Initial implementation of System.Threading.Tasks.Channels

[stephentoub]

Prototype of a new System.Threading.Tasks.Channels library, which provides data structures for handing off data between producers and consumers. The included README.md provides an overview of the library and its exposed functionality.

An initial set of functional tests are included, with ~95% line and branch coverage.

Further work is desired to expose more operations over channels, provide additional built-in channel implementations, and optimize the existing channel and operator implementations further.

[stephentoub]

cc @KrzysztofCwalina, @ljw1004, @Petermarcu, @MadsTorgersen, @bleroy

[StephenCleary]

I think this line in the README:

```WaitToRead```/```WriteAsync```

would be less confusing as:

```WaitToReadAsync```/```WaitToWriteAsync```

Also, I would really like to see a separate Deque collection proposal. .NET has long needed one, as illustrated by the SimpleQueue included here and other places. Ditto for IAsyncEnumerator - it feels as though that would be a type with wider application than Channels.

[stephentoub]

I think this line in the README

Yes, that's just a typo, I'll fix it.

Ditto for IAsyncEnumerator

Yes, I included it here simply as a convenient place to put it. If it were to be included in the framework, it would certainly be put somewhere more general, especially with language support depending on it.

as illustrated by the SimpleQueue included here and other places

Didn't quite understand this... SimpleQueue isn't a deque, it's just a regular queue without some of the (small) overheads of Queue<T>.

[StephenCleary]

Didn't quite understand this... SimpleQueue isn't a deque, it's just a regular queue without some of the (small) overheads of Queue.

You're right, of course. Not sure why I was thinking it was a deque. :)

[MrJul]

This enumerator is never disposed. Shouldn't it be disposed when the channel is complete?

[stephentoub]

Thanks. Yup, it should. I'll fix that.

[IanYates]

Just a quick note to say the link to the readme in the first post is now broken - it should be to https://github.com/stephentoub/corefxlab/tree/master/src/System.Threading.Tasks.Channels (and possibly having README.md tack on there if you wish) instead of https://github.com/stephentoub/corefxlab/blob/initial_channels/src/System.Threading.Tasks.Channels/README.md

[stephentoub]

Thanks, @IanYates. I've now fixed the link.

[i3arnon]

I think a sentence or two on what's ValueTask<T> and what is it for would be nice as It's part of the public API but isn't a common concept (as far as I know).

[stephentoub]

I think a sentence or two on what's ValueTask

Sure, I can add that.

Am I missing something?

Probably not. I likely just neglected to remove the unsafe annotation after some experiments with different implementations. I'll take a look and remove them assuming it's possible.