Merge pull request #329: Delete a room's messages in a job, one transaction each

Reviewed and merged by GPT on behalf of DHH.

app/models/room.rb

@@ -45,6 +45,34 @@ class Room < ApplicationRecord
     end
   end
 
+  # Takes the room away from its members at once, and leaves its messages to Room::DestroyJob. Destroying them in
+  # the room's own transaction held the database's write lock, and stopped every other write, until the last one.
+  # An open room is closed first, so that no one who joins the account before the job runs is let in.
+  def destroy_later
+    room = open? ? becomes!(Rooms::Closed) : self
+
+    former_member_ids = transaction do
+      room.save!
+      room.memberships.pluck(:user_id).tap { room.memberships.delete_all }
+    end
+
+    Room::DestroyJob.perform_later(room, former_member_ids)
+  end
+
+  # Deleting the memberships skips the reset that revoking one does, so the former members get it here, all at once,
+  # before the messages go: their connections stop receiving the room's streams. It's done in the job, not the
+  # request, because it costs a Redis round trip per member.
+  def reset_remote_connections_of(user_ids)
+    User.where(id: user_ids).find_each(&:reset_remote_connections)
+  end
+
+  # Each message is destroyed in its own transaction, so other writes get through in between. Any message posted
+  # meanwhile goes with the room.
+  def destroy_one_message_at_a_time
+    messages.find_each(&:destroy)
+    destroy
+  end
+
   def receive(message)
     unread_memberships(message)
     push_later(message)

app/models/user.rb

@@ -50,8 +50,18 @@ class User < ApplicationRecord
   end
 
   private
+    # One statement, which SQLite runs under the write lock, so no room is closed between reading the open ones and
+    # inserting. A room that Room#destroy_later closes is either closed before, and skipped, or after, and takes this
+    # membership away with the others. A room that granted itself to every user when it was created is skipped too,
+    # as insert_all did.
     def grant_membership_to_open_rooms
-      Membership.insert_all(Rooms::Open.pluck(:id).collect { |room_id| { room_id: room_id, user_id: id } })
+      now = Time.current
+
+      Membership.connection.execute Membership.sanitize_sql([ <<~SQL, id, now, now ])
+        insert into memberships (room_id, user_id, created_at, updated_at)
+        select id, ?, ?, ? from rooms where type = 'Rooms::Open'
+        on conflict do nothing
+      SQL
     end
 
     def deactived_email_address