[RFC] Batched low level index operations

Trước tiên, hãy đọc PR #907 cùng các entry point hiện có là Index.Add() và Index.Remove(). So sánh API Index.Update/IndexUpdater được đề xuất với yêu cầu rằng repository phải nhất quán giữa các lần gọi; để được xem là hoàn tất sẽ cần một design được maintainer phê duyệt và phạm vi triển khai được xác định, nhưng RFC này hiện vẫn chưa cung cấp các nội dung đó.

In the light of https://github.com/libgit2/libgit2sharp/pull/907, I was wondering if we should take this approach one step further.

Each call to repo.Stage() will

• Perform a diff between the working directory and the Index in order to determine what additions/removals should be promoted to the staging area
• Perform additions/deletions in the in memory Index
• Eventually persist the in memory Index to the disk

Each call to repo.Index.Add() (or Remove()) will

• Perform an addition (or a deletion) in the in memory Index
• Persist the in memory Index to the disk

As such, invoking repo.Index.Add() or repo.Index.Remove() will be more efficient than a call to repo.Stage().

We want to allow the user to switch to the command line git.git, between each call to LibGit2Sharp, and find the repository in a coherent state. This is the requirement that compels us to persist the in-memory Index at the very end of each Index related method.

However, would clients of the library require to perform many sequential calls to the lower level methods repo.Index.Add()|Remove(), the Index would be persisted as part of each call. Which may not be very efficient.

Considering this above, would a need for an IndexUpdater exist?

We may expose something like in the Index type:

public virtual void Update(params Action<IndexUpdater>[] actions)

which would allow the client to write such code:

repo.Index.Update(i => i.Add(path), i => i.Remove(otherPath))

Or even some more elaborate custom thingies such as

private void StageAllAdditionsRemovalsAndModifications(IRepository repo)
{
    // Compare the Workdir against the Index without attempting at detecting Copies/Renames
    var changes = repo.Diff.Compare<TreeChanges>(null, true,
        compareOptions: new CompareOptions { Similarity = SimilarityOptions.None });

    repo.Index.Update(i =>
    {
        // We first clean the Index from unwanted entries...
        foreach (TreeEntryChanges treeEntryChanges in changes
            .Where(tec => tec.Status == ChangeKind.Deleted))
        {
            i.Remove(treeEntryChanges.Path);
        }

        // ...then insert in the Index the additions/modifications.
        foreach (TreeEntryChanges treeEntryChanges in changes
            .Where(tec => tec.Status == ChangeKind.Added || tec.Status == ChangeKind.Modified))
        {
            i.Add(treeEntryChanges.Path);
        }
    });
}

And allow us to only persist the Index only once, when all the actions have been performed by the IndexUpdater.

Thoughts?