net: force cgo for myhostname and mdns nss modules for LookupAddr on unix

Currently there is a small bug in the LookupAddr for unix systems
that causes the use of go resolver instead of the cgo one.
Example for nss myhostname:

func main() {
        fmt.Println(net.LookupAddr(os.Args[1]))
}

root@arch:~# cat /etc/nsswitch.conf | grep host
hosts:          myhostname dns
root@arch:~# GODEBUG=netdns=+3 go run main.go 192.168.1.200
go package net: confVal.netCgo = false  netGo = false
go package net: dynamic selection of DNS resolver
go package net: hostLookupOrder() = dns
[] lookup 200.1.168.192.in-addr.arpa. on 8.8.8.8:53: no such host
root@arch:~# GODEBUG=netdns=go+3 go run main.go 192.168.1.200
go package net: confVal.netCgo = false  netGo = true
go package net: GODEBUG setting forcing use of Go's resolver
go package net: hostLookupOrder() = dns
[] lookup 200.1.168.192.in-addr.arpa. on 8.8.8.8:53: no such host
root@arch:~# GODEBUG=netdns=cgo+3 go run main.go 192.168.1.200
go package net: confVal.netCgo = true  netGo = false
go package net: using cgo DNS resolver
go package net: hostLookupOrder() = cgo
[arch] <nil>

The problem come from that we are only checking for hostnames that the
myhostname can resolve, but not for the addrs that it can also.

man  nss-myhostname:
       Please keep in mind that nss-myhostname (and nss-resolve) also
       resolve in the other direction — from locally attached IP
       addresses to hostnames.

Change-Id: Ic18a9f99a2214b2938463e9a95f7f3ca5db1c01b
GitHub-Last-Rev: ade40fd3e3057de418b9b6a79f79fb9a53fb6c09
GitHub-Pull-Request: golang/go#59921
Reviewed-on: https://go-review.googlesource.com/c/go/+/491235
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Mateusz Poliwczak <mpoliwczak34@gmail.com>

1 files changed, 36 insertions, 1 deletions

diff --git a/src/net/conf_test.go b/src/net/conf_test.go
index 08d774bfe2..d2cdac8083 100644
--- a/src/net/conf_test.go
+++ b/src/net/conf_test.go
@@ -304,7 +304,6 @@ func TestConfHostLookupOrder(t *testing.T) {
 				{"anything.localhost.localdomain", "myhostname", hostLookupCgo},
 				{"Anything.Localhost.Localdomain", "myhostname", hostLookupCgo},
 				{"somehostname", "myhostname", hostLookupFilesDNS},
-				{"", "myhostname", hostLookupFilesDNS}, // Issue 13623
 			},
 		},
 		{
@@ -392,6 +391,42 @@ func TestConfHostLookupOrder(t *testing.T) {
 	}
 }
 
+func TestAddrLookupOrder(t *testing.T) {
+	// This test is written for a system with cgo available,
+	// without using the netgo tag.
+	if netGoBuildTag {
+		t.Skip("skipping test because net package built with netgo tag")
+	}
+	if !cgoAvailable {
+		t.Skip("skipping test because cgo resolver not available")
+	}
+
+	defer setSystemNSS(getSystemNSS(), 0)
+	c, err := newResolvConfTest()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer c.teardown()
+
+	if !c.forceUpdateConf(defaultResolvConf, time.Now().Add(time.Hour)) {
+		t.Fatal("failed to change resolv config")
+	}
+
+	setSystemNSS(nssStr(t, "hosts: files myhostname dns"), time.Hour)
+	cnf := &conf{}
+	order, _ := cnf.addrLookupOrder(nil, "192.0.2.1")
+	if order != hostLookupCgo {
+		t.Errorf("addrLookupOrder returned: %v, want cgo", order)
+	}
+
+	setSystemNSS(nssStr(t, "hosts: files mdns4 dns"), time.Hour)
+	order, _ = cnf.addrLookupOrder(nil, "192.0.2.1")
+	if order != hostLookupCgo {
+		t.Errorf("addrLookupOrder returned: %v, want cgo", order)
+	}
+
+}
+
 func setSystemNSS(nss *nssConf, addDur time.Duration) {
 	nssConfig.mu.Lock()
 	nssConfig.nssConf = nss