Add BinarySemaphore (error checking semaphore with max value 1).

spec/process_shared/binary_semaphore_spec.rb

@@ -0,0 +1,13 @@
+require 'spec_helper'
+require 'process_shared/binary_semaphore'
+
+module ProcessShared
+  describe BinarySemaphore do
+    it 'raises exception with double post' do
+      BinarySemaphore.open(0) do |sem|
+        sem.post
+        proc { sem.post }.must_raise(ProcessError)
+      end
+    end
+  end
+end