Re: [coreutils] feature request: make chown/chmod/chgrp --verbose/--changes additionally show original property

[William Plusnick]

Thanks for the suggestion.

>From a user perspective, I can see the attraction of this functionality
(and generally of --verbose), but from an implementation perspective,
even --verbose is not welcome because it adds a nontrivial amount of
logic (and the threat of a large performance hit) for only marginal gain.
The worst part is that must of what ch??? --verbose prints is not even
guaranteed to be true, so it cannot be used reliably to infer prior state.
While working on ch???.c over the years, --verbose support has turned
out be the source of enough hassles that I have seriously considered
deprecating-then-removing the option.  Thus, the relatively-new
chcon.c (which is very similar to the other ch*.c programs) does *not*
provide a --verbose option.

Here's a big part of the problem:

 - what chown/chgrp/chmod --verbose reports with your proposed
   addition may not be true.  There is a guaranteed race: the
   original owner/group/mode may be changed between when the
   tool records it and when it comes time to actually change it.
   It could end up not being changed at all, or it could be changed
   from some other value than the one printed in the diagnostic.

With that, you get a feel for why I'm tempted to remove --verbose
support.  Even existing --verbose output may be inaccurate due to
this implicit race condition.  For example, if I run chgrp --verbose G f,
it may print "changed group of `f' to G", while in actuality it may not
have changed the group at all, due to the unavoidable race when using
--verbose.

If you really want the prior state, you should simply record it
separately, rather than hoping to get something consistent and
usable from --verbose.

In summary I'd rather remove --verbose from ch???

I definitely see your point. While I didn't notice that there was a race condition, I definitely did see, in chown-core.c anyway, a fair amount of the logic was devoted toward actually printing the correct (or, as you have pointed out, possibly incorrect, depending on the outcome of the race condition) information. Deprecating seems like the way to go if there aren't too many scripts that will break because of it. (I don't know how often it is used, I really don't use ch??? much because I am the only user of my computers.)
 
Are there any other small-median features, such as this one that someone would like done that you could see a practical reason for implementing?
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