MDL-56488 boost: validate scss settings for parsable scss

* Parses the scss and fails validation if a parse error is found

* Does not detect the situation when variables are not present - this
  would involve us parsing the entire tree and would be slow. It could
  also change over time, depending on whats defined in the scss
  from themes.

* Introduces a new admin_setting_scsscode to do this

lang/en/admin.php

@@ -969,6 +969,7 @@
 $string['rssglobaldisabled'] = 'Disabled at server level';
 $string['save'] = 'Save';
 $string['savechanges'] = 'Save changes';
+$string['scssinvalid'] = 'SCSS code is not valid, fails with: {$a}';
 $string['search'] = 'Search';
 $string['searchalldeleted'] = 'All indexed contents have been deleted';
 $string['searchareaenabled'] = 'Search area enabled';

lib/adminlib.php

@@ -10180,3 +10180,41 @@ public function output_html($data, $query='') {
     }
 
 }
+
+/**
+ * Used to validate the contents of SCSS code and ensuring they are parsable.
+ *
+ * It does not attempt to detect undefined SCSS variables because it is designed
+ * to be used without knowledge of other config/scss included.
+ *
+ * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ * @copyright 2016 Dan Poltawski <dan@moodle.com>
+ */
+class admin_setting_scsscode extends admin_setting_configtextarea {
+
+    /**
+     * Validate the contents of the SCSS to ensure its parsable. Does not
+     * attempt to detect undefined scss variables.
+     *
+     * @param string $data The scss code from text field.
+     * @return mixed bool true for success or string:error on failure.
+     */
+    public function validate($data) {
+        if (empty($data)) {
+            return true;
+        }
+
+        $scss = new core_scss();
+        try {
+            $scss->compile($data);
+        } catch (Leafo\ScssPhp\Exception\ParserException $e) {
+            return get_string('scssinvalid', 'admin', $e->getMessage());
+        } catch (Leafo\ScssPhp\Exception\CompilerException $e) {
+            // Silently ignore this - it could be a scss variable defined from somewhere
+            // else which we are not examining here.
+            return true;
+        }
+
+        return true;
+    }
+}

theme/boost/settings.php

@@ -73,13 +73,13 @@
     $page = new admin_settingpage('theme_boost_advanced', get_string('advancedsettings', 'theme_boost'));
 
     // Raw SCSS to include before the content.
-    $setting = new admin_setting_configtextarea('theme_boost/scsspre',
+    $setting = new admin_setting_scsscode('theme_boost/scsspre',
         get_string('rawscsspre', 'theme_boost'), get_string('rawscsspre_desc', 'theme_boost'), '', PARAM_RAW);
     $setting->set_updatedcallback('theme_reset_all_caches');
     $page->add($setting);
 
     // Raw SCSS to include after the content.
-    $setting = new admin_setting_configtextarea('theme_boost/scss', get_string('rawscss', 'theme_boost'),
+    $setting = new admin_setting_scsscode('theme_boost/scss', get_string('rawscss', 'theme_boost'),
         get_string('rawscss_desc', 'theme_boost'), '', PARAM_RAW);
     $setting->set_updatedcallback('theme_reset_all_caches');
     $page->add($setting);