[PATCH 1/1] winex11.drv: Fix a possible desktop window message queue hang.

[Zhiyi Zhang]

From: Zhiyi Zhang

After an X11 event handler queues messages to another thread, TRUE should be returned and eventually
propagated to X11DRV_ProcessEvents().

When FALSE is always returned in xrandr14_device_change_handler(), a possible hang can happen for
the desktop message queue as follows:

1. The explorer.exe calls GetMessageW() -> NtUserGetMessage() -> wait_objects() -> wait_message().
2. In wait_message(), user_driver->pProcessEvents() gets called in the desktop window thread to
handle RRNotify events and calls xrandr14_device_change_handler() -> display_mode_changed(FALSE)
-> send_message(get_desktop_window(), WM_DISPLAYCHANGE, ...) -> desktop_window_proc() ->
send_message_timeout() -> send_client_message() -> process_message() -> broadcast_message() ->
send_message_timeout() -> send_client_message() -> process_message() -> send_inter_thread_message()
-> wait_message_reply() -> a server set_queue_mask() with skip_wait being 1 -> wake_mask and
changed_mask are set to 0.
3. In wait_message(), user_driver->pProcessEvents() returns FALSE from xrandr14_device_change_handler().
So wait_message() continues to call NtWaitForMultipleObjects().
4. Now NtWaitForMultipleObjects() hangs for INFINITE timeout because wake_mask and changed_mask
for the message queue are set to 0 so the thread is not woke up.

The hang is sensitive to message ordering and only happens in this specific case so it's hard to
reproduce with tests. I believe some of the past test timeouts on TestBots can be attributed to this
bug.
---
 dlls/winex11.drv/xrandr.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/dlls/winex11.drv/xrandr.c b/dlls/winex11.drv/xrandr.c
index edad0712927..1cf8c7c5070 100644
--- a/dlls/winex11.drv/xrandr.c
+++ b/dlls/winex11.drv/xrandr.c
@@ -1212,15 +1212,19 @@ static void xrandr14_free_monitors( struct gdi_monitor *monitors, int count )

static BOOL xrandr14_device_change_handler( HWND hwnd, XEvent *event )
 {
+    BOOL ret = FALSE;
     RECT rect;

xrandr14_invalidate_current_mode_cache();
     if (hwnd == NtUserGetDesktopWindow() && NtUserGetWindowThread( hwnd, NULL ) == GetCurrentThreadId())
+    {
         NtUserCallNoParam( NtUserCallNoParam_DisplayModeChanged );
+        ret = TRUE;
+    }
     /* Update xinerama monitors for xinerama_get_fullscreen_monitors() */
     rect = get_host_primary_monitor_rect();
     xinerama_init( rect.right - rect.left, rect.bottom - rect.top );
-    return FALSE;
+    return ret;
 }

static void xrandr14_register_event_handlers(void)