// Package cron reads every scheduled job on the host into one list: /etc/crontab,
// /etc/cron.d/*, per-user crontabs, the run-parts directories they drive, and systemd
// timers. It computes next runs from the cron expressions, pulls last runs from the
// journal (cron logs "(user) CMD (command)" via syslog), and can toggle, edit and run
// jobs. Reads/writes that need root go through passwordless sudo when available.
package cron

import (
	"bufio"
	"bytes"
	"context"
	"crypto/sha1"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"os/exec"
	"os/user"
	"path/filepath"
	"regexp"
	"sort"
	"strconv"
	"strings"
	"sync"
	"time"
)

type Job struct {
	ID       string `json:"id"`
	Source   string `json:"source"` // "/etc/crontab", "/etc/cron.d/x", "crontab:user", "/etc/cron.daily"
	Line     int    `json:"line"`   // 1-based line in Source (0 for run-parts scripts)
	User     string `json:"user"`
	Schedule string `json:"schedule"`
	Command  string `json:"command"`
	Enabled  bool   `json:"enabled"`
	Next     int64  `json:"next,omitempty"` // unix ms
	Last     int64  `json:"last,omitempty"`
	OutFile  string `json:"out_file,omitempty"` // parsed from a ">> file" redirect
	Note     string `json:"note,omitempty"`
	Editable bool   `json:"editable"`
	raw      string
}

type Timer struct {
	Unit        string `json:"unit"`
	Activates   string `json:"activates"`
	Description string `json:"description"`
	Active      string `json:"active"`
	Sub         string `json:"sub"`
	Enabled     string `json:"enabled"`
	Next        int64  `json:"next,omitempty"`
	Last        int64  `json:"last,omitempty"`
}

type Overview struct {
	Jobs    []Job   `json:"jobs"`
	Timers  []Timer `json:"timers"`
	Logging bool    `json:"logging"` // cron lines seen in the journal
	Anacron bool    `json:"anacron"`
	Sudo    bool    `json:"sudo"`
}

type Run struct {
	ID       string    `json:"id"`
	JobID    string    `json:"job_id"`
	Command  string    `json:"command"`
	User     string    `json:"user"`
	Started  time.Time `json:"started"`
	Finished time.Time `json:"finished,omitempty"`
	Running  bool      `json:"running"`
	ExitCode int       `json:"exit_code"`
	Output   string    `json:"output"`
	mu       sync.Mutex
	buf      bytes.Buffer
}

type Manager struct {
	sudo        bool
	journalSudo bool
	self        string
	mu          sync.Mutex
	lastRuns    map[string]int64 // user\x00command -> unix ms
	lastAt      time.Time
	logging     bool
	runs        map[string]*Run
}

func New(sudo, journalSudo bool) *Manager {
	m := &Manager{sudo: sudo, journalSudo: journalSudo, runs: map[string]*Run{}}
	if u, err := user.Current(); err == nil {
		m.self = u.Username
	}
	return m
}

// ---- helpers ----

func run(ctx context.Context, stdin string, args ...string) (string, error) {
	ctx, cancel := context.WithTimeout(ctx, 20*time.Second)
	defer cancel()
	cmd := exec.CommandContext(ctx, args[0], args[1:]...)
	if stdin != "" {
		cmd.Stdin = strings.NewReader(stdin)
	}
	var out, errb bytes.Buffer
	cmd.Stdout, cmd.Stderr = &out, &errb
	if err := cmd.Run(); err != nil {
		msg := strings.TrimSpace(errb.String())
		if msg == "" {
			msg = err.Error()
		}
		return out.String(), errors.New(msg)
	}
	return out.String(), nil
}

func (m *Manager) sudoArgs(args ...string) []string {
	if m.sudo {
		return append([]string{"sudo", "-n"}, args...)
	}
	return args
}

func jobID(source, raw string) string {
	h := sha1.Sum([]byte(source + "\n" + raw))
	return hex.EncodeToString(h[:6])
}

var userRe = regexp.MustCompile(`^[a-z_][a-z0-9_-]{0,31}$`)
var cronDName = regexp.MustCompile(`^[A-Za-z0-9_-]+$`) // cron ignores other names in /etc/cron.d

// ---- parsing ----

// parseLine splits one crontab line. withUser is true for system crontabs (6th field is
// the user). Returns ok=false for comments, blanks, env assignments and garbage.
func parseLine(line string, withUser bool) (schedule, usr, command string, ok bool) {
	t := strings.TrimSpace(line)
	if t == "" || strings.HasPrefix(t, "#") {
		return
	}
	if i := strings.Index(t, "="); i > 0 && !strings.ContainsAny(t[:i], " \t*") { // NAME=value
		return
	}
	var fields []string
	if strings.HasPrefix(t, "@") {
		f := strings.Fields(t)
		if len(f) < 2 {
			return
		}
		fields = []string{f[0]}
		t = strings.TrimSpace(t[len(f[0]):])
	} else {
		f := strings.Fields(t)
		if len(f) < 6 {
			return
		}
		fields = f[:5]
		for _, x := range fields {
			t = strings.TrimSpace(t[len(x):])
		}
	}
	schedule = strings.Join(fields, " ")
	if _, err := parseSchedule(schedule); err != nil {
		return
	}
	if withUser {
		f := strings.Fields(t)
		if len(f) < 2 || !userRe.MatchString(f[0]) {
			return
		}
		usr = f[0]
		t = strings.TrimSpace(t[len(f[0]):])
	}
	if t == "" {
		return
	}
	return schedule, usr, t, true
}

var redirectRe = regexp.MustCompile(`(?:^|\s)(?:>>|>|2>&1\s*>>|&>>|&>)\s*("?)([^\s"&|;]+)`)

func outFile(cmd string) string {
	for _, m := range redirectRe.FindAllStringSubmatch(cmd, -1) {
		if p := m[2]; strings.HasPrefix(p, "/") && p != "/dev/null" {
			return p
		}
	}
	return ""
}

// parseSource turns crontab text into jobs. Commented-out lines that still parse as a
// job are reported disabled, so a "# 0 5 * * * ..." example shows up as such.
func (m *Manager) parseSource(source, text string, withUser, editable bool, defaultUser string, now time.Time, anacron bool) []Job {
	var jobs []Job
	lines := strings.Split(text, "\n")
	isComment := func(i int) bool {
		return i >= 0 && i < len(lines) && strings.HasPrefix(strings.TrimSpace(lines[i]), "#")
	}
	for i, line := range lines {
		enabled := true
		probe := line
		if t := strings.TrimSpace(line); strings.HasPrefix(t, "#") {
			// A commented-out job sits on its own; a job-shaped line inside a block of
			// comments is documentation (Debian's stock header has two such examples).
			if isComment(i-1) && isComment(i+1) {
				continue
			}
			enabled = false
			probe = strings.TrimLeft(t, "# ")
		}
		schedule, usr, command, ok := parseLine(probe, withUser)
		if !ok {
			continue
		}
		if usr == "" {
			usr = defaultUser
		}
		if !enabled {
			if _, err := user.Lookup(usr); err != nil {
				continue // "user-name" placeholder and the like
			}
		}
		j := Job{ID: jobID(source, line), Source: source, Line: i + 1, User: usr, Schedule: schedule, Command: command,
			Enabled: enabled, OutFile: outFile(command), Editable: editable, raw: line}
		if enabled {
			if next, ok := nextRun(schedule, now); ok {
				j.Next = next.UnixMilli()
			}
		}
		if strings.Contains(command, "run-parts") {
			if dir := runPartsDir(command); dir != "" {
				if anacron && strings.Contains(command, "anacron") {
					j.Note = "skipped: anacron is installed and runs " + dir + " instead"
				}
				jobs = append(jobs, j)
				jobs = append(jobs, m.runPartsJobs(dir, j)...)
				continue
			}
		}
		jobs = append(jobs, j)
	}
	return jobs
}

var runPartsRe = regexp.MustCompile(`run-parts\s+(?:--\S+\s+)*(/\S+)`)

func runPartsDir(cmd string) string {
	if m := runPartsRe.FindStringSubmatch(cmd); m != nil {
		return m[1]
	}
	return ""
}

// runPartsJobs lists the executable scripts a run-parts line will run, as read-only jobs.
func (m *Manager) runPartsJobs(dir string, parent Job) []Job {
	ents, err := os.ReadDir(dir)
	if err != nil {
		return nil
	}
	var out []Job
	for _, e := range ents {
		if e.IsDir() || !cronDName.MatchString(e.Name()) {
			continue
		}
		info, err := e.Info()
		if err != nil || info.Mode()&0o111 == 0 {
			continue
		}
		p := filepath.Join(dir, e.Name())
		out = append(out, Job{ID: jobID(dir, e.Name()), Source: dir, User: parent.User, Schedule: parent.Schedule, Command: p,
			Enabled: parent.Enabled, Next: parent.Next, Note: "run by " + parent.Source + " via run-parts", raw: e.Name()})
	}
	return out
}

// ---- sources ----

type source struct {
	name     string // job Source
	withUser bool
	user     string // default user for user crontabs
	editable bool
}

func (m *Manager) readSource(ctx context.Context, s source) (string, error) {
	if strings.HasPrefix(s.name, "crontab:") {
		u := s.user
		if u == m.self {
			out, err := run(ctx, "", "crontab", "-l")
			if err != nil && strings.Contains(err.Error(), "no crontab") {
				return "", nil
			}
			return out, err
		}
		if !m.sudo {
			return "", errors.New("needs sudo")
		}
		out, err := run(ctx, "", "sudo", "-n", "crontab", "-u", u, "-l")
		if err != nil && strings.Contains(err.Error(), "no crontab") {
			return "", nil
		}
		return out, err
	}
	b, err := os.ReadFile(s.name)
	if err != nil && m.sudo {
		out, err2 := run(ctx, "", "sudo", "-n", "cat", s.name)
		return out, err2
	}
	return string(b), err
}

func (m *Manager) sources(ctx context.Context) []source {
	var out []source
	if _, err := os.Stat("/etc/crontab"); err == nil {
		out = append(out, source{name: "/etc/crontab", withUser: true, editable: m.sudo})
	}
	if ents, err := os.ReadDir("/etc/cron.d"); err == nil {
		for _, e := range ents {
			if !e.IsDir() && cronDName.MatchString(e.Name()) {
				out = append(out, source{name: "/etc/cron.d/" + e.Name(), withUser: true, editable: m.sudo})
			}
		}
	}
	// per-user crontabs: Debian keeps them in /var/spool/cron/crontabs, cronie in /var/spool/cron
	users := map[string]bool{}
	if m.sudo {
		for _, dir := range []string{"/var/spool/cron/crontabs", "/var/spool/cron"} {
			if out, err := run(ctx, "", "sudo", "-n", "ls", "-1", dir); err == nil {
				for _, n := range strings.Split(out, "\n") {
					if n = strings.TrimSpace(n); userRe.MatchString(n) {
						users[n] = true
					}
				}
				break
			}
		}
	}
	if m.self != "" {
		users[m.self] = true
	}
	names := make([]string, 0, len(users))
	for u := range users {
		names = append(names, u)
	}
	sort.Strings(names)
	for _, u := range names {
		out = append(out, source{name: "crontab:" + u, user: u, editable: u == m.self || m.sudo})
	}
	return out
}

func (m *Manager) findSource(ctx context.Context, name string) (source, bool) {
	for _, s := range m.sources(ctx) {
		if s.name == name {
			return s, true
		}
	}
	return source{}, false
}

// ---- overview ----

func (m *Manager) Overview(ctx context.Context) Overview {
	now := time.Now()
	_, anacronErr := os.Stat("/usr/sbin/anacron")
	anacron := anacronErr == nil
	ov := Overview{Jobs: []Job{}, Timers: []Timer{}, Anacron: anacron, Sudo: m.sudo}
	for _, s := range m.sources(ctx) {
		text, err := m.readSource(ctx, s)
		if err != nil {
			continue
		}
		ov.Jobs = append(ov.Jobs, m.parseSource(s.name, text, s.withUser, s.editable, s.user, now, anacron)...)
	}
	last := m.lastRunsFromJournal(ctx)
	ov.Logging = m.logging
	for i := range ov.Jobs {
		j := &ov.Jobs[i]
		if t, ok := last[j.User+"\x00"+j.Command]; ok {
			j.Last = t
		}
	}
	ov.Timers = m.timers(ctx)
	return ov
}

var cronLogRe = regexp.MustCompile(`^\((\S+)\) CMD \((.*)\)$`)

// lastRunsFromJournal maps user+command to the most recent "(user) CMD (command)" line.
func (m *Manager) lastRunsFromJournal(ctx context.Context) map[string]int64 {
	m.mu.Lock()
	if time.Since(m.lastAt) < 5*time.Second && m.lastRuns != nil {
		r := m.lastRuns
		m.mu.Unlock()
		return r
	}
	m.mu.Unlock()
	args := []string{"journalctl", "-t", "CRON", "-t", "cron", "-t", "crond", "-o", "json", "--no-pager", "-q", "--since", "-30d", "-n", "20000"}
	if m.journalSudo {
		args = append([]string{"sudo", "-n"}, args...)
	}
	out, _ := run(ctx, "", args...)
	res := map[string]int64{}
	seen := false
	sc := bufio.NewScanner(strings.NewReader(out))
	sc.Buffer(make([]byte, 0, 64<<10), 4<<20)
	for sc.Scan() {
		var raw struct {
			Msg any    `json:"MESSAGE"`
			TS  string `json:"__REALTIME_TIMESTAMP"`
		}
		if json.Unmarshal(sc.Bytes(), &raw) != nil {
			continue
		}
		seen = true
		msg, _ := raw.Msg.(string)
		mm := cronLogRe.FindStringSubmatch(msg)
		if mm == nil {
			continue
		}
		us, _ := strconv.ParseInt(raw.TS, 10, 64)
		k := mm[1] + "\x00" + mm[2]
		if us/1000 > res[k] {
			res[k] = us / 1000
		}
	}
	m.mu.Lock()
	m.lastRuns, m.lastAt, m.logging = res, time.Now(), seen
	m.mu.Unlock()
	return res
}

// JobLogs returns the journal lines for one job's command (most recent last).
func (m *Manager) JobLogs(ctx context.Context, j Job, n int) ([]map[string]any, error) {
	args := []string{"journalctl", "-t", "CRON", "-t", "cron", "-t", "crond", "-o", "json", "--no-pager", "-q", "-n", "20000"}
	if m.journalSudo {
		args = append([]string{"sudo", "-n"}, args...)
	}
	out, err := run(ctx, "", args...)
	if err != nil {
		return nil, err
	}
	var res []map[string]any
	sc := bufio.NewScanner(strings.NewReader(out))
	sc.Buffer(make([]byte, 0, 64<<10), 4<<20)
	for sc.Scan() {
		var raw struct {
			Msg any    `json:"MESSAGE"`
			TS  string `json:"__REALTIME_TIMESTAMP"`
			PID string `json:"_PID"`
		}
		if json.Unmarshal(sc.Bytes(), &raw) != nil {
			continue
		}
		msg, _ := raw.Msg.(string)
		if !strings.Contains(msg, "("+j.User+") CMD ("+j.Command+")") {
			continue
		}
		us, _ := strconv.ParseInt(raw.TS, 10, 64)
		res = append(res, map[string]any{"t": us / 1000, "p": 6, "ident": "CRON", "pid": raw.PID, "msg": msg})
	}
	if len(res) > n {
		res = res[len(res)-n:]
	}
	if res == nil {
		res = []map[string]any{}
	}
	return res, nil
}

// OutputTail returns the last n lines of the job's redirect target.
func (m *Manager) OutputTail(ctx context.Context, j Job, n int) (string, error) {
	if j.OutFile == "" {
		return "", errors.New("job has no output file")
	}
	out, err := run(ctx, "", m.sudoArgs("tail", "-n", strconv.Itoa(n), j.OutFile)...)
	return out, err
}

// ---- timers ----

func (m *Manager) timers(ctx context.Context) []Timer {
	out, err := run(ctx, "", "systemctl", "list-timers", "--all", "--output=json", "--no-pager")
	if err != nil {
		return []Timer{}
	}
	var lt []struct {
		Next      int64  `json:"next"`
		Last      int64  `json:"last"`
		Unit      string `json:"unit"`
		Activates string `json:"activates"`
	}
	_ = json.Unmarshal([]byte(out), &lt)
	units, _ := run(ctx, "", "systemctl", "list-units", "--type=timer", "--all", "--output=json", "--no-pager", "--plain")
	var lu []struct{ Unit, Active, Sub, Description string }
	_ = json.Unmarshal([]byte(units), &lu)
	meta := map[string]struct{ Active, Sub, Description string }{}
	for _, u := range lu {
		meta[u.Unit] = struct{ Active, Sub, Description string }{u.Active, u.Sub, u.Description}
	}
	files, _ := run(ctx, "", "systemctl", "list-unit-files", "--type=timer", "--output=json", "--no-pager")
	var uf []struct {
		UnitFile string `json:"unit_file"`
		State    string `json:"state"`
	}
	_ = json.Unmarshal([]byte(files), &uf)
	enabled := map[string]string{}
	for _, f := range uf {
		enabled[f.UnitFile] = f.State
	}
	res := make([]Timer, 0, len(lt))
	for _, t := range lt {
		x := Timer{Unit: t.Unit, Activates: t.Activates, Enabled: enabled[t.Unit]}
		if mt, ok := meta[t.Unit]; ok {
			x.Active, x.Sub, x.Description = mt.Active, mt.Sub, mt.Description
		}
		if t.Next > 0 {
			x.Next = t.Next / 1000
		}
		if t.Last > 0 {
			x.Last = t.Last / 1000
		}
		res = append(res, x)
	}
	sort.Slice(res, func(i, j int) bool { return res[i].Unit < res[j].Unit })
	return res
}

var timerRe = regexp.MustCompile(`^[A-Za-z0-9][A-Za-z0-9@._:\\-]{0,255}\.timer$`)

func (m *Manager) TimerAction(ctx context.Context, unit, action string) error {
	if !timerRe.MatchString(unit) {
		return errors.New("invalid timer name")
	}
	switch action {
	case "start", "stop", "enable", "disable":
		_, err := run(ctx, "", m.sudoArgs("systemctl", action, unit)...)
		return err
	case "run": // fire the service the timer activates, right now
		out, err := run(ctx, "", "systemctl", "show", unit, "-p", "Triggers", "--value")
		if err != nil {
			return err
		}
		svc := strings.TrimSpace(strings.Split(out, " ")[0])
		if svc == "" {
			return errors.New("timer triggers nothing")
		}
		_, err = run(ctx, "", m.sudoArgs("systemctl", "start", svc)...)
		return err
	}
	return errors.New("unknown action")
}

// ---- job lookup and editing ----

func (m *Manager) FindJob(ctx context.Context, id string) (Job, bool) {
	for _, j := range m.Overview(ctx).Jobs {
		if j.ID == id {
			return j, true
		}
	}
	return Job{}, false
}

// Source returns a crontab's raw text for the editor.
func (m *Manager) Source(ctx context.Context, name string) (text string, editable bool, err error) {
	s, ok := m.findSource(ctx, name)
	if !ok {
		return "", false, errors.New("unknown source")
	}
	text, err = m.readSource(ctx, s)
	return text, s.editable, err
}

// validate rejects any line that is neither blank, comment, env assignment, nor a job.
func validate(text string, withUser bool) error {
	for i, line := range strings.Split(text, "\n") {
		t := strings.TrimSpace(line)
		if t == "" || strings.HasPrefix(t, "#") {
			continue
		}
		if k, _, ok := strings.Cut(t, "="); ok && !strings.ContainsAny(strings.TrimSpace(k), " \t*") {
			continue
		}
		if _, _, _, ok := parseLine(t, withUser); !ok {
			return fmt.Errorf("line %d: not a valid cron entry", i+1)
		}
	}
	return nil
}

// WriteSource replaces a crontab. User crontabs go through crontab(1) so cron reloads
// them; system files are written in place with their permissions preserved.
func (m *Manager) WriteSource(ctx context.Context, name, text string) error {
	s, ok := m.findSource(ctx, name)
	if !ok {
		return errors.New("unknown source")
	}
	if !s.editable {
		return errors.New("source is read-only here (no sudo)")
	}
	if !strings.HasSuffix(text, "\n") {
		text += "\n"
	}
	if err := validate(text, s.withUser); err != nil {
		return err
	}
	if strings.HasPrefix(name, "crontab:") {
		if s.user == m.self {
			_, err := run(ctx, text, "crontab", "-")
			return err
		}
		_, err := run(ctx, text, "sudo", "-n", "crontab", "-u", s.user, "-")
		return err
	}
	if name != "/etc/crontab" && !strings.HasPrefix(name, "/etc/cron.d/") {
		return errors.New("refusing to write outside /etc/crontab and /etc/cron.d")
	}
	_, err := run(ctx, text, "sudo", "-n", "tee", name)
	m.mu.Lock()
	m.lastAt = time.Time{}
	m.mu.Unlock()
	return err
}

// Toggle comments or uncomments a job's line. The line must still read as the job did
// when it was listed, so a concurrent edit can't be clobbered.
func (m *Manager) Toggle(ctx context.Context, j Job, enable bool) error {
	if j.Line == 0 || !j.Editable {
		return errors.New("this job can't be toggled here")
	}
	s, ok := m.findSource(ctx, j.Source)
	if !ok {
		return errors.New("unknown source")
	}
	text, err := m.readSource(ctx, s)
	if err != nil {
		return err
	}
	lines := strings.Split(text, "\n")
	if j.Line > len(lines) || lines[j.Line-1] != j.raw {
		return errors.New("crontab changed underneath; reload and try again")
	}
	if enable {
		lines[j.Line-1] = strings.TrimLeft(strings.TrimSpace(j.raw), "# ")
	} else {
		lines[j.Line-1] = "#" + j.raw
	}
	return m.WriteSource(ctx, j.Source, strings.Join(lines, "\n"))
}

// ---- run now ----

// StartRun executes the job's command as its user, the way cron would (sh -c, cwd = home,
// '%' turned into newlines/stdin), in the background. Output is kept in memory.
func (m *Manager) StartRun(ctx context.Context, j Job) (*Run, error) {
	if j.User != m.self && !m.sudo {
		return nil, errors.New("needs sudo to run as " + j.User)
	}
	cmdline, stdin := cronPercent(j.Command)
	args := []string{"sh", "-c", cmdline}
	if j.User != m.self {
		args = append([]string{"sudo", "-n", "-u", j.User, "-H"}, args...)
	}
	r := &Run{ID: jobID("run", strconv.FormatInt(time.Now().UnixNano(), 10)), JobID: j.ID, Command: j.Command, User: j.User, Started: time.Now(), Running: true}
	runCtx, cancel := context.WithTimeout(context.Background(), 30*time.Minute)
	cmd := exec.CommandContext(runCtx, args[0], args[1:]...)
	if stdin != "" {
		cmd.Stdin = strings.NewReader(stdin)
	}
	w := &capWriter{r: r}
	cmd.Stdout, cmd.Stderr = w, w
	if err := cmd.Start(); err != nil {
		cancel()
		return nil, err
	}
	m.mu.Lock()
	m.runs[r.ID] = r
	for id, old := range m.runs { // keep the map small
		if !old.Running && time.Since(old.Finished) > time.Hour {
			delete(m.runs, id)
		}
	}
	m.mu.Unlock()
	go func() {
		defer cancel()
		err := cmd.Wait()
		r.mu.Lock()
		r.Running, r.Finished = false, time.Now()
		if err != nil {
			if ee, ok := err.(*exec.ExitError); ok {
				r.ExitCode = ee.ExitCode()
			} else {
				r.ExitCode = -1
			}
		}
		r.mu.Unlock()
		m.mu.Lock()
		m.lastAt = time.Time{}
		m.mu.Unlock()
	}()
	return r, nil
}

func (m *Manager) GetRun(id string) (*Run, bool) {
	m.mu.Lock()
	r, ok := m.runs[id]
	m.mu.Unlock()
	return r, ok
}

// Snapshot returns a copy safe to encode.
func (r *Run) Snapshot() Run {
	r.mu.Lock()
	defer r.mu.Unlock()
	return Run{ID: r.ID, JobID: r.JobID, Command: r.Command, User: r.User, Started: r.Started, Finished: r.Finished,
		Running: r.Running, ExitCode: r.ExitCode, Output: r.buf.String()}
}

type capWriter struct{ r *Run }

func (w *capWriter) Write(p []byte) (int, error) {
	w.r.mu.Lock()
	defer w.r.mu.Unlock()
	w.r.buf.Write(p)
	if w.r.buf.Len() > 256<<10 { // keep the tail
		b := w.r.buf.Bytes()
		w.r.buf = *bytes.NewBuffer(append([]byte("…\n"), b[len(b)-200<<10:]...))
	}
	return len(p), nil
}

// cronPercent applies crontab(5) rules: an unescaped % ends the command; later ones are
// newlines fed to stdin. "\%" is a literal percent.
func cronPercent(cmd string) (string, string) {
	var out, in strings.Builder
	cur := &out
	esc := false
	for _, c := range cmd {
		switch {
		case esc:
			if c != '%' {
				cur.WriteRune('\\')
			}
			cur.WriteRune(c)
			esc = false
		case c == '\\':
			esc = true
		case c == '%':
			if cur == &out {
				cur = &in
			} else {
				in.WriteRune('\n')
			}
		default:
			cur.WriteRune(c)
		}
	}
	return out.String(), in.String()
}