Persistence is not thread-safe

[cneumuel]

Serializing multiple (different) objects at the same time may result in the following exception:

System.ArgumentException occurred
  Message=An item with the same key has already been added.
  Source=mscorlib
  StackTrace:
       at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
       at HeuristicLab.Persistence.Default.CompositeSerializers.NumberEnumerable2StringSerializer.GetGenericEnumerableInterface(Type type)
  InnerException: 

in HeuristicLab.Persistence.Default.CompositeSerializers.NumberEnumerable2StringSerializer:60.

This happens due to a race condition in the method GetGenericEnumerableInterface.

Here is some code to reproduce the issue:

    private static void TestPersistenceThreadSafe() {
      int n = 20;
      Task[] tasks = new Task[n];
      for (int i = 0; i < n; i++) {
        tasks[i] = Task.Factory.StartNew((idx) => {
          var alg = new GeneticAlgorithm { Engine = new SequentialEngine.SequentialEngine(), Problem = new SingleObjectiveTestFunctionProblem() };
          byte[] data = Serialize(alg);
          Console.WriteLine("Object #{0} serialized", idx);
        }, i);
      }
      Task.WaitAll(tasks);
    }
    public static byte[] Serialize(object obj) {
      MemoryStream memStream = new MemoryStream();
      XmlGenerator.Serialize(obj, memStream);
      byte[] jobByteArray = memStream.ToArray();
      memStream.Dispose();
      return jobByteArray;
    }

This issue is rated with high priority, because it has been identified to be causing problems in HeuristicLab Hive both on Slaves and Clients where parallel serialization is common.

[epitzer]

Added additional locks to improve concurrent persistence (r6356)