FIX: never attempt to log invalid post numbers

Previously in some cases we would queue logging of invalid post numbers

The impact would be we would miss logging an incoming link and would leak
an error.

app/controllers/topics_controller.rb

@@ -806,7 +806,7 @@ class TopicsController < ApplicationController
         host: request.host,
         current_user: current_user,
         topic_id: @topic_view.topic.id,
-        post_number: params[:post_number],
+        post_number: @topic_view.current_post_number,
         username: request['u'],
         ip_address: request.remote_ip
       }

app/serializers/topic_view_serializer.rb

@@ -174,11 +174,11 @@ class TopicViewSerializer < ApplicationSerializer
   end
 
   def current_post_number
-    [object.post_number, object.highest_post_number].min
+    object.current_post_number
   end
 
   def include_current_post_number?
-    object.highest_post_number.present?
+    object.current_post_number.present?
   end
 
   def highest_post_number

lib/topic_view.rb

@@ -481,6 +481,14 @@ class TopicView
     @filtered_posts.order(sort_order: :desc).limit(1).pluck(:id).first
   end
 
+  def current_post_number
+    if highest_post_number.present?
+      post_number > highest_post_number ? highest_post_number : post_number
+    else
+      nil
+    end
+  end
+
   protected
 
   def read_posts_set

spec/requests/topics_controller_spec.rb

@@ -1095,6 +1095,18 @@ RSpec.describe TopicsController do
       end.to change(TopicViewItem, :count).by(1)
     end
 
+    it 'records a view to invalid post_number' do
+      user = Fabricate(:user)
+
+      expect do
+        get "/t/#{topic.slug}/#{topic.id}/#{256**4}", params: {
+          u: user.username
+        }
+        expect(response.status).to eq(200)
+      end.to change { IncomingLink.count }.by(1)
+
+    end
+
     it 'records incoming links' do
       user = Fabricate(:user)