managed_db_test.go

package badger

import (
	"fmt"
	"io/ioutil"
	"math"
	"math/rand"
	"runtime"
	"sync"
	"sync/atomic"
	"testing"
	"time"

	"github.com/dgraph-io/badger/v3/y"
	"github.com/dgraph-io/ristretto/z"
	"github.com/stretchr/testify/require"
)

func val(large bool) []byte {
	var buf []byte
	if large {
		buf = make([]byte, 8192)
	} else {
		buf = make([]byte, 16)
	}
	rand.Read(buf)
	return buf
}

func numKeys(db *DB) int {
	var count int
	err := db.View(func(txn *Txn) error {
		itr := txn.NewIterator(DefaultIteratorOptions)
		defer itr.Close()

		for itr.Rewind(); itr.Valid(); itr.Next() {
			count++
		}
		return nil
	})
	y.Check(err)
	return count
}

func numKeysManaged(db *DB, readTs uint64) int {
	txn := db.NewTransactionAt(readTs, false)
	defer txn.Discard()

	itr := txn.NewIterator(DefaultIteratorOptions)
	defer itr.Close()

	var count int
	for itr.Rewind(); itr.Valid(); itr.Next() {
		count++
	}
	return count
}

func TestDropAllManaged(t *testing.T) {
	dir, err := ioutil.TempDir("", "badger-test")
	require.NoError(t, err)
	defer removeDir(dir)
	opts := getTestOptions(dir)
	opts.managedTxns = true
	opts.ValueLogFileSize = 5 << 20
	db, err := Open(opts)
	require.NoError(t, err)

	N := uint64(10000)
	populate := func(db *DB, start uint64) {
		var wg sync.WaitGroup
		for i := start; i < start+N; i++ {
			wg.Add(1)
			txn := db.NewTransactionAt(math.MaxUint64, true)
			require.NoError(t, txn.SetEntry(NewEntry([]byte(key("key", int(i))), val(true))))
			require.NoError(t, txn.CommitAt(uint64(i), func(err error) {
				require.NoError(t, err)
				wg.Done()
			}))
		}
		wg.Wait()
	}

	populate(db, N)
	require.Equal(t, int(N), numKeysManaged(db, math.MaxUint64))

	require.NoError(t, db.DropAll())
	require.NoError(t, db.DropAll()) // Just call it twice, for fun.
	require.Equal(t, 0, numKeysManaged(db, math.MaxUint64))

	// Check that we can still write to db, and using lower timestamps.
	populate(db, 1)
	require.Equal(t, int(N), numKeysManaged(db, math.MaxUint64))
	require.NoError(t, db.Close())

	// Ensure that value log is correctly replayed, that we are preserving badgerHead.
	opts.managedTxns = true
	db2, err := Open(opts)
	require.NoError(t, err)
	require.Equal(t, int(N), numKeysManaged(db2, math.MaxUint64))
	require.NoError(t, db2.Close())
}

func TestDropAll(t *testing.T) {
	dir, err := ioutil.TempDir("", "badger-test")
	require.NoError(t, err)
	defer removeDir(dir)
	opts := getTestOptions(dir)
	opts.ValueLogFileSize = 5 << 20
	db, err := Open(opts)
	require.NoError(t, err)

	N := uint64(10000)
	populate := func(db *DB) {
		writer := db.NewWriteBatch()
		for i := uint64(0); i < N; i++ {
			require.NoError(t, writer.Set([]byte(key("key", int(i))), val(true)))
		}
		require.NoError(t, writer.Flush())
	}

	populate(db)
	require.Equal(t, int(N), numKeys(db))

	require.NoError(t, db.DropAll())
	require.Equal(t, 0, numKeys(db))

	// Check that we can still write to mdb, and using lower timestamps.
	populate(db)
	require.Equal(t, int(N), numKeys(db))
	require.NoError(t, db.Close())

	// Ensure that value log is correctly replayed.
	db2, err := Open(opts)
	require.NoError(t, err)
	require.Equal(t, int(N), numKeys(db2))
	require.NoError(t, db2.Close())
}

func TestDropAllTwice(t *testing.T) {
	test := func(t *testing.T, opts Options) {
		db, err := Open(opts)

		require.NoError(t, err)
		defer func() {
			require.NoError(t, db.Close())
		}()

		N := uint64(10000)
		populate := func(db *DB) {
			writer := db.NewWriteBatch()
			for i := uint64(0); i < N; i++ {
				require.NoError(t, writer.Set([]byte(key("key", int(i))), val(false)))
			}
			require.NoError(t, writer.Flush())
		}

		populate(db)
		require.Equal(t, int(N), numKeys(db))

		require.NoError(t, db.DropAll())
		require.Equal(t, 0, numKeys(db))

		// Call DropAll again.
		require.NoError(t, db.DropAll())
		require.NoError(t, db.Close())
	}
	t.Run("disk mode", func(t *testing.T) {
		dir, err := ioutil.TempDir("", "badger-test")
		require.NoError(t, err)
		defer removeDir(dir)
		opts := getTestOptions(dir)
		opts.ValueLogFileSize = 5 << 20
		test(t, opts)
	})
	t.Run("InMemory mode", func(t *testing.T) {
		opts := getTestOptions("")
		opts.InMemory = true
		test(t, opts)
	})
}

func TestDropAllWithPendingTxn(t *testing.T) {
	dir, err := ioutil.TempDir("", "badger-test")
	require.NoError(t, err)
	defer removeDir(dir)
	opts := getTestOptions(dir)
	opts.ValueLogFileSize = 5 << 20
	db, err := Open(opts)
	require.NoError(t, err)
	defer func() {
		require.NoError(t, db.Close())
	}()

	N := uint64(10000)
	populate := func(db *DB) {
		writer := db.NewWriteBatch()
		for i := uint64(0); i < N; i++ {
			require.NoError(t, writer.Set([]byte(key("key", int(i))), val(true)))
		}
		require.NoError(t, writer.Flush())
	}

	populate(db)
	require.Equal(t, int(N), numKeys(db))

	txn := db.NewTransaction(true)

	var wg sync.WaitGroup
	wg.Add(1)
	go func() {
		defer wg.Done()
		itr := txn.NewIterator(DefaultIteratorOptions)
		defer itr.Close()

		var keys []string
		for {
			var count int
			for itr.Rewind(); itr.Valid(); itr.Next() {
				count++
				item := itr.Item()
				keys = append(keys, string(item.KeyCopy(nil)))
				_, err := item.ValueCopy(nil)
				if err != nil {
					t.Logf("Got error during value copy: %v", err)
					return
				}
			}
			t.Logf("Got number of keys: %d\n", count)
			for _, key := range keys {
				item, err := txn.Get([]byte(key))
				if err != nil {
					t.Logf("Got error during key lookup: %v", err)
					return
				}
				if _, err := item.ValueCopy(nil); err != nil {
					t.Logf("Got error during second value copy: %v", err)
					return
				}
			}
		}
	}()
	// Do not cancel txn.

	wg.Add(1)
	go func() {
		defer wg.Done()
		time.Sleep(2 * time.Second)
		require.NoError(t, db.DropAll())
	}()
	wg.Wait()
}

func TestDropReadOnly(t *testing.T) {
	dir, err := ioutil.TempDir("", "badger-test")
	require.NoError(t, err)
	defer removeDir(dir)
	opts := getTestOptions(dir)
	opts.ValueLogFileSize = 5 << 20
	db, err := Open(opts)
	require.NoError(t, err)
	N := uint64(1000)
	populate := func(db *DB) {
		writer := db.NewWriteBatch()
		for i := uint64(0); i < N; i++ {
			require.NoError(t, writer.Set([]byte(key("key", int(i))), val(true)))
		}
		require.NoError(t, writer.Flush())
	}

	populate(db)
	require.Equal(t, int(N), numKeys(db))
	require.NoError(t, db.Close())

	opts.ReadOnly = true
	db2, err := Open(opts)
	// acquireDirectoryLock returns ErrWindowsNotSupported on Windows. It can be ignored safely.
	if runtime.GOOS == "windows" {
		require.Equal(t, err, ErrWindowsNotSupported)
	} else {
		require.NoError(t, err)
		require.Panics(t, func() { db2.DropAll() })
		require.NoError(t, db2.Close())
	}
}

func TestWriteAfterClose(t *testing.T) {
	dir, err := ioutil.TempDir("", "badger-test")
	require.NoError(t, err)
	defer removeDir(dir)
	opts := getTestOptions(dir)
	opts.ValueLogFileSize = 5 << 20
	db, err := Open(opts)
	require.NoError(t, err)
	N := uint64(1000)
	populate := func(db *DB) {
		writer := db.NewWriteBatch()
		for i := uint64(0); i < N; i++ {
			require.NoError(t, writer.Set([]byte(key("key", int(i))), val(true)))
		}
		require.NoError(t, writer.Flush())
	}

	populate(db)
	require.Equal(t, int(N), numKeys(db))
	require.NoError(t, db.Close())
	err = db.Update(func(txn *Txn) error {
		return txn.SetEntry(NewEntry([]byte("a"), []byte("b")))
	})
	require.Equal(t, ErrDBClosed, err)
}

func TestDropAllRace(t *testing.T) {
	dir, err := ioutil.TempDir("", "badger-test")
	require.NoError(t, err)
	defer removeDir(dir)
	opts := getTestOptions(dir)
	opts.managedTxns = true
	db, err := Open(opts)
	require.NoError(t, err)

	N := 10000
	// Start a goroutine to keep trying to write to DB while DropAll happens.
	closer := z.NewCloser(1)
	go func() {
		defer closer.Done()
		ticker := time.NewTicker(time.Millisecond)
		defer ticker.Stop()

		i := N + 1 // Writes would happen above N.
		var errors int32
		for {
			select {
			case <-ticker.C:
				i++
				txn := db.NewTransactionAt(math.MaxUint64, true)
				require.NoError(t, txn.SetEntry(NewEntry([]byte(key("key", i)), val(false))))
				if err := txn.CommitAt(uint64(i), func(err error) {
					if err != nil {
						atomic.AddInt32(&errors, 1)
					}
				}); err != nil {
					atomic.AddInt32(&errors, 1)
				}
			case <-closer.HasBeenClosed():
				// The following causes a data race.
				// t.Logf("i: %d. Number of (expected) write errors: %d.\n", i, errors)
				return
			}
		}
	}()

	var wg sync.WaitGroup
	for i := 1; i <= N; i++ {
		wg.Add(1)
		txn := db.NewTransactionAt(math.MaxUint64, true)
		require.NoError(t, txn.SetEntry(NewEntry([]byte(key("key", i)), val(false))))
		require.NoError(t, txn.CommitAt(uint64(i), func(err error) {
			require.NoError(t, err)
			wg.Done()
		}))
	}
	wg.Wait()

	before := numKeysManaged(db, math.MaxUint64)
	require.True(t, before > N)

	require.NoError(t, db.DropAll())
	closer.SignalAndWait()

	after := numKeysManaged(db, math.MaxUint64)
	t.Logf("Before: %d. After dropall: %d\n", before, after)
	require.True(t, after < before)
	db.Close()
}

func TestDropPrefix(t *testing.T) {
	dir, err := ioutil.TempDir("", "badger-test")
	require.NoError(t, err)
	defer removeDir(dir)
	opts := getTestOptions(dir)
	opts.ValueLogFileSize = 5 << 20
	db, err := Open(opts)
	require.NoError(t, err)

	N := uint64(10000)
	populate := func(db *DB) {
		writer := db.NewWriteBatch()
		for i := uint64(0); i < N; i++ {
			require.NoError(t, writer.Set([]byte(key("key", int(i))), val(true)))
		}
		require.NoError(t, writer.Flush())
	}

	populate(db)
	require.Equal(t, int(N), numKeys(db))

	require.NoError(t, db.DropPrefix([]byte("key000")))
	require.Equal(t, int(N)-10, numKeys(db))

	require.NoError(t, db.DropPrefix([]byte("key00")))
	require.Equal(t, int(N)-100, numKeys(db))

	expected := int(N)
	for i := 0; i < 10; i++ {
		require.NoError(t, db.DropPrefix([]byte(fmt.Sprintf("key%d", i))))
		expected -= 1000
		require.Equal(t, expected, numKeys(db))
	}
	require.NoError(t, db.DropPrefix([]byte("key1")))
	require.Equal(t, 0, numKeys(db))
	require.NoError(t, db.DropPrefix([]byte("key")))
	require.Equal(t, 0, numKeys(db))

	// Check that we can still write to mdb, and using lower timestamps.
	populate(db)
	require.Equal(t, int(N), numKeys(db))
	require.NoError(t, db.DropPrefix([]byte("key")))
	require.Equal(t, 0, numKeys(db))
	require.NoError(t, db.Close())

	// Ensure that value log is correctly replayed.
	db2, err := Open(opts)
	require.NoError(t, err)
	require.Equal(t, 0, numKeys(db2))
	require.NoError(t, db2.Close())
}

func TestDropPrefixWithPendingTxn(t *testing.T) {
	dir, err := ioutil.TempDir("", "badger-test")
	require.NoError(t, err)
	defer removeDir(dir)
	opts := getTestOptions(dir)
	opts.ValueLogFileSize = 5 << 20
	db, err := Open(opts)
	require.NoError(t, err)
	defer func() {
		require.NoError(t, db.Close())
	}()

	N := uint64(10000)
	populate := func(db *DB) {
		writer := db.NewWriteBatch()
		for i := uint64(0); i < N; i++ {
			require.NoError(t, writer.Set([]byte(key("key", int(i))), val(true)))
		}
		require.NoError(t, writer.Flush())
	}

	populate(db)
	require.Equal(t, int(N), numKeys(db))

	txn := db.NewTransaction(true)

	var wg sync.WaitGroup
	wg.Add(2)
	go func() {
		defer wg.Done()
		itr := txn.NewIterator(DefaultIteratorOptions)
		defer itr.Close()

		var keys []string
		for {
			var count int
			for itr.Rewind(); itr.Valid(); itr.Next() {
				count++
				item := itr.Item()
				keys = append(keys, string(item.KeyCopy(nil)))
				_, err := item.ValueCopy(nil)
				if err != nil {
					t.Logf("Got error during value copy: %v", err)
					return
				}
			}
			t.Logf("Got number of keys: %d\n", count)
			for _, key := range keys {
				item, err := txn.Get([]byte(key))
				if err != nil {
					t.Logf("Got error during key lookup: %v", err)
					return
				}
				if _, err := item.ValueCopy(nil); err != nil {
					t.Logf("Got error during second value copy: %v", err)
					return
				}
			}
		}
	}()
	// Do not cancel txn.

	go func() {
		defer wg.Done()
		time.Sleep(2 * time.Second)
		require.NoError(t, db.DropPrefix([]byte("key0")))
		require.NoError(t, db.DropPrefix([]byte("key00")))
		require.NoError(t, db.DropPrefix([]byte("key")))
	}()
	wg.Wait()
}

func TestDropPrefixReadOnly(t *testing.T) {
	dir, err := ioutil.TempDir("", "badger-test")
	require.NoError(t, err)
	defer removeDir(dir)
	opts := getTestOptions(dir)
	opts.ValueLogFileSize = 5 << 20
	db, err := Open(opts)
	require.NoError(t, err)
	N := uint64(1000)
	populate := func(db *DB) {
		writer := db.NewWriteBatch()
		for i := uint64(0); i < N; i++ {
			require.NoError(t, writer.Set([]byte(key("key", int(i))), val(true)))
		}
		require.NoError(t, writer.Flush())
	}

	populate(db)
	require.Equal(t, int(N), numKeys(db))
	require.NoError(t, db.Close())

	opts.ReadOnly = true
	db2, err := Open(opts)
	// acquireDirectoryLock returns ErrWindowsNotSupported on Windows. It can be ignored safely.
	if runtime.GOOS == "windows" {
		require.Equal(t, err, ErrWindowsNotSupported)
	} else {
		require.NoError(t, err)
		require.Panics(t, func() { db2.DropPrefix([]byte("key0")) })
		require.NoError(t, db2.Close())
	}
}

func TestDropPrefixRace(t *testing.T) {
	dir, err := ioutil.TempDir("", "badger-test")
	require.NoError(t, err)
	defer removeDir(dir)
	opts := getTestOptions(dir)
	opts.managedTxns = true
	db, err := Open(opts)
	require.NoError(t, err)

	N := 10000
	// Start a goroutine to keep trying to write to DB while DropPrefix happens.
	closer := z.NewCloser(1)
	go func() {
		defer closer.Done()
		ticker := time.NewTicker(time.Millisecond)
		defer ticker.Stop()

		i := N + 1 // Writes would happen above N.
		var errors int32
		for {
			select {
			case <-ticker.C:
				i++
				txn := db.NewTransactionAt(math.MaxUint64, true)
				require.NoError(t, txn.SetEntry(NewEntry([]byte(key("key", i)), val(false))))
				if err := txn.CommitAt(uint64(i), func(err error) {
					if err != nil {
						atomic.AddInt32(&errors, 1)
					}
				}); err != nil {
					atomic.AddInt32(&errors, 1)
				}
			case <-closer.HasBeenClosed():
				// The following causes a data race.
				// t.Logf("i: %d. Number of (expected) write errors: %d.\n", i, errors)
				return
			}
		}
	}()

	var wg sync.WaitGroup
	for i := 1; i <= N; i++ {
		wg.Add(1)
		txn := db.NewTransactionAt(math.MaxUint64, true)
		require.NoError(t, txn.SetEntry(NewEntry([]byte(key("key", i)), val(false))))
		require.NoError(t, txn.CommitAt(uint64(i), func(err error) {
			require.NoError(t, err)
			wg.Done()
		}))
	}
	wg.Wait()

	before := numKeysManaged(db, math.MaxUint64)
	require.True(t, before > N)

	require.NoError(t, db.DropPrefix([]byte("key00")))
	require.NoError(t, db.DropPrefix([]byte("key1")))
	require.NoError(t, db.DropPrefix([]byte("key")))
	closer.SignalAndWait()

	after := numKeysManaged(db, math.MaxUint64)
	t.Logf("Before: %d. After dropprefix: %d\n", before, after)
	require.True(t, after < before)
	require.NoError(t, db.Close())
}

func TestWriteBatchManagedMode(t *testing.T) {
	key := func(i int) []byte {
		return []byte(fmt.Sprintf("%10d", i))
	}
	val := func(i int) []byte {
		return []byte(fmt.Sprintf("%128d", i))
	}
	opt := DefaultOptions("")
	opt.managedTxns = true
	opt.BaseTableSize = 1 << 20 // This would create multiple transactions in write batch.
	runBadgerTest(t, &opt, func(t *testing.T, db *DB) {
		wb := db.NewWriteBatchAt(1)
		defer wb.Cancel()

		N, M := 50000, 1000
		start := time.Now()

		for i := 0; i < N; i++ {
			require.NoError(t, wb.Set(key(i), val(i)))
		}
		for i := 0; i < M; i++ {
			require.NoError(t, wb.Delete(key(i)))
		}
		require.NoError(t, wb.Flush())
		t.Logf("Time taken for %d writes (w/ test options): %s\n", N+M, time.Since(start))

		err := db.View(func(txn *Txn) error {
			itr := txn.NewIterator(DefaultIteratorOptions)
			defer itr.Close()

			i := M
			for itr.Rewind(); itr.Valid(); itr.Next() {
				item := itr.Item()
				require.Equal(t, string(key(i)), string(item.Key()))
				require.Equal(t, item.Version(), uint64(1))
				valcopy, err := item.ValueCopy(nil)
				require.NoError(t, err)
				require.Equal(t, val(i), valcopy)
				i++
			}
			require.Equal(t, N, i)
			return nil
		})
		require.NoError(t, err)
	})
}
func TestWriteBatchManaged(t *testing.T) {
	key := func(i int) []byte {
		return []byte(fmt.Sprintf("%10d", i))
	}
	val := func(i int) []byte {
		return []byte(fmt.Sprintf("%128d", i))
	}
	opt := DefaultOptions("")
	opt.managedTxns = true
	opt.BaseTableSize = 1 << 15 // This would create multiple transactions in write batch.
	runBadgerTest(t, &opt, func(t *testing.T, db *DB) {
		wb := db.NewManagedWriteBatch()
		defer wb.Cancel()

		N, M := 50000, 1000
		start := time.Now()

		for i := 0; i < N; i++ {
			require.NoError(t, wb.SetEntryAt(&Entry{Key: key(i), Value: val(i)}, 1))
		}
		for i := 0; i < M; i++ {
			require.NoError(t, wb.DeleteAt(key(i), 2))
		}
		require.NoError(t, wb.Flush())
		t.Logf("Time taken for %d writes (w/ test options): %s\n", N+M, time.Since(start))

		err := db.View(func(txn *Txn) error {
			itr := txn.NewIterator(DefaultIteratorOptions)
			defer itr.Close()

			i := M
			for itr.Rewind(); itr.Valid(); itr.Next() {
				item := itr.Item()
				require.Equal(t, string(key(i)), string(item.Key()))
				require.Equal(t, item.Version(), uint64(1))
				valcopy, err := item.ValueCopy(nil)
				require.NoError(t, err)
				require.Equal(t, val(i), valcopy)
				i++
			}
			require.Equal(t, N, i)
			return nil
		})
		require.NoError(t, err)
	})
}

func TestWriteBatchDuplicate(t *testing.T) {
	N := 10
	k := []byte("key")
	v := []byte("val")
	readVerify := func(t *testing.T, db *DB, n int, versions []int) {
		err := db.View(func(txn *Txn) error {
			iopt := DefaultIteratorOptions
			iopt.AllVersions = true
			itr := txn.NewIterator(iopt)
			defer itr.Close()

			i := 0
			for itr.Rewind(); itr.Valid(); itr.Next() {
				item := itr.Item()
				require.Equal(t, k, item.Key())
				require.Equal(t, uint64(versions[i]), item.Version())
				err := item.Value(func(val []byte) error {
					require.Equal(t, v, val)
					return nil
				})
				require.NoError(t, err)
				i++
			}
			require.Equal(t, n, i)
			return nil
		})
		require.NoError(t, err)
	}

	t.Run("writebatch", func(t *testing.T) {
		opt := DefaultOptions("")
		opt.BaseTableSize = 1 << 15 // This would create multiple transactions in write batch.

		runBadgerTest(t, &opt, func(t *testing.T, db *DB) {
			wb := db.NewWriteBatch()
			defer wb.Cancel()

			for i := uint64(0); i < uint64(N); i++ {
				// Multiple versions of the same key.
				require.NoError(t, wb.SetEntry(&Entry{Key: k, Value: v}))
			}
			require.NoError(t, wb.Flush())
			readVerify(t, db, 1, []int{1})
		})
	})
	t.Run("writebatch at", func(t *testing.T) {
		opt := DefaultOptions("")
		opt.BaseTableSize = 1 << 15 // This would create multiple transactions in write batch.
		opt.managedTxns = true

		runBadgerTest(t, &opt, func(t *testing.T, db *DB) {
			wb := db.NewWriteBatchAt(10)
			defer wb.Cancel()

			for i := uint64(0); i < uint64(N); i++ {
				// Multiple versions of the same key.
				require.NoError(t, wb.SetEntry(&Entry{Key: k, Value: v}))
			}
			require.NoError(t, wb.Flush())
			readVerify(t, db, 1, []int{10})
		})

	})
	t.Run("managed writebatch", func(t *testing.T) {
		opt := DefaultOptions("")
		opt.managedTxns = true
		opt.BaseTableSize = 1 << 15 // This would create multiple transactions in write batch.
		runBadgerTest(t, &opt, func(t *testing.T, db *DB) {
			wb := db.NewManagedWriteBatch()
			defer wb.Cancel()

			for i := uint64(1); i <= uint64(N); i++ {
				// Multiple versions of the same key.
				require.NoError(t, wb.SetEntryAt(&Entry{Key: k, Value: v}, i))
			}
			require.NoError(t, wb.Flush())
			readVerify(t, db, N, []int{10, 9, 8, 7, 6, 5, 4, 3, 2, 1})
		})
	})
}

func TestWriteViaSkip(t *testing.T) {
	key := func(i int) []byte {
		return []byte(fmt.Sprintf("%10d", i))
	}
	val := func(i int) []byte {
		return []byte(fmt.Sprintf("%128d", i))
	}
	opt := DefaultOptions("")
	opt.managedTxns = true
	runBadgerTest(t, &opt, func(t *testing.T, db *DB) {
		s := db.NewSkiplist()
		for i := 0; i < 100; i++ {
			s.Put(y.KeyWithTs(key(i), math.MaxUint64), y.ValueStruct{Value: val(i)})
		}
		{
			// Update key timestamps by directly changing them in the skiplist.
			itr := s.NewUniIterator(false)
			defer itr.Close()
			itr.Rewind()
			for itr.Valid() {
				y.SetKeyTs(itr.Key(), 101)
				itr.Next()
			}
		}

		// Hand over skiplist to Badger.
		require.NoError(t, db.HandoverSkiplist(s, nil))

		// Read the data back.
		txn := db.NewTransactionAt(101, false)
		defer txn.Discard()
		itr := txn.NewIterator(DefaultIteratorOptions)
		defer itr.Close()

		i := 0
		for itr.Rewind(); itr.Valid(); itr.Next() {
			item := itr.Item()
			require.Equal(t, string(key(i)), string(item.Key()))
			require.Equal(t, item.Version(), uint64(101))
			valcopy, err := item.ValueCopy(nil)
			require.NoError(t, err)
			require.Equal(t, val(i), valcopy)
			i++
		}
		require.Equal(t, 100, i)
	})
}

func TestZeroDiscardStats(t *testing.T) {
	N := uint64(10000)
	populate := func(t *testing.T, db *DB) {
		writer := db.NewWriteBatch()
		for i := uint64(0); i < N; i++ {
			require.NoError(t, writer.Set([]byte(key("key", int(i))), val(true)))
		}
		require.NoError(t, writer.Flush())
	}

	t.Run("after rewrite", func(t *testing.T) {
		opts := getTestOptions("")
		opts.ValueLogFileSize = 5 << 20
		opts.ValueThreshold = 1 << 10
		opts.MemTableSize = 1 << 15
		runBadgerTest(t, &opts, func(t *testing.T, db *DB) {
			populate(t, db)
			require.Equal(t, int(N), numKeys(db))

			fids := db.vlog.sortedFids()
			for _, fid := range fids {
				db.vlog.discardStats.Update(uint32(fid), 1)
			}

			// Ensure we have some valid fids.
			require.True(t, len(fids) > 2)
			fid := fids[0]
			require.NoError(t, db.vlog.rewrite(db.vlog.filesMap[fid]))
			// All data should still be present.
			require.Equal(t, int(N), numKeys(db))

			db.vlog.discardStats.Iterate(func(id, val uint64) {
				// Vlog with id=fid has been re-written, it's discard stats should be zero.
				if uint32(id) == fid {
					require.Zero(t, val)
				}
			})
		})
	})
	t.Run("after dropall", func(t *testing.T) {
		opts := getTestOptions("")
		opts.ValueLogFileSize = 5 << 20
		runBadgerTest(t, &opts, func(t *testing.T, db *DB) {
			populate(t, db)
			require.Equal(t, int(N), numKeys(db))

			// Fill discard stats. Normally these are filled by compaction.
			fids := db.vlog.sortedFids()
			for _, fid := range fids {
				db.vlog.discardStats.Update(uint32(fid), 1)
			}

			db.vlog.discardStats.Iterate(func(id, val uint64) { require.NotZero(t, val) })
			require.NoError(t, db.DropAll())
			require.Equal(t, 0, numKeys(db))
			// We've deleted everything. DS should be zero.
			db.vlog.discardStats.Iterate(func(id, val uint64) { require.Zero(t, val) })
		})
	})
}