Review Memory<T> architecture

[KrzysztofCwalina]

Not a detailed design, but we should review general architecture of OwnedMemory, Memory, etc.

Here are the main points to discuss:

1. Relationship between main types: OwnedMemory, Memory, ROMemory, IMemory, IROMemory, IReferenceCounted
2. Do we want the abstractions? how much abstraction?
3. Reservation IDs
4. Reference counting

After the review, I will write down the spec.

[buybackoff]

Could you please add a disposable ReservedMemory<T> with a field Memory<T> instead of DisposableReservation? Similar pattern to this. Now I have to combine DisposableReservation and Memory<T> in a new struct to pass Memory<T> around. Not sure it is the supposed way, and it would be nice to have a built-in API for that.

    public struct ReservedMemory<T> : IDisposable {
        private DisposableReservation _reservation;

        public ReservedMemory(Memory<T> memory) {
            Memory = memory;
            _reservation = memory.Reserve();
        }

        public Memory<T> Memory { get; }

        public void Dispose() {
            _reservation.Dispose();
        }
    }

[benaadams]

@buybackoff would #944 meet those needs? In particular ReservedMemory

[buybackoff]

@benaadams Exactly, thanks! Haven't noticed that PR.

[buybackoff]

Currently Memory<T> equality is missing and struct's equality is used by default. Probably the default way should be what Span.ReferenceEqual is doing, e.g. only owner, offset and length comparison.

[KrzysztofCwalina]

The reservation token cannot have Memory property because Memory is mutable. We would need to have R/O and R/W reservation tokens.

[benaadams]

Already in #944 as ReservedReadOnlyMemory

[mjp41]

@KrzysztofCwalina I have a question about the OwnedMemory class.

https://github.com/dotnet/corefxlab/blob/master/src/System.Slices/System/Buffers/OwnedMemory.cs#L113-L116

         protected internal virtual DisposableReservation Reserve(ref ReadOnlyMemory<T> memory) 
         { 
             return new DisposableReservation(this, Id); 
         } 

In this function, what is the purpose of memory? It is not used?

It is passed as a parameter in
https://github.com/dotnet/corefxlab/blob/master/src/System.Slices/System/Buffers/ReadOnlyMemory.cs#L54

           public DisposableReservation Reserve() 
           { 
             return _owner.Reserve(ref this); 
           } 

[KrzysztofCwalina]

The idea was that if you all Reserve on a small slice of large memory buffer, the implementation can copy just this small slice to a new buffer and return it in reserve. See https://github.com/dotnet/corefxlab/blob/master/tests/System.Slices.Tests/MemoryTests.cs#L105

But, I am not convinced we want this feature. @davidfowl was going to use it at some point, but I am not sure he does.

[davidfowl]

It no longer needs to be part of OwnedMemory<T>, ReadableBuffer already has a different version of it.

[KrzysztofCwalina]

closing as I think we settled on the architecture we want.

[ahsonkhan]

After the review, I will write down the spec.

Tracked by https://github.com/dotnet/corefx/issues/23863