Fix default block race condition.

By the time `elsif !key?(key)` is called another thread might have
created a `key` mapping, then `!key?(key)` check would then fail
resulting in `[]` returning `nil` an incorrect `value`.

This is most likely to be triggered by a code like this:

  ThreadSafe::Cache.new {|cache, key| cache[key] = some_value}

Related: rails/rails#13961.

Author: thedarkone

lib/thread_safe/cache.rb

@@ -35,9 +35,13 @@ def initialize(options = nil, &block)
     end
 
     def [](key)
-      if value = super
+      if value = super # non-falsy value is an existing mapping, return it right away
         value
-      elsif @default_proc && !key?(key)
+      # re-check is done with get_or_default(key, NULL) instead of a simple !key?(key) in order to avoid a race condition, whereby by the time the current thread gets to the key?(key) call
+      # a key => value mapping might have already been created by a different thread (key?(key) would then return true, this elsif branch wouldn't be taken and an incorrent +nil+ value
+      # would be returned)
+      # note: nil == value check is not technically necessary
+      elsif @default_proc && nil == value && NULL == (value = get_or_default(key, NULL))
         @default_proc.call(self, key)
       else
         value