Fix deadlocks while updating tags counts.

Sometimes uploads fail with this error:

    ActiveRecord::QueryCanceled - PG::QueryCanceled: ERROR: canceling
    statement due to statement timeout CONTEXT: while updating tuple
    (2808,110) in relation "tags"

This is caused by two uploads trying to update tag counts for the same
tags at the same time. The UPDATE statement doesn't guarantee the tags
will be updated in any particular order, which can cause deadlocks when
two UPDATEs try to update the same tags in a conflicting order. For
example, if one process tries to increment '1girl' and 'solo' (in that
order), while at the same time another process tries to increment 'solo'
and '1girl' (in that order), the result will be a deadlock.

The solution is to explicitly lock the tags in a consistent order before
the update statement.

app/models/tag.rb

@@ -91,12 +91,19 @@ class Tag < ApplicationRecord
     extend ActiveSupport::Concern
 
     module ClassMethods
+      # Lock the tags first in alphabetical order to avoid deadlocks under concurrent updates.
+      #
+      # https://stackoverflow.com/questions/44660368/postgres-update-with-order-by-how-to-do-it
+      # https://www.postgresql.org/message-id/flat/freemail.20070030161126.43285%40fm10.freemail.hu
+      # https://www.postgresql.org/message-id/flat/CAKOSWNkb3Zy_YFQzwyRw3MRrU10LrMj04%2BHdByfQu6M1S5B7mg%40mail.gmail.com#9dc514507357472bdf22d3109d9c7957
       def increment_post_counts(tag_names)
-        Tag.where(:name => tag_names).update_all("post_count = post_count + 1")
+        Tag.where(name: tag_names).order(:name).lock("FOR UPDATE").pluck(1)
+        Tag.where(name: tag_names).update_all("post_count = post_count + 1")
       end
 
       def decrement_post_counts(tag_names)
-        Tag.where(:name => tag_names).update_all("post_count = post_count - 1")
+        Tag.where(name: tag_names).order(:name).lock("FOR UPDATE").pluck(1)
+        Tag.where(name: tag_names).update_all("post_count = post_count - 1")
       end
 
       def regenerate_post_counts!