netutil: add func ListenHostPort

pkg/netutil/netutil.go

@@ -44,6 +44,8 @@ func AwaitReachable(addr string, maxWait time.Duration) error {
 // to passing to net.Dial, with the port set as the scheme's default port if
 // absent.
 func HostPort(urlStr string) (string, error) {
+	// TODO: rename this function to URLHostPort instead, like
+	// ListenHostPort below.
 	u, err := url.Parse(urlStr)
 	if err != nil {
 		return "", fmt.Errorf("could not parse %q as a url: %v", urlStr, err)
@@ -69,6 +71,22 @@ func HostPort(urlStr string) (string, error) {
 	return hostPort, nil
 }
 
+// ListenHostPort maps a listen address into a host:port string.
+// If the host part in listenAddr is empty or 0.0.0.0, localhost
+// is used instead.
+func ListenHostPort(listenAddr string) (string, error) {
+	hp := listenAddr
+	if strings.HasPrefix(hp, ":") {
+		hp = "localhost" + hp
+	} else if strings.HasPrefix(hp, "0.0.0.0:") {
+		hp = "localhost:" + hp[len("0.0.0.0:"):]
+	}
+	if _, _, err := net.SplitHostPort(hp); err != nil {
+		return "", err
+	}
+	return hp, nil
+}
+
 // ListenOnLocalRandomPort returns a TCP listener on a random
 // localhost port.
 func ListenOnLocalRandomPort() (net.Listener, error) {

pkg/netutil/netutil_test.go

@@ -123,6 +123,29 @@ func TestHostPort(t *testing.T) {
 	}
 }
 
+func TestListenHostPort(t *testing.T) {
+	tests := []struct {
+		in   string
+		want string // or "ERR:"
+	}{
+		{":80", "localhost:80"},
+		{"0.0.0.0:80", "localhost:80"},
+		{"foo:80", "foo:80"},
+		{"foo:0", "foo:0"},
+		{"", "ERR"},
+	}
+	for _, tt := range tests {
+		got, err := ListenHostPort(tt.in)
+		if err != nil {
+			got = "ERR"
+		}
+		if got != tt.want {
+			t.Errorf("ListenHostPort(%q) = %q, %v; want %q", tt.in, got, err, tt.want)
+		}
+	}
+
+}
+
 func testLocalhostResolver(t *testing.T, resolve func() net.IP) {
 	ip := resolve()
 	if ip == nil {