bug#50097: gzip: add "--keep" option to keep original files unchanged

[Paul Eggert]

On 8/19/21 1:43 PM, Krzysztof Żelechowski wrote:
The command { gzip b; } fails because it would compress [a] without renaming
it.
The command { gzip -k b; } would not compress [a], so there is no reason to
fail.

There is a safety reason to fail. A naive user might do this:

gzip -k b && gzip -cd b.gz >b

which would trash the hard-linked file as well.

There's a similar safety issue with symbolic links:

$ ln -s a b
$ touch a
$ gzip -k b && gzip -cd b.gz >b
gzip: b: Too many levels of symbolic links

where gzip's failure prevents the naive user from trashing the linked-to 
file.

Of course one can use -f to go ahead and compress anyway.

Similar diagnostics say "file ignored" or "ignored", and that is clearer 
than saying "unchanged", so I installed the attached patch.
From 4b2f69227db5739b63e22a72f6db2b13972affb5 Mon Sep 17 00:00:00 2001
From: Paul Eggert <egg...@cs.ucla.edu>
Date: Thu, 19 Aug 2021 14:26:38 -0700
Subject: [PATCH] gzip: clarify "other links" diagnostic

* gzip.c (treat_file): Instead of saying
"gzip: FOO has 1 other link  -- unchanged", say
"gzip: FOO has 1 other link -- file ignored".
This is clearer when -k is also used (Bug#50097).
---
 gzip.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/gzip.c b/gzip.c
index 0123f82..f75b614 100644
--- a/gzip.c
+++ b/gzip.c
@@ -942,10 +942,10 @@ local void treat_file(iname)
               }
             if (2 <= istat.st_nlink)
               {
-                WARN ((stderr, "%s: %s has %lu other link%c -- unchanged\n",
+                WARN ((stderr, "%s: %s has %lu other link%s -- file ignored\n",
                        program_name, ifname,
                        (unsigned long int) istat.st_nlink - 1,
-                       istat.st_nlink == 2 ? ' ' : 's'));
+                       istat.st_nlink == 2 ? "" : "s"));
                 close (ifd);
                 return;
               }
-- 
2.30.2