From: Piotr Caban <piotr@codeweavers.com>
Subject: Re: [PATCH] server: Don't wait for low level hook result when queueing hardware message.
Message-Id: <a2c92d8e-3a1b-d86a-9a2a-6596da59f549@codeweavers.com>
Date: Tue, 14 Sep 2021 18:38:38 +0200
In-Reply-To: <ada6d040-2217-4756-5871-2d2bbb29a149@codeweavers.com>
References: <ada6d040-2217-4756-5871-2d2bbb29a149@codeweavers.com>

I'm attaching a simple application that may be used to observe the 
problem. The application creates a window and waits for WM_KEYDOWN 
message. It deadlocks without the patch. Unfortunately there's no way of 
testing it without the ability to generate non-injected hardware 
messages. I can add an interactive test if it's preferred.

Thanks,
Piotr

#include <windows.h>
#include <stdio.h>

HANDLE start_event, end_event;
HANDLE wake_up, sleep;

static LRESULT WINAPI hook_proc(int code, WPARAM wparam, LPARAM lparam)
{
    printf("hook called %x %x %lx\n", code, wparam, lparam);
    return CallNextHookEx(0, code, wparam, lparam);
}

static DWORD WINAPI hook_thread(void *arg)
{
    HANDLE wait[2] = {wake_up, end_event};
    HHOOK hook;
    DWORD ret;
    MSG msg;

    hook = SetWindowsHookExA(WH_KEYBOARD_LL, hook_proc, 0, 0);
    SetEvent(start_event);

    while(1) {
        ret = WaitForMultipleObjects(2, wait, FALSE, INFINITE);
        if (ret == WAIT_OBJECT_0 + 1) break;

        PeekMessageA(&msg, 0, 0, 0, 0);
        SetEvent(sleep);
    }

    UnhookWindowsHookEx(hook);
    return 0;
}

int main() {
    HANDLE thread;
    HWND hwnd;
    MSG msg;

    start_event = CreateEventA(0, 0, 0, 0);
    end_event = CreateEventA(0, 0, 0, 0);
    wake_up = CreateEventA(0, 0, 0, 0);
    sleep = CreateEventA(0, 0, 0, 0);

    thread = CreateThread(0, 0, hook_thread, 0, 0, 0);
    WaitForSingleObject(start_event, INFINITE);

    hwnd = CreateWindowA("button", "button", WS_POPUP | WS_VISIBLE, 50, 50, 100, 100, 0, 0, 0, 0);
    while(1)
    {
        /* make sure that hook_thread message loop is not hung */
        DWORD ret = MsgWaitForMultipleObjects(0, NULL, 0, 100, QS_ALLINPUT);
        msg.message = 0;
        if (ret == WAIT_OBJECT_0)
        {
            while(PeekMessageA(&msg, 0, 0, 0, PM_REMOVE))
            {
                if (msg.message == WM_KEYDOWN)
                {
                    printf("got WM_KEYDOWN\n");
                    break;
                }
                DispatchMessageA(&msg);
            }
        }
        if (msg.message == WM_KEYDOWN) break;

        SetEvent(wake_up);
        WaitForSingleObject(sleep, INFINITE);
    }

    SetEvent(end_event);
    WaitForSingleObject(thread, INFINITE);
    CloseHandle(hook_thread);
    CloseHandle(start_event);
    CloseHandle(end_event);
    CloseHandle(wake_up);
    CloseHandle(sleep);
    return 0;
}