frostfs-s3-gw/cmd/posix.go

/*
 * MinIO Cloud Storage, (C) 2016-2020 MinIO, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package cmd

import (
	"bufio"
	"bytes"
	"context"
	"crypto/rand"
	"encoding/hex"
	"errors"
	"io"
	"io/ioutil"
	"os"
	slashpath "path"
	"path/filepath"
	"runtime"
	"sort"
	"strings"
	"sync"
	"sync/atomic"
	"syscall"
	"time"

	humanize "github.com/dustin/go-humanize"
	jsoniter "github.com/json-iterator/go"
	"github.com/klauspost/readahead"
	"github.com/minio/minio/cmd/logger"
	"github.com/minio/minio/pkg/disk"
	xioutil "github.com/minio/minio/pkg/ioutil"
	"github.com/minio/minio/pkg/mountinfo"
)

const (
	diskMinFreeSpace  = 900 * humanize.MiByte // Min 900MiB free space.
	diskMinTotalSpace = diskMinFreeSpace      // Min 900MiB total space.
	readBlockSize     = 4 * humanize.MiByte   // Default read block size 4MiB.

	// On regular files bigger than this;
	readAheadSize = 16 << 20
	// Read this many buffers ahead.
	readAheadBuffers = 4
	// Size of each buffer.
	readAheadBufSize = 1 << 20

	// Wait interval to check if active IO count is low
	// to proceed crawling to compute data usage
	lowActiveIOWaitTick = 100 * time.Millisecond
)

// isValidVolname verifies a volname name in accordance with object
// layer requirements.
func isValidVolname(volname string) bool {
	if len(volname) < 3 {
		return false
	}

	if runtime.GOOS == "windows" {
		// Volname shouldn't have reserved characters in Windows.
		return !strings.ContainsAny(volname, `\:*?\"<>|`)
	}

	return true
}

// posix - implements StorageAPI interface.
type posix struct {
	// Disk usage metrics
	totalUsed uint64 // ref: https://golang.org/pkg/sync/atomic/#pkg-note-BUG

	maxActiveIOCount int32
	activeIOCount    int32

	diskPath string
	hostname string

	pool sync.Pool

	diskMount bool // indicates if the path is an actual mount.

	diskID string

	formatFileInfo  os.FileInfo
	formatLastCheck time.Time
	// Disk usage metrics
	stopUsageCh chan struct{}

	sync.RWMutex
}

// checkPathLength - returns error if given path name length more than 255
func checkPathLength(pathName string) error {
	// Apple OS X path length is limited to 1016
	if runtime.GOOS == "darwin" && len(pathName) > 1016 {
		return errFileNameTooLong
	}

	// Disallow more than 1024 characters on windows, there
	// are no known name_max limits on Windows.
	if runtime.GOOS == "windows" && len(pathName) > 1024 {
		return errFileNameTooLong
	}

	// On Unix we reject paths if they are just '.', '..' or '/'
	if pathName == "." || pathName == ".." || pathName == slashSeparator {
		return errFileAccessDenied
	}

	// Check each path segment length is > 255 on all Unix
	// platforms, look for this value as NAME_MAX in
	// /usr/include/linux/limits.h
	var count int64
	for _, p := range pathName {
		switch p {
		case '/':
			count = 0 // Reset
		case '\\':
			if runtime.GOOS == globalWindowsOSName {
				count = 0
			}
		default:
			count++
			if count > 255 {
				return errFileNameTooLong
			}
		}
	} // Success.
	return nil
}

func getValidPath(path string, requireDirectIO bool) (string, error) {
	if path == "" {
		return path, errInvalidArgument
	}

	var err error
	// Disallow relative paths, figure out absolute paths.
	path, err = filepath.Abs(path)
	if err != nil {
		return path, err
	}

	fi, err := os.Stat(path)
	if err != nil && !os.IsNotExist(err) {
		return path, err
	}
	if os.IsNotExist(err) {
		// Disk not found create it.
		if err = os.MkdirAll(path, 0777); err != nil {
			return path, err
		}
	}
	if fi != nil && !fi.IsDir() {
		return path, syscall.ENOTDIR
	}

	di, err := getDiskInfo(path)
	if err != nil {
		return path, err
	}
	if err = checkDiskMinTotal(di); err != nil {
		return path, err
	}

	// check if backend is writable.
	var rnd [8]byte
	_, _ = rand.Read(rnd[:])

	fn := pathJoin(path, ".writable-check-"+hex.EncodeToString(rnd[:])+".tmp")
	defer os.Remove(fn)

	var file *os.File

	if requireDirectIO {
		file, err = disk.OpenFileDirectIO(fn, os.O_CREATE|os.O_EXCL, 0666)
	} else {
		file, err = os.OpenFile(fn, os.O_CREATE|os.O_EXCL, 0666)
	}

	// open file in direct I/O and use default umask, this also verifies
	// if direct i/o failed.
	if err != nil {
		if isSysErrInvalidArg(err) {
			return path, errUnsupportedDisk
		}
		return path, err
	}
	file.Close()

	return path, nil
}

// isDirEmpty - returns whether given directory is empty or not.
func isDirEmpty(dirname string) bool {
	f, err := os.Open((dirname))
	if err != nil {
		if !os.IsNotExist(err) {
			logger.LogIf(GlobalContext, err)
		}

		return false
	}
	defer f.Close()
	// List one entry.
	_, err = f.Readdirnames(1)
	if err != io.EOF {
		if !os.IsNotExist(err) {
			logger.LogIf(GlobalContext, err)
		}

		return false
	}
	// Returns true if we have reached EOF, directory is indeed empty.
	return true
}

// Initialize a new storage disk.
func newPosix(path string, hostname string) (*posix, error) {
	var err error
	if path, err = getValidPath(path, true); err != nil {
		return nil, err
	}
	_, err = os.Stat(path)
	if err != nil {
		return nil, err
	}
	p := &posix{
		diskPath: path,
		hostname: hostname,
		pool: sync.Pool{
			New: func() interface{} {
				b := disk.AlignedBlock(readBlockSize)
				return &b
			},
		},
		stopUsageCh: make(chan struct{}),
		diskMount:   mountinfo.IsLikelyMountPoint(path),
		// Allow disk usage crawler to run with up to 2 concurrent
		// I/O ops, if and when activeIOCount reaches this
		// value disk usage routine suspends the crawler
		// and waits until activeIOCount reaches below this threshold.
		maxActiveIOCount: 3,
	}

	// Success.
	return p, nil
}

// getDiskInfo returns given disk information.
func getDiskInfo(diskPath string) (di disk.Info, err error) {
	if err = checkPathLength(diskPath); err == nil {
		di, err = disk.GetInfo(diskPath)
	}

	switch {
	case os.IsNotExist(err):
		err = errDiskNotFound
	case isSysErrTooLong(err):
		err = errFileNameTooLong
	case isSysErrIO(err):
		err = errFaultyDisk
	}

	return di, err
}

// List of operating systems where we ignore disk space
// verification.
var ignoreDiskFreeOS = []string{
	globalWindowsOSName,
	globalNetBSDOSName,
}

// check if disk total has minimum required size.
func checkDiskMinTotal(di disk.Info) (err error) {
	// Remove 5% from total space for cumulative disk space
	// used for journalling, inodes etc.
	totalDiskSpace := float64(di.Total) * 0.95
	if int64(totalDiskSpace) <= diskMinTotalSpace {
		return errMinDiskSize
	}
	return nil
}

// check if disk free has minimum required size.
func checkDiskMinFree(di disk.Info) error {
	// Remove 5% from free space for cumulative disk space used for journalling, inodes etc.
	availableDiskSpace := float64(di.Free) * 0.95
	if int64(availableDiskSpace) <= diskMinFreeSpace {
		return errDiskFull
	}

	// Success.
	return nil
}

// checkDiskFree verifies if disk path has sufficient minimum free disk space and files.
func checkDiskFree(diskPath string, neededSpace int64) (err error) {
	// We don't validate disk space or inode utilization on windows.
	// Each windows call to 'GetVolumeInformationW' takes around
	// 3-5seconds. And StatDISK is not supported by Go for solaris
	// and netbsd.
	if contains(ignoreDiskFreeOS, runtime.GOOS) {
		return nil
	}

	var di disk.Info
	di, err = getDiskInfo((diskPath))
	if err != nil {
		return err
	}

	if err = checkDiskMinFree(di); err != nil {
		return err
	}

	// Check if we have enough space to store data
	if neededSpace > int64(float64(di.Free)*0.95) {
		return errDiskFull
	}

	return nil
}

// Implements stringer compatible interface.
func (s *posix) String() string {
	return s.diskPath
}

func (s *posix) Hostname() string {
	return s.hostname
}

func (s *posix) Close() error {
	close(s.stopUsageCh)
	return nil
}

func (s *posix) IsOnline() bool {
	return true
}

func (s *posix) IsLocal() bool {
	return true
}

func (s *posix) waitForLowActiveIO() {
	for atomic.LoadInt32(&s.activeIOCount) >= s.maxActiveIOCount {
		time.Sleep(lowActiveIOWaitTick)
	}
}

func (s *posix) CrawlAndGetDataUsage(ctx context.Context, cache dataUsageCache) (dataUsageCache, error) {
	// Check if the current bucket has a configured lifecycle policy
	lc, err := globalLifecycleSys.Get(cache.Info.Name)
	if err == nil && lc.HasActiveRules("", true) {
		cache.Info.lifeCycle = lc
	}

	// Get object api
	objAPI := newObjectLayerWithoutSafeModeFn()
	if objAPI == nil {
		return cache, errors.New("object layer not initialized")
	}
	dataUsageInfo, err := crawlDataFolder(ctx, s.diskPath, cache, s.waitForLowActiveIO, func(item crawlItem) (int64, error) {
		// Look for `xl.json' at the leaf.
		if !strings.HasSuffix(item.Path, SlashSeparator+xlMetaJSONFile) {
			// if no xl.json found, skip the file.
			return 0, errSkipFile
		}

		xlMetaBuf, err := ioutil.ReadFile(item.Path)
		if err != nil {
			return 0, errSkipFile
		}

		meta, err := xlMetaV1UnmarshalJSON(ctx, xlMetaBuf)
		if err != nil {
			return 0, errSkipFile
		}

		// Remove filename which is the meta file.
		item.transformMetaDir()

		return item.applyActions(ctx, objAPI,
			actionMeta{oi: meta.ToObjectInfo(item.bucket, item.objectPath()),
				meta: meta.Meta,
			}), nil
	})
	if err != nil {
		return dataUsageInfo, err
	}

	dataUsageInfo.Info.LastUpdate = time.Now()
	total := dataUsageInfo.sizeRecursive(dataUsageInfo.Info.Name)
	if total == nil {
		total = &dataUsageEntry{}
	}
	atomic.StoreUint64(&s.totalUsed, uint64(total.Size))
	return dataUsageInfo, nil
}

// DiskInfo is an extended type which returns current
// disk usage per path.
type DiskInfo struct {
	Total     uint64
	Free      uint64
	Used      uint64
	RootDisk  bool
	MountPath string
	Error     string // reports any error returned by underlying disk
}

// DiskInfo provides current information about disk space usage,
// total free inodes and underlying filesystem.
func (s *posix) DiskInfo() (info DiskInfo, err error) {
	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	di, err := getDiskInfo(s.diskPath)
	if err != nil {
		return info, err
	}

	used := di.Total - di.Free
	if !s.diskMount {
		used = atomic.LoadUint64(&s.totalUsed)
	}

	rootDisk, err := disk.IsRootDisk(s.diskPath)
	if err != nil {
		return info, err
	}

	return DiskInfo{
		Total:     di.Total,
		Free:      di.Free,
		Used:      used,
		RootDisk:  rootDisk,
		MountPath: s.diskPath,
	}, nil
}

// getVolDir - will convert incoming volume names to
// corresponding valid volume names on the backend in a platform
// compatible way for all operating systems. If volume is not found
// an error is generated.
func (s *posix) getVolDir(volume string) (string, error) {
	if volume == "" || volume == "." || volume == ".." {
		return "", errVolumeNotFound
	}
	volumeDir := pathJoin(s.diskPath, volume)
	return volumeDir, nil
}

// GetDiskID - returns the cached disk uuid
func (s *posix) GetDiskID() (string, error) {
	s.RLock()
	diskID := s.diskID
	fileInfo := s.formatFileInfo
	lastCheck := s.formatLastCheck
	s.RUnlock()

	// check if we have a valid disk ID that is less than 1 second old.
	if fileInfo != nil && diskID != "" && time.Now().Before(lastCheck.Add(time.Second)) {
		return diskID, nil
	}

	s.Lock()
	defer s.Unlock()

	// If somebody else updated the disk ID and changed the time, return what they got.
	if !lastCheck.IsZero() && !s.formatLastCheck.Equal(lastCheck) && diskID != "" {
		// Somebody else got the lock first.
		return diskID, nil
	}
	formatFile := pathJoin(s.diskPath, minioMetaBucket, formatConfigFile)
	fi, err := os.Stat(formatFile)
	if err != nil {
		// If the disk is still not initialized.
		if os.IsNotExist(err) {
			return "", errUnformattedDisk
		}
		return "", errCorruptedFormat
	}

	if xioutil.SameFile(fi, fileInfo) && diskID != "" {
		// If the file has not changed, just return the cached diskID information.
		s.formatLastCheck = time.Now()
		return diskID, nil
	}

	b, err := ioutil.ReadFile(formatFile)
	if err != nil {
		return "", errCorruptedFormat
	}
	format := &formatXLV3{}
	var json = jsoniter.ConfigCompatibleWithStandardLibrary
	if err = json.Unmarshal(b, &format); err != nil {
		return "", errCorruptedFormat
	}
	s.diskID = format.XL.This
	s.formatFileInfo = fi
	s.formatLastCheck = time.Now()
	return s.diskID, nil
}

// Make a volume entry.
func (s *posix) SetDiskID(id string) {
	// NO-OP for posix as it is handled either by posixDiskIDCheck{} for local disks or
	// storage rest server for remote disks.
}

func (s *posix) MakeVolBulk(volumes ...string) (err error) {
	for _, volume := range volumes {
		if err = s.MakeVol(volume); err != nil {
			if os.IsPermission(err) {
				return errVolumeAccessDenied
			}
		}
	}
	return nil
}

// Make a volume entry.
func (s *posix) MakeVol(volume string) (err error) {
	if !isValidVolname(volume) {
		return errInvalidArgument
	}

	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	volumeDir, err := s.getVolDir(volume)
	if err != nil {
		return err
	}

	if _, err := os.Stat(volumeDir); err != nil {
		// Volume does not exist we proceed to create.
		if os.IsNotExist(err) {
			// Make a volume entry, with mode 0777 mkdir honors system umask.
			err = os.MkdirAll(volumeDir, 0777)
		}
		if os.IsPermission(err) {
			return errDiskAccessDenied
		} else if isSysErrIO(err) {
			return errFaultyDisk
		}
		return err
	}

	// Stat succeeds we return errVolumeExists.
	return errVolumeExists
}

// ListVols - list volumes.
func (s *posix) ListVols() (volsInfo []VolInfo, err error) {
	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	volsInfo, err = listVols(s.diskPath)
	if err != nil {
		if isSysErrIO(err) {
			return nil, errFaultyDisk
		}
		return nil, err
	}
	for i, vol := range volsInfo {
		volInfo := VolInfo{
			Name:    vol.Name,
			Created: vol.Created,
		}
		volsInfo[i] = volInfo
	}
	return volsInfo, nil
}

// List all the volumes from diskPath.
func listVols(dirPath string) ([]VolInfo, error) {
	if err := checkPathLength(dirPath); err != nil {
		return nil, err
	}
	entries, err := readDir(dirPath)
	if err != nil {
		return nil, errDiskNotFound
	}
	var volsInfo []VolInfo
	for _, entry := range entries {
		if !HasSuffix(entry, SlashSeparator) || !isValidVolname(slashpath.Clean(entry)) {
			// Skip if entry is neither a directory not a valid volume name.
			continue
		}
		var fi os.FileInfo
		fi, err = os.Stat(pathJoin(dirPath, entry))
		if err != nil {
			// If the file does not exist, skip the entry.
			if os.IsNotExist(err) {
				continue
			} else if isSysErrIO(err) {
				return nil, errFaultyDisk
			}
			return nil, err
		}
		volsInfo = append(volsInfo, VolInfo{
			Name: fi.Name(),
			// As os.Stat() doesn't carry other than ModTime(), use
			// ModTime() as CreatedTime.
			Created: fi.ModTime(),
		})
	}
	return volsInfo, nil
}

// StatVol - get volume info.
func (s *posix) StatVol(volume string) (volInfo VolInfo, err error) {
	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	// Verify if volume is valid and it exists.
	volumeDir, err := s.getVolDir(volume)
	if err != nil {
		return VolInfo{}, err
	}
	// Stat a volume entry.
	var st os.FileInfo
	st, err = os.Stat(volumeDir)
	if err != nil {
		if os.IsNotExist(err) {
			return VolInfo{}, errVolumeNotFound
		} else if isSysErrIO(err) {
			return VolInfo{}, errFaultyDisk
		}
		return VolInfo{}, err
	}
	// As os.Stat() doesn't carry other than ModTime(), use ModTime()
	// as CreatedTime.
	createdTime := st.ModTime()
	return VolInfo{
		Name:    volume,
		Created: createdTime,
	}, nil
}

// DeleteVol - delete a volume.
func (s *posix) DeleteVol(volume string, forceDelete bool) (err error) {
	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	// Verify if volume is valid and it exists.
	volumeDir, err := s.getVolDir(volume)
	if err != nil {
		return err
	}

	if forceDelete {
		err = os.RemoveAll(volumeDir)
	} else {
		err = os.Remove(volumeDir)
	}

	if err != nil {
		switch {
		case os.IsNotExist(err):
			return errVolumeNotFound
		case isSysErrNotEmpty(err):
			return errVolumeNotEmpty
		case os.IsPermission(err):
			return errDiskAccessDenied
		case isSysErrIO(err):
			return errFaultyDisk
		default:
			return err
		}
	}
	return nil
}

const guidSplunk = "guidSplunk"

// ListDirSplunk - return all the entries at the given directory path.
// If an entry is a directory it will be returned with a trailing SlashSeparator.
func (s *posix) ListDirSplunk(volume, dirPath string, count int) (entries []string, err error) {
	guidIndex := strings.Index(dirPath, guidSplunk)
	if guidIndex != -1 {
		return nil, nil
	}

	const receiptJSON = "receipt.json"

	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	// Verify if volume is valid and it exists.
	volumeDir, err := s.getVolDir(volume)
	if err != nil {
		return nil, err
	}

	if _, err = os.Stat(volumeDir); err != nil {
		if os.IsNotExist(err) {
			return nil, errVolumeNotFound
		} else if isSysErrIO(err) {
			return nil, errFaultyDisk
		}
		return nil, err
	}

	dirPath = pathJoin(volumeDir, dirPath)
	if count > 0 {
		entries, err = readDirN(dirPath, count)
	} else {
		entries, err = readDir(dirPath)
	}
	if err != nil {
		return nil, err
	}

	for i, entry := range entries {
		if entry != receiptJSON {
			continue
		}
		if _, serr := os.Stat(pathJoin(dirPath, entry, xlMetaJSONFile)); serr == nil {
			entries[i] = strings.TrimSuffix(entry, SlashSeparator)
		}
	}

	return entries, nil
}

// WalkSplunk - is a sorted walker which returns file entries in lexically
// sorted order, additionally along with metadata about each of those entries.
// Implemented specifically for Splunk backend structure and List call with
// delimiter as "guidSplunk"
func (s *posix) WalkSplunk(volume, dirPath, marker string, endWalkCh <-chan struct{}) (ch chan FileInfo, err error) {
	// Verify if volume is valid and it exists.
	volumeDir, err := s.getVolDir(volume)
	if err != nil {
		return nil, err
	}

	// Stat a volume entry.
	_, err = os.Stat(volumeDir)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, errVolumeNotFound
		} else if isSysErrIO(err) {
			return nil, errFaultyDisk
		}
		return nil, err
	}

	ch = make(chan FileInfo)
	go func() {
		defer close(ch)
		listDir := func(volume, dirPath, dirEntry string) (bool, []string) {
			entries, err := s.ListDirSplunk(volume, dirPath, -1)
			if err != nil {
				return false, nil
			}
			if len(entries) == 0 {
				return true, nil
			}
			sort.Strings(entries)
			return false, filterMatchingPrefix(entries, dirEntry)
		}

		walkResultCh := startTreeWalk(GlobalContext, volume, dirPath, marker, true, listDir, endWalkCh)
		for {
			walkResult, ok := <-walkResultCh
			if !ok {
				return
			}
			var fi FileInfo
			if HasSuffix(walkResult.entry, SlashSeparator) {
				fi = FileInfo{
					Volume: volume,
					Name:   walkResult.entry,
					Mode:   os.ModeDir,
				}
			} else {
				xlMetaBuf, err := ioutil.ReadFile(pathJoin(volumeDir, walkResult.entry, xlMetaJSONFile))
				if err != nil {
					continue
				}
				fi = readMetadata(xlMetaBuf, volume, walkResult.entry)
			}
			select {
			case ch <- fi:
			case <-endWalkCh:
				return
			}
		}
	}()

	return ch, nil
}

// Walk - is a sorted walker which returns file entries in lexically
// sorted order, additionally along with metadata about each of those entries.
func (s *posix) Walk(volume, dirPath, marker string, recursive bool, leafFile string,
	readMetadataFn readMetadataFunc, endWalkCh <-chan struct{}) (ch chan FileInfo, err error) {

	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	// Verify if volume is valid and it exists.
	volumeDir, err := s.getVolDir(volume)
	if err != nil {
		return nil, err
	}

	// Stat a volume entry.
	_, err = os.Stat(volumeDir)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, errVolumeNotFound
		} else if isSysErrIO(err) {
			return nil, errFaultyDisk
		}
		return nil, err
	}

	// buffer channel matches the S3 ListObjects implementation
	ch = make(chan FileInfo, maxObjectList)
	go func() {
		defer close(ch)
		listDir := func(volume, dirPath, dirEntry string) (bool, []string) {
			entries, err := s.ListDir(volume, dirPath, -1, leafFile)
			if err != nil {
				return false, nil
			}
			if len(entries) == 0 {
				return true, nil
			}
			sort.Strings(entries)
			return false, filterMatchingPrefix(entries, dirEntry)
		}

		walkResultCh := startTreeWalk(GlobalContext, volume, dirPath, marker, recursive, listDir, endWalkCh)
		for {
			walkResult, ok := <-walkResultCh
			if !ok {
				return
			}
			var fi FileInfo
			if HasSuffix(walkResult.entry, SlashSeparator) {
				fi = FileInfo{
					Volume: volume,
					Name:   walkResult.entry,
					Mode:   os.ModeDir,
				}
			} else {
				xlMetaBuf, err := ioutil.ReadFile(pathJoin(volumeDir, walkResult.entry, leafFile))
				if err != nil {
					continue
				}
				fi = readMetadataFn(xlMetaBuf, volume, walkResult.entry)
			}
			select {
			case ch <- fi:
			case <-endWalkCh:
				return
			}
		}
	}()

	return ch, nil
}

// ListDir - return all the entries at the given directory path.
// If an entry is a directory it will be returned with a trailing SlashSeparator.
func (s *posix) ListDir(volume, dirPath string, count int, leafFile string) (entries []string, err error) {
	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	// Verify if volume is valid and it exists.
	volumeDir, err := s.getVolDir(volume)
	if err != nil {
		return nil, err
	}

	if _, err = os.Stat(volumeDir); err != nil {
		if os.IsNotExist(err) {
			return nil, errVolumeNotFound
		} else if isSysErrIO(err) {
			return nil, errFaultyDisk
		}
		return nil, err
	}

	dirPath = pathJoin(volumeDir, dirPath)
	if count > 0 {
		entries, err = readDirN(dirPath, count)
	} else {
		entries, err = readDir(dirPath)
	}
	if err != nil {
		return nil, err
	}

	// If leaf file is specified, filter out the entries.
	if leafFile != "" {
		for i, entry := range entries {
			if _, serr := os.Stat(pathJoin(dirPath, entry, leafFile)); serr == nil {
				entries[i] = strings.TrimSuffix(entry, SlashSeparator)
			}
		}
	}

	return entries, err
}

// ReadAll reads from r until an error or EOF and returns the data it read.
// A successful call returns err == nil, not err == EOF. Because ReadAll is
// defined to read from src until EOF, it does not treat an EOF from Read
// as an error to be reported.
// This API is meant to be used on files which have small memory footprint, do
// not use this on large files as it would cause server to crash.
func (s *posix) ReadAll(volume, path string) (buf []byte, err error) {
	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	volumeDir, err := s.getVolDir(volume)
	if err != nil {
		return nil, err
	}
	// Stat a volume entry.
	_, err = os.Stat((volumeDir))
	if err != nil {
		if os.IsNotExist(err) {
			return nil, errVolumeNotFound
		} else if isSysErrIO(err) {
			return nil, errFaultyDisk
		} else if isSysErrTooManyFiles(err) {
			return nil, errTooManyOpenFiles
		}
		return nil, err
	}

	// Validate file path length, before reading.
	filePath := pathJoin(volumeDir, path)
	if err = checkPathLength((filePath)); err != nil {
		return nil, err
	}

	// Open the file for reading.
	buf, err = ioutil.ReadFile((filePath))
	if err != nil {
		if os.IsNotExist(err) {
			return nil, errFileNotFound
		} else if os.IsPermission(err) {
			return nil, errFileAccessDenied
		} else if errors.Is(err, syscall.ENOTDIR) || errors.Is(err, syscall.EISDIR) {
			return nil, errFileNotFound
		} else if isSysErrHandleInvalid(err) {
			// This case is special and needs to be handled for windows.
			return nil, errFileNotFound
		} else if isSysErrIO(err) {
			return nil, errFaultyDisk
		}
		return nil, err
	}
	return buf, nil
}

// ReadFile reads exactly len(buf) bytes into buf. It returns the
// number of bytes copied. The error is EOF only if no bytes were
// read. On return, n == len(buf) if and only if err == nil. n == 0
// for io.EOF.
//
// If an EOF happens after reading some but not all the bytes,
// ReadFile returns ErrUnexpectedEOF.
//
// If the BitrotVerifier is not nil or not verified ReadFile
// tries to verify whether the disk has bitrot.
//
// Additionally ReadFile also starts reading from an offset. ReadFile
// semantics are same as io.ReadFull.
func (s *posix) ReadFile(volume, path string, offset int64, buffer []byte, verifier *BitrotVerifier) (int64, error) {
	if offset < 0 {
		return 0, errInvalidArgument
	}

	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	volumeDir, err := s.getVolDir(volume)
	if err != nil {
		return 0, err
	}

	var n int

	// Stat a volume entry.
	_, err = os.Stat((volumeDir))
	if err != nil {
		if os.IsNotExist(err) {
			return 0, errVolumeNotFound
		} else if isSysErrIO(err) {
			return 0, errFaultyDisk
		}
		return 0, err
	}

	// Validate effective path length before reading.
	filePath := pathJoin(volumeDir, path)
	if err = checkPathLength((filePath)); err != nil {
		return 0, err
	}

	// Open the file for reading.
	file, err := os.Open((filePath))
	if err != nil {
		switch {
		case os.IsNotExist(err):
			return 0, errFileNotFound
		case os.IsPermission(err):
			return 0, errFileAccessDenied
		case isSysErrNotDir(err):
			return 0, errFileAccessDenied
		case isSysErrIO(err):
			return 0, errFaultyDisk
		case isSysErrTooManyFiles(err):
			return 0, errTooManyOpenFiles
		default:
			return 0, err
		}
	}

	// Close the file descriptor.
	defer file.Close()

	st, err := file.Stat()
	if err != nil {
		return 0, err
	}

	// Verify it is a regular file, otherwise subsequent Seek is
	// undefined.
	if !st.Mode().IsRegular() {
		return 0, errIsNotRegular
	}

	if verifier == nil {
		n, err = file.ReadAt(buffer, offset)
		return int64(n), err
	}

	bufp := s.pool.Get().(*[]byte)
	defer s.pool.Put(bufp)

	h := verifier.algorithm.New()
	if _, err = io.CopyBuffer(h, io.LimitReader(file, offset), *bufp); err != nil {
		return 0, err
	}

	if n, err = io.ReadFull(file, buffer); err != nil {
		return int64(n), err
	}

	if _, err = h.Write(buffer); err != nil {
		return 0, err
	}

	if _, err = io.CopyBuffer(h, file, *bufp); err != nil {
		return 0, err
	}

	if !bytes.Equal(h.Sum(nil), verifier.sum) {
		return 0, errFileCorrupt
	}

	return int64(len(buffer)), nil
}

func (s *posix) openFile(volume, path string, mode int) (f *os.File, err error) {
	volumeDir, err := s.getVolDir(volume)
	if err != nil {
		return nil, err
	}
	// Stat a volume entry.
	_, err = os.Stat((volumeDir))
	if err != nil {
		if os.IsNotExist(err) {
			return nil, errVolumeNotFound
		} else if isSysErrIO(err) {
			return nil, errFaultyDisk
		}
		return nil, err
	}

	filePath := pathJoin(volumeDir, path)
	if err = checkPathLength((filePath)); err != nil {
		return nil, err
	}

	// Verify if the file already exists and is not of regular type.
	var st os.FileInfo
	if st, err = os.Stat(filePath); err == nil {
		if !st.Mode().IsRegular() {
			return nil, errIsNotRegular
		}
	} else {
		// Create top level directories if they don't exist.
		// with mode 0777 mkdir honors system umask.
		if err = mkdirAll(slashpath.Dir(filePath), 0777); err != nil {
			return nil, err
		}
	}

	w, err := os.OpenFile(filePath, mode, 0666)
	if err != nil {
		// File path cannot be verified since one of the parents is a file.
		switch {
		case isSysErrNotDir(err):
			return nil, errFileAccessDenied
		case os.IsPermission(err):
			return nil, errFileAccessDenied
		case isSysErrIO(err):
			return nil, errFaultyDisk
		case isSysErrTooManyFiles(err):
			return nil, errTooManyOpenFiles
		default:
			return nil, err
		}
	}

	return w, nil
}

// ReadFileStream - Returns the read stream of the file.
func (s *posix) ReadFileStream(volume, path string, offset, length int64) (io.ReadCloser, error) {
	if offset < 0 {
		return nil, errInvalidArgument
	}

	volumeDir, err := s.getVolDir(volume)
	if err != nil {
		return nil, err
	}
	// Stat a volume entry.
	_, err = os.Stat((volumeDir))
	if err != nil {
		if os.IsNotExist(err) {
			return nil, errVolumeNotFound
		} else if isSysErrIO(err) {
			return nil, errFaultyDisk
		}
		return nil, err
	}

	// Validate effective path length before reading.
	filePath := pathJoin(volumeDir, path)
	if err = checkPathLength((filePath)); err != nil {
		return nil, err
	}

	// Open the file for reading.
	file, err := os.Open((filePath))
	if err != nil {
		switch {
		case os.IsNotExist(err):
			return nil, errFileNotFound
		case os.IsPermission(err):
			return nil, errFileAccessDenied
		case isSysErrNotDir(err):
			return nil, errFileAccessDenied
		case isSysErrIO(err):
			return nil, errFaultyDisk
		case isSysErrTooManyFiles(err):
			return nil, errTooManyOpenFiles
		default:
			return nil, err
		}
	}

	st, err := file.Stat()
	if err != nil {
		return nil, err
	}

	// Verify it is a regular file, otherwise subsequent Seek is
	// undefined.
	if !st.Mode().IsRegular() {
		return nil, errIsNotRegular
	}

	if _, err = file.Seek(offset, io.SeekStart); err != nil {
		return nil, err
	}

	atomic.AddInt32(&s.activeIOCount, 1)
	r := struct {
		io.Reader
		io.Closer
	}{Reader: io.LimitReader(file, length), Closer: closeWrapper(func() error {
		atomic.AddInt32(&s.activeIOCount, -1)
		return file.Close()
	})}

	// Add readahead to big reads
	if length >= readAheadSize {
		rc, err := readahead.NewReadCloserSize(r, readAheadBuffers, readAheadBufSize)
		if err != nil {
			r.Close()
			return nil, err
		}
		return rc, nil
	}

	// Just add a small 64k buffer.
	r.Reader = bufio.NewReaderSize(r.Reader, 64<<10)
	return r, nil
}

// closeWrapper converts a function to an io.Closer
type closeWrapper func() error

// Close calls the wrapped function.
func (c closeWrapper) Close() error {
	return c()
}

// CreateFile - creates the file.
func (s *posix) CreateFile(volume, path string, fileSize int64, r io.Reader) (err error) {
	if fileSize < -1 {
		return errInvalidArgument
	}

	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	// Validate if disk is indeed free.
	if err = checkDiskFree(s.diskPath, fileSize); err != nil {
		if isSysErrIO(err) {
			return errFaultyDisk
		}
		return err
	}

	volumeDir, err := s.getVolDir(volume)
	if err != nil {
		return err
	}
	// Stat a volume entry.
	_, err = os.Stat((volumeDir))
	if err != nil {
		if os.IsNotExist(err) {
			return errVolumeNotFound
		} else if isSysErrIO(err) {
			return errFaultyDisk
		}
		return err
	}

	filePath := pathJoin(volumeDir, path)
	if err = checkPathLength((filePath)); err != nil {
		return err
	}

	// Create top level directories if they don't exist.
	// with mode 0777 mkdir honors system umask.
	if err = mkdirAll(slashpath.Dir(filePath), 0777); err != nil {
		if errors.Is(err, &os.PathError{}) {
			return errFileAccessDenied
		}
		return err
	}

	w, err := disk.OpenFileDirectIO(filePath, os.O_CREATE|os.O_WRONLY|os.O_EXCL, 0666)
	if err != nil {
		switch {
		case os.IsPermission(err):
			return errFileAccessDenied
		case os.IsExist(err):
			return errFileAccessDenied
		case isSysErrIO(err):
			return errFaultyDisk
		case isSysErrInvalidArg(err):
			return errUnsupportedDisk
		default:
			return err
		}
	}

	var e error
	if fileSize > 0 {
		// Allocate needed disk space to append data
		e = Fallocate(int(w.Fd()), 0, fileSize)
	}

	// Ignore errors when Fallocate is not supported in the current system
	if e != nil && !isSysErrNoSys(e) && !isSysErrOpNotSupported(e) {
		switch {
		case isSysErrNoSpace(e):
			err = errDiskFull
		case isSysErrIO(e):
			err = errFaultyDisk
		default:
			// For errors: EBADF, EINTR, EINVAL, ENODEV, EPERM, ESPIPE  and ETXTBSY
			// Appending was failed anyway, returns unexpected error
			err = errUnexpected
		}
		return err
	}

	defer func() {
		disk.Fdatasync(w) // Only interested in flushing the size_t not mtime/atime
		w.Close()
	}()

	bufp := s.pool.Get().(*[]byte)
	defer s.pool.Put(bufp)

	written, err := xioutil.CopyAligned(w, r, *bufp, fileSize)
	if err != nil {
		return err
	}

	if written < fileSize {
		return errLessData
	} else if written > fileSize {
		return errMoreData
	}

	return nil
}

func (s *posix) WriteAll(volume, path string, reader io.Reader) (err error) {
	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	// Create file if not found. Note that it is created with os.O_EXCL flag as the file
	// always is supposed to be created in the tmp directory with a unique file name.
	w, err := s.openFile(volume, path, os.O_CREATE|os.O_SYNC|os.O_WRONLY|os.O_EXCL)
	if err != nil {
		return err
	}

	defer w.Close()

	bufp := s.pool.Get().(*[]byte)
	defer s.pool.Put(bufp)

	_, err = io.CopyBuffer(w, reader, *bufp)
	return err
}

// AppendFile - append a byte array at path, if file doesn't exist at
// path this call explicitly creates it.
func (s *posix) AppendFile(volume, path string, buf []byte) (err error) {
	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	var w *os.File
	// Create file if not found. Not doing O_DIRECT here to avoid the code that does buffer aligned writes.
	// AppendFile() is only used by healing code to heal objects written in old format.
	w, err = s.openFile(volume, path, os.O_CREATE|os.O_SYNC|os.O_APPEND|os.O_WRONLY)
	if err != nil {
		return err
	}

	if _, err = w.Write(buf); err != nil {
		return err
	}

	return w.Close()
}

// StatFile - get file info.
func (s *posix) StatFile(volume, path string) (file FileInfo, err error) {
	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	volumeDir, err := s.getVolDir(volume)
	if err != nil {
		return FileInfo{}, err
	}
	// Stat a volume entry.
	_, err = os.Stat((volumeDir))
	if err != nil {
		if os.IsNotExist(err) {
			return FileInfo{}, errVolumeNotFound
		}
		return FileInfo{}, err
	}

	filePath := slashpath.Join(volumeDir, path)
	if err = checkPathLength((filePath)); err != nil {
		return FileInfo{}, err
	}
	st, err := os.Stat((filePath))
	if err != nil {
		switch {
		case os.IsNotExist(err):
			// File is really not found.
			return FileInfo{}, errFileNotFound
		case isSysErrIO(err):
			return FileInfo{}, errFaultyDisk
		case isSysErrNotDir(err):
			// File path cannot be verified since one of the parents is a file.
			return FileInfo{}, errFileNotFound
		default:
			// Return all errors here.
			return FileInfo{}, err
		}
	}
	// If its a directory its not a regular file.
	if st.Mode().IsDir() {
		return FileInfo{}, errFileNotFound
	}
	return FileInfo{
		Volume:  volume,
		Name:    path,
		ModTime: st.ModTime(),
		Size:    st.Size(),
		Mode:    st.Mode(),
	}, nil
}

// deleteFile deletes a file or a directory if its empty unless recursive
// is set to true. If the target is successfully deleted, it will recursively
// move up the tree, deleting empty parent directories until it finds one
// with files in it. Returns nil for a non-empty directory even when
// recursive is set to false.
func deleteFile(basePath, deletePath string, recursive bool) error {
	if basePath == "" || deletePath == "" {
		return nil
	}
	basePath = filepath.Clean(basePath)
	deletePath = filepath.Clean(deletePath)
	if !strings.HasPrefix(deletePath, basePath) || deletePath == basePath {
		return nil
	}

	var err error
	if recursive {
		err = os.RemoveAll(deletePath)
	} else {
		err = os.Remove(deletePath)
	}
	if err != nil {
		switch {
		case isSysErrNotEmpty(err):
			// Ignore errors if the directory is not empty. The server relies on
			// this functionality, and sometimes uses recursion that should not
			// error on parent directories.
			return nil
		case os.IsNotExist(err):
			return errFileNotFound
		case os.IsPermission(err):
			return errFileAccessDenied
		case isSysErrIO(err):
			return errFaultyDisk
		default:
			return err
		}
	}

	deletePath = filepath.Dir(deletePath)

	// Delete parent directory obviously not recursively. Errors for
	// parent directories shouldn't trickle down.
	deleteFile(basePath, deletePath, false)

	return nil
}

// DeletePrefixes forcibly deletes all the contents of a set of specified paths.
// Parent directories are automatically removed if they become empty. err can
// bil nil while errs can contain some errors for corresponding objects. No error
// is set if a specified prefix path does not exist.
func (s *posix) DeletePrefixes(volume string, paths []string) (errs []error, err error) {
	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	volumeDir, err := s.getVolDir(volume)
	if err != nil {
		return nil, err
	}

	// Stat a volume entry.
	_, err = os.Stat(volumeDir)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, errVolumeNotFound
		} else if os.IsPermission(err) {
			return nil, errVolumeAccessDenied
		} else if isSysErrIO(err) {
			return nil, errFaultyDisk
		}
		return nil, err
	}

	errs = make([]error, len(paths))
	// Following code is needed so that we retain SlashSeparator
	// suffix if any in path argument.
	for idx, path := range paths {
		filePath := pathJoin(volumeDir, path)
		errs[idx] = checkPathLength(filePath)
		if errs[idx] != nil {
			continue
		}
		// Delete file or a directory recursively, delete parent
		// directory as well if its empty.
		errs[idx] = deleteFile(volumeDir, filePath, true)
	}

	return
}

// DeleteFile - delete a file at path.
func (s *posix) DeleteFile(volume, path string) (err error) {
	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	volumeDir, err := s.getVolDir(volume)
	if err != nil {
		return err
	}

	// Stat a volume entry.
	_, err = os.Stat(volumeDir)
	if err != nil {
		if os.IsNotExist(err) {
			return errVolumeNotFound
		} else if os.IsPermission(err) {
			return errVolumeAccessDenied
		} else if isSysErrIO(err) {
			return errFaultyDisk
		}
		return err
	}

	// Following code is needed so that we retain SlashSeparator suffix if any in
	// path argument.
	filePath := pathJoin(volumeDir, path)
	if err = checkPathLength(filePath); err != nil {
		return err
	}

	// Delete file and delete parent directory as well if its empty.
	return deleteFile(volumeDir, filePath, false)
}

func (s *posix) DeleteFileBulk(volume string, paths []string) (errs []error, err error) {
	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	volumeDir, err := s.getVolDir(volume)
	if err != nil {
		return nil, err
	}

	// Stat a volume entry.
	_, err = os.Stat(volumeDir)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, errVolumeNotFound
		} else if os.IsPermission(err) {
			return nil, errVolumeAccessDenied
		} else if isSysErrIO(err) {
			return nil, errFaultyDisk
		}
		return nil, err
	}

	errs = make([]error, len(paths))
	// Following code is needed so that we retain SlashSeparator
	// suffix if any in path argument.
	for idx, path := range paths {
		filePath := pathJoin(volumeDir, path)
		errs[idx] = checkPathLength(filePath)
		if errs[idx] != nil {
			continue
		}
		// Delete file and delete parent directory as well if its empty.
		errs[idx] = deleteFile(volumeDir, filePath, false)
	}
	return
}

// RenameFile - rename source path to destination path atomically.
func (s *posix) RenameFile(srcVolume, srcPath, dstVolume, dstPath string) (err error) {
	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	srcVolumeDir, err := s.getVolDir(srcVolume)
	if err != nil {
		return err
	}
	dstVolumeDir, err := s.getVolDir(dstVolume)
	if err != nil {
		return err
	}
	// Stat a volume entry.
	_, err = os.Stat(srcVolumeDir)
	if err != nil {
		if os.IsNotExist(err) {
			return errVolumeNotFound
		} else if isSysErrIO(err) {
			return errFaultyDisk
		}
		return err
	}
	_, err = os.Stat(dstVolumeDir)
	if err != nil {
		if os.IsNotExist(err) {
			return errVolumeNotFound
		} else if isSysErrIO(err) {
			return errFaultyDisk
		}
		return err
	}

	srcIsDir := HasSuffix(srcPath, SlashSeparator)
	dstIsDir := HasSuffix(dstPath, SlashSeparator)
	// Either src and dst have to be directories or files, else return error.
	if !(srcIsDir && dstIsDir || !srcIsDir && !dstIsDir) {
		return errFileAccessDenied
	}
	srcFilePath := slashpath.Join(srcVolumeDir, srcPath)
	if err = checkPathLength(srcFilePath); err != nil {
		return err
	}
	dstFilePath := slashpath.Join(dstVolumeDir, dstPath)
	if err = checkPathLength(dstFilePath); err != nil {
		return err
	}
	if srcIsDir {
		// If source is a directory, we expect the destination to be non-existent but we
		// we still need to allow overwriting an empty directory since it represents
		// an object empty directory.
		_, err = os.Stat(dstFilePath)
		if isSysErrIO(err) {
			return errFaultyDisk
		}
		if err == nil && !isDirEmpty(dstFilePath) {
			return errFileAccessDenied
		}
		if err != nil && !os.IsNotExist(err) {
			return err
		}
		// Empty destination remove it before rename.
		if isDirEmpty(dstFilePath) {
			if err = os.Remove(dstFilePath); err != nil {
				if isSysErrNotEmpty(err) {
					return errFileAccessDenied
				}
				return err
			}
		}
	}

	if err = renameAll(srcFilePath, dstFilePath); err != nil {
		if isSysErrIO(err) {
			return errFaultyDisk
		}
		return err
	}

	// Remove parent dir of the source file if empty
	if parentDir := slashpath.Dir(srcFilePath); isDirEmpty(parentDir) {
		deleteFile(srcVolumeDir, parentDir, false)
	}

	return nil
}

func (s *posix) VerifyFile(volume, path string, fileSize int64, algo BitrotAlgorithm, sum []byte, shardSize int64) (err error) {
	atomic.AddInt32(&s.activeIOCount, 1)
	defer func() {
		atomic.AddInt32(&s.activeIOCount, -1)
	}()

	volumeDir, err := s.getVolDir(volume)
	if err != nil {
		return err
	}

	// Stat a volume entry.
	_, err = os.Stat(volumeDir)
	if err != nil {
		if os.IsNotExist(err) {
			return errVolumeNotFound
		} else if isSysErrIO(err) {
			return errFaultyDisk
		} else if os.IsPermission(err) {
			return errVolumeAccessDenied
		}
		return err
	}

	// Validate effective path length before reading.
	filePath := pathJoin(volumeDir, path)
	if err = checkPathLength(filePath); err != nil {
		return err
	}

	// Open the file for reading.
	file, err := os.Open(filePath)
	if err != nil {
		return osErrToFSFileErr(err)
	}

	// Close the file descriptor.
	defer file.Close()

	if algo != HighwayHash256S {
		bufp := s.pool.Get().(*[]byte)
		defer s.pool.Put(bufp)

		h := algo.New()
		if _, err = io.CopyBuffer(h, file, *bufp); err != nil {
			// Premature failure in reading the object,file is corrupt.
			return errFileCorrupt
		}
		if !bytes.Equal(h.Sum(nil), sum) {
			return errFileCorrupt
		}
		return nil
	}

	buf := make([]byte, shardSize)
	h := algo.New()
	hashBuf := make([]byte, h.Size())
	fi, err := file.Stat()
	if err != nil {
		// Unable to stat on the file, return an expected error
		// for healing code to fix this file.
		return err
	}

	size := fi.Size()

	// Calculate the size of the bitrot file and compare
	// it with the actual file size.
	if size != bitrotShardFileSize(fileSize, shardSize, algo) {
		return errFileCorrupt
	}

	var n int
	for {
		if size == 0 {
			return nil
		}
		h.Reset()
		n, err = file.Read(hashBuf)
		if err != nil {
			// Read's failed for object with right size, file is corrupt.
			return err
		}
		size -= int64(n)
		if size < int64(len(buf)) {
			buf = buf[:size]
		}
		n, err = file.Read(buf)
		if err != nil {
			// Read's failed for object with right size, at different offsets.
			return err
		}
		size -= int64(n)
		h.Write(buf)
		if !bytes.Equal(h.Sum(nil), hashBuf) {
			return errFileCorrupt
		}
	}
}