Issue 5956 - Catch exception when enumerating stylesheets

lib/content/elemHideEmulation.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 397 matching lines @@
     let cssStyles = [];
 
     let stylesheetOnlyChange = !!stylesheets;
     if (!stylesheets)
       stylesheets = this.document.styleSheets;
 
     try
     {
       for (let stylesheet of stylesheets)
       {
-        // Explicitly ignore third-party stylesheets to ensure consistent behavior
-        // between Firefox and Chrome.
-        if (!this.isSameOrigin(stylesheet))
-          continue;
-
-        let rules = stylesheet.cssRules;
-        if (!rules)
-          continue;
-
-        for (let rule of rules)
-        {
-          if (rule.type != rule.STYLE_RULE)
+        try
+        {
+          // Explicitly ignore third-party stylesheets to ensure consistent behavior

[kzar, 2017/11/03 10:20:50]

Nit: This line is too long, did you check the code lints?

+          // between Firefox and Chrome.
+          if (!this.isSameOrigin(stylesheet))
             continue;
 
-          cssStyles.push(stringifyStyle(rule));
+          let rules = stylesheet.cssRules;
+          if (!rules)
+            continue;
+
+          for (let rule of rules)
+          {
+            if (rule.type != rule.STYLE_RULE)
+              continue;
+
+            cssStyles.push(stringifyStyle(rule));
+          }
+        }
+        catch (e)
+        {
+          // On Firefox we seem to get a InvalidAccessError when

[kzar, 2017/11/03 10:20:50]

Nit: "...get an...".

+          // accessing the stylesheet under some circumstances.
         }
       }
     }
     catch (e)
     {
-      // Make sure to not interrupt.
       // On Firefox we seem to get a InvalidAccessError when

[kzar, 2017/11/03 10:20:50]

Maybe combine these two identical comments and add some more details like the
Firefox version or a Mozilla issue number?

-      // enumerating the stylesheets.
+      // enumerating the stylesheet under some circumstances.
     }
 
     let patterns = this.patterns.slice();
     let pattern = null;
     let generator = null;
 
     let processPatterns = () =>
     {
       let cycleStart = performance.now();
 
@@ skipping 149 matching lines @@
           characterData: true,
           subtree: true
         }
       );
       this.document.addEventListener("load", this.onLoad.bind(this), true);
     }
   }
 };
 
 exports.ElemHideEmulation = ElemHideEmulation;