Issue 6622 - Implement $rewrite filter option

lib/requestBlocker.js

Index: lib/requestBlocker.js
===================================================================
--- a/lib/requestBlocker.js
+++ b/lib/requestBlocker.js
@@ -65,20 +65,21 @@
 
   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.
+  // POPUP, CSP, REWRITE and ELEMHIDE filters aren't mapped to resource types.
   yield "POPUP";
   yield "ELEMHIDE";
   yield "CSP";
+  yield "REWRITE";
 }());
 
 function getDocumentInfo(page, frame, originUrl)
 {
   return [
     extractHostFromFrame(frame, originUrl),
     getKey(page, frame, originUrl),
     !!checkWhitelisted(page, frame, originUrl,
@@ -189,16 +190,33 @@
     return;
 
   let type = resourceTypes.get(details.type) || "OTHER";
   let [docDomain, sitekey, specificOnly] = getDocumentInfo(page, frame,
                                                            originUrl);
   let [filter, urlString, thirdParty] = matchRequest(url, type, docDomain,
                                                      sitekey, specificOnly);
 
+  if (filter instanceof BlockingFilter && filter.rewrite)
+  {
+    let rewritten = filter.doRewrite(urlString);
+    if (rewritten && rewritten != urlString)
+    {
+      getRelatedTabIds(details).then(tabIds =>
+      {
+        logRequest(tabIds, urlString, "REWRITE", docDomain,
+                   thirdParty, sitekey, specificOnly, filter);
+      });
+
+      return {redirectUrl: rewritten};
+    }
+    // we couldn't do the rewrite, so just let it through.
+    return;
+  }
+
   getRelatedTabIds(details).then(tabIds =>
   {
     logRequest(tabIds, urlString, type, docDomain,
                thirdParty, sitekey, specificOnly, filter);
   });
 
   if (filter instanceof BlockingFilter)
     return {cancel: true};