net: respect go vs cgo resolver selection in all lookup routines

This is especially important for LookupAddr, which used to be pure Go
(lightweight, one goroutine per call) and without this CL is now
unconditionally cgo (heavy, one thread per call).

Fixes #12190.

Change-Id: I43436a942bc1838b024225893e156f280a1e80cf
Reviewed-on: https://go-review.googlesource.com/13698Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/conf.go

@@ -19,7 +19,7 @@ type conf struct {
 	// forceCgoLookupHost forces CGO to always be used, if available.
 	forceCgoLookupHost bool
 
-	netGo  bool // "netgo" build tag in use (or no cgo)
+	netGo  bool // go DNS resolution forced
 	netCgo bool // cgo DNS resolution forced
 
 	// machine has an /etc/mdns.allow file
@@ -112,6 +112,12 @@ func initConfVal() {
 	}
 }
 
+// canUseCgo reports whether calling cgo functions is allowed
+// for non-hostname lookups.
+func (c *conf) canUseCgo() bool {
+	return c.hostLookupOrder("") == hostLookupCgo
+}
+
 // hostLookupOrder determines which strategy to use to resolve hostname.
 func (c *conf) hostLookupOrder(hostname string) (ret hostLookupOrder) {
 	if c.dnsDebugLevel > 1 {

src/net/lookup_unix.go

@@ -74,19 +74,21 @@ func lookupIP(host string) (addrs []IPAddr, err error) {
 }
 
 func lookupPort(network, service string) (int, error) {
-	port, err, ok := cgoLookupPort(network, service)
-	if !ok {
-		port, err = goLookupPort(network, service)
+	if systemConf().canUseCgo() {
+		if port, err, ok := cgoLookupPort(network, service); ok {
+			return port, err
+		}
 	}
-	return port, err
+	return goLookupPort(network, service)
 }
 
 func lookupCNAME(name string) (string, error) {
-	cname, err, ok := cgoLookupCNAME(name)
-	if !ok {
-		cname, err = goLookupCNAME(name)
+	if systemConf().canUseCgo() {
+		if cname, err, ok := cgoLookupCNAME(name); ok {
+			return cname, err
+		}
 	}
-	return cname, err
+	return goLookupCNAME(name)
 }
 
 func lookupSRV(service, proto, name string) (string, []*SRV, error) {
@@ -148,9 +150,10 @@ func lookupTXT(name string) ([]string, error) {
 }
 
 func lookupAddr(addr string) ([]string, error) {
-	ptrs, err, ok := cgoLookupPTR(addr)
-	if !ok {
-		ptrs, err = goLookupPTR(addr)
+	if systemConf().canUseCgo() {
+		if ptrs, err, ok := cgoLookupPTR(addr); ok {
+			return ptrs, err
+		}
 	}
-	return ptrs, err
+	return goLookupPTR(addr)
 }