bug#6268: Suggestion: truncate should allow -r and -s options together

[Pádraig Brady]

> what about --ref=non-regular-file ?
> 
> Perhaps truncate should refuse to use st_size info from
> a non-regular file.
> 
> Here's a pathological example:
> 
>   $ echo abcdefgh > bar
>   $ strace -e ftruncate ./truncate --ref=/dev/tty bar
>   ftruncate(3, 0)                         = 0
>   $ wc -c bar
>   0 bar
> 
> It's obvious that such an example is not likely in practice, but since
> currently truncate's --ref uses stat, it would follow a symlink, too.
> 
> For a directory you'd get unportable and probably surprising results.
> Using such a reference size deserves at least a warning.

Yes that's safer. st_size is only defined for regular files
(or shared mem), so I'll only allow regular files.
I'll push a separate patch soon.

Since we're only referencing the size, there is the argument
that we should be trying harder to get the size of block devices
for example.  There are portability issues with that though,
and doing outside of truncate is easy enough, so I'll leave that for now.

cheers,
Pádraig.