FEATURE: when moving posts to a PM also add acting user (#36789)

Without acting user being allowed it can be very confusing, and the
acting
user can just remove themselves from the PM

---------

Co-authored-by: Martin Brennan <martin@discourse.org>

app/models/post_mover.rb

@@ -75,6 +75,7 @@ class PostMover
   def move_posts_to(topic)
     Guardian.new(user).ensure_can_see! topic
     @destination_topic = topic
+    ensure_acting_user_is_allowed_in_destination
 
     # when a topic contains some posts after moving posts to another topic we shouldn't close it
     # two types of posts should prevent a topic from closing:
@@ -130,6 +131,16 @@ class PostMover
     destination_topic
   end
 
+  def ensure_acting_user_is_allowed_in_destination
+    return if !@move_to_pm
+    return if destination_topic.archetype != Archetype.private_message
+    return if user.id.blank? || user.bot?
+    return if destination_topic.topic_allowed_users.exists?(user_id: user.id)
+
+    destination_topic.topic_allowed_users.create!(user_id: user.id)
+    destination_topic.notifier.watch!(user.id)
+  end
+
   def create_temp_table
     DB.exec("DROP TABLE IF EXISTS temp_moved_posts") if Rails.env.test?
 

spec/models/post_mover_spec.rb

@@ -2958,6 +2958,7 @@ RSpec.describe PostMover do
     end
 
     context "with freeze_original option" do
+      fab!(:admin)
       fab!(:original_topic, :topic)
       fab!(:destination_topic, :topic)
       fab!(:op) { Fabricate(:post, topic: original_topic, raw: "op of original topic") }
@@ -3210,6 +3211,22 @@ RSpec.describe PostMover do
         expect(pm.posts.map(&:raw)).to include(*moving_posts.map(&:raw))
       end
 
+      it "adds the acting user to the new PM" do
+        moving_posts = [first_post, second_post]
+        pm =
+          PostMover.new(
+            original_topic,
+            admin,
+            moving_posts.map(&:id),
+            move_to_pm: true,
+            options: {
+              freeze_original: true,
+            },
+          ).to_new_topic("Hi I'm a new PM, with a copy of the old posts")
+
+        expect(pm.topic_allowed_users.pluck(:user_id)).to include(admin.id)
+      end
+
       it "keep all posts when moving to an existing PM" do
         pm = Fabricate(:private_message_topic)
         pm_with_posts = Fabricate(:private_message_topic)