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
@@ -521,26 +521,37 @@
 
     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)
+      try
       {
-        if (rule.type != rule.STYLE_RULE)
+        let rules = stylesheet.cssRules;

[kzar, 2018/03/19 15:52:07]

If this is the only line that could cause the exception to be thrown, perhaps we
should only wrap this line in the try?

[hub, 2018/03/20 14:53:53]

Done.

+        if (!rules)
           continue;
 
-        cssStyles.push(stringifyStyle(rule));
+        for (let rule of rules)
+        {
+          if (rule.type != rule.STYLE_RULE)
+            continue;
+
+          cssStyles.push(stringifyStyle(rule));
+        }
+      }
+      catch (e)
+      {
+        // On Firefox, there is a chance that an InvalidAccessError

[kzar, 2018/03/19 15:52:07]

How come this happens? Is there a Firefox bug / did you file one?

[hub, 2018/03/20 16:16:02]

It's not a bug.

https://searchfox.org/mozilla-central/source/layout/style/StyleSheet.cpp#732

The stylesheet haven't been completely loaded yet. AreRulesAvailable() is called
when GetCssRules() is called.

I'm not sure if there is anything we can do to ensure rules are available, but
catching the exception allow to not cancel the processing for the rest.

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

Acknowledged.

In fact that link you gave is pretty useful, perhaps add that instead of the
link to our issue tracker? (Well a permalink for the right line like
https://searchfox.org/mozilla-central/rev/f65d7528e34ef1a7665b4a1a7b7cdb1388f...
)

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

Done.

+        // get thrown when accessing cssRules. Just skip the stylesheet
+        // in that case.
+        // See https://issues.adblockplus.org/ticket/6382
+        continue;
       }
     }
 
     let patterns = this.patterns.slice();
     let pattern = null;
     let generator = null;
 
     let processPatterns = () =>