[Bug go/63731] Fallback to netgo does not work

["yohei at jp dot ibm.com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63731

--- Comment #10 from Yohei Ueda <yohei at jp dot ibm.com> ---
https://code.google.com/p/go/source/browse/src/net/lookup_unix.go#63

func lookupIP(host string) (addrs []IP, err error) {
        addrs, err, ok := cgoLookupIP(host)
        if !ok {
                addrs, err = goLookupIP(host)
        }
        return
}

This code shows how fallback works. If cgoLookup returns ok == false, then
pure-Go goLookupIP is called.

When the netgo tag is set, ok is always false.
https://code.google.com/p/go/source/browse/src/net/cgo_stub.go#5

When the netgo tag is not set, ok is always true.
https://code.google.com/p/go/source/browse/src/net/cgo_unix.go#147
https://code.google.com/p/go/source/browse/src/net/cgo_unix.go#84

If cgoLookupIP can also return ok == false when no nss is available, goLookupIP
will be called. This is my first idea.
https://code.google.com/p/go/source/browse/src/net/cgo_unix.go#116

However, I realized that we cannot easily distinguish the "not found" errors.
getaddrinfo returns "not found" in both cases of invalid hostname lookups and
static binaries.

If cgoLookup returns ok == false even when an invalid host name is looked up
with nss available, goLookupIP also looks it up again, so IP lookup occurs
twice, and both fails with "not found" error. I don't think this is desired
behavior.