Throw CancellationError instead of returning nil during early cancellation.

[dnadoba]

Motivation:

Follow up PR for #2399

We currently still return nil if the current Task is canceled before the first call to NIOThrowingAsyncSequenceProducer.AsyncIterator.next() but it should throw CancellationError too.

In addition, the generic Failure type turns out to be a problem. Just throwing a CancellationError without checking that Failure type is any Swift.Error or CancellationError introduced a type safety violation as we throw an unrelated type.

Modifications:

• throw CancellationError on eager cancellation
• deprecates the generic Failure type of NIOThrowingAsyncSequenceProducer. It now must always be any Swift.Error. For backward compatibility we will still return nil if Failure is not any Swift.Error or CancellationError.

Result:

CancellationError is now correctly thrown instead of returning nil on eager cancelation. Generic Failure type is deprecated.

[Lukasa]

We clearly have no tests for the typed error case, as we haven't had to deprecate anything. Can you add tests for that case that confirm we appropriately do not throw CancellationError in them, thereby testing the new function?

[dnadoba]

@Lukasa tests added in 28e4c70

[Lukasa]

Can we make sure to use withExtendedLifetime to preserve the producer side? That'll help me be confident that this code doesn't have this effect because the producer is being dropped.

[dnadoba]

Added in c7863f6
We can't wrap the whole thing in withExtendedLifetime because we call an async method and the closure withExtendedLifetime takes isn't async. (another use case for reasync)

[Lukasa]

Thanks. I'm happy with the patch as written. More broadly, if you ever want to say "I need a value to live to this point in the code", the idiom has ended up becoming to just write withExtendedLifetime(theValue) { }. Your version works too, so I'm happy with this as-is, but just an FYI for the future that you could have simply written that at the bottom of the file.

[dnadoba]

Ah, nice. Thank you!