FIX: handle link tracking correctly for cdn based urls

(usually attachments)

app/models/topic_link_click.rb

@@ -29,6 +29,15 @@ class TopicLinkClick < ActiveRecord::Base
     urls << uri.path if uri.try(:host) == Discourse.current_hostname
     urls << url.sub(/\?.*$/, '') if url.include?('?')
 
+    # add a cdn link
+    if uri && Discourse.asset_host.present?
+      cdn_uri = URI.parse(Discourse.asset_host) rescue nil
+      if cdn_uri && cdn_uri.hostname == uri.hostname && uri.path.starts_with?(cdn_uri.path)
+        is_cdn_link = true
+        urls << uri.path[(cdn_uri.path.length)..-1]
+      end
+    end
+
     link = TopicLink.select([:id, :user_id])
 
     # test for all possible URLs
@@ -54,14 +63,7 @@ class TopicLinkClick < ActiveRecord::Base
       return nil unless uri
 
       # Only redirect to whitelisted hostnames
-      return url if WHITELISTED_REDIRECT_HOSTNAMES.include?(uri.hostname)
-
-      if Discourse.asset_host.present?
-        cdn_uri = URI.parse(Discourse.asset_host) rescue nil
-        if cdn_uri
-          return url if cdn_uri.hostname == uri.hostname && uri.path.starts_with?(cdn_uri.path)
-        end
-      end
+      return url if WHITELISTED_REDIRECT_HOSTNAMES.include?(uri.hostname) || is_cdn_link
 
       return nil
     end

spec/models/topic_link_click_spec.rb

@@ -122,6 +122,24 @@ describe TopicLinkClick do
               ip: '127.0.0.3')
 
             expect(url).to eq(nil)
+
+            # cdn better link track
+            path = "/uploads/site/29/5b585f848d8761d5.xls"
+
+            post = Fabricate(:post, topic: @topic, raw: "[test](#{path})")
+            TopicLink.extract_from(post)
+
+            url = TopicLinkClick.create_from(
+              url: "https://cdn.discourse.org/stuff#{path}",
+              topic_id: post.topic_id,
+              post_id: post.id,
+              ip: '127.0.0.3')
+
+            expect(url).to eq("https://cdn.discourse.org/stuff#{path}")
+
+            click = TopicLinkClick.order('id desc').first
+
+            expect(click.topic_link_id).to eq(TopicLink.order('id desc').first.id)
           end
 
         end