MDL-60209 mathjax: Do not load broken accessibility extensions

The Accessible.js config loads accessibility-menu extension which in
turn loads mathjax-sre.js library. There is a bug in this library
https://github.com/zorkow/speech-rule-engine/issues/182 that makes it
fail in RequireJS environments. This has been reported upstream as
https://github.com/mathjax/MathJax/issues/1854

As an immediate solution for now, this patch changes the default MathJax
configuration so that it does not load accessibility extensions at all.

filter/mathjaxloader/db/upgrade.php

@@ -140,5 +140,34 @@ function xmldb_filter_mathjaxloader_upgrade($oldversion) {
         upgrade_plugin_savepoint(true, 2017100900, 'filter', 'mathjaxloader');
     }
 
+    if ($oldversion < 2017101200) {
+        // Update default MathJax configuration so that it does not use the Accessible.js config (causes JS errors due to upstream bug).
+        $previousdefault = '
+MathJax.Hub.Config({
+    config: ["Accessible.js", "Safe.js"],
+    errorSettings: { message: ["!"] },
+    skipStartupTypeset: true,
+    messageStyle: "none"
+});
+';
+
+        $newdefault = '
+MathJax.Hub.Config({
+    config: ["default.js", "MMLorHTML.js", "Safe.js"],
+    errorSettings: { message: ["!"] },
+    skipStartupTypeset: true,
+    messageStyle: "none"
+});
+';
+
+        $mathjaxconfig = get_config('filter_mathjaxloader', 'mathjaxconfig');
+
+        if (empty($mathjaxconfig) || filter_mathjaxloader_upgrade_mathjaxconfig_equal($mathjaxconfig, $previousdefault)) {
+            set_config('mathjaxconfig', $newdefault, 'filter_mathjaxloader');
+        }
+
+        upgrade_plugin_savepoint(true, 2017101200, 'filter', 'mathjaxloader');
+    }
+
     return true;
 }

filter/mathjaxloader/db/upgradelib.php

@@ -64,3 +64,32 @@ function filter_mathjaxloader_upgrade_cdn_cloudflare($mathjaxurl, $httponly = fa
 
     return $newcdnurl;
 }
+
+/**
+ * Compares two values of the 'mathjaxconfig' config option.
+ *
+ * This is used during the upgrade to see if the two text values of the 'mathjaxconfig' config option should be
+ * considered equal of different. The strings are normalized so that EOL characters and whitespace is not significant.
+ *
+ * @param string $val1 value
+ * @param string $val2 value
+ * @return bool true if the texts should be considered equals, false otherwise
+ */
+function filter_mathjaxloader_upgrade_mathjaxconfig_equal($val1, $val2) {
+
+    $val1lines = preg_split("/[\r\n]/", $val1);
+    $val2lines = preg_split("/[\r\n]/", $val2);
+
+    $val1lines = array_map('trim', $val1lines);
+    $val2lines = array_map('trim', $val2lines);
+
+    $val1lines = array_filter($val1lines, function($value) {
+        return $value !== '';
+    });
+
+    $val2lines = array_filter($val2lines, function($value) {
+        return $value !== '';
+    });
+
+    return (implode(' ', $val1lines) === implode(' ', $val2lines));
+}

filter/mathjaxloader/readme_moodle.txt

@@ -17,3 +17,10 @@ Upgrading the default MathJax version
    previous default.
 3. Check and eventually update the list of language mappings in filter.php.
    Also see the unit test for the language mappings.
+
+Changes
+-------
+
+* The MathJax 2.7.2 seems to have a bug causing the accessibility extensions
+  fail in web apps using RequireJS (such as Moodle). We had to stop using the
+  Accessible.js config for that reason. See MDL-60209 for details.

filter/mathjaxloader/settings.php

@@ -45,7 +45,7 @@ if ($ADMIN->fulltree) {
 
     $default = '
 MathJax.Hub.Config({
-    config: ["Accessible.js", "Safe.js"],
+    config: ["default.js", "MMLorHTML.js", "Safe.js"],
     errorSettings: { message: ["!"] },
     skipStartupTypeset: true,
     messageStyle: "none"

filter/mathjaxloader/tests/upgradelib_test.php

@@ -37,7 +37,10 @@ require_once($CFG->dirroot . '/filter/mathjaxloader/db/upgradelib.php');
  */
 class filter_mathjax_upgradelib_testcase extends advanced_testcase {
 
-    public function test_upgradelib() {
+    /**
+     * Tests for {@link filter_mathjaxloader_upgrade_cdn_cloudflare()} function.
+     */
+    public function test_filter_mathjaxloader_upgrade_cdn_cloudflare() {
         $current = 'https://cdn.mathjax.org/mathjax/2.7-latest/MathJax.js?...';
         $expected = 'https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.0/MathJax.js?...';
         $this->assertEquals($expected, filter_mathjaxloader_upgrade_cdn_cloudflare($current));
@@ -87,5 +90,35 @@ class filter_mathjax_upgradelib_testcase extends advanced_testcase {
         $expected = 'https://cdn.mathjax.org/mathjax/2.7-latest';
         $this->assertEquals($expected, filter_mathjaxloader_upgrade_cdn_cloudflare($current));
     }
+
+    /**
+     * Tests for {@link filter_mathjaxloader_upgrade_mathjaxconfig_equal()} function.
+     */
+    public function test_filter_mathjaxloader_upgrade_mathjaxconfig_equal() {
+
+        $val1 = '';
+        $val2 = "\n \r \r\n \t    ";
+        $this->assertTrue(filter_mathjaxloader_upgrade_mathjaxconfig_equal($val1, $val2));
+
+        $val1 = '0';
+        $val2 = '';
+        $this->assertFalse(filter_mathjaxloader_upgrade_mathjaxconfig_equal($val1, $val2));
+
+        $val1 = 'Hello Unittest, my old friend    '.PHP_EOL."I've come to play with you again  \r\n\r\n  \t ";
+        $val2 = '   Hello Unittest, my old friend '."\r\n\r\n"."  I've come to play with you again \n\n\n  ";
+        $this->assertTrue(filter_mathjaxloader_upgrade_mathjaxconfig_equal($val1, $val2));
+
+        $val1 = "\n".'MathJax.Hub.Config({'."\n".'    config: ["Accessible.js", "Safe.js"]'."\n".'});'."\n";
+        $val2 = 'MathJax.Hub.Config({'."\r".'config: ["Accessible.js", "Safe.js"]'."\r".'});';
+        $this->assertTrue(filter_mathjaxloader_upgrade_mathjaxconfig_equal($val1, $val2));
+
+        $val1 = "\r\n\t".'MathJax.Hub.Config({'."\r\n\t".' config: ["Accessible.js", "Safe.js"]'."\r\n".'});  '."\r\n\r\n";
+        $val2 = 'MathJax.Hub.Config({'."\n".'config: ["Accessible.js", "Safe.js"]'."\r".'});';
+        $this->assertTrue(filter_mathjaxloader_upgrade_mathjaxconfig_equal($val1, $val2));
+
+        $val2 = 'MathJax.Hub.Config({'."\n".'config: ["Significant.js"]'."\n".'});';
+        $val2 = 'MathJax.Hub.Config({'."\n".'config: ["Signi ficant.js", "Safe.js"]'."\n".'});';
+        $this->assertFalse(filter_mathjaxloader_upgrade_mathjaxconfig_equal($val1, $val2));
+    }
 }
 

filter/mathjaxloader/version.php

@@ -24,6 +24,6 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$plugin->version  = 2017100900;
+$plugin->version  = 2017101200;
 $plugin->requires = 2017050500;  // Requires this Moodle version.
 $plugin->component= 'filter_mathjaxloader';