[tz] zic fails for soft-links-only target volume

[Paul Eggert]

On 11/03/2016 07:55 AM, Tom Lane wrote:

> It appears to me that the problem is that commit 35484e98c broke the case of linking to an existing symlink.

That's right. Thanks for the diagnosis.

> There's no reason to disallow that case

Unfortunately the 'link' system call has system-dependent behavior when 
linking to a symlink. Some systems follow the symlink, and others don't. 
For the systems that don't, hardlinking to a relative symlink can cause 
trouble, since the symlink is resolved relative to its parent directory, 
and this interpretation can differ in the source vs the destination 
directory.

I fixed this in the attached patch by using linkat with 
AT_SYMLINK_FOLLOW, as this is immune to the problem of resolving 
symlinks and is the intended meaning here anyway. On older systems 
lacking linkat, the code checks whether the source is a symlink and 
refuses to hard-link to it; this should be good enough.

> So a less error-prone solution would be to revert 35484e98c's changes 
> to itsdir(), taking it back to doing only what the name suggests, and 
> invent an itssymlink() function that has a similar API but tests for 
> symlink-ness not directory-ness.

Thanks for this suggestion too. Done in the attached patch.

> I would actually say this should be
>
> -			if (err != EEXIST && itsdir(name) < 0) {
> +			if (err != EEXIST || itsdir(name) <= 0) {
>
> as we really don't want the code to keep going unless there is a directory
> there.

Yes, that sounds right, and is also in the attached patch.

> Maybe even drop the test on what the err code is, and only check
> itsdir()?
If errno == EEXIST (a common case), there should be no need to check 
itsdir, as the callers of mkdir do not consider an existing 
non-directory to be trouble (they'll diagnose that themselves). So 
unless I'm missing something, I'm inclined to keep the err code test, as 
a minor optimization.

Thanks again for the diagnosis and suggestions. Please try the attached 
patch, which I've installed into the development repository.
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