net: make Resolver.PreferGo work more as documented

Fixes #24393

Change-Id: I8bcee34cdf30472663d866ed6056301d8445215c
Reviewed-on: https://go-review.googlesource.com/100875
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/conf.go

@@ -114,18 +114,19 @@ func initConfVal() {
 // canUseCgo reports whether calling cgo functions is allowed
 // for non-hostname lookups.
 func (c *conf) canUseCgo() bool {
-	return c.hostLookupOrder("") == hostLookupCgo
+	return c.hostLookupOrder(nil, "") == hostLookupCgo
 }
 
 // hostLookupOrder determines which strategy to use to resolve hostname.
-func (c *conf) hostLookupOrder(hostname string) (ret hostLookupOrder) {
+// The provided Resolver is optional. nil means to not consider its options.
+func (c *conf) hostLookupOrder(r *Resolver, hostname string) (ret hostLookupOrder) {
 	if c.dnsDebugLevel > 1 {
 		defer func() {
 			print("go package net: hostLookupOrder(", hostname, ") = ", ret.String(), "\n")
 		}()
 	}
 	fallbackOrder := hostLookupCgo
-	if c.netGo {
+	if c.netGo || (r != nil && r.PreferGo) {
 		fallbackOrder = hostLookupFilesDNS
 	}
 	if c.forceCgoLookupHost || c.resolv.unknownOpt || c.goos == "android" {

src/net/conf_test.go

@@ -33,6 +33,7 @@ func TestConfHostLookupOrder(t *testing.T) {
 	tests := []struct {
 		name      string
 		c         *conf
+		resolver  *Resolver
 		hostTests []nssHostTest
 	}{
 		{
@@ -322,6 +323,21 @@ func TestConfHostLookupOrder(t *testing.T) {
 				{"x.com", "myhostname", hostLookupCgo},
 			},
 		},
+		// Issue 24393: make sure "Resolver.PreferGo = true" acts like netgo.
+		{
+			name:     "resolver-prefergo",
+			resolver: &Resolver{PreferGo: true},
+			c: &conf{
+				goos:               "darwin",
+				forceCgoLookupHost: true, // always true for darwin
+				resolv:             defaultResolvConf,
+				nss:                nssStr(""),
+				netCgo:             true,
+			},
+			hostTests: []nssHostTest{
+				{"localhost", "myhostname", hostLookupFilesDNS},
+			},
+		},
 	}
 
 	origGetHostname := getHostname
@@ -331,7 +347,7 @@ func TestConfHostLookupOrder(t *testing.T) {
 		for _, ht := range tt.hostTests {
 			getHostname = func() (string, error) { return ht.localhost, nil }
 
-			gotOrder := tt.c.hostLookupOrder(ht.host)
+			gotOrder := tt.c.hostLookupOrder(tt.resolver, ht.host)
 			if gotOrder != ht.want {
 				t.Errorf("%s: hostLookupOrder(%q) = %v; want %v", tt.name, ht.host, gotOrder, ht.want)
 			}

src/net/dnsclient_unix.go

@@ -524,7 +524,7 @@ func goLookupIPFiles(name string) (addrs []IPAddr) {
 // goLookupIP is the native Go implementation of LookupIP.
 // The libc versions are in cgo_*.go.
 func (r *Resolver) goLookupIP(ctx context.Context, host string) (addrs []IPAddr, err error) {
-	order := systemConf().hostLookupOrder(host)
+	order := systemConf().hostLookupOrder(r, host)
 	addrs, _, err = r.goLookupIPCNAMEOrder(ctx, host, order)
 	return
 }
@@ -676,7 +676,7 @@ func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, name string, order
 
 // goLookupCNAME is the native Go (non-cgo) implementation of LookupCNAME.
 func (r *Resolver) goLookupCNAME(ctx context.Context, host string) (string, error) {
-	order := systemConf().hostLookupOrder(host)
+	order := systemConf().hostLookupOrder(r, host)
 	_, cname, err := r.goLookupIPCNAMEOrder(ctx, host, order)
 	return cname.String(), err
 }

src/net/lookup_unix.go

@@ -74,7 +74,7 @@ func (r *Resolver) dial(ctx context.Context, network, server string) (Conn, erro
 }
 
 func (r *Resolver) lookupHost(ctx context.Context, host string) (addrs []string, err error) {
-	order := systemConf().hostLookupOrder(host)
+	order := systemConf().hostLookupOrder(r, host)
 	if !r.PreferGo && order == hostLookupCgo {
 		if addrs, err, ok := cgoLookupHost(ctx, host); ok {
 			return addrs, err
@@ -89,7 +89,7 @@ func (r *Resolver) lookupIP(ctx context.Context, host string) (addrs []IPAddr, e
 	if r.PreferGo {
 		return r.goLookupIP(ctx, host)
 	}
-	order := systemConf().hostLookupOrder(host)
+	order := systemConf().hostLookupOrder(r, host)
 	if order == hostLookupCgo {
 		if addrs, err, ok := cgoLookupIP(ctx, host); ok {
 			return addrs, err