bug#49283: [PATCH] 27.2; `(call-process "program" null-device ...)' fails over TRAMP from local MS Windows

[Jim Porter <jporterbugs@gmail.com>]

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On Wed, Jun 30, 2021 at 12:24 AM Michael Albinus <michael.albinus@gmx.de> wrote:
>
> Jim Porter <jporterbugs@gmail.com> writes:
>
> > I noticed an issue when trying to use flyspell (or ispell) using
> > hunspell from a local MS Windows system on a TRAMP file. It results in
> > an error that it can't find the file "/sshx:server:/path/to/NUL". I
> > narrowed this down to the fact that
> > `ispell-find-hunspell-dictionaries' calls `call-process' with `infile'
> > set to `null-device'.
>
> `call-process' doesn't know of remote files. You must use `process-file' instead.

That's not a problem; it's actually the right thing to do in this
case, I think. flyspell/ispell is trying to use my local version of
hunspell on the contents of a remote buffer. Since flyspell/ispell
just look at the buffer contents and not the actual file, it can use
`call-process'.

> It is not a good idea to use `null-device' as INFILE, just use nil. At
> least in the `process-file' case, Tramp shall know which value to take
> for `null-device'.

That fix would also work (see the attached patch).

However, when I read the `call-process' documentation, it says that
when `default-directory' is remote, it runs the program from "~".
That's fine overall, and means it's a good way to be sure you're
always running a process locally. It's just that when you do this,
INFILE's path is expanded relative to the remote directory. I don't
think that can ever work, since `call-process' doesn't know how to
open a TRAMP file. Because of that, it makes more sense to me that
you'd expand INFILE's path relative to wherever PROGRAM will be run
from. That means that when `default-directory' is remote, both PROGRAM
and INFILE are expanded relative to "~". That's more consistent, and
my first patch would hopefully prevent similar errors anytime
`call-process' is used from a remote buffer.

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From 2eee9b4e70fe489eb46fb94ab000c6d0b7262e28 Mon Sep 17 00:00:00 2001
From: Jim Porter <jporterbugs@gmail.com>
Date: Wed, 30 Jun 2021 10:01:32 -0700
Subject: [PATCH] Don't pass 'null-device' to 'call-process' in ispell

* lisp/textmodes/ispell.el (ispell-find-hunspell-dictionaries):
Replace 'null-device' with nil.
---
 lisp/textmodes/ispell.el | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lisp/textmodes/ispell.el b/lisp/textmodes/ispell.el
index ce5a572085..0a82bf5a2d 100644
--- a/lisp/textmodes/ispell.el
+++ b/lisp/textmodes/ispell.el
@@ -1076,7 +1076,7 @@ ispell-find-hunspell-dictionaries
           (split-string
            (with-temp-buffer
              (ispell-call-process ispell-program-name
-                            null-device
+                            nil
                             t
                             nil
                             "-D"
-- 
2.25.1