[C#] Add support for the ping pong test, bring support for futures that return strings, i.e. types returned via pointers.

[yowl]

As per the title, this PR adds more support to the C# wit-bindgen to get the ping-pong test to pass. As part of this I've:

• Added more things to the VTable: size, alignment, lower and lift.
• Added support for lifting and lowering, tested for strings only.
• Removed ICancelableRead and ICancelableWrite and replaced with IVTable
• Removed the FreeBuffer method and replaced with cleanup actions

[dicej]

Thanks for continuing to push this forward!

[dicej]

Looks like a stray debug print statement.

[yowl]

removed

[dicej]

I'm surprised to see a SubtaskDrop call here for e.SubtaskStatus.IsStarted. Don't we want to wait until it reaches the e.SubtaskStatus.IsReturned state?

[dicej]

Also, the line above seems like it should only apply to e.SubtaskStatus.IsReturned as well.

[yowl]

Ah, yes, I misread the go equivalent, not realising it doesn't use a break statement between cases.

[dicej]

There's a lot of code duplicated between the two overloads of Callback. Would it be practical to combine them, e.g. by having the non-generic one defer to the generic one using a dummy type of some sort?

[yowl]

Turns out that we don't need the generic version at all as the lift func parameter is not used here. Was an idea that later got dropped.

[dicej]

I'll admit I don't understand why this is needed or what it is doing. Could you elaborate?

[yowl]

ah yes, I meant to talk about this in the description. This stems from the chat we had and the idea to use an async function for the exported function. In C# UnmanagedCallersOnly functions cannot be marked async so what I did was from the UCO function, we call to an async function so we can do the cleanups easily. Without this strip_lift function, that pattern ends up like this

[global::System.Runtime.InteropServices.UnmanagedCallersOnlyAttribute(EntryPoint = "[async-lift]a:b/i#one-argument")]
public static unsafe int wasmExportOneArgument(int p0) {

    var task = OneArgumentAsync((unchecked((uint)(p0))));
    if (task.IsCompletedSuccessfully)
    {
        return (int)CallbackCode.Exit;
    }

    // TODO: Defer dropping borrowed resources until a result is returned.
    ContextTask* contextTaskPtr = AsyncSupport.ContextGet();

    return (int)CallbackCode.Wait | (int)(contextTaskPtr->WaitableSetHandle << 4);
}

public static async Task OneArgumentAsync(uint unchecked((uint)(p0)))
{
    var cleanups = new global::System.Collections.Generic.List<global::System.Action>();

    await IExportsImpl.OneArgument((unchecked((uint)(p0))));
    // TODO: task_cancel.forget();
    OneArgumentTaskReturn();

    foreach (var cleanup in cleanups)
    {
        cleanup();
    }

}

The signature to OneArgumentAsync has the lifted parameter expression and is not valid.

[dicej]

I see, thanks for explaining. Perhaps you could expand the comment to include an example of what the input and output of the regex would be, e.g. unchecked((uint)(p0))->p0.

[yowl]

done.

[dicej]

Did you mean to leave this in here?

[yowl]

Removed this and other debugs

[dicej]

FYI, this will probably need to be refined; e.g. resource handles and other types may not need pointers either.

[yowl]

Thanks, added a TODO for now.