FIX: corrently handle hidden tags when checking for edit conflicts

In 806e37aaec, I improved the conflict handling when editing a post to account for title and tags.

This fixes an edge cases when a topic has a hidden tag the current editor can't see. When they submit their edit, we automatically add the hidden tags before checking with the tags stored in the database.

Reported in https://meta.discourse.org/t/341375

app/controllers/drafts_controller.rb

@@ -108,8 +108,15 @@ class DraftsController < ApplicationController
 
           if post.post_number == 1
             conflict ||= original_title.present? && original_title != post.topic.title
-            conflict ||=
-              original_tags.present? && original_tags.sort != post.topic.tags.pluck(:name).sort
+
+            # Since the topic might have hidden tags the current editor can't see,
+            # we need to check for conflicts even though there might not be any visible tags in the editor
+            if !conflict
+              original_tags = (original_tags || []).map(&:downcase).to_set
+              current_tags = post.topic.tags.pluck(:name).to_set
+              hidden_tags = DiscourseTagging.hidden_tag_names(@guardian).to_set
+              conflict = original_tags != (current_tags - hidden_tags)
+            end
           end
 
           if conflict

spec/requests/drafts_controller_spec.rb

@@ -164,6 +164,39 @@ RSpec.describe DraftsController do
       expect(response.parsed_body["conflict_user"]["id"]).to eq(post.last_editor.id)
     end
 
+    it "handles hidden tags when checking for tag conflict" do
+      sign_in(user)
+
+      regular_tag = Fabricate(:tag)
+      admin_only_tag_one = Fabricate(:tag)
+      admin_only_tag_two = Fabricate(:tag)
+
+      admin_only_tag_group = Fabricate(:tag_group)
+      admin_only_tag_group.tags = [admin_only_tag_one, admin_only_tag_two]
+      admin_only_tag_group.permissions = [
+        [Group::AUTO_GROUPS[:admins], TagGroupPermission.permission_types[:full]],
+      ]
+      admin_only_tag_group.save!
+
+      post = Fabricate(:post, user:)
+      post.topic.tags = [regular_tag, admin_only_tag_one]
+
+      post "/drafts.json",
+           params: {
+             draft_key: "topic",
+             sequence: 0,
+             data: {
+               postId: post.id,
+               original_text: post.raw,
+               original_tags: [regular_tag.name],
+               action: "edit",
+             }.to_json,
+           }
+
+      expect(response.status).to eq(200)
+      expect(response.parsed_body["conflict_user"]).to eq(nil)
+    end
+
     it "cant trivially resolve conflicts without interaction" do
       sign_in(user)