//go:build windows

// Package service integrates gpu-turnstile with the Windows Service
// Control Manager: running as a service with graceful stop, plus
// install/remove helpers. Installed services always run as the virtual
// account NT SERVICE\gpu-turnstile — a per-service low-privilege identity
// managed by the SCM, with no password and no admin rights.
package service

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

	"golang.org/x/sys/windows/svc"
	"golang.org/x/sys/windows/svc/mgr"

	"gpu-turnstile/internal/config"
)

// Name is the Windows service name.
const Name = "gpu-turnstile"

// virtualAccount is the per-service identity the service runs as. The SCM
// manages it: no password, automatic "log on as a service" right, gone
// when the service is removed.
const virtualAccount = `NT SERVICE\` + Name

// IsService reports whether the process is running as a Windows service.
func IsService() bool {
	isSvc, err := svc.IsWindowsService()
	return err == nil && isSvc
}

// Run executes run as a Windows service. SCM Stop and Shutdown cancel the
// context passed to run, triggering the same graceful shutdown as SIGTERM
// in interactive mode.
func Run(run func(ctx context.Context) error) error {
	return svc.Run(Name, &handler{run: run})
}

type handler struct {
	run func(ctx context.Context) error
}

func (h *handler) Execute(_ []string, requests <-chan svc.ChangeRequest, status chan<- svc.Status) (bool, uint32) {
	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()

	status <- svc.Status{State: svc.StartPending}
	errCh := make(chan error, 1)
	go func() { errCh <- h.run(ctx) }()
	status <- svc.Status{State: svc.Running, Accepts: svc.AcceptStop | svc.AcceptShutdown}

	for {
		select {
		case err := <-errCh:
			status <- svc.Status{State: svc.Stopped}
			if err != nil {
				return true, 1
			}
			return false, 0
		case c := <-requests:
			switch c.Cmd {
			case svc.Interrogate:
				status <- c.CurrentStatus
			case svc.Stop, svc.Shutdown:
				status <- svc.Status{State: svc.StopPending}
				cancel()
			}
		}
	}
}

// Install registers gpu-turnstile as an auto-start Windows service running
// as the NT SERVICE\gpu-turnstile virtual account, whose binPath loads the
// given config file. Recovery actions restart the service after 5s on
// failure — this is also what brings up a staged update after the updater
// exits with a non-zero code. After registering, the virtual account is
// granted modify access to the install directory (self-updates rewrite the
// exe) and to the LOG_FILE directory, and read access to the config file
// if it lives elsewhere. The grants must come after CreateService: the
// virtual account's SID only exists once the service is registered.
func Install(configPath string) error {
	exe, err := os.Executable()
	if err != nil {
		return err
	}
	if configPath != "" {
		if abs, absErr := filepath.Abs(configPath); absErr == nil {
			configPath = abs
		}
	}

	m, err := mgr.Connect()
	if err != nil {
		return fmt.Errorf("connect to service manager (run as administrator): %w", err)
	}
	defer m.Disconnect()

	binPath := fmt.Sprintf(`"%s" -config "%s"`, exe, configPath)
	s, err := m.CreateService(Name, binPath, mgr.Config{
		StartType:        mgr.StartAutomatic,
		DisplayName:      "gpu-turnstile",
		Description:      "GPU arbitration proxy for Ollama and ComfyUI",
		ServiceStartName: virtualAccount,
	})
	if err != nil {
		return fmt.Errorf("create service: %w", err)
	}
	defer s.Close()

	restart := mgr.RecoveryAction{Type: mgr.ServiceRestart, Delay: 5 * time.Second}
	if err := s.SetRecoveryActions([]mgr.RecoveryAction{restart, restart, restart}, 24*60*60); err != nil {
		return fmt.Errorf("set recovery actions: %w", err)
	}
	if err := s.SetRecoveryActionsOnNonCrashFailures(true); err != nil {
		return fmt.Errorf("set failure actions flag: %w", err)
	}

	if err := grantAll(exe, configPath); err != nil {
		s.Delete() // roll back so a retry starts clean
		return err
	}
	return nil
}

// grantAll gives the virtual account every ACL the service needs.
func grantAll(exe, configPath string) error {
	exeDir := filepath.Dir(exe)
	if err := grantAccess(exeDir, "(OI)(CI)(M)"); err != nil {
		return err
	}
	if configPath != "" && !strings.HasPrefix(strings.ToLower(configPath), strings.ToLower(exeDir)+`\`) {
		if err := grantAccess(configPath, "(R)"); err != nil {
			return err
		}
	}
	if logFile := configuredLogFile(configPath); logFile != "" {
		dir := filepath.Dir(logFile)
		if err := os.MkdirAll(dir, 0o755); err == nil {
			if err := grantAccess(dir, "(OI)(CI)(M)"); err != nil {
				return err
			}
		}
	}
	return nil
}

// Remove stops (if running) and unregisters the service. The virtual
// account ceases to exist with it; the ACL grants on the install and log
// directories are left in place (harmless without the account).
func Remove() error {
	m, err := mgr.Connect()
	if err != nil {
		return fmt.Errorf("connect to service manager (run as administrator): %w", err)
	}
	defer m.Disconnect()
	s, err := m.OpenService(Name)
	if err != nil {
		return fmt.Errorf("open service: %w", err)
	}
	defer s.Close()
	s.Control(svc.Stop) // ignore error: may already be stopped
	if err := s.Delete(); err != nil {
		return fmt.Errorf("delete service: %w", err)
	}
	return nil
}

// grantAccess gives the virtual account the icacls permission set (e.g.
// "(OI)(CI)(M)") on path.
func grantAccess(path, perms string) error {
	out, err := exec.Command("icacls", path, "/grant", virtualAccount+":"+perms).CombinedOutput()
	if err != nil {
		return fmt.Errorf("grant %s access to %s: %w (%s)", virtualAccount, path, err, strings.TrimSpace(string(out)))
	}
	return nil
}

// configuredLogFile reads LOG_FILE from the config file so the installer
// can pre-create and ACL the log directory. "" when unset or unreadable.
func configuredLogFile(configPath string) string {
	f, err := os.Open(configPath)
	if err != nil {
		return ""
	}
	defer f.Close()
	values, err := config.ParseEnvFile(f)
	if err != nil {
		return ""
	}
	return values["LOG_FILE"]
}