Delete a room's messages in a job, one transaction each

Room#destroy destroyed every message inside the room's own
transaction, which holds SQLite's write lock until the last one: on a
room with many messages, every other write in the app waited and
failed. The request now takes the room away from its members and
leaves the rest to Room::DestroyJob, which destroys the messages one
at a time, each in its own short transaction, and then the room.

An open room is closed in the request, so that someone who joins the
account before the job ends isn't given it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bj8KnxpTf9sj2Ysa8aLAVa

test/models/room_test.rb

@@ -30,8 +30,54 @@ class RoomTest < ActiveSupport::TestCase
     assert Rooms::Closed.new.closed?
   end
 
+  test "an open room destroyed later lets no one in who joins the account before the job runs" do
+    room = rooms(:pets)
+
+    room.destroy_later
+    newcomer = User.create!(name: "Newcomer", email_address: "newcomer@example.com", password: "secret123456")
+
+    assert_not newcomer.memberships.exists?(room_id: room.id)
+    perform_enqueued_jobs only: Room::DestroyJob
+    assert_not Room.exists?(room.id)
+  end
+
+  test "destroying one message at a time leaves nothing of the room behind" do
+    room = rooms(:designers)
+    searchable = room.messages.create!(body: "Kept in the search index", creator: users(:david))
+    searchable.attachment.attach io: StringIO.new("hello"), filename: "hello.txt", content_type: "text/plain"
+    message_ids = room.messages.ids
+    assert Boost.where(message_id: message_ids).exists?
+    assert_equal 1, search_index_rows(searchable)
+
+    room.destroy_one_message_at_a_time
+
+    assert_not Room.exists?(room.id)
+    assert_empty Message.where(id: message_ids)
+    assert_empty Boost.where(message_id: message_ids)
+    assert_empty ActionText::RichText.where(record_type: "Message", record_id: message_ids)
+    assert_empty ActiveStorage::Attachment.where(record_type: "Message", record_id: message_ids)
+    assert_enqueued_jobs 1, only: ActiveStorage::PurgeJob
+    assert_equal 0, search_index_rows(searchable)
+  end
+
+  test "each message is destroyed in its own transaction, so other writes get through in between" do
+    room = rooms(:designers)
+    messages = room.messages.count
+    transactions = 0
+    count_transactions = ->(*, payload) { transactions += 1 if payload[:sql].start_with?("RELEASE SAVEPOINT") }
+
+    ActiveSupport::Notifications.subscribed(count_transactions, "sql.active_record") { room.destroy_one_message_at_a_time }
+
+    assert_equal messages + 1, transactions
+  end
+
   test "default involvement for new users" do
     room = Rooms::Closed.create_for({ name: "Hello!", creator: users(:david) }, users: [ users(:kevin), users(:david) ])
     assert room.memberships.all? { |m| m.involved_in_mentions? }
   end
+
+  private
+    def search_index_rows(message)
+      Message.connection.select_value("select count(*) from message_search_index where rowid = #{message.id}")
+    end
 end