### Motivation:
Follow up PR for https://github.com/apple/swift-nio/pull/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.
# Motivation
This test has been flaky for some time. This was due to the fact that the consuming Task could have deinited the iterator which has resulted in a call to `didTerminate`.
# Modification
This PR, adds a small sleep in the consuming task to avoid this race.
# Result
No more flakiness in this test.
Co-authored-by: David Nadoba <d_nadoba@apple.com>
* Call finish once the Source is deinited
# Motivation
We **MUST** call `finish()` when the `Source` deinits otherwise we can have a suspended continuation that never gets resumed.
# Modification
Introduce an internal class to both `Source`s and call `finish()` in their `deinit`s.
# Result
We are now resuming all continuations.
* Remove @unchecked
* Add throwing version of `NIOAsyncSequenceProducer`
# Motivation
We recently introduced a `NIOAsyncSequenceProducer` to bridge a stream of elements from the NIO world into the async world. The introduced type was a non-throwing `AsyncSequence`. To support all use-cases we also need to offer a throwing variant of the type.
# Modification
- Introduce a new `NIOThrowingAsyncSequenceProducer` that is identical to the `NIOAsyncSequenceProducer` except that it has a `Failure` generic parameter and that the `next()` method is throwing.
- Extract the `StateMachine` from both `AsyncSequenceProducer`s and unify them.
- There is one modification in behaviour: `didTerminate` is now only called after `nil` or the error has been consumed from the sequence.
# Result
We now have a throwing variant of the `NIOAsyncSequenceProducer`.
* Code review and fix CI
* Remove duplicated code
Co-authored-by: Cory Benfield <lukasa@apple.com>
David Nadoba