github.com/cockroachdb/pebble@v0.0.0-20231214172447-ab4952c5f87b/metamorphic/generator.go

// Copyright 2019 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 metamorphic

import (
	"bytes"
	"fmt"
	"os"
	"slices"

	"github.com/cockroachdb/pebble"
	"github.com/cockroachdb/pebble/internal/randvar"
	"github.com/cockroachdb/pebble/internal/testkeys"
	"golang.org/x/exp/rand"
)

const maxValueSize = 20

type iterOpts struct {
	lower    []byte
	upper    []byte
	keyTypes uint32 // pebble.IterKeyType
	// maskSuffix may be set if keyTypes is IterKeyTypePointsAndRanges to
	// configure IterOptions.RangeKeyMasking.Suffix.
	maskSuffix []byte

	// If filterMax is >0, this iterator will filter out any keys that have
	// suffixes that don't fall within the range [filterMin,filterMax).
	// Additionally, the iterator will be constructed with a block-property
	// filter that filters out blocks accordingly. Not all OPTIONS hook up the
	// corresponding block property collector, so block-filtering may still be
	// effectively disabled in some runs. The iterator operations themselves
	// however will always skip past any points that should be filtered to
	// ensure determinism.
	filterMin uint64
	filterMax uint64

	// see IterOptions.UseL6Filters.
	useL6Filters bool

	// NB: If adding or removing fields, ensure IsZero is in sync.
}

func (o iterOpts) IsZero() bool {
	return o.lower == nil && o.upper == nil && o.keyTypes == 0 &&
		o.maskSuffix == nil && o.filterMin == 0 && o.filterMax == 0 && !o.useL6Filters
}

type generator struct {
	cfg config
	rng *rand.Rand

	init *initOp
	ops  []op

	// keyManager tracks the state of keys a operation generation time.
	keyManager *keyManager
	dbs        objIDSlice
	// Unordered sets of object IDs for live objects. Used to randomly select on
	// object when generating an operation. There are 4 concrete objects: the DB
	// (of which there is exactly 1), batches, iterators, and snapshots.
	//
	// liveBatches contains the live indexed and write-only batches.
	liveBatches objIDSlice
	// liveIters contains the live iterators.
	liveIters     objIDSlice
	itersLastOpts map[objID]iterOpts
	// liveReaders contains the DB, and any live indexed batches and snapshots. The DB is always
	// at index 0.
	liveReaders objIDSlice
	// liveSnapshots contains the live snapshots.
	liveSnapshots objIDSlice
	// liveWriters contains the DB, and any live batches. The DB is always at index 0.
	liveWriters objIDSlice

	// Maps used to find associated objects during generation. These maps are not
	// needed during test execution.
	//
	// batchID -> batch iters: used to keep track of the open iterators on an
	// indexed batch. The iter set value will also be indexed by the readers map.
	batches map[objID]objIDSet
	// iterID -> reader iters: used to keep track of all of the open
	// iterators. The iter set value will also be indexed by either the batches
	// or snapshots maps.
	iters map[objID]objIDSet
	// objectID -> db: used to keep track of the DB a batch, iter, or snapshot
	// was created on.
	objDB map[objID]objID
	// readerID -> reader iters: used to keep track of the open iterators on a
	// reader. The iter set value will also be indexed by either the batches or
	// snapshots maps. This map is the union of batches and snapshots maps.
	readers map[objID]objIDSet
	// snapshotID -> snapshot iters: used to keep track of the open iterators on
	// a snapshot. The iter set value will also be indexed by the readers map.
	snapshots map[objID]objIDSet
	// snapshotID -> bounds of the snapshot: only populated for snapshots that
	// are constrained by bounds.
	snapshotBounds map[objID][]pebble.KeyRange
	// iterSequenceNumber is the metaTimestamp at which the iter was created.
	iterCreationTimestamp map[objID]int
	// iterReaderID is a map from an iterID to a readerID.
	iterReaderID map[objID]objID
}

func newGenerator(rng *rand.Rand, cfg config, km *keyManager) *generator {
	g := &generator{
		cfg:                   cfg,
		rng:                   rng,
		init:                  &initOp{dbSlots: uint32(cfg.numInstances)},
		keyManager:            km,
		liveReaders:           objIDSlice{makeObjID(dbTag, 1)},
		liveWriters:           objIDSlice{makeObjID(dbTag, 1)},
		dbs:                   objIDSlice{makeObjID(dbTag, 1)},
		objDB:                 make(map[objID]objID),
		batches:               make(map[objID]objIDSet),
		iters:                 make(map[objID]objIDSet),
		readers:               make(map[objID]objIDSet),
		snapshots:             make(map[objID]objIDSet),
		snapshotBounds:        make(map[objID][]pebble.KeyRange),
		itersLastOpts:         make(map[objID]iterOpts),
		iterCreationTimestamp: make(map[objID]int),
		iterReaderID:          make(map[objID]objID),
	}
	for i := 1; i < cfg.numInstances; i++ {
		g.liveReaders = append(g.liveReaders, makeObjID(dbTag, uint32(i+1)))
		g.liveWriters = append(g.liveWriters, makeObjID(dbTag, uint32(i+1)))
		g.dbs = append(g.dbs, makeObjID(dbTag, uint32(i+1)))
	}
	// Note that the initOp fields are populated during generation.
	g.ops = append(g.ops, g.init)
	return g
}

func generate(rng *rand.Rand, count uint64, cfg config, km *keyManager) []op {
	g := newGenerator(rng, cfg, km)

	generators := []func(){
		batchAbort:                  g.batchAbort,
		batchCommit:                 g.batchCommit,
		dbCheckpoint:                g.dbCheckpoint,
		dbCompact:                   g.dbCompact,
		dbFlush:                     g.dbFlush,
		dbRatchetFormatMajorVersion: g.dbRatchetFormatMajorVersion,
		dbRestart:                   g.dbRestart,
		iterClose:                   g.randIter(g.iterClose),
		iterFirst:                   g.randIter(g.iterFirst),
		iterLast:                    g.randIter(g.iterLast),
		iterNext:                    g.randIter(g.iterNext),
		iterNextWithLimit:           g.randIter(g.iterNextWithLimit),
		iterNextPrefix:              g.randIter(g.iterNextPrefix),
		iterCanSingleDelete:         g.randIter(g.iterCanSingleDelete),
		iterPrev:                    g.randIter(g.iterPrev),
		iterPrevWithLimit:           g.randIter(g.iterPrevWithLimit),
		iterSeekGE:                  g.randIter(g.iterSeekGE),
		iterSeekGEWithLimit:         g.randIter(g.iterSeekGEWithLimit),
		iterSeekLT:                  g.randIter(g.iterSeekLT),
		iterSeekLTWithLimit:         g.randIter(g.iterSeekLTWithLimit),
		iterSeekPrefixGE:            g.randIter(g.iterSeekPrefixGE),
		iterSetBounds:               g.randIter(g.iterSetBounds),
		iterSetOptions:              g.randIter(g.iterSetOptions),
		newBatch:                    g.newBatch,
		newIndexedBatch:             g.newIndexedBatch,
		newIter:                     g.newIter,
		newIterUsingClone:           g.newIterUsingClone,
		newSnapshot:                 g.newSnapshot,
		readerGet:                   g.readerGet,
		replicate:                   g.replicate,
		snapshotClose:               g.snapshotClose,
		writerApply:                 g.writerApply,
		writerDelete:                g.writerDelete,
		writerDeleteRange:           g.writerDeleteRange,
		writerIngest:                g.writerIngest,
		writerIngestAndExcise:       g.writerIngestAndExcise,
		writerMerge:                 g.writerMerge,
		writerRangeKeyDelete:        g.writerRangeKeyDelete,
		writerRangeKeySet:           g.writerRangeKeySet,
		writerRangeKeyUnset:         g.writerRangeKeyUnset,
		writerSet:                   g.writerSet,
		writerSingleDelete:          g.writerSingleDelete,
	}

	// TPCC-style deck of cards randomization. Every time the end of the deck is
	// reached, we shuffle the deck.
	deck := randvar.NewDeck(g.rng, cfg.ops...)

	defer func() {
		if r := recover(); r != nil {
			fmt.Fprintln(os.Stderr, formatOps(g.ops))
			panic(r)
		}
	}()
	for i := uint64(0); i < count; i++ {
		generators[deck.Int()]()
	}

	g.dbClose()
	return g.ops
}

func (g *generator) add(op op) {
	g.ops = append(g.ops, op)
	g.keyManager.update(op)
}

// randKeyToWrite returns a key for any write other than SingleDelete.
//
// TODO(peter): make the size and distribution of keys configurable. See
// keyDist and keySizeDist in config.go.
func (g *generator) randKeyToWrite(newKey float64) []byte {
	return g.randKeyHelper(g.keyManager.knownKeys(), newKey, nil)
}

// prefixKeyRange generates a [start, end) pair consisting of two prefix keys.
func (g *generator) prefixKeyRange() ([]byte, []byte) {
	start := g.randPrefixToWrite(0.001)
	end := g.randPrefixToWrite(0.001)
	for g.cmp(start, end) == 0 {
		end = g.randPrefixToWrite(0.05)
	}
	if g.cmp(start, end) > 0 {
		start, end = end, start
	}
	return start, end
}

// randPrefixToWrite returns a prefix key (a key with no suffix) for a range key
// write operation.
func (g *generator) randPrefixToWrite(newPrefix float64) []byte {
	prefixes := g.keyManager.prefixes()
	if len(prefixes) > 0 && g.rng.Float64() > newPrefix {
		// Use an existing prefix.
		p := g.rng.Intn(len(prefixes))
		return prefixes[p]
	}

	// Use a new prefix.
	var prefix []byte
	for {
		prefix = g.randKeyHelperSuffix(nil, 4, 12, 0)
		if !g.keyManager.prefixExists(prefix) {
			if !g.keyManager.addNewKey(prefix) {
				panic("key must not exist if prefix doesn't exist")
			}
			return prefix
		}
	}
}

// randSuffixToWrite generates a random suffix according to the configuration's suffix
// distribution. It takes a probability 0 ≤ p ≤ 1.0 indicating the probability
// with which the generator should increase the max suffix generated by the
// generator.
//
// randSuffixToWrite may return a nil suffix, with the probability the
// configuration's suffix distribution assigns to the zero suffix.
func (g *generator) randSuffixToWrite(incMaxProb float64) []byte {
	if g.rng.Float64() < incMaxProb {
		g.cfg.writeSuffixDist.IncMax(1)
	}
	return suffixFromInt(int64(g.cfg.writeSuffixDist.Uint64(g.rng)))
}

// randSuffixToRead generates a random suffix used during reads. The suffixes
// generated by this function are within the same range as suffixes generated by
// randSuffixToWrite, however randSuffixToRead pulls from a uniform
// distribution.
func (g *generator) randSuffixToRead() []byte {
	// When reading, don't apply the recency skewing in order to better exercise
	// a reading a mix of older and newer keys.
	max := g.cfg.writeSuffixDist.Max()
	return suffixFromInt(g.rng.Int63n(int64(max)))
}

func suffixFromInt(suffix int64) []byte {
	// Treat the zero as no suffix to match the behavior during point key
	// generation in randKeyHelper.
	if suffix == 0 {
		return nil
	}
	return testkeys.Suffix(suffix)
}

func (g *generator) randKeyToSingleDelete(id objID) []byte {
	keys := g.keyManager.eligibleSingleDeleteKeys(id)
	length := len(keys)
	if length == 0 {
		return nil
	}
	return keys[g.rng.Intn(length)]
}

// randKeyToRead returns a key for read operations.
func (g *generator) randKeyToRead(newKey float64) []byte {
	return g.randKeyHelper(g.keyManager.knownKeys(), newKey, nil)
}

// randKeyToReadInRange returns a key for read operations within the provided
// key range. The bounds of the provided key range must span a prefix boundary.
func (g *generator) randKeyToReadInRange(newKey float64, kr pebble.KeyRange) []byte {
	return g.randKeyHelper(g.keyManager.knownKeysInRange(kr), newKey, &kr)
}

func (g *generator) randKeyHelper(
	keys [][]byte, newKey float64, newKeyBounds *pebble.KeyRange,
) []byte {
	switch {
	case len(keys) > 0 && g.rng.Float64() > newKey:
		// Use an existing user key.
		return keys[g.rng.Intn(len(keys))]

	case len(keys) > 0 && g.rng.Float64() > g.cfg.newPrefix:
		// Use an existing prefix but a new suffix, producing a new user key.
		prefixes := g.keyManager.prefixes()

		// If we're constrained to a key range, find which existing prefixes
		// fall within that key range.
		if newKeyBounds != nil {
			s, _ := slices.BinarySearchFunc(prefixes, newKeyBounds.Start, g.cmp)
			e, _ := slices.BinarySearchFunc(prefixes, newKeyBounds.End, g.cmp)
			prefixes = prefixes[s:e]
		}

		if len(prefixes) > 0 {
			for {
				// Pick a prefix on each iteration in case most or all suffixes are
				// already in use for any individual prefix.
				p := g.rng.Intn(len(prefixes))
				suffix := int64(g.cfg.writeSuffixDist.Uint64(g.rng))

				var key []byte
				if suffix > 0 {
					key = resizeBuffer(key, len(prefixes[p]), testkeys.SuffixLen(suffix))
					n := copy(key, prefixes[p])
					testkeys.WriteSuffix(key[n:], suffix)
				} else {
					key = resizeBuffer(key, len(prefixes[p]), 0)
					copy(key, prefixes[p])
				}

				if (newKeyBounds == nil || (g.cmp(key, newKeyBounds.Start) >= 0 && g.cmp(key, newKeyBounds.End) < 0)) &&
					g.keyManager.addNewKey(key) {
					return key
				}

				// If the generated key already existed, or the generated key
				// fell outside the provided bounds, increase the suffix
				// distribution and loop.
				g.cfg.writeSuffixDist.IncMax(1)
			}
		}
		// Otherwise fall through to generating a new prefix.
		fallthrough

	default:
		// Use a new prefix, producing a new user key.

		var key []byte

		suffix := int64(g.cfg.writeSuffixDist.Uint64(g.rng))

		// If we have bounds in which we need to generate the key, use
		// testkeys.RandomSeparator to generate a key between the bounds.
		if newKeyBounds != nil {
			targetLength := 4 + g.rng.Intn(8)
			key = testkeys.RandomSeparator(nil, g.prefix(newKeyBounds.Start), g.prefix(newKeyBounds.End),
				suffix, targetLength, g.rng)
		} else {
			for {
				key = g.randKeyHelperSuffix(nil, 4, 12, suffix)
				if !g.keyManager.prefixExists(key[:testkeys.Comparer.Split(key)]) {
					if !g.keyManager.addNewKey(key) {
						panic("key must not exist if prefix doesn't exist")
					}
					break
				}
			}
		}
		return key
	}
}

// randKeyHelperSuffix is a helper function for randKeyHelper, and should not be
// invoked directly.
func (g *generator) randKeyHelperSuffix(
	dst []byte, minPrefixLen, maxPrefixLen int, suffix int64,
) []byte {
	n := minPrefixLen
	if maxPrefixLen > minPrefixLen {
		n += g.rng.Intn(maxPrefixLen - minPrefixLen)
	}
	// In order to test a mix of suffixed and unsuffixed keys, omit the zero
	// suffix.
	if suffix == 0 {
		dst = resizeBuffer(dst, n, 0)
		g.fillRand(dst)
		return dst
	}
	suffixLen := testkeys.SuffixLen(suffix)
	dst = resizeBuffer(dst, n, suffixLen)
	g.fillRand(dst[:n])
	testkeys.WriteSuffix(dst[n:], suffix)
	return dst
}

func resizeBuffer(buf []byte, prefixLen, suffixLen int) []byte {
	if cap(buf) >= prefixLen+suffixLen {
		return buf[:prefixLen+suffixLen]
	}
	return make([]byte, prefixLen+suffixLen)
}

// TODO(peter): make the value size configurable. See valueSizeDist in
// config.go.
func (g *generator) randValue(min, max int) []byte {
	n := min
	if max > min {
		n += g.rng.Intn(max - min)
	}
	if n == 0 {
		return nil
	}
	buf := make([]byte, n)
	g.fillRand(buf)
	return buf
}

func (g *generator) fillRand(buf []byte) {
	// NB: The actual random values are not particularly important. We only use
	// lowercase letters because that makes visual determination of ordering
	// easier, rather than having to remember the lexicographic ordering of
	// uppercase vs lowercase, or letters vs numbers vs punctuation.
	const letters = "abcdefghijklmnopqrstuvwxyz"
	const lettersLen = uint64(len(letters))
	const lettersCharsPerRand = 12 // floor(log(math.MaxUint64)/log(lettersLen))

	var r uint64
	var q int
	for i := 0; i < len(buf); i++ {
		if q == 0 {
			r = g.rng.Uint64()
			q = lettersCharsPerRand
		}
		buf[i] = letters[r%lettersLen]
		r = r / lettersLen
		q--
	}
}

func (g *generator) newBatch() {
	batchID := makeObjID(batchTag, g.init.batchSlots)
	g.init.batchSlots++
	g.liveBatches = append(g.liveBatches, batchID)
	g.liveWriters = append(g.liveWriters, batchID)
	dbID := g.dbs.rand(g.rng)
	g.objDB[batchID] = dbID

	g.add(&newBatchOp{
		dbID:    dbID,
		batchID: batchID,
	})
}

func (g *generator) newIndexedBatch() {
	batchID := makeObjID(batchTag, g.init.batchSlots)
	g.init.batchSlots++
	g.liveBatches = append(g.liveBatches, batchID)
	g.liveReaders = append(g.liveReaders, batchID)
	g.liveWriters = append(g.liveWriters, batchID)

	iters := make(objIDSet)
	g.batches[batchID] = iters
	g.readers[batchID] = iters
	dbID := g.dbs.rand(g.rng)
	g.objDB[batchID] = dbID

	g.add(&newIndexedBatchOp{
		dbID:    dbID,
		batchID: batchID,
	})
}

// removeFromBatchGenerator will not generate a closeOp for the target batch as
// not every batch that is removed from the generator should be closed. For
// example, running a closeOp before an ingestOp that contains the closed batch
// will cause an error.
func (g *generator) removeBatchFromGenerator(batchID objID) {
	g.liveBatches.remove(batchID)
	iters := g.batches[batchID]
	delete(g.batches, batchID)

	if iters != nil {
		g.liveReaders.remove(batchID)
		delete(g.readers, batchID)
	}
	g.liveWriters.remove(batchID)
	for _, id := range iters.sorted() {
		g.liveIters.remove(id)
		delete(g.iters, id)
		g.add(&closeOp{objID: id, derivedDBID: g.objDB[batchID]})
	}
}

func (g *generator) batchAbort() {
	if len(g.liveBatches) == 0 {
		return
	}

	batchID := g.liveBatches.rand(g.rng)
	g.removeBatchFromGenerator(batchID)

	g.add(&closeOp{objID: batchID, derivedDBID: g.objDB[batchID]})
}

func (g *generator) batchCommit() {
	if len(g.liveBatches) == 0 {
		return
	}

	batchID := g.liveBatches.rand(g.rng)
	dbID := g.objDB[batchID]
	g.removeBatchFromGenerator(batchID)

	// The batch we're applying may contain single delete tombstones that when
	// applied to the writer result in nondeterminism in the deleted key. If
	// that's the case, we can restore determinism by first deleting the key
	// from the writer.
	//
	// Generating additional operations here is not ideal, but it simplifies
	// single delete invariants significantly.
	singleDeleteConflicts := g.keyManager.checkForSingleDelConflicts(batchID, dbID, false /* collapsed */)
	for _, conflict := range singleDeleteConflicts {
		g.add(&deleteOp{
			writerID:    dbID,
			key:         conflict,
			derivedDBID: dbID,
		})
	}

	g.add(&batchCommitOp{
		dbID:    dbID,
		batchID: batchID,
	})
	g.add(&closeOp{objID: batchID, derivedDBID: dbID})

}

func (g *generator) dbClose() {
	// Close any live iterators and snapshots, so that we can close the DB
	// cleanly.
	for len(g.liveIters) > 0 {
		g.randIter(g.iterClose)()
	}
	for len(g.liveSnapshots) > 0 {
		g.snapshotClose()
	}
	for len(g.liveBatches) > 0 {
		batchID := g.liveBatches[0]
		dbID := g.objDB[batchID]
		g.removeBatchFromGenerator(batchID)
		g.add(&closeOp{objID: batchID, derivedDBID: dbID})
	}
	for len(g.dbs) > 0 {
		db := g.dbs[0]
		g.dbs = g.dbs[1:]
		g.add(&closeOp{objID: db})
	}
}

func (g *generator) dbCheckpoint() {
	// 1/2 of the time we don't restrict the checkpoint;
	// 1/4 of the time we restrict to 1 span;
	// 1/8 of the time we restrict to 2 spans; etc.
	numSpans := 0
	var spans []pebble.CheckpointSpan
	for g.rng.Intn(2) == 0 {
		numSpans++
	}
	if numSpans > 0 {
		spans = make([]pebble.CheckpointSpan, numSpans)
	}
	for i := range spans {
		start := g.randKeyToRead(0.01)
		end := g.randKeyToRead(0.01)
		if g.cmp(start, end) > 0 {
			start, end = end, start
		}
		spans[i].Start = start
		spans[i].End = end
	}
	dbID := g.dbs.rand(g.rng)
	g.add(&checkpointOp{
		dbID:  dbID,
		spans: spans,
	})
}

func (g *generator) dbCompact() {
	// Generate new key(s) with a 1% probability.
	start := g.randKeyToRead(0.01)
	end := g.randKeyToRead(0.01)
	if g.cmp(start, end) > 0 {
		start, end = end, start
	}
	dbID := g.dbs.rand(g.rng)
	g.add(&compactOp{
		dbID:        dbID,
		start:       start,
		end:         end,
		parallelize: g.rng.Float64() < 0.5,
	})
}

func (g *generator) dbFlush() {
	g.add(&flushOp{g.dbs.rand(g.rng)})
}

func (g *generator) dbRatchetFormatMajorVersion() {
	// Ratchet to a random format major version between the minimum the
	// metamorphic tests support and the newest. At runtime, the generated
	// version may be behind the database's format major version, in which case
	// RatchetFormatMajorVersion should deterministically error.

	dbID := g.dbs.rand(g.rng)
	n := int(newestFormatMajorVersionToTest - minimumFormatMajorVersion)
	vers := pebble.FormatMajorVersion(g.rng.Intn(n+1)) + minimumFormatMajorVersion
	g.add(&dbRatchetFormatMajorVersionOp{dbID: dbID, vers: vers})
}

func (g *generator) dbRestart() {
	// Close any live iterators and snapshots, so that we can close the DB
	// cleanly.
	dbID := g.dbs.rand(g.rng)
	for len(g.liveIters) > 0 {
		g.randIter(g.iterClose)()
	}
	for len(g.liveSnapshots) > 0 {
		g.snapshotClose()
	}
	// Close the batches.
	for len(g.liveBatches) > 0 {
		batchID := g.liveBatches[0]
		dbID := g.objDB[batchID]
		g.removeBatchFromGenerator(batchID)
		g.add(&closeOp{objID: batchID, derivedDBID: dbID})
	}
	if len(g.liveReaders) != len(g.dbs) || len(g.liveWriters) != len(g.dbs) {
		panic(fmt.Sprintf("unexpected counts: liveReaders %d, liveWriters: %d",
			len(g.liveReaders), len(g.liveWriters)))
	}
	g.add(&dbRestartOp{dbID: dbID})
}

// maybeSetSnapshotIterBounds must be called whenever creating a new iterator or
// modifying the bounds of an iterator. If the iterator is backed by a snapshot
// that only guarantees consistency within a limited set of key spans, then the
// iterator must set bounds within one of the snapshot's consistent keyspans. It
// returns true if the provided readerID is a bounded snapshot and bounds were
// set.
func (g *generator) maybeSetSnapshotIterBounds(readerID objID, opts *iterOpts) bool {
	snapBounds, isBoundedSnapshot := g.snapshotBounds[readerID]
	if !isBoundedSnapshot {
		return false
	}
	// Pick a random keyrange within one of the snapshot's key ranges.
	parentBounds := snapBounds[g.rng.Intn(len(snapBounds))]
	// With 10% probability, use the parent start bound as-is.
	if g.rng.Float64() <= 0.1 {
		opts.lower = parentBounds.Start
	} else {
		opts.lower = testkeys.RandomSeparator(
			nil, /* dst */
			parentBounds.Start,
			parentBounds.End,
			0, /* suffix */
			4+g.rng.Intn(8),
			g.rng,
		)
	}
	// With 10% probability, use the parent end bound as-is.
	if g.rng.Float64() <= 0.1 {
		opts.upper = parentBounds.End
	} else {
		opts.upper = testkeys.RandomSeparator(
			nil, /* dst */
			opts.lower,
			parentBounds.End,
			0, /* suffix */
			4+g.rng.Intn(8),
			g.rng,
		)
	}
	return true
}

func (g *generator) newIter() {
	iterID := makeObjID(iterTag, g.init.iterSlots)
	g.init.iterSlots++
	g.liveIters = append(g.liveIters, iterID)

	readerID := g.liveReaders.rand(g.rng)
	if iters := g.readers[readerID]; iters != nil {
		iters[iterID] = struct{}{}
		g.iters[iterID] = iters
		//lint:ignore SA9003 - readability
	} else {
		// NB: the DB object does not track its open iterators because it never
		// closes.
	}
	g.iterReaderID[iterID] = readerID
	dbID := g.deriveDB(iterID)

	var opts iterOpts
	if !g.maybeSetSnapshotIterBounds(readerID, &opts) {
		// Generate lower/upper bounds with a 10% probability.
		if g.rng.Float64() <= 0.1 {
			// Generate a new key with a .1% probability.
			opts.lower = g.randKeyToRead(0.001)
		}
		if g.rng.Float64() <= 0.1 {
			// Generate a new key with a .1% probability.
			opts.upper = g.randKeyToRead(0.001)
		}
		if g.cmp(opts.lower, opts.upper) > 0 {
			opts.lower, opts.upper = opts.upper, opts.lower
		}
	}
	opts.keyTypes, opts.maskSuffix = g.randKeyTypesAndMask()

	// With 10% probability, enable automatic filtering of keys with suffixes
	// not in the provided range. This filtering occurs both through
	// block-property filtering and explicitly within the iterator operations to
	// ensure determinism.
	if g.rng.Float64() <= 0.1 {
		max := g.cfg.writeSuffixDist.Max()
		opts.filterMin, opts.filterMax = g.rng.Uint64n(max)+1, g.rng.Uint64n(max)+1
		if opts.filterMin > opts.filterMax {
			opts.filterMin, opts.filterMax = opts.filterMax, opts.filterMin
		} else if opts.filterMin == opts.filterMax {
			opts.filterMax = opts.filterMin + 1
		}
	}

	// Enable L6 filters with a 10% probability.
	if g.rng.Float64() <= 0.1 {
		opts.useL6Filters = true
	}

	g.itersLastOpts[iterID] = opts
	g.iterCreationTimestamp[iterID] = g.keyManager.nextMetaTimestamp()
	g.iterReaderID[iterID] = readerID
	g.add(&newIterOp{
		readerID:    readerID,
		iterID:      iterID,
		iterOpts:    opts,
		derivedDBID: dbID,
	})
}

func (g *generator) randKeyTypesAndMask() (keyTypes uint32, maskSuffix []byte) {
	// Iterate over different key types.
	p := g.rng.Float64()
	switch {
	case p < 0.2: // 20% probability
		keyTypes = uint32(pebble.IterKeyTypePointsOnly)
	case p < 0.8: // 60% probability
		keyTypes = uint32(pebble.IterKeyTypePointsAndRanges)
		// With 50% probability, enable masking.
		if g.rng.Intn(2) == 1 {
			maskSuffix = g.randSuffixToRead()
		}
	default: // 20% probability
		keyTypes = uint32(pebble.IterKeyTypeRangesOnly)
	}
	return keyTypes, maskSuffix
}

func (g *generator) deriveDB(readerID objID) objID {
	if readerID.tag() == iterTag {
		readerID = g.iterReaderID[readerID]
	}
	dbParentID := readerID
	if dbParentID.tag() != dbTag {
		dbParentID = g.objDB[dbParentID]
	}
	g.objDB[readerID] = dbParentID
	return dbParentID
}

func (g *generator) newIterUsingClone() {
	if len(g.liveIters) == 0 {
		return
	}
	existingIterID := g.liveIters.rand(g.rng)
	iterID := makeObjID(iterTag, g.init.iterSlots)
	g.init.iterSlots++
	g.liveIters = append(g.liveIters, iterID)
	if iters := g.iters[existingIterID]; iters != nil {
		iters[iterID] = struct{}{}
		g.iters[iterID] = iters
		//lint:ignore SA9003 - readability
	} else {
		// NB: the DB object does not track its open iterators because it never
		// closes.
	}
	readerID := g.iterReaderID[existingIterID]
	g.iterReaderID[iterID] = readerID
	g.deriveDB(iterID)

	var refreshBatch bool
	if readerID.tag() == batchTag {
		refreshBatch = g.rng.Intn(2) == 1
	}

	opts := g.itersLastOpts[existingIterID]
	// With 50% probability, consider modifying the iterator options used by the
	// clone.
	if g.rng.Intn(2) == 1 {
		g.maybeMutateOptions(readerID, &opts)
	}
	g.itersLastOpts[iterID] = opts

	g.iterCreationTimestamp[iterID] = g.keyManager.nextMetaTimestamp()
	g.iterReaderID[iterID] = g.iterReaderID[existingIterID]
	g.add(&newIterUsingCloneOp{
		existingIterID:  existingIterID,
		iterID:          iterID,
		refreshBatch:    refreshBatch,
		iterOpts:        opts,
		derivedReaderID: readerID,
	})
}

func (g *generator) iterClose(iterID objID) {
	g.liveIters.remove(iterID)
	if readerIters, ok := g.iters[iterID]; ok {
		delete(g.iters, iterID)
		delete(readerIters, iterID)
		//lint:ignore SA9003 - readability
	} else {
		// NB: the DB object does not track its open iterators because it never
		// closes.
	}

	readerID := g.iterReaderID[iterID]
	g.add(&closeOp{objID: iterID, derivedDBID: g.objDB[readerID]})
}

func (g *generator) iterSetBounds(iterID objID) {
	iterLastOpts := g.itersLastOpts[iterID]
	newOpts := iterLastOpts
	// TODO(jackson): The logic to increase the probability of advancing bounds
	// monotonically only applies if the snapshot is not bounded. Refactor to
	// allow bounded snapshots to benefit too, when possible.
	if !g.maybeSetSnapshotIterBounds(g.iterReaderID[iterID], &newOpts) {
		var lower, upper []byte
		genLower := g.rng.Float64() <= 0.9
		genUpper := g.rng.Float64() <= 0.9
		// When one of ensureLowerGE, ensureUpperLE is true, the new bounds
		// don't overlap with the previous bounds.
		var ensureLowerGE, ensureUpperLE bool
		if genLower && iterLastOpts.upper != nil && g.rng.Float64() <= 0.9 {
			ensureLowerGE = true
		}
		if (!ensureLowerGE || g.rng.Float64() < 0.5) && genUpper && iterLastOpts.lower != nil {
			ensureUpperLE = true
			ensureLowerGE = false
		}
		attempts := 0
		for {
			attempts++
			if genLower {
				// Generate a new key with a .1% probability.
				lower = g.randKeyToRead(0.001)
			}
			if genUpper {
				// Generate a new key with a .1% probability.
				upper = g.randKeyToRead(0.001)
			}
			if g.cmp(lower, upper) > 0 {
				lower, upper = upper, lower
			}
			if ensureLowerGE && g.cmp(iterLastOpts.upper, lower) > 0 {
				if attempts < 25 {
					continue
				}
				lower = iterLastOpts.upper
				upper = lower
				break
			}
			if ensureUpperLE && g.cmp(upper, iterLastOpts.lower) > 0 {
				if attempts < 25 {
					continue
				}
				upper = iterLastOpts.lower
				lower = upper
				break
			}
			break
		}
		newOpts.lower = lower
		newOpts.upper = upper
	}
	g.itersLastOpts[iterID] = newOpts
	g.add(&iterSetBoundsOp{
		iterID: iterID,
		lower:  newOpts.lower,
		upper:  newOpts.upper,
	})
	// Additionally seek the iterator in a manner consistent with the bounds,
	// and do some steps (Next/Prev). The seeking exercises typical
	// CockroachDB behavior when using iterators and the steps are trying to
	// stress the region near the bounds. Ideally, we should not do this as
	// part of generating a single op, but this is easier than trying to
	// control future op generation via generator state.
	doSeekLT := newOpts.upper != nil && g.rng.Float64() < 0.5
	doSeekGE := newOpts.lower != nil && g.rng.Float64() < 0.5
	if doSeekLT && doSeekGE {
		// Pick the seek.
		if g.rng.Float64() < 0.5 {
			doSeekGE = false
		} else {
			doSeekLT = false
		}
	}
	if doSeekLT {
		g.add(&iterSeekLTOp{
			iterID:          iterID,
			key:             newOpts.upper,
			derivedReaderID: g.iterReaderID[iterID],
		})
		if g.rng.Float64() < 0.5 {
			g.iterNext(iterID)
		}
		if g.rng.Float64() < 0.5 {
			g.iterNext(iterID)
		}
		if g.rng.Float64() < 0.5 {
			g.iterPrev(iterID)
		}
	} else if doSeekGE {
		g.add(&iterSeekGEOp{
			iterID:          iterID,
			key:             newOpts.lower,
			derivedReaderID: g.iterReaderID[iterID],
		})
		if g.rng.Float64() < 0.5 {
			g.iterPrev(iterID)
		}
		if g.rng.Float64() < 0.5 {
			g.iterPrev(iterID)
		}
		if g.rng.Float64() < 0.5 {
			g.iterNext(iterID)
		}
	}
}

func (g *generator) iterSetOptions(iterID objID) {
	opts := g.itersLastOpts[iterID]
	g.maybeMutateOptions(g.iterReaderID[iterID], &opts)
	g.itersLastOpts[iterID] = opts
	g.add(&iterSetOptionsOp{
		iterID:          iterID,
		iterOpts:        opts,
		derivedReaderID: g.iterReaderID[iterID],
	})

	// Additionally, perform a random absolute positioning operation. The
	// SetOptions contract requires one before the next relative positioning
	// operation. Ideally, we should not do this as part of generating a single
	// op, but this is easier than trying to control future op generation via
	// generator state.
	g.pickOneUniform(
		g.iterFirst,
		g.iterLast,
		g.iterSeekGE,
		g.iterSeekGEWithLimit,
		g.iterSeekPrefixGE,
		g.iterSeekLT,
		g.iterSeekLTWithLimit,
	)(iterID)
}

func (g *generator) iterSeekGE(iterID objID) {
	g.add(&iterSeekGEOp{
		iterID:          iterID,
		key:             g.randKeyToRead(0.001), // 0.1% new keys
		derivedReaderID: g.iterReaderID[iterID],
	})
}

func (g *generator) iterSeekGEWithLimit(iterID objID) {
	// 0.1% new keys
	key, limit := g.randKeyToRead(0.001), g.randKeyToRead(0.001)
	if g.cmp(key, limit) > 0 {
		key, limit = limit, key
	}
	g.add(&iterSeekGEOp{
		iterID:          iterID,
		key:             key,
		limit:           limit,
		derivedReaderID: g.iterReaderID[iterID],
	})
}

func (g *generator) randKeyToReadWithinBounds(lower, upper []byte, readerID objID) []*keyMeta {
	var inRangeKeys []*keyMeta
	for _, keyMeta := range g.keyManager.byObj[readerID] {
		posKey := keyMeta.key
		if g.cmp(posKey, lower) < 0 || g.cmp(posKey, upper) >= 0 {
			continue
		}
		inRangeKeys = append(inRangeKeys, keyMeta)
	}
	return inRangeKeys
}

func (g *generator) iterSeekPrefixGE(iterID objID) {
	lower := g.itersLastOpts[iterID].lower
	upper := g.itersLastOpts[iterID].upper
	iterCreationTimestamp := g.iterCreationTimestamp[iterID]
	var key []byte

	// We try to make sure that the SeekPrefixGE key is within the iter bounds,
	// and that the iter can read the key. If the key was created on a batch
	// which deleted the key, then the key will still be considered visible
	// by the current logic. We're also not accounting for keys written to
	// batches which haven't been presisted to the DB. But we're only picking
	// keys in a best effort manner, and the logic is better than picking a
	// random key.
	if g.rng.Intn(10) >= 1 {
		possibleKeys := make([][]byte, 0, 100)
		inRangeKeys := g.randKeyToReadWithinBounds(lower, upper, g.objDB[iterID])
		for _, keyMeta := range inRangeKeys {
			visibleHistory := keyMeta.history.before(iterCreationTimestamp)

			// Check if the last op on this key set a value, (eg SETs, MERGEs).
			// If the key should be visible to the iterator and it would make a
			// good candidate for a SeekPrefixGE.
			if visibleHistory.hasVisibleValue() {
				possibleKeys = append(possibleKeys, keyMeta.key)
			}
		}

		if len(possibleKeys) > 0 {
			key = []byte(possibleKeys[g.rng.Int31n(int32(len(possibleKeys)))])
		}
	}

	if key == nil {
		// TODO(bananabrick): We should try and use keys within the bounds,
		// even if we couldn't find any keys visible to the iterator. However,
		// doing this in experiments didn't really increase the valid
		// SeekPrefixGE calls by much.
		key = g.randKeyToRead(0) // 0% new keys
	}

	g.add(&iterSeekPrefixGEOp{
		iterID:          iterID,
		key:             key,
		derivedReaderID: g.iterReaderID[iterID],
	})
}

func (g *generator) iterSeekLT(iterID objID) {
	g.add(&iterSeekLTOp{
		iterID:          iterID,
		key:             g.randKeyToRead(0.001), // 0.1% new keys
		derivedReaderID: g.iterReaderID[iterID],
	})
}

func (g *generator) iterSeekLTWithLimit(iterID objID) {
	// 0.1% new keys
	key, limit := g.randKeyToRead(0.001), g.randKeyToRead(0.001)
	if g.cmp(limit, key) > 0 {
		key, limit = limit, key
	}
	g.add(&iterSeekLTOp{
		iterID:          iterID,
		key:             key,
		limit:           limit,
		derivedReaderID: g.iterReaderID[iterID],
	})
}

// randIter performs partial func application ("currying"), returning a new
// function that supplies the given func with a random iterator.
func (g *generator) randIter(gen func(objID)) func() {
	return func() {
		if len(g.liveIters) == 0 {
			return
		}
		gen(g.liveIters.rand(g.rng))
	}
}

func (g *generator) iterFirst(iterID objID) {
	g.add(&iterFirstOp{
		iterID:          iterID,
		derivedReaderID: g.iterReaderID[iterID],
	})
}

func (g *generator) iterLast(iterID objID) {
	g.add(&iterLastOp{
		iterID:          iterID,
		derivedReaderID: g.iterReaderID[iterID],
	})
}

func (g *generator) iterNext(iterID objID) {
	g.add(&iterNextOp{
		iterID:          iterID,
		derivedReaderID: g.iterReaderID[iterID],
	})
}

func (g *generator) iterPrev(iterID objID) {
	g.add(&iterPrevOp{
		iterID:          iterID,
		derivedReaderID: g.iterReaderID[iterID],
	})
}

func (g *generator) iterNextWithLimit(iterID objID) {
	g.add(&iterNextOp{
		iterID:          iterID,
		limit:           g.randKeyToRead(0.001), // 0.1% new keys
		derivedReaderID: g.iterReaderID[iterID],
	})
}

func (g *generator) iterNextPrefix(iterID objID) {
	g.add(&iterNextPrefixOp{
		iterID:          iterID,
		derivedReaderID: g.iterReaderID[iterID],
	})
}

func (g *generator) iterCanSingleDelete(iterID objID) {
	g.add(&iterCanSingleDelOp{
		iterID:          iterID,
		derivedReaderID: g.iterReaderID[iterID],
	})
}

func (g *generator) iterPrevWithLimit(iterID objID) {
	g.add(&iterPrevOp{
		iterID:          iterID,
		limit:           g.randKeyToRead(0.001), // 0.1% new keys
		derivedReaderID: g.iterReaderID[iterID],
	})
}

func (g *generator) readerGet() {
	if len(g.liveReaders) == 0 {
		return
	}

	readerID := g.liveReaders.rand(g.rng)

	// If the chosen reader is a snapshot created with user-specified key
	// ranges, restrict the read to fall within one of the provided key ranges.
	var key []byte
	if bounds := g.snapshotBounds[readerID]; len(bounds) > 0 {
		kr := bounds[g.rng.Intn(len(bounds))]
		key = g.randKeyToReadInRange(0.001, kr) // 0.1% new keys
	} else {
		key = g.randKeyToRead(0.001) // 0.1% new keys
	}
	derivedDBID := objID(0)
	if readerID.tag() == batchTag || readerID.tag() == snapTag {
		derivedDBID = g.deriveDB(readerID)
	}
	g.add(&getOp{readerID: readerID, key: key, derivedDBID: derivedDBID})
}

func (g *generator) replicate() {
	if len(g.dbs) < 2 {
		return
	}

	source := g.dbs.rand(g.rng)
	dest := source
	for dest == source {
		dest = g.dbs.rand(g.rng)
	}

	startKey, endKey := g.prefixKeyRange()
	g.add(&replicateOp{
		source: source,
		dest:   dest,
		start:  startKey,
		end:    endKey,
	})
}

// generateDisjointKeyRanges generates n disjoint key ranges.
func (g *generator) generateDisjointKeyRanges(n int) []pebble.KeyRange {
	bounds := make([][]byte, 2*n)
	used := map[string]bool{}
	for i := 0; i < len(bounds); i++ {
		k := g.prefix(g.randKeyToRead(0.1))
		for used[string(k)] {
			k = g.prefix(g.randKeyToRead(0.1))
		}
		bounds[i] = k
		used[string(k)] = true
	}
	slices.SortFunc(bounds, g.cmp)
	keyRanges := make([]pebble.KeyRange, n)
	for i := range keyRanges {
		keyRanges[i] = pebble.KeyRange{
			Start: bounds[i*2],
			End:   bounds[i*2+1],
		}
	}
	return keyRanges
}

func (g *generator) newSnapshot() {
	snapID := makeObjID(snapTag, g.init.snapshotSlots)
	g.init.snapshotSlots++
	g.liveSnapshots = append(g.liveSnapshots, snapID)
	g.liveReaders = append(g.liveReaders, snapID)
	dbID := g.dbs.rand(g.rng)
	g.objDB[snapID] = dbID

	iters := make(objIDSet)
	g.snapshots[snapID] = iters
	g.readers[snapID] = iters

	s := &newSnapshotOp{
		dbID:   dbID,
		snapID: snapID,
	}

	// Impose bounds on the keys that may be read with the snapshot. Setting bounds
	// allows some runs of the metamorphic test to use a EventuallyFileOnlySnapshot
	// instead of a Snapshot, testing equivalence between the two for reads within
	// those bounds.
	s.bounds = g.generateDisjointKeyRanges(
		g.rng.Intn(5) + 1, /* between 1-5 */
	)
	g.snapshotBounds[snapID] = s.bounds
	g.add(s)
}

func (g *generator) snapshotClose() {
	if len(g.liveSnapshots) == 0 {
		return
	}

	snapID := g.liveSnapshots.rand(g.rng)
	g.liveSnapshots.remove(snapID)
	iters := g.snapshots[snapID]
	delete(g.snapshots, snapID)
	g.liveReaders.remove(snapID)
	delete(g.readers, snapID)

	for _, id := range iters.sorted() {
		g.liveIters.remove(id)
		delete(g.iters, id)
		g.add(&closeOp{objID: id, derivedDBID: g.objDB[snapID]})
	}

	g.add(&closeOp{objID: snapID, derivedDBID: g.objDB[snapID]})
}

func (g *generator) writerApply() {
	if len(g.liveBatches) == 0 {
		return
	}
	if len(g.liveWriters) < 2 {
		panic(fmt.Sprintf("insufficient liveWriters (%d) to apply batch", len(g.liveWriters)))
	}

	batchID := g.liveBatches.rand(g.rng)
	dbID := g.objDB[batchID]

	var writerID objID
	for {
		// NB: The writer we're applying to, as well as the batch we're applying,
		// must be from the same DB. The writer could be the db itself. Applying
		// a batch from one DB on another DB results in a panic, so avoid that.
		writerID = g.liveWriters.rand(g.rng)
		writerDBID := writerID
		if writerID.tag() != dbTag {
			writerDBID = g.objDB[writerID]
		}
		if writerID != batchID && writerDBID == dbID {
			break
		}
	}

	// The batch we're applying may contain single delete tombstones that when
	// applied to the writer result in nondeterminism in the deleted key. If
	// that's the case, we can restore determinism by first deleting the key
	// from the writer.
	//
	// Generating additional operations here is not ideal, but it simplifies
	// single delete invariants significantly.
	singleDeleteConflicts := g.keyManager.checkForSingleDelConflicts(batchID, writerID, false /* collapsed */)
	for _, conflict := range singleDeleteConflicts {
		g.add(&deleteOp{
			writerID:    writerID,
			key:         conflict,
			derivedDBID: dbID,
		})
	}

	g.removeBatchFromGenerator(batchID)

	g.add(&applyOp{
		writerID: writerID,
		batchID:  batchID,
	})
	g.add(&closeOp{
		objID:       batchID,
		derivedDBID: dbID,
	})
}

func (g *generator) writerDelete() {
	if len(g.liveWriters) == 0 {
		return
	}

	writerID := g.liveWriters.rand(g.rng)
	derivedDBID := writerID
	if derivedDBID.tag() != dbTag {
		derivedDBID = g.objDB[writerID]
	}
	g.add(&deleteOp{
		writerID:    writerID,
		key:         g.randKeyToWrite(0.001), // 0.1% new keys
		derivedDBID: derivedDBID,
	})
}

func (g *generator) writerDeleteRange() {
	if len(g.liveWriters) == 0 {
		return
	}

	start := g.randKeyToWrite(0.001)
	end := g.randKeyToWrite(0.001)
	if g.cmp(start, end) > 0 {
		start, end = end, start
	}

	writerID := g.liveWriters.rand(g.rng)
	g.add(&deleteRangeOp{
		writerID: writerID,
		start:    start,
		end:      end,
	})
}

func (g *generator) writerRangeKeyDelete() {
	if len(g.liveWriters) == 0 {
		return
	}
	start, end := g.prefixKeyRange()

	writerID := g.liveWriters.rand(g.rng)
	g.add(&rangeKeyDeleteOp{
		writerID: writerID,
		start:    start,
		end:      end,
	})
}

func (g *generator) writerRangeKeySet() {
	if len(g.liveWriters) == 0 {
		return
	}
	start, end := g.prefixKeyRange()

	// 90% of the time, set a suffix.
	var suffix []byte
	if g.rng.Float64() < 0.90 {
		// Increase the max suffix 5% of the time.
		suffix = g.randSuffixToWrite(0.05)
	}

	writerID := g.liveWriters.rand(g.rng)
	g.add(&rangeKeySetOp{
		writerID: writerID,
		start:    start,
		end:      end,
		suffix:   suffix,
		value:    g.randValue(0, maxValueSize),
	})
}

func (g *generator) writerRangeKeyUnset() {
	if len(g.liveWriters) == 0 {
		return
	}
	start, end := g.prefixKeyRange()

	// 90% of the time, set a suffix.
	var suffix []byte
	if g.rng.Float64() < 0.90 {
		// Increase the max suffix 5% of the time.
		suffix = g.randSuffixToWrite(0.05)
	}

	// TODO(jackson): Increase probability of effective unsets? Purely random
	// unsets are unlikely to remove an active range key.

	writerID := g.liveWriters.rand(g.rng)
	g.add(&rangeKeyUnsetOp{
		writerID: writerID,
		start:    start,
		end:      end,
		suffix:   suffix,
	})
}

func (g *generator) writerIngest() {
	if len(g.liveBatches) == 0 {
		return
	}

	// Ingest between 1 and 3 batches.
	dbID := g.dbs.rand(g.rng)
	n := min(1+g.rng.Intn(3), len(g.liveBatches))
	batchIDs := make([]objID, n)
	derivedDBIDs := make([]objID, n)
	for i := 0; i < n; i++ {
		batchID := g.liveBatches.rand(g.rng)
		batchIDs[i] = batchID
		derivedDBIDs[i] = g.objDB[batchIDs[i]]
		g.removeBatchFromGenerator(batchID)
	}

	// Ingestions may fail if the ingested sstables overlap one another.
	// Either it succeeds and its keys are committed to the DB, or it fails and
	// the keys are not committed.
	if !g.keyManager.doObjectBoundsOverlap(batchIDs) {
		// This ingestion will succeed.
		//
		// The batches we're ingesting may contain single delete tombstones that
		// when applied to the writer result in nondeterminism in the deleted key.
		// If that's the case, we can restore determinism by first deleting the keys
		// from the writer.
		//
		// Generating additional operations here is not ideal, but it simplifies
		// single delete invariants significantly.
		for _, batchID := range batchIDs {
			singleDeleteConflicts := g.keyManager.checkForSingleDelConflicts(batchID, dbID, true /* collapsed */)
			for _, conflict := range singleDeleteConflicts {
				g.add(&deleteOp{
					writerID:    dbID,
					key:         conflict,
					derivedDBID: dbID,
				})
			}
		}
	}
	g.add(&ingestOp{
		dbID:         dbID,
		batchIDs:     batchIDs,
		derivedDBIDs: derivedDBIDs,
	})
}

func (g *generator) writerIngestAndExcise() {
	if len(g.liveBatches) == 0 {
		return
	}

	dbID := g.dbs.rand(g.rng)
	batchID := g.liveBatches.rand(g.rng)
	g.removeBatchFromGenerator(batchID)

	start, end := g.prefixKeyRange()
	derivedDBID := g.objDB[batchID]

	g.add(&ingestAndExciseOp{
		dbID:        dbID,
		batchID:     batchID,
		derivedDBID: derivedDBID,
		exciseStart: start,
		exciseEnd:   end,
	})
}

func (g *generator) writerMerge() {
	if len(g.liveWriters) == 0 {
		return
	}

	writerID := g.liveWriters.rand(g.rng)
	g.add(&mergeOp{
		writerID: writerID,
		// 20% new keys.
		key:   g.randKeyToWrite(0.2),
		value: g.randValue(0, maxValueSize),
	})
}

func (g *generator) writerSet() {
	if len(g.liveWriters) == 0 {
		return
	}

	writerID := g.liveWriters.rand(g.rng)
	g.add(&setOp{
		writerID: writerID,
		// 50% new keys.
		key:   g.randKeyToWrite(0.5),
		value: g.randValue(0, maxValueSize),
	})
}

func (g *generator) writerSingleDelete() {
	if len(g.liveWriters) == 0 {
		return
	}

	writerID := g.liveWriters.rand(g.rng)
	key := g.randKeyToSingleDelete(writerID)
	if key == nil {
		return
	}
	g.add(&singleDeleteOp{
		writerID: writerID,
		key:      key,
		// Keys eligible for single deletes can be removed with a regular
		// delete. Mutate a percentage of SINGLEDEL ops into DELETEs. Note that
		// here we are only determining whether the replacement *could* happen.
		// At test runtime, the `replaceSingleDelete` test option must also be
		// set to true for the single delete to be replaced.
		maybeReplaceDelete: g.rng.Float64() < 0.25,
	})
}

func (g *generator) maybeMutateOptions(readerID objID, opts *iterOpts) {
	// With 95% probability, allow changes to any options at all. This ensures
	// that in 5% of cases there are no changes, and SetOptions hits its fast
	// path.
	if g.rng.Intn(100) >= 5 {
		if !g.maybeSetSnapshotIterBounds(readerID, opts) {
			// With 1/3 probability, clear existing bounds.
			if opts.lower != nil && g.rng.Intn(3) == 0 {
				opts.lower = nil
			}
			if opts.upper != nil && g.rng.Intn(3) == 0 {
				opts.upper = nil
			}
			// With 1/3 probability, update the bounds.
			if g.rng.Intn(3) == 0 {
				// Generate a new key with a .1% probability.
				opts.lower = g.randKeyToRead(0.001)
			}
			if g.rng.Intn(3) == 0 {
				// Generate a new key with a .1% probability.
				opts.upper = g.randKeyToRead(0.001)
			}
			if g.cmp(opts.lower, opts.upper) > 0 {
				opts.lower, opts.upper = opts.upper, opts.lower
			}
		}

		// With 1/3 probability, update the key-types/mask.
		if g.rng.Intn(3) == 0 {
			opts.keyTypes, opts.maskSuffix = g.randKeyTypesAndMask()
		}

		// With 1/3 probability, clear existing filter.
		if opts.filterMax > 0 && g.rng.Intn(3) == 0 {
			opts.filterMax, opts.filterMin = 0, 0
		}
		// With 10% probability, set a filter range.
		if g.rng.Intn(10) == 1 {
			max := g.cfg.writeSuffixDist.Max()
			opts.filterMin, opts.filterMax = g.rng.Uint64n(max)+1, g.rng.Uint64n(max)+1
			if opts.filterMin > opts.filterMax {
				opts.filterMin, opts.filterMax = opts.filterMax, opts.filterMin
			} else if opts.filterMin == opts.filterMax {
				opts.filterMax = opts.filterMin + 1
			}
		}
		// With 10% probability, flip enablement of L6 filters.
		if g.rng.Float64() <= 0.1 {
			opts.useL6Filters = !opts.useL6Filters
		}
	}
}

func (g *generator) pickOneUniform(options ...func(objID)) func(objID) {
	i := g.rng.Intn(len(options))
	return options[i]
}

func (g *generator) cmp(a, b []byte) int {
	return g.keyManager.comparer.Compare(a, b)
}

func (g *generator) equal(a, b []byte) bool {
	return g.keyManager.comparer.Equal(a, b)
}

func (g *generator) split(a []byte) int {
	return g.keyManager.comparer.Split(a)
}

func (g *generator) prefix(a []byte) []byte {
	return a[:g.split(a)]
}

func (g *generator) String() string {
	var buf bytes.Buffer
	for _, op := range g.ops {
		fmt.Fprintf(&buf, "%s\n", op)
	}
	return buf.String()
}