MDL-57102 auth: Add new signup_is_enabled function

And apply the function in:
- login/signup.php
- blocks/login/block_login.php

auth/upgrade.txt

@@ -9,6 +9,7 @@ information provided here is intended especially for developers.
 * The authentication plugin auth_radius has been moved to https://github.com/moodlehq/moodle-auth_radius
 * New auth_email::user_signup_with_confirmation() method has a new optional parameter $confirmationurl to provide a different
   confirmation URL.
+* New signup_is_enabled() function available in lib/authlib.php to safely check if sign-up is enabled in the site.
 
 === 3.0 ===
 

blocks/login/block_login.php

@@ -33,6 +33,8 @@ class block_login extends block_base {
 
     function get_content () {
         global $USER, $CFG, $SESSION, $OUTPUT;
+        require_once($CFG->libdir . '/authlib.php');
+
         $wwwroot = '';
         $signup = '';
 
@@ -48,11 +50,8 @@ class block_login extends block_base {
             $wwwroot = str_replace("http://", "https://", $CFG->wwwroot);
         }
 
-        if (!empty($CFG->registerauth)) {
-            $authplugin = get_auth_plugin($CFG->registerauth);
-            if ($authplugin->can_signup()) {
-                $signup = $wwwroot . '/login/signup.php';
-            }
+        if (signup_is_enabled()) {
+            $signup = $wwwroot . '/login/signup.php';
         }
         // TODO: now that we have multiauth it is hard to find out if there is a way to change password
         $forgot = $wwwroot . '/login/forgot_password.php';

lib/authlib.php

@@ -916,3 +916,21 @@ function signup_get_user_confirmation_authplugin() {
     }
     return $authplugin;
 }
+
+/**
+ * Check if sign-up is enabled in the site. If is enabled, the function will return the authplugin instance.
+ *
+ * @return mixed false if sign-up is not enabled, the authplugin instance otherwise.
+ * @since  Moodle 3.2
+ */
+function signup_is_enabled() {
+    global $CFG;
+
+    if (!empty($CFG->registerauth)) {
+        $authplugin = get_auth_plugin($CFG->registerauth);
+        if ($authplugin->can_signup()) {
+            return $authplugin;
+        }
+    }
+    return false;
+}

login/signup.php

@@ -26,6 +26,7 @@
 
 require('../config.php');
 require_once($CFG->dirroot . '/user/editlib.php');
+require_once($CFG->libdir . '/authlib.php');
 
 // Try to prevent searching for sites that allow sign-up.
 if (!isset($CFG->additionalhtmlhead)) {
@@ -33,12 +34,7 @@ if (!isset($CFG->additionalhtmlhead)) {
 }
 $CFG->additionalhtmlhead .= '<meta name="robots" content="noindex" />';
 
-if (empty($CFG->registerauth)) {
-    print_error('notlocalisederrormessage', 'error', '', 'Sorry, you may not use this page.');
-}
-$authplugin = get_auth_plugin($CFG->registerauth);
-
-if (!$authplugin->can_signup()) {
+if (!$authplugin = signup_is_enabled()) {
     print_error('notlocalisederrormessage', 'error', '', 'Sorry, you may not use this page.');
 }