bug#69083: Emacs's keyboard hook state is not reset on session lock (Windows)

[Raffael Stocker <r.stocker@mnet-mail.de>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Raffael Stocker <r.stocker@mnet-mail.de>
>> Cc: 69083@debbugs.gnu.org
>> Date: Wed, 14 Feb 2024 21:20:35 +0100
>> 
>> > So we need to load that DLL at run time via LoadLibrary, ...
>> 
>> I have added that.
>
> Thanks.  I suggest one optimization: instead of probing whether
> LoadLibrary succeeds in two places, do it only once, where you call
> WTSRegisterSessionNotification, and at that time also record the
> function pointer for WTSUnRegisterSessionNotification; then use the
> 2nd function pointer, if non-NULL, to call the unregister API.

Yes, I can do that.  I will leave the initialisation in the
setup_w32_kbdhook function as the branch with the call to LoadLibrary
should be executed only once.

I experimented a bit today and found that the remove_w32_kbdhook
function is not actually getting called when Emacs is being killed.  It
is only called when the Emacs window receives a WM_DESTROY message.  But
we get relevant WM_* messages, when

   - creating second frame with ‘C-x 5 2’ and closing it using ‘C-x 5 0’:
     WM_EMACS_DESTROYWINDOW, then WM_DESTROY
   - creating second frame with ‘C-x 5 2’ and closing it by clicking ‘X’
     in frame decoration:
     WM_CLOSE, then WM_EMACS_DESTROYWINDOW, then WM_DESTROY
   - closing the last frame by clicking ‘X’ in frame decoration: WM_CLOSE
   - killing Emacs with ‘C-x C-c’: nothing

At least, if Emacs is not run as daemon.  Then, the WM_DESTROY is only
handled when there is still another frame, and in this case
‘remove_w32_kbdhook’ will not remove the hook as kbdhook.hook_count > 1.

If Emacs is run as daemon, it cleans up the keyboard hook when closing
the last window.

Is there a better place where the remove_w32_kbdhook call could go such
that cleanup can always happen?  Or should we just not care?

> This URL:
>
>   https://stackoverflow.com/questions/8606300/how-to-detect-windows-is-locked
>
> seems to indicate that WTSRegisterSessionNotification requires
> elevation on Windows 10/11.  Did you get the UAC prompt when calling
> that API?  Are you running with admin privileges when you test this
> code?  Triggering UAC prompts when starting Emacs would be a nuisance
> to our users.

I have read this post, but it seems to be wrong.  I don't have any
elevated privileges and it works without showing any prompt, at least on
Windows 10.  I'll try to have my colleague test it as well, she runs
Windows 11.

>> Unfortunately, this didn't work for me.  I tried calling
>> ‘EnumWindows(find_child_console, ...)’ with a ‘child_process’ instance
>> containing the current process id as returned by ‘GetCurrentProcessId’,
>> but I don't seem to get a useful window handle.
>
> What do you mean?  What is the result of using find_child_console?
> does the condition in find_child_console, which looks at the
> process_id of all windows, never match the process ID of the Emacs
> session running with -nw?  Or what else goes wrong?

I'm not quite sure myself what I mean.  I will experiment with this
a bit more and try to find out what's happening.

Regards,
Raffael