Feature: Offer CoreCLR-compatible portable library

As discussed in #874:

I’m willing to take a look at converting libgit2sharp to a portable library that can run on CoreCLR. My exploration is in the portable branch of my fork.

I’d like an active issue to track this exploration where I can share ideas for making this work with the project owners, and verify owners’ willingness to ultimately accept a PR when the work is done.

Work ongoing in PR #1318

Issue	Proposal/Resolution
Maintain net40 compatibility	Maintain the net40 targeted class library. Move source code to a Shared Project and add a Portable library project to add support for net45 genre portable library
Serializable exceptions	Put #if blocks around desktop-only serializable support
ICustomMarshaler	CoreCLR does not support this attribute. Replace all uses with thunking method wrappers. Keep tax and extra code very low by leveraging code gen during build, the way the PInvoke project does.
ReliabilityContract	Put #if blocks around these
X509Certificate	Offered via a corefx nuget package
SecureString	Offered via corefx NuGet package
BufferedStream	Cheap portable imitation
CriticalFinalizerObject	Put #if around base class declaration
Environment.OSVersion	#if Desktop keep, #else use CoreCLR RuntimeInformation
Trace	Put #ifblocks around these

Code generation design

The p/invoke signature that is hand-written changes from this:

[DllImport(libgit2)]
internal static extern unsafe int git_blob_create_fromchunks(
    ref GitOid oid,
    git_repository* repositoryPtr,
    [MarshalAs(UnmanagedType.CustomMarshaler, MarshalCookie = UniqueId.UniqueIdentifier, MarshalTypeRef = typeof(StrictFilePathMarshaler))] FilePath hintpath,
    source_callback fileCallback,
    IntPtr data);

To this:

[DllImport(libgit2)]
internal static extern unsafe int git_blob_create_fromchunks(
    ref GitOid oid,
    git_repository* repositoryPtr,
    [CustomMarshaler(typeof(StrictFilePathMarshaler))] byte* hintpath,
    source_callback fileCallback,
    IntPtr data);

Notice the change from using the MarshalAs attribute to using CustomMarshaler. This attribute causes another overload of this method to be automatically generated during the build (and available to Intellisense):

internal static unsafe int git_blob_create_fromchunks(ref GitOid oid, git_repository* repositoryPtr, string hintpath, source_callback fileCallback, IntPtr data)
{
    ICustomMarshaler marshaler = new StrictFilePathMarshaler();
    IntPtr p_hintpath = marshaler.MarshalManagedToNative(hintpath);
    try
    {
        return git_blob_create_fromchunks(ref oid, repositoryPtr, (byte*)p_hintpath, fileCallback, data);
    }
    finally
    {
        marshaler.CleanUpNativeData(p_hintpath);
    }
}

Notice this generated overload uses string as the marshaled parameter type, keeping the ease of use that we had before. It then emulates what the desktop marshaler would have done (at least, as far as I understand it).

[shiftkey]

@Jaykul it’s in flight, but it’s been a tough review to slog through so it’s kind of stalled: https://github.com/libgit2/libgit2sharp/pull/1318

[pawchen]

@AArnott Thanks but that button would show ‘Unsubscribe’ if you already watched the repository, so I just don’t trust it.