FIX: update PM tracking state when marking messages as unread (#36258)

When marking a private message as unread via "destroy_timings", two
  issues prevented old PMs from appearing in the Unread list:

1. PostTiming.destroy_for used Topic.listable_topics which excludes PMs,
so first_unread_pm_at was never updated. Now it explicitly handles PMs
by calling set_minimum_first_unread_pm!

2. The client-side PM tracking state wasn't notified of the change.
Unlike regular topics which sync state on navigation, PM tracking relies
on MessageBus for updates. Added publish_read call to notify the client.

Internal ref - t/155983

app/controllers/topics_controller.rb

@@ -341,6 +341,19 @@ class TopicsController < ApplicationController
 
     last_notification.update!(read: false) if last_notification
 
+    topic = Topic.find_by(id: topic_id)
+
+    if topic&.private_message?
+      topic_user = TopicUser.find_by(user: current_user, topic:)
+
+      PrivateMessageTopicTrackingState.publish_read(
+        topic_id,
+        topic_user&.last_read_post_number,
+        current_user,
+        topic_user&.notification_level,
+      )
+    end
+
     render body: nil
   end
 

app/models/post_timing.rb

@@ -108,9 +108,15 @@ class PostTiming < ActiveRecord::Base
 
       Post.where(topic_id: topic_ids).update_all("reads = reads - 1")
 
-      date = Topic.listable_topics.where(id: topic_ids).minimum(:updated_at)
+      topics = Topic.where(id: topic_ids)
 
-      set_minimum_first_unread!(user_id: user_id, date: date) if date
+      if (date = topics.listable_topics.minimum(:updated_at))
+        set_minimum_first_unread!(user_id:, date:)
+      end
+
+      topics.private_messages.find_each do |topic|
+        set_minimum_first_unread_pm!(topic:, user_id:, date: topic.updated_at)
+      end
     end
   end
 

spec/models/post_timing_spec.rb

@@ -226,4 +226,30 @@ RSpec.describe PostTiming do
       )
     end
   end
+
+  describe ".destroy_for" do
+    it "updates first unread for a user correctly when topic is a pm" do
+      post = Fabricate(:private_message_post)
+      post.topic.update!(updated_at: 10.minutes.ago)
+      PostTiming.process_timings(post.user, post.topic_id, 1, [[post.post_number, 100]])
+
+      PostTiming.destroy_for(post.user.id, [post.topic_id])
+
+      expect(post.user.user_stat.reload.first_unread_pm_at).to eq_time(post.topic.updated_at)
+    end
+
+    it "updates first unread for a user correctly when topic is a group pm" do
+      topic = Fabricate(:private_message_topic, updated_at: 10.minutes.ago)
+      post = Fabricate(:post, topic:)
+      user = Fabricate(:user)
+      group = Fabricate(:group)
+      group.add(user)
+      topic.allowed_groups << group
+      PostTiming.process_timings(user, topic.id, 1, [[post.post_number, 100]])
+
+      PostTiming.destroy_for(user.id, [topic.id])
+
+      expect(GroupUser.find_by(user:, group:).first_unread_pm_at).to eq_time(topic.updated_at)
+    end
+  end
 end

spec/requests/topics_controller_spec.rb

@@ -1489,6 +1489,34 @@ RSpec.describe TopicsController do
         }.from([1, 1]).to([0, 0])
       end
     end
+
+    context "for private messages" do
+      fab!(:pm_post, :private_message_post)
+      fab!(:pm_topic) { pm_post.topic }
+      fab!(:pm_user) { pm_topic.user }
+
+      before do
+        sign_in(pm_user)
+        TopicUser.create!(
+          topic: pm_topic,
+          user: pm_user,
+          last_read_post_number: 1,
+          notification_level: TopicUser.notification_levels[:watching],
+        )
+        PostTiming.create!(topic: pm_topic, user: pm_user, post_number: 1, msecs: 1000)
+      end
+
+      it "publishes a message to update the client-side tracking state" do
+        messages =
+          MessageBus.track_publish(PrivateMessageTopicTrackingState.user_channel(pm_user.id)) do
+            delete "/t/#{pm_topic.id}/timings.json"
+          end
+
+        expect(messages.size).to eq(1)
+        expect(messages.first.data["message_type"]).to eq("read")
+        expect(messages.first.data["topic_id"]).to eq(pm_topic.id)
+      end
+    end
   end
 
   describe "#mute/unmute" do