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

从 Repository.ObjectDatabase.CreateBlob 开始，针对 bare repository 运行 C# 重现。跟踪相同内容的并行路径，以及报告的 FILE_EXISTS 和 null-blob 结果。并发调用在创建预期 blob 的同时完成，且不出现任一失败，即表示完成。

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!