Fix Mutex to allow lock from different Ruby (green) thread in process

spec/process_shared/mutex_spec.rb

@@ -24,10 +24,37 @@ module ProcessShared
       ::Process.wait(pid)
     end
 
-    it 'raises exception when locked twice by same process' do
+    it 'raises exception when unlocked by other thread in same process' do
+      t = Thread.new do
+        @lock.lock
+        sleep 0.2
+        @lock.unlock
+      end
+
+      sleep 0.1
+      proc { @lock.unlock }.must_raise(ProcessError)
+
+      t.join
+    end
+
+    it 'raises exception when locked twice by same process and thread' do
       @lock.lock
       proc { @lock.lock }.must_raise(ProcessError)
       @lock.unlock
     end
+
+    it 'does not raise when locked by different threads on same process' do
+      t = Thread.new do
+        @lock.lock
+        sleep 0.2
+        @lock.unlock
+      end
+
+      sleep 0.1
+      @lock.synchronize { }
+
+      t.join
+    end
+
   end
 end