Performance regression when reading files from the tree in parallel

Reproduction steps

1. Add the following test cases in CommitFixture.cs:

[Fact]
public void CanReadCommit()
{
    var timer = new Stopwatch();
    var fileContents = new ConcurrentStack<string>();
    var path = SandboxStandardTestRepo();
    using (var repo = new Repository(path))
    {
        var latestCommit = repo.Head.Tip;
        var tree = latestCommit.Tree;
        timer.Start();
        for (var i = 0; i < 100000; i++)
        {
            fileContents.Push(ReadEntry("1.txt", tree));
            fileContents.Push(ReadEntry("README", tree));
            fileContents.Push(ReadEntry("new.txt", tree));
        }
        timer.Stop();
        testOutputHelper.WriteLine($"Took: {timer.ElapsedMilliseconds.ToString()}ms");
        Assert.Equal(300000, fileContents.Count);
    }
}

[Fact]
public void CanReadCommitParallel()
{
    var timer = new Stopwatch();
    var fileContents = new ConcurrentStack<string>();
    var path = SandboxStandardTestRepo();
    using (var repo = new Repository(path))
    {
        var latestCommit = repo.Head.Tip;
        var tree = latestCommit.Tree;
        timer.Start();
        var fileNames = new List<string>() {"1.txt", "README", "new.txt"}.AsEnumerable();
        Parallel.ForEach(fileNames, (fileName) =>
        {
            for (var i = 0; i < 100000; i++)
            {
                fileContents.Push(ReadEntry(fileName, tree));
            }
        });
        timer.Stop();
        testOutputHelper.WriteLine($"Took: {timer.ElapsedMilliseconds.ToString()}ms");
        Assert.Equal(300000, fileContents.Count);
    }
}

private static string ReadEntry(string name, Tree tree)
{
    var treeEntry = tree[name];
    if (treeEntry != null && treeEntry.Target is Blob blob)
    {
        return blob.GetContentText();
    }

    throw new InvalidOperationException($"{name} must be a Blob");
}

2. Run against version 0.27.0-preview-0119 (commit: 6329bea). On my machine, CanReadCommitParallel takes 1417ms and CanReadCommit takes 2971ms (which is fine)
3. Checkout the latest version or the latest release
4. Run the test again. On my machine, CanReadCommitParallel takes 3731ms and CanReadCommit takes 3250ms!

Expected behavior

Reading files in parallel should be faster than reading files sequentially from the git tree.

Actual behavior

It seems that reading files from the tree in parallel (multi-thread) is not faster. I did a git bisect and it seems that this regression was introduced in https://github.com/libgit2/libgit2sharp/commit/21d4f13ac7c739a5526cf088fbd8765d4ad12f57

Version of LibGit2Sharp (release number or SHA1)

Versions after https://github.com/libgit2/libgit2sharp/commit/21d4f13ac7c739a5526cf088fbd8765d4ad12f57

Operating system(s) tested; .NET runtime tested

.NET 6 and .NET 7.

CommitFixture.cs で、レポートに記載されているバージョンに対する逐次実行と Parallel.ForEach の再現コードを追加して実行するところから始めてください。Commit 21d4f13ac7c739a5526cf088fbd8765d4ad12f57 前後の tree と blob の読み取り動作を .NET 6 と .NET 7 で比較してください。並列読み取りで正しい件数が維持され、比較可能な実行で逐次読み取りより高速であれば完了です。