navi/internal/singleinstance/singleinstance_windows.go

//go:build windows

package singleinstance

import (
	"fmt"
	"unsafe"

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

var (
	user32            = windows.NewLazySystemDLL("user32.dll")
	procFindWindow    = user32.NewProc("FindWindowW")
	procShowWindow    = user32.NewProc("ShowWindow")
	procSetForeground = user32.NewProc("SetForegroundWindow")
	procIsIconic      = user32.NewProc("IsIconic")
)

const (
	swShow    = 5
	swRestore = 9
)

// Lock acquires a per-user mutex. If another Navis holds it, activates that
// window (title "Navis") and returns ErrAlreadyRunning.
func Lock(appName string) (func(), error) {
	if appName == "" {
		appName = "Navis"
	}
	name, err := windows.UTF16PtrFromString("Local\\" + appName + "SingleInstance")
	if err != nil {
		return nil, err
	}
	handle, err := windows.CreateMutex(nil, false, name)
	if err == windows.ERROR_ALREADY_EXISTS {
		if handle != 0 {
			_ = windows.CloseHandle(handle)
		}
		activateWindow(appName)
		return nil, ErrAlreadyRunning
	}
	if err != nil {
		return nil, fmt.Errorf("singleinstance: %w", err)
	}
	return func() {
		_ = windows.ReleaseMutex(handle)
		_ = windows.CloseHandle(handle)
	}, nil
}

func activateWindow(title string) {
	t, err := windows.UTF16PtrFromString(title)
	if err != nil {
		return
	}
	hwnd, _, _ := procFindWindow.Call(0, uintptr(unsafe.Pointer(t)))
	if hwnd == 0 {
		return
	}
	iconic, _, _ := procIsIconic.Call(hwnd)
	if iconic != 0 {
		procShowWindow.Call(hwnd, swRestore)
	} else {
		procShowWindow.Call(hwnd, swShow)
	}
	procSetForeground.Call(hwnd)
}