bug#37564: [PATCH] don't export LINES and COLUMNS env vars in term to fix ncurses applications

[Eli Zaretskii <eliz@gnu.org>]

> From: Matthew Leach <matthew@mattleach.net>
> Cc: 37564@debbugs.gnu.org
> Date: Tue, 01 Oct 2019 19:33:39 +0100
> 
> >> I'm curious as to which programs you are referring? AFAIK, if a program
> >> tries to read the LINES and COLUMNS environment variables, using
> >> `getenv()', they don't exist.
> >
> > What makes you say that? 
> 
> If I run the C program in my previous post in an xterm and resize I get:
> 
> getenv: (null) (null)
> ioctl:  24 80
> getenv: (null) (null)
> ioctl:  42 169
> 
> You can see that doing getenv() on LINES and COLUMNS returns a null
> pointer.  I've tested this same behaviour on gnome-terminal and numerous
> other terminal emulators.

I simply tried "env | fgrep LINES" and I do see the variable, although
it wasn't there before I launched Emacs.  'env' is just another
program accessing the environment, right?  I have no idea why your
test program doesn't see that, but then I'm far from being an expert
on these issues.

> > Emacs exports these variables into the environment that is passed to
> > child subprocesses, so those subprocesses will definitely see them
> > using getenv.
> 
> I'm no expert on how these things work but it doesn't appear to be the
> way that it's done, on my machine anyway.  Perhaps there are programs
> that uses these variables on MacOS or Windows?

I tried the above on a Trisquel GNU/Windows machine, FWIW.

> I'm testing this on the latest Emacs Git version. I can see that
> window-adjust-process-window-size-function is used in term.el however,
> this doesn't appear to update the environment variables exported to the
> terminal process.

AFAIR, it's supposed to send the corresponding ioctl command, in
addition to setting LINES and COLUMNS.

> If I run the same program with term on Emacs git and
> resize I get:
> 
> getenv: 31 80
> ioctl:  30 89
> getenv: 31 80
> ioctl:  30 90
> getenv: 31 80
> ioctl:  30 91
> getenv: 31 80
> ioctl:  31 92
> getenv: 31 80
> ioctl:  31 93
> getenv: 31 80
> ioctl:  31 94
> ...
> 
> Notice that the values returned by getenv don't change.

Again, the "env | fgrep LINES" method does show the change when I
resize the window on the machine I tried this, so I'm unsure why you
don't see it.

> Given that this functionality is broken, I'm suggesting that we don't
> export the variables.

I don't agree that it's broken; it isn't for me.  I think someone who
understands more than I do about this stuff should chime in and
explain why we see such different results.