Add spec for thread-safe autoload

spec/ruby/core/module/autoload_spec.rb

@@ -903,6 +903,20 @@ class ModuleSpecs::Autoload::Z < ModuleSpecs::Autoload::ZZ
       t2_exc.should be_nil
     end
 
+    it "blocks other threads until the file is done loading so no partial modules are seen" do
+      ModuleSpecs::Autoload.autoload :ThreadSafe, fixture(__FILE__, "autoload_thread_safe.rb")
+      barrier = ModuleSpecs::CyclicBarrier.new 2
+      ScratchPad.record(barrier)
+
+      thread = Thread.new { # starts the autoload
+        ModuleSpecs::Autoload::ThreadSafe.foo.should == 42
+      }
+      barrier.await
+      ModuleSpecs::Autoload::ThreadSafe.foo.should == 42 # ThreadSafe should only be published once the whole file is loaded
+
+      thread.join
+    end
+
     # https://bugs.ruby-lang.org/issues/10892
     it "blocks others threads while doing an autoload" do
       file_path     = fixture(__FILE__, "repeated_concurrent_autoload.rb")

spec/ruby/core/module/fixtures/autoload_thread_safe.rb

@@ -0,0 +1,20 @@
+module ModuleSpecs::Autoload
+  class ThreadSafe
+    def self.bar # to illustrate partial loading
+    end
+
+    barrier = ScratchPad.recorded
+    barrier.await
+
+    # the main thread should be waiting for this file to be fully loaded
+    Thread.pass until Thread.main.stop?
+    10.times do
+      Thread.pass
+      Thread.main.should.stop?
+    end
+
+    def self.foo
+      42
+    end
+  end
+end

spec/ruby/core/module/fixtures/classes.rb

@@ -475,7 +475,7 @@ def extend_object(obj)
   end
 
   class CyclicBarrier
-    def initialize(count = 1)
+    def initialize(count)
       @count = count
       @state = 0
       @mutex = Mutex.new