SafeHandle marshalling

[jkoritzinsky]

Implement default marshalling for SafeHandles using both CoreCLR and CoreRT's implementations as a baseline.

Introduce a new phase : "LeakSafeUnmarshal" to represent unmarshalling steps that cause leaks if skipped in exceptional cases.

Since we don't have access to a Compilation object in MarshallingGenerators.TryCreate and plumbing one down there requires passing it through 3 or 4 layers of the API, I've renamed the MarshallingAttributeInfo distriminated union to MarshallingInfo and extended it to also represent marshalling info that requires a Compilation object to calculate it.

I've added some xunit-like execution tests in the Demo project that the Main method just calls directly until we decide how we want to do our execution testing.

[jkotas]

It may be useful to have a comment that describes the overall structure of the method to make it clear how the stages fit together. Something like:

... Setup ...
try
{
    ... Marshal ...
    fixed (... Pin ...)
    {
        ... Invoke ...
    }
    ... Unmarshal ...
}
finally
{
    ... Cleanup...
}

... KeepAlive ...

[AaronRobinsonMSFT]

This is something we are attempting to push into the design documents: https://github.com/dotnet/runtimelab/blob/DllImportGenerator/DllImportGenerator/designs/Pipeline.md

Which also need to be updated with these new stages.

[AaronRobinsonMSFT]

This is something we are attempting to push into the design documents: https://github.com/dotnet/runtimelab/blob/DllImportGenerator/DllImportGenerator/designs/Pipeline.md

Which also need to be updated with these new stages.

[jkotas]

an exception can throw in a finally

I think exception thrown during cleanup should be a non-scenario.

[jkoritzinsky]

Any more feedback?

[christianscheuer]

Being a guest here, but happened to see this is the only that doesn't currently use nameof?

[jkoritzinsky]

The MarshalEx type isn't a type in the .NET Standard 2.0 contract, so we can't use nameof for any of its methods. It's currently in Ancillary.Interop. In the future its methods will move into dotnet/runtime in some fashion.