Example of shared_connection hack causing "Mysql2::Error: This connection is still waiting for a result"

.ruby-version

1.9.3-p194

database.rb

require "active_record"
require "mysql2"

ActiveRecord::Base.establish_connection(
  :adapter  => "mysql2",
  :host     => "localhost",
  :username => "root",
  :database => "capybara1047")

# mysql -u root
# CREATE DATABASE capybara1047;
# USE capybara1047;
# CREATE TABLE `users` (`id` integer NOT NULL PRIMARY KEY AUTO_INCREMENT, `whatever` text);

class User < ActiveRecord::Base
end

unless User.any?
  5000.times do |n|
    User.create(whatever: rand(1000))
  end
end

Gemfile

source "https://rubygems.org"
gem "connection_pool"
gem "activerecord"
gem "mysql2"

results.txt

adammck@leela [rbenv:1.9.3-p194] 
~/projects/capybara-pr-1047$ bundle exec ruby with-connection-pool.rb 
7058629314

adammck@leela [rbenv:1.9.3-p194] 
~/projects/capybara-pr-1047$ bundle exec ruby without-connection-pool.rb        
/Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/connection_adapters/abstract_mysql_adapter.rb:245:in `query': Mysql2::Error: This connection is still waiting for a result, try again once you have the result: SELECT `users`.* FROM `users`  (ActiveRecord::StatementInvalid)
  from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/connection_adapters/abstract_mysql_adapter.rb:245:in `block in execute'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/connection_adapters/abstract_adapter.rb:280:in `block in log'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activesupport-3.2.13/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/connection_adapters/abstract_adapter.rb:275:in `log'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/connection_adapters/abstract_mysql_adapter.rb:245:in `execute'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/connection_adapters/mysql2_adapter.rb:211:in `execute'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/connection_adapters/mysql2_adapter.rb:215:in `exec_query'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/connection_adapters/mysql2_adapter.rb:224:in `select'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/connection_adapters/abstract/database_statements.rb:18:in `select_all'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/connection_adapters/abstract/query_cache.rb:63:in `select_all'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/querying.rb:38:in `block in find_by_sql'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/explain.rb:26:in `logging_query_plan'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/querying.rb:37:in `find_by_sql'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/relation.rb:171:in `exec_queries'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/relation.rb:160:in `block in to_a'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/explain.rb:26:in `logging_query_plan'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/relation.rb:159:in `to_a'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/relation/finder_methods.rb:159:in `all'
	from /Users/adammck/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.13/lib/active_record/querying.rb:5:in `all'
	from /Users/adammck/projects/capybara-pr-1047/test.rb:4:in `block (2 levels) in run_test'

test.rb

def run_test
  $threads = 10.times.map do |n|
    Thread.new do

      # Deliberately waste time
      User.all.to_a.sample.update_attributes(whatever: rand(1000))
      ActiveRecord::Base.connection.close
      print n
    end
  end

  $threads.each do |thread|
    thread.join
  end
end

with-connection-pool.rb

require "connection_pool"
require "./database"
require "./test"

class ActiveRecord::Base
  mattr_accessor :shared_connection
  @@shared_connection = nil

  def self.connection
    @@shared_connection || ConnectionPool::Wrapper.new(:size => 1) { retrieve_connection }
  end
end

ActiveRecord::Base.shared_connection = ActiveRecord::Base.connection

run_test

without-connection-pool.rb

require "./database"
require "./test"

class ActiveRecord::Base
  mattr_accessor :shared_connection
  @@shared_connection = nil

  def self.connection
    @@shared_connection || retrieve_connection
  end
end

ActiveRecord::Base.shared_connection = ActiveRecord::Base.connection

run_test