Emacs 25.1 RC2 - sporadic display errors on Linux virtual terminal.

Emacs 25.1 RC2 - sporadic display errors on Linux virtual terminal.

[Alan Mackenzie]

Hello, Emacs.

On the Emacs 25.1-rc2, I'm seeing worrying display anomalies on a Linux
virtual terminal.

My Emacs is configured with:

    ./configure  -with-gif=no --with-tiff=no --with-gpm

.

What I see is on full speed typing (I type at, perhaps, 10 characters
per second), a character fails to be displayed, the following character
takes its place, and a space is left afterwards.  For example, I just
now typed

    deal

.  This was displayed (in what is basically Text Mode) as

    da l

.  On typing C-l, the display becomes correct again.

It appears that Emacs goes into some sort of mode where this happens.  It
happened to me a few days ago (at the end of an hours long session), and
I just put it down to "one of these things".  It's just happened to me
again.  I am currently getting one of these errors, perhaps once every
two lines of typed text (where a line is ~78 characters).

Does anybody have any idea what might be happening?

-- 
Alan Mackenzie (Nuremberg, Germany).

Re: Emacs 25.1 RC2 - sporadic display errors on Linux virtual terminal.

[Eli Zaretskii]

> Date: Wed, 7 Sep 2016 15:59:27 +0000
> From: Alan Mackenzie <acm@muc.de>
> 
> On the Emacs 25.1-rc2, I'm seeing worrying display anomalies on a Linux
> virtual terminal.

Are you saying this happens only with that one build of Emacs, but not
with other versions?  Because it sounds like a system-wide problem
that has nothing to do with Emacs.

If you open a termscript file and then type, do you see the missing
characters in the termscript file, which means they were sent to the
screen?

Re: Emacs 25.1 RC2 - sporadic display errors on Linux virtual terminal.

[Alan Mackenzie]

Hello, Eli.

On Wed, Sep 07, 2016 at 07:35:49PM +0300, Eli Zaretskii wrote:
> > Date: Wed, 7 Sep 2016 15:59:27 +0000
> > From: Alan Mackenzie <acm@muc.de>

> > On the Emacs 25.1-rc2, I'm seeing worrying display anomalies on a Linux
> > virtual terminal.

> Are you saying this happens only with that one build of Emacs, but not
> with other versions?  Because it sounds like a system-wide problem
> that has nothing to do with Emacs.

I've not noticed it happening in any other build.

I've discovered I can trigger the problem by repeatedly hitting two
adjacent keys "at once" in the left hand, then the same in the right
hand as fast as possible.

> If you open a termscript file and then type, do you see the missing
> characters in the termscript file, which means they were sent to the
> screen?

No, the termscript file contained what I typed, not what I saw on the
screen.

I'll see if I can create the problem in other versions of Emacs.

-- 
Alan Mackenzie (Nuremberg, Germany).

Re: Emacs 25.1 RC2 - sporadic display errors on Linux virtual terminal.

[Eli Zaretskii]

> Date: Wed, 7 Sep 2016 16:53:38 +0000
> Cc: emacs-devel@gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> > If you open a termscript file and then type, do you see the missing
> > characters in the termscript file, which means they were sent to the
> > screen?
> 
> No, the termscript file contained what I typed, not what I saw on the
> screen.

So this means Emacs writes the stuff correctly to the screen, and the
problem happens between terminal driver (or its emulation) and the
glass.

Re: Emacs 25.1 RC2 - sporadic display errors on Linux virtual terminal.

[Alan Mackenzie]

Hello, Eli.

On Wed, Sep 07, 2016 at 08:17:24PM +0300, Eli Zaretskii wrote:
> > Date: Wed, 7 Sep 2016 16:53:38 +0000
> > Cc: emacs-devel@gnu.org
> > From: Alan Mackenzie <acm@muc.de>

> > > If you open a termscript file and then type, do you see the missing
> > > characters in the termscript file, which means they were sent to the
> > > screen?

> > No, the termscript file contained what I typed, not what I saw on the
> > screen.

> So this means Emacs writes the stuff correctly to the screen, and the
> problem happens between terminal driver (or its emulation) and the
> glass.

Yes, so it would seem.

Going back to an earlier question, I fired up both Emacs 24.5 and Emacs
25.1-rc1 without being able to reproduce the problem.  I tried both with
-Q and loading my full .emacs and desktop.  I've restarted Emacs
25.1-rc2, where I saw the problem.  I now can't reproduce it here,
either.

This suggests that Emacs gets "tired" after several hours, whatever that
might mean.  Perhaps a buffer is getting depleted somehow, or something
like that.

-- 
Alan Mackenzie (Nuremberg, Germany).

Re: Emacs 25.1 RC2 - sporadic display errors on Linux virtual terminal.

[Eli Zaretskii]

> Date: Wed, 7 Sep 2016 17:39:25 +0000
> Cc: emacs-devel@gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> This suggests that Emacs gets "tired" after several hours, whatever that
> might mean.  Perhaps a buffer is getting depleted somehow, or something
> like that.

What buffer did you have in mind?

Re: Emacs 25.1 RC2 - sporadic display errors on Linux virtual terminal.

[Alan Mackenzie]

Hello, Eli.

On Wed, Sep 07, 2016 at 09:04:55PM +0300, Eli Zaretskii wrote:
> > Date: Wed, 7 Sep 2016 17:39:25 +0000
> > Cc: emacs-devel@gnu.org
> > From: Alan Mackenzie <acm@muc.de>

> > This suggests that Emacs gets "tired" after several hours, whatever that
> > might mean.  Perhaps a buffer is getting depleted somehow, or something
> > like that.

> What buffer did you have in mind?

I don't have any particular buffer in mind.  I simply don't know this
level of the software at all.  (At least, not yet).

Maybe there's some circular buffer somewhere which, when it becomes
full, spuriously outputs a space to the screen.  The buffer might well
not be in Emacs.  Can a circular buffer gradually (over hours) become
smaller?  That seems a bit unlikely.

I really haven't much idea - I've reported what I've observed, and I'm
speculating.

-- 
Alan Mackenzie (Nuremberg, Germany).

Re: Emacs 25.1 RC2 - sporadic display errors on Linux virtual terminal.

[Kaushal Modi]

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Sounds sort of like this issue (
https://lists.gnu.org/archive/html/help-gnu-emacs/2016-08/msg00010.html )
someone else reported on help-gnu-emacs.

@Alan Do the symptoms there match what you see?

FYI I haven't seen that issue happen for me.

On Wed, Sep 7, 2016 at 2:16 PM Alan Mackenzie <acm@muc.de> wrote:

> Hello, Eli.
>
> On Wed, Sep 07, 2016 at 09:04:55PM +0300, Eli Zaretskii wrote:
> > > Date: Wed, 7 Sep 2016 17:39:25 +0000
> > > Cc: emacs-devel@gnu.org
> > > From: Alan Mackenzie <acm@muc.de>
>
> > > This suggests that Emacs gets "tired" after several hours, whatever
> that
> > > might mean.  Perhaps a buffer is getting depleted somehow, or something
> > > like that.
>
> > What buffer did you have in mind?
>
> I don't have any particular buffer in mind.  I simply don't know this
> level of the software at all.  (At least, not yet).
>
> Maybe there's some circular buffer somewhere which, when it becomes
> full, spuriously outputs a space to the screen.  The buffer might well
> not be in Emacs.  Can a circular buffer gradually (over hours) become
> smaller?  That seems a bit unlikely.
>
> I really haven't much idea - I've reported what I've observed, and I'm
> speculating.
>
-- 

Kaushal Modi

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Re: Emacs 25.1 RC2 - sporadic display errors on Linux virtual terminal.

[Eli Zaretskii]

> Date: Wed, 7 Sep 2016 18:16:06 +0000
> Cc: emacs-devel@gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> > What buffer did you have in mind?
> 
> I don't have any particular buffer in mind.  I simply don't know this
> level of the software at all.  (At least, not yet).
> 
> Maybe there's some circular buffer somewhere which, when it becomes
> full, spuriously outputs a space to the screen.  The buffer might well
> not be in Emacs.  Can a circular buffer gradually (over hours) become
> smaller?  That seems a bit unlikely.

You are probably talking about something that happens in the terminal
emulator or driver.  Emacs simply uses fwrite to send the text and
tputs to send termcap/terminfo commands to the terminal.  You can see
that in term.c.

Re: Emacs 25.1 RC2 - sporadic display errors on Linux virtual terminal.

[Alan Mackenzie]

Hello, Kaushal.

On Wed, Sep 07, 2016 at 06:24:43PM +0000, Kaushal Modi wrote:
> Sounds sort of like this issue (
> https://lists.gnu.org/archive/html/help-gnu-emacs/2016-08/msg00010.html )
> someone else reported on help-gnu-emacs.

> @Alan Do the symptoms there match what you see?

I don't think they do.  The OP on that thread was using a GUI, I'm on a
text terminal.

> FYI I haven't seen that issue happen for me.

Well, at least I've got a workaround, save everything and restart Emacs.

> On Wed, Sep 7, 2016 at 2:16 PM Alan Mackenzie <acm@muc.de> wrote:

> > On Wed, Sep 07, 2016 at 09:04:55PM +0300, Eli Zaretskii wrote:
> > > > Date: Wed, 7 Sep 2016 17:39:25 +0000
> > > > Cc: emacs-devel@gnu.org
> > > > From: Alan Mackenzie <acm@muc.de>

> > > > This suggests that Emacs gets "tired" after several hours, whatever
> > > > that might mean.  Perhaps a buffer is getting depleted somehow,
> > > > or something like that.

> > > What buffer did you have in mind?

> > I don't have any particular buffer in mind.  I simply don't know this
> > level of the software at all.  (At least, not yet).

> > Maybe there's some circular buffer somewhere which, when it becomes
> > full, spuriously outputs a space to the screen.  The buffer might well
> > not be in Emacs.  Can a circular buffer gradually (over hours) become
> > smaller?  That seems a bit unlikely.

> > I really haven't much idea - I've reported what I've observed, and I'm
> > speculating.

> -- 

> Kaushal Modi

-- 
Alan Mackenzie (Nuremberg, Germany).

Re: Emacs 25.1 RC2 - sporadic display errors on Linux virtual terminal.

[Stefan Monnier]

>> > If you open a termscript file and then type, do you see the missing
>> > characters in the termscript file, which means they were sent to the
>> > screen?
>> No, the termscript file contained what I typed, not what I saw on the
>> screen.
> So this means Emacs writes the stuff correctly to the screen, and the
> problem happens between terminal driver (or its emulation) and the
> glass.

Indeed.  Alan, what Eli is saying here is that the problem is unrelated
to Emacs itself, it's a bug in your xterm program.


        Stefan

Re: Emacs 25.1 RC2 - sporadic display errors on Linux virtual terminal.

[Pip Cet]

On Wed, Sep 7, 2016 at 7:10 PM, Stefan Monnier <monnier@iro.umontreal.ca> wrote:
> Indeed.  Alan, what Eli is saying here is that the problem is unrelated
> to Emacs itself, it's a bug in your xterm program.

Not necessarily; maybe the problem is as simple as another program
writing to the terminal and its output getting interspersed with the
Emacs output?

Running
while sleep 1; do echo -n " "; done & DISPLAY=: emacs -Q
results in behavior similar to what Alan describes.

Re: Emacs 25.1 RC2 - sporadic display errors on Linux virtual terminal.

[Alan Mackenzie]

Hello, Stefan.

On Wed, Sep 07, 2016 at 03:10:58PM -0400, Stefan Monnier wrote:
> >> > If you open a termscript file and then type, do you see the missing
> >> > characters in the termscript file, which means they were sent to the
> >> > screen?
> >> No, the termscript file contained what I typed, not what I saw on the
> >> screen.
> > So this means Emacs writes the stuff correctly to the screen, and the
> > problem happens between terminal driver (or its emulation) and the
> > glass.

> Indeed.  Alan, what Eli is saying here is that the problem is unrelated
> to Emacs itself, it's a bug in your xterm program.

Yes, thanks.  This has the advantage that we don't have to fix it, but
also the disadvantage that we can't fix it.

It's something of a relief that it's not Emacs's bug, so close to the
25.1 release.  I think I'm just going to learn to live with it - after
all, it only happens after many hours of Emacsing, and I have a
workaround in restarting Emacs.

Assuming the bug is in the Linux virtual terminal, the effort to get
useful debugging info to the maintainers would be excessive.

Alternatively, it's already been fixed, and I just need to upgrade my
kernel.

>         Stefan

-- 
Alan Mackenzie (Nuremberg, Germany).

Re: Emacs 25.1 RC2 - sporadic display errors on Linux virtual terminal.

[Stefan Monnier]

>> Indeed.  Alan, what Eli is saying here is that the problem is unrelated
>> to Emacs itself, it's a bug in your xterm program.
> Not necessarily; maybe the problem is as simple as another program
> writing to the terminal and its output getting interspersed with the
> Emacs output?

Right, but the end-result is the same: there's no point looking for the
source of the problem in Emacs.


        Stefan