/compact"
	"github.com/cockroachdb/pebble/internal/keyspan"
	"github.com/cockroachdb/pebble/internal/keyspan/keyspanimpl"
	"github.com/cockroachdb/pebble/internal/manifest"
	"github.com/cockroachdb/pebble/internal/sstableinternal"
	"github.com/cockroachdb/pebble/objstorage"
	"github.com/cockroachdb/pebble/objstorage/objstorageprovider/objiotracing"
	"github.com/cockroachdb/pebble/objstorage/remote"
	"github.com/cockroachdb/pebble/sstable"
	"github.com/cockroachdb/pebble/vfs"
)

var errEmptyTable = errors.New("pebble: empty table")

// ErrCancelledCompaction is returned if a compaction is cancelled by a
// concurrent excise or ingest-split operation.
var ErrCancelledCompaction = errors.New("pebble: compaction cancelled by a concurrent operation, will retry compaction")

var compactLabels = pprof.Labels("pebble", "compact")
var flushLabels = pprof.Labels("pebble", "flush")
var gcLabels = pprof.Labels("pebble", "gc")

// expandedCompactionByteSizeLimit is the maximum number of bytes in all
// compacted files. We avoid expanding the lower level file set of a compaction
// if it would make the total compaction cover more than this many bytes.
func expandedCompactionByteSizeLimit(opts *Options, level int, availBytes uint64) uint64 {
	v := uint64(25 * opts.Level(level).TargetFileSize)

	// Never expand a compaction beyond half the available capacity, divided
	// by the maximum number of concurrent compactions. Each of the concurrent
	// compactions may expand up to this limit, so this attempts to limit
	// compactions to half of available disk space. Note that this will not
	// prevent compaction picking from pursuing compactions that are larger
	// than this threshold before expansion.
	diskMax := (availBytes / 2) / uint64(opts.MaxConcurrentCompactions())
	if v > diskMax {
		v = diskMax
	}
	return v
}

// maxGrandparentOverlapBytes is the maximum bytes of overlap with level+1
// before we stop building a single file in a level-1 to level compaction.
func maxGrandparentOverlapBytes(opts *Options, level int) uint64 {
	return uint64(10 * opts.Level(level).TargetFileSize)
}

// maxReadCompactionBytes is used to prevent read compactions which
// are too wide.
func maxReadCompactionBytes(opts *Options, level int) uint64 {
	return uint64(10 * opts.Level(level).TargetFileSize)
}

// noCloseIter wraps around a FragmentIterator, intercepting and eliding
// calls to Close. It is used during compaction to ensure that rangeDelIters
// are not closed prematurely.
type noCloseIter struct {
	keyspan.FragmentIterator
}

func (i *noCloseIter) Close() {}

type compactionLevel struct {
	level int
	files manifest.LevelSlice
	// l0SublevelInfo contains information about L0 sublevels being compacted.
	// It's only set for the start level of a compaction starting out of L0 and
	// is nil for all other compactions.
	l0SublevelInfo []sublevelInfo
}

func (cl compactionLevel) Clone() compactionLevel {
	newCL := compactionLevel{
		level: cl.level,
		files: cl.files,
	}
	return newCL
}
func (cl compactionLevel) String() string {
	return fmt.Sprintf(`Level %d, Files %s`, cl.level, cl.files)
}

// compactionWritable is a objstorage.Writable wrapper that, on every write,
// updates a metric in `versions` on bytes written by in-progress compactions so
// far. It also increments a per-compaction `written` int.
type compactionWritable struct {
	objstorage.Writable

	versions *versionSet
	written  *int64
}

// Write is part of the objstorage.Writable interface.
func (c *compactionWritable) Write(p []byte) error {
	if err := c.Writable.Write(p); err != nil {
		return err
	}

	*c.written += int64(len(p))
	c.versions.incrementCompactionBytes(int64(len(p)))
	return nil
}

type compactionKind int

// The ordering of these compaction kinds is important - it is used to
// determine the priority of a pickedCompaction when multiple DBs are waiting
// to schedule a compaction. A compactionKind is considered as being strictly
// higher priority than the one below it.
//
// The ordering of these compactionKinds should mirror the order in which
// compaction types are picked in compactionPicker.
const (
	compactionKindFlush compactionKind = iota
	// compactionKindDeleteOnly denotes a compaction that only deletes input
	// files. It can occur when wide range tombstones completely contain sstables.
	compactionKindDeleteOnly
	// compactionKindMove denotes a move compaction where the input file is
	// retained and linked in a new level without being obsoleted.
	compactionKindMove
	compactionKindDefault
	// compactionKindCopy denotes a copy compaction where the input file is
	// copied byte-by-byte into a new file with a new FileNum in the output level.
	compactionKindCopy
	compactionKindTombstoneDensity
	compactionKindElisionOnly
	compactionKindRead
	compactionKindRewrite
	compactionKindIngestedFlushable
)

func (k compactionKind) String() string {
	switch k {
	case compactionKindDefault:
		return "default"
	case compactionKindFlush:
		return "flush"
	case compactionKindMove:
		return "move"
	case compactionKindDeleteOnly:
		return "delete-only"
	case compactionKindElisionOnly:
		return "elision-only"
	case compactionKindRead:
		return "read"
	case compactionKindTombstoneDensity:
		return "tombstone-density"
	case compactionKindRewrite:
		return "rewrite"
	case compactionKindIngestedFlushable:
		return "ingested-flushable"
	case compactionKindCopy:
		return "copy"
	}
	return "?"
}

// compaction is a table compaction from one level to the next, starting from a
// given version.
type compaction struct {
	// cancel is a bool that can be used by other goroutines to signal a compaction
	// to cancel, such as if a conflicting excise operation raced it to manifest
	// application. Only holders of the manifest lock will write to this atomic.
	cancel atomic.Bool

	kind compactionKind
	// isDownload is true if this compaction was started as part of a Download
	// operation. In this case kind is compactionKindCopy or
	// compactionKindRewrite.
	isDownload bool

	cmp       Compare
	equal     Equal
	comparer  *base.Comparer
	formatKey base.FormatKey
	logger    Logger
	version   *version
	stats     base.InternalIteratorStats
	beganAt   time.Time
	// versionEditApplied is set to true when a compaction has completed and the
	// resulting version has been installed (if successful), but the compaction
	// goroutine is still cleaning up (eg, deleting obsolete files).
	versionEditApplied bool
	bufferPool         sstable.BufferPool

	// startLevel is the level that is being compacted. Inputs from startLevel
	// and outputLevel will be merged to produce a set of outputLevel files.
	startLevel *compactionLevel

	// outputLevel is the level that files are being produced in. outputLevel is
	// equal to startLevel+1 except when:
	//    - if startLevel is 0, the output level equals compactionPicker.baseLevel().
	//    - in multilevel compaction, the output level is the lowest level involved in
	//      the compaction
	// A compaction's outputLevel is nil for delete-only compactions.
	outputLevel *compactionLevel

	// extraLevels point to additional levels in between the input and output
	// levels that get compacted in multilevel compactions
	extraLevels []*compactionLevel

	inputs []compactionLevel

	// maxOutputFileSize is the maximum size of an individual table created
	// during compaction.
	maxOutputFileSize uint64
	// maxOverlapBytes is the maximum number of bytes of overlap allowed for a
	// single output table with the tables in the grandparent level.
	maxOverlapBytes uint64

	// flushing contains the flushables (aka memtables) that are being flushed.
	flushing flushableList
	// bytesWritten contains the number of bytes that have been written to outputs.
	bytesWritten int64

	// The boundaries of the input data.
	smallest InternalKey
	largest  InternalKey

	// A list of fragment iterators to close when the compaction finishes. Used by
	// input iteration to keep rangeDelIters open for the lifetime of the
	// compaction, and only close them when the compaction finishes.
	closers []*noCloseIter

	// grandparents are the tables in level+2 that overlap with the files being
	// compacted. Used to determine output table boundaries. Do not assume that the actual files
	// in the grandparent when this compaction finishes will be the same.
	grandparents manifest.LevelSlice

	// Boundaries at which flushes to L0 should be split. Determined by
	// L0Sublevels. If nil, flushes aren't split.
	l0Limits [][]byte

	delElision      compact.TombstoneElision
	rangeKeyElision compact.TombstoneElision

	// allowedZeroSeqNum is true if seqnums can be zeroed if there are no
	// snapshots requiring them to be kept. This determination is made by
	// looking for an sstable which overlaps the bounds of the compaction at a
	// lower level in the LSM during runCompaction.
	allowedZeroSeqNum bool

	metrics map[int]*LevelMetrics

	pickerMetrics compactionPickerMetrics
}

// inputLargestSeqNumAbsolute returns the maximum LargestSeqNumAbsolute of any
// input sstables.
func (c *compaction) inputLargestSeqNumAbsolute() base.SeqNum {
	var seqNum base.SeqNum
	for _, cl := range c.inputs {
		cl.files.Each(func(m *manifest.FileMetadata) {
			seqNum = max(seqNum, m.LargestSeqNumAbsolute)
		})
	}
	return seqNum
}

func (c *compaction) makeInfo(jobID JobID) CompactionInfo {
	info := CompactionInfo{
		JobID:       int(jobID),
		Reason:      c.kind.String(),
		Input:       make([]LevelInfo, 0, len(c.inputs)),
		Annotations: []string{},
	}
	if c.isDownload {
		info.Reason = "download," + info.Reason
	}
	for _, cl := range c.inputs {
		inputInfo := LevelInfo{Level: cl.level, Tables: nil}
		iter := cl.files.Iter()
		for m := iter.First(); m != nil; m = iter.Next() {
			inputInfo.Tables = append(inputInfo.Tables, m.TableInfo())
		}
		info.Input = append(info.Input, inputInfo)
	}
	if c.outputLevel != nil {
		info.Output.Level = c.outputLevel.level

		// If there are no inputs from the output level (eg, a move
		// compaction), add an empty LevelInfo to info.Input.
		if len(c.inputs) > 0 && c.inputs[len(c.inputs)-1].level != c.outputLevel.level {
			info.Input = append(info.Input, LevelInfo{Level: c.outputLevel.level})
		}
	} else {
		// For a delete-only compaction, set the output level to L6. The
		// output level is not meaningful here, but complicating the
		// info.Output interface with a pointer doesn't seem worth the
		// semantic distinction.
		info.Output.Level = numLevels - 1
	}

	for i, score := range c.pickerMetrics.scores {
		info.Input[i].Score = score
	}
	info.SingleLevelOverlappingRatio = c.pickerMetrics.singleLevelOverlappingRatio
	info.MultiLevelOverlappingRatio = c.pickerMetrics.multiLevelOverlappingRatio
	if len(info.Input) > 2 {
		info.Annotations = append(info.Annotations, "multilevel")
	}
	return info
}

func (c *compaction) userKeyBounds() base.UserKeyBounds {
	return base.UserKeyBoundsFromInternal(c.smallest, c.largest)
}

func newCompaction(
	pc *pickedCompaction, opts *Options, beganAt time.Time, provider objstorage.Provider,
) *compaction {
	c := &compaction{
		kind:              compactionKindDefault,
		cmp:               pc.cmp,
		equal:             opts.Comparer.Equal,
		comparer:          opts.Comparer,
		formatKey:         opts.Comparer.FormatKey,
		inputs:            pc.inputs,
		smallest:          pc.smallest,
		largest:           pc.largest,
		logger:            opts.Logger,
		version:           pc.version,
		beganAt:           beganAt,
		maxOutputFileSize: pc.maxOutputFileSize,
		maxOverlapBytes:   pc.maxOverlapBytes,
		pickerMetrics:     pc.pickerMetrics,
	}
	c.startLevel = &c.inputs[0]
	if pc.startLevel.l0SublevelInfo != nil {
		c.startLevel.l0SublevelInfo = pc.startLevel.l0SublevelInfo
	}
	c.outputLevel = &c.inputs[1]

	if len(pc.extraLevels) > 0 {
		c.extraLevels = pc.extraLevels
		c.outputLevel = &c.inputs[len(c.inputs)-1]
	}
	// Compute the set of outputLevel+1 files that overlap this compaction (these
	// are the grandparent sstables).
	if c.outputLevel.level+1 < numLevels {
		c.grandparents = c.version.Overlaps(c.outputLevel.level+1, c.userKeyBounds())
	}
	c.delElision, c.rangeKeyElision = compact.SetupTombstoneElision(
		c.cmp, c.version, c.outputLevel.level, base.UserKeyBoundsFromInternal(c.smallest, c.largest),
	)
	c.kind = pc.kind

	if c.kind == compactionKindDefault && c.outputLevel.files.Empty() && !c.hasExtraLevelData() &&
		c.startLevel.files.Len() == 1 && c.grandparents.SizeSum() <= c.maxOverlapBytes {
		// This compaction can be converted into a move or copy from one level
		// to the next. We avoid such a move if there is lots of overlapping
		// grandparent data. Otherwise, the move could create a parent file
		// that will require a very expensive merge later on.
		iter := c.startLevel.files.Iter()
		meta := iter.First()
		isRemote := false
		// We should always be passed a provider, except in some unit tests.
		if provider != nil {
			isRemote = !objstorage.IsLocalTable(provider, meta.FileBacking.DiskFileNum)
		}
		// Avoid a trivial move or copy if all of these are true, as rewriting a
		// new file is better:
		//
		// 1) The source file is a virtual sstable
		// 2) The existing file `meta` is on non-remote storage
		// 3) The output level prefers shared storage
		mustCopy := !isRemote && remote.ShouldCreateShared(opts.Experimental.CreateOnShared, c.outputLevel.level)
		if mustCopy {
			// If the source is virtual, it's best to just rewrite the file as all
			// conditions in the above comment are met.
			if !meta.Virtual {
				c.kind = compactionKindCopy
			}
		} else {
			c.kind = compactionKindMove
		}
	}
	return c
}

func newDeleteOnlyCompaction(
	opts *Options, cur *version, inputs []compactionLevel, beganAt time.Time,
) *compaction {
	c := &compaction{
		kind:      compactionKindDeleteOnly,
		cmp:       opts.Comparer.Compare,
		equal:     opts.Comparer.Equal,
		comparer:  opts.Comparer,
		formatKey: opts.Comparer.FormatKey,
		logger:    opts.Logger,
		version:   cur,
		beganAt:   beganAt,
		inputs:    inputs,
	}

	// Set c.smallest, c.largest.
	files := make([]manifest.LevelIterator, 0, len(inputs))
	for _, in := range inputs {
		files = append(files, in.files.Iter())
	}
	c.smallest, c.largest = manifest.KeyRange(opts.Comparer.Compare, files...)
	return c
}

func adjustGrandparentOverlapBytesForFlush(c *compaction, flushingBytes uint64) {
	// Heuristic to place a lower bound on compaction output file size
	// caused by Lbase. Prior to this heuristic we have observed an L0 in
	// production with 310K files of which 290K files were < 10KB in size.
	// Our hypothesis is that it was caused by L1 having 2600 files and
	// ~10GB, such that each flush got split into many tiny files due to
	// overlapping with most of the files in Lbase.
	//
	// The computation below is general in that it accounts
	// for flushing different volumes of data (e.g. we may be flushing
	// many memtables). For illustration, we consider the typical
	// example of flushing a 64MB memtable. So 12.8MB output,
	// based on the compression guess below. If the compressed bytes
	// guess is an over-estimate we will end up with smaller files,
	// and if an under-estimate we will end up with larger files.
	// With a 2MB target file size, 7 files. We are willing to accept
	// 4x the number of files, if it results in better write amplification
	// when later compacting to Lbase, i.e., ~450KB files (target file
	// size / 4).
	//
	// Note that this is a pessimistic heuristic in that
	// fileCountUpperBoundDueToGrandparents could be far from the actual
	// number of files produced due to the grandparent limits. For
	// example, in the extreme, consider a flush that overlaps with 1000
	// files in Lbase f0...f999, and the initially calculated value of
	// maxOverlapBytes will cause splits at f10, f20,..., f990, which
	// means an upper bound file count of 100 files. Say the input bytes
	// in the flush are such that acceptableFileCount=10. We will fatten
	// up maxOverlapBytes by 10x to ensure that the upper bound file count
	// drops to 10. However, it is possible that in practice, even without
	// this change, we would have produced no more than 10 files, and that
	// this change makes the files unnecessarily wide. Say the input bytes
	// are distributed such that 10% are in f0...f9, 10% in f10...f19, ...
	// 10% in f80...f89 and 10% in f990...f999. The original value of
	// maxOverlapBytes would have actually produced only 10 sstables. But
	// by increasing maxOverlapBytes by 10x, we may produce 1 sstable that
	// spans f0...f89, i.e., a much wider sstable than necessary.
	//
	// We could produce a tighter estimate of
	// fileCountUpperBoundDueToGrandparents if we had knowledge of the key
	// distribution of the flush. The 4x multiplier mentioned earlier is
	// a way to try to compensate for this pessimism.
	//
	// TODO(sumeer): we don't have compression info for the data being
	// flushed, but it is likely that existing files that overlap with
	// this flush in Lbase are representative wrt compression ratio. We
	// could store the uncompressed size in FileMetadata and estimate
	// the compression ratio.
	const approxCompressionRatio = 0.2
	approxOutputBytes := approxCompressionRatio * float64(flushingBytes)
	approxNumFilesBasedOnTargetSize :=
		int(math.Ceil(approxOutputBytes / float64(c.maxOutputFileSize)))
	acceptableFileCount := float64(4 * approxNumFilesBasedOnTargetSize)
	// The byte calculation is linear in numGrandparentFiles, but we will
	// incur this linear cost in compact.Runner.TableSplitLimit() too, so we are
	// also willing to pay it now. We could approximate this cheaply by using the
	// mean file size of Lbase.
	grandparentFileBytes := c.grandparents.SizeSum()
	fileCountUpperBoundDueToGrandparents :=
		float64(grandparentFileBytes) / float64(c.maxOverlapBytes)
	if fileCountUpperBoundDueToGrandparents > acceptableFileCount {
		c.maxOverlapBytes = uint64(
			float64(c.maxOverlapBytes) *
				(fileCountUpperBoundDueToGrandparents / acceptableFileCount))
	}
}

func newFlush(
	opts *Options, cur *version, baseLevel int, flushing flushableList, beganAt time.Time,
) (*compaction, error) {
	c := &compaction{
		kind:              compactionKindFlush,
		cmp:               opts.Comparer.Compare,
		equal:             opts.Comparer.Equal,
		comparer:          opts.Comparer,
		formatKey:         opts.Comparer.FormatKey,
		logger:            opts.Logger,
		version:           cur,
		beganAt:           beganAt,
		inputs:            []compactionLevel{{level: -1}, {level: 0}},
		maxOutputFileSize: math.MaxUint64,
		maxOverlapBytes:   math.MaxUint64,
		flushing:          flushing,
	}
	c.startLevel = &c.inputs[0]
	c.outputLevel = &c.inputs[1]

	if len(flushing) > 0 {
		if _, ok := flushing[0].flushable.(*ingestedFlushable); ok {
			if len(flushing) != 1 {
				panic("pebble: ingestedFlushable must be flushed one at a time.")
			}
			c.kind = compactionKindIngestedFlushable
			return c, nil
		}
	}

	// Make sure there's no ingestedFlushable after the first flushable in the
	// list.
	for _, f := range flushing {
		if _, ok := f.flushable.(*ingestedFlushable); ok {
			panic("pebble: flushing shouldn't contain ingestedFlushable flushable")
		}
	}

	if cur.L0Sublevels != nil {
		c.l0Limits = cur.L0Sublevels.FlushSplitKeys()
	}

	smallestSet, largestSet := false, false
	updatePointBounds := func(iter internalIterator) {
		if kv := iter.First(); kv != nil {
			if !smallestSet ||
				base.InternalCompare(c.cmp, c.smallest, kv.K) > 0 {
				smallestSet = true
				c.smallest = kv.K.Clone()
			}
		}
		if kv := iter.Last(); kv != nil {
			if !largestSet ||
				base.InternalCompare(c.cmp, c.largest, kv.K) < 0 {
				largestSet = true
				c.largest = kv.K.Clone()
			}
		}
	}

	updateRangeBounds := func(iter keyspan.FragmentIterator) error {
		// File bounds require s != nil && !s.Empty(). We only need to check for
		// s != nil here, as the memtable's FragmentIterator would never surface
		// empty spans.
		if s, err := iter.First(); err != nil {
			return err
		} else if s != nil {
			if key := s.SmallestKey(); !smallestSet ||
				base.InternalCompare(c.cmp, c.smallest, key) > 0 {
				smallestSet = true
				c.smallest = key.Clone()
			}
		}
		if s, err := iter.Last(); err != nil {
			return err
		} else if s != nil {
			if key := s.LargestKey(); !largestSet ||
				base.InternalCompare(c.cmp, c.largest, key) < 0 {
				largestSet = true
				c.largest = key.Clone()
			}
		}
		return nil
	}

	var flushingBytes uint64
	for i := range flushing {
		f := flushing[i]
		updatePointBounds(f.newIter(nil))
		if rangeDelIter := f.newRangeDelIter(nil); rangeDelIter != nil {
			if err := updateRangeBounds(rangeDelIter); err != nil {
				return nil, err
			}
		}
		if rangeKeyIter := f.newRangeKeyIter(nil); rangeKeyIter != nil {
			if err := updateRangeBounds(rangeKeyIter); err != nil {
				return nil, err
			}
		}
		flushingBytes += f.inuseBytes()
	}

	if opts.FlushSplitBytes > 0 {
		c.maxOutputFileSize = uint64(opts.Level(0).TargetFileSize)
		c.maxOverlapBytes = maxGrandparentOverlapBytes(opts, 0)
		c.grandparents = c.version.Overlaps(baseLevel, c.userKeyBounds())
		adjustGrandparentOverlapBytesForFlush(c, flushingBytes)
	}

	// We don't elide tombstones for flushes.
	c.delElision, c.rangeKeyElision = compact.NoTombstoneElision(), compact.NoTombstoneElision()
	return c, nil
}

func (c *compaction) hasExtraLevelData() bool {
	if len(c.extraLevels) == 0 {
		// not a multi level compaction
		return false
	} else if c.extraLevels[0].files.Empty() {
		// a multi level compaction without data in the intermediate input level;
		// e.g. for a multi level compaction with levels 4,5, and 6, this could
		// occur if there is no files to compact in 5, or in 5 and 6 (i.e. a move).
		return false
	}
	return true
}

// errorOnUserKeyOverlap returns an error if the last two written sstables in
// this compaction have revisions of the same user key present in both sstables,
// when it shouldn't (eg. when splitting flushes).
func (c *compaction) errorOnUserKeyOverlap(ve *versionEdit) error {
	if n := len(ve.NewFiles); n > 1 {
		meta := ve.NewFiles[n-1].Meta
		prevMeta := ve.NewFiles[n-2].Meta
		if !prevMeta.Largest.IsExclusiveSentinel() &&
			c.cmp(prevMeta.Largest.UserKey, meta.Smallest.UserKey) >= 0 {
			return errors.Errorf("pebble: compaction split user key across two sstables: %s in %s and %s",
				prevMeta.Largest.Pretty(c.formatKey),
				prevMeta.FileNum,
				meta.FileNum)
		}
	}
	return nil
}

// allowZeroSeqNum returns true if seqnum's can be zeroed if there are no
// snapshots requiring them to be kept. It performs this determination by
// looking at the TombstoneElision values which are set up based on sstables
// which overlap the bounds of the compaction at a lower level in the LSM.
func (c *compaction) allowZeroSeqNum() bool {
	// TODO(peter): we disable zeroing of seqnums during flushing to match
	// RocksDB behavior and to avoid generating overlapping sstables during
	// DB.replayWAL. When replaying WAL files at startup, we flush after each
	// WAL is replayed building up a single version edit that is
	// applied. Because we don't apply the version edit after each flush, this
	// code doesn't know that L0 contains files and zeroing of seqnums should
	// be disabled. That is fixable, but it seems safer to just match the
	// RocksDB behavior for now.
	return len(c.flushing) == 0 && c.delElision.ElidesEverything() && c.rangeKeyElision.ElidesEverything()
}

// newInputIters returns an iterator over all the input tables in a compaction.
func (c *compaction) newInputIters(
	newIters tableNewIters, newRangeKeyIter keyspanimpl.TableNewSpanIter,
) (
	pointIter internalIterator,
	rangeDelIter, rangeKeyIter keyspan.FragmentIterator,
	retErr error,
) {
	// Validate the ordering of compaction input files for defense in depth.
	if len(c.flushing) == 0 {
		if c.startLevel.level >= 0 {
			err := manifest.CheckOrdering(c.cmp, c.formatKey,
				manifest.Level(c.startLevel.level), c.startLevel.files.Iter())
			if err != nil {
				return nil, nil, nil, err
			}
		}
		err := manifest.CheckOrdering(c.cmp, c.formatKey,
			manifest.Level(c.outputLevel.level), c.outputLevel.files.Iter())
		if err != nil {
			return nil, nil, nil, err
		}
		if c.startLevel.level == 0 {
			if c.startLevel.l0SublevelInfo == nil {
				panic("l0SublevelInfo not created for compaction out of L0")
			}
			for _, info := range c.startLevel.l0SublevelInfo {
				err := manifest.CheckOrdering(c.cmp, c.formatKey,
					info.sublevel, info.Iter())
				if err != nil {
					return nil, nil, nil, err
				}
			}
		}
		if len(c.extraLevels) > 0 {
			if len(c.extraLevels) > 1 {
				panic("n>2 multi level compaction not implemented yet")
			}
			interLevel := c.extraLevels[0]
			err := manifest.CheckOrdering(c.cmp, c.formatKey,
				manifest.Level(interLevel.level), interLevel.files.Iter())
			if err != nil {
				return nil, nil, nil, err
			}
		}
	}

	// There are three classes of keys that a compaction needs to process: point
	// keys, range deletion tombstones and range keys. Collect all iterators for
	// all these classes of keys from all the levels. We'll aggregate them
	// together farther below.
	//
	// numInputLevels is an approximation of the number of iterator levels. Due
	// to idiosyncrasies in iterator construction, we may (rarely) exceed this
	// initial capacity.
	numInputLevels := max(len(c.flushing), len(c.inputs))
	iters := make([]internalIterator, 0, numInputLevels)
	rangeDelIters := make([]keyspan.FragmentIterator, 0, numInputLevels)
	rangeKeyIters := make([]keyspan.FragmentIterator, 0, numInputLevels)

	// If construction of the iterator inputs fails, ensure that we close all
	// the consitutent iterators.
	defer func() {
		if retErr != nil {
			for _, iter := range iters {
				if iter != nil {
					iter.Close()
				}
			}
			for _, rangeDelIter := range rangeDelIters {
				rangeDelIter.Close()
			}
		}
	}()
	iterOpts := IterOptions{
		CategoryAndQoS: sstable.CategoryAndQoS{
			Category: "pebble-compaction",
			QoSLevel: sstable.NonLatencySensitiveQoSLevel,
		},
		logger: c.logger,
	}

	// Populate iters, rangeDelIters and rangeKeyIters with the appropriate
	// constituent iterators. This depends on whether this is a flush or a
	// compaction.
	if len(c.flushing) != 0 {
		// If flushing, we need to build the input iterators over the memtables
		// stored in c.flushing.
		for i := range c.flushing {
			f := c.flushing[i]
			iters = append(iters, f.newFlushIter(nil))
			rangeDelIter := f.newRangeDelIter(nil)
			if rangeDelIter != nil {
				rangeDelIters = append(rangeDelIters, rangeDelIter)
			}
			if rangeKeyIter := f.newRangeKeyIter(nil); rangeKeyIter != nil {
				rangeKeyIters = append(rangeKeyIters, rangeKeyIter)
			}
		}
	} else {
		addItersForLevel := func(level *compactionLevel, l manifest.Layer) error {
			// Add a *levelIter for point iterators. Because we don't call
			// initRangeDel, the levelIter will close and forget the range
			// deletion iterator when it steps on to a new file. Surfacing range
			// deletions to compactions are handled below.
			iters = append(iters, newLevelIter(context.Background(),
				iterOpts, c.comparer, newIters, level.files.Iter(), l, internalIterOpts{
					compaction: true,
					bufferPool: &c.bufferPool,
				}))
			// TODO(jackson): Use keyspanimpl.LevelIter to avoid loading all the range
			// deletions into memory upfront. (See #2015, which reverted this.) There
			// will be no user keys that are split between sstables within a level in
			// Cockroach 23.1, which unblocks this optimization.

			// Add the range deletion iterator for each file as an independent level
			// in mergingIter, as opposed to making a levelIter out of those. This
			// is safer as levelIter expects all keys coming from underlying
			// iterators to be in order. Due to compaction / tombstone writing
			// logic in finishOutput(), it is possible for range tombstones to not
			// be strictly ordered across all files in one level.
			//
			// Consider this example from the metamorphic tests (also repeated in
			// finishOutput()), consisting of three L3 files with their bounds
			// specified in square brackets next to the file name:
			//
			// ./000240.sst   [tmgc#391,MERGE-tmgc#391,MERGE]
			// tmgc#391,MERGE [786e627a]
			// tmgc-udkatvs#331,RANGEDEL
			//
			// ./000241.sst   [tmgc#384,MERGE-tmgc#384,MERGE]
			// tmgc#384,MERGE [666c7070]
			// tmgc-tvsalezade#383,RANGEDEL
			// tmgc-tvsalezade#331,RANGEDEL
			//
			// ./000242.sst   [tmgc#383,RANGEDEL-tvsalezade#72057594037927935,RANGEDEL]
			// tmgc-tvsalezade#383,RANGEDEL
			// tmgc#375,SET [72646c78766965616c72776865676e79]
			// tmgc-tvsalezade#356,RANGEDEL
			//
			// Here, the range tombstone in 000240.sst falls "after" one in
			// 000241.sst, despite 000240.sst being ordered "before" 000241.sst for
			// levelIter's purposes. While each file is still consistent before its
			// bounds, it's safer to have all rangedel iterators be visible to
			// mergingIter.
			iter := level.files.Iter()
			for f := iter.First(); f != nil; f = iter.Next() {
				rangeDelIter, err := c.newRangeDelIter(newIters, iter.Take(), iterOpts, l)
				if err != nil {
					// The error will already be annotated with the BackingFileNum, so
					// we annotate it with the FileNum.
					return errors.Wrapf(err, "pebble: could not open table %s", errors.Safe(f.FileNum))
				}
				if rangeDelIter == nil {
					continue
				}
				rangeDelIters = append(rangeDelIters, rangeDelIter)
				c.closers = append(c.closers, rangeDelIter)
			}

			// Check if this level has any range keys.
			hasRangeKeys := false
			for f := iter.First(); f != nil; f = iter.Next() {
				if f.HasRangeKeys {
					hasRangeKeys = true
					break
				}
			}
			if hasRangeKeys {
				newRangeKeyIterWrapper := func(ctx context.Context, file *manifest.FileMetadata, iterOptions keyspan.SpanIterOptions) (keyspan.FragmentIterator, error) {
					rangeKeyIter, err := newRangeKeyIter(ctx, file, iterOptions)
					if err != nil {
						return nil, err
					} else if rangeKeyIter == nil {
						return emptyKeyspanIter, nil
					}
					// Ensure that the range key iter is not closed until the compaction is
					// finished. This is necessary because range key processing
					// requires the range keys to be held in memory for up to the
					// lifetime of the compaction.
					noCloseIter := &noCloseIter{rangeKeyIter}
					c.closers = append(c.closers, noCloseIter)

					// We do not need to truncate range keys to sstable boundaries, or
					// only read within the file's atomic compaction units, unlike with
					// range tombstones. This is because range keys were added after we
					// stopped splitting user keys across sstables, so all the range keys
					// in this sstable must wholly lie within the file's bounds.
					return noCloseIter, err
				}
				li := keyspanimpl.NewLevelIter(
					context.Background(), keyspan.SpanIterOptions{}, c.cmp,
					newRangeKeyIterWrapper, level.files.Iter(), l, manifest.KeyTypeRange,
				)
				rangeKeyIters = append(rangeKeyIters, li)
			}
			return nil
		}

		for i := range c.inputs {
			// If the level is annotated with l0SublevelInfo, expand it into one
			// level per sublevel.
			// TODO(jackson): Perform this expansion even earlier when we pick the
			// compaction?
			if len(c.inputs[i].l0SublevelInfo) > 0 {
				for _, info := range c.startLevel.l0SublevelInfo {
					sublevelCompactionLevel := &compactionLevel{0, info.LevelSlice, nil}
					if err := addItersForLevel(sublevelCompactionLevel, info.sublevel); err != nil {
						return nil, nil, nil, err
					}
				}
				continue
			}
			if err := addItersForLevel(&c.inputs[i], manifest.Level(c.inputs[i].level)); err != nil {
				return nil, nil, nil, err
			}
		}
	}

	// If there's only one constituent point iterator, we can avoid the overhead
	// of a *mergingIter. This is possible, for example, when performing a flush
	// of a single memtable. Otherwise, combine all the iterators into a merging
	// iter.
	pointIter = iters[0]
	if len(iters) > 1 {
		pointIter = newMergingIter(c.logger, &c.stats, c.cmp, nil, iters...)
	}

	// In normal operation, levelIter iterates over the point operations in a
	// level, and initializes a rangeDelIter pointer for the range deletions in
	// each table. During compaction, we want to iterate over the merged view of
	// point operations and range deletions. In order to do this we create one
	// levelIter per level to iterate over the point operations, and collect up
	// all the range deletion files.
	//
	// The range deletion levels are combined with a keyspanimpl.MergingIter. The
	// resulting merged rangedel iterator is then included using an
	// InterleavingIter.
	// TODO(jackson): Consider using a defragmenting iterator to stitch together
	// logical range deletions that were fragmented due to previous file
	// boundaries.
	if len(rangeDelIters) > 0 {
		mi := &keyspanimpl.MergingIter{}
		mi.Init(c.comparer, keyspan.NoopTransform, new(keyspanimpl.MergingBuffers), rangeDelIters...)
		rangeDelIter = mi
	}

	// If there are range key iterators, we need to combine them using
	// keyspanimpl.MergingIter, and then interleave them among the points.
	if len(rangeKeyIters) > 0 {
		mi := &keyspanimpl.MergingIter{}
		mi.Init(c.comparer, keyspan.NoopTransform, new(keyspanimpl.MergingBuffers), rangeKeyIters...)
		// TODO(radu): why do we have a defragmenter here but not above?
		di := &keyspan.DefragmentingIter{}
		di.Init(c.comparer, mi, keyspan.DefragmentInternal, keyspan.StaticDefragmentReducer, new(keyspan.DefragmentingBuffers))
		rangeKeyIter = di
	}
	return pointIter, rangeDelIter, rangeKeyIter, nil
}

func (c *compaction) newRangeDelIter(
	newIters tableNewIters, f manifest.LevelFile, opts IterOptions, l manifest.Layer,
) (*noCloseIter, error) {
	opts.layer = l
	iterSet, err := newIters(context.Background(), f.FileMetadata, &opts,
		internalIterOpts{
			compaction: true,
			bufferPool: &c.bufferPool,
		}, iterRangeDeletions)
	if err != nil {
		return nil, err
	} else if iterSet.rangeDeletion == nil {
		// The file doesn't contain any range deletions.
		return nil, nil
	}
	// Ensure that rangeDelIter is not closed until the compaction is
	// finished. This is necessary because range tombstone processing
	// requires the range tombstones to be held in memory for up to the
	// lifetime of the compaction.
	return &noCloseIter{iterSet.rangeDeletion}, nil
}

func (c *compaction) String() string {
	if len(c.flushing) != 0 {
		return "flush\n"
	}

	var buf bytes.Buffer
	for level := c.startLevel.level; level <= c.outputLevel.level; level++ {
		i := level - c.startLevel.level
		fmt.Fprintf(&buf, "%d:", level)
		iter := c.inputs[i].files.Iter()
		for f := iter.First(); f != nil; f = iter.Next() {
			fmt.Fprintf(&buf, " %s:%s-%s", f.FileNum, f.Smallest, f.Largest)
		}
		fmt.Fprintf(&buf, "\n")
	}
	return buf.String()
}

type manualCompaction struct {
	// Count of the retries either due to too many concurrent compactions, or a
	// concurrent compaction to overlapping levels.
	retries     int
	level       int
	outputLevel int
	done        chan error
	start       []byte
	end         []byte
	split       bool
}

type readCompaction struct {
	level int
	// [start, end] key ranges are used for de-duping.
	start []byte
	end   []byte

	// The file associated with the compaction.
	// If the file no longer belongs in the same
	// level, then we skip the compaction.
	fileNum base.FileNum
}

func (d *DB) addInProgressCompaction(c *compaction) {
	d.mu.compact.inProgress[c] = struct{}{}
	var isBase, isIntraL0 bool
	for _, cl := range c.inputs {
		iter := cl.files.Iter()
		for f := iter.First(); f != nil; f = iter.Next() {
			if f.IsCompacting() {
				d.opts.Logger.Fatalf("L%d->L%d: %s already being compacted", c.startLevel.level, c.outputLevel.level, f.FileNum)
			}
			f.SetCompactionState(manifest.CompactionStateCompacting)
			if c.startLevel != nil && c.outputLevel != nil && c.startLevel.level == 0 {
				if c.outputLevel.level == 0 {
					f.IsIntraL0Compacting = true
					isIntraL0 = true
				} else {
					isBase = true
				}
			}
		}
	}

	if (isIntraL0 || isBase) && c.version.L0Sublevels != nil {
		l0Inputs := []manifest.LevelSlice{c.startLevel.files}
		if isIntraL0 {
			l0Inputs = append(l0Inputs, c.outputLevel.files)
		}
		if err := c.version.L0Sublevels.UpdateStateForStartedCompaction(l0Inputs, isBase); err != nil {
			d.opts.Logger.Fatalf("could not update state for compaction: %s", err)
		}
	}
}

// Removes compaction markers from files in a compaction. The rollback parameter
// indicates whether the compaction state should be rolled back to its original
// state in the case of an unsuccessful compaction.
//
// DB.mu must be held when calling this method, however this method can drop and
// re-acquire that mutex. All writes to the manifest for this compaction should
// have completed by this point.
func (d *DB) clearCompactingState(c *compaction, rollback bool) {
	c.versionEditApplied = true
	for _, cl := range c.inputs {
		iter := cl.files.Iter()
		for f := iter.First(); f != nil; f = iter.Next() {
			if !f.IsCompacting() {
				d.opts.Logger.Fatalf("L%d->L%d: %s not being compacted", c.startLevel.level, c.outputLevel.level, f.FileNum)
			}
			if !rollback {
				// On success all compactions other than move-compactions transition the
				// file into the Compacted state. Move-compacted files become eligible
				// for compaction again and transition back to NotCompacting.
				if c.kind != compactionKindMove {
					f.SetCompactionState(manifest.CompactionStateCompacted)
				} else {
					f.SetCompactionState(manifest.CompactionStateNotCompacting)
				}
			} else {
				// Else, on rollback, all input files unconditionally transition back to
				// NotCompacting.
				f.SetCompactionState(manifest.CompactionStateNotCompacting)
			}
			f.IsIntraL0Compacting = false
		}
	}
	l0InProgress := inProgressL0Compactions(d.getInProgressCompactionInfoLocked(c))
	func() {
		// InitCompactingFileInfo requires that no other manifest writes be
		// happening in parallel with it, i.e. we're not in the midst of installing
		// another version. Otherwise, it's possible that we've created another
		// L0Sublevels instance, but not added it to the versions list, causing
		// all the indices in FileMetadata to be inaccurate. To ensure this,
		// grab the manifest lock.
		d.mu.versions.logLock()
		defer d.mu.versions.logUnlock()
		d.mu.versions.currentVersion().L0Sublevels.InitCompactingFileInfo(l0InProgress)
	}()
}

func (d *DB) calculateDiskAvailableBytes() uint64 {
	if space, err := d.opts.FS.GetDiskUsage(d.dirname); err == nil {
		d.diskAvailBytes.Store(space.AvailBytes)
		return space.AvailBytes
	} else if !errors.Is(err, vfs.ErrUnsupported) {
		d.opts.EventListener.BackgroundError(err)
	}
	return d.diskAvailBytes.Load()
}

// maybeScheduleFlush schedules a flush if necessary.
//
// d.mu must be held when calling this.
func (d *DB) maybeScheduleFlush() {
	if d.mu.compact.flushing || d.closed.Load() != nil || d.opts.ReadOnly {
		return
	}
	if len(d.mu.mem.queue) <= 1 {
		return
	}

	if !d.passedFlushThreshold() {
		return
	}

	d.mu.compact.flushing = true
	go d.flush()
}

func (d *DB) passedFlushThreshold() bool {
	var n int
	var size uint64
	for ; n < len(d.mu.mem.queue)-1; n++ {
		if !d.mu.mem.queue[n].readyForFlush() {
			break
		}
		if d.mu.mem.queue[n].flushForced {
			// A flush was forced. Pretend the memtable size is the configured
			// size. See minFlushSize below.
			size += d.opts.MemTableSize
		} else {
			size += d.mu.mem.queue[n].totalBytes()
		}
	}
	if n == 0 {
		// None of the immutable memtables are ready for flushing.
		return false
	}

	// Only flush once the sum of the queued memtable sizes exceeds half the
	// configured memtable size. This prevents flushing of memtables at startup
	// while we're undergoing the ramp period on the memtable size. See
	// DB.newMemTable().
	minFlushSize := d.opts.MemTableSize / 2
	return size >= minFlushSize
}

func (d *DB) maybeScheduleDelayedFlush(tbl *memTable, dur time.Duration) {
	var mem *flushableEntry
	for _, m := range d.mu.mem.queue {
		if m.flushable == tbl {
			mem = m
			break
		}
	}
	if mem == nil || mem.flushForced {
		return
	}
	deadline := d.timeNow().Add(dur)
	if !mem.delayedFlushForcedAt.IsZero() && deadline.After(mem.delayedFlushForcedAt) {
		// Already scheduled to flush sooner than within `dur`.
		return
	}
	mem.delayedFlushForcedAt = deadline
	go func() {
		timer := time.NewTimer(dur)
		defer timer.Stop()

		select {
		case <-d.closedCh:
			return
		case <-mem.flushed:
			return
		case <-timer.C:
			d.commit.mu.Lock()
			defer d.commit.mu.Unlock()
			d.mu.Lock()
			defer d.mu.Unlock()

			// NB: The timer may fire concurrently with a call to Close.  If a
			// Close call beat us to acquiring d.mu, d.closed holds ErrClosed,
			// and it's too late to flush anything. Otherwise, the Close call
			// will block on locking d.mu until we've finished scheduling the
			// flush and set `d.mu.compact.flushing` to true. Close will wait
			// for the current flush to complete.
			if d.closed.Load() != nil {
				return
			}

			if d.mu.mem.mutable == tbl {
				d.makeRoomForWrite(nil)
			} else {
				mem.flushForced = true
			}
			d.maybeScheduleFlush()
		}
	}()
}

func (d *DB) flush() {
	pprof.Do(context.Background(), flushLabels, func(context.Context) {
		flushingWorkStart := time.Now()
		d.mu.Lock()
		defer d.mu.Unlock()
		idleDuration := flushingWorkStart.Sub(d.mu.compact.noOngoingFlushStartTime)
		var bytesFlushed uint64
		var err error
		if bytesFlushed, err = d.flush1(); err != nil {
			// TODO(peter): count consecutive flush errors and backoff.
			d.opts.EventListener.BackgroundError(err)
		}
		d.mu.compact.flushing = false
		d.mu.compact.noOngoingFlushStartTime = time.Now()
		workDuration := d.mu.compact.noOngoingFlushStartTime.Sub(flushingWorkStart)
		d.mu.compact.flushWriteThroughput.Bytes += int64(bytesFlushed)
		d.mu.compact.flushWriteThroughput.WorkDuration += workDuration
		d.mu.compact.flushWriteThroughput.IdleDuration += idleDuration
		// More flush work may have arrived while we were flushing, so schedule
		// another flush if needed.
		d.maybeScheduleFlush()
		// The flush may have produced too many files in a level, so schedule a
		// compaction if needed.
		d.maybeScheduleCompaction()
		d.mu.compact.cond.Broadcast()
	})
}

// runIngestFlush is used to generate a flush version edit for sstables which
// were ingested as flushables. Both DB.mu and the manifest lock must be held
// while runIngestFlush is called.
func (d *DB) runIngestFlush(c *compaction) (*manifest.VersionEdit, error) {
	if len(c.flushing) != 1 {
		panic("pebble: ingestedFlushable must be flushed one at a time.")
	}

	// Construct the VersionEdit, levelMetrics etc.
	c.metrics = make(map[int]*LevelMetrics, numLevels)
	// Finding the target level for ingestion must use the latest version
	// after the logLock has been acquired.
	c.version = d.mu.versions.currentVersion()

	baseLevel := d.mu.versions.picker.getBaseLevel()
	ve := &versionEdit{}
	var ingestSplitFiles []ingestSplitFile
	ingestFlushable := c.flushing[0].flushable.(*ingestedFlushable)

	updateLevelMetricsOnExcise := func(m *fileMetadata, level int, added []newFileEntry) {
		levelMetrics := c.metrics[level]
		if levelMetrics == nil {
			levelMetrics = &LevelMetrics{}
			c.metrics[level] = levelMetrics
		}
		levelMetrics.NumFiles--
		levelMetrics.Size -= int64(m.Size)
		for i := range added {
			levelMetrics.NumFiles++
			levelMetrics.Size += int64(added[i].Meta.Size)
		}
	}

	suggestSplit := d.opts.Experimental.IngestSplit != nil && d.opts.Experimental.IngestSplit() &&
		d.FormatMajorVersion() >= FormatVirtualSSTables

	if suggestSplit || ingestFlushable.exciseSpan.Valid() {
		// We could add deleted files to ve.
		ve.DeletedFiles = make(map[manifest.DeletedFileEntry]*manifest.FileMetadata)
	}

	ctx := context.Background()
	overlapChecker := &overlapChecker{
		comparer: d.opts.Comparer,
		newIters: d.newIters,
		opts: IterOptions{
			logger: d.opts.Logger,
			CategoryAndQoS: sstable.CategoryAndQoS{
				Category: "pebble-ingest",
				QoSLevel: sstable.LatencySensitiveQoSLevel,
			},
		},
		v: c.version,
	}
	replacedFiles := make(map[base.FileNum][]newFileEntry)
	for _, file := range ingestFlushable.files {
		var fileToSplit *fileMetadata
		var level int

		// This file fits perfectly within the excise span, so we can slot it at L6.
		if ingestFlushable.exciseSpan.Valid() &&
			ingestFlushable.exciseSpan.Contains(d.cmp, file.FileMetadata.Smallest) &&
			ingestFlushable.exciseSpan.Contains(d.cmp, file.FileMetadata.Largest) {
			level = 6
		} else {
			// TODO(radu): this can perform I/O; we should not do this while holding DB.mu.
			lsmOverlap, err := overlapChecker.DetermineLSMOverlap(ctx, file.UserKeyBounds())
			if err != nil {
				return nil, err
			}
			level, fileToSplit, err = ingestTargetLevel(
				ctx, d.cmp, lsmOverlap, baseLevel, d.mu.compact.inProgress, file.FileMetadata, suggestSplit,
			)
			if err != nil {
				return nil, err
			}
		}

		// Add the current flushableIngest file to the version.
		ve.NewFiles = append(ve.NewFiles, newFileEntry{Level: level, Meta: file.FileMetadata})
		if fileToSplit != nil {
			ingestSplitFiles = append(ingestSplitFiles, ingestSplitFile{
				ingestFile: file.FileMetadata,
				splitFile:  fileToSplit,
				level:      level,
			})
		}
		levelMetrics := c.metrics[level]
		if levelMetrics == nil {
			levelMetrics = &LevelMetrics{}
			c.metrics[level] = levelMetrics
		}
		levelMetrics.BytesIngested += file.Size
		levelMetrics.TablesIngested++
	}
	if ingestFlushable.exciseSpan.Valid() {
		// Iterate through all levels and find files that intersect with exciseSpan.
		for l := range c.version.Levels {
			overlaps := c.version.Overlaps(l, base.UserKeyBoundsEndExclusive(ingestFlushable.exciseSpan.Start, ingestFlushable.exciseSpan.End))
			iter := overlaps.Iter()

			for m := iter.First(); m != nil; m = iter.Next() {
				newFiles, err := d.excise(context.TODO(), ingestFlushable.exciseSpan.UserKeyBounds(), m, ve, l)
				if err != nil {
					return nil, err
				}

				if _, ok := ve.DeletedFiles[deletedFileEntry{
					Level:   l,
					FileNum: m.FileNum,
				}]; !ok {
					// We did not excise this file.
					continue
				}
				replacedFiles[m.FileNum] = newFiles
				updateLevelMetricsOnExcise(m, l, newFiles)
			}
		}
	}

	if len(ingestSplitFiles) > 0 {
		if err := d.ingestSplit(context.TODO(), ve, updateLevelMetricsOnExcise, ingestSplitFiles, replacedFiles); err != nil {
			return nil, err
		}
	}

	return ve, nil
}

// flush runs a compaction that copies the immutable memtables from memory to
// disk.
//
// d.mu must be held when calling this, but the mutex may be dropped and
// re-acquired during the course of this method.
func (d *DB) flush1() (bytesFlushed uint64, err error) {
	// NB: The flushable queue can contain flushables of type ingestedFlushable.
	// The sstables in ingestedFlushable.files must be placed into the appropriate
	// level in the lsm. Let's say the flushable queue contains a prefix of
	// regular immutable memtables, then an ingestedFlushable, and then the
	// mutable memtable. When the flush of the ingestedFlushable is performed,
	// it needs an updated view of the lsm. That is, the prefix of immutable
	// memtables must have already been flushed. Similarly, if there are two
	// contiguous ingestedFlushables in the queue, then the first flushable must
	// be flushed, so that the second flushable can see an updated view of the
	// lsm.
	//
	// Given the above, we restrict flushes to either some prefix of regular
	// memtables, or a single flushable of type ingestedFlushable. The DB.flush
	// function will call DB.maybeScheduleFlush again, so a new flush to finish
	// the remaining flush work should be scheduled right away.
	//
	// NB: Large batches placed in the flushable queue share the WAL with the
	// previous memtable in the queue. We must ensure the property that both the
	// large batch and the memtable with which it shares a WAL are flushed
	// together. The property ensures that the minimum unflushed log number
	// isn't incremented incorrectly. Since a flushableBatch.readyToFlush always
	// returns true, and since the large batch will always be placed right after
	// the memtable with which it shares a WAL, the property is naturally
	// ensured. The large batch will always be placed after the memtable with
	// which it shares a WAL because we ensure it in DB.commitWrite by holding
	// the commitPipeline.mu and then holding DB.mu. As an extra defensive
	// measure, if we try to flush the memtable without also flushing the
	// flushable batch in the same flush, since the memtable and flushableBatch
	// have the same logNum, the logNum invariant check below will trigger.
	var n, inputs int
	var inputBytes uint64
	var ingest bool
	for ; n < len(d.mu.mem.queue)-1; n++ {
		if f, ok := d.mu.mem.queue[n].flushable.(*ingestedFlushable); ok {
			if n == 0 {
				// The first flushable is of type ingestedFlushable. Since these
				// must be flushed individually, we perform a flush for just
				// this.
				if !f.readyForFlush() {
					// This check is almost unnecessary, but we guard against it
					// just in case this invariant changes in the future.
					panic("pebble: ingestedFlushable should always be ready to flush.")
				}
				// By setting n = 1, we ensure that the first flushable(n == 0)
				// is scheduled for a flush. The number of tables added is equal to the
				// number of files in the ingest operation.
				n = 1
				inputs = len(f.files)
				ingest = true
				break
			} else {
				// There was some prefix of flushables which weren't of type
				// ingestedFlushable. So, perform a flush for those.
				break
			}
		}
		if !d.mu.mem.queue[n].readyForFlush() {
			break
		}
		inputBytes += d.mu.mem.queue[n].inuseBytes()
	}
	if n == 0 {
		// None of the immutable memtables are ready for flushing.
		return 0, nil
	}
	if !ingest {
		// Flushes of memtables add the prefix of n memtables from the flushable
		// queue.
		inputs = n
	}

	// Require that every memtable being flushed has a log number less than the
	// new minimum unflushed log number.
	minUnflushedLogNum := d.mu.mem.queue[n].logNum
	if !d.opts.DisableWAL {
		for i := 0; i < n; i++ {
			if logNum := d.mu.mem.queue[i].logNum; logNum >= minUnflushedLogNum {
				panic(errors.AssertionFailedf("logNum invariant violated: flushing %d items; %d:type=%T,logNum=%d; %d:type=%T,logNum=%d",
					n,
					i, d.mu.mem.queue[i].flushable, logNum,
					n, d.mu.mem.queue[n].flushable, minUnflushedLogNum))
			}
		}
	}

	c, err := newFlush(d.opts, d.mu.versions.currentVersion(),
		d.mu.versions.picker.getBaseLevel(), d.mu.mem.queue[:n], d.timeNow())
	if err != nil {
		return 0, err
	}
	d.addInProgressCompaction(c)

	jobID := d.newJobIDLocked()
	d.opts.EventListener.FlushBegin(FlushInfo{
		JobID:      int(jobID),
		Input:      inputs,
		InputBytes: inputBytes,
		Ingest:     ingest,
	})
	startTime := d.timeNow()

	var ve *manifest.VersionEdit
	var stats compact.Stats
	// To determine the target level of the files in the ingestedFlushable, we
	// need to acquire the logLock, and not release it for that duration. Since,
	// we need to acquire the logLock below to perform the logAndApply step
	// anyway, we create the VersionEdit for ingestedFlushable outside of
	// runCompaction. For all other flush cases, we construct the VersionEdit
	// inside runCompaction.
	if c.kind != compactionKindIngestedFlushable {
		ve, stats, err = d.runCompaction(jobID, c)
	}

	// Acquire logLock. This will be released either on an error, by way of
	// logUnlock, or through a call to logAndApply if there is no error.
	d.mu.versions.logLock()

	if c.kind == compactionKindIngestedFlushable {
		ve, err = d.runIngestFlush(c)
	}

	info := FlushInfo{
		JobID:      int(jobID),
		Input:      inputs,
		InputBytes: inputBytes,
		Duration:   d.timeNow().Sub(startTime),
		Done:       true,
		Ingest:     ingest,
		Err:        err,
	}
	if err == nil {
		validateVersionEdit(ve, d.opts.Experimental.KeyValidationFunc, d.opts.Comparer.FormatKey, d.opts.Logger)
		for i := range ve.NewFiles {
			e := &ve.NewFiles[i]
			info.Output = append(info.Output, e.Meta.TableInfo())
			// Ingested tables are not necessarily flushed to L0. Record the level of
			// each ingested file explicitly.
			if ingest {
				info.IngestLevels = append(info.IngestLevels, e.Level)
			}
		}
		if len(ve.NewFiles) == 0 {
			info.Err = errEmptyTable
		}

		// The flush succeeded or it produced an empty sstable. In either case we
		// want to bump the minimum unflushed log number to the log number of the
		// oldest unflushed memtable.
		ve.MinUnflushedLogNum = minUnflushedLogNum
		if c.kind != compactionKindIngestedFlushable {
			metrics := c.metrics[0]
			if d.opts.DisableWAL {
				// If the WAL is disabled, every flushable has a zero [logSize],
				// resulting in zero bytes in. Instead, use the number of bytes we
				// flushed as the BytesIn. This ensures we get a reasonable w-amp
				// calculation even when the WAL is disabled.
				metrics.BytesIn = metrics.BytesFlushed
			} else {
				for i := 0; i < n; i++ {
					metrics.BytesIn += d.mu.mem.queue[i].logSize
				}
			}
		} else {
			// c.kind == compactionKindIngestedFlushable && we could have deleted files due
			// to ingest-time splits or excises.
			ingestFlushable := c.flushing[0].flushable.(*ingestedFlushable)
			for c2 := range d.mu.compact.inProgress {
				// Check if this compaction overlaps with the excise span. Note that just
				// checking if the inputs individually overlap with the excise span
				// isn't sufficient; for instance, a compaction could have [a,b] and [e,f]
				// as inputs and write it all out as [a,b,e,f] in one sstable. If we're
				// doing a [c,d) excise at the same time as this compaction, we will have
				// to error out the whole compaction as we can't guarantee it hasn't/won't
				// write a file overlapping with the excise span.
				if ingestFlushable.exciseSpan.OverlapsInternalKeyRange(d.cmp, c2.smallest, c2.largest) {
					c2.cancel.Store(true)
					continue
				}
			}

			if len(ve.DeletedFiles) > 0 {
				// Iterate through all other compactions, and check if their inputs have
				// been replaced due to an ingest-time split or excise. In that case,
				// cancel the compaction.
				for c2 := range d.mu.compact.inProgress {
					for i := range c2.inputs {
						iter := c2.inputs[i].files.Iter()
						for f := iter.First(); f != nil; f = iter.Next() {
							if _, ok := ve.DeletedFiles[deletedFileEntry{FileNum: f.FileNum, Level: c2.inputs[i].level}]; ok {
								c2.cancel.Store(true)
								break
							}
						}
					}
				}
			}
		}
		err = d.mu.versions.logAndApply(jobID, ve, c.metrics, false, /* forceRotation */
			func() []compactionInfo { return d.getInProgressCompactionInfoLocked(c) })
		if err != nil {
			info.Err = err
		}
	} else {
		// We won't be performing the logAndApply step because of the error,
		// so logUnlock.
		d.mu.versions.logUnlock()
	}

	// If err != nil, then the flush will be retried, and we will recalculate
	// these metrics.
	if err == nil {
		d.mu.snapshots.cumulativePinnedCount += stats.CumulativePinnedKeys
		d.mu.snapshots.cumulativePinnedSize += stats.CumulativePinnedSize
		d.mu.versions.metrics.Keys.MissizedTombstonesCount += stats.CountMissizedDels
	}

	d.clearCompactingState(c, err != nil)
	delete(d.mu.compact.inProgress, c)
	d.mu.versions.incrementCompactions(c.kind, c.extraLevels, c.pickerMetrics)

	var flushed flushableList
	if err == nil {
		flushed = d.mu.mem.queue[:n]
		d.mu.mem.queue = d.mu.mem.queue[n:]
		d.updateReadStateLocked(d.opts.DebugCheck)
		d.updateTableStatsLocked(ve.NewFiles)
		if ingest {
			d.mu.versions.metrics.Flush.AsIngestCount++
			for _, l := range c.metrics {
				d.mu.versions.metrics.Flush.AsIngestBytes += l.BytesIngested
				d.mu.versions.metrics.Flush.AsIngestTableCount += l.TablesIngested
			}
		}
		d.maybeTransitionSnapshotsToFileOnlyLocked()

	}
	// Signal FlushEnd after installing the new readState. This helps for unit
	// tests that use the callback to trigger a read using an iterator with
	// IterOptions.OnlyReadGuaranteedDurable.
	info.TotalDuration = d.timeNow().Sub(startTime)
	d.opts.EventListener.FlushEnd(info)

	// The order of these operations matters here for ease of testing.
	// Removing the reader reference first allows tests to be guaranteed that
	// the memtable reservation has been released by the time a synchronous
	// flush returns. readerUnrefLocked may also produce obsolete files so the
	// call to deleteObsoleteFiles must happen after it.
	for i := range flushed {
		flushed[i].readerUnrefLocked(true)
	}

	d.deleteObsoleteFiles(jobID)

	// Mark all the memtables we flushed as flushed.
	for i := range flushed {
		close(flushed[i].flushed)
	}

	return inputBytes, err
}

// maybeTransitionSnapshotsToFileOnlyLocked transitions any "eventually
// file-only" snapshots to be file-only if all their visible state has been
// flushed to sstables.
//
// REQUIRES: d.mu.
func (d *DB) maybeTransitionSnapshotsToFileOnlyLocked() {
	earliestUnflushedSeqNum := d.getEarliestUnflushedSeqNumLocked()
	currentVersion := d.mu.versions.currentVersion()
	for s := d.mu.snapshots.root.next; s != &d.mu.snapshots.root; {
		if s.efos == nil {
			s = s.next
			continue
		}
		overlapsFlushable := false
		if base.Visible(earliestUnflushedSeqNum, s.efos.seqNum, base.SeqNumMax) {
			// There are some unflushed keys that are still visible to the EFOS.
			// Check if any memtables older than the EFOS contain keys within a
			// protected range of the EFOS. If no, we can transition.
			protectedRanges := make([]bounded, len(s.efos.protectedRanges))
			for i := range s.efos.protectedRanges {
				protectedRanges[i] = s.efos.protectedRanges[i]
			}
			for i := range d.mu.mem.queue {
				if !base.Visible(d.mu.mem.queue[i].logSeqNum, s.efos.seqNum, base.SeqNumMax) {
					// All keys in this memtable are newer than the EFOS. Skip this
					// memtable.
					continue
				}
				// NB: computePossibleOverlaps could have false positives, such as if
				// the flushable is a flushable ingest and not a memtable. In that
				// case we don't open the sstables to check; we just pessimistically
				// assume an overlap.
				d.mu.mem.queue[i].computePossibleOverlaps(func(b bounded) shouldContinue {
					overlapsFlushable = true
					return stopIteration
				}, protectedRanges...)
				if overlapsFlushable {
					break
				}
			}
		}
		if overlapsFlushable {
			s = s.next
			continue
		}
		currentVersion.Ref()

		// NB: s.efos.transitionToFileOnlySnapshot could close s, in which
		// case s.next would be nil. Save it before calling it.
		next := s.next
		_ = s.efos.transitionToFileOnlySnapshot(currentVersion)
		s = next
	}
}

// maybeScheduleCompactionAsync should be used when
// we want to possibly schedule a compaction, but don't
// want to eat the cost of running maybeScheduleCompaction.
// This method should be launched in a separate goroutine.
// d.mu must not be held when this is called.
func (d *DB) maybeScheduleCompactionAsync() {
	defer d.compactionSchedulers.Done()

	d.mu.Lock()
	d.maybeScheduleCompaction()
	d.mu.Unlock()
}

// tryScheduleDownloadCompactions tries to start download compactions.
//
// Requires d.mu to be held. Updates d.mu.compact.downloads.
func (d *DB) tryScheduleDownloadCompactions(env compactionEnv, maxConcurrentDownloads int) {
	vers := d.mu.versions.currentVersion()
	for i := 0; i < len(d.mu.compact.downloads); {
		if d.mu.compact.downloadingCount >= maxConcurrentDownloads {
			break
		}
		download := d.mu.compact.downloads[i]
		switch d.tryLaunchDownloadCompaction(download, vers, env, maxConcurrentDownloads) {
		case launchedCompaction:
			continue
		case didNotLaunchCompaction:
			// See if we can launch a compaction for another download task.
			i++
		case downloadTaskCompleted:
			// Task is completed and must be removed.
			d.mu.compact.downloads = slices.Delete(d.mu.compact.downloads, i, i+1)
		}
	}
}

// makeCompactionEnv attempts to acquire d.mu.versions.logLock in order
// to provide the proper mutual exclusion necessary during compaction picking.
// If the DB is closed or marked as read-only, makeCompactionEnv returns nil to
// indicate that compactions may not be performed. Otherwise, a new
// compactionEnv is constructed using the current DB state.
//
// Compaction picking needs a coherent view of a Version. In particular, we
// need to exclude concurrent ingestions from making a decision on which level
// to ingest into that conflicts with our compaction
// decision. versionSet.logLock provides the necessary mutual exclusion.
//
// NOTE: makeCompactionEnv does not call d.mu.versions.logUnlock; it is the
// caller's responsibility to ensure that the manifest is unlocked.
//
// Requires d.mu to be held.
func (d *DB) makeCompactionEnv() *compactionEnv {
	if d.closed.Load() != nil || d.opts.ReadOnly {
		return nil
	}

	d.mu.versions.logLock()

	// Check for the closed flag again, in case the DB was closed while we were
	// waiting for logLock().
	if d.closed.Load() != nil {
		d.mu.versions.logUnlock()
		return nil
	}

	return &compactionEnv{
		diskAvailBytes:          d.diskAvailBytes.Load(),
		earliestSnapshotSeqNum:  d.mu.snapshots.earliest(),
		earliestUnflushedSeqNum: d.getEarliestUnflushedSeqNumLocked(),
		inProgressCompactions:   d.getInProgressCompactionInfoLocked(nil),
		readCompactionEnv: readCompactionEnv{
			readCompactions:          &d.mu.compact.readCompactions,
			flushing:                 d.mu.compact.flushing || d.passedFlushThreshold(),
			rescheduleReadCompaction: &d.mu.compact.rescheduleReadCompaction,
		},
	}
}

// pickAnyCompaction tries to pick a manual or automatic compaction.
func (d *DB) pickAnyCompaction(env compactionEnv) (pc *pickedCompaction) {
	pc = d.pickManualCompaction(env)
	if pc == nil && !d.opts.DisableAutomaticCompactions {
		pc = d.mu.versions.picker.pickAuto(env)
	}
	return pc
}

// runPickedCompaction kicks off the provided pickedCompaction. In case the
// pickedCompaction is a manual compaction, the corresponding manualCompaction
// is removed from d.mu.compact.manual.
func (d *DB) runPickedCompaction(pc *pickedCompaction) {
	var doneChannel chan error
	if pc.isManual {
		doneChannel = d.mu.compact.manual[0].done
		d.mu.compact.manual = d.mu.compact.manual[1:]
	}

	d.mu.compact.compactingCount++
	compaction := newCompaction(pc, d.opts, d.timeNow(), d.ObjProvider())
	d.addInProgressCompaction(compaction)
	go func() {
		d.compact(compaction, doneChannel)
		d.compactionPool.CompactionFinished(pc)
	}()
}

// CompactionPool is responsible for scheduling both automatic and manual
// compactions. In the case of multiple DB instances (i.e. a multi-store
// configuration), implementations of CompactionPool may or may not enforce
// a global maximum compaction concurrency.
type CompactionPool interface {
	// AddWaitingDB signals to the CompactionPool that the provided DB might
	// have compaction(s) that need to be scheduled. Implementations of
	// CompactionPool may decide when a compaction will actually be picked
	// and run from this DB.
	//
	// DB.mu should NOT be held for any DB (including d) when AddWaitingDB is
	// called.
	AddWaitingDB(d *DB)
	// CompactionFinished signals to the CompactionPool that the provided
	// pickedCompaction has now finished running.
	CompactionFinished(pc *pickedCompaction)
}

var defaultCompactionPool = &UnlimitedCompactionPool{}

// UnlimitedCompactionPool implements CompactionPool. It does not enforce any
// global maximum compaction concurrency when multiple DBs attempt to schedule
// compactions.
type UnlimitedCompactionPool struct{}

func (ucp *UnlimitedCompactionPool) AddWaitingDB(d *DB) {
	d.mu.Lock()
	env := d.makeCompactionEnv()
	if env == nil {
		d.mu.Unlock()
		return
	}

	pc := d.pickAnyCompaction(*env)
	if pc != nil {
		d.runPickedCompaction(pc)
		// We might be able to schedule more compactions.
		defer ucp.AddWaitingDB(d)
	}

	d.mu.versions.logUnlock()
	d.mu.Unlock()
}

func (ucp *UnlimitedCompactionPool) CompactionFinished(pc *pickedCompaction) {
	// No-op for an UnlimitedCompactionPool.
}

// PrioritizingCompactionPool enforces a global max compaction concurrency
// in a multi-store configuration. If multiple DBs are waiting to perform a
// compaction, it prioritizes the DB whose pickedCompaction has the highest
// priority.
type PrioritizingCompactionPool struct {
	mu sync.Mutex
	// compactingCount is the current number of running compactions across
	// all DBs.
	compactingCount int
	// waiting contains all DBs which might have compactions that need to be
	// scheduled. The value stored for each DB may be nil to indicate that
	// a compaction needs to be picked from the DB.
	waiting map[*DB]*pickedCompaction
	// maxCompactionConcurrency defines the global max compaction concurrency
	// across all DBs.
	maxCompactionConcurrency int
}

// NewPrioritizingCompactionPool creates a new PrioritizingCompactionPool
// with the specified maxCompactionConcurrency.
func NewPrioritizingCompactionPool(maxCompactionConcurrency int) *PrioritizingCompactionPool {
	if maxCompactionConcurrency <= 0 {
		panic("pebble: maxCompactionConcurrency for a CompactionPool must be greater than 0")
	}
	return &PrioritizingCompactionPool{
		maxCompactionConcurrency: maxCompactionConcurrency,
		waiting:                  make(map[*DB]*pickedCompaction),
	}
}

// shouldLimitConcurrency returns true if the provided pickedCompaction should
// be counted towards this PrioritizingCompactionPool's compactingCount.
func (pcp *PrioritizingCompactionPool) shouldLimitConcurrency(pc *pickedCompaction) bool {
	return pc.kind != compactionKindDeleteOnly && pc.kind != compactionKindMove
}

// comparePickedCompactions returns true if pc1 is higher priority than pc2.
func comparePickedCompactions(pc1 *pickedCompaction, pc2 *pickedCompaction) bool {
	if pc1 == nil {
		return false
	} else if pc2 == nil {
		return true
	}

	// If pc1 and pc2 are of different compactionKinds, pc1 is higher priority
	// only if its compactionKind is higher priority. The relative priorities
	// of compactionKinds are determined by the order they are defined in. See
	// the comment below compactionKind.
	if pc1.kind < pc2.kind {
		return true
	}
	// Otherwise, use the pickedCompaction's score to break ties. This is the
	// score of the level in the case of score-based compactions; other
	// compaction types assign the score differently.
	return pc1.score > pc2.score
}

// maybeScheduleWaitingCompactionLocked attempts to schedule a waiting
// compaction from the list of waiting DBs. It prioritizes the DB with the
// highest priority pickedCompaction as defined by comparePickedCompactions.
//
// c.mu must be held. DB.mu must not be held for any DB.
func (pcp *PrioritizingCompactionPool) maybeScheduleWaitingCompactionLocked() {
	if pcp.compactingCount >= pcp.maxCompactionConcurrency {
		return
	}

	unlockDB := func(d *DB) {
		d.mu.versions.logUnlock()
		d.mu.Unlock()
	}

	var selectedDB *DB
	// We need to find the highest-priority compaction across all waiting DBs.
	// This is tricky because concurrent flushes, ingestions, etc. from each DB
	// may invalidate a compaction after it is picked. For this reason, the
	// below loop maintains the invariant that both selectedDB.mu and
	// selectedDB.mu.versions.logLock are continuously held until we either find
	// a higher priority compaction from one of the other DBs, or we finish
	// iteration and start the compaction.
	for d := range pcp.waiting {
		d.mu.Lock()

		env := d.makeCompactionEnv()
		if env == nil {
			// This DB is read-only or closed; skip it.
			delete(pcp.waiting, d)
			d.mu.Unlock()
			continue
		}

		if pcp.waiting[d] == nil {
			if pcp.waiting[d] = d.pickAnyCompaction(*env); pcp.waiting[d] == nil {
				// There are no compactions that can be scheduled from this DB.
				// Mark it as no longer waiting.
				delete(pcp.waiting, d)
				unlockDB(d)
				continue
			}
		}
		if selectedDB == nil {
			selectedDB = d
		} else if comparePickedCompactions(pcp.waiting[d], pcp.waiting[selectedDB]) {
			// We've found a higher priority pickedCompaction - first unlock the old
			// selectedDB, and then swap it out for the current DB.
			unlockDB(selectedDB)
			selectedDB = d
		} else {
			// This DB's pickedCompaction is lower priority than that of selectedDB.
			// Release locks and continue.
			unlockDB(d)
		}
	}

	if selectedDB == nil {
		return
	}

	// At this point, locks are held only for selectedDB.mu.
	pc := pcp.waiting[selectedDB]
	if pcp.shouldLimitConcurrency(pc) {
		pcp.compactingCount++
	}
	selectedDB.runPickedCompaction(pc)
	pcp.waiting[selectedDB] = nil

	unlockDB(selectedDB)
	pcp.maybeScheduleWaitingCompactionLocked()
}

func (pcp *PrioritizingCompactionPool) CompactionFinished(pc *pickedCompaction) {
	if pcp.shouldLimitConcurrency(pc) {
		pcp.mu.Lock()
		defer pcp.mu.Unlock()

		pcp.compactingCount--
		pcp.maybeScheduleWaitingCompactionLocked()
	}
}

func (pcp *PrioritizingCompactionPool) AddWaitingDB(d *DB) {
	pcp.mu.Lock()
	defer pcp.mu.Unlock()
	// Mark this DB as waiting, but also invalidate any existing
	// pickedCompaction from this DB since a higher priority compaction
	// could be picked.
	pcp.waiting[d] = nil
	pcp.maybeScheduleWaitingCompactionLocked()
}

// maybeScheduleCompaction schedules a compaction if necessary.
//
// Requires d.mu to be held.
func (d *DB) maybeScheduleCompaction() {
	env := d.makeCompactionEnv()
	if env == nil {
		return
	}
	// Delete-only compactions are expected to be cheap and reduce future
	// compaction work, so schedule them directly instead of using
	// d.compactionPool.
	d.tryScheduleDeleteOnlyCompaction()
	// Download compactions have their own concurrency.
	d.tryScheduleDownloadCompactions(*env, d.opts.MaxConcurrentDownloads())
	d.mu.versions.logUnlock()

	// NB: we must release d.mu to avoid deadlock when calling
	// addWaitingDB below.
	d.mu.Unlock()
	d.compactionPool.AddWaitingDB(d)
	d.mu.Lock()
}

// tryScheduleDeleteOnlyCompaction tries to kick off a delete-only compaction
// for all files that can be deleted as suggested by deletionHints.
//
// Requires d.mu to be held. Updates d.mu.compact.deletionHints.
func (d *DB) tryScheduleDeleteOnlyCompaction() {
	if d.opts.private.disableDeleteOnlyCompactions || d.opts.DisableAutomaticCompactions || d.mu.compact.compactingCount >= d.opts.MaxConcurrentCompactions() || len(d.mu.compact.deletionHints) == 0 {
		return
	}

	v := d.mu.versions.currentVersion()
	snapshots := d.mu.snapshots.toSlice()
	inputs, unresolvedHints := checkDeleteCompactionHints(d.cmp, v, d.mu.compact.deletionHints, snapshots)
	d.mu.compact.deletionHints = unresolvedHints

	if len(inputs) > 0 {
		c := newDeleteOnlyCompaction(d.opts, v, inputs, d.timeNow())
		d.mu.compact.compactingCount++
		d.addInProgressCompaction(c)
		go d.compact(c, nil)
	}
}

func (d *DB) pickManualCompaction(env compactionEnv) (pc *pickedCompaction) {
	v := d.mu.versions.currentVersion()
	for len(d.mu.compact.manual) > 0 {
		manual := d.mu.compact.manual[0]
		if d.mu.compact.compactingCount >= d.opts.MaxConcurrentCompactions() {
			manual.retries++
			return nil
		}

		pc, retryLater := newPickedManualCompaction(v, d.opts, env, d.mu.versions.picker.getBaseLevel(), manual)
		if pc != nil {
			return pc
		}
		if retryLater {
			// We are not able to run this manual compaction at this time.
			manual.retries++
			return nil
		}
		// Manual compaction is a no-op. Signal that it's complete.
		manual.done <- nil
		d.mu.compact.manual = d.mu.compact.manual[1:]
	}
	return nil
}

// deleteCompactionHintType indicates whether the deleteCompactionHint was
// generated from a span containing a range del (point key only), a range key
// delete (range key only), or both a point and range key.
type deleteCompactionHintType uint8

const (
	// NOTE: While these are primarily used as enumeration types, they are also
	// used for some bitwise operations. Care should be taken when updating.
	deleteCompactionHintTypeUnknown deleteCompactionHintType = iota
	deleteCompactionHintTypePointKeyOnly
	deleteCompactionHintTypeRangeKeyOnly
	deleteCompactionHintTypePointAndRangeKey
)

// String implements fmt.Stringer.
func (h deleteCompactionHintType) String() string {
	switch h {
	case deleteCompactionHintTypeUnknown:
		return "unknown"
	case deleteCompactionHintTypePointKeyOnly:
		return "point-key-only"
	case deleteCompactionHintTypeRangeKeyOnly:
		return "range-key-only"
	case deleteCompactionHintTypePointAndRangeKey:
		return "point-and-range-key"
	default:
		panic(fmt.Sprintf("unknown hint type: %d", h))
	}
}

// compactionHintFromKeys returns a deleteCompactionHintType given a slice of
// keyspan.Keys.
func compactionHintFromKeys(keys []keyspan.Key) deleteCompactionHintType {
	var hintType deleteCompactionHintType
	for _, k := range keys {
		switch k.Kind() {
		case base.InternalKeyKindRangeDelete:
			hintType |= deleteCompactionHintTypePointKeyOnly
		case base.InternalKeyKindRangeKeyDelete:
			hintType |= deleteCompactionHintTypeRangeKeyOnly
		default:
			panic(fmt.Sprintf("unsupported key kind: %s", k.Kind()))
		}
	}
	return hintType
}

// A deleteCompactionHint records a user key and sequence number span that has been
// deleted by a range tombstone. A hint is recorded if at least one sstable
// falls completely within both the user key and sequence number spans.
// Once the tombstones and the observed completely-contained sstables fall
// into the same snapshot stripe, a delete-only compaction may delete any
// sstables within the range.
type deleteCompactionHint struct {
	// The type of key span that generated this hint (point key, range key, or
	// both).
	hintType deleteCompactionHintType
	// start and end are user keys specifying a key range [start, end) of
	// deleted keys.
	start []byte
	end   []byte
	// The level of the file containing the range tombstone(s) when the hint
	// was created. Only lower levels need to be searched for files that may
	// be deleted.
	tombstoneLevel int
	// The file containing the range tombstone(s) that created the hint.
	tombstoneFile *fileMetadata
	// The smallest and largest sequence numbers of the abutting tombstones
	// merged to form this hint. All of a tables' keys must be less than the
	// tombstone smallest sequence number to be deleted. All of a tables'
	// sequence numbers must fall into the same snapshot stripe as the
	// tombstone largest sequence number to be deleted.
	tombstoneLargestSeqNum  base.SeqNum
	tombstoneSmallestSeqNum base.SeqNum
	// The smallest sequence number of a sstable that was found to be covered
	// by this hint. The hint cannot be resolved until this sequence number is
	// in the same snapshot stripe as the largest tombstone sequence number.
	// This is set when a hint is created, so the LSM may look different and
	// notably no longer contain the sstable that contained the key at this
	// sequence number.
	fileSmallestSeqNum base.SeqNum
}

func (h deleteCompactionHint) String() string {
	return fmt.Sprintf(
		"L%d.%s %s-%s seqnums(tombstone=%d-%d, file-smallest=%d, type=%s)",
		h.tombstoneLevel, h.tombstoneFile.FileNum, h.start, h.end,
		h.tombstoneSmallestSeqNum, h.tombstoneLargestSeqNum, h.fileSmallestSeqNum,
		h.hintType,
	)
}

func (h *deleteCompactionHint) canDelete(
	cmp Compare, m *fileMetadata, snapshots compact.Snapshots,
) bool {
	// The file can only be deleted if all of its keys are older than the
	// earliest tombstone aggregated into the hint. Note that we use
	// m.LargestSeqNumAbsolute, not m.LargestSeqNum. Consider a compaction that
	// zeroes sequence numbers. A compaction may zero the sequence number of a
	// key with a sequence number > h.tombstoneSmallestSeqNum and set it to
	// zero. If we looked at m.LargestSeqNum, the resulting output file would
	// appear to not contain any keys more recent than the oldest tombstone. To
	// avoid this error, the largest pre-zeroing sequence number is maintained
	// in LargestSeqNumAbsolute and used here to make the determination whether
	// the file's keys are older than all of the hint's tombstones.
	if m.LargestSeqNumAbsolute >= h.tombstoneSmallestSeqNum || m.SmallestSeqNum < h.fileSmallestSeqNum {
		return false
	}

	// The file's oldest key must  be in the same snapshot stripe as the
	// newest tombstone. NB: We already checked the hint's sequence numbers,
	// but this file's oldest sequence number might be lower than the hint's
	// smallest sequence number despite the file falling within the key range
	// if this file was constructed after the hint by a compaction.
	if snapshots.Index(h.tombstoneLargestSeqNum) != snapshots.Index(m.SmallestSeqNum) {
		return false
	}

	switch h.hintType {
	case deleteCompactionHintTypePointKeyOnly:
		// A hint generated by a range del span cannot delete tables that contain
		// range keys.
		if m.HasRangeKeys {
			return false
		}
	case deleteCompactionHintTypeRangeKeyOnly:
		// A hint generated by a range key del span cannot delete tables that
		// contain point keys.
		if m.HasPointKeys {
			return false
		}
	case deleteCompactionHintTypePointAndRangeKey:
		// A hint from a span that contains both range dels *and* range keys can
		// only be deleted if both bounds fall within the hint. The next check takes
		// care of this.
	default:
		panic(fmt.Sprintf("pebble: unknown delete compaction hint type: %d", h.hintType))
	}

	// The file's keys must be completely contained within the hint range.
	return cmp(h.start, m.Smallest.UserKey) <= 0 && cmp(m.Largest.UserKey, h.end) < 0
}

func checkDeleteCompactionHints(
	cmp Compare, v *version, hints []deleteCompactionHint, snapshots compact.Snapshots,
) ([]compactionLevel, []deleteCompactionHint) {
	var files map[*fileMetadata]bool
	var byLevel [numLevels][]*fileMetadata

	unresolvedHints := hints[:0]
	for _, h := range hints {
		// Check each compaction hint to see if it's resolvable. Resolvable
		// hints are removed and trigger a delete-only compaction if any files
		// in the current LSM still meet their criteria. Unresolvable hints
		// are saved and don't trigger a delete-only compaction.
		//
		// When a compaction hint is created, the sequence numbers of the
		// range tombstones and the covered file with the oldest key are
		// recorded. The largest tombstone sequence number and the smallest
		// file sequence number must be in the same snapshot stripe for the
		// hint to be resolved. The below graphic models a compaction hint
		// covering the keyspace [b, r). The hint completely contains two
		// files, 000002 and 000003. The file 000003 contains the lowest
		// covered sequence number at #90. The tombstone b.RANGEDEL.230:h has
		// the highest tombstone sequence number incorporated into the hint.
		// The hint may be resolved only once the snapshots at #100, #180 and
		// #210 are all closed. File 000001 is not included within the hint
		// because it extends beyond the range tombstones in user key space.
		//
		// 250
		//
		//       |-b...230:h-|
		// _____________________________________________________ snapshot #210
		// 200               |--h.RANGEDEL.200:r--|
		//
		// _____________________________________________________ snapshot #180
		//
		// 150                     +--------+
		//           +---------+   | 000003 |
		//           | 000002  |   |        |
		//           +_________+   |        |
		// 100_____________________|________|___________________ snapshot #100
		//                         +--------+
		// _____________________________________________________ snapshot #70
		//                             +---------------+
		//  50                         | 000001        |
		//                             |               |
		//                             +---------------+
		// ______________________________________________________________
		//     a b c d e f g h i j k l m n o p q r s t u v w x y z

		if snapshots.Index(h.tombstoneLargestSeqNum) != snapshots.Index(h.fileSmallestSeqNum) {
			// Cannot resolve yet.
			unresolvedHints = append(unresolvedHints, h)
			continue
		}

		// The hint h will be resolved and dropped, regardless of whether
		// there are any tables that can be deleted.
		for l := h.tombstoneLevel + 1; l < numLevels; l++ {
			overlaps := v.Overlaps(l, base.UserKeyBoundsEndExclusive(h.start, h.end))
			iter := overlaps.Iter()
			for m := iter.First(); m != nil; m = iter.Next() {
				if m.IsCompacting() || !h.canDelete(cmp, m, snapshots) || files[m] {
					continue
				}
				if files == nil {
					// Construct files lazily, assuming most calls will not
					// produce delete-only compactions.
					files = make(map[*fileMetadata]bool)
				}
				files[m] = true
				byLevel[l] = append(byLevel[l], m)
			}
		}
	}

	var compactLevels []compactionLevel
	for l, files := range byLevel {
		if len(files) == 0 {
			continue
		}
		compactLevels = append(compactLevels, compactionLevel{
			level: l,
			files: manifest.NewLevelSliceKeySorted(cmp, files),
		})
	}
	return compactLevels, unresolvedHints
}

// compact runs one compaction and maybe schedules another call to compact.
func (d *DB) compact(c *compaction, errChannel chan error) {
	pprof.Do(context.Background(), compactLabels, func(context.Context) {
		d.mu.Lock()
		defer d.mu.Unlock()
		if err := d.compact1(c, errChannel); err != nil {
			// TODO(peter): count consecutive compaction errors and backoff.
			d.opts.EventListener.BackgroundError(err)
		}
		if c.isDownload {
			d.mu.compact.downloadingCount--
		} else {
			d.mu.compact.compactingCount--
		}
		delete(d.mu.compact.inProgress, c)
		// Add this compaction's duration to the cumulative duration. NB: This
		// must be atomic with the above removal of c from
		// d.mu.compact.InProgress to ensure Metrics.Compact.Duration does not
		// miss or double count a completing compaction's duration.
		d.mu.compact.duration += d.timeNow().Sub(c.beganAt)

		// The previous compaction may have produced too many files in a
		// level, so reschedule another compaction if needed.
		d.maybeScheduleCompaction()
		d.mu.compact.cond.Broadcast()
	})
}

// compact1 runs one compaction.
//
// d.mu must be held when calling this, but the mutex may be dropped and
// re-acquired during the course of this method.
func (d *DB) compact1(c *compaction, errChannel chan error) (err error) {
	if errChannel != nil {
		defer func() {
			errChannel <- err
		}()
	}

	jobID := d.newJobIDLocked()
	info := c.makeInfo(jobID)
	d.opts.EventListener.CompactionBegin(info)
	startTime := d.timeNow()

	ve, stats, err := d.runCompaction(jobID, c)

	info.Duration = d.timeNow().Sub(startTime)
	if err == nil {
		validateVersionEdit(ve, d.opts.Experimental.KeyValidationFunc, d.opts.Comparer.FormatKey, d.opts.Logger)
		err = func() error {
			var err error
			d.mu.versions.logLock()
			// Check if this compaction had a conflicting operation (eg. a d.excise())
			// that necessitates it restarting from scratch. Note that since we hold
			// the manifest lock, we don't expect this bool to change its value
			// as only the holder of the manifest lock will ever write to it.
			if c.cancel.Load() {
				err = firstError(err, ErrCancelledCompaction)
			}
			if err != nil {
				// logAndApply calls logUnlock. If we didn't call it, we need to call
				// logUnlock ourselves.
				d.mu.versions.logUnlock()
				return err
			}
			return d.mu.versions.logAndApply(jobID, ve, c.metrics, false /* forceRotation */, func() []compactionInfo {
				return d.getInProgressCompactionInfoLocked(c)
			})
		}()
	}

	info.Done = true
	info.Err = err
	if err == nil {
		for i := range ve.NewFiles {
			e := &ve.NewFiles[i]
			info.Output.Tables = append(info.Output.Tables, e.Meta.TableInfo())
		}
		d.mu.snapshots.cumulativePinnedCount += stats.CumulativePinnedKeys
		d.mu.snapshots.cumulativePinnedSize += stats.CumulativePinnedSize
		d.mu.versions.metrics.Keys.MissizedTombstonesCount += stats.CountMissizedDels
	}

	// NB: clearing compacting state must occur before updating the read state;
	// L0Sublevels initialization depends on it.
	d.clearCompactingState(c, err != nil)
	d.mu.versions.incrementCompactions(c.kind, c.extraLevels, c.pickerMetrics)
	d.mu.versions.incrementCompactionBytes(-c.bytesWritten)

	info.TotalDuration = d.timeNow().Sub(c.beganAt)
	d.opts.EventListener.CompactionEnd(info)

	// Update the read state before deleting obsolete files because the
	// read-state update will cause the previous version to be unref'd and if
	// there are no references obsolete tables will be added to the obsolete
	// table list.
	if err == nil {
		d.updateReadStateLocked(d.opts.DebugCheck)
		d.updateTableStatsLocked(ve.NewFiles)
	}
	d.deleteObsoleteFiles(jobID)

	return err
}

// runCopyCompaction runs a copy compaction where a new FileNum is created that
// is a byte-for-byte copy of the input file or span thereof in some cases. This
// is used in lieu of a move compaction when a file is being moved across the
// local/remote storage boundary. It could also be used in lieu of a rewrite
// compaction as part of a Download() call, which allows copying only a span of
// the external file, provided the file does not contain range keys or value
// blocks (see sstable.CopySpan).
//
// d.mu must be held when calling this method. The mutex will be released when
// doing IO.
func (d *DB) runCopyCompaction(
	jobID JobID, c *compaction,
) (ve *versionEdit, stats compact.Stats, _ error) {
	iter := c.startLevel.files.Iter()
	inputMeta := iter.First()
	if iter.Next() != nil {
		return nil, compact.Stats{}, base.AssertionFailedf("got more than one file for a move compaction")
	}
	if c.cancel.Load() {
		return nil, compact.Stats{}, ErrCancelledCompaction
	}
	ve = &versionEdit{
		DeletedFiles: map[deletedFileEntry]*fileMetadata{
			{Level: c.startLevel.level, FileNum: inputMeta.FileNum}: inputMeta,
		},
	}

	objMeta, err := d.objProvider.Lookup(fileTypeTable, inputMeta.FileBacking.DiskFileNum)
	if err != nil {
		return nil, compact.Stats{}, err
	}
	if !objMeta.IsExternal() {
		if objMeta.IsRemote() || !remote.ShouldCreateShared(d.opts.Experimental.CreateOnShared, c.outputLevel.level) {
			panic("pebble: scheduled a copy compaction that is not actually moving files to shared storage")
		}
		// Note that based on logic in the compaction picker, we're guaranteed
		// inputMeta.Virtual is false.
		if inputMeta.Virtual {
			panic(errors.AssertionFailedf("cannot do a copy compaction of a virtual sstable across local/remote storage"))
		}
	}

	// We are in the relatively more complex case where we need to copy this
	// file to remote storage. Drop the db mutex while we do the copy
	//
	// To ease up cleanup of the local file and tracking of refs, we create
	// a new FileNum. This has the potential of making the block cache less
	// effective, however.
	newMeta := &fileMetadata{
		Size:                  inputMeta.Size,
		CreationTime:          inputMeta.CreationTime,
		SmallestSeqNum:        inputMeta.SmallestSeqNum,
		LargestSeqNum:         inputMeta.LargestSeqNum,
		LargestSeqNumAbsolute: inputMeta.LargestSeqNumAbsolute,
		Stats:                 inputMeta.Stats,
		Virtual:               inputMeta.Virtual,
		SyntheticPrefix:       inputMeta.SyntheticPrefix,
		SyntheticSuffix:       inputMeta.SyntheticSuffix,
	}
	if inputMeta.HasPointKeys {
		newMeta.ExtendPointKeyBounds(c.cmp, inputMeta.SmallestPointKey, inputMeta.LargestPointKey)
	}
	if inputMeta.HasRangeKeys {
		newMeta.ExtendRangeKeyBounds(c.cmp, inputMeta.SmallestRangeKey, inputMeta.LargestRangeKey)
	}
	newMeta.FileNum = d.mu.versions.getNextFileNum()
	if objMeta.IsExternal() {
		// external -> local/shared copy. File must be virtual.
		// We will update this size later after we produce the new backing file.
		newMeta.InitProviderBacking(base.DiskFileNum(newMeta.FileNum), inputMeta.FileBacking.Size)
	} else {
		// local -> shared copy. New file is guaranteed to not be virtual.
		newMeta.InitPhysicalBacking()
	}

	// Before dropping the db mutex, grab a ref to the current version. This
	// prevents any concurrent excises from deleting files that this compaction
	// needs to read/maintain a reference to.
	vers := d.mu.versions.currentVersion()
	vers.Ref()
	defer vers.UnrefLocked()

	// NB: The order here is reversed, lock after unlock. This is similar to
	// runCompaction.
	d.mu.Unlock()
	defer d.mu.Lock()

	deleteOnExit := false
	defer func() {
		if deleteOnExit {
			_ = d.objProvider.Remove(fileTypeTable, newMeta.FileBacking.DiskFileNum)
		}
	}()

	// If the src obj is external, we're doing an external to local/shared copy.
	if objMeta.IsExternal() {
		ctx := context.TODO()
		src, err := d.objProvider.OpenForReading(
			ctx, fileTypeTable, inputMeta.FileBacking.DiskFileNum, objstorage.OpenOptions{},
		)
		if err != nil {
			return nil, compact.Stats{}, err
		}
		defer func() {
			if src != nil {
				src.Close()
			}
		}()

		w, _, err := d.objProvider.Create(
			ctx, fileTypeTable, newMeta.FileBacking.DiskFileNum,
			objstorage.CreateOptions{
				PreferSharedStorage: remote.ShouldCreateShared(d.opts.Experimental.CreateOnShared, c.outputLevel.level),
			},
		)
		if err != nil {
			return nil, compact.Stats{}, err
		}
		deleteOnExit = true

		start, end := newMeta.Smallest, newMeta.Largest
		if newMeta.SyntheticPrefix.IsSet() {
			start.UserKey = newMeta.SyntheticPrefix.Invert(start.UserKey)
			end.UserKey = newMeta.SyntheticPrefix.Invert(end.UserKey)
		}
		if newMeta.SyntheticSuffix.IsSet() {
			// Extend the bounds as necessary so that the keys don't include suffixes.
			start.UserKey = start.UserKey[:c.comparer.Split(start.UserKey)]
			if n := c.comparer.Split(end.UserKey); n < len(end.UserKey) {
				end = base.MakeRangeDeleteSentinelKey(c.comparer.ImmediateSuccessor(nil, end.UserKey[:n]))
			}
		}

		// NB: external files are always virtual.
		var wrote uint64
		err = d.tableCache.withVirtualReader(inputMeta.VirtualMeta(), func(r sstable.VirtualReader) error {
			var err error
			wrote, err = sstable.CopySpan(ctx,
				src, r.UnsafeReader(), d.opts.MakeReaderOptions(),
				w, d.opts.MakeWriterOptions(c.outputLevel.level, d.FormatMajorVersion().MaxTableFormat()),
				start, end,
			)
			return err
		})

		src = nil // We passed src to CopySpan; it's responsible for closing it.
		if err != nil {
			if errors.Is(err, sstable.ErrEmptySpan) {
				// The virtual table was empty. Just remove the backing file.
				// Note that deleteOnExit is true so we will delete the created object.
				c.metrics = map[int]*LevelMetrics{
					c.outputLevel.level: {
						BytesIn: inputMeta.Size,
					},
				}
				return ve, compact.Stats{}, nil
			}
			return nil, compact.Stats{}, err
		}
		newMeta.FileBacking.Size = wrote
		newMeta.Size = wrote
	} else {
		_, err := d.objProvider.LinkOrCopyFromLocal(context.TODO(), d.opts.FS,
			d.objProvider.Path(objMeta), fileTypeTable, newMeta.FileBacking.DiskFileNum,
			objstorage.CreateOptions{PreferSharedStorage: true})
		if err != nil {
			return nil, compact.Stats{}, err
		}
		deleteOnExit = true
	}
	ve.NewFiles = []newFileEntry{{
		Level: c.outputLevel.level,
		Meta:  newMeta,
	}}
	if newMeta.Virtual {
		ve.CreatedBackingTables = []*fileBacking{newMeta.FileBacking}
	}
	c.metrics = map[int]*LevelMetrics{
		c.outputLevel.level: {
			BytesIn:         inputMeta.Size,
			BytesCompacted:  newMeta.Size,
			TablesCompacted: 1,
		},
	}

	if err := d.objProvider.Sync(); err != nil {
		return nil, compact.Stats{}, err
	}
	deleteOnExit = false
	return ve, compact.Stats{}, nil
}

func (d *DB) runDeleteOnlyCompaction(
	jobID JobID, c *compaction,
) (ve *versionEdit, stats compact.Stats, retErr error) {
	c.metrics = make(map[int]*LevelMetrics, len(c.inputs))
	ve = &versionEdit{
		DeletedFiles: map[deletedFileEntry]*fileMetadata{},
	}
	for _, cl := range c.inputs {
		levelMetrics := &LevelMetrics{}
		iter := cl.files.Iter()
		for f := iter.First(); f != nil; f = iter.Next() {
			ve.DeletedFiles[deletedFileEntry{
				Level:   cl.level,
				FileNum: f.FileNum,
			}] = f
		}
		c.metrics[cl.level] = levelMetrics
	}
	return ve, stats, nil
}

func (d *DB) runMoveCompaction(
	jobID JobID, c *compaction,
) (ve *versionEdit, stats compact.Stats, _ error) {
	iter := c.startLevel.files.Iter()
	meta := iter.First()
	if iter.Next() != nil {
		return nil, stats, base.AssertionFailedf("got more than one file for a move compaction")
	}
	if c.cancel.Load() {
		return ve, stats, ErrCancelledCompaction
	}
	c.metrics = map[int]*LevelMetrics{
		c.outputLevel.level: {
			BytesMoved:  meta.Size,
			TablesMoved: 1,
		},
	}
	ve = &versionEdit{
		DeletedFiles: map[deletedFileEntry]*fileMetadata{
			{Level: c.startLevel.level, FileNum: meta.FileNum}: meta,
		},
		NewFiles: []newFileEntry{
			{Level: c.outputLevel.level, Meta: meta},
		},
	}

	return ve, stats, nil
}

// runCompaction runs a compaction that produces new on-disk tables from
// memtables or old on-disk tables.
//
// runCompaction cannot be used for compactionKindIngestedFlushable.
//
// d.mu must be held when calling this, but the mutex may be dropped and
// re-acquired during the course of this method.
func (d *DB) runCompaction(
	jobID JobID, c *compaction,
) (ve *versionEdit, stats compact.Stats, retErr error) {
	switch c.kind {
	case compactionKindDeleteOnly:
		return d.runDeleteOnlyCompaction(jobID, c)
	case compactionKindMove:
		return d.runMoveCompaction(jobID, c)
	case compactionKindCopy:
		return d.runCopyCompaction(jobID, c)
	case compactionKindIngestedFlushable:
		panic("pebble: runCompaction cannot handle compactionKindIngestedFlushable.")
	}

	snapshots := d.mu.snapshots.toSlice()

	if c.flushing == nil {
		// Before dropping the db mutex, grab a ref to the current version. This
		// prevents any concurrent excises from deleting files that this compaction
		// needs to read/maintain a reference to.
		//
		// Note that unlike user iterators, compactionIter does not maintain a ref
		// of the version or read state.
		vers := d.mu.versions.currentVersion()
		vers.Ref()
		defer vers.UnrefLocked()
	}

	if c.cancel.Load() {
		return ve, stats, ErrCancelledCompaction
	}

	// The table is typically written at the maximum allowable format implied by
	// the current format major version of the DB.
	tableFormat := d.FormatMajorVersion().MaxTableFormat()
	// In format major versions with maximum table formats of Pebblev3, value
	// blocks were conditional on an experimental setting. In format major
	// versions with maximum table formats of Pebblev4 and higher, value blocks
	// are always enabled.
	if tableFormat == sstable.TableFormatPebblev3 &&
		(d.opts.Experimental.EnableValueBlocks == nil || !d.opts.Experimental.EnableValueBlocks()) {
		tableFormat = sstable.TableFormatPebblev2
	}

	// Release the d.mu lock while doing I/O.
	// Note the unusual order: Unlock and then Lock.
	d.mu.Unlock()
	defer d.mu.Lock()

	result := d.compactAndWrite(jobID, c, snapshots, tableFormat)
	if result.Err == nil {
		ve, result.Err = c.makeVersionEdit(result)
	}
	if result.Err != nil {
		// Delete any created tables.
		for i := range result.Tables {
			_ = d.objProvider.Remove(fileTypeTable, result.Tables[i].ObjMeta.DiskFileNum)
		}
	}
	// Refresh the disk available statistic whenever a compaction/flush
	// completes, before re-acquiring the mutex.
	d.calculateDiskAvailableBytes()
	return ve, result.Stats, result.Err
}

// compactAndWrite runs the data part of a compaction, where we set up a
// compaction iterator and use it to write output tables.
func (d *DB) compactAndWrite(
	jobID JobID, c *compaction, snapshots compact.Snapshots, tableFormat sstable.TableFormat,
) (result compact.Result) {
	// Compactions use a pool of buffers to read blocks, avoiding polluting the
	// block cache with blocks that will not be read again. We initialize the
	// buffer pool with a size 12. This initial size does not need to be
	// accurate, because the pool will grow to accommodate the maximum number of
	// blocks allocated at a given time over the course of the compaction. But
	// choosing a size larger than that working set avoids any additional
	// allocations to grow the size of the pool over the course of iteration.
	//
	// Justification for initial size 12: In a two-level compaction, at any
	// given moment we'll have 2 index blocks in-use and 2 data blocks in-use.
	// Additionally, when decoding a compressed block, we'll temporarily
	// allocate 1 additional block to hold the compressed buffer. In the worst
	// case that all input sstables have two-level index blocks (+2), value
	// blocks (+2), range deletion blocks (+n) and range key blocks (+n), we'll
	// additionally require 2n+4 blocks where n is the number of input sstables.
	// Range deletion and range key blocks are relatively rare, and the cost of
	// an additional allocation or two over the course of the compaction is
	// considered to be okay. A larger initial size would cause the pool to hold
	// on to more memory, even when it's not in-use because the pool will
	// recycle buffers up to the current capacity of the pool. The memory use of
	// a 12-buffer pool is expected to be within reason, even if all the buffers
	// grow to the typical size of an index block (256 KiB) which would
	// translate to 3 MiB per compaction.
	c.bufferPool.Init(12)
	defer c.bufferPool.Release()

	pointIter, rangeDelIter, rangeKeyIter, err := c.newInputIters(d.newIters, d.tableNewRangeKeyIter)
	defer func() {
		for _, closer := range c.closers {
			closer.FragmentIterator.Close()
		}
	}()
	if err != nil {
		return compact.Result{Err: err}
	}
	c.allowedZeroSeqNum = c.allowZeroSeqNum()
	cfg := compact.IterConfig{
		Comparer:                               c.comparer,
		Merge:                                  d.merge,
		TombstoneElision:                       c.delElision,
		RangeKeyElision:                        c.rangeKeyElision,
		Snapshots:                              snapshots,
		AllowZeroSeqNum:                        c.allowedZeroSeqNum,
		IneffectualSingleDeleteCallback:        d.opts.Experimental.IneffectualSingleDeleteCallback,
		SingleDeleteInvariantViolationCallback: d.opts.Experimental.SingleDeleteInvariantViolationCallback,
	}
	iter := compact.NewIter(cfg, pointIter, rangeDelIter, rangeKeyIter)

	runnerCfg := compact.RunnerConfig{
		CompactionBounds:           base.UserKeyBoundsFromInternal(c.smallest, c.largest),
		L0SplitKeys:                c.l0Limits,
		Grandparents:               c.grandparents,
		MaxGrandparentOverlapBytes: c.maxOverlapBytes,
		TargetOutputFileSize:       c.maxOutputFileSize,
	}
	runner := compact.NewRunner(runnerCfg, iter)
	for runner.MoreDataToWrite() {
		if c.cancel.Load() {
			return runner.Finish().WithError(ErrCancelledCompaction)
		}
		// Create a new table.
		writerOpts := d.opts.MakeWriterOptions(c.outputLevel.level, tableFormat)
		objMeta, tw, cpuWorkHandle, err := d.newCompactionOutput(jobID, c, writerOpts)
		if err != nil {
			return runner.Finish().WithError(err)
		}
		runner.WriteTable(objMeta, tw)
		d.opts.Experimental.CPUWorkPermissionGranter.CPUWorkDone(cpuWorkHandle)
	}
	result = runner.Finish()
	if result.Err == nil {
		result.Err = d.objProvider.Sync()
	}
	return result
}

// makeVersionEdit creates the version edit for a compaction, based on the
// tables in compact.Result.
func (c *compaction) makeVersionEdit(result compact.Result) (*versionEdit, error) {
	ve := &versionEdit{
		DeletedFiles: map[deletedFileEntry]*fileMetadata{},
	}
	for _, cl := range c.inputs {
		iter := cl.files.Iter()
		for f := iter.First(); f != nil; f = iter.Next() {
			ve.DeletedFiles[deletedFileEntry{
				Level:   cl.level,
				FileNum: f.FileNum,
			}] = f
		}
	}

	startLevelBytes := c.startLevel.files.SizeSum()
	outputMetrics := &LevelMetrics{
		BytesIn:   startLevelBytes,
		BytesRead: c.outputLevel.files.SizeSum(),
	}
	if len(c.extraLevels) > 0 {
		outputMetrics.BytesIn += c.extraLevels[0].files.SizeSum()
	}
	outputMetrics.BytesRead += outputMetrics.BytesIn

	c.metrics = map[int]*LevelMetrics{
		c.outputLevel.level: outputMetrics,
	}
	if len(c.flushing) == 0 && c.metrics[c.startLevel.level] == nil {
		c.metrics[c.startLevel.level] = &LevelMetrics{}
	}
	if len(c.extraLevels) > 0 {
		c.metrics[c.extraLevels[0].level] = &LevelMetrics{}
		outputMetrics.MultiLevel.BytesInTop = startLevelBytes
		outputMetrics.MultiLevel.BytesIn = outputMetrics.BytesIn
		outputMetrics.MultiLevel.BytesRead = outputMetrics.BytesRead
	}

	inputLargestSeqNumAbsolute := c.inputLargestSeqNumAbsolute()
	ve.NewFiles = make([]newFileEntry, len(result.Tables))
	for i := range result.Tables {
		t := &result.Tables[i]

		fileMeta := &fileMetadata{
			FileNum:        base.PhysicalTableFileNum(t.ObjMeta.DiskFileNum),
			CreationTime:   t.CreationTime.Unix(),
			Size:           t.WriterMeta.Size,
			SmallestSeqNum: t.WriterMeta.SmallestSeqNum,
			LargestSeqNum:  t.WriterMeta.LargestSeqNum,
		}
		if c.flushing == nil {
			// Set the file's LargestSeqNumAbsolute to be the maximum value of any
			// of the compaction's input sstables.
			// TODO(jackson): This could be narrowed to be the maximum of input
			// sstables that overlap the output sstable's key range.
			fileMeta.LargestSeqNumAbsolute = inputLargestSeqNumAbsolute
		} else {
			fileMeta.LargestSeqNumAbsolute = t.WriterMeta.LargestSeqNum
		}
		fileMeta.InitPhysicalBacking()

		// If the file didn't contain any range deletions, we can fill its
		// table stats now, avoiding unnecessarily loading the table later.
		maybeSetStatsFromProperties(
			fileMeta.PhysicalMeta(), &t.WriterMeta.Properties,
		)

		if t.WriterMeta.HasPointKeys {
			fileMeta.ExtendPointKeyBounds(c.cmp, t.WriterMeta.SmallestPoint, t.WriterMeta.LargestPoint)
		}
		if t.WriterMeta.HasRangeDelKeys {
			fileMeta.ExtendPointKeyBounds(c.cmp, t.WriterMeta.SmallestRangeDel, t.WriterMeta.LargestRangeDel)
		}
		if t.WriterMeta.HasRangeKeys {
			fileMeta.ExtendRangeKeyBounds(c.cmp, t.WriterMeta.SmallestRangeKey, t.WriterMeta.LargestRangeKey)
		}

		ve.NewFiles[i] = newFileEntry{
			Level: c.outputLevel.level,
			Meta:  fileMeta,
		}

		// Update metrics.
		if c.flushing == nil {
			outputMetrics.TablesCompacted++
			outputMetrics.BytesCompacted += fileMeta.Size
		} else {
			outputMetrics.TablesFlushed++
			outputMetrics.BytesFlushed += fileMeta.Size
		}
		outputMetrics.Size += int64(fileMeta.Size)
		outputMetrics.NumFiles++
		outputMetrics.Additional.BytesWrittenDataBlocks += t.WriterMeta.Properties.DataSize
		outputMetrics.Additional.BytesWrittenValueBlocks += t.WriterMeta.Properties.ValueBlocksSize
	}

	// Sanity check that the tables are ordered and don't overlap.
	for i := 1; i < len(ve.NewFiles); i++ {
		if ve.NewFiles[i-1].Meta.UserKeyBounds().End.IsUpperBoundFor(c.cmp, ve.NewFiles[i].Meta.Smallest.UserKey) {
			return nil, base.AssertionFailedf("pebble: compaction output tables overlap: %s and %s",
				ve.NewFiles[i-1].Meta.DebugString(c.formatKey, true),
				ve.NewFiles[i].Meta.DebugString(c.formatKey, true),
			)
		}
	}

	return ve, nil
}

// newCompactionOutput creates an object for a new table produced by a
// compaction or flush.
func (d *DB) newCompactionOutput(
	jobID JobID, c *compaction, writerOpts sstable.WriterOptions,
) (objstorage.ObjectMetadata, *sstable.RawWriter, CPUWorkHandle, error) {
	d.mu.Lock()
	diskFileNum := d.mu.versions.getNextDiskFileNum()
	d.mu.Unlock()

	var writeCategory vfs.DiskWriteCategory
	if d.opts.EnableSQLRowSpillMetrics {
		// In the scenario that the Pebble engine is used for SQL row spills the
		// data written to the memtable will correspond to spills to disk and
		// should be categorized as such.
		writeCategory = "sql-row-spill"
	} else if c.kind == compactionKindFlush {
		writeCategory = "pebble-memtable-flush"
	} else {
		writeCategory = "pebble-compaction"
	}

	var reason string
	if c.kind == compactionKindFlush {
		reason = "flushing"
	} else {
		reason = "compacting"
	}

	ctx := context.TODO()
	if objiotracing.Enabled {
		ctx = objiotracing.WithLevel(ctx, c.outputLevel.level)
		if c.kind == compactionKindFlush {
			ctx = objiotracing.WithReason(ctx, objiotracing.ForFlush)
		} else {
			ctx = objiotracing.WithReason(ctx, objiotracing.ForCompaction)
		}
	}

	// Prefer shared storage if present.
	createOpts := objstorage.CreateOptions{
		PreferSharedStorage: remote.ShouldCreateShared(d.opts.Experimental.CreateOnShared, c.outputLevel.level),
		WriteCategory:       writeCategory,
	}
	writable, objMeta, err := d.objProvider.Create(ctx, fileTypeTable, diskFileNum, createOpts)
	if err != nil {
		return objstorage.ObjectMetadata{}, nil, nil, err
	}

	if c.kind != compactionKindFlush {
		writable = &compactionWritable{
			Writable: writable,
			versions: d.mu.versions,
			written:  &c.bytesWritten,
		}
	}
	d.opts.EventListener.TableCreated(TableCreateInfo{
		JobID:   int(jobID),
		Reason:  reason,
		Path:    d.objProvider.Path(objMeta),
		FileNum: diskFileNum,
	})

	writerOpts.SetInternal(sstableinternal.WriterOptions{
		CacheOpts: sstableinternal.CacheOptions{
			Cache:   d.opts.Cache,
			CacheID: d.cacheID,
			FileNum: diskFileNum,
		},
	})

	const MaxFileWriteAdditionalCPUTime = time.Millisecond * 100
	cpuWorkHandle := d.opts.Experimental.CPUWorkPermissionGranter.GetPermission(
		MaxFileWriteAdditionalCPUTime,
	)
	writerOpts.Parallelism =
		d.opts.Experimental.MaxWriterConcurrency > 0 &&
			(cpuWorkHandle.Permitted() || d.opts.Experimental.ForceWriterParallelism)

	tw := sstable.NewRawWriter(writable, writerOpts)
	return objMeta, tw, cpuWorkHandle, nil
}

// validateVersionEdit validates that start and end keys across new and deleted
// files in a versionEdit pass the given validation function.
func validateVersionEdit(
	ve *versionEdit, validateFn func([]byte) error, format base.FormatKey, logger Logger,
) {
	validateKey := func(f *manifest.FileMetadata, key []byte) {
		if err := validateFn(key); err != nil {
			logger.Fatalf("pebble: version edit validation failed (key=%s file=%s): %v", format(key), f, err)
		}
	}

	// Validate both new and deleted files.
	for _, f := range ve.NewFiles {
		validateKey(f.Meta, f.Meta.Smallest.UserKey)
		validateKey(f.Meta, f.Meta.Largest.UserKey)
	}
	for _, m := range ve.DeletedFiles {
		validateKey(m, m.Smallest.UserKey)
		validateKey(m, m.Largest.UserKey)
	}
}