bug#71289: 30.0.50; cmcheckmagic aborts when tty_write_glyphs writes "Garbage collecting..." in some cases

[Eli Zaretskii <eliz@gnu.org>]

> From: Daniel Clemente <n142857@gmail.com>
> Date: Mon, 3 Jun 2024 15:35:20 +0000
> Cc: 71289@debbugs.gnu.org
> 
> > So you are saying that the terminal lies to us?  It has 3 rows, but
> > somehow wraps the cursor to the 4th row?  What are the window and
> > frame dimensions at this point?
> 
> I don't know enough to say whether the terminal is providing wrong numbers.
> But it seems that the positions and terminal dimensions aren't wrong, they're just outdated. They were right a
> moment ago (i.e. the terminal was really as large as reported) but I was resizing the window during a slow
> operation (GC) that was trying to display a message due to garbage-collection-messages t, and it seems that
> the GC message is using outdated information about the terminal size. That's my hypothesis.

That hypothesis needs to be explained.  Which is why I asked to
investigate what happens when the SIGWINCH handler is called.

> > What are the window and
> > frame dimensions at this point?
> 
> (I'm using the example mentioned above: 13, vs. 4 - 1). 
> The frame seems 4 lines 80 columns.
> I'm not sure how to obtain the window dimensions from gdb, since there are several fields. Here are two
> attempts to get it, but total_cols/total_lines are 0 so it seems I'm not looking at the right fields.

We determine the actual dimensions of the frame in the SIGWINCH
handler, and the new dimensions are recorded by the signal handler in
f->new_width and f->new_height, see change_frame_size_1.