18554165489

Committed 16 Oct 2025 07:47AM UTC coverage: 87.793% (-0.2%) from 88.039%

Build # 18554165489

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DeRuina

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ci: staticcheck (Go 1.22 compatible)

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/timberjack.go

// Package timberjack provides a rolling logger with size-based and time-based rotation.
//
// Timberjack is a simple, pluggable component for log rotation. It can rotate
// the active log file when any of the following occur:
//   - the file grows beyond MaxSize (size-based)
//   - the configured RotationInterval elapses (interval-based)
//   - a scheduled time is reached via RotateAt or RotateAtMinutes (clock-based)
//   - rotation is triggered explicitly via Rotate() (manual)
//
// Rotated files can optionally be compressed with gzip or zstd.
// Cleanup is handled automatically. Old log files are removed based on MaxBackups and MaxAge.
//
// Import:
//
//        import "github.com/DeRuina/timberjack"
//
// Timberjack works with any logger that writes to an io.Writer, including the
// standard library’s log package.
//
// Concurrency note: timberjack assumes a single process writes to the target
// files. Reusing the same Logger configuration across multiple processes on the
// same machine may lead to improper behavior.
//
// Source code: https://github.com/DeRuina/timberjack
package timberjack

import (
        "cmp"
        "compress/gzip"
        "errors"
        "fmt"
        "io"
        "math"
        "os"
        "path/filepath"
        "slices"
        "sort"
        "strconv"
        "strings"
        "sync"
        "sync/atomic"
        "time"
        "unicode"

        "github.com/klauspost/compress/zstd"
)

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

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

// safeClose is a generic function that safely closes a channel of any type.
// It prevents "panic: close of closed channel" and "panic: close of nil channel".
//
// The type parameter [T any] allows this function to work with channels of any element type,
// for example, chan int, chan string, chan struct{}, etc.
func safeClose[T any](ch chan T) {
        defer func() {
                recover()
        }()
        close(ch)
}

type rotateAt [2]int

// Logger is an io.WriteCloser that writes to the specified filename.
//
// Logger opens or creates the logfile on the first Write.
// If the file exists and is smaller than MaxSize megabytes, timberjack will open and append to that file.
// If the file's size exceeds MaxSize, or if the configured RotationInterval has elapsed since the last rotation,
// the file is closed, renamed with a timestamp, and a new logfile is created using the original filename.
//
// 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-<reason>.ext` where `name` is the filename without the extension,
// `timestamp` is the time of rotation formatted as `2006-01-02T15-04-05.000`,
// `reason` is "size" or "time" (Rotate/auto), or a custom tag (RotateWithReason), and `ext` 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
// due to size would use the filename `/var/log/foo/server-2016-11-04T18-30-00.000-size.log`.
//
// # Cleaning Up Old Log Files
//
// Whenever a new logfile is created, old log files may be deleted based on MaxBackups and MaxAge.
// The most recent files (according to the timestamp) will be retained up to MaxBackups (or all files if MaxBackups is 0).
// Any files with a timestamp older than MaxAge days are deleted, regardless of MaxBackups.
// Note that the timestamp is the rotation time, not necessarily the last write time.
//
// If MaxBackups and MaxAge are both 0, no old log files will be deleted.
//
// timberjack assumes only a single process is writing to the log files at a time.
type Logger struct {
        // Filename is the file to write logs to.  Backup log files will be retained
        // in the same directory.  It uses <processname>-timberjack.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 counts distinct rotation events (timestamps).
        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"`

        // Deprecated: use Compression instead ("none" | "gzip" | "zstd").
        Compress bool `json:"compress,omitempty" yaml:"compress,omitempty"`

        // Compression selects the algorithm. If empty, legacy Compress is used.
        // Allowed values: "none", "gzip", "zstd". Unknown => "none" (with a warning).
        Compression string `json:"compression,omitempty" yaml:"compression,omitempty"`

        // RotationInterval is the maximum duration between log rotations.
        // If the elapsed time since the last rotation exceeds this interval,
        // the log file is rotated, even if the file size has not reached MaxSize.
        // The minimum recommended value is 1 minute. If set to 0, time-based rotation is disabled.
        //
        // Example: RotationInterval = time.Hour * 24 will rotate logs daily.
        RotationInterval time.Duration `json:"rotationinterval" yaml:"rotationinterval"`

        // BackupTimeFormat defines the layout for the timestamp appended to rotated file names.
        // While other formats are allowed, it is recommended to follow the standard Go time layout
        // (https://pkg.go.dev/time#pkg-constants). Use the ValidateBackupTimeFormat() method to check
        // if the value is valid. It is recommended to call this method before using the Logger instance.
        //
        // WARNING: This field is assumed to be constant after initialization.
        // WARNING: If invalid value is supplied then default format `2006-01-02T15-04-05.000` will be used.
        //
        // Example:
        // BackupTimeFormat = `2006-01-02-15-04-05`
        // will generate rotated backup files in the format:
        // <logfilename>-2006-01-02-15-04-05-<rotationCriterion>-timberjack.log
        // where `rotationCriterion` could be `time` or `size`.
        BackupTimeFormat string `json:"backuptimeformat" yaml:"backuptimeformat"`

        // RotateAtMinutes defines specific minutes within an hour (0-59) to trigger a rotation.
        // For example, []int{0} for top of the hour, []int{0, 30} for top and half-past the hour.
        // Rotations are aligned to the clock minute (second 0).
        // This operates in addition to RotationInterval and MaxSize.
        // If multiple rotation conditions are met, the first one encountered typically triggers.
        RotateAtMinutes []int `json:"rotateAtMinutes" yaml:"rotateAtMinutes"`

        // RotateAt defines specific time within a day to trigger a rotation.
        // For example, []string{'00:00'} for midnight, []string{'00:00', '12:00'} for
        // midnight and midday.
        // Rotations are aligned to the clock minute (second 0).
        // This operates in addition to RotationInterval and MaxSize.
        // If multiple rotation conditions are met, the first one encountered typically triggers.
        RotateAt []string `json:"rotateAt" yaml:"rotateAt"`

        // AppendTimeAfterExt controls where the timestamp/reason go.
        // false (default):  <name>-<timestamp>-<reason>.log
        // true:             <name>.log-<timestamp>-<reason>
        AppendTimeAfterExt bool `json:"appendTimeAfterExt" yaml:"appendTimeAfterExt"`

        // Internal fields
        size                   int64     // current size of the log file
        file                   *os.File  // current log file
        lastRotationTime       time.Time // records the last time a rotation happened (for interval/scheduled).
        logStartTime           time.Time // start time of the current logging period (used for backup filename timestamp).
        cfgOnce                sync.Once
        resolvedBackupLayout   string
        resolvedAppendAfterExt bool
        resolvedLocalTime      bool
        resolvedCompression    string

        mu sync.Mutex // ensures atomic writes and rotations

        // For mill goroutine (backups, compression cleanup)
        millCh        chan bool      // channel to signal the mill goroutine
        startMill     sync.Once      // ensures mill goroutine is started only once
        millWg        sync.WaitGroup // waits for the mill goroutine to finish
        millWGStarted bool

        // For scheduled rotation goroutine (RotateAt)
        startScheduledRotationOnce sync.Once      // ensures scheduled rotation goroutine is started only once
        scheduledRotationQuitCh    chan struct{}  // channel to signal the scheduled rotation goroutine to stop
        scheduledRotationWg        sync.WaitGroup // waits for the scheduled rotation goroutine to finish
        processedRotateAt          []rotateAt     // internal storage for sorted and validated RotateAt
        isClosed                   uint32

        // snapshots of globals to avoid races
        resolvedTimeNow func() time.Time
        resolvedStat    func(string) (os.FileInfo, error)
        resolvedRename  func(string, string) error
        resolvedRemove  func(string) error
}

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

        // osStat exists so it can be mocked out by tests.
        osStat = 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

        osRename = os.Rename

        osRemove = os.Remove

        // empty BackupTimeFormatField
        ErrEmptyBackupTimeFormatField = errors.New("empty backupformat field")
)

func (l *Logger) resolveConfigLocked() {
        l.cfgOnce.Do(func() {
                // Resolve time format
                layout := l.BackupTimeFormat
                if layout == "" {
                        layout = backupTimeFormat
                } else if err := l.ValidateBackupTimeFormat(); err != nil {
                        fmt.Fprintf(os.Stderr,
                                "timberjack: invalid BackupTimeFormat: %v — falling back to default format: %s\n",
                                err, backupTimeFormat)
                        layout = backupTimeFormat
                }
                l.resolvedBackupLayout = layout
                l.resolvedAppendAfterExt = l.AppendTimeAfterExt
                l.resolvedLocalTime = l.LocalTime

                // snapshot global funcs so tests can fiddle with the globals safely
                l.resolvedTimeNow = currentTime
                l.resolvedStat = osStat
                l.resolvedRename = osRename
                l.resolvedRemove = osRemove

                // Freeze compression (prevents races if toggled later)
                l.resolvedCompression = l.effectiveCompression()
        })
}

// Write implements io.Writer.
// It writes the provided bytes to the current log file.
// If the log file exceeds MaxSize after writing, or if the configured RotationInterval has elapsed
// since the last rotation, or if a scheduled rotation time (RotateAtMinutes) has been reached,
// the file is closed, renamed to include a timestamp, and a new log file is created
// using the original filename.
// If the size of a single write exceeds MaxSize, the write is rejected and an error is returned.
func (l *Logger) Write(p []byte) (n int, err error) {
        l.mu.Lock()
        defer l.mu.Unlock()

        // Ensure configuration is resolved once
        l.resolveConfigLocked()

        // Handle writes to a closed logger.
        if atomic.LoadUint32(&l.isClosed) == 1 {
                // The logger is closed. To ensure the write succeeds, we perform a
                // single open-write-close cycle. This does not perform rotation
                // and does not restart the background goroutines. l.file remains nil.
                file, openErr := os.OpenFile(l.filename(), os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0644)
                if openErr != nil {
                        return 0, fmt.Errorf("timberjack: write on closed logger failed to open file: %w", openErr)
                }

                n, writeErr := file.Write(p)

                closeErr := file.Close()

                if writeErr != nil {
                        return n, writeErr
                }
                return n, closeErr
        }

        // Ensure the scheduled-rotation goroutine is running (if you've still got one).
        l.ensureScheduledRotationLoopRunning()

        // Snapshot
        now := l.resolvedTimeNow().In(l.location())

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

        // Open (or create) the file on first write.
        if l.file == nil {
                if err = l.openExistingOrNew(len(p)); err != nil {
                        return 0, err
                }
                if l.lastRotationTime.IsZero() {
                        // Initialize to 'now' so interval/minute checks start from here.
                        l.lastRotationTime = now
                }
        }

        // 1) Interval-based rotation
        if l.RotationInterval > 0 && now.Sub(l.lastRotationTime) >= l.RotationInterval {
                if err := l.rotate("time"); err != nil {
                        return 0, fmt.Errorf("interval rotation failed: %w", err)
                }
                l.lastRotationTime = now
        }

        // 2) Scheduled time based rotation (RotateAt)
        if len(l.processedRotateAt) > 0 {
                for _, m := range l.processedRotateAt {
                        mark := time.Date(now.Year(), now.Month(), now.Day(),
                                m[0], m[1], 0, 0, l.location())
                        // If we've crossed that mark since the last rotation, fire one rotation.
                        if l.lastRotationTime.Before(mark) && (mark.Before(now) || mark.Equal(now)) {
                                if err := l.rotate("time"); err != nil {
                                        return 0, fmt.Errorf("scheduled-minute rotation failed: %w", err)
                                }
                                // Record the logical mark—so we don’t rerun until next slot.
                                l.lastRotationTime = mark
                                break
                        }
                }
        }

        // 3) Size-based rotation
        if l.size+writeLen > l.max() {
                if err := l.rotate("size"); err != nil {
                        return 0, fmt.Errorf("size rotation failed: %w", err)
                }
                // Note: we leave lastRotationTime untouched for size rotations.
        }

        // Finally, write the bytes and update size.
        n, err = l.file.Write(p)
        l.size += int64(n)
        return n, err
}

// ValidateBackupTimeFormat checks if the configured BackupTimeFormat is a valid time layout.
// While other formats are allowed, it is recommended to follow the standard time layout
// rules as defined here: https://pkg.go.dev/time#pkg-constants
//
// WARNING: Assumes that BackupTimeFormat value remains constant after initialization.
func (l *Logger) ValidateBackupTimeFormat() error {
        if len(l.BackupTimeFormat) == 0 {
                return ErrEmptyBackupTimeFormatField
        }
        // 2025-05-22 23:41:59.987654321 +0000 UTC
        now := time.Date(2025, 05, 22, 23, 41, 59, 987_654_321, time.UTC)

        layoutPrecision := countDigitsAfterDot(l.BackupTimeFormat)

        now, err := truncateFractional(now, layoutPrecision)

        if err != nil {
                return err
        }
        formatted := now.Format(l.BackupTimeFormat)
        parsedT, err := time.Parse(l.BackupTimeFormat, formatted)
        if err != nil {
                return fmt.Errorf("invalid BackupTimeFormat: %w", err)
        }
        if !parsedT.Equal(now) {
                return errors.New("invalid BackupTimeFormat: time.Time parsed from the format does not match the time.Time supplied")
        }

        return nil
}

// location returns the time.Location (UTC or Local) to use for timestamps in backup filenames.
func (l *Logger) location() *time.Location {
        l.resolveConfigLocked()
        if l.resolvedLocalTime {
                return time.Local
        }
        return time.UTC
}

func parseTime(s string) (*rotateAt, error) {
        parts := strings.Split(s, ":")
        if len(parts) != 2 {
                return nil, errors.New("invalid time")
        }
        hs, ms := parts[0], parts[1]
        h, err := strconv.Atoi(hs)
        if err != nil {
                return nil, err
        }
        m, err := strconv.Atoi(ms)
        if err != nil {
                return nil, err
        }

        if m < 0 || m > 59 || h < 0 || h > 23 {
                return nil, errors.New("invalid time")
        }

        return &rotateAt{h, m}, nil
}

func compareTime(a, b rotateAt) int {
        h1, m1 := a[0], a[1]
        h2, m2 := b[0], b[1]
        if h1 == h2 {
                return cmp.Compare(m1, m2)
        }
        return cmp.Compare(h1, h2)
}

// ensureScheduledRotationLoopRunning starts the scheduled rotation goroutine if RotateAtMinutes is configured
// and the goroutine is not already running.
func (l *Logger) ensureScheduledRotationLoopRunning() {
        if len(l.RotateAtMinutes)+len(l.RotateAt) == 0 {
                return // No scheduled rotations configured
        }

        l.startScheduledRotationOnce.Do(func() {
                var processedRotateAt []rotateAt
                for _, m := range l.RotateAtMinutes {
                        if m < 0 || m > 59 {
                                fmt.Fprintf(os.Stderr, "timberjack: [%d] No valid minute specified for RotateAtMinutes.\n", m)
                                continue
                        }
                        for h := 0; h < 24; h++ {
                                processedRotateAt = append(processedRotateAt, rotateAt{h, m})
                        }
                }

                for _, t := range l.RotateAt {
                        r, err := parseTime(t)
                        if err != nil {
                                fmt.Fprintf(os.Stderr, "timberjack: [%s] No valid time specified for RotateAt.\n", t)
                                continue
                        }
                        processedRotateAt = append(processedRotateAt, *r)
                }

                if len(processedRotateAt) == 0 {
                        // Optionally log that no valid minutes were found, preventing goroutine start
                        // fmt.Fprintf(os.Stderr, "timberjack: [%s] No valid minutes specified for RotateAtMinutes.\n", l.Filename)
                        return
                }

                // Sort for predictable order in calculating next rotation
                slices.SortFunc(processedRotateAt, compareTime)
                processedRotateAt = slices.CompactFunc(processedRotateAt, func(a, b rotateAt) bool {
                        return compareTime(a, b) == 0
                })

                l.processedRotateAt = processedRotateAt
                quitCh := make(chan struct{})
                l.scheduledRotationQuitCh = quitCh

                // Snapshot immutable inputs for the goroutine to avoid reading fields later.
                slots := l.processedRotateAt
                loc := l.location()
                nowFn := l.resolvedTimeNow

                l.scheduledRotationWg.Add(1)
                go l.runScheduledRotations(quitCh, slots, loc, nowFn)
        })
}

// runScheduledRotations is the main loop for handling rotations at specific minute marks
// as defined in RotateAtMinutes. It runs in a separate goroutine.
func (l *Logger) runScheduledRotations(quit <-chan struct{}, slots []rotateAt, loc *time.Location, nowFn func() time.Time) {
        defer l.scheduledRotationWg.Done()

        // No slots to process, exit immediately.
        if len(slots) == 0 {
                return
        }

        timer := time.NewTimer(0) // Timer will be reset with the correct duration in the loop
        if !timer.Stop() {
                // Drain the channel if the timer fired prematurely (e.g., duration was 0 on first NewTimer)
                select {
                case <-timer.C:
                default:
                }
        }

        for {
                now := nowFn()
                nowInLocation := now.In(loc)
                nextRotationAbsoluteTime := time.Time{}
                foundNextSlot := false

        determineNextSlot:
                // Calculate the next rotation time based on the current time and processedRotateAt.
                // Iterate through the current hour, then subsequent hours (up to 24h ahead for robustness
                // against system sleep or large clock jumps).
                for hourOffset := 0; hourOffset <= 24; hourOffset++ {
                        // Base time for the hour we are checking (e.g., if now is 10:35, current hour base is 10:00)
                        hourToCheck := time.Date(nowInLocation.Year(), nowInLocation.Month(), nowInLocation.Day(), nowInLocation.Hour(), 0, 0, 0, loc).Add(time.Duration(hourOffset) * time.Hour)

                        for _, mark := range slots {
                                candidateTime := time.Date(hourToCheck.Year(), hourToCheck.Month(), hourToCheck.Day(), mark[0], mark[1], 0, 0, loc)

                                if candidateTime.After(now) { // Found the earliest future slot
                                        nextRotationAbsoluteTime = candidateTime
                                        foundNextSlot = true
                                        break determineNextSlot // Exit both loops
                                }
                        }
                }

                if !foundNextSlot {
                        // This should ideally not happen if processedRotateAt is valid and non-empty.
                        // Could occur if currentTime() is unreliable or jumps massively backward.
                        // Log an error and retry calculation after a fallback delay.
                        fmt.Fprintf(os.Stderr, "timberjack: [%s] Could not determine next scheduled rotation time for %v with marks %v. Retrying calculation in 1 minute.\n", l.Filename, nowInLocation, slots)
                        select {
                        case <-time.After(time.Minute): // Wait a bit before retrying calculation
                                continue // Restart the outer loop to recalculate
                        case <-quit:
                                return
                        }
                }

                sleepDuration := nextRotationAbsoluteTime.Sub(now)
                timer.Reset(sleepDuration)

                select {
                case <-timer.C: // Timer fired, it's time for a scheduled rotation
                        l.mu.Lock()
                        // Only rotate if the last rotation time was before this specific scheduled mark.
                        // This prevents redundant rotations if another rotation (e.g., size/interval) happened
                        // very close to, but just before or at, this scheduled time for the same mark.
                        if l.lastRotationTime.Before(nextRotationAbsoluteTime) {
                                if err := l.rotate("time"); err != nil { // Scheduled rotations are "time" based for filename
                                        fmt.Fprintf(os.Stderr, "timberjack: [%s] scheduled rotation failed: %v\n", l.Filename, err)
                                } else {
                                        l.lastRotationTime = nowFn()
                                }
                        }
                        l.mu.Unlock()
                // Loop will continue and recalculate the next slot from the new "now"

                case <-quit: // Signal to quit from Close()
                        if !timer.Stop() {
                                // If Stop() returns false, the timer has already fired or been stopped.
                                // If it fired, its channel might have a value, so drain it.
                                select {
                                case <-timer.C:
                                default:
                                }
                        }
                        return // Exit goroutine
                }
        }
}

// Close implements io.Closer, and closes the current logfile.
// It also signals any running goroutines (like scheduled rotation or mill) to stop.
func (l *Logger) Close() error {
        l.mu.Lock()

        if atomic.LoadUint32(&l.isClosed) == 1 {
                l.mu.Unlock()
                return nil
        }
        atomic.StoreUint32(&l.isClosed, 1)

        // Stop the scheduled rotation goroutine
        var quitCh chan struct{}
        if l.scheduledRotationQuitCh != nil {
                quitCh = l.scheduledRotationQuitCh
                l.scheduledRotationQuitCh = nil // clear under lock
        }

        // Stop the mill goroutine (doesn't use l.mu, no deadlock risk)
        var millCh chan bool
        if l.millCh != nil {
                millCh = l.millCh
                // don't nil it; startMill.Do prevents restarts
        }

        // Close file under lock (safe and quick)
        err := l.closeFile()

        l.mu.Unlock()

        // Now signal and wait outside the lock
        if quitCh != nil {
                safeClose(quitCh)
                l.scheduledRotationWg.Wait()
        }
        if millCh != nil {
                safeClose(millCh)
                l.millWg.Wait()
        }

        return err
}

// closeFile closes the file if it is open. This is an internal method.
// It expects l.mu to be held.
func (l *Logger) closeFile() error {
        if l.file == nil {
                return nil
        }
        err := l.file.Close()
        l.file = nil // Set to nil to indicate it's closed.
        return err
}

// Rotate forces an immediate rotation using the legacy auto-reason logic.
// (empty reason => "time" if an interval rotation is due, otherwise "size")
func (l *Logger) Rotate() error {
        return l.RotateWithReason("")
}

// 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 (mill).
// It expects l.mu to be held by the caller.
// Takes an explicit reason for the rotation which is used in the backup filename.
func (l *Logger) rotate(reason string) error {
        if err := l.closeFile(); err != nil {
                return err
        }
        // Pass the determined reason to openNew so it's used in the backup filename
        if err := l.openNew(reason); err != nil {
                return err
        }
        l.mill()
        return nil
}

// RotateWithReason forces a rotation immediately and tags the backup filename
// with the provided reason (after sanitization). If the sanitized reason is
// empty, it falls back to the default behavior used by Rotate(): "time" if an
// interval rotation is due, otherwise "size".
//
// NOTE: Like Rotate(), this does not modify lastRotationTime. If an interval
// rotation is already due, a subsequent write may still trigger another
// interval-based rotation.
func (l *Logger) RotateWithReason(reason string) error {
        l.mu.Lock()
        defer l.mu.Unlock()

        if atomic.LoadUint32(&l.isClosed) == 1 {
                return errors.New("logger closed")
        }

        r := sanitizeReason(reason)
        if r == "" {
                // keep legacy Rotate() semantics
                r = "size"
                if l.shouldTimeRotate() {
                        r = "time"
                }
        }

        return l.rotate(r)
}

func backupNameWithResolved(name string, local bool, reason string, t time.Time, layout string, afterExt bool) string {
        dir := filepath.Dir(name)
        filename := filepath.Base(name)
        ext := filepath.Ext(filename)
        prefix := filename[:len(filename)-len(ext)]

        loc := time.UTC
        if local {
                loc = time.Local
        }
        ts := t.In(loc).Format(layout)

        if afterExt {
                // <name><ext>-<ts>-<reason>
                return filepath.Join(dir, fmt.Sprintf("%s%s-%s-%s", prefix, ext, ts, reason))
        }
        // <name>-<ts>-<reason><ext>
        return filepath.Join(dir, fmt.Sprintf("%s-%s-%s%s", prefix, ts, reason, ext))
}

// openNew creates a new log file for writing.
// If an old log file already exists, it is moved aside by renaming it with a timestamp.
// This method assumes that l.mu is held and the old file (if any) has already been closed.
// The reasonForBackup parameter is used in the backup filename.
func (l *Logger) openNew(reasonForBackup string) error {
        l.resolveConfigLocked() // no-op after first time

        if err := os.MkdirAll(l.dir(), 0755); err != nil {
                return fmt.Errorf("can't make directories for new logfile: %s", err)
        }

        name := l.filename()
        finalMode := os.FileMode(0640)
        var oldInfo os.FileInfo

        info, err := l.resolvedStat(name)
        if err == nil {
                oldInfo = info
                finalMode = oldInfo.Mode()

                rotationTimeForBackup := l.resolvedTimeNow()

                // Build the rotated name from the immutable snapshot (no public field writes).
                newname := backupNameWithResolved(
                        name,
                        l.resolvedLocalTime,
                        reasonForBackup,
                        rotationTimeForBackup,
                        l.resolvedBackupLayout,
                        l.resolvedAppendAfterExt,
                )

                if errRename := l.resolvedRename(name, newname); errRename != nil {
                        return fmt.Errorf("can't rename log file: %s", errRename)
                }
                l.logStartTime = rotationTimeForBackup
        } else if os.IsNotExist(err) {
                l.logStartTime = l.resolvedTimeNow()
                oldInfo = nil
        } else {
                return fmt.Errorf("failed to stat log file %s: %w", name, err)
        }

        // Create and open the new log file at path `name`.
        f, err := os.OpenFile(name, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, finalMode)
        if err != nil {
                return fmt.Errorf("can't open new logfile %s: %s", name, err)
        }
        l.file = f
        l.size = 0

        // Try to chown the new file to match the old file's owner/group (if there was an old file).
        if oldInfo != nil {
                if errChown := chown(name, oldInfo); errChown != nil {
                        fmt.Fprintf(os.Stderr, "timberjack: [%s] failed to chown new log file %s: %v\n", l.Filename, name, errChown)
                }
        }

        return nil
}

// shouldTimeRotate checks if the time-based rotation interval has elapsed
// since the last rotation. This is used for RotationInterval logic.
func (l *Logger) shouldTimeRotate() bool {
        l.resolveConfigLocked()
        if l.RotationInterval == 0 { // Time-based rotation (interval) is disabled
                return false
        }
        // If lastRotationTime is zero (e.g., logger just started, no writes/rotations yet),
        // then it's not yet time for an interval-based rotation.
        if l.lastRotationTime.IsZero() {
                return false
        }
        return l.resolvedTimeNow().Sub(l.lastRotationTime) >= l.RotationInterval
}

// backupName creates a new backup filename by inserting a timestamp and a rotation reason
// ("time" or "size") between the filename prefix and the extension.
// It uses the local time if requested (otherwise UTC).
func backupName(name string, local bool, reason string, t time.Time, fileTimeFormat string, appendTimeAfterExt bool) string {

        dir := filepath.Dir(name)
        filename := filepath.Base(name)
        ext := filepath.Ext(filename)
        prefix := filename[:len(filename)-len(ext)]

        currentLoc := time.UTC
        if local {
                currentLoc = time.Local
        }
        // Format the timestamp for the backup file.
        timestamp := t.In(currentLoc).Format(fileTimeFormat)

        if appendTimeAfterExt {
                // <name><ext>-<ts>-<reason>
                // e.g. httpd.log-2025-01-01T00-00-00.000-size
                return filepath.Join(dir, fmt.Sprintf("%s%s-%s-%s", prefix, ext, timestamp, reason))
        }

        // default: <name>-<ts>-<reason><ext>
        return filepath.Join(dir, fmt.Sprintf("%s-%s-%s%s", prefix, timestamp, reason, ext))
}

// openExistingOrNew opens the existing logfile if it exists and the current write
// would not cause it to exceed MaxSize. If the file does not exist, or if writing
// would exceed MaxSize, the current file is rotated (if it exists) and a new logfile is created.
// It expects l.mu to be held by the caller.
func (l *Logger) openExistingOrNew(writeLen int) error {
        l.resolveConfigLocked()
        l.mill() // Perform house-keeping for old logs (compression, deletion) first.

        filename := l.filename()
        info, err := l.resolvedStat(filename)
        if os.IsNotExist(err) {
                // File doesn't exist, so openNew is creating a new file.
                // The 'reason' passed to openNew here ("initial") won't affect a backup filename
                // as no backup (renaming) is happening.
                return l.openNew("initial")
        }
        if err != nil {
                return fmt.Errorf("error getting log file info: %s", err)
        }

        // Check if rotation is needed due to size before opening/appending.
        if info.Size()+int64(writeLen) >= l.max() {
                return l.rotate("size") // This rotation is explicitly due to "size"
        }

        // Open existing file for appending.
        file, err := os.OpenFile(filename, os.O_APPEND|os.O_WRONLY, 0644) // Mode 0644 is common for append.
        if err != nil {
                // If opening existing fails (e.g., permissions, corruption), try to create a new one.
                return l.openNew("initial") // Fallback if append fails
        }
        l.file = file
        l.size = info.Size()
        // Note: l.logStartTime is NOT updated here if we successfully open an existing file without rotating.
        // It retains its value from when this current log segment was created (by a previous openNew).
        // l.lastRotationTime is also NOT updated here; it's handled by rotation trigger logic.
        return nil
}

// filename returns the current log filename, using the configured Filename,
// or a default based on the process name if Filename is empty.
func (l *Logger) filename() string {
        if l.Filename != "" {
                return l.Filename
        }
        name := filepath.Base(os.Args[0]) + "-timberjack.log"
        return filepath.Join(os.TempDir(), name)
}

// millRunOnce performs one cycle of compression and removal of old log files.
// If compression is enabled, uncompressed backups are compressed using gzip.
// Old backup files are deleted to enforce MaxBackups and MaxAge limits.
func (l *Logger) millRunOnce() error {
        l.resolveConfigLocked()
        comp := l.resolvedCompression
        if l.MaxBackups == 0 && l.MaxAge == 0 && comp == "none" {
                return nil // Nothing to do if all cleanup options are disabled.
        }

        now := l.resolvedTimeNow()

        files, err := l.oldLogFiles() // Gets LogInfo structs, sorted newest first by timestamp
        if err != nil {
                return err
        }

        var filesToProcess = files  // Start with all found old log files
        var filesToRemove []logInfo // Accumulates files to be deleted

        // MaxBackups filtering: Keep files belonging to the MaxBackups newest distinct timestamps
        if l.MaxBackups > 0 {
                uniqueTimestamps := make([]time.Time, 0)
                timestampMap := make(map[time.Time]bool)
                for _, f := range filesToProcess { // filesToProcess is sorted newest first
                        if !timestampMap[f.timestamp] {
                                timestampMap[f.timestamp] = true
                                uniqueTimestamps = append(uniqueTimestamps, f.timestamp)
                        }
                }

                if len(uniqueTimestamps) > l.MaxBackups {
                        // Determine the set of timestamps to keep (the MaxBackups newest ones)
                        keptTimestampsSet := make(map[time.Time]bool)
                        for i := 0; i < l.MaxBackups; i++ {
                                keptTimestampsSet[uniqueTimestamps[i]] = true
                        }

                        var filteredFiles []logInfo // Files that pass this MaxBackups filter
                        for _, f := range filesToProcess {
                                if keptTimestampsSet[f.timestamp] {
                                        filteredFiles = append(filteredFiles, f)
                                } else {
                                        filesToRemove = append(filesToRemove, f) // Mark for removal
                                }
                        }
                        filesToProcess = filteredFiles // Update filesToProcess for subsequent filters
                }
                // If len(uniqueTimestamps) <= l.MaxBackups, all files pass this MaxBackups filter.
        }

        // MaxAge filtering (operates on files that passed MaxBackups filter)
        if l.MaxAge > 0 {
                diff := time.Duration(int64(24*time.Hour) * int64(l.MaxAge))
                cutoff := now.Add(-diff)
                var filteredFiles []logInfo // Files that pass this MaxAge filter
                for _, f := range filesToProcess {
                        if f.timestamp.Before(cutoff) {
                                // avoid duplicates
                                isAlreadyMarked := false
                                for _, rmf := range filesToRemove {
                                        if rmf.Name() == f.Name() {
                                                isAlreadyMarked = true
                                                break
                                        }
                                }
                                if !isAlreadyMarked {
                                        filesToRemove = append(filesToRemove, f)
                                }
                        } else {
                                filteredFiles = append(filteredFiles, f)
                        }
                }
                filesToProcess = filteredFiles
        }

        // Compression task identification (operates on files that passed MaxBackups and MaxAge)
        var filesToCompress []logInfo
        if comp != "none" {
                for _, f := range filesToProcess {
                        name := f.Name()
                        if strings.HasSuffix(name, compressSuffix) || strings.HasSuffix(name, zstdSuffix) {
                                continue // already compressed
                        }
                        isMarked := false
                        for _, rmf := range filesToRemove {
                                if rmf.Name() == name {
                                        isMarked = true
                                        break
                                }
                        }
                        if !isMarked {
                                filesToCompress = append(filesToCompress, f)
                        }
                }
        }

        // Execute removals (ensure unique removals)
        finalUniqueRemovals := make(map[string]logInfo)
        for _, f := range filesToRemove {
                finalUniqueRemovals[f.Name()] = f
        }
        for _, f := range finalUniqueRemovals {
                errRemove := l.resolvedRemove(filepath.Join(l.dir(), f.Name()))
                if errRemove != nil && !os.IsNotExist(errRemove) {
                        fmt.Fprintf(os.Stderr, "timberjack: [%s] failed to remove old log file %s: %v\n", l.Filename, f.Name(), errRemove)
                }
        }

        // Execute compressions (suffix also comes from snapshot via compressedSuffix)
        suffix := l.compressedSuffix()
        for _, f := range filesToCompress {
                fn := filepath.Join(l.dir(), f.Name())
                if errCompress := l.compressLogFile(fn, fn+suffix); errCompress != nil {
                        fmt.Fprintf(os.Stderr, "timberjack: [%s] failed to compress log file %s: %v\n", l.Filename, f.Name(), errCompress)
                }
        }
        return nil
}

// millRun runs in a goroutine to manage post-rotation compression and removal
// of old log files. It listens on millCh for signals to run millRunOnce.
func (l *Logger) millRun() {
        if l.millWGStarted {
                defer l.millWg.Done()
        }
        ch := l.millCh
        for range ch {
                _ = l.millRunOnce()
        }
}

// mill performs post-rotation compression and removal of stale log files,
// starting the mill goroutine if necessary and sending a signal to it.
func (l *Logger) mill() {
        if atomic.LoadUint32(&l.isClosed) == 1 {
                return
        }
        l.startMill.Do(func() {
                l.millWGStarted = true
                l.millCh = make(chan bool, 1)
                l.millWg.Add(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 their embedded timestamp (newest first).
func (l *Logger) oldLogFiles() ([]logInfo, error) {
        entries, err := os.ReadDir(l.dir()) // ReadDir is generally preferred over ReadFile for directory listings
        if err != nil {
                return nil, fmt.Errorf("can't read log file directory: %s", err)
        }
        var logFiles []logInfo

        prefix, ext := l.prefixAndExt() // Get prefix like "filename-" and original extension like ".log"

        for _, e := range entries {
                if e.IsDir() { // Skip directories
                        continue
                }
                name := e.Name()
                info, errInfo := e.Info() // Get FileInfo for modification time and other details
                if errInfo != nil {
                        // fmt.Fprintf(os.Stderr, "timberjack: failed to get FileInfo for %s: %v\n", name, errInfo)
                        continue // Skip files we can't stat
                }

                // Attempt to parse timestamp from filename (e.g., from "filename-timestamp-reason.log")
                if t, errTime := l.timeFromName(name, prefix, ext); errTime == nil {
                        logFiles = append(logFiles, logInfo{t, info})
                        continue
                }
                // Attempt to parse timestamp from compressed gzip filename (e.g., from "filename-timestamp-reason.log.gz")
                if t, errTime := l.timeFromName(name, prefix, ext+compressSuffix); errTime == nil {
                        logFiles = append(logFiles, logInfo{t, info})
                        continue
                }
                // Attempt to parse timestamp from compressed zstd filename (e.g., from "filename-timestamp-reason.log.zst")
                if t, errTime := l.timeFromName(name, prefix, ext+zstdSuffix); errTime == nil {
                        logFiles = append(logFiles, logInfo{t, info})
                        continue
                }
                // Files that don't match the expected backup pattern are ignored.
        }

        sort.Sort(byFormatTime(logFiles)) // Sorts newest first based on parsed timestamp
        return logFiles, nil
}

// timeFromName extracts the formatted timestamp from the backup filename.
// It expects filenames like "prefix-YYYY-MM-DDTHH-MM-SS.mmm-reason.ext" or "prefix.ext-YYYY-MM-DDTHH-MM-SS.mmm-reason[.gz]"
func (l *Logger) timeFromName(filename, prefix, ext string) (time.Time, error) {
        layout := l.resolvedBackupLayout
        if layout == "" {
                // defensive default if called very early
                layout = backupTimeFormat
        }
        loc := time.UTC
        if l.resolvedLocalTime {
                loc = time.Local
        }

        if !l.resolvedAppendAfterExt {

                // Keep legacy behavior for error messages to satisfy existing tests
                if !strings.HasPrefix(filename, prefix) {
                        return time.Time{}, errors.New("mismatched prefix")
                }
                if !strings.HasSuffix(filename, ext) {
                        return time.Time{}, errors.New("mismatched extension")
                }
                // "<prefix><timestamp>-<reason><ext>"
                trimmed := filename[len(prefix) : len(filename)-len(ext)]
                lastHyphenIdx := strings.LastIndex(trimmed, "-")
                if lastHyphenIdx == -1 {
                        return time.Time{}, fmt.Errorf("malformed backup filename: missing reason separator in %q", trimmed)
                }
                ts := trimmed[:lastHyphenIdx]
                return time.ParseInLocation(layout, ts, loc)
        }

        // After-ext parsing:
        // base is "<name><ext>" (e.g., "foo.log")
        base := prefix[:len(prefix)-1] + ext

        // Allow optional trailing compression suffix (".gz" or ".zst")
        nameNoComp := trimCompressionSuffix(filename)

        // nameNoComp must start with "<base>-"
        if !strings.HasPrefix(nameNoComp, base+"-") {
                return time.Time{}, fmt.Errorf("malformed backup filename: %q", filename)
        }

        // nameNoComp = "<base>-<timestamp>-<reason>"
        trimmed := nameNoComp[len(base)+1:]

        lastHyphenIdx := strings.LastIndex(trimmed, "-")
        if lastHyphenIdx == -1 {
                return time.Time{}, fmt.Errorf("malformed backup filename: %q", filename)
        }
        ts := trimmed[:lastHyphenIdx]
        return time.ParseInLocation(layout, ts, loc)
}

// max returns the maximum size in bytes of log files before rolling.
func (l *Logger) max() int64 {
        if l.MaxSize == 0 { // If MaxSize is 0, use default.
                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 (up to the extension, with a trailing dash for backups)
// and extension part from the Logger's filename.
// e.g., for "foo.log", returns "foo-", ".log"
func (l *Logger) prefixAndExt() (prefix, ext string) {
        filename := filepath.Base(l.filename())
        ext = filepath.Ext(filename)
        prefix = filename[:len(filename)-len(ext)] + "-" // Add dash as backup filenames include it after original prefix
        return prefix, ext
}

// countDigitsAfterDot returns the number of consecutive digit characters
// immediately following the first '.' in the input.
// It skips all characters before the '.' and stops counting at the first non-digit
// character after the '.'.

// Example: `prefix.0012304123suffix` would return 10
// Example: `prefix.0012304_middle_123_suffix` would return 7
func countDigitsAfterDot(layout string) int {
        for i, ch := range layout {
                if ch == '.' {
                        count := 0
                        for _, c := range layout[i+1:] {
                                if unicode.IsDigit(c) {
                                        count++
                                } else {
                                        break
                                }
                        }
                        return count
                }
        }
        return 0 // no '.' found or no digits after dot
}

// truncateFractional truncates time t to n fractional digits of seconds.
// n=0 → truncate to seconds, n=3 → milliseconds, n=6 → microseconds, etc.
func truncateFractional(t time.Time, n int) (time.Time, error) {
        if n < 0 || n > 9 {
                return time.Time{}, fmt.Errorf("unsupported fractional precision: %d", n)
        }

        // number of nanoseconds to keep
        factor := math.Pow10(9 - n) // e.g. for n=3, factor=10^(9-3)=1,000,000

        nanos := t.Nanosecond()
        truncatedNanos := int((int64(nanos) / int64(factor)) * int64(factor))

        return time.Date(
                t.Year(), t.Month(), t.Day(),
                t.Hour(), t.Minute(), t.Second(),
                truncatedNanos,
                t.Location(),
        ), nil
}

// compressLogFile compresses the given source log file (src) to a destination file (dst),
// removing the source file if compression is successful.
func (l *Logger) compressLogFile(src, dst string) error {
        srcFile, err := os.Open(src)
        if err != nil {
                return fmt.Errorf("failed to open source log file %s for compression: %v", src, err)
        }
        defer srcFile.Close()

        srcInfo, err := l.resolvedStat(src)
        if err != nil {
                return fmt.Errorf("failed to stat source log file %s: %v", src, err)
        }

        // Create or open the destination file for writing the compressed content
        dstFile, err := os.OpenFile(dst, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, srcInfo.Mode())
        if err != nil {
                return fmt.Errorf("failed to open destination compressed log file %s: %v", dst, err)
        }
        // No `defer dstFile.Close()` here, explicit closing in sequence is critical.

        var copyErr error // To capture error from io.Copy

        // Choose compression algorithm based on dst suffix
        // Default to gzip if no recognized suffix
        // This allows future extension to other algorithms by checking dst suffix
        if strings.HasSuffix(dst, zstdSuffix) {
                enc, err := zstd.NewWriter(dstFile)
                if err != nil { // Error creating zstd writer
                        _ = dstFile.Close() // Close dstFile before removing
                        _ = l.resolvedRemove(dst)
                        return fmt.Errorf("failed to init zstd writer for %s: %v", dst, err)
                }
                _, copyErr = io.Copy(enc, srcFile) // Copy data from source file to zstd writer
                closeErr := enc.Close()            // Close zstd writer to flush data
                if copyErr == nil && closeErr != nil {
                        copyErr = closeErr
                }
        } else {
                gz := gzip.NewWriter(dstFile)     // Default to gzip
                _, copyErr = io.Copy(gz, srcFile) // Copy data from source file to gzip writer
                closeErr := gz.Close()            // Close gzip writer to flush data
                if copyErr == nil && closeErr != nil {
                        copyErr = closeErr
                }
        }

        if copyErr != nil { // Error during copy or close
                _ = dstFile.Close()       // Try to close destination file
                _ = l.resolvedRemove(dst) // Try to remove potentially partial destination file
                return fmt.Errorf("failed to write compressed data to %s: %w", dst, copyErr)
        }

        if err := dstFile.Close(); err != nil { // Close destination file
                // Data is likely written and compressor closed successfully, but closing the file descriptor failed.
                // The destination file might still be valid on disk. We typically wouldn't remove dst here
                // as the data might be recoverable or fully written despite the close error.
                return fmt.Errorf("failed to close destination compressed file %s: %w", dst, err)
        }

        if errChown := chown(dst, srcInfo); errChown != nil { // Attempt to chown the destination file
                // Log the chown error, but don't make it a fatal error for the compression process itself,
                // as the compressed file is valid. The original source file will still be removed.
                fmt.Fprintf(os.Stderr, "timberjack: [%s] failed to chown compressed log file %s: %v (source %s)\n",
                        filepath.Base(src), dst, errChown, src)
                // Note: Depending on requirements, a chown failure could be considered critical.
                // For now, it's logged, and compression proceeds to remove the source.
        }

        // Finally, after successful compression and closing (and optional chown), remove the original source file.
        if err = l.resolvedRemove(src); err != nil {
                // This is a more significant error if the original isn't removed, as it might be re-processed.
                return fmt.Errorf("failed to remove original source log file %s after compression: %w", src, err)
        }
        return nil // Compression successful

}

// effectiveCompression returns "none" | "gzip" | "zstd".
// Rule: if Compression is set, it wins; if empty, fallback to legacy Compress.
// Unknown strings default to "none" (and warn once).
func (l *Logger) effectiveCompression() string {
        alg := strings.ToLower(strings.TrimSpace(l.Compression))
        switch alg {
        case "gzip", "zstd":
                return alg
        case "none", "":
                if alg == "" && l.Compress {
                        return "gzip"
                }
                return "none"
        default:
                fmt.Fprintf(os.Stderr, "timberjack: invalid compression %q — using none\n", alg)
                return "none"
        }
}

// compressedSuffix returns ".gz" / ".zst" or "" if none.
func (l *Logger) compressedSuffix() string {
        switch l.resolvedCompression {
        case "gzip":
                return compressSuffix
        case "zstd":
                return zstdSuffix
        default:
                return ""
        }
}

// trimCompressionSuffix strips one known compression suffix (".gz" or ".zst").
func trimCompressionSuffix(name string) string {
        name = strings.TrimSuffix(name, compressSuffix)
        name = strings.TrimSuffix(name, zstdSuffix)
        return name
}

// sanitizeReason turns an arbitrary string into a safe, short tag for filenames.
// Allowed: [a-z0-9_-]. Everything else becomes '-'. Collapses repeats, trims edges.
// Returns empty string if nothing usable remains.
func sanitizeReason(s string) string {
        s = strings.TrimSpace(strings.ToLower(s))
        if s == "" {
                return ""
        }
        const max = 32

        var b strings.Builder
        lastDash := false
        for _, r := range s {
                ok := (r >= 'a' && r <= 'z') || (r >= '0' && r <= '9') || r == '-' || r == '_'
                if ok {
                        b.WriteRune(r)
                        lastDash = (r == '-')
                } else {
                        // replace anything else (including whitespace) with a single '-'
                        if !lastDash && b.Len() > 0 {
                                b.WriteByte('-')
                                lastDash = true
                        }
                }
                if b.Len() >= max {
                        break
                }
        }
        out := strings.Trim(b.String(), "-_")
        return out
}

// logInfo is a convenience struct to return the filename and its embedded
// timestamp, along with its os.FileInfo.
type logInfo struct {
        timestamp   time.Time // Parsed timestamp from the filename
        os.FileInfo           // Full FileInfo
}

// byFormatTime sorts a slice of logInfo structs by their parsed timestamp in descending order (newest first).
type byFormatTime []logInfo

func (b byFormatTime) Less(i, j int) bool {
        // Handle cases where timestamps might be zero (e.g., parsing failed, though timeFromName should error out)
        if b[i].timestamp.IsZero() && !b[j].timestamp.IsZero() {
                return false
        } // Treat zero time as oldest
        if !b[i].timestamp.IsZero() && b[j].timestamp.IsZero() {
                return true
        } // Non-zero is newer than zero
        if b[i].timestamp.IsZero() && b[j].timestamp.IsZero() {
                return false
        } // Equal if both are zero (order doesn't matter)
        return b[i].timestamp.After(b[j].timestamp) // Sort newest first
}
func (b byFormatTime) Swap(i, j int) { b[i], b[j] = b[j], b[i] }
func (b byFormatTime) Len() int      { return len(b) }

DeRuina / timberjack / 18554165489

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