//go:build windows

package instance

import (
	"errors"
	"fmt"

	"golang.org/x/sys/windows"
)

// ErrAlreadyRunning means another instance holds the single-instance mutex.
var ErrAlreadyRunning = errors.New("another instance of ipswap is already running")

// mutexName is session-local rather than Global\: ipswap is a per-user tray
// app, and two users logged into the same machine should each get their own.
const mutexName = `Local\ipswap-single-instance`

type winLock struct{ h windows.Handle }

func (l winLock) release() {
	if l.h != 0 {
		windows.ReleaseMutex(l.h)
		windows.CloseHandle(l.h)
	}
}

// lockProcess claims the single-instance slot.
//
// CreateMutexW succeeds even when the mutex already exists, so the existing
// object is detected via GetLastError rather than the return value. The handle
// is kept for the process lifetime; Windows releases it automatically if we
// crash, which is what stops a hard kill from locking the user out.
func lockProcess() (releaser, error) {
	name, err := windows.UTF16PtrFromString(mutexName)
	if err != nil {
		return nil, err
	}

	h, err := windows.CreateMutex(nil, false, name)
	if err != nil {
		if errors.Is(err, windows.ERROR_ALREADY_EXISTS) {
			if h != 0 {
				windows.CloseHandle(h)
			}
			return nil, ErrAlreadyRunning
		}
		return nil, fmt.Errorf("creating the single-instance mutex: %w", err)
	}
	return winLock{h: h}, nil
}