bug#62385: POSIX behavior of readlink, realpath

On 03/08/2025 05:33, Collin Funk wrote:
> Hi Eric,
> 
> You said:
> 
>> Among other things, I can see the following changes that coreutils
>> will need to make to become compliant, or else we need to push back on
>> the POSIX folks if we have strong reasons to complain that their
>> specification will break things:
>>
>> POSIX wants 'readlink non-symlink' to output a diagnostic; that is, it
>> looks like POSIX wants us to behave like '-v' is enabled by default
>> (our current behavior of -q by default will be non-compliant).
> 
> I have attached a patch that I think is correct for readlink.
> 
> It is a bit more complex than just behaving the same as 'readlink -v'
> previously would. This is because the behavior of non-POSIX
> options. Here are some commands:
> 
>      $ touch file
>      $ ln -s symlink file
>      
>      # Example 1
>      $ readlink -v file
>      readlink: file: Invalid argument
> 
>      # Example 2
>      $ readlink -v -f file
>      /home/collin/file
> 
> If 'readlink -v' where 'POSIXLY_CORRECT=1 readlink' then example 1 would
> conform. I think that example 2 would violate POSIX.
> 
> So, when POSIXLY_CORRECT is enabled we can call 'readlink', check for
> errors, and then canonicalize if it is successful.

I'd say any option apart from -n implicitly disables strict POSIX conformance,
since those options are not in POSIX at all. So all I'd do is enable -v
if POSIXLY_CORRECT is set.  There is the argument for defaulting to -v,
but I see FreeBSD does not output an error either if file is not a symlink,
so it's probably best to only enable -v if POSIXLY_CORRECT is set.

In NEWS this should be under "Changes in behavior" rather than "New Features".

Thanks for following up on this.

Padraig.