Issue 6382 - Catch InvalidAccessError exception when checking styles.

lib/content/elemHideEmulation.js

Index: lib/content/elemHideEmulation.js
===================================================================
--- a/lib/content/elemHideEmulation.js
+++ b/lib/content/elemHideEmulation.js
@@ -603,17 +603,30 @@
 
     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;
+      let rules;
+      try
+      {
+        rules = stylesheet.cssRules;
+      }
+      catch (e)
+      {
+        // On Firefox, there is a chance that an InvalidAccessError
+        // get thrown when accessing cssRules. Just skip the stylesheet
+        // in that case.
+        // See https://issues.adblockplus.org/ticket/6382
+        continue;

[kzar, 2018/04/18 16:12:30]

I wonder if we should check that the exception really is an InvalidAccessError
before continuing? Then we wouldn't ignore other exceptions, also we could
shorten the comment since the code would already be clear about the type of
exception we're worrying about.

[hub, 2018/04/18 18:48:23]

The intent here is to catch the exception to not interrupt the processing. There
is nothing much we can do instead so I don't think it matters.

[kzar, 2018/04/19 07:29:29]

Acknowledged.

+      }
+
       if (!rules)
         continue;
 
       for (let rule of rules)
       {
         if (rule.type != rule.STYLE_RULE)
           continue;