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.

0001-gzip-clarify-other-links-diagnostic.patch
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