FIX: Correctly place moderator post for full topic move with freeze_original (#30324)

When freeze_original option is passed to PostMover, and we are moving all posts there is an issue. We attempt to put the small_action right after the last moved post. The issue is when there is an existing small action after the last moved "real" post. We then try to put the moderator post at the same location of the existing small action, which causes an index conflict and the move fails.

This makes sure that we place the moderator post at the verrrrrry end of the topic :)

app/models/post_mover.rb

@@ -97,11 +97,15 @@ class PostMover
     @first_post_number_moved =
       posts.first.is_first_post? ? posts[1]&.post_number : posts.first.post_number
 
-    if @options[:freeze_original] # in this case we need to add the moderator post after the last copied post
-      from_posts = @original_topic.ordered_posts.where("post_number > ?", posts.last.post_number)
-      shift_post_numbers(from_posts) if !@full_move
-
-      @first_post_number_moved = posts.last.post_number + 1
+    if @options[:freeze_original]
+      # in this case we need to add the moderator post after the last copied post
+      if @full_move
+        @first_post_number_moved = @original_topic.ordered_posts.last.post_number + 1
+      else
+        from_posts = @original_topic.ordered_posts.where("post_number > ?", posts.last.post_number)
+        shift_post_numbers(from_posts)
+        @first_post_number_moved = posts.last.post_number + 1
+      end
     end
 
     move_each_post

spec/models/post_mover_spec.rb

@@ -2898,7 +2898,7 @@ RSpec.describe PostMover do
         ).to eq(true)
       end
 
-      it "creates the moderator message in the correct position" do
+      it "creates the moderator message in the correct position for partial move" do
         PostMover.new(
           original_topic,
           Discourse.system_user,
@@ -2908,11 +2908,34 @@ RSpec.describe PostMover do
           },
         ).to_topic(destination_topic.id)
 
-        moderator_post =
-          original_topic.reload.ordered_posts.find_by(post_number: second_post.post_number + 1) # the next post
-        expect(moderator_post).to be_present
-        expect(moderator_post.post_type).to eq(Post.types[:small_action])
-        expect(moderator_post.action_code).to eq("split_topic")
+        # Moderator post is right after the second_post since it was the last post moved
+        expect(
+          original_topic.ordered_posts.find_by(
+            post_number: second_post.post_number + 1,
+            post_type: Post.types[:small_action],
+            action_code: "split_topic",
+          ),
+        ).to be_present
+      end
+
+      it "creates the moderator message in the correct position for full move" do
+        small_action = Fabricate(:small_action, topic: original_topic)
+        PostMover.new(
+          original_topic,
+          Discourse.system_user,
+          [op.id, first_post.id, second_post.id, third_post.id],
+          options: {
+            freeze_original: true,
+          },
+        ).to_topic(destination_topic.id)
+
+        expect(
+          original_topic.ordered_posts.find_by(
+            post_number: small_action.post_number + 1,
+            post_type: Post.types[:small_action],
+            action_code: "split_topic",
+          ),
+        ).to be_present
       end
 
       context "with `post_mover_create_moderator_post` modifier" do