vendor/gopkg.in/natefinch/lumberjack.v2/lumberjack.go - cloudstack-kubernetes-provider - Git at Google

 // Package lumberjack provides a rolling logger.
 //
 // Note that this is v2.0 of lumberjack, and should be imported using gopkg.in
 // thusly:
 //
 //   import "gopkg.in/natefinch/lumberjack.v2"
 //
 // The package name remains simply lumberjack, and the code resides at
 // https://github.com/natefinch/lumberjack under the v2.0 branch.
 //
 // Lumberjack is intended to be one part of a logging infrastructure.
 // It is not an all-in-one solution, but instead is a pluggable
 // component at the bottom of the logging stack that simply controls the files
 // to which logs are written.
 //
 // Lumberjack plays well with any logging package that can write to an
 // io.Writer, including the standard library's log package.
 //
 // Lumberjack assumes that only one process is writing to the output files.
 // Using the same lumberjack configuration from multiple processes on the same
 // machine will result in improper behavior.
 package lumberjack

 import (
 	"compress/gzip"
 	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"
 	"sort"
 	"strings"
 	"sync"
 	"time"
 )

 const (
 	backupTimeFormat = "2006-01-02T15-04-05.000"
 	compressSuffix   = ".gz"
 	defaultMaxSize   = 100
 )

 // ensure we always implement io.WriteCloser
 var _ io.WriteCloser = (*Logger)(nil)

 // Logger is an io.WriteCloser that writes to the specified filename.
 //
 // Logger opens or creates the logfile on first Write.  If the file exists and
 // is less than MaxSize megabytes, lumberjack will open and append to that file.
 // If the file exists and its size is >= MaxSize megabytes, the file is renamed
 // by putting the current time in a timestamp in the name immediately before the
 // file's extension (or the end of the filename if there's no extension). A new
 // log file is then created using original filename.
 //
 // Whenever a write would cause the current log file exceed MaxSize megabytes,
 // the current file is closed, renamed, and a new log file created with the
 // original name. Thus, the filename you give Logger is always the "current" log
 // file.
 //
 // Backups use the log file name given to Logger, in the form
 // `name-timestamp.ext` where name is the filename without the extension,
 // timestamp is the time at which the log was rotated formatted with the
 // time.Time format of `2006-01-02T15-04-05.000` and the extension is the
 // original extension.  For example, if your Logger.Filename is
 // `/var/log/foo/server.log`, a backup created at 6:30pm on Nov 11 2016 would
 // use the filename `/var/log/foo/server-2016-11-04T18-30-00.000.log`
 //
 // Cleaning Up Old Log Files
 //
 // Whenever a new logfile gets created, old log files may be deleted.  The most
 // recent files according to the encoded timestamp will be retained, up to a
 // number equal to MaxBackups (or all of them if MaxBackups is 0).  Any files
 // with an encoded timestamp older than MaxAge days are deleted, regardless of
 // MaxBackups.  Note that the time encoded in the timestamp is the rotation
 // time, which may differ from the last time that file was written to.
 //
 // If MaxBackups and MaxAge are both 0, no old log files will be deleted.
 type Logger struct {
 	// Filename is the file to write logs to.  Backup log files will be retained
 	// in the same directory.  It uses <processname>-lumberjack.log in
 	// os.TempDir() if empty.
 	Filename string `json:"filename" yaml:"filename"`

 	// MaxSize is the maximum size in megabytes of the log file before it gets
 	// rotated. It defaults to 100 megabytes.
 	MaxSize int `json:"maxsize" yaml:"maxsize"`

 	// MaxAge is the maximum number of days to retain old log files based on the
 	// timestamp encoded in their filename.  Note that a day is defined as 24
 	// hours and may not exactly correspond to calendar days due to daylight
 	// savings, leap seconds, etc. The default is not to remove old log files
 	// based on age.
 	MaxAge int `json:"maxage" yaml:"maxage"`

 	// MaxBackups is the maximum number of old log files to retain.  The default
 	// is to retain all old log files (though MaxAge may still cause them to get
 	// deleted.)
 	MaxBackups int `json:"maxbackups" yaml:"maxbackups"`

 	// LocalTime determines if the time used for formatting the timestamps in
 	// backup files is the computer's local time.  The default is to use UTC
 	// time.
 	LocalTime bool `json:"localtime" yaml:"localtime"`

 	// Compress determines if the rotated log files should be compressed
 	// using gzip.
 	Compress bool `json:"compress" yaml:"compress"`

 	size int64
 	file *os.File
 	mu   sync.Mutex

 	millCh    chan bool
 	startMill sync.Once
 }

 var (
 	// currentTime exists so it can be mocked out by tests.
 	currentTime = time.Now

 	// os_Stat exists so it can be mocked out by tests.
 	os_Stat = os.Stat

 	// megabyte is the conversion factor between MaxSize and bytes.  It is a
 	// variable so tests can mock it out and not need to write megabytes of data
 	// to disk.
 	megabyte = 1024 * 1024
 )

 // Write implements io.Writer.  If a write would cause the log file to be larger
 // than MaxSize, the file is closed, renamed to include a timestamp of the
 // current time, and a new log file is created using the original log file name.
 // If the length of the write is greater than MaxSize, an error is returned.
 func (l *Logger) Write(p []byte) (n int, err error) {
 	l.mu.Lock()
 	defer l.mu.Unlock()

 	writeLen := int64(len(p))
 	if writeLen > l.max() {
 		return 0, fmt.Errorf(
 			"write length %d exceeds maximum file size %d", writeLen, l.max(),
 		)
 	}

 	if l.file == nil {
 		if err = l.openExistingOrNew(len(p)); err != nil {
 			return 0, err
 		}
 	}

 	if l.size+writeLen > l.max() {
 		if err := l.rotate(); err != nil {
 			return 0, err
 		}
 	}

 	n, err = l.file.Write(p)
 	l.size += int64(n)

 	return n, err
 }

 // Close implements io.Closer, and closes the current logfile.
 func (l *Logger) Close() error {
 	l.mu.Lock()
 	defer l.mu.Unlock()
 	return l.close()
 }

 // close closes the file if it is open.
 func (l *Logger) close() error {
 	if l.file == nil {
 		return nil
 	}
 	err := l.file.Close()
 	l.file = nil
 	return err
 }

 // Rotate causes Logger to close the existing log file and immediately create a
 // new one.  This is a helper function for applications that want to initiate
 // rotations outside of the normal rotation rules, such as in response to
 // SIGHUP.  After rotating, this initiates compression and removal of old log
 // files according to the configuration.
 func (l *Logger) Rotate() error {
 	l.mu.Lock()
 	defer l.mu.Unlock()
 	return l.rotate()
 }

 // rotate closes the current file, moves it aside with a timestamp in the name,
 // (if it exists), opens a new file with the original filename, and then runs
 // post-rotation processing and removal.
 func (l *Logger) rotate() error {
 	if err := l.close(); err != nil {
 		return err
 	}
 	if err := l.openNew(); err != nil {
 		return err
 	}
 	l.mill()
 	return nil
 }

 // openNew opens a new log file for writing, moving any old log file out of the
 // way.  This methods assumes the file has already been closed.
 func (l *Logger) openNew() error {
 	err := os.MkdirAll(l.dir(), 0744)
 	if err != nil {
 		return fmt.Errorf("can't make directories for new logfile: %s", err)
 	}

 	name := l.filename()
 	mode := os.FileMode(0644)
 	info, err := os_Stat(name)
 	if err == nil {
 		// Copy the mode off the old logfile.
 		mode = info.Mode()
 		// move the existing file
 		newname := backupName(name, l.LocalTime)
 		if err := os.Rename(name, newname); err != nil {
 			return fmt.Errorf("can't rename log file: %s", err)
 		}

 		// this is a no-op anywhere but linux
 		if err := chown(name, info); err != nil {
 			return err
 		}
 	}

 	// we use truncate here because this should only get called when we've moved
 	// the file ourselves. if someone else creates the file in the meantime,
 	// just wipe out the contents.
 	f, err := os.OpenFile(name, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, mode)
 	if err != nil {
 		return fmt.Errorf("can't open new logfile: %s", err)
 	}
 	l.file = f
 	l.size = 0
 	return nil
 }

 // backupName creates a new filename from the given name, inserting a timestamp
 // between the filename and the extension, using the local time if requested
 // (otherwise UTC).
 func backupName(name string, local bool) string {
 	dir := filepath.Dir(name)
 	filename := filepath.Base(name)
 	ext := filepath.Ext(filename)
 	prefix := filename[:len(filename)-len(ext)]
 	t := currentTime()
 	if !local {
 		t = t.UTC()
 	}

 	timestamp := t.Format(backupTimeFormat)
 	return filepath.Join(dir, fmt.Sprintf("%s-%s%s", prefix, timestamp, ext))
 }

 // openExistingOrNew opens the logfile if it exists and if the current write
 // would not put it over MaxSize.  If there is no such file or the write would
 // put it over the MaxSize, a new file is created.
 func (l *Logger) openExistingOrNew(writeLen int) error {
 	l.mill()

 	filename := l.filename()
 	info, err := os_Stat(filename)
 	if os.IsNotExist(err) {
 		return l.openNew()
 	}
 	if err != nil {
 		return fmt.Errorf("error getting log file info: %s", err)
 	}

 	if info.Size()+int64(writeLen) >= l.max() {
 		return l.rotate()
 	}

 	file, err := os.OpenFile(filename, os.O_APPEND|os.O_WRONLY, 0644)
 	if err != nil {
 		// if we fail to open the old log file for some reason, just ignore
 		// it and open a new log file.
 		return l.openNew()
 	}
 	l.file = file
 	l.size = info.Size()
 	return nil
 }

 // genFilename generates the name of the logfile from the current time.
 func (l *Logger) filename() string {
 	if l.Filename != "" {
 		return l.Filename
 	}
 	name := filepath.Base(os.Args[0]) + "-lumberjack.log"
 	return filepath.Join(os.TempDir(), name)
 }

 // millRunOnce performs compression and removal of stale log files.
 // Log files are compressed if enabled via configuration and old log
 // files are removed, keeping at most l.MaxBackups files, as long as
 // none of them are older than MaxAge.
 func (l *Logger) millRunOnce() error {
 	if l.MaxBackups == 0 && l.MaxAge == 0 && !l.Compress {
 		return nil
 	}

 	files, err := l.oldLogFiles()
 	if err != nil {
 		return err
 	}

 	var compress, remove []logInfo

 	if l.MaxBackups > 0 && l.MaxBackups < len(files) {
 		preserved := make(map[string]bool)
 		var remaining []logInfo
 		for _, f := range files {
 			// Only count the uncompressed log file or the
 			// compressed log file, not both.
 			fn := f.Name()
 			if strings.HasSuffix(fn, compressSuffix) {
 				fn = fn[:len(fn)-len(compressSuffix)]
 			}
 			preserved[fn] = true

 			if len(preserved) > l.MaxBackups {
 				remove = append(remove, f)
 			} else {
 				remaining = append(remaining, f)
 			}
 		}
 		files = remaining
 	}
 	if l.MaxAge > 0 {
 		diff := time.Duration(int64(24*time.Hour) * int64(l.MaxAge))
 		cutoff := currentTime().Add(-1 * diff)

 		var remaining []logInfo
 		for _, f := range files {
 			if f.timestamp.Before(cutoff) {
 				remove = append(remove, f)
 			} else {
 				remaining = append(remaining, f)
 			}
 		}
 		files = remaining
 	}

 	if l.Compress {
 		for _, f := range files {
 			if !strings.HasSuffix(f.Name(), compressSuffix) {
 				compress = append(compress, f)
 			}
 		}
 	}

 	for _, f := range remove {
 		errRemove := os.Remove(filepath.Join(l.dir(), f.Name()))
 		if err == nil && errRemove != nil {
 			err = errRemove
 		}
 	}
 	for _, f := range compress {
 		fn := filepath.Join(l.dir(), f.Name())
 		errCompress := compressLogFile(fn, fn+compressSuffix)
 		if err == nil && errCompress != nil {
 			err = errCompress
 		}
 	}

 	return err
 }

 // millRun runs in a goroutine to manage post-rotation compression and removal
 // of old log files.
 func (l *Logger) millRun() {
 	for _ = range l.millCh {
 		// what am I going to do, log this?
 		_ = l.millRunOnce()
 	}
 }

 // mill performs post-rotation compression and removal of stale log files,
 // starting the mill goroutine if necessary.
 func (l *Logger) mill() {
 	l.startMill.Do(func() {
 		l.millCh = make(chan bool, 1)
 		go l.millRun()
 	})
 	select {
 	case l.millCh <- true:
 	default:
 	}
 }

 // oldLogFiles returns the list of backup log files stored in the same
 // directory as the current log file, sorted by ModTime
 func (l *Logger) oldLogFiles() ([]logInfo, error) {
 	files, err := ioutil.ReadDir(l.dir())
 	if err != nil {
 		return nil, fmt.Errorf("can't read log file directory: %s", err)
 	}
 	logFiles := []logInfo{}

 	prefix, ext := l.prefixAndExt()

 	for _, f := range files {
 		if f.IsDir() {
 			continue
 		}
 		if t, err := l.timeFromName(f.Name(), prefix, ext); err == nil {
 			logFiles = append(logFiles, logInfo{t, f})
 			continue
 		}
 		if t, err := l.timeFromName(f.Name(), prefix, ext+compressSuffix); err == nil {
 			logFiles = append(logFiles, logInfo{t, f})
 			continue
 		}
 		// error parsing means that the suffix at the end was not generated
 		// by lumberjack, and therefore it's not a backup file.
 	}

 	sort.Sort(byFormatTime(logFiles))

 	return logFiles, nil
 }

 // timeFromName extracts the formatted time from the filename by stripping off
 // the filename's prefix and extension. This prevents someone's filename from
 // confusing time.parse.
 func (l *Logger) timeFromName(filename, prefix, ext string) (time.Time, error) {
 	if !strings.HasPrefix(filename, prefix) {
 		return time.Time{}, errors.New("mismatched prefix")
 	}
 	if !strings.HasSuffix(filename, ext) {
 		return time.Time{}, errors.New("mismatched extension")
 	}
 	ts := filename[len(prefix) : len(filename)-len(ext)]
 	return time.Parse(backupTimeFormat, ts)
 }

 // max returns the maximum size in bytes of log files before rolling.
 func (l *Logger) max() int64 {
 	if l.MaxSize == 0 {
 		return int64(defaultMaxSize * megabyte)
 	}
 	return int64(l.MaxSize) * int64(megabyte)
 }

 // dir returns the directory for the current filename.
 func (l *Logger) dir() string {
 	return filepath.Dir(l.filename())
 }

 // prefixAndExt returns the filename part and extension part from the Logger's
 // filename.
 func (l *Logger) prefixAndExt() (prefix, ext string) {
 	filename := filepath.Base(l.filename())
 	ext = filepath.Ext(filename)
 	prefix = filename[:len(filename)-len(ext)] + "-"
 	return prefix, ext
 }

 // compressLogFile compresses the given log file, removing the
 // uncompressed log file if successful.
 func compressLogFile(src, dst string) (err error) {
 	f, err := os.Open(src)
 	if err != nil {
 		return fmt.Errorf("failed to open log file: %v", err)
 	}
 	defer f.Close()

 	fi, err := os_Stat(src)
 	if err != nil {
 		return fmt.Errorf("failed to stat log file: %v", err)
 	}

 	if err := chown(dst, fi); err != nil {
 		return fmt.Errorf("failed to chown compressed log file: %v", err)
 	}

 	// If this file already exists, we presume it was created by
 	// a previous attempt to compress the log file.
 	gzf, err := os.OpenFile(dst, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, fi.Mode())
 	if err != nil {
 		return fmt.Errorf("failed to open compressed log file: %v", err)
 	}
 	defer gzf.Close()

 	gz := gzip.NewWriter(gzf)

 	defer func() {
 		if err != nil {
 			os.Remove(dst)
 			err = fmt.Errorf("failed to compress log file: %v", err)
 		}
 	}()

 	if _, err := io.Copy(gz, f); err != nil {
 		return err
 	}
 	if err := gz.Close(); err != nil {
 		return err
 	}
 	if err := gzf.Close(); err != nil {
 		return err
 	}

 	if err := f.Close(); err != nil {
 		return err
 	}
 	if err := os.Remove(src); err != nil {
 		return err
 	}

 	return nil
 }

 // logInfo is a convenience struct to return the filename and its embedded
 // timestamp.
 type logInfo struct {
 	timestamp time.Time
 	os.FileInfo
 }

 // byFormatTime sorts by newest time formatted in the name.
 type byFormatTime []logInfo

 func (b byFormatTime) Less(i, j int) bool {
 	return b[i].timestamp.After(b[j].timestamp)
 }

 func (b byFormatTime) Swap(i, j int) {
 	b[i], b[j] = b[j], b[i]
 }

 func (b byFormatTime) Len() int {
 	return len(b)
 }
	// Package lumberjack provides a rolling logger.
	//
	// Note that this is v2.0 of lumberjack, and should be imported using gopkg.in
	// thusly:
	//
	// import "gopkg.in/natefinch/lumberjack.v2"
	//
	// The package name remains simply lumberjack, and the code resides at
	// https://github.com/natefinch/lumberjack under the v2.0 branch.
	//
	// Lumberjack is intended to be one part of a logging infrastructure.
	// It is not an all-in-one solution, but instead is a pluggable
	// component at the bottom of the logging stack that simply controls the files
	// to which logs are written.
	//
	// Lumberjack plays well with any logging package that can write to an
	// io.Writer, including the standard library's log package.
	//
	// Lumberjack assumes that only one process is writing to the output files.
	// Using the same lumberjack configuration from multiple processes on the same
	// machine will result in improper behavior.
	package lumberjack

	import (
	"compress/gzip"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"sync"
	"time"
	)

	const (
	backupTimeFormat = "2006-01-02T15-04-05.000"
	compressSuffix = ".gz"
	defaultMaxSize = 100
	)

	// ensure we always implement io.WriteCloser
	var _ io.WriteCloser = (*Logger)(nil)

	// Logger is an io.WriteCloser that writes to the specified filename.
	//
	// Logger opens or creates the logfile on first Write. If the file exists and
	// is less than MaxSize megabytes, lumberjack will open and append to that file.
	// If the file exists and its size is >= MaxSize megabytes, the file is renamed
	// by putting the current time in a timestamp in the name immediately before the
	// file's extension (or the end of the filename if there's no extension). A new
	// log file is then created using original filename.
	//
	// Whenever a write would cause the current log file exceed MaxSize megabytes,
	// the current file is closed, renamed, and a new log file created with the
	// original name. Thus, the filename you give Logger is always the "current" log
	// file.
	//
	// Backups use the log file name given to Logger, in the form
	// `name-timestamp.ext` where name is the filename without the extension,
	// timestamp is the time at which the log was rotated formatted with the
	// time.Time format of `2006-01-02T15-04-05.000` and the extension is the
	// original extension. For example, if your Logger.Filename is
	// `/var/log/foo/server.log`, a backup created at 6:30pm on Nov 11 2016 would
	// use the filename `/var/log/foo/server-2016-11-04T18-30-00.000.log`
	//
	// Cleaning Up Old Log Files
	//
	// Whenever a new logfile gets created, old log files may be deleted. The most
	// recent files according to the encoded timestamp will be retained, up to a
	// number equal to MaxBackups (or all of them if MaxBackups is 0). Any files
	// with an encoded timestamp older than MaxAge days are deleted, regardless of
	// MaxBackups. Note that the time encoded in the timestamp is the rotation
	// time, which may differ from the last time that file was written to.
	//
	// If MaxBackups and MaxAge are both 0, no old log files will be deleted.
	type Logger struct {
	// Filename is the file to write logs to. Backup log files will be retained
	// in the same directory. It uses <processname>-lumberjack.log in
	// os.TempDir() if empty.
	Filename string `json:"filename" yaml:"filename"`

	// MaxSize is the maximum size in megabytes of the log file before it gets
	// rotated. It defaults to 100 megabytes.
	MaxSize int `json:"maxsize" yaml:"maxsize"`

	// MaxAge is the maximum number of days to retain old log files based on the
	// timestamp encoded in their filename. Note that a day is defined as 24
	// hours and may not exactly correspond to calendar days due to daylight
	// savings, leap seconds, etc. The default is not to remove old log files
	// based on age.
	MaxAge int `json:"maxage" yaml:"maxage"`

	// MaxBackups is the maximum number of old log files to retain. The default
	// is to retain all old log files (though MaxAge may still cause them to get
	// deleted.)
	MaxBackups int `json:"maxbackups" yaml:"maxbackups"`

	// LocalTime determines if the time used for formatting the timestamps in
	// backup files is the computer's local time. The default is to use UTC
	// time.
	LocalTime bool `json:"localtime" yaml:"localtime"`

	// Compress determines if the rotated log files should be compressed
	// using gzip.
	Compress bool `json:"compress" yaml:"compress"`

	size int64
	file *os.File
	mu sync.Mutex

	millCh chan bool
	startMill sync.Once
	}

	var (
	// currentTime exists so it can be mocked out by tests.
	currentTime = time.Now

	// os_Stat exists so it can be mocked out by tests.
	os_Stat = os.Stat

	// megabyte is the conversion factor between MaxSize and bytes. It is a
	// variable so tests can mock it out and not need to write megabytes of data
	// to disk.
	megabyte = 1024 * 1024
	)

	// Write implements io.Writer. If a write would cause the log file to be larger
	// than MaxSize, the file is closed, renamed to include a timestamp of the
	// current time, and a new log file is created using the original log file name.
	// If the length of the write is greater than MaxSize, an error is returned.
	func (l *Logger) Write(p []byte) (n int, err error) {
	l.mu.Lock()
	defer l.mu.Unlock()

	writeLen := int64(len(p))
	if writeLen > l.max() {
	return 0, fmt.Errorf(
	"write length %d exceeds maximum file size %d", writeLen, l.max(),
	)
	}

	if l.file == nil {
	if err = l.openExistingOrNew(len(p)); err != nil {
	return 0, err
	}
	}

	if l.size+writeLen > l.max() {
	if err := l.rotate(); err != nil {
	return 0, err
	}
	}

	n, err = l.file.Write(p)
	l.size += int64(n)

	return n, err
	}

	// Close implements io.Closer, and closes the current logfile.
	func (l *Logger) Close() error {
	l.mu.Lock()
	defer l.mu.Unlock()
	return l.close()
	}

	// close closes the file if it is open.
	func (l *Logger) close() error {
	if l.file == nil {
	return nil
	}
	err := l.file.Close()
	l.file = nil
	return err
	}

	// Rotate causes Logger to close the existing log file and immediately create a
	// new one. This is a helper function for applications that want to initiate
	// rotations outside of the normal rotation rules, such as in response to
	// SIGHUP. After rotating, this initiates compression and removal of old log
	// files according to the configuration.
	func (l *Logger) Rotate() error {
	l.mu.Lock()
	defer l.mu.Unlock()
	return l.rotate()
	}

	// rotate closes the current file, moves it aside with a timestamp in the name,
	// (if it exists), opens a new file with the original filename, and then runs
	// post-rotation processing and removal.
	func (l *Logger) rotate() error {
	if err := l.close(); err != nil {
	return err
	}
	if err := l.openNew(); err != nil {
	return err
	}
	l.mill()
	return nil
	}

	// openNew opens a new log file for writing, moving any old log file out of the
	// way. This methods assumes the file has already been closed.
	func (l *Logger) openNew() error {
	err := os.MkdirAll(l.dir(), 0744)
	if err != nil {
	return fmt.Errorf("can't make directories for new logfile: %s", err)
	}

	name := l.filename()
	mode := os.FileMode(0644)
	info, err := os_Stat(name)
	if err == nil {
	// Copy the mode off the old logfile.
	mode = info.Mode()
	// move the existing file
	newname := backupName(name, l.LocalTime)
	if err := os.Rename(name, newname); err != nil {
	return fmt.Errorf("can't rename log file: %s", err)
	}

	// this is a no-op anywhere but linux
	if err := chown(name, info); err != nil {
	return err
	}
	}

	// we use truncate here because this should only get called when we've moved
	// the file ourselves. if someone else creates the file in the meantime,
	// just wipe out the contents.
	f, err := os.OpenFile(name, os.O_CREATE\|os.O_WRONLY\|os.O_TRUNC, mode)
	if err != nil {
	return fmt.Errorf("can't open new logfile: %s", err)
	}
	l.file = f
	l.size = 0
	return nil
	}

	// backupName creates a new filename from the given name, inserting a timestamp
	// between the filename and the extension, using the local time if requested
	// (otherwise UTC).
	func backupName(name string, local bool) string {
	dir := filepath.Dir(name)
	filename := filepath.Base(name)
	ext := filepath.Ext(filename)
	prefix := filename[:len(filename)-len(ext)]
	t := currentTime()
	if !local {
	t = t.UTC()
	}

	timestamp := t.Format(backupTimeFormat)
	return filepath.Join(dir, fmt.Sprintf("%s-%s%s", prefix, timestamp, ext))
	}

	// openExistingOrNew opens the logfile if it exists and if the current write
	// would not put it over MaxSize. If there is no such file or the write would
	// put it over the MaxSize, a new file is created.
	func (l *Logger) openExistingOrNew(writeLen int) error {
	l.mill()

	filename := l.filename()
	info, err := os_Stat(filename)
	if os.IsNotExist(err) {
	return l.openNew()
	}
	if err != nil {
	return fmt.Errorf("error getting log file info: %s", err)
	}

	if info.Size()+int64(writeLen) >= l.max() {
	return l.rotate()
	}

	file, err := os.OpenFile(filename, os.O_APPEND\|os.O_WRONLY, 0644)
	if err != nil {
	// if we fail to open the old log file for some reason, just ignore
	// it and open a new log file.
	return l.openNew()
	}
	l.file = file
	l.size = info.Size()
	return nil
	}

	// genFilename generates the name of the logfile from the current time.
	func (l *Logger) filename() string {
	if l.Filename != "" {
	return l.Filename
	}
	name := filepath.Base(os.Args[0]) + "-lumberjack.log"
	return filepath.Join(os.TempDir(), name)
	}

	// millRunOnce performs compression and removal of stale log files.
	// Log files are compressed if enabled via configuration and old log
	// files are removed, keeping at most l.MaxBackups files, as long as
	// none of them are older than MaxAge.
	func (l *Logger) millRunOnce() error {
	if l.MaxBackups == 0 && l.MaxAge == 0 && !l.Compress {
	return nil
	}

	files, err := l.oldLogFiles()
	if err != nil {
	return err
	}

	var compress, remove []logInfo

	if l.MaxBackups > 0 && l.MaxBackups < len(files) {
	preserved := make(map[string]bool)
	var remaining []logInfo
	for _, f := range files {
	// Only count the uncompressed log file or the
	// compressed log file, not both.
	fn := f.Name()
	if strings.HasSuffix(fn, compressSuffix) {
	fn = fn[:len(fn)-len(compressSuffix)]
	}
	preserved[fn] = true

	if len(preserved) > l.MaxBackups {
	remove = append(remove, f)
	} else {
	remaining = append(remaining, f)
	}
	}
	files = remaining
	}
	if l.MaxAge > 0 {
	diff := time.Duration(int64(24time.Hour) int64(l.MaxAge))
	cutoff := currentTime().Add(-1 * diff)

	var remaining []logInfo
	for _, f := range files {
	if f.timestamp.Before(cutoff) {
	remove = append(remove, f)
	} else {
	remaining = append(remaining, f)
	}
	}
	files = remaining
	}

	if l.Compress {
	for _, f := range files {
	if !strings.HasSuffix(f.Name(), compressSuffix) {
	compress = append(compress, f)
	}
	}
	}

	for _, f := range remove {
	errRemove := os.Remove(filepath.Join(l.dir(), f.Name()))
	if err == nil && errRemove != nil {
	err = errRemove
	}
	}
	for _, f := range compress {
	fn := filepath.Join(l.dir(), f.Name())
	errCompress := compressLogFile(fn, fn+compressSuffix)
	if err == nil && errCompress != nil {
	err = errCompress
	}
	}

	return err
	}

	// millRun runs in a goroutine to manage post-rotation compression and removal
	// of old log files.
	func (l *Logger) millRun() {
	for _ = range l.millCh {
	// what am I going to do, log this?
	_ = l.millRunOnce()
	}
	}

	// mill performs post-rotation compression and removal of stale log files,
	// starting the mill goroutine if necessary.
	func (l *Logger) mill() {
	l.startMill.Do(func() {
	l.millCh = make(chan bool, 1)
	go l.millRun()
	})
	select {
	case l.millCh <- true:
	default:
	}
	}

	// oldLogFiles returns the list of backup log files stored in the same
	// directory as the current log file, sorted by ModTime
	func (l *Logger) oldLogFiles() ([]logInfo, error) {
	files, err := ioutil.ReadDir(l.dir())
	if err != nil {
	return nil, fmt.Errorf("can't read log file directory: %s", err)
	}
	logFiles := []logInfo{}

	prefix, ext := l.prefixAndExt()

	for _, f := range files {
	if f.IsDir() {
	continue
	}
	if t, err := l.timeFromName(f.Name(), prefix, ext); err == nil {
	logFiles = append(logFiles, logInfo{t, f})
	continue
	}
	if t, err := l.timeFromName(f.Name(), prefix, ext+compressSuffix); err == nil {
	logFiles = append(logFiles, logInfo{t, f})
	continue
	}
	// error parsing means that the suffix at the end was not generated
	// by lumberjack, and therefore it's not a backup file.
	}

	sort.Sort(byFormatTime(logFiles))

	return logFiles, nil
	}

	// timeFromName extracts the formatted time from the filename by stripping off
	// the filename's prefix and extension. This prevents someone's filename from
	// confusing time.parse.
	func (l *Logger) timeFromName(filename, prefix, ext string) (time.Time, error) {
	if !strings.HasPrefix(filename, prefix) {
	return time.Time{}, errors.New("mismatched prefix")
	}
	if !strings.HasSuffix(filename, ext) {
	return time.Time{}, errors.New("mismatched extension")
	}
	ts := filename[len(prefix) : len(filename)-len(ext)]
	return time.Parse(backupTimeFormat, ts)
	}

	// max returns the maximum size in bytes of log files before rolling.
	func (l *Logger) max() int64 {
	if l.MaxSize == 0 {
	return int64(defaultMaxSize * megabyte)
	}
	return int64(l.MaxSize) * int64(megabyte)
	}

	// dir returns the directory for the current filename.
	func (l *Logger) dir() string {
	return filepath.Dir(l.filename())
	}

	// prefixAndExt returns the filename part and extension part from the Logger's
	// filename.
	func (l *Logger) prefixAndExt() (prefix, ext string) {
	filename := filepath.Base(l.filename())
	ext = filepath.Ext(filename)
	prefix = filename[:len(filename)-len(ext)] + "-"
	return prefix, ext
	}

	// compressLogFile compresses the given log file, removing the
	// uncompressed log file if successful.
	func compressLogFile(src, dst string) (err error) {
	f, err := os.Open(src)
	if err != nil {
	return fmt.Errorf("failed to open log file: %v", err)
	}
	defer f.Close()

	fi, err := os_Stat(src)
	if err != nil {
	return fmt.Errorf("failed to stat log file: %v", err)
	}

	if err := chown(dst, fi); err != nil {
	return fmt.Errorf("failed to chown compressed log file: %v", err)
	}

	// If this file already exists, we presume it was created by
	// a previous attempt to compress the log file.
	gzf, err := os.OpenFile(dst, os.O_CREATE\|os.O_TRUNC\|os.O_WRONLY, fi.Mode())
	if err != nil {
	return fmt.Errorf("failed to open compressed log file: %v", err)
	}
	defer gzf.Close()

	gz := gzip.NewWriter(gzf)

	defer func() {
	if err != nil {
	os.Remove(dst)
	err = fmt.Errorf("failed to compress log file: %v", err)
	}
	}()

	if _, err := io.Copy(gz, f); err != nil {
	return err
	}
	if err := gz.Close(); err != nil {
	return err
	}
	if err := gzf.Close(); err != nil {
	return err
	}

	if err := f.Close(); err != nil {
	return err
	}
	if err := os.Remove(src); err != nil {
	return err
	}

	return nil
	}

	// logInfo is a convenience struct to return the filename and its embedded
	// timestamp.
	type logInfo struct {
	timestamp time.Time
	os.FileInfo
	}

	// byFormatTime sorts by newest time formatted in the name.
	type byFormatTime []logInfo

	func (b byFormatTime) Less(i, j int) bool {
	return b[i].timestamp.After(b[j].timestamp)
	}

	func (b byFormatTime) Swap(i, j int) {
	b[i], b[j] = b[j], b[i]
	}

	func (b byFormatTime) Len() int {
	return len(b)
	}