Be consistent about caching behavior of function resolutions through IDynamicInterfaceCastable

[davidwrighton]

Based on discussion in #106843 (comment) we need to validate the caching behavior here and not cause the runtimes to do variable amounts of work here.

[davidwrighton]

We need to write some tests to validate that my thoughts on the caching behavior are correct, and then if I am right, we should be able to remove the special casing in NativeAOT cached interface dispatch path.

[dotnet-policy-service]

Tagging subscribers to this area: @agocke, @MichalStrehovsky, @jkotas
See info in area-owners.md if you want to be subscribed.

[MichalStrehovsky]

I'm very confused about how the caching here works on the VM side of things:

using System;
using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

for (int i = 0; i < 2; i++)
{
    Callsite1();
    Callsite2();

    static void Callsite1()
    {
        IFoo d = GetDyn();

        for (int j = 0; j < 5; j++)
            d.Frob();
    }

    static void Callsite2()
    {
        IFoo d = GetDyn();

        for (int j = 0; j < 5; j++)
            d.Frob();
    }
}

[MethodImpl(MethodImplOptions.NoInlining)]
static IFoo GetDyn() => (IFoo)new Dyn();


class Dyn : IDynamicInterfaceCastable
{
    static int counter;

    public RuntimeTypeHandle GetInterfaceImplementation(RuntimeTypeHandle interfaceType)
        => (counter++ % 6) switch
        {
            0 => typeof(IA).TypeHandle,
            1 => typeof(IB).TypeHandle,
            2 => typeof(IC).TypeHandle,
            3 => typeof(ID).TypeHandle,
            4 => typeof(IE).TypeHandle,
            _ => typeof(IF).TypeHandle,
        };

    public bool IsInterfaceImplemented(RuntimeTypeHandle interfaceType, bool throwIfNotImplemented) => true;
}

interface IFoo
{
    public void Frob();
}

[DynamicInterfaceCastableImplementation]
interface IA : IFoo
{
    void IFoo.Frob() => Console.WriteLine("IA");
}

[DynamicInterfaceCastableImplementation]
interface IB : IFoo
{
    void IFoo.Frob() => Console.WriteLine("IB");
}

[DynamicInterfaceCastableImplementation]
interface IC : IFoo
{
    void IFoo.Frob() => Console.WriteLine("IC");
}

[DynamicInterfaceCastableImplementation]
interface ID : IFoo
{
    void IFoo.Frob() => Console.WriteLine("ID");
}

[DynamicInterfaceCastableImplementation]
interface IE : IFoo
{
    void IFoo.Frob() => Console.WriteLine("IE");
}

[DynamicInterfaceCastableImplementation]
interface IF : IFoo
{
    void IFoo.Frob() => Console.WriteLine("IF");
}

This program will print:

IA
IB
IC
ID
IE
IF
IA
IA
IA
IA
IA
IA
IA
IA
IA
IA
IA
IA
IA
IA

Wat.

[MichalStrehovsky]

Here is another one:

using System;
using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

for (int i = 0; i < 2; i++)
{
    IFoo d = GetDyn();

    for (int j = 0; j < 10; j++)
        d.Frob();
}

[MethodImpl(MethodImplOptions.NoInlining)]
static IFoo GetDyn() => (IFoo)new Dyn();


class Dyn : IDynamicInterfaceCastable
{
    static int counter;

    public RuntimeTypeHandle GetInterfaceImplementation(RuntimeTypeHandle interfaceType)
        => counter++ % 2 == 0 ? typeof(IBar).TypeHandle : typeof(IBaz).TypeHandle;

    public bool IsInterfaceImplemented(RuntimeTypeHandle interfaceType, bool throwIfNotImplemented) => true;
}

interface IFoo
{
    public void Frob();
}

[DynamicInterfaceCastableImplementation]
interface IBar : IFoo
{
    void IFoo.Frob() => Console.WriteLine("IBar");
}

[DynamicInterfaceCastableImplementation]
interface IBaz : IFoo
{
    void IFoo.Frob() => Console.WriteLine("IBaz");
}

This will print:

IBar
IBaz
IBar
IBar
IBar
IBar
IBar
IBar
IBar
IBar
IBar
IBar
IBar
IBar
IBar
IBar
IBar
IBar
IBar
IBar

Yes, the second one is IBaz, otherwise all IBar. Wat.