Parallel calls to Repository.ObjectDatabase.CreateBlob with identical contents fails.

I would like to insert blobs from memory into an object database in parallel (based on the technique described at http://stackoverflow.com/a/16244234). This works fine most of the time, but is subject to race conditions if the contents are the same. There are two classes of errors I observe:
1. Exception due to FILE_EXISTS conflict when trying to create the directory with the first two characters of the OID under objects.
2. NULL blob returned from CreateBlob.

Here's some example code to reproduce the problem:

using System;
using System.Text;
using System.IO;
using System.Threading.Tasks;
using LibGit2Sharp;

namespace GitODBStress
{
    class Program
    {
        static void Main()
        {

            string repoPath = Path.Combine(Path.GetTempPath(), "Git_Parallel_Odb");
            int iterationCount = 100;

            if (!Directory.Exists(repoPath))
            {
                Directory.CreateDirectory(repoPath);
            }

            var path = Repository.Init(repoPath, isBare:true);
            Console.WriteLine("Made repo at {0}", path);

            var repo = new Repository(path);

            var tasks = new Task[2];
            for (int i = 0; i < iterationCount; ++i)
            {
                var iter = i;
                byte[] buf = Encoding.ASCII.GetBytes("foo" + iter.ToString());
                for (int taskNo = 0; taskNo < 2; ++taskNo)
                {
                    tasks[taskNo] = Task.Factory.StartNew(() =>
                        {
                            MemoryStream ms = new MemoryStream(buf);
                            for (; ; )
                            {
                                try
                                {
                                    var blob = repo.ObjectDatabase.CreateBlob(ms);
                                    if (blob == null)
                                    {
                                        Console.WriteLine("ERROR: NULL blob on iteration {0}", iter);
                                        continue;
                                    }

                                    break;
                                }
                                catch (LibGit2SharpException e)
                                {
                                    Console.WriteLine("ERROR: Caught LibGit2SharpException on iteration {0}: {1}", iter, e.Message);
                                    if (e.Message.Contains("Cannot create a file when that file already exists."))
                                    {
                                        continue;
                                    }
                                }
                            }
                        });
                }

                Task.WaitAll(tasks);
            }

            repo.Dispose();

            // clean up temp repo files
            var files = Directory.GetFiles(repoPath, "*.*", SearchOption.AllDirectories);
            foreach (var f in files)
            {
                var attrib = File.GetAttributes(f);
                if (attrib.HasFlag(FileAttributes.ReadOnly))
                {
                    File.SetAttributes(f, attrib & ~FileAttributes.ReadOnly);
                }
            }

            Directory.Delete(repoPath, true);
        }
    }
}

Thanks!

從 Repository.ObjectDatabase.CreateBlob 開始，針對 bare repository 執行 C# 重現。追蹤相同內容的平行路徑，以及回報的 FILE_EXISTS 和 null-blob 結果。並行呼叫在建立預期 blob 的同時完成，且不出現任一失敗，即表示完成。