bug#8789: 23.3; debug backtrace buffer changes window on step-through

[Eli Zaretskii]

> From: "Drew Adams" <drew.adams@oracle.com>
> Date: Thu, 20 Sep 2012 11:08:26 -0700
> Cc: 8789@debbugs.gnu.org
> 
> >  > It's caused by the calculation of the mode-line.  Try this:
> >  >   (setq-default mode-line-remote nil)
> >  > and the problem is gone.
> > 
> > So the solution is probably to never enable `debug-on-entry' for
> > functions that run through the mode-line mechanism.
> 
> That does not sound like the solution to me.

Please suggest another solution, then, for a function that is
constantly called as part of redisplay (which is what happens here,
see below).  AFAIU, this is why you "cannot exit the debugger":
anything that you do redisplays the mode line, and re-enters the
debugger.

> What does "run through the mode-line mechanism" even mean,

It means functions that are called when some Lisp form is evaluated as
part of displaying the mode line.

> and how does a user tell whether a given function might "run through..."? 

By examining the C-level call stack, I presume.

> And
> for such functions (whatever that might mean), what workaround do you propose,
> to get the effect of `debug-on-entry'.

Why do you need a work-around?  The feature works, it just has nasty
side-effects due to the fact that the function is constantly called.

> This is a regression, and should just be fixed.

How do you "fix"  situation where a function you want to debug is
called at a very high frequency?  The only way I know if is to stop
calling it -- which is precisely what was suggested: set
mode-line-remote to nil, which avoids the invocation of file-remote-p
as part of mode-line redisplay.

>  > M-x debug-on-entry file-remote-p
>  >
>  > That puts me immediately into the debugger, with a call to 
> `file-remote-p'.  Why
>  > does debugging file-remote-p cause the debugger to enter even for that M-x
>  > invocation?

Typing "M-x" causes redisplay of certain parts of the Emacs frame,
including the mode line.  And redisplaying the mode line calls
file-remote-p because of mode-line-remote, which puts an indication of
a file being remote on the mode line.

>  > There is not even any reading of a file name.

The file name in question is buffer-file-name.

>  > Thereafter, I cannot exit the debugger, no matter what I do.  `c' and `q' 
> seem
>  > to do nothing.  `d' just steps into `file-remote-p'.  Even `C-]' does not 
> exit
>  > the debugger.  I can use C-x k to kill the *Backtrace* buffer, but that 
> leaves
>  > Emacs in a recursive edit, and `C-]' then brings Emacs back into the 

You just re-enter the debugger again and again, and the indication is:

>  > After the `C-x k', *Messages* shows this multiple times, always ending by
>  > entering:
>  >
>  > Entering debugger...
>  > Quit
>  > Entering debugger...
>  > Quit
>  > Entering debugger...
>  > Quit
>  > Entering debugger...
>  > Quit
>  > Entering debugger...