This code was added in April in CL 166297, for #12524.
This CL fixes the following problems in the code:

 - The test for failure in the assembly stubs checked for
   64-bit -1 instead of 32-bit -1 to decide to fetch errno.

 - These C routines (res_init and res_search) don't set errno anyway,
   so the Go code using errno to decide success is incorrect.
   (The routines set h_errno, which is a racy global variable
   that can't safely be consulted, storing values in a different
   error space.)

 - The Go call passed res_search a non-NUL-terminated name.

 - The C res_search rejects calls asking for TypeALL as opposed to
   more specific answers like TypeA/TypeAAAA/TypeCNAME,
   breaking cgoLookupHost in all cases and cgoLookupIP
   except with IP-version-specific networks.

 - The DNS response packet was parsed twice, once with msg.Unpack
   (discarded), and once with the lower-level dnsmessage.Parser.
   The Parser loop was missing a call to p.SkipAllQuestions, with the
   result that no DNS response packet would ever parse successfully.

 - The parsing of the DNS response answers, if reached, behaved as if
   that the AResource and AAAAResource record contained textual
   IP addresses, while in fact they contain binary ones. The calls to
   parseIPv4 and parseIPv6 therefore would always returns nil,
   so that no useful result would be returned from the resolver.

With these fixes, cgoLookupIP can correctly resolve google.com
and return both the A and AAAA addresses.

Even after fixing all these things, TestGoLookupIP still fails,
because it is testing that in non-cgo builds the cgo stubs
correctly report "I can't handle the lookup", and as written the
code intentionally violates that expectation.

This CL adds new direct tests of the pseudo-cgo routines.
The direct IP address lookups succeed, but the CNAME query
causes res_search to hang, and the PTR query fails unconditionally
(a trivial C program confirms these behaviors are due to res_search itself).

Traditionally, res_search is only intended for single-threaded use.
It is unclear whether this one is safe for use from multiple goroutines.
If you run net.test under lldb, that causes syslog messages to be
printed to standard error suggesting double-free bugs:

	2019-06-05 19:52:43.505246-0400 net.test[6256:6831076] dnssd_clientstub DNSServiceRefDeallocate called with invalid DNSServiceRef 0x5c000f0 FFFFFFFF DDDDDDDD
	2019-06-05 19:52:43.505274-0400 net.test[6256:6831076] dnssd_clientstub DNSServiceRefDeallocate called with invalid DNSServiceRef 0x5c000f0 FFFFFFFF DDDDDDDD
	2019-06-05 19:52:43.505303-0400 net.test[6256:6831076] dnssd_clientstub DNSServiceRefDeallocate called with invalid DNSServiceRef 0x5c000f0 FFFFFFFF DDDDDDDD
	2019-06-05 19:52:43.505329-0400 net.test[6256:6831076] dnssd_clientstub DNSServiceRefDeallocate called with invalid DNSServiceRef 0x5c000f0 FFFFFFFF DDDDDDDD

This res_search is from libsystem_info; a normal C program would
get res_search (#defined to res_9_search) from libresolv instead.
It is unclear what the relation between the two is.
Issue #12524 was about supporting the /etc/resolver directory tree,
but only libresolv contains code for that; libsystem_info does not.
So this code probably does not enable use of /etc/resolver.

In short:

 - Before this CL, the code clearly had never run successfully.
 - The code appears not to improve upon the usual non-cgo fallback.
 - The code carries with it no tests of improved behavior.
 - The code breaks existing tests.
 - Calling res_search does not work for PTR/CNAME queries,
   so the code breaks existing behavior, even after this CL.
 - It's unclear whether res_search is safe to call from multiple threads.
 - It's unclear whether res_search is used by any other macOS programs.

Given this, it probably makes sense to delete this code rather
than rejigger the test. This CL fixes the code first, so that there
is a working copy to bring back later if we find out that it really
is necessary.

For #31705.

Change-Id: Id2e11e8ade43098b0f90dd4d16a62ca86a7a244a
Reviewed-on: https://go-review.googlesource.com/c/go/+/180842
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>