[#958652] Set EAI_SYSTEM only when h_errno is NETDB_INTERNAL

[Siddhesh Poyarekar]

On Thu, May 16, 2013 at 07:20:22PM +0530, Siddhesh Poyarekar wrote:
> > What would help in the review of this, and the upstream review, is
> > to have a detailed breakdown of the analysis you did while fixing
> > this.
> > 
> > Do you have such an analysis?
> 
> I don't have a written detailed breakdown right now, but I don't mind
> doing it.  I'll post an analysis of this soon.  Meanwhile, here's the
> original patch which caused this breakage, in case that helps build
> some of the context:
> 
> http://sourceware.org/ml/libc-alpha/2012-10/msg00611.html
> 

OK, so here's the background of all these changes:

The original request was to have getaddrinfo return EAI_SYSTEM if the
errno from the resulting request was EMFILE or ENFILE during the DNS
host resolution, to differentiate those conditions from the usual
EAI_NONAME.  The reason behind this was that they could be treated
differently from the default error conditions.  To satisfy this, I
looked at the two places that these were implemented,
i.e. get_XX_by_YY.c (for AF_INET) and dns-host.c (for AF_UNSPEC and
AF_INET6).  The places that set this errno were during module load
and/or during socket creation in both those places.  I set nss_status
to NSS_STATUS_UNAVAILABLE in those cases to signify that the error was
something other than a standard failure and used it in gaih_inet() to
set the return code as EAI_SYSTEM.

There were two problems with that patch.  The last part (of setting
NSS_STATUS_UNAVAILABLE and checking on it) was wrong since the default
failure was NSS_STATUS_UNAVAILABLE.  As a result, all failures were
returning EAI_SYSTEM instead of EAI_NONAME.  The correct check here
would have been to look for h_errno to be NETDB_INTERNAL since I set
that explicitly.  Secondly, I had not checked for module load errors
due to AF_UNSPEC or AF_INET6, which would occur in gaih_inet() itself.
This needed the second patch (i.e. the one I'm requesting review for)
which checks h_errno to check failure and ensures that ENOENT does not
qualify for an EAI_SYSTEM, since the latter case is documented to be a
valid reason for EAI_NONAME.

Finally, the tweak for #958652.  This was a minor oversight TBH - I
was testing for errno != ENOENT, but the module open code resets errno
to 0 if there is a failure in dlopen'ing the module.  As a result this
check also needs a check to ensure that errno is actually set, to
signal a failure.

I hope this is sufficient.  Please let me know if you need any further
clarification.

Siddhesh