net: make LookupPort with empty service mean 0

Fixes #13610

Change-Id: I9c8f924dc1ad515a9697291e981ece34fdbec8b7
Reviewed-on: https://go-review.googlesource.com/17755
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/lookup.go

@@ -123,6 +123,11 @@ func lookupIPDeadline(host string, deadline time.Time) (addrs []IPAddr, err erro
 
 // LookupPort looks up the port for the given network and service.
 func LookupPort(network, service string) (port int, err error) {
+	if service == "" {
+		// Lock in the legacy behavior that an empty string
+		// means port 0. See Issue 13610.
+		return 0, nil
+	}
 	port, _, ok := dtoi(service, 0)
 	if !ok && port != big && port != -big {
 		port, err = lookupPort(network, service)

src/net/lookup_test.go

@@ -591,6 +591,12 @@ var lookupPortTests = []struct {
 	{"tcp", "65536", 0, false},
 	{"udp", "-1", 0, false},
 	{"udp", "65536", 0, false},
+
+	// Issue 13610: LookupPort("tcp", "")
+	{"tcp", "", 0, true},
+	{"tcp6", "", 0, true},
+	{"tcp4", "", 0, true},
+	{"udp", "", 0, true},
 }
 
 func TestLookupPort(t *testing.T) {