bug#45052: 27.1; GDB completion moves point

bug#45052: 27.1; GDB completion moves point

[Philip K.]

When I'm in GDB, and I try to invoke completion, the completion
mechanism might move the point, and ultimately insert the completion at
the wrong place. This issue only occurs when the point is currently in
the last few lines of the window, so that when the completion pop-up
appears, it is moved, while the buffer stays in the same place.

One way to circumvent this problem is to use the mouse to select the
completion.

I've tested it with the default completion framework, but remember that
the same issue also existed with other completion frameworks such as
ivy.

Steps to reproduce  in emacs -Q (also tested on the master branch):

1. M-x gdb
2. Press enter a few times, so that the buffer can't fit into the window
3. Ensure that the point is in the last few lines (C-l)
4. Enter "l" and then press M-tab.

The point should now have been moved. This is not expected, and not seen
any other mode.

In GNU Emacs 27.1 (build 1, x86_64-redhat-linux-gnu, GTK+ Version 3.24.22, cairo version 1.16.0)
 of 2020-08-21 built on buildhw-x86-03.iad2.fedoraproject.org
Windowing system distributor 'Fedora Project', version 11.0.12009000
System Description: Fedora 33 (Workstation Edition)

Recent messages:
Quit [2 times]
Mark set
user-error: No window right from selected window
M-RET is undefined
Saving file /home/phi/doc/uni/cs/uebung/assignment2/lwt/lwt.h...
Wrote /home/phi/doc/uni/cs/uebung/assignment2/lwt/lwt.h
Saving file /home/phi/doc/uni/cs/uebung/assignment2/lwt/lwt.h...
Wrote /home/phi/doc/uni/cs/uebung/assignment2/lwt/lwt.h
Saving file /home/phi/doc/uni/cs/uebung/assignment2/lwt/lwt.h...
Wrote /home/phi/doc/uni/cs/uebung/assignment2/lwt/lwt.h

Configured using:
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Important settings:
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Load-path shadows:
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-- 
	Philip K.

bug#45052: 27.1; GDB completion moves point

[Eli Zaretskii]

> From: "Philip K." <philipk@posteo.net>
> Date: Sat, 05 Dec 2020 13:10:46 +0100
> 
> 1. M-x gdb
> 2. Press enter a few times, so that the buffer can't fit into the window
> 3. Ensure that the point is in the last few lines (C-l)
> 4. Enter "l" and then press M-tab.
> 
> The point should now have been moved. This is not expected, and not seen
> any other mode.

Does the patch below give good results, and doesn't produce any
adverse effects in other use cases?

diff --git a/lisp/progmodes/gdb-mi.el b/lisp/progmodes/gdb-mi.el
index 9030056..2b9a9c1 100644
--- a/lisp/progmodes/gdb-mi.el
+++ b/lisp/progmodes/gdb-mi.el
@@ -3010,7 +3010,7 @@ def-gdb-auto-update-handler
        (,custom-defun)
        (gdb-update-buffer-name)
        ,@(when (not nopreserve)
-          '((set-window-start window start)
+          '((set-window-start window start t)
             (set-window-point window p))))))
 
 (defmacro def-gdb-trigger-and-handler (trigger-name gdb-command

bug#45052: 27.1; GDB completion moves point

[Philip K.]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: "Philip K." <philipk@posteo.net>
>> Date: Sat, 05 Dec 2020 13:10:46 +0100
>> 
>> 1. M-x gdb
>> 2. Press enter a few times, so that the buffer can't fit into the window
>> 3. Ensure that the point is in the last few lines (C-l)
>> 4. Enter "l" and then press M-tab.
>> 
>> The point should now have been moved. This is not expected, and not seen
>> any other mode.
>
> Does the patch below give good results, and doesn't produce any
> adverse effects in other use cases?

It seems to work, and I can't see any unintended side-effects!

> diff --git a/lisp/progmodes/gdb-mi.el b/lisp/progmodes/gdb-mi.el
> index 9030056..2b9a9c1 100644
> --- a/lisp/progmodes/gdb-mi.el
> +++ b/lisp/progmodes/gdb-mi.el
> @@ -3010,7 +3010,7 @@ def-gdb-auto-update-handler
>         (,custom-defun)
>         (gdb-update-buffer-name)
>         ,@(when (not nopreserve)
> -          '((set-window-start window start)
> +          '((set-window-start window start t)
>              (set-window-point window p))))))
>  
>  (defmacro def-gdb-trigger-and-handler (trigger-name gdb-command
>

-- 
	Philip K.

bug#45052: 27.1; GDB completion moves point

[Eli Zaretskii]

> From: "Philip K." <philipk@posteo.net>
> Cc: 45052@debbugs.gnu.org
> Date: Sat, 05 Dec 2020 15:01:28 +0100
> 
> > Does the patch below give good results, and doesn't produce any
> > adverse effects in other use cases?
> 
> It seems to work, and I can't see any unintended side-effects!

Thanks.  If you use gdb-mi.el a lot, how about if you run with this
change for a week and report back if you saw any adverse effects?
Then I'll install this if no problems are uncovered.

bug#45052: 27.1; GDB completion moves point

[Philip K.]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: "Philip K." <philipk@posteo.net>
>> Cc: 45052@debbugs.gnu.org
>> Date: Sat, 05 Dec 2020 15:01:28 +0100
>> 
>> > Does the patch below give good results, and doesn't produce any
>> > adverse effects in other use cases?
>> 
>> It seems to work, and I can't see any unintended side-effects!
>
> Thanks.  If you use gdb-mi.el a lot, how about if you run with this
> change for a week and report back if you saw any adverse effects?
> Then I'll install this if no problems are uncovered.

Will do. And thanks for finding the bug!


-- 
	Philip K.

bug#45052: 27.1; GDB completion moves point

[Eli Zaretskii]

> From: "Philip K." <philipk@posteo.net>
> Cc: 45052@debbugs.gnu.org
> Date: Sat, 05 Dec 2020 15:48:01 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> It seems to work, and I can't see any unintended side-effects!
> >
> > Thanks.  If you use gdb-mi.el a lot, how about if you run with this
> > change for a week and report back if you saw any adverse effects?
> > Then I'll install this if no problems are uncovered.
> 
> Will do. And thanks for finding the bug!

Not a bug, more like an unintended consequence.  The completion
machinery happens in this case to call the gdb-mi handlers for
processing responses to GDB commands, and those handlers force
redisplay to show the GUD window with the same window-start point,
which is impossible when the completions window pops up and point is
close to the bottom of the GUD window.  So I made the window-start
setting less forceful.  My fear is that when those handlers are called
in other situations, you will now see sub-optimal display after
sending GDB commands.  So please pay attention to any such effects on
the gdb-mi related windows.

If my "simple" solution has adverse effects, we will need to come up
with a more thorough one, which will likely require changes on the C
level.

Martin, do we have any way of preventing the original problem via
display-buffer actions, per chance?  I couldn't find anything to that
effect, FWIW.

bug#45052: 27.1; GDB completion moves point

[martin rudalics]

 > Martin, do we have any way of preventing the original problem via
 > display-buffer actions, per chance?  I couldn't find anything to that
 > effect, FWIW.

There is `set-window-buffer-start-and-point' for this purpose but it is
not used by `display-buffer'.  I'm afraid that with these new completion
frameworks proliferating we may have to check our sources for more
incarnations of `set-window-start/-point' couplings that do not set
NOFORCE.

martin

bug#45052: 27.1; GDB completion moves point

[Eli Zaretskii]

> Cc: 45052@debbugs.gnu.org
> From: martin rudalics <rudalics@gmx.at>
> Date: Sat, 5 Dec 2020 16:21:15 +0100
> 
>  > Martin, do we have any way of preventing the original problem via
>  > display-buffer actions, per chance?  I couldn't find anything to that
>  > effect, FWIW.
> 
> There is `set-window-buffer-start-and-point' for this purpose but it is
> not used by `display-buffer'.

OK, thanks.

> I'm afraid that with these new completion frameworks proliferating
> we may have to check our sources for more incarnations of
> `set-window-start/-point' couplings that do not set NOFORCE.

Maybe we should teach these functions to set the optional_new_start
flag of the window, not only the force_start, given some special value
of NOFORCE.  That'd be the next thing I'd try if Philip (or someone
else) reports some adverse effects from the simplistic change I
proposed.

bug#45052: 27.1; GDB completion moves point

[Philip K.]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: "Philip K." <philipk@posteo.net>
>> Cc: 45052@debbugs.gnu.org
>> Date: Sat, 05 Dec 2020 15:01:28 +0100
>> 
>> > Does the patch below give good results, and doesn't produce any
>> > adverse effects in other use cases?
>> 
>> It seems to work, and I can't see any unintended side-effects!
>
> Thanks.  If you use gdb-mi.el a lot, how about if you run with this
> change for a week and report back if you saw any adverse effects?
> Then I'll install this if no problems are uncovered.

Just wanted to report back, that after using the patch for a bit, at
least in my case, there were no unintended consequences. Completion
behaved just the way I would have expected it to from other modes.

-- 
	Philip K.

bug#45052: 27.1; GDB completion moves point

[Eli Zaretskii]

> From: "Philip K." <philipk@posteo.net>
> Cc: 45052@debbugs.gnu.org
> Date: Mon, 14 Dec 2020 09:23:34 +0100
> 
> > Thanks.  If you use gdb-mi.el a lot, how about if you run with this
> > change for a week and report back if you saw any adverse effects?
> > Then I'll install this if no problems are uncovered.
> 
> Just wanted to report back, that after using the patch for a bit, at
> least in my case, there were no unintended consequences. Completion
> behaved just the way I would have expected it to from other modes.

Thanks, I've now installed the change on the emacs-27 branch, and I'm
marking this bug done.