HTMLEntities race condition

Ever ran into this error?

/usr/lib/ruby/gems/1.8/gems/htmlentities-4.0.0/lib/htmlentities.rb:110:in `map': uninitialized constant HTMLEntities::MAPPINGS (NameError)

or this?

/usr/lib/ruby/gems/1.8/gems/htmlentities-4.0.0/lib/htmlentities.rb:141:in `reverse_map': undefined method `invert' for nil:NilClass (NoMethodError)

Well you might have if you’re using the HTMLEntities gem in threaded ruby code. And if you are but haven’t gotten one of those errors, they might just be waiting for the stars to align correctly before they manifest themselves.

The errors are due to a race condition in the code:

  def map
    @map ||= (require "htmlentities/#{@flavor}"; HTMLEntities::MAPPINGS[@flavor])
  end

The issue arises in the following scenario:
Thread1 calls require and begins processing the required file, thread2 calls require which returns immediately because ruby has already marked the file as required even though it hasn’t finished loading. So then when thread2 tries to access HTMLEntities::MAPPINGS it is either undefined or empty because thread1 hasn’t actually finished evaling the required file yet.

Quick monkeypatch to fix the issue:

require 'rubygems'
require 'htmlentities'
class HTMLEntities
  alias_method :unsafe_map, :map
  @@map_guard = Mutex.new
  def map
    @@map_guard.synchronize { unsafe_map }
  end
end

You could also require all the htmlentities/* files at the beginning of your application but monkeypatching is so much more fun :)