package bbolt

import (
	"errors"
	"fmt"
	"io"
	"os"
	"runtime"
	"sync"
	"time"
	"unsafe"

	berrors "go.etcd.io/bbolt/errors"
	"go.etcd.io/bbolt/internal/common"
	fl "go.etcd.io/bbolt/internal/freelist"
)

// The time elapsed between consecutive file locking attempts.
const flockRetryTimeout = 50 * time.Millisecond

// FreelistType is the type of the freelist backend
type FreelistType string

// TODO(ahrtr): eventually we should (step by step)
//  1. default to `FreelistMapType`;
//  2. remove the `FreelistArrayType`, do not export `FreelistMapType`
//     and remove field `FreelistType' from both `DB` and `Options`;
const (
	// FreelistArrayType indicates backend freelist type is array
	FreelistArrayType = FreelistType("array")
	// FreelistMapType indicates backend freelist type is hashmap
	FreelistMapType = FreelistType("hashmap")
)

// DB represents a collection of buckets persisted to a file on disk.
// All data access is performed through transactions which can be obtained through the DB.
// All the functions on DB will return a ErrDatabaseNotOpen if accessed before Open() is called.
type DB struct {
	// Put `stats` at the first field to ensure it's 64-bit aligned. Note that
	// the first word in an allocated struct can be relied upon to be 64-bit
	// aligned. Refer to https://pkg.go.dev/sync/atomic#pkg-note-BUG. Also
	// refer to discussion in https://github.com/etcd-io/bbolt/issues/577.
	stats Stats

	// When enabled, the database will perform a Check() after every commit.
	// A panic is issued if the database is in an inconsistent state. This
	// flag has a large performance impact so it should only be used for
	// debugging purposes.
	StrictMode bool

	// Setting the NoSync flag will cause the database to skip fsync()
	// calls after each commit. This can be useful when bulk loading data
	// into a database and you can restart the bulk load in the event of
	// a system failure or database corruption. Do not set this flag for
	// normal use.
	//
	// If the package global IgnoreNoSync constant is true, this value is
	// ignored.  See the comment on that constant for more details.
	//
	// THIS IS UNSAFE. PLEASE USE WITH CAUTION.
	NoSync bool

	// When true, skips syncing freelist to disk. This improves the database
	// write performance under normal operation, but requires a full database
	// re-sync during recovery.
	NoFreelistSync bool

	// FreelistType sets the backend freelist type. There are two options. Array which is simple but endures
	// dramatic performance degradation if database is large and fragmentation in freelist is common.
	// The alternative one is using hashmap, it is faster in almost all circumstances
	// but it doesn't guarantee that it offers the smallest page id available. In normal case it is safe.
	// The default type is array
	FreelistType FreelistType

	// When true, skips the truncate call when growing the database.
	// Setting this to true is only safe on non-ext3/ext4 systems.
	// Skipping truncation avoids preallocation of hard drive space and
	// bypasses a truncate() and fsync() syscall on remapping.
	//
	// https://github.com/boltdb/bolt/issues/284
	NoGrowSync bool

	// When `true`, bbolt will always load the free pages when opening the DB.
	// When opening db in write mode, this flag will always automatically
	// set to `true`.
	PreLoadFreelist bool

	// If you want to read the entire database fast, you can set MmapFlag to
	// syscall.MAP_POPULATE on Linux 2.6.23+ for sequential read-ahead.
	MmapFlags int

	// MaxBatchSize is the maximum size of a batch. Default value is
	// copied from DefaultMaxBatchSize in Open.
	//
	// If <=0, disables batching.
	//
	// Do not change concurrently with calls to Batch.
	MaxBatchSize int

	// MaxBatchDelay is the maximum delay before a batch starts.
	// Default value is copied from DefaultMaxBatchDelay in Open.
	//
	// If <=0, effectively disables batching.
	//
	// Do not change concurrently with calls to Batch.
	MaxBatchDelay time.Duration

	// AllocSize is the amount of space allocated when the database
	// needs to create new pages. This is done to amortize the cost
	// of truncate() and fsync() when growing the data file.
	AllocSize int

	// Mlock locks database file in memory when set to true.
	// It prevents major page faults, however used memory can't be reclaimed.
	//
	// Supported only on Unix via mlock/munlock syscalls.
	Mlock bool

	logger Logger

	path     string
	openFile func(string, int, os.FileMode) (*os.File, error)
	file     *os.File
	// `dataref` isn't used at all on Windows, and the golangci-lint
	// always fails on Windows platform.
	//nolint
	dataref  []byte // mmap'ed readonly, write throws SEGV
	data     *[maxMapSize]byte
	datasz   int
	meta0    *common.Meta
	meta1    *common.Meta
	pageSize int
	opened   bool
	rwtx     *Tx
	txs      []*Tx

	freelist     fl.Interface
	freelistLoad sync.Once

	pagePool sync.Pool

	batchMu sync.Mutex
	batch   *batch

	rwlock   sync.Mutex   // Allows only one writer at a time.
	metalock sync.Mutex   // Protects meta page access.
	mmaplock sync.RWMutex // Protects mmap access during remapping.
	statlock sync.RWMutex // Protects stats access.

	ops struct {
		writeAt func(b []byte, off int64) (n int, err error)
	}

	// Read only mode.
	// When true, Update() and Begin(true) return ErrDatabaseReadOnly immediately.
	readOnly bool
}

// Path returns the path to currently open database file.
func (db *DB) Path() string {
	return db.path
}

// GoString returns the Go string representation of the database.
func (db *DB) GoString() string {
	return fmt.Sprintf("bolt.DB{path:%q}", db.path)
}

// String returns the string representation of the database.
func (db *DB) String() string {
	return fmt.Sprintf("DB<%q>", db.path)
}

// Open creates and opens a database at the given path with a given file mode.
// If the file does not exist then it will be created automatically with a given file mode.
// Passing in nil options will cause Bolt to open the database with the default options.
// Note: For read/write transactions, ensure the owner has write permission on the created/opened database file, e.g. 0600
func Open(path string, mode os.FileMode, options *Options) (db *DB, err error) {
	db = &DB{
		opened: true,
	}

	// Set default options if no options are provided.
	if options == nil {
		options = DefaultOptions
	}
	db.NoSync = options.NoSync
	db.NoGrowSync = options.NoGrowSync
	db.MmapFlags = options.MmapFlags
	db.NoFreelistSync = options.NoFreelistSync
	db.PreLoadFreelist = options.PreLoadFreelist
	db.FreelistType = options.FreelistType
	db.Mlock = options.Mlock

	// Set default values for later DB operations.
	db.MaxBatchSize = common.DefaultMaxBatchSize
	db.MaxBatchDelay = common.DefaultMaxBatchDelay
	db.AllocSize = common.DefaultAllocSize

	if options.Logger == nil {
		db.logger = getDiscardLogger()
	} else {
		db.logger = options.Logger
	}

	lg := db.Logger()
	if lg != discardLogger {
		lg.Infof("Opening db file (%s) with mode %s and with options: %s", path, mode, options)
		defer func() {
			if err != nil {
				lg.Errorf("Opening bbolt db (%s) failed: %v", path, err)
			} else {
				lg.Infof("Opening bbolt db (%s) successfully", path)
			}
		}()
	}

	flag := os.O_RDWR
	if options.ReadOnly {
		flag = os.O_RDONLY
		db.readOnly = true
	} else {
		// always load free pages in write mode
		db.PreLoadFreelist = true
		flag |= os.O_CREATE
	}

	db.openFile = options.OpenFile
	if db.openFile == nil {
		db.openFile = os.OpenFile
	}

	// Open data file and separate sync handler for metadata writes.
	if db.file, err = db.openFile(path, flag, mode); err != nil {
		_ = db.close()
		lg.Errorf("failed to open db file (%s): %v", path, err)
		return nil, err
	}
	db.path = db.file.Name()

	// Lock file so that other processes using Bolt in read-write mode cannot
	// use the database  at the same time. This would cause corruption since
	// the two processes would write meta pages and free pages separately.
	// The database file is locked exclusively (only one process can grab the lock)
	// if !options.ReadOnly.
	// The database file is locked using the shared lock (more than one process may
	// hold a lock at the same time) otherwise (options.ReadOnly is set).
	if err = flock(db, !db.readOnly, options.Timeout); err != nil {
		_ = db.close()
		lg.Errorf("failed to lock db file (%s), readonly: %t, error: %v", path, db.readOnly, err)
		return nil, err
	}

	// Default values for test hooks
	db.ops.writeAt = db.file.WriteAt

	if db.pageSize = options.PageSize; db.pageSize == 0 {
		// Set the default page size to the OS page size.
		db.pageSize = common.DefaultPageSize
	}

	// Initialize the database if it doesn't exist.
	if info, statErr := db.file.Stat(); statErr != nil {
		_ = db.close()
		lg.Errorf("failed to get db file's stats (%s): %v", path, err)
		return nil, statErr
	} else if info.Size() == 0 {
		// Initialize new files with meta pages.
		if err = db.init(); err != nil {
			// clean up file descriptor on initialization fail
			_ = db.close()
			lg.Errorf("failed to initialize db file (%s): %v", path, err)
			return nil, err
		}
	} else {
		// try to get the page size from the metadata pages
		if db.pageSize, err = db.getPageSize(); err != nil {
			_ = db.close()
			lg.Errorf("failed to get page size from db file (%s): %v", path, err)
			return nil, err
		}
	}

	// Initialize page pool.
	db.pagePool = sync.Pool{
		New: func() interface{} {
			return make([]byte, db.pageSize)
		},
	}

	// Memory map the data file.
	if err = db.mmap(options.InitialMmapSize); err != nil {
		_ = db.close()
		lg.Errorf("failed to map db file (%s): %v", path, err)
		return nil, err
	}

	if db.PreLoadFreelist {
		db.loadFreelist()
	}

	if db.readOnly {
		return db, nil
	}

	// Flush freelist when transitioning from no sync to sync so
	// NoFreelistSync unaware boltdb can open the db later.
	if !db.NoFreelistSync && !db.hasSyncedFreelist() {
		tx, txErr := db.Begin(true)
		if tx != nil {
			txErr = tx.Commit()
		}
		if txErr != nil {
			lg.Errorf("starting readwrite transaction failed: %v", txErr)
			_ = db.close()
			return nil, txErr
		}
	}

	// Mark the database as opened and return.
	return db, nil
}

// getPageSize reads the pageSize from the meta pages. It tries
// to read the first meta page firstly. If the first page is invalid,
// then it tries to read the second page using the default page size.
func (db *DB) getPageSize() (int, error) {
	var (
		meta0CanRead, meta1CanRead bool
	)

	// Read the first meta page to determine the page size.
	if pgSize, canRead, err := db.getPageSizeFromFirstMeta(); err != nil {
		// We cannot read the page size from page 0, but can read page 0.
		meta0CanRead = canRead
	} else {
		return pgSize, nil
	}

	// Read the second meta page to determine the page size.
	if pgSize, canRead, err := db.getPageSizeFromSecondMeta(); err != nil {
		// We cannot read the page size from page 1, but can read page 1.
		meta1CanRead = canRead
	} else {
		return pgSize, nil
	}

	// If we can't read the page size from both pages, but can read
	// either page, then we assume it's the same as the OS or the one
	// given, since that's how the page size was chosen in the first place.
	//
	// If both pages are invalid, and (this OS uses a different page size
	// from what the database was created with or the given page size is
	// different from what the database was created with), then we are out
	// of luck and cannot access the database.
	if meta0CanRead || meta1CanRead {
		return db.pageSize, nil
	}

	return 0, berrors.ErrInvalid
}

// getPageSizeFromFirstMeta reads the pageSize from the first meta page
func (db *DB) getPageSizeFromFirstMeta() (int, bool, error) {
	var buf [0x1000]byte
	var metaCanRead bool
	if bw, err := db.file.ReadAt(buf[:], 0); err == nil && bw == len(buf) {
		metaCanRead = true
		if m := db.pageInBuffer(buf[:], 0).Meta(); m.Validate() == nil {
			return int(m.PageSize()), metaCanRead, nil
		}
	}
	return 0, metaCanRead, berrors.ErrInvalid
}

// getPageSizeFromSecondMeta reads the pageSize from the second meta page
func (db *DB) getPageSizeFromSecondMeta() (int, bool, error) {
	var (
		fileSize    int64
		metaCanRead bool
	)

	// get the db file size
	if info, err := db.file.Stat(); err != nil {
		return 0, metaCanRead, err
	} else {
		fileSize = info.Size()
	}

	// We need to read the second meta page, so we should skip the first page;
	// but we don't know the exact page size yet, it's chicken & egg problem.
	// The solution is to try all the possible page sizes, which starts from 1KB
	// and until 16MB (1024<<14) or the end of the db file
	//
	// TODO: should we support larger page size?
	for i := 0; i <= 14; i++ {
		var buf [0x1000]byte
		var pos int64 = 1024 << uint(i)
		if pos >= fileSize-1024 {
			break
		}
		bw, err := db.file.ReadAt(buf[:], pos)
		if (err == nil && bw == len(buf)) || (err == io.EOF && int64(bw) == (fileSize-pos)) {
			metaCanRead = true
			if m := db.pageInBuffer(buf[:], 0).Meta(); m.Validate() == nil {
				return int(m.PageSize()), metaCanRead, nil
			}
		}
	}

	return 0, metaCanRead, berrors.ErrInvalid
}

// loadFreelist reads the freelist if it is synced, or reconstructs it
// by scanning the DB if it is not synced. It assumes there are no
// concurrent accesses being made to the freelist.
func (db *DB) loadFreelist() {
	db.freelistLoad.Do(func() {
		db.freelist = newFreelist(db.FreelistType)
		if !db.hasSyncedFreelist() {
			// Reconstruct free list by scanning the DB.
			db.freelist.Init(db.freepages())
		} else {
			// Read free list from freelist page.
			db.freelist.Read(db.page(db.meta().Freelist()))
		}
		db.stats.FreePageN = db.freelist.FreeCount()
	})
}

func (db *DB) hasSyncedFreelist() bool {
	return db.meta().Freelist() != common.PgidNoFreelist
}

func (db *DB) fileSize() (int, error) {
	info, err := db.file.Stat()
	if err != nil {
		return 0, fmt.Errorf("file stat error: %w", err)
	}
	sz := int(info.Size())
	if sz < db.pageSize*2 {
		return 0, fmt.Errorf("file size too small %d", sz)
	}
	return sz, nil
}

// mmap opens the underlying memory-mapped file and initializes the meta references.
// minsz is the minimum size that the new mmap can be.
func (db *DB) mmap(minsz int) (err error) {
	db.mmaplock.Lock()
	defer db.mmaplock.Unlock()

	lg := db.Logger()

	// Ensure the size is at least the minimum size.
	var fileSize int
	fileSize, err = db.fileSize()
	if err != nil {
		lg.Errorf("getting file size failed: %w", err)
		return err
	}
	var size = fileSize
	if size < minsz {
		size = minsz
	}
	size, err = db.mmapSize(size)
	if err != nil {
		lg.Errorf("getting map size failed: %w", err)
		return err
	}

	if db.Mlock {
		// Unlock db memory
		if err := db.munlock(fileSize); err != nil {
			return err
		}
	}

	// Dereference all mmap references before unmapping.
	if db.rwtx != nil {
		db.rwtx.root.dereference()
	}

	// Unmap existing data before continuing.
	if err = db.munmap(); err != nil {
		return err
	}

	// Memory-map the data file as a byte slice.
	// gofail: var mapError string
	// return errors.New(mapError)
	if err = mmap(db, size); err != nil {
		lg.Errorf("[GOOS: %s, GOARCH: %s] mmap failed, size: %d, error: %v", runtime.GOOS, runtime.GOARCH, size, err)
		return err
	}

	// Perform unmmap on any error to reset all data fields:
	// dataref, data, datasz, meta0 and meta1.
	defer func() {
		if err != nil {
			if unmapErr := db.munmap(); unmapErr != nil {
				err = fmt.Errorf("%w; rollback unmap also failed: %v", err, unmapErr)
			}
		}
	}()

	if db.Mlock {
		// Don't allow swapping of data file
		if err := db.mlock(fileSize); err != nil {
			return err
		}
	}

	// Save references to the meta pages.
	db.meta0 = db.page(0).Meta()
	db.meta1 = db.page(1).Meta()

	// Validate the meta pages. We only return an error if both meta pages fail
	// validation, since meta0 failing validation means that it wasn't saved
	// properly -- but we can recover using meta1. And vice-versa.
	err0 := db.meta0.Validate()
	err1 := db.meta1.Validate()
	if err0 != nil && err1 != nil {
		lg.Errorf("both meta pages are invalid, meta0: %v, meta1: %v", err0, err1)
		return err0
	}

	return nil
}

func (db *DB) invalidate() {
	db.dataref = nil
	db.data = nil
	db.datasz = 0

	db.meta0 = nil
	db.meta1 = nil
}

// munmap unmaps the data file from memory.
func (db *DB) munmap() error {
	defer db.invalidate()

	// gofail: var unmapError string
	// return errors.New(unmapError)
	if err := munmap(db); err != nil {
		db.Logger().Errorf("[GOOS: %s, GOARCH: %s] munmap failed, db.datasz: %d, error: %v", runtime.GOOS, runtime.GOARCH, db.datasz, err)
		return fmt.Errorf("unmap error: %v", err.Error())
	}

	return nil
}

// mmapSize determines the appropriate size for the mmap given the current size
// of the database. The minimum size is 32KB and doubles until it reaches 1GB.
// Returns an error if the new mmap size is greater than the max allowed.
func (db *DB) mmapSize(size int) (int, error) {
	// Double the size from 32KB until 1GB.
	for i := uint(15); i <= 30; i++ {
		if size <= 1<<i {
			return 1 << i, nil
		}
	}

	// Verify the requested size is not above the maximum allowed.
	if size > maxMapSize {
		return 0, errors.New("mmap too large")
	}

	// If larger than 1GB then grow by 1GB at a time.
	sz := int64(size)
	if remainder := sz % int64(common.MaxMmapStep); remainder > 0 {
		sz += int64(common.MaxMmapStep) - remainder
	}

	// Ensure that the mmap size is a multiple of the page size.
	// This should always be true since we're incrementing in MBs.
	pageSize := int64(db.pageSize)
	if (sz % pageSize) != 0 {
		sz = ((sz / pageSize) + 1) * pageSize
	}

	// If we've exceeded the max size then only grow up to the max size.
	if sz > maxMapSize {
		sz = maxMapSize
	}

	return int(sz), nil
}

func (db *DB) munlock(fileSize int) error {
	// gofail: var munlockError string
	// return errors.New(munlockError)
	if err := munlock(db, fileSize); err != nil {
		db.Logger().Errorf("[GOOS: %s, GOARCH: %s] munlock failed, fileSize: %d, db.datasz: %d, error: %v", runtime.GOOS, runtime.GOARCH, fileSize, db.datasz, err)
		return fmt.Errorf("munlock error: %v", err.Error())
	}
	return nil
}

func (db *DB) mlock(fileSize int) error {
	// gofail: var mlockError string
	// return errors.New(mlockError)
	if err := mlock(db, fileSize); err != nil {
		db.Logger().Errorf("[GOOS: %s, GOARCH: %s] mlock failed, fileSize: %d, db.datasz: %d, error: %v", runtime.GOOS, runtime.GOARCH, fileSize, db.datasz, err)
		return fmt.Errorf("mlock error: %v", err.Error())
	}
	return nil
}

func (db *DB) mrelock(fileSizeFrom, fileSizeTo int) error {
	if err := db.munlock(fileSizeFrom); err != nil {
		return err
	}
	if err := db.mlock(fileSizeTo); err != nil {
		return err
	}
	return nil
}

// init creates a new database file and initializes its meta pages.
func (db *DB) init() error {
	// Create two meta pages on a buffer.
	buf := make([]byte, db.pageSize*4)
	for i := 0; i < 2; i++ {
		p := db.pageInBuffer(buf, common.Pgid(i))
		p.SetId(common.Pgid(i))
		p.SetFlags(common.MetaPageFlag)

		// Initialize the meta page.
		m := p.Meta()
		m.SetMagic(common.Magic)
		m.SetVersion(common.Version)
		m.SetPageSize(uint32(db.pageSize))
		m.SetFreelist(2)
		m.SetRootBucket(common.NewInBucket(3, 0))
		m.SetPgid(4)
		m.SetTxid(common.Txid(i))
		m.SetChecksum(m.Sum64())
	}

	// Write an empty freelist at page 3.
	p := db.pageInBuffer(buf, common.Pgid(2))
	p.SetId(2)
	p.SetFlags(common.FreelistPageFlag)
	p.SetCount(0)

	// Write an empty leaf page at page 4.
	p = db.pageInBuffer(buf, common.Pgid(3))
	p.SetId(3)
	p.SetFlags(common.LeafPageFlag)
	p.SetCount(0)

	// Write the buffer to our data file.
	if _, err := db.ops.writeAt(buf, 0); err != nil {
		db.Logger().Errorf("writeAt failed: %w", err)
		return err
	}
	if err := fdatasync(db); err != nil {
		db.Logger().Errorf("[GOOS: %s, GOARCH: %s] fdatasync failed: %w", runtime.GOOS, runtime.GOARCH, err)
		return err
	}

	return nil
}

// Close releases all database resources.
// It will block waiting for any open transactions to finish
// before closing the database and returning.
func (db *DB) Close() error {
	db.rwlock.Lock()
	defer db.rwlock.Unlock()

	db.metalock.Lock()
	defer db.metalock.Unlock()

	db.mmaplock.Lock()
	defer db.mmaplock.Unlock()

	return db.close()
}

func (db *DB) close() error {
	if !db.opened {
		return nil
	}

	db.opened = false

	db.freelist = nil

	// Clear ops.
	db.ops.writeAt = nil

	var errs []error
	// Close the mmap.
	if err := db.munmap(); err != nil {
		errs = append(errs, err)
	}

	// Close file handles.
	if db.file != nil {
		// No need to unlock read-only file.
		if !db.readOnly {
			// Unlock the file.
			if err := funlock(db); err != nil {
				errs = append(errs, fmt.Errorf("bolt.Close(): funlock error: %w", err))
			}
		}

		// Close the file descriptor.
		if err := db.file.Close(); err != nil {
			errs = append(errs, fmt.Errorf("db file close: %w", err))
		}
		db.file = nil
	}

	db.path = ""

	if len(errs) > 0 {
		return errs[0]
	}
	return nil
}

// Begin starts a new transaction.
// Multiple read-only transactions can be used concurrently but only one
// write transaction can be used at a time. Starting multiple write transactions
// will cause the calls to block and be serialized until the current write
// transaction finishes.
//
// Transactions should not be dependent on one another. Opening a read
// transaction and a write transaction in the same goroutine can cause the
// writer to deadlock because the database periodically needs to re-mmap itself
// as it grows and it cannot do that while a read transaction is open.
//
// If a long running read transaction (for example, a snapshot transaction) is
// needed, you might want to set DB.InitialMmapSize to a large enough value
// to avoid potential blocking of write transaction.
//
// IMPORTANT: You must close read-only transactions after you are finished or
// else the database will not reclaim old pages.
func (db *DB) Begin(writable bool) (t *Tx, err error) {
	if lg := db.Logger(); lg != discardLogger {
		lg.Debugf("Starting a new transaction [writable: %t]", writable)
		defer func() {
			if err != nil {
				lg.Errorf("Starting a new transaction [writable: %t] failed: %v", writable, err)
			} else {
				lg.Debugf("Starting a new transaction [writable: %t] successfully", writable)
			}
		}()
	}

	if writable {
		return db.beginRWTx()
	}
	return db.beginTx()
}

func (db *DB) Logger() Logger {
	if db == nil || db.logger == nil {
		return getDiscardLogger()
	}
	return db.logger
}

func (db *DB) beginTx() (*Tx, error) {
	// Lock the meta pages while we initialize the transaction. We obtain
	// the meta lock before the mmap lock because that's the order that the
	// write transaction will obtain them.
	db.metalock.Lock()

	// Obtain a read-only lock on the mmap. When the mmap is remapped it will
	// obtain a write lock so all transactions must finish before it can be
	// remapped.
	db.mmaplock.RLock()

	// Exit if the database is not open yet.
	if !db.opened {
		db.mmaplock.RUnlock()
		db.metalock.Unlock()
		return nil, berrors.ErrDatabaseNotOpen
	}

	// Exit if the database is not correctly mapped.
	if db.data == nil {
		db.mmaplock.RUnlock()
		db.metalock.Unlock()
		return nil, berrors.ErrInvalidMapping
	}

	// Create a transaction associated with the database.
	t := &Tx{}
	t.init(db)

	// Keep track of transaction until it closes.
	db.txs = append(db.txs, t)
	n := len(db.txs)
	if db.freelist != nil {
		db.freelist.AddReadonlyTXID(t.meta.Txid())
	}

	// Unlock the meta pages.
	db.metalock.Unlock()

	// Update the transaction stats.
	db.statlock.Lock()
	db.stats.TxN++
	db.stats.OpenTxN = n
	db.statlock.Unlock()

	return t, nil
}

func (db *DB) beginRWTx() (*Tx, error) {
	// If the database was opened with Options.ReadOnly, return an error.
	if db.readOnly {
		return nil, berrors.ErrDatabaseReadOnly
	}

	// Obtain writer lock. This is released by the transaction when it closes.
	// This enforces only one writer transaction at a time.
	db.rwlock.Lock()

	// Once we have the writer lock then we can lock the meta pages so that
	// we can set up the transaction.
	db.metalock.Lock()
	defer db.metalock.Unlock()

	// Exit if the database is not open yet.
	if !db.opened {
		db.rwlock.Unlock()
		return nil, berrors.ErrDatabaseNotOpen
	}

	// Exit if the database is not correctly mapped.
	if db.data == nil {
		db.rwlock.Unlock()
		return nil, berrors.ErrInvalidMapping
	}

	// Create a transaction associated with the database.
	t := &Tx{writable: true}
	t.init(db)
	db.rwtx = t
	db.freelist.ReleasePendingPages()
	return t, nil
}

// removeTx removes a transaction from the database.
func (db *DB) removeTx(tx *Tx) {
	// Release the read lock on the mmap.
	db.mmaplock.RUnlock()

	// Use the meta lock to restrict access to the DB object.
	db.metalock.Lock()

	// Remove the transaction.
	for i, t := range db.txs {
		if t == tx {
			last := len(db.txs) - 1
			db.txs[i] = db.txs[last]
			db.txs[last] = nil
			db.txs = db.txs[:last]
			break
		}
	}
	n := len(db.txs)
	if db.freelist != nil {
		db.freelist.RemoveReadonlyTXID(tx.meta.Txid())
	}

	// Unlock the meta pages.
	db.metalock.Unlock()

	// Merge statistics.
	db.statlock.Lock()
	db.stats.OpenTxN = n
	db.stats.TxStats.add(&tx.stats)
	db.statlock.Unlock()
}

// Update executes a function within the context of a read-write managed transaction.
// If no error is returned from the function then the transaction is committed.
// If an error is returned then the entire transaction is rolled back.
// Any error that is returned from the function or returned from the commit is
// returned from the Update() method.
//
// Attempting to manually commit or rollback within the function will cause a panic.
func (db *DB) Update(fn func(*Tx) error) error {
	t, err := db.Begin(true)
	if err != nil {
		return err
	}

	// Make sure the transaction rolls back in the event of a panic.
	defer func() {
		if t.db != nil {
			t.rollback()
		}
	}()

	// Mark as a managed tx so that the inner function cannot manually commit.
	t.managed = true

	// If an error is returned from the function then rollback and return error.
	err = fn(t)
	t.managed = false
	if err != nil {
		_ = t.Rollback()
		return err
	}

	return t.Commit()
}

// View executes a function within the context of a managed read-only transaction.
// Any error that is returned from the function is returned from the View() method.
//
// Attempting to manually rollback within the function will cause a panic.
func (db *DB) View(fn func(*Tx) error) error {
	t, err := db.Begin(false)
	if err != nil {
		return err
	}

	// Make sure the transaction rolls back in the event of a panic.
	defer func() {
		if t.db != nil {
			t.rollback()
		}
	}()

	// Mark as a managed tx so that the inner function cannot manually rollback.
	t.managed = true

	// If an error is returned from the function then pass it through.
	err = fn(t)
	t.managed = false
	if err != nil {
		_ = t.Rollback()
		return err
	}

	return t.Rollback()
}

// Batch calls fn as part of a batch. It behaves similar to Update,
// except:
//
// 1. concurrent Batch calls can be combined into a single Bolt
// transaction.
//
// 2. the function passed to Batch may be called multiple times,
// regardless of whether it returns error or not.
//
// This means that Batch function side effects must be idempotent and
// take permanent effect only after a successful return is seen in
// caller.
//
// The maximum batch size and delay can be adjusted with DB.MaxBatchSize
// and DB.MaxBatchDelay, respectively.
//
// Batch is only useful when there are multiple goroutines calling it.
func (db *DB) Batch(fn func(*Tx) error) error {
	errCh := make(chan error, 1)

	db.batchMu.Lock()
	if (db.batch == nil) || (db.batch != nil && len(db.batch.calls) >= db.MaxBatchSize) {
		// There is no existing batch, or the existing batch is full; start a new one.
		db.batch = &batch{
			db: db,
		}
		db.batch.timer = time.AfterFunc(db.MaxBatchDelay, db.batch.trigger)
	}
	db.batch.calls = append(db.batch.calls, call{fn: fn, err: errCh})
	if len(db.batch.calls) >= db.MaxBatchSize {
		// wake up batch, it's ready to run
		go db.batch.trigger()
	}
	db.batchMu.Unlock()

	err := <-errCh
	if err == trySolo {
		err = db.Update(fn)
	}
	return err
}

type call struct {
	fn  func(*Tx) error
	err chan<- error
}

type batch struct {
	db    *DB
	timer *time.Timer
	start sync.Once
	calls []call
}

// trigger runs the batch if it hasn't already been run.
func (b *batch) trigger() {
	b.start.Do(b.run)
}

// run performs the transactions in the batch and communicates results
// back to DB.Batch.
func (b *batch) run() {
	b.db.batchMu.Lock()
	b.timer.Stop()
	// Make sure no new work is added to this batch, but don't break
	// other batches.
	if b.db.batch == b {
		b.db.batch = nil
	}
	b.db.batchMu.Unlock()

retry:
	for len(b.calls) > 0 {
		var failIdx = -1
		err := b.db.Update(func(tx *Tx) error {
			for i, c := range b.calls {
				if err := safelyCall(c.fn, tx); err != nil {
					failIdx = i
					return err
				}
			}
			return nil
		})

		if failIdx >= 0 {
			// take the failing transaction out of the batch. it's
			// safe to shorten b.calls here because db.batch no longer
			// points to us, and we hold the mutex anyway.
			c := b.calls[failIdx]
			b.calls[failIdx], b.calls = b.calls[len(b.calls)-1], b.calls[:len(b.calls)-1]
			// tell the submitter re-run it solo, continue with the rest of the batch
			c.err <- trySolo
			continue retry
		}

		// pass success, or bolt internal errors, to all callers
		for _, c := range b.calls {
			c.err <- err
		}
		break retry
	}
}

// trySolo is a special sentinel error value used for signaling that a
// transaction function should be re-run. It should never be seen by
// callers.
var trySolo = errors.New("batch function returned an error and should be re-run solo")

type panicked struct {
	reason interface{}
}

func (p panicked) Error() string {
	if err, ok := p.reason.(error); ok {
		return err.Error()
	}
	return fmt.Sprintf("panic: %v", p.reason)
}

func safelyCall(fn func(*Tx) error, tx *Tx) (err error) {
	defer func() {
		if p := recover(); p != nil {
			err = panicked{p}
		}
	}()
	return fn(tx)
}

// Sync executes fdatasync() against the database file handle.
//
// This is not necessary under normal operation, however, if you use NoSync
// then it allows you to force the database file to sync against the disk.
func (db *DB) Sync() (err error) {
	if lg := db.Logger(); lg != discardLogger {
		lg.Debug("Syncing bbolt db (%s)", db.path)
		defer func() {
			if err != nil {
				lg.Errorf("[GOOS: %s, GOARCH: %s] syncing bbolt db (%s) failed: %v", runtime.GOOS, runtime.GOARCH, db.path, err)
			} else {
				lg.Debugf("Syncing bbolt db (%s) successfully", db.path)
			}
		}()
	}

	return fdatasync(db)
}

// Stats retrieves ongoing performance stats for the database.
// This is only updated when a transaction closes.
func (db *DB) Stats() Stats {
	db.statlock.RLock()
	defer db.statlock.RUnlock()
	return db.stats
}

// This is for internal access to the raw data bytes from the C cursor, use
// carefully, or not at all.
func (db *DB) Info() *Info {
	common.Assert(db.data != nil, "database file isn't correctly mapped")
	return &Info{uintptr(unsafe.Pointer(&db.data[0])), db.pageSize}
}

// page retrieves a page reference from the mmap based on the current page size.
func (db *DB) page(id common.Pgid) *common.Page {
	pos := id * common.Pgid(db.pageSize)
	return (*common.Page)(unsafe.Pointer(&db.data[pos]))
}

// pageInBuffer retrieves a page reference from a given byte array based on the current page size.
func (db *DB) pageInBuffer(b []byte, id common.Pgid) *common.Page {
	return (*common.Page)(unsafe.Pointer(&b[id*common.Pgid(db.pageSize)]))
}

// meta retrieves the current meta page reference.
func (db *DB) meta() *common.Meta {
	// We have to return the meta with the highest txid which doesn't fail
	// validation. Otherwise, we can cause errors when in fact the database is
	// in a consistent state. metaA is the one with the higher txid.
	metaA := db.meta0
	metaB := db.meta1
	if db.meta1.Txid() > db.meta0.Txid() {
		metaA = db.meta1
		metaB = db.meta0
	}

	// Use higher meta page if valid. Otherwise, fallback to previous, if valid.
	if err := metaA.Validate(); err == nil {
		return metaA
	} else if err := metaB.Validate(); err == nil {
		return metaB
	}

	// This should never be reached, because both meta1 and meta0 were validated
	// on mmap() and we do fsync() on every write.
	panic("bolt.DB.meta(): invalid meta pages")
}

// allocate returns a contiguous block of memory starting at a given page.
func (db *DB) allocate(txid common.Txid, count int) (*common.Page, error) {
	// Allocate a temporary buffer for the page.
	var buf []byte
	if count == 1 {
		buf = db.pagePool.Get().([]byte)
	} else {
		buf = make([]byte, count*db.pageSize)
	}
	p := (*common.Page)(unsafe.Pointer(&buf[0]))
	p.SetOverflow(uint32(count - 1))

	// Use pages from the freelist if they are available.
	p.SetId(db.freelist.Allocate(txid, count))
	if p.Id() != 0 {
		return p, nil
	}

	// Resize mmap() if we're at the end.
	p.SetId(db.rwtx.meta.Pgid())
	var minsz = int((p.Id()+common.Pgid(count))+1) * db.pageSize
	if minsz >= db.datasz {
		if err := db.mmap(minsz); err != nil {
			return nil, fmt.Errorf("mmap allocate error: %s", err)
		}
	}

	// Move the page id high water mark.
	curPgid := db.rwtx.meta.Pgid()
	db.rwtx.meta.SetPgid(curPgid + common.Pgid(count))

	return p, nil
}

// grow grows the size of the database to the given sz.
func (db *DB) grow(sz int) error {
	// Ignore if the new size is less than available file size.
	lg := db.Logger()
	fileSize, err := db.fileSize()
	if err != nil {
		lg.Errorf("getting file size failed: %w", err)
		return err
	}
	if sz <= fileSize {
		return nil
	}

	// If the data is smaller than the alloc size then only allocate what's needed.
	// Once it goes over the allocation size then allocate in chunks.
	if db.datasz <= db.AllocSize {
		sz = db.datasz
	} else {
		sz += db.AllocSize
	}

	// Truncate and fsync to ensure file size metadata is flushed.
	// https://github.com/boltdb/bolt/issues/284
	if !db.NoGrowSync && !db.readOnly {
		if runtime.GOOS != "windows" {
			// gofail: var resizeFileError string
			// return errors.New(resizeFileError)
			if err := db.file.Truncate(int64(sz)); err != nil {
				lg.Errorf("[GOOS: %s, GOARCH: %s] truncating file failed, size: %d, db.datasz: %d, error: %v", runtime.GOOS, runtime.GOARCH, sz, db.datasz, err)
				return fmt.Errorf("file resize error: %s", err)
			}
		}
		if err := db.file.Sync(); err != nil {
			lg.Errorf("[GOOS: %s, GOARCH: %s] syncing file failed, db.datasz: %d, error: %v", runtime.GOOS, runtime.GOARCH, db.datasz, err)
			return fmt.Errorf("file sync error: %s", err)
		}
		if db.Mlock {
			// unlock old file and lock new one
			if err := db.mrelock(fileSize, sz); err != nil {
				return fmt.Errorf("mlock/munlock error: %s", err)
			}
		}
	}

	return nil
}

func (db *DB) IsReadOnly() bool {
	return db.readOnly
}

func (db *DB) freepages() []common.Pgid {
	tx, err := db.beginTx()
	defer func() {
		err = tx.Rollback()
		if err != nil {
			panic("freepages: failed to rollback tx")
		}
	}()
	if err != nil {
		panic("freepages: failed to open read only tx")
	}

	reachable := make(map[common.Pgid]*common.Page)
	nofreed := make(map[common.Pgid]bool)
	ech := make(chan error)
	go func() {
		for e := range ech {
			panic(fmt.Sprintf("freepages: failed to get all reachable pages (%v)", e))
		}
	}()
	tx.recursivelyCheckBucket(&tx.root, reachable, nofreed, HexKVStringer(), ech)
	close(ech)

	// TODO: If check bucket reported any corruptions (ech) we shouldn't proceed to freeing the pages.

	var fids []common.Pgid
	for i := common.Pgid(2); i < db.meta().Pgid(); i++ {
		if _, ok := reachable[i]; !ok {
			fids = append(fids, i)
		}
	}
	return fids
}

func newFreelist(freelistType FreelistType) fl.Interface {
	if freelistType == FreelistMapType {
		return fl.NewHashMapFreelist()
	}
	return fl.NewArrayFreelist()
}

// Options represents the options that can be set when opening a database.
type Options struct {
	// Timeout is the amount of time to wait to obtain a file lock.
	// When set to zero it will wait indefinitely.
	Timeout time.Duration

	// Sets the DB.NoGrowSync flag before memory mapping the file.
	NoGrowSync bool

	// Do not sync freelist to disk. This improves the database write performance
	// under normal operation, but requires a full database re-sync during recovery.
	NoFreelistSync bool

	// PreLoadFreelist sets whether to load the free pages when opening
	// the db file. Note when opening db in write mode, bbolt will always
	// load the free pages.
	PreLoadFreelist bool

	// FreelistType sets the backend freelist type. There are two options. Array which is simple but endures
	// dramatic performance degradation if database is large and fragmentation in freelist is common.
	// The alternative one is using hashmap, it is faster in almost all circumstances
	// but it doesn't guarantee that it offers the smallest page id available. In normal case it is safe.
	// The default type is array
	FreelistType FreelistType

	// Open database in read-only mode. Uses flock(..., LOCK_SH |LOCK_NB) to
	// grab a shared lock (UNIX).
	ReadOnly bool

	// Sets the DB.MmapFlags flag before memory mapping the file.
	MmapFlags int

	// InitialMmapSize is the initial mmap size of the database
	// in bytes. Read transactions won't block write transaction
	// if the InitialMmapSize is large enough to hold database mmap
	// size. (See DB.Begin for more information)
	//
	// If <=0, the initial map size is 0.
	// If initialMmapSize is smaller than the previous database size,
	// it takes no effect.
	InitialMmapSize int

	// PageSize overrides the default OS page size.
	PageSize int

	// NoSync sets the initial value of DB.NoSync. Normally this can just be
	// set directly on the DB itself when returned from Open(), but this option
	// is useful in APIs which expose Options but not the underlying DB.
	NoSync bool

	// OpenFile is used to open files. It defaults to os.OpenFile. This option
	// is useful for writing hermetic tests.
	OpenFile func(string, int, os.FileMode) (*os.File, error)

	// Mlock locks database file in memory when set to true.
	// It prevents potential page faults, however
	// used memory can't be reclaimed. (UNIX only)
	Mlock bool

	// Logger is the logger used for bbolt.
	Logger Logger
}

func (o *Options) String() string {
	if o == nil {
		return "{}"
	}

	return fmt.Sprintf("{Timeout: %s, NoGrowSync: %t, NoFreelistSync: %t, PreLoadFreelist: %t, FreelistType: %s, ReadOnly: %t, MmapFlags: %x, InitialMmapSize: %d, PageSize: %d, NoSync: %t, OpenFile: %p, Mlock: %t, Logger: %p}",
		o.Timeout, o.NoGrowSync, o.NoFreelistSync, o.PreLoadFreelist, o.FreelistType, o.ReadOnly, o.MmapFlags, o.InitialMmapSize, o.PageSize, o.NoSync, o.OpenFile, o.Mlock, o.Logger)

}

// DefaultOptions represent the options used if nil options are passed into Open().
// No timeout is used which will cause Bolt to wait indefinitely for a lock.
var DefaultOptions = &Options{
	Timeout:      0,
	NoGrowSync:   false,
	FreelistType: FreelistArrayType,
}

// Stats represents statistics about the database.
type Stats struct {
	// Put `TxStats` at the first field to ensure it's 64-bit aligned. Note
	// that the first word in an allocated struct can be relied upon to be
	// 64-bit aligned. Refer to https://pkg.go.dev/sync/atomic#pkg-note-BUG.
	// Also refer to discussion in https://github.com/etcd-io/bbolt/issues/577.
	TxStats TxStats // global, ongoing stats.

	// Freelist stats
	FreePageN     int // total number of free pages on the freelist
	PendingPageN  int // total number of pending pages on the freelist
	FreeAlloc     int // total bytes allocated in free pages
	FreelistInuse int // total bytes used by the freelist

	// Transaction stats
	TxN     int // total number of started read transactions
	OpenTxN int // number of currently open read transactions
}

// Sub calculates and returns the difference between two sets of database stats.
// This is useful when obtaining stats at two different points and time and
// you need the performance counters that occurred within that time span.
func (s *Stats) Sub(other *Stats) Stats {
	if other == nil {
		return *s
	}
	var diff Stats
	diff.FreePageN = s.FreePageN
	diff.PendingPageN = s.PendingPageN
	diff.FreeAlloc = s.FreeAlloc
	diff.FreelistInuse = s.FreelistInuse
	diff.TxN = s.TxN - other.TxN
	diff.TxStats = s.TxStats.Sub(&other.TxStats)
	return diff
}

type Info struct {
	Data     uintptr
	PageSize int
}