test: reproduce possible Hash inconsistency when [] / []= concurrently

... a sometimes reproducer  for ruby-i18n#51 (needs a GIL free Ruby such as JRuby)

test/backend/memoize_test.rb

@@ -44,4 +44,39 @@ def test_resets_available_locales_on_store_translations
     assert I18n.available_locales.include?(:copa)
     assert_equal 1, I18n.backend.spy_calls
   end
+
+  module TestLookup
+    def lookup(locale, key, scope = [], options = {})
+      keys = I18n.normalize_keys(locale, key, scope, options[:separator])
+      keys.inspect
+    end
+  end
+
+  def test_lookup_concurrent_consistency
+    backend_impl = Class.new(I18n::Backend::Simple) do
+      include TestLookup
+      include I18n::Backend::Memoize
+    end
+    backend = backend_impl.new
+
+    memoized_lookup = backend.send(:memoized_lookup)
+    # make the 'default_proc' execution artificially slower to help reproduce :
+    default_proc = memoized_lookup.default_proc
+    memoized_lookup.default_proc = Proc.new { |h, k| sleep 0.1; default_proc.call(h, k) }
+
+    assert_equal "[:foo, :scoped, :sample]", backend.translate('foo', scope = [:scoped, :sample])
+
+    results = []
+    30.times.inject([]) do |memo, i|
+      memo << Thread.new do
+        backend.translate('bar', scope); backend.translate(:baz, scope)
+      end
+    end.each(&:join)
+
+    memoized_lookup = backend.send(:memoized_lookup)
+    puts memoized_lookup.inspect if $VERBOSE
+    assert_equal 3, memoized_lookup.size, "NON-THREAD-SAFE lookup memoization backend: #{memoized_lookup.class}"
+    # if a plain Hash is used might eventually end up in a weird (inconsistent) state
+  end
+
 end