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.
+  // These filter types 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,
@@ -189,24 +190,41 @@
     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);
 
+  let result;
+
+  if (filter instanceof BlockingFilter)
+  {
+    if (filter.rewrite)
+    {
+      let rewritten = filter.rewriteUrl(urlString);
+      if (!rewritten || rewritten == urlString)

[Manish Jethani, 2018/05/03 02:51:16]

This should probably be updated now, since rewriteUrl never returns a falsey
value. Actually we know at this point that filter.regexp matches, this means the
rewrite is happening even if the result is the same as the original URL. That's
OK, we can just remove this check here entirely.

[hub, 2018/05/03 18:43:48]

Done.

+        // we couldn't do the rewrite, so just let it through.
+        return;
+
+      type = "REWRITE";
+      result = {redirectUrl: rewritten};
+    }
+    else
+      result = {cancel: true};
+  }
+
   getRelatedTabIds(details).then(tabIds =>
   {
     logRequest(tabIds, urlString, type, docDomain,
                thirdParty, sitekey, specificOnly, 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;