Issue 6622 - Implement $rewrite filter option

lib/requestBlocker.js

Index: lib/requestBlocker.js
===================================================================
--- a/lib/requestBlocker.js
+++ b/lib/requestBlocker.js
@@ -65,17 +65,17 @@
 
   for (let type in browser.webRequest.ResourceType)
     yield resourceTypes.get(browser.webRequest.ResourceType[type]) || "OTHER";
 
   // WEBRTC gets addressed through a workaround, even if the webRequest API is
   // lacking support to block this kind of a request.
   yield "WEBRTC";
 
-  // POPUP, CSP and ELEMHIDE filters aren't mapped to resource types.
+  // These filter types aren't mapped to resource types.
   yield "POPUP";
   yield "ELEMHIDE";
   yield "CSP";
 }());
 
 function getDocumentInfo(page, frame, originUrl)
 {
   return [
@@ -180,27 +180,46 @@
     return;
 
   let type = resourceTypes.get(details.type) || "OTHER";
   let [docDomain, sitekey, specificOnly] = getDocumentInfo(page, frame,
                                                            originUrl);
   let [filter, thirdParty] = matchRequest(url, type, docDomain,
                                           sitekey, specificOnly);
 
+  let result;
+  let rewritten;
+
+  if (filter instanceof BlockingFilter)
+  {
+    if (filter.rewrite)
+    {
+      rewritten = filter.rewriteUrl(details.url);
+      // If no rewrite happened (error, diff origin), we'll
+      // return undefined in order to avoid an "infinite" loop.
+      if (rewritten != details.url)
+        result = {redirectUrl: rewritten};
+    }
+    else
+      result = {cancel: true};
+  }
+
   getRelatedTabIds(details).then(tabIds =>
   {
     logRequest(
       tabIds,
-      {url: details.url, type, docDomain, thirdParty, sitekey, specificOnly},
+      {
+        url: details.url, type, docDomain, thirdParty,
+        sitekey, specificOnly, rewrittenTo: rewritten

[Sebastian Noack, 2018/05/10 15:09:52]

Nit: How about just calling the variable rewritteTo for consistency?

[hub, 2018/05/10 16:08:48]

Done.

+      },
       filter
     );
   });
 
-  if (filter instanceof BlockingFilter)
-    return {cancel: true};
+  return result;
 }, {urls: ["<all_urls>"]}, ["blocking"]);
 
 port.on("filters.collapse", (message, sender) =>
 {
   let {page, frame} = sender;
 
   if (checkWhitelisted(page, frame))
     return false;