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

[Raffael Stocker]

Eli Zaretskii <eliz@gnu.org> writes:

> So we need to load that DLL at run time via LoadLibrary, ...

I have added that.

> (I'm a bit surprised that Remote Desktop Services need to be used for
> this purpose.  Are you sure there's no other way for Emacs to know
> that the system is going to be locked?  Where did you read about the
> need to reset the keyboard hook state in that case, and the way to do
> it?)

For me, the need to reset the hook state is implied by the existence of
the ‘reset_w32_kbdhook_state’ function defined in ‘w32fns.c’ and its
call site.  This function has been there since 2016, but was never
called, because the ‘WM_WTSSESSION_CHANGE’ message wasn't received.
>From [0]:

MS> This message is sent only to applications that have registered to
MS> receive this message by calling WTSRegisterSessionNotification.

This seems to be the only way to find out it's being locked; at least I
couldn't find anything indicating otherwise on the Microsoft website and
a quick web search didn't turn up anything either.

>> To receive session notifications, one must provide a window handle,
>> which is fine if Emacs does not run in console mode.  I don't know
>> whether it is possible to get these notifications in console Emacs; at
>> least using the console handle didn't work for me.
>
> Please try the technique used by the function find_child_console which
> is defined in w32proc.c.

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.  I must be doing
something wrong here.  OTOH, [1] says:

MS> Note For Windows 8 and later, EnumWindows enumerates only top-level
MS> windows of desktop apps.

Does this mean that ‘EnumWindows’ it doesn't work for console windows or
is this remark irrelevant here?  I am not familiar with the Windows
terminology.

Come to think of it, don't I need the window handle of the console
window that Emacs is running in, which would imply that a handle for
Emacs' pid doesn't exist?  I was starting Emacs in cmd.exe using ‘emacs
-Q -nw’, so I would probably have to find the pid of the ‘cmd’ process
first, right?

>> I also noticed that while the keyboard hook is set up in console mode
>> using ‘setup_w32_kbdhook’, there does not seem to be a corresponding
>> call to ‘remove_w32_kbdhook’.
>
> Doesn't Windows remove the hook when the process exits?

According to [2] we have to remove the hook:

MS> Before terminating, an application must call the UnhookWindowsHookEx
MS> function function to free system resources associated with the hook.

>> Also, in console mode the keyboard hook is always installed, while
>> in GUI Emacs it is only installed when ‘WINDOWSNT’ is defined.
>
> That's because w32console.c is not compiled into the Cygwin build
> (which does not define WINDOWSNT), whereas w32fns.c is.

Ok, that makes sense, I have removed my superfluous #ifdef.

> Do we really need to use WTSRegisterSessionNotificationEx?  Can't we
> use WTSRegisterSessionNotification instead?  AFAIK, the latter is
> available since Windows XP, whereas the former only since Vista.

Yes, ‘WTSRegisterSessionNotification’ works, too.  However, [3] says
that the minimum supported client here is Windows Vista, as with the
‘-Ex’ version.

I attached my current patch version.

Regards,
Raffael

[0] 
https://learn.microsoft.com/en-us/windows/win32/termserv/wm-wtssession-change
[1] 
https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-enumwindows
[2] 
https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-setwindowshookexa
[3] 
https://learn.microsoft.com/en-us/windows/win32/api/wtsapi32/nf-wtsapi32-wtsregistersessionnotification

reset_kbdhook_1.patch
Description: reset kbdhook patch