Fix SNIPacket.ReadFromStreamAsync to not consume same ValueTask twice

[stephentoub]

Port dotnet/corefx#42338

[David-Engel]

Nice find. Any ideas on when and how this would manifest itself to the application and how to add a test for it?

[stephentoub]

Any ideas on when and how this would manifest itself to the application and how to add a test for it?

I don't know at a high-level when this ReadFromStreamAsync method is invoked. When it is invoked, though, if:

• The read on the underlying stream completed synchronously or otherwise really fast such that it was already completed by the time we check, and
• If the result "length" was <= 0 (which assuming a well behaving stream would mean 0 and that we hit EOF), and
• If the underlying stream was implemented with pooled IValueTaskSource instances (uncommon right now, more common in the future)

then it's possible we could end up using an instance backed by a pooled object already being used concurrently by someone else... most likely outcome would be this caller getting an InvalidOperationException stating that the object was being reused incorrectly.

So, it's unlikely to be an impactful issue, and also relatively hard to test well. But worth fixing in my opinion regardless.