pools: lock pool when adding/removing posts (fixes #3091).

Adding a post id to a pool's post_ids string is non-atomic, hence we
must lock the pool to avoid a race condition.

Adding a pool to a post's pool_string is likewise non-atomic, hence we
must lock the post as well.

app/models/pool.rb

@@ -246,20 +246,24 @@ class Pool < ActiveRecord::Base
     return if contains?(post.id)
     return if is_deleted?
 
+    with_lock do
       update_attributes(:post_ids => add_number_to_string(post.id, post_ids), :post_count => post_count + 1)
       post.add_pool!(self, true)
       clear_post_id_array
     end
+  end
 
   def remove!(post)
     return unless contains?(post.id)
     return unless CurrentUser.user.can_remove_from_pools?
     return if is_deleted?
 
+    with_lock do
       update_attributes(:post_ids => remove_number_from_string(post.id, post_ids), :post_count => post_count - 1)
       post.remove_pool!(self, true)
       clear_post_id_array
     end
+  end
 
   def add_number_to_string(number, string)
     "#{string} #{number}"

app/models/post.rb

@@ -1042,23 +1042,27 @@ class Post < ActiveRecord::Base
     def add_pool!(pool, force = false)
       return if belongs_to_pool?(pool)
       return if pool.is_deleted? && !force
-      reload
+
+      with_lock do
         self.pool_string = "#{pool_string} pool:#{pool.id}".strip
         set_pool_category_pseudo_tags
         update_column(:pool_string, pool_string) unless new_record?
         pool.add!(self)
       end
+    end
 
     def remove_pool!(pool, force = false)
       return unless belongs_to_pool?(pool)
       return unless CurrentUser.user.can_remove_from_pools?
       return if pool.is_deleted? && !force
-      reload
+
+      with_lock do
         self.pool_string = pool_string.gsub(/(?:\A| )pool:#{pool.id}(?:\Z| )/, " ").strip
         set_pool_category_pseudo_tags
         update_column(:pool_string, pool_string) unless new_record?
         pool.remove!(self)
       end
+    end
 
     def remove_from_all_pools
       pools.find_each do |pool|