urls: helper function for extracting domain from url

urls.go

@@ -24,3 +24,14 @@ func CreateURL(origin, path string, params map[string]string) *url.URL {
 	u.RawQuery = query.Encode()
 	return u
 }
+
+// GetDomain extracts the domain name from a given url s.
+//
+// If s is not a valid url, the empty string is returned.
+func GetDomain(s string) string {
+	u, uerr := url.Parse(s)
+	if uerr != nil {
+		return ""
+	}
+	return u.Host
+}

urls_test.go

@@ -18,3 +18,11 @@ func TestCreateURL(t *testing.T) {
 	u := CreateURL(orig, "/hello", params)
 	must.Eq(t, "http://example.org:8000/hello?key=abc123&offset=3", u.String())
 }
+
+func TestGetDomain(t *testing.T) {
+	t.Parallel()
+
+	orig := "http://stage.example.org/foo/bar"
+	result := GetDomain(orig)
+	must.Eq(t, "stage.example.org", result)
+}