CancellableReadStream

[jnm2]

@AArnott Would this be a good home for an implementation of a stream which implements ReadAsync cancellation around a base stream that may have no support or partial support for cancellation?

I have a prototype with tests that I can contribute which works by caching the previous ReadAsync task if the base stream doesn't cancel when the wrapper does. It's been useful for wrapping streams such as Console.OpenStandardInput() which are synchronously implemented and the base Stream.ReadAsync method is in play. Such streams can't cancel mid-read, but some stream consumption scenarios need to be able to cancel and continue later.

[AArnott]

Thanks for reaching out. What concerns me is that Streams are not concurrency safe, although some can read and write concurrently. If we pretend we support cancellation in the wrapper, then Stream-based code may try to perform some subsequent operation which would end up in parallel with the first. We could perhaps guard against that using a semaphore, but the reads may be incompatible. For example if ReadBlock is called underneath the first time, that won't return until the buffer is filled. If we fake up cancellation, and the next call is to ReadByte, that will be expected to return after just one byte, yet the original stream is still blocked by the original ReadBlock call waiting for perhaps 1K bytes.

So while I think in very special circumstances, something like what you're talking about sounds useful and perhaps even safe, in general I think it would not be, so I don't think would be a good fit for this general purpose library.

If I'm missing something, let me know.