bug#57129: 29.0.50; Improve behavior of conditionals in Eshell

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 15 Aug 2022 13:55:35 -0700
> Cc: larsi@gnus.org, 57129@debbugs.gnu.org, Jim Porter
>  <jporterbugs@gmail.com>, Gnulib bugs <bug-gnulib@gnu.org>
> From: Paul Eggert <eggert@cs.ucla.edu>
> 
> On 8/15/22 11:58, Eli Zaretskii wrote:
> 
> > Ah, okay.  It's a (mis)feature of Gnulib's gen_tempname function
> > (which is the guts of make-temp-file) in its implementation for
> > MS-Windows (and maybe other platforms?): it always begins from the
> > same "random" characters in the file name, and only generates other
> > random characters if there's already a file by that name.
> 
> Not sure I'd call it a misfeature

I didn't say "misfeature".  Evidently, for you and perhaps others it's
a feature.

> as gen_tempname is generating a uniquely-named file that is
> exclusive to Emacs, which is all it's supposed to do.

Then perhaps calling it from make-temp-file in Emacs is a mistake,
because that function's doc string says

  Create a temporary file.
  The returned file name (created by appending some random characters at the end
  of PREFIX, and expanding against ‘temporary-file-directory’ if necessary),
  is guaranteed to point to a newly created file.

When you tell someone that the file name will include "some random
characters", they don't expect that they will be the same "random
characters" every call, just like saying that some function generates
random numbers doesn't imply it will be the same number every call.

> I'm not sure I'm entirely understanding the Emacs problem, but it 
> appears to be that Emacs has its own set of filenames that it thinks it 
> knows about, and Emacs wants the new temporary file's name to not be a 
> member of that set. If I'm right, does the second attached patch (this 
> patch is to Emacs) address the problem? I haven't tested or installed it.

I don't think that's the problem, no.  The problem is that callers of
make-temp-file reasonably expect it to return a new name every call.
And evidently, it does that on GNU/Linux (not sure how, given that the
tempname.c code is supposed to be in glibc?).  So if there's no
intention to change gen_tempname on non-glibc platforms so that it
doesn't repeat the same "random characters" upon each call, I think
Emacs should call a different function to generate "random" names of
temporary files, for consistency across platforms if not for other
reasons.