Fixes #369 thread issue when calling translate with fallbacks

[chadrschroeder]

This pull request is a fix for the thread issues discuss in #369.

In Version 0.7.0 the I18n::Backend::Fallbacks#translate method had an internal option called fallback. This was set to true when the fallback logic was in progress and was used as a way to prevent fallbacks from getting stuck in an infinite loop:

def translate(locale, key, options = {})
  return super if options[:fallback]
  ...
  options[:fallback] = true
  I18n.fallbacks[locale].each do |fallback|
    ...
  end
  options.delete(:fallback)

Although fallback was an internal option, this wiki made reference to a fallbacks flag that could be used to disable fallbacks.

Unfortunately the fallbacks flag didn't do anything so pull request #354 made a change to allow for passing fallback: false as a way to disable the fallback logic. When that was done this commit added an instance variable called @fallback_locked which breaks thread-safety:

def translate(locale, key, options = {})
  return super unless options.fetch(:fallback, true)

  # Depending on the timing, a given thread can incorrectly short-circuit out of the 
  # fallback flow on the next line if @fallback_locked has been set to true but another 
  # thread
  return super if (@fallback_locked ||= false)  # <---
  ...

  begin
    @fallback_locked = true
     I18n.fallbacks[locale].each do |fallback|
  ...

  ensure
    @fallback_locked = false

This pull request will remove the instance variable and revert back to the local options hash, using a new internal option called :fallback_in_progress to control the fallback looping structure in a way that is thread-safe.

[radar]

Thank you for taking the time to build this PR and to write up such a great description of the bug it fixes. I think this looks good, so I'm going to go ahead and merge it.

[radar]

I will try to do a 0.8.5 release tomorrow morning. I would do it now but I am currently on a train which has poor internet quality.

[radar]

0.8.5 has been released with this patch included.