// Package runit generates and manages runit service directories.
//
// Each unit is a directory /etc/sv/<name> (run, finish) linked into the
// supervised service dir. Services run at system level (root only). Units with
// a user run as that user through chpst, with HOME and XDG_RUNTIME_DIR set.
//
// Install links the service with a `down` file: it is supervised but does not
// start until started or enabled. Enable removes `down`, Disable restores it.
package runit

import (
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
	"time"

	"github.com/warkanum/unitdore/config"
	"github.com/warkanum/unitdore/service"
)

// Overridable for tests.
var (
	svDir = "/etc/sv"

	// serviceDirCandidates are probed in order when no service_dir is set.
	serviceDirCandidates = []string{"/var/service", "/etc/service"}

	// superviseWait is how long Start waits for runsvdir to pick up a new link.
	superviseWait = 10 * time.Second

	runCmd = func(name string, args ...string) error {
		cmd := exec.Command(name, args...)
		cmd.Stdout = os.Stdout
		cmd.Stderr = os.Stderr
		return cmd.Run()
	}
	outCmd = func(name string, args ...string) ([]byte, error) {
		return exec.Command(name, args...).CombinedOutput()
	}
)

// Backend is the runit implementation of the init interface.
type Backend struct {
	ServiceDir string // supervised dir; empty = autodetect
}

func (Backend) Name() string { return "runit" }

// SupportsLinger is false: runit services need no linger.
func (Backend) SupportsLinger() bool { return false }

func (Backend) ServiceName(u config.Unit, prefix, suffix string) string {
	return service.BaseName(u, prefix, suffix)
}

// UnitPath is the service definition directory under /etc/sv.
func (b Backend) UnitPath(u config.Unit, prefix, suffix string) (string, error) {
	return filepath.Join(svDir, b.ServiceName(u, prefix, suffix)), nil
}

func (b Backend) IsInstalled(u config.Unit, prefix, suffix string) bool {
	dir, _ := b.UnitPath(u, prefix, suffix)
	_, err := os.Stat(filepath.Join(dir, "run"))
	return err == nil
}

// serviceDir returns the supervised dir that gets the symlinks.
func (b Backend) serviceDir() string {
	if b.ServiceDir != "" {
		return b.ServiceDir
	}
	for _, d := range serviceDirCandidates {
		if fi, err := os.Stat(d); err == nil && fi.IsDir() {
			return d
		}
	}
	return serviceDirCandidates[0]
}

func (b Backend) link(u config.Unit, prefix, suffix string) string {
	return filepath.Join(b.serviceDir(), b.ServiceName(u, prefix, suffix))
}

type file struct {
	name    string
	content string
}

// Generate produces the run and finish scripts, separated by marker lines.
func (b Backend) Generate(u config.Unit, prefix, suffix string) (string, error) {
	files, err := b.files(u, prefix, suffix)
	if err != nil {
		return "", err
	}
	dir, _ := b.UnitPath(u, prefix, suffix)
	var w strings.Builder
	for _, f := range files {
		fmt.Fprintf(&w, "# ===== %s =====\n%s\n", filepath.Join(dir, f.name), f.content)
	}
	return w.String(), nil
}

func (b Backend) files(u config.Unit, prefix, suffix string) ([]file, error) {
	if err := service.Validate(u); err != nil {
		return nil, err
	}
	user, err := service.LookupUser(u.User)
	if err != nil {
		return nil, err
	}
	spec := service.Resolve(u)

	start, err := service.ShellCommand(spec.Start)
	if err != nil {
		return nil, fmt.Errorf("unit %s: %w", u.Name, err)
	}
	stop := ""
	if spec.Stop != "" {
		if stop, err = service.ShellCommand(spec.Stop); err != nil {
			return nil, fmt.Errorf("unit %s: %w", u.Name, err)
		}
	}

	header := "#!/bin/sh\n# " + b.ServiceName(u, prefix, suffix) + " — generated by unitdore, do not edit manually\n"

	run := header + "exec 2>&1\n" + b.waitFor(u, prefix, suffix) + preamble(user, spec.WorkingDir)
	if spec.Type == service.TypeOneshot {
		// Run once, then stay up (like RemainAfterExit) until stopped
		run += asUser(user, start) + " || exit 1\nexec tail -f /dev/null\n"
	} else {
		run += "exec " + asUser(user, start) + "\n"
	}

	files := []file{{"run", run}}
	if fin := finishScript(header, u, user, spec.WorkingDir, stop); fin != "" {
		files = append(files, file{"finish", fin})
	}
	return files, nil
}

// predecessorWait is how long run waits for each unit of the previous group.
const predecessorWait = 30

// waitFor holds the service until the previous order group is up, then sleeps
// the unit's delay. `sv check` never starts a service, and returns at once for
// one that is down or not installed.
func (b Backend) waitFor(u config.Unit, prefix, suffix string) string {
	var s string
	for _, n := range u.After {
		link := b.link(config.Unit{Name: n}, prefix, suffix)
		s += fmt.Sprintf("sv -w %d check %s >/dev/null 2>&1 || true\n", predecessorWait, service.ShellQuote(link))
	}
	if secs := int((u.StartDelay + time.Second - 1) / time.Second); secs > 0 {
		s += fmt.Sprintf("sleep %d\n", secs)
	}
	return s
}

// preamble prepares the environment: working dir and rootless user runtime.
func preamble(user *service.UserEnv, workDir string) string {
	var s string
	if workDir != "" {
		s += "cd " + service.ShellQuote(workDir) + " || exit 1\n"
	}
	if user != nil {
		s += fmt.Sprintf("[ -d %[1]s ] || install -d -m 0700 -o %[2]s %[1]s\n",
			service.ShellQuote(user.RuntimeDir), service.ShellQuote(user.Name))
		s += "export HOME=" + service.ShellQuote(user.Home) + "\n"
		s += "export XDG_RUNTIME_DIR=" + service.ShellQuote(user.RuntimeDir) + "\n"
	}
	return s
}

func asUser(user *service.UserEnv, cmd string) string {
	if user == nil {
		return cmd
	}
	return "chpst -u " + service.ShellQuote(user.Name) + " " + cmd
}

// finishScript runs after the service exits: best-effort stop command, then
// the restart policy. runit restarts by default, so no-restart stops it.
func finishScript(header string, u config.Unit, user *service.UserEnv, workDir, stop string) string {
	var body string
	if workDir != "" {
		body += "cd " + service.ShellQuote(workDir) + " 2>/dev/null\n"
	}
	if stop != "" {
		body += asUser(user, stop) + " || true\n"
	}
	switch {
	case !u.Restart:
		body += "exec sv down .\n"
	default:
		// Restart=on-failure: a clean exit stays down
		body += "[ \"$1\" = 0 ] && exec sv down .\n"
		if u.RestartSec > 0 {
			body += fmt.Sprintf("sleep %d\n", u.RestartSec)
		}
	}
	return header + body
}

func (b Backend) Install(u config.Unit, prefix, suffix string) error {
	if err := service.RequireRoot("runit"); err != nil {
		return err
	}
	files, err := b.files(u, prefix, suffix)
	if err != nil {
		return err
	}
	dir, _ := b.UnitPath(u, prefix, suffix)
	link := b.link(u, prefix, suffix)

	// A re-install keeps an already enabled service enabled
	wasEnabled := isLinked(link, dir) && !exists(filepath.Join(dir, "down"))

	if err := os.MkdirAll(dir, 0755); err != nil {
		return fmt.Errorf("creating service dir: %w", err)
	}
	for _, f := range files {
		if err := os.WriteFile(filepath.Join(dir, f.name), []byte(f.content), 0755); err != nil {
			return fmt.Errorf("writing %s: %w", f.name, err)
		}
	}
	if !hasFile(files, "finish") {
		_ = os.Remove(filepath.Join(dir, "finish"))
	}
	if !wasEnabled {
		if err := setDown(dir, true); err != nil {
			return err
		}
	}
	return ensureLink(dir, link)
}

func (b Backend) Uninstall(u config.Unit, prefix, suffix string) error {
	if err := service.RequireRoot("runit"); err != nil {
		return err
	}
	dir, _ := b.UnitPath(u, prefix, suffix)
	link := b.link(u, prefix, suffix)

	_ = runCmd("sv", "stop", link) // not running is fine
	if err := os.Remove(link); err != nil && !os.IsNotExist(err) {
		return fmt.Errorf("removing service link: %w", err)
	}
	if err := os.RemoveAll(dir); err != nil {
		return fmt.Errorf("removing service dir: %w", err)
	}
	return nil
}

// Enable makes the service start with runsvdir and starts it now.
func (b Backend) Enable(u config.Unit, prefix, suffix string) error {
	if err := service.RequireRoot("runit"); err != nil {
		return err
	}
	dir, _ := b.UnitPath(u, prefix, suffix)
	link := b.link(u, prefix, suffix)
	if err := setDown(dir, false); err != nil {
		return err
	}
	if err := ensureLink(dir, link); err != nil {
		return err
	}
	return startLink(link)
}

// Disable stops the service and keeps it from starting with runsvdir.
func (b Backend) Disable(u config.Unit, prefix, suffix string) error {
	if err := service.RequireRoot("runit"); err != nil {
		return err
	}
	dir, _ := b.UnitPath(u, prefix, suffix)
	if err := setDown(dir, true); err != nil {
		return err
	}
	_ = runCmd("sv", "stop", b.link(u, prefix, suffix)) // not running is fine
	return nil
}

func (b Backend) Start(u config.Unit, prefix, suffix string) error {
	if err := service.RequireRoot("runit"); err != nil {
		return err
	}
	dir, _ := b.UnitPath(u, prefix, suffix)
	link := b.link(u, prefix, suffix)
	if err := ensureLink(dir, link); err != nil {
		return err
	}
	return startLink(link)
}

func (b Backend) Stop(u config.Unit, prefix, suffix string) error {
	if err := service.RequireRoot("runit"); err != nil {
		return err
	}
	return runCmd("sv", "stop", b.link(u, prefix, suffix))
}

func (b Backend) Status(u config.Unit, prefix, suffix string) string {
	out, _ := outCmd("sv", "status", b.link(u, prefix, suffix))
	return parseStatus(string(out))
}

// parseStatus maps `sv status` output to a systemd-like state.
func parseStatus(out string) string {
	out = strings.TrimSpace(out)
	switch {
	case strings.HasPrefix(out, "run:"):
		return "active"
	case strings.HasPrefix(out, "down:"):
		return "inactive"
	case strings.HasPrefix(out, "finish:"):
		return "deactivating"
	case strings.Contains(out, "runsv not running"):
		return "inactive"
	default:
		return "unknown"
	}
}

// startLink waits until runsvdir supervises the link, then starts it.
func startLink(link string) error {
	deadline := time.Now().Add(superviseWait)
	for !exists(filepath.Join(link, "supervise", "ok")) {
		if time.Now().After(deadline) {
			return fmt.Errorf("%s is not supervised yet (is runsvdir running?)", link)
		}
		time.Sleep(200 * time.Millisecond)
	}
	return runCmd("sv", "start", link)
}

func ensureLink(dir, link string) error {
	if isLinked(link, dir) {
		return nil
	}
	if err := os.MkdirAll(filepath.Dir(link), 0755); err != nil {
		return fmt.Errorf("creating service dir: %w", err)
	}
	if err := os.Symlink(dir, link); err != nil {
		return fmt.Errorf("linking service: %w", err)
	}
	return nil
}

func isLinked(link, dir string) bool {
	target, err := os.Readlink(link)
	return err == nil && target == dir
}

func setDown(dir string, down bool) error {
	path := filepath.Join(dir, "down")
	if !down {
		if err := os.Remove(path); err != nil && !os.IsNotExist(err) {
			return fmt.Errorf("removing down file: %w", err)
		}
		return nil
	}
	if err := os.WriteFile(path, nil, 0644); err != nil {
		return fmt.Errorf("writing down file: %w", err)
	}
	return nil
}

func exists(path string) bool {
	_, err := os.Stat(path)
	return err == nil
}

func hasFile(files []file, name string) bool {
	for _, f := range files {
		if f.name == name {
			return true
		}
	}
	return false
}