Creation of singleton dependencies is not thread-safe

[KevinBrowne]

Sinject::Container#get has no critical sections, meaning that requesting a singleton dependency is not guaranteed to return the same object each time. Though unlikely in practice, timing issues may result in the generation of multiple instances of a supposed singleton.

Steps to reproduce:

(A contrived example, admittedly.)

require 'sinject'
container = Sinject::Container.new(false)
# obvious contrivance!
container.register(key: :foo, class: Array, singleton: true) { sleep(1); [] }

objects = Set.new.compare_by_identity
mutex = Mutex.new
3500.times.map do
  Thread.new do
    o = container.get(:foo)
    mutex.synchronize { objects.add(o) }
  end
end.each(&:join)
objects.size

Expected:

objects.size is 1.

Actual:

objects.size is greater than 1, the exact number being subject to the timing of the threads' execution.

Where objects.size is less than the number of threads spawned, this shows that eventually #get does return the same instance each time.