Re: [bug-diffutils] diff: support for --no-dereference option

[Andreas Gruenbacher]

On Sat, 2012-01-07 at 12:51 -0800, Paul Eggert wrote:
> Thanks for the patch!  I would like to fold something like
> this in.
> 
> Two thoughts.
> 
> First, many programs (e.g., cp) have three options:
> -H -L -P.  These distinctions seem to be useful for diff.
> This suggests that the new --no-dereference flag should
> be -P, not -h.  -h has a subtly different meaning from
> -P, in programs like chown.
> 
> Changing the patch to use -P instead of -h should be easy;
> we can implement -H and -L later as need be.

I wonder if comparing symlinks (instead of what they point to) shouldn't
become the default once diff supports it.  At the very least, diff
should warn when following symlinks though.

> Second, and more important, the output of --no-dereference
> should be something that we can feed to an (augmented)
> 'patch' so that it can alter a copy of the old tree,
> symlinks and all, so that it looks like the new tree,
> symlinks and all.  This suggests that the output of
> 'diff' needs to contain the symlink contents, and needs
> to distinguish symlinks from regular files, so that 'patch'
> can reconstruct the symlinks.

Binary files, file permissions, copied/renamed files, creation/deletion
of empty files also are not supported by diff currently.  Alpha versions
of patch already mostly support git's extended diff format [*] which
addresses these shortcomings, so this would be a good place to start.

[*] https://github.com/gitster/git/blob/master/Documentation/
        diff-generate-patch.txt

Thanks,
Andreas