XLib event loop canceling

[data-niklas]

This PR allows canceling the XLib event loop by sending a custom ClientMessage on an invisible Window.
See #15
For further XLib documentation refer to Tronche's documentation.

[changkun]

Hi, thanks for this great contribution.

I just tested this change with an example:

func TestHotkey_Unregister(t *testing.T) {
	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()

	hk := hotkey.New([]hotkey.Modifier{hotkey.ModCtrl, hotkey.Mod2, hotkey.Mod4}, hotkey.KeyA)
	if err := hk.Register(); err != nil {
		t.Errorf("failed to register hotkey: %v", err)
		return
	}
	if err := hk.Unregister(); err != nil {
		t.Errorf("failed to unregister hotkey: %v", err)
		return
	}

	for {
		select {
		case <-ctx.Done():
			return
		case <-hk.Keydown():
			t.Fatalf("hotkey should not be registered but actually triggered.")
		}
	}
}

It actually crashes:

=== RUN   TestHotkey_Unregister
fatal error: unexpected signal during runtime execution
[signal SIGSEGV: segmentation violation code=0x1 addr=0x968 pc=0x7f2ab8dac414]

runtime stack:
runtime.throw({0x534755?, 0x39?})
        /home/changkun/goes/go1.18/src/runtime/panic.go:992 +0x71
runtime.sigpanic()
        /home/changkun/goes/go1.18/src/runtime/signal_unix.go:802 +0x3a9

goroutine 18 [syscall]:
runtime.cgocall(0x4fa410, 0xc00006be28)
        /home/changkun/goes/go1.18/src/runtime/cgocall.go:157 +0x5c fp=0xc00006be00 sp=0xc00006bdc8 pc=0x4058fc
golang.design/x/hotkey._Cfunc_sendCancel(0x0, 0x0)
        _cgo_gotypes.go:110 +0x45 fp=0xc00006be28 sp=0xc00006be00 pc=0x4f86a5
golang.design/x/hotkey.(*Hotkey).unregister.func1(0x4657ee?)
        /home/changkun/dev/golang.design/hotkey/hotkey_linux.go:85 +0x56 fp=0xc00006be68 sp=0xc00006be28 pc=0x4f8cd6
golang.design/x/hotkey.(*Hotkey).unregister(0xc000146080)
        /home/changkun/dev/golang.design/hotkey/hotkey_linux.go:85 +0x8a fp=0xc00006beb8 sp=0xc00006be68 pc=0x4f8baa
golang.design/x/hotkey.(*Hotkey).Unregister(0xc000146080)
        /home/changkun/dev/golang.design/hotkey/hotkey.go:100 +0x25 fp=0xc00006bed0 sp=0xc00006beb8 pc=0x4f7f25
golang.design/x/hotkey_test.TestHotkey_Unregister(0xc0001024e0)
        /home/changkun/dev/golang.design/hotkey/hotkey_linux_test.go:52 +0x125 fp=0xc00006bf70 sp=0xc00006bed0 pc=0x4f9d05
testing.tRunner(0xc0001024e0, 0x537100)
        /home/changkun/goes/go1.18/src/testing/testing.go:1439 +0x102 fp=0xc00006bfc0 sp=0xc00006bf70 pc=0x4bb562
testing.(*T).Run.func1()
        /home/changkun/goes/go1.18/src/testing/testing.go:1486 +0x2a fp=0xc00006bfe0 sp=0xc00006bfc0 pc=0x4bc40a
runtime.goexit()
        /home/changkun/goes/go1.18/src/runtime/asm_amd64.s:1571 +0x1 fp=0xc00006bfe8 sp=0xc00006bfe0 pc=0x4679c1
created by testing.(*T).Run
        /home/changkun/goes/go1.18/src/testing/testing.go:1486 +0x35f

goroutine 1 [select, locked to thread]:
golang.design/x/mainthread.Init(0xc000072500)
        /home/changkun/dev/golang.design/hotkey/vendor/golang.design/x/mainthread/mainthread.go:100 +0x16b
golang.design/x/hotkey/mainthread.Init(...)
        /home/changkun/dev/golang.design/hotkey/mainthread/os.go:19
golang.design/x/hotkey_test.TestMain(0xc0000aa1e0?)
        /home/changkun/dev/golang.design/hotkey/hotkey_test.go:19 +0x56
main.main()
        _testmain.go:53 +0x1d3

goroutine 6 [chan receive]:
testing.(*T).Run(0xc000102340, {0x52f9cb?, 0x4bb2c5?}, 0x537100)
        /home/changkun/goes/go1.18/src/testing/testing.go:1487 +0x37a
testing.runTests.func1(0xc00010e0f0?)
        /home/changkun/goes/go1.18/src/testing/testing.go:1839 +0x6e
testing.tRunner(0xc000102340, 0xc00006ad70)
        /home/changkun/goes/go1.18/src/testing/testing.go:1439 +0x102
testing.runTests(0xc0000aa1e0?, {0x5f5580, 0x2, 0x2}, {0x0?, 0x0?, 0x612dc0?})
        /home/changkun/goes/go1.18/src/testing/testing.go:1837 +0x457
testing.(*M).Run(0xc0000aa1e0)
        /home/changkun/goes/go1.18/src/testing/testing.go:1719 +0x5d9
golang.design/x/hotkey_test.TestMain.func1()
        /home/changkun/dev/golang.design/hotkey/hotkey_test.go:19 +0x1d
golang.design/x/mainthread.Init.func1()
        /home/changkun/dev/golang.design/hotkey/vendor/golang.design/x/mainthread/mainthread.go:96 +0x5b
created by golang.design/x/mainthread.Init
        /home/changkun/dev/golang.design/hotkey/vendor/golang.design/x/mainthread/mainthread.go:92 +0x10a

goroutine 19 [runnable]:
golang.design/x/hotkey.newEventChan.func1()
        /home/changkun/dev/golang.design/hotkey/hotkey.go:127
created by golang.design/x/hotkey.newEventChan
        /home/changkun/dev/golang.design/hotkey/hotkey.go:127 +0x92

goroutine 20 [runnable]:
golang.design/x/hotkey.newEventChan.func1()
        /home/changkun/dev/golang.design/hotkey/hotkey.go:127
created by golang.design/x/hotkey.newEventChan
        /home/changkun/dev/golang.design/hotkey/hotkey.go:127 +0x92

goroutine 21 [runnable]:
golang.design/x/hotkey.(*Hotkey).register.func1()
        /home/changkun/dev/golang.design/hotkey/hotkey_linux.go:74
runtime.goexit()
        /home/changkun/goes/go1.18/src/runtime/asm_amd64.s:1571 +0x1
created by golang.design/x/hotkey.(*Hotkey).register
        /home/changkun/dev/golang.design/hotkey/hotkey_linux.go:74 +0x165
exit status 2
FAIL    golang.design/x/hotkey  0.004s

[data-niklas]

Thanks for testing the code! (It worked on my machine in my limited tests, haha). I will try to reproduce the error and stabilize the PR.

[data-niklas]

I think that 'unregister' might be called before the ´Display´ and ´Window´ are initialized in the handle method.

[data-niklas]

I added your test.

[changkun]

I don't feel confident about this approach. What if the display is allocated after this check? What if the Unregister happens on a different goroutine to Register? This will lead to leaking resources, and race conditions of hk.display.

I think we will need some sort of synchronization here to guarantee that unregister can be called only after display and window are allocated.