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//===----------------------------------------------------------------------===//
//
// This source file is part of the SwiftNIO open source project
//
// Copyright (c) 2023 Apple Inc. and the SwiftNIO project authors
// Licensed under Apache License v2.0
//
// See LICENSE.txt for license information
// See CONTRIBUTORS.txt for the list of SwiftNIO project authors
//
// SPDX-License-Identifier: Apache-2.0
//
//===----------------------------------------------------------------------===//
#if os(macOS) || os(iOS) || os(tvOS) || os(watchOS) || os(Linux) || os(Android)
import DequeModule
import NIOConcurrencyHelpers
/// An asynchronous sequence generated from an error-throwing closure that
/// calls a continuation to produce new elements.
///
/// `BufferedStream` conforms to `AsyncSequence`, providing a convenient
/// way to create an asynchronous sequence without manually implementing an
/// asynchronous iterator. In particular, an asynchronous stream is well-suited
/// to adapt callback- or delegation-based APIs to participate with
/// `async`-`await`.
///
/// In contrast to `AsyncStream`, this type can throw an error from the awaited
/// `next()`, which terminates the stream with the thrown error.
///
/// You initialize an `BufferedStream` with a closure that receives an
/// `BufferedStream.Continuation`. Produce elements in this closure, then
/// provide them to the stream by calling the continuation's `yield(_:)` method.
/// When there are no further elements to produce, call the continuation's
/// `finish()` method. This causes the sequence iterator to produce a `nil`,
/// which terminates the sequence. If an error occurs, call the continuation's
/// `finish(throwing:)` method, which causes the iterator's `next()` method to
/// throw the error to the awaiting call point. The continuation is `Sendable`,
/// which permits calling it from concurrent contexts external to the iteration
/// of the `BufferedStream`.
///
/// An arbitrary source of elements can produce elements faster than they are
/// consumed by a caller iterating over them. Because of this, `BufferedStream`
/// defines a buffering behavior, allowing the stream to buffer a specific
/// number of oldest or newest elements. By default, the buffer limit is
/// `Int.max`, which means it's unbounded.
///
/// ### Adapting Existing Code to Use Streams
///
/// To adapt existing callback code to use `async`-`await`, use the callbacks
/// to provide values to the stream, by using the continuation's `yield(_:)`
/// method.
///
/// Consider a hypothetical `QuakeMonitor` type that provides callers with
/// `Quake` instances every time it detects an earthquake. To receive callbacks,
/// callers set a custom closure as the value of the monitor's
/// `quakeHandler` property, which the monitor calls back as necessary. Callers
/// can also set an `errorHandler` to receive asynchronous error notifications,
/// such as the monitor service suddenly becoming unavailable.
///
/// class QuakeMonitor {
/// var quakeHandler: ((Quake) -> Void)?
/// var errorHandler: ((Error) -> Void)?
///
/// func startMonitoring() {…}
/// func stopMonitoring() {…}
/// }
///
/// To adapt this to use `async`-`await`, extend the `QuakeMonitor` to add a
/// `quakes` property, of type `BufferedStream<Quake>`. In the getter for
/// this property, return an `BufferedStream`, whose `build` closure --
/// called at runtime to create the stream -- uses the continuation to
/// perform the following steps:
///
/// 1. Creates a `QuakeMonitor` instance.
/// 2. Sets the monitor's `quakeHandler` property to a closure that receives
/// each `Quake` instance and forwards it to the stream by calling the
/// continuation's `yield(_:)` method.
/// 3. Sets the monitor's `errorHandler` property to a closure that receives
/// any error from the monitor and forwards it to the stream by calling the
/// continuation's `finish(throwing:)` method. This causes the stream's
/// iterator to throw the error and terminate the stream.
/// 4. Sets the continuation's `onTermination` property to a closure that
/// calls `stopMonitoring()` on the monitor.
/// 5. Calls `startMonitoring` on the `QuakeMonitor`.
///
/// ```
/// extension QuakeMonitor {
///
/// static var throwingQuakes: BufferedStream<Quake, Error> {
/// BufferedStream { continuation in
/// let monitor = QuakeMonitor()
/// monitor.quakeHandler = { quake in
/// continuation.yield(quake)
/// }
/// monitor.errorHandler = { error in
/// continuation.finish(throwing: error)
/// }
/// continuation.onTermination = { @Sendable _ in
/// monitor.stopMonitoring()
/// }
/// monitor.startMonitoring()
/// }
/// }
/// }
/// ```
///
///
/// Because the stream is an `AsyncSequence`, the call point uses the
/// `for`-`await`-`in` syntax to process each `Quake` instance as produced by the stream:
///
/// do {
/// for try await quake in quakeStream {
/// print("Quake: \(quake.date)")
/// }
/// print("Stream done.")
/// } catch {
/// print("Error: \(error)")
/// }
///
@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
internal struct BufferedStream<Element> {
final class _Backing: Sendable {
let storage: _BackPressuredStorage
init(storage: _BackPressuredStorage) {
self.storage = storage
}
deinit {
self.storage.sequenceDeinitialized()
}
}
enum _Implementation: Sendable {
/// This is the implementation with backpressure based on the Source
case backpressured(_Backing)
}
let implementation: _Implementation
}
@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
extension BufferedStream: AsyncSequence {
/// The asynchronous iterator for iterating an asynchronous stream.
///
/// This type is not `Sendable`. Don't use it from multiple
/// concurrent contexts. It is a programmer error to invoke `next()` from a
/// concurrent context that contends with another such call, which
/// results in a call to `fatalError()`.
internal struct Iterator: AsyncIteratorProtocol {
final class _Backing {
let storage: _BackPressuredStorage
init(storage: _BackPressuredStorage) {
self.storage = storage
self.storage.iteratorInitialized()
}
deinit {
self.storage.iteratorDeinitialized()
}
}
enum _Implementation {
/// This is the implementation with backpressure based on the Source
case backpressured(_Backing)
}
var implementation: _Implementation
/// The next value from the asynchronous stream.
///
/// When `next()` returns `nil`, this signifies the end of the
/// `BufferedStream`.
///
/// It is a programmer error to invoke `next()` from a concurrent context
/// that contends with another such call, which results in a call to
/// `fatalError()`.
///
/// If you cancel the task this iterator is running in while `next()` is
/// awaiting a value, the `BufferedStream` terminates. In this case,
/// `next()` may return `nil` immediately, or else return `nil` on
/// subsequent calls.
internal mutating func next() async throws -> Element? {
switch self.implementation {
case .backpressured(let backing):
return try await backing.storage.next()
}
}
}
/// Creates the asynchronous iterator that produces elements of this
/// asynchronous sequence.
internal func makeAsyncIterator() -> Iterator {
switch self.implementation {
case .backpressured(let backing):
return Iterator(implementation: .backpressured(.init(storage: backing.storage)))
}
}
}
@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
extension BufferedStream: Sendable where Element: Sendable {}
internal struct _ManagedCriticalState<State>: @unchecked Sendable {
let lock: NIOLockedValueBox<State>
internal init(_ initial: State) {
self.lock = .init(initial)
}
internal func withCriticalRegion<R>(
_ critical: (inout State) throws -> R
) rethrows -> R {
try self.lock.withLockedValue(critical)
}
}
internal struct AlreadyFinishedError: Error {}
@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
extension BufferedStream {
/// A mechanism to interface between producer code and an asynchronous stream.
///
/// Use this source to provide elements to the stream by calling one of the `write` methods, then terminate the stream normally
/// by calling the `finish()` method. You can also use the source's `finish(throwing:)` method to terminate the stream by
/// throwing an error.
internal struct Source: Sendable {
/// A strategy that handles the backpressure of the asynchronous stream.
internal struct BackPressureStrategy: Sendable {
/// When the high watermark is reached producers will be suspended. All producers will be resumed again once
/// the low watermark is reached.
internal static func watermark(low: Int, high: Int) -> BackPressureStrategy {
BackPressureStrategy(
internalBackPressureStrategy: .watermark(.init(low: low, high: high))
)
}
private init(internalBackPressureStrategy: _InternalBackPressureStrategy) {
self._internalBackPressureStrategy = internalBackPressureStrategy
}
fileprivate let _internalBackPressureStrategy: _InternalBackPressureStrategy
}
/// A type that indicates the result of writing elements to the source.
@frozen
internal enum WriteResult: Sendable {
/// A token that is returned when the asynchronous stream's backpressure strategy indicated that production should
/// be suspended. Use this token to enqueue a callback by calling the ``enqueueCallback(_:)`` method.
internal struct CallbackToken: Sendable {
let id: UInt
}
/// Indicates that more elements should be produced and written to the source.
case produceMore
/// Indicates that a callback should be enqueued.
///
/// The associated token should be passed to the ``enqueueCallback(_:)`` method.
case enqueueCallback(CallbackToken)
}
/// Backing class for the source used to hook a deinit.
final class _Backing: Sendable {
let storage: _BackPressuredStorage
init(storage: _BackPressuredStorage) {
self.storage = storage
}
deinit {
self.storage.sourceDeinitialized()
}
}
/// A callback to invoke when the stream finished.
///
/// The stream finishes and calls this closure in the following cases:
/// - No iterator was created and the sequence was deinited
/// - An iterator was created and deinited
/// - After ``finish(throwing:)`` was called and all elements have been consumed
/// - The consuming task got cancelled
internal var onTermination: (@Sendable () -> Void)? {
set {
self._backing.storage.onTermination = newValue
}
get {
self._backing.storage.onTermination
}
}
private var _backing: _Backing
internal init(storage: _BackPressuredStorage) {
self._backing = .init(storage: storage)
}
/// Writes new elements to the asynchronous stream.
///
/// If there is a task consuming the stream and awaiting the next element then the task will get resumed with the
/// first element of the provided sequence. If the asynchronous stream already terminated then this method will throw an error
/// indicating the failure.
///
/// - Parameter sequence: The elements to write to the asynchronous stream.
/// - Returns: The result that indicates if more elements should be produced at this time.
internal func write<S>(contentsOf sequence: S) throws -> WriteResult
where Element == S.Element, S: Sequence {
try self._backing.storage.write(contentsOf: sequence)
}
/// Write the element to the asynchronous stream.
///
/// If there is a task consuming the stream and awaiting the next element then the task will get resumed with the
/// provided element. If the asynchronous stream already terminated then this method will throw an error
/// indicating the failure.
///
/// - Parameter element: The element to write to the asynchronous stream.
/// - Returns: The result that indicates if more elements should be produced at this time.
internal func write(_ element: Element) throws -> WriteResult {
try self._backing.storage.write(contentsOf: CollectionOfOne(element))
}
/// Enqueues a callback that will be invoked once more elements should be produced.
///
/// Call this method after ``write(contentsOf:)`` or ``write(:)`` returned ``WriteResult/enqueueCallback(_:)``.
///
/// - Important: Enqueueing the same token multiple times is not allowed.
///
/// - Parameters:
/// - callbackToken: The callback token.
/// - onProduceMore: The callback which gets invoked once more elements should be produced.
internal func enqueueCallback(
callbackToken: WriteResult.CallbackToken,
onProduceMore: @escaping @Sendable (Result<Void, Error>) -> Void
) {
self._backing.storage.enqueueProducer(
callbackToken: callbackToken,
onProduceMore: onProduceMore
)
}
/// Cancel an enqueued callback.
///
/// Call this method to cancel a callback enqueued by the ``enqueueCallback(callbackToken:onProduceMore:)`` method.
///
/// - Note: This methods supports being called before ``enqueueCallback(callbackToken:onProduceMore:)`` is called and
/// will mark the passed `callbackToken` as cancelled.
///
/// - Parameter callbackToken: The callback token.
internal func cancelCallback(callbackToken: WriteResult.CallbackToken) {
self._backing.storage.cancelProducer(callbackToken: callbackToken)
}
/// Write new elements to the asynchronous stream and provide a callback which will be invoked once more elements should be produced.
///
/// If there is a task consuming the stream and awaiting the next element then the task will get resumed with the
/// first element of the provided sequence. If the asynchronous stream already terminated then `onProduceMore` will be invoked with
/// a `Result.failure`.
///
/// - Parameters:
/// - sequence: The elements to write to the asynchronous stream.
/// - onProduceMore: The callback which gets invoked once more elements should be produced. This callback might be
/// invoked during the call to ``write(contentsOf:onProduceMore:)``.
internal func write<S>(
contentsOf sequence: S,
onProduceMore: @escaping @Sendable (Result<Void, Error>) -> Void
) where Element == S.Element, S: Sequence {
do {
let writeResult = try self.write(contentsOf: sequence)
switch writeResult {
case .produceMore:
onProduceMore(Result<Void, Error>.success(()))
case .enqueueCallback(let callbackToken):
self.enqueueCallback(callbackToken: callbackToken, onProduceMore: onProduceMore)
}
} catch {
onProduceMore(.failure(error))
}
}
/// Writes the element to the asynchronous stream.
///
/// If there is a task consuming the stream and awaiting the next element then the task will get resumed with the
/// provided element. If the asynchronous stream already terminated then `onProduceMore` will be invoked with
/// a `Result.failure`.
///
/// - Parameters:
/// - sequence: The element to write to the asynchronous stream.
/// - onProduceMore: The callback which gets invoked once more elements should be produced. This callback might be
/// invoked during the call to ``write(_:onProduceMore:)``.
internal func write(
_ element: Element,
onProduceMore: @escaping @Sendable (Result<Void, Error>) -> Void
) {
self.write(contentsOf: CollectionOfOne(element), onProduceMore: onProduceMore)
}
/// Write new elements to the asynchronous stream.
///
/// If there is a task consuming the stream and awaiting the next element then the task will get resumed with the
/// first element of the provided sequence. If the asynchronous stream already terminated then this method will throw an error
/// indicating the failure.
///
/// This method returns once more elements should be produced.
///
/// - Parameters:
/// - sequence: The elements to write to the asynchronous stream.
internal func write<S>(contentsOf sequence: S) async throws
where Element == S.Element, S: Sequence {
let writeResult = try { try self.write(contentsOf: sequence) }()
switch writeResult {
case .produceMore:
return
case .enqueueCallback(let callbackToken):
try await withTaskCancellationHandler {
try await withCheckedThrowingContinuation { continuation in
self.enqueueCallback(
callbackToken: callbackToken,
onProduceMore: { result in
switch result {
case .success():
continuation.resume(returning: ())
case .failure(let error):
continuation.resume(throwing: error)
}
}
)
}
} onCancel: {
self.cancelCallback(callbackToken: callbackToken)
}
}
}
/// Write new element to the asynchronous stream.
///
/// If there is a task consuming the stream and awaiting the next element then the task will get resumed with the
/// provided element. If the asynchronous stream already terminated then this method will throw an error
/// indicating the failure.
///
/// This method returns once more elements should be produced.
///
/// - Parameters:
/// - sequence: The element to write to the asynchronous stream.
internal func write(_ element: Element) async throws {
try await self.write(contentsOf: CollectionOfOne(element))
}
/// Write the elements of the asynchronous sequence to the asynchronous stream.
///
/// This method returns once the provided asynchronous sequence or the the asynchronous stream finished.
///
/// - Important: This method does not finish the source if consuming the upstream sequence terminated.
///
/// - Parameters:
/// - sequence: The elements to write to the asynchronous stream.
internal func write<S>(contentsOf sequence: S) async throws
where Element == S.Element, S: AsyncSequence {
for try await element in sequence {
try await self.write(contentsOf: CollectionOfOne(element))
}
}
/// Indicates that the production terminated.
///
/// After all buffered elements are consumed the next iteration point will return `nil` or throw an error.
///
/// Calling this function more than once has no effect. After calling finish, the stream enters a terminal state and doesn't accept
/// new elements.
///
/// - Parameters:
/// - error: The error to throw, or `nil`, to finish normally.
internal func finish(throwing error: Error?) {
self._backing.storage.finish(error)
}
}
/// Initializes a new ``BufferedStream`` and an ``BufferedStream/Source``.
///
/// - Parameters:
/// - elementType: The element type of the stream.
/// - failureType: The failure type of the stream.
/// - backPressureStrategy: The backpressure strategy that the stream should use.
/// - Returns: A tuple containing the stream and its source. The source should be passed to the
/// producer while the stream should be passed to the consumer.
internal static func makeStream(
of elementType: Element.Type = Element.self,
throwing failureType: Error.Type = Error.self,
backPressureStrategy: Source.BackPressureStrategy
) -> (`Self`, Source) where Error == Error {
let storage = _BackPressuredStorage(
backPressureStrategy: backPressureStrategy._internalBackPressureStrategy
)
let source = Source(storage: storage)
return (.init(storage: storage), source)
}
init(storage: _BackPressuredStorage) {
self.implementation = .backpressured(.init(storage: storage))
}
}
@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
extension BufferedStream {
struct _WatermarkBackPressureStrategy {
/// The low watermark where demand should start.
private let _low: Int
/// The high watermark where demand should be stopped.
private let _high: Int
/// Initializes a new ``_WatermarkBackPressureStrategy``.
///
/// - Parameters:
/// - low: The low watermark where demand should start.
/// - high: The high watermark where demand should be stopped.
init(low: Int, high: Int) {
precondition(low <= high)
self._low = low
self._high = high
}
func didYield(bufferDepth: Int) -> Bool {
// We are demanding more until we reach the high watermark
return bufferDepth < self._high
}
func didConsume(bufferDepth: Int) -> Bool {
// We start demanding again once we are below the low watermark
return bufferDepth < self._low
}
}
enum _InternalBackPressureStrategy {
case watermark(_WatermarkBackPressureStrategy)
mutating func didYield(bufferDepth: Int) -> Bool {
switch self {
case .watermark(let strategy):
return strategy.didYield(bufferDepth: bufferDepth)
}
}
mutating func didConsume(bufferDepth: Int) -> Bool {
switch self {
case .watermark(let strategy):
return strategy.didConsume(bufferDepth: bufferDepth)
}
}
}
}
@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
extension BufferedStream {
// We are unchecked Sendable since we are protecting our state with a lock.
final class _BackPressuredStorage: Sendable {
/// The state machine
let _stateMachine: _ManagedCriticalState<_StateMachine>
var onTermination: (@Sendable () -> Void)? {
set {
self._stateMachine.withCriticalRegion {
$0._onTermination = newValue
}
}
get {
self._stateMachine.withCriticalRegion {
$0._onTermination
}
}
}
init(
backPressureStrategy: _InternalBackPressureStrategy
) {
self._stateMachine = .init(.init(backPressureStrategy: backPressureStrategy))
}
func sequenceDeinitialized() {
let action = self._stateMachine.withCriticalRegion {
$0.sequenceDeinitialized()
}
switch action {
case .callOnTermination(let onTermination):
onTermination?()
case .failProducersAndCallOnTermination(let producerContinuations, let onTermination):
for producerContinuation in producerContinuations {
producerContinuation(.failure(AlreadyFinishedError()))
}
onTermination?()
case .none:
break
}
}
func iteratorInitialized() {
self._stateMachine.withCriticalRegion {
$0.iteratorInitialized()
}
}
func iteratorDeinitialized() {
let action = self._stateMachine.withCriticalRegion {
$0.iteratorDeinitialized()
}
switch action {
case .callOnTermination(let onTermination):
onTermination?()
case .failProducersAndCallOnTermination(let producerContinuations, let onTermination):
for producerContinuation in producerContinuations {
producerContinuation(.failure(AlreadyFinishedError()))
}
onTermination?()
case .none:
break
}
}
func sourceDeinitialized() {
let action = self._stateMachine.withCriticalRegion {
$0.sourceDeinitialized()
}
switch action {
case .callOnTermination(let onTermination):
onTermination?()
case .failProducersAndCallOnTermination(let producerContinuations, let onTermination):
for producerContinuation in producerContinuations {
producerContinuation(.failure(AlreadyFinishedError()))
}
onTermination?()
case .failProducers(let producerContinuations):
for producerContinuation in producerContinuations {
producerContinuation(.failure(AlreadyFinishedError()))
}
case .none:
break
}
}
func write(
contentsOf sequence: some Sequence<Element>
) throws -> Source.WriteResult {
let action = self._stateMachine.withCriticalRegion {
return $0.write(sequence)
}
switch action {
case .returnProduceMore:
return .produceMore
case .returnEnqueue(let callbackToken):
return .enqueueCallback(callbackToken)
case .resumeConsumerAndReturnProduceMore(let continuation, let element):
continuation.resume(returning: element)
return .produceMore
case .resumeConsumerAndReturnEnqueue(let continuation, let element, let callbackToken):
continuation.resume(returning: element)
return .enqueueCallback(callbackToken)
case .throwFinishedError:
throw AlreadyFinishedError()
}
}
func enqueueProducer(
callbackToken: Source.WriteResult.CallbackToken,
onProduceMore: @escaping @Sendable (Result<Void, Error>) -> Void
) {
let action = self._stateMachine.withCriticalRegion {
$0.enqueueProducer(callbackToken: callbackToken, onProduceMore: onProduceMore)
}
switch action {
case .resumeProducer(let onProduceMore):
onProduceMore(Result<Void, Error>.success(()))
case .resumeProducerWithError(let onProduceMore, let error):
onProduceMore(Result<Void, Error>.failure(error))
case .none:
break
}
}
func cancelProducer(callbackToken: Source.WriteResult.CallbackToken) {
let action = self._stateMachine.withCriticalRegion {
$0.cancelProducer(callbackToken: callbackToken)
}
switch action {
case .resumeProducerWithCancellationError(let onProduceMore):
onProduceMore(Result<Void, Error>.failure(CancellationError()))
case .none:
break
}
}
func finish(_ failure: Error?) {
let action = self._stateMachine.withCriticalRegion {
$0.finish(failure)
}
switch action {
case .callOnTermination(let onTermination):
onTermination?()
case .resumeConsumerAndCallOnTermination(
let consumerContinuation,
let failure,
let onTermination
):
switch failure {
case .some(let error):
consumerContinuation.resume(throwing: error)
case .none:
consumerContinuation.resume(returning: nil)
}
onTermination?()
case .resumeProducers(let producerContinuations):
for producerContinuation in producerContinuations {
producerContinuation(.failure(AlreadyFinishedError()))
}
case .none:
break
}
}
func next() async throws -> Element? {
let action = self._stateMachine.withCriticalRegion {
$0.next()
}
switch action {
case .returnElement(let element):
return element
case .returnElementAndResumeProducers(let element, let producerContinuations):
for producerContinuation in producerContinuations {
producerContinuation(Result<Void, Error>.success(()))
}
return element
case .returnErrorAndCallOnTermination(let failure, let onTermination):
onTermination?()
switch failure {
case .some(let error):
throw error
case .none:
return nil
}
case .returnNil:
return nil
case .suspendTask:
return try await self.suspendNext()
}
}
func suspendNext() async throws -> Element? {
return try await withTaskCancellationHandler {
return try await withCheckedThrowingContinuation { continuation in
let action = self._stateMachine.withCriticalRegion {
$0.suspendNext(continuation: continuation)
}
switch action {
case .resumeConsumerWithElement(let continuation, let element):
continuation.resume(returning: element)
case .resumeConsumerWithElementAndProducers(
let continuation,
let element,
let producerContinuations
):
continuation.resume(returning: element)
for producerContinuation in producerContinuations {
producerContinuation(Result<Void, Error>.success(()))
}
case .resumeConsumerWithErrorAndCallOnTermination(
let continuation,
let failure,
let onTermination
):
switch failure {
case .some(let error):
continuation.resume(throwing: error)
case .none:
continuation.resume(returning: nil)
}
onTermination?()
case .resumeConsumerWithNil(let continuation):
continuation.resume(returning: nil)
case .none:
break
}
}
} onCancel: {
let action = self._stateMachine.withCriticalRegion {
$0.cancelNext()
}
switch action {
case .resumeConsumerWithCancellationErrorAndCallOnTermination(
let continuation,
let onTermination
):
continuation.resume(throwing: CancellationError())
onTermination?()
case .failProducersAndCallOnTermination(
let producerContinuations,
let onTermination
):
for producerContinuation in producerContinuations {
producerContinuation(.failure(AlreadyFinishedError()))
}
onTermination?()
case .none:
break
}
}
}
}
}
@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
extension BufferedStream {
/// The state machine of the backpressured async stream.
struct _StateMachine {
enum _State {
struct Initial {
/// The backpressure strategy.
var backPressureStrategy: _InternalBackPressureStrategy
/// Indicates if the iterator was initialized.
var iteratorInitialized: Bool
/// The onTermination callback.
var onTermination: (@Sendable () -> Void)?
}
struct Streaming {
/// The backpressure strategy.
var backPressureStrategy: _InternalBackPressureStrategy
/// Indicates if the iterator was initialized.
var iteratorInitialized: Bool
/// The onTermination callback.
var onTermination: (@Sendable () -> Void)?
/// The buffer of elements.
var buffer: Deque<Element>
/// The optional consumer continuation.
var consumerContinuation: CheckedContinuation<Element?, Error>?
/// The producer continuations.
var producerContinuations: Deque<(UInt, (Result<Void, Error>) -> Void)>
/// The producers that have been cancelled.
var cancelledAsyncProducers: Deque<UInt>
/// Indicates if we currently have outstanding demand.
var hasOutstandingDemand: Bool
}
struct SourceFinished {
/// Indicates if the iterator was initialized.
var iteratorInitialized: Bool
/// The buffer of elements.
var buffer: Deque<Element>
/// The failure that should be thrown after the last element has been consumed.
var failure: Error?
/// The onTermination callback.
var onTermination: (@Sendable () -> Void)?
}
case initial(Initial)
/// The state once either any element was yielded or `next()` was called.
case streaming(Streaming)
/// The state once the underlying source signalled that it is finished.
case sourceFinished(SourceFinished)
/// The state once there can be no outstanding demand. This can happen if:
/// 1. The iterator was deinited
/// 2. The underlying source finished and all buffered elements have been consumed
case finished(iteratorInitialized: Bool)
/// An intermediate state to avoid CoWs.
case modify
}
/// The state machine's current state.
var _state: _State
// The ID used for the next CallbackToken.
var nextCallbackTokenID: UInt = 0
var _onTermination: (@Sendable () -> Void)? {
set {
switch self._state {
case .initial(var initial):
initial.onTermination = newValue
self._state = .initial(initial)
case .streaming(var streaming):
streaming.onTermination = newValue
self._state = .streaming(streaming)
case .sourceFinished(var sourceFinished):
sourceFinished.onTermination = newValue
self._state = .sourceFinished(sourceFinished)
case .finished:
break
case .modify:
fatalError("AsyncStream internal inconsistency")
}
}
get {
switch self._state {
case .initial(let initial):
return initial.onTermination
case .streaming(let streaming):
return streaming.onTermination
case .sourceFinished(let sourceFinished):
return sourceFinished.onTermination
case .finished:
return nil
case .modify:
fatalError("AsyncStream internal inconsistency")
}
}
}
/// Initializes a new `StateMachine`.
///
/// We are passing and holding the back-pressure strategy here because
/// it is a customizable extension of the state machine.
///
/// - Parameter backPressureStrategy: The back-pressure strategy.
init(
backPressureStrategy: _InternalBackPressureStrategy
) {
self._state = .initial(
.init(
backPressureStrategy: backPressureStrategy,
iteratorInitialized: false,
onTermination: nil
)
)
}
/// Generates the next callback token.
mutating func nextCallbackToken() -> Source.WriteResult.CallbackToken {
let id = self.nextCallbackTokenID
self.nextCallbackTokenID += 1
return .init(id: id)
}
/// Actions returned by `sequenceDeinitialized()`.
enum SequenceDeinitializedAction {
/// Indicates that `onTermination` should be called.
case callOnTermination((@Sendable () -> Void)?)
/// Indicates that all producers should be failed and `onTermination` should be called.
case failProducersAndCallOnTermination(
[(Result<Void, Error>) -> Void],
(@Sendable () -> Void)?
)
}
mutating func sequenceDeinitialized() -> SequenceDeinitializedAction? {
switch self._state {
case .initial(let initial):
if initial.iteratorInitialized {
// An iterator was created and we deinited the sequence.
// This is an expected pattern and we just continue on normal.
return .none
} else {
// No iterator was created so we can transition to finished right away.
self._state = .finished(iteratorInitialized: false)
return .callOnTermination(initial.onTermination)
}
case .streaming(let streaming):
if streaming.iteratorInitialized {
// An iterator was created and we deinited the sequence.
// This is an expected pattern and we just continue on normal.
return .none
} else {
// No iterator was created so we can transition to finished right away.
self._state = .finished(iteratorInitialized: false)
return .failProducersAndCallOnTermination(
Array(streaming.producerContinuations.map { $0.1 }),
streaming.onTermination
)
}
case .sourceFinished(let sourceFinished):
if sourceFinished.iteratorInitialized {
// An iterator was created and we deinited the sequence.
// This is an expected pattern and we just continue on normal.
return .none
} else {
// No iterator was created so we can transition to finished right away.
self._state = .finished(iteratorInitialized: false)
return .callOnTermination(sourceFinished.onTermination)
}
case .finished:
// We are already finished so there is nothing left to clean up.
// This is just the references dropping afterwards.
return .none
case .modify:
fatalError("AsyncStream internal inconsistency")
}
}
mutating func iteratorInitialized() {
switch self._state {
case .initial(var initial):
if initial.iteratorInitialized {
// Our sequence is a unicast sequence and does not support multiple AsyncIterator's
fatalError("Only a single AsyncIterator can be created")
} else {
// The first and only iterator was initialized.
initial.iteratorInitialized = true
self._state = .initial(initial)
}
case .streaming(var streaming):
if streaming.iteratorInitialized {
// Our sequence is a unicast sequence and does not support multiple AsyncIterator's
fatalError("Only a single AsyncIterator can be created")
} else {
// The first and only iterator was initialized.
streaming.iteratorInitialized = true
self._state = .streaming(streaming)
}
case .sourceFinished(var sourceFinished):
if sourceFinished.iteratorInitialized {
// Our sequence is a unicast sequence and does not support multiple AsyncIterator's
fatalError("Only a single AsyncIterator can be created")
} else {
// The first and only iterator was initialized.
sourceFinished.iteratorInitialized = true
self._state = .sourceFinished(sourceFinished)
}
case .finished(iteratorInitialized: true):
// Our sequence is a unicast sequence and does not support multiple AsyncIterator's
fatalError("Only a single AsyncIterator can be created")
case .finished(iteratorInitialized: false):
// It is strange that an iterator is created after we are finished
// but it can definitely happen, e.g.
// Sequence.init -> source.finish -> sequence.makeAsyncIterator
self._state = .finished(iteratorInitialized: true)
case .modify:
fatalError("AsyncStream internal inconsistency")
}
}
/// Actions returned by `iteratorDeinitialized()`.
enum IteratorDeinitializedAction {
/// Indicates that `onTermination` should be called.
case callOnTermination((@Sendable () -> Void)?)
/// Indicates that all producers should be failed and `onTermination` should be called.
case failProducersAndCallOnTermination(
[(Result<Void, Error>) -> Void],
(@Sendable () -> Void)?
)
}
mutating func iteratorDeinitialized() -> IteratorDeinitializedAction? {
switch self._state {
case .initial(let initial):
if initial.iteratorInitialized {
// An iterator was created and deinited. Since we only support
// a single iterator we can now transition to finish.
self._state = .finished(iteratorInitialized: true)
return .callOnTermination(initial.onTermination)
} else {
// An iterator needs to be initialized before it can be deinitialized.
fatalError("AsyncStream internal inconsistency")
}
case .streaming(let streaming):
if streaming.iteratorInitialized {
// An iterator was created and deinited. Since we only support
// a single iterator we can now transition to finish.
self._state = .finished(iteratorInitialized: true)
return .failProducersAndCallOnTermination(
Array(streaming.producerContinuations.map { $0.1 }),
streaming.onTermination
)
} else {
// An iterator needs to be initialized before it can be deinitialized.
fatalError("AsyncStream internal inconsistency")
}
case .sourceFinished(let sourceFinished):
if sourceFinished.iteratorInitialized {
// An iterator was created and deinited. Since we only support
// a single iterator we can now transition to finish.
self._state = .finished(iteratorInitialized: true)
return .callOnTermination(sourceFinished.onTermination)
} else {
// An iterator needs to be initialized before it can be deinitialized.
fatalError("AsyncStream internal inconsistency")
}
case .finished:
// We are already finished so there is nothing left to clean up.
// This is just the references dropping afterwards.
return .none
case .modify:
fatalError("AsyncStream internal inconsistency")
}
}
/// Actions returned by `sourceDeinitialized()`.
enum SourceDeinitializedAction {
/// Indicates that `onTermination` should be called.
case callOnTermination((() -> Void)?)
/// Indicates that all producers should be failed and `onTermination` should be called.
case failProducersAndCallOnTermination(
[(Result<Void, Error>) -> Void],
(@Sendable () -> Void)?
)
/// Indicates that all producers should be failed.
case failProducers([(Result<Void, Error>) -> Void])
}
mutating func sourceDeinitialized() -> SourceDeinitializedAction? {
switch self._state {
case .initial(let initial):
// The source got deinited before anything was written
self._state = .finished(iteratorInitialized: initial.iteratorInitialized)
return .callOnTermination(initial.onTermination)
case .streaming(let streaming):
if streaming.buffer.isEmpty {
// We can transition to finished right away since the buffer is empty now
self._state = .finished(iteratorInitialized: streaming.iteratorInitialized)
return .failProducersAndCallOnTermination(
Array(streaming.producerContinuations.map { $0.1 }),
streaming.onTermination
)
} else {
// The continuation must be `nil` if the buffer has elements
precondition(streaming.consumerContinuation == nil)
self._state = .sourceFinished(
.init(
iteratorInitialized: streaming.iteratorInitialized,
buffer: streaming.buffer,
failure: nil,
onTermination: streaming.onTermination
)
)
return .failProducers(
Array(streaming.producerContinuations.map { $0.1 })
)
}
case .sourceFinished, .finished:
// This is normal and we just have to tolerate it
return .none
case .modify:
fatalError("AsyncStream internal inconsistency")
}
}
/// Actions returned by `write()`.
enum WriteAction {
/// Indicates that the producer should be notified to produce more.
case returnProduceMore
/// Indicates that the producer should be suspended to stop producing.
case returnEnqueue(
callbackToken: Source.WriteResult.CallbackToken
)
/// Indicates that the consumer should be resumed and the producer should be notified to produce more.
case resumeConsumerAndReturnProduceMore(
continuation: CheckedContinuation<Element?, Error>,
element: Element
)
/// Indicates that the consumer should be resumed and the producer should be suspended.
case resumeConsumerAndReturnEnqueue(
continuation: CheckedContinuation<Element?, Error>,
element: Element,
callbackToken: Source.WriteResult.CallbackToken
)
/// Indicates that the producer has been finished.
case throwFinishedError
init(
callbackToken: Source.WriteResult.CallbackToken?,
continuationAndElement: (CheckedContinuation<Element?, Error>, Element)? = nil
) {
switch (callbackToken, continuationAndElement) {
case (.none, .none):
self = .returnProduceMore
case (.some(let callbackToken), .none):
self = .returnEnqueue(callbackToken: callbackToken)
case (.none, .some((let continuation, let element))):
self = .resumeConsumerAndReturnProduceMore(
continuation: continuation,
element: element
)
case (.some(let callbackToken), .some((let continuation, let element))):
self = .resumeConsumerAndReturnEnqueue(
continuation: continuation,
element: element,
callbackToken: callbackToken
)
}
}
}
mutating func write(_ sequence: some Sequence<Element>) -> WriteAction {
switch self._state {
case .initial(var initial):
var buffer = Deque<Element>()
buffer.append(contentsOf: sequence)
let shouldProduceMore = initial.backPressureStrategy.didYield(
bufferDepth: buffer.count
)
let callbackToken = shouldProduceMore ? nil : self.nextCallbackToken()
self._state = .streaming(
.init(
backPressureStrategy: initial.backPressureStrategy,
iteratorInitialized: initial.iteratorInitialized,
onTermination: initial.onTermination,
buffer: buffer,
consumerContinuation: nil,
producerContinuations: .init(),
cancelledAsyncProducers: .init(),
hasOutstandingDemand: shouldProduceMore
)
)
return .init(callbackToken: callbackToken)
case .streaming(var streaming):
self._state = .modify
streaming.buffer.append(contentsOf: sequence)
// We have an element and can resume the continuation
let shouldProduceMore = streaming.backPressureStrategy.didYield(
bufferDepth: streaming.buffer.count
)
streaming.hasOutstandingDemand = shouldProduceMore
let callbackToken = shouldProduceMore ? nil : self.nextCallbackToken()
if let consumerContinuation = streaming.consumerContinuation {
guard let element = streaming.buffer.popFirst() else {
// We got a yield of an empty sequence. We just tolerate this.
self._state = .streaming(streaming)
return .init(callbackToken: callbackToken)
}
// We got a consumer continuation and an element. We can resume the consumer now
streaming.consumerContinuation = nil
self._state = .streaming(streaming)
return .init(
callbackToken: callbackToken,
continuationAndElement: (consumerContinuation, element)
)
} else {
// We don't have a suspended consumer so we just buffer the elements
self._state = .streaming(streaming)
return .init(
callbackToken: callbackToken
)
}
case .sourceFinished, .finished:
// If the source has finished we are dropping the elements.
return .throwFinishedError
case .modify:
fatalError("AsyncStream internal inconsistency")
}
}
/// Actions returned by `enqueueProducer()`.
enum EnqueueProducerAction {
/// Indicates that the producer should be notified to produce more.
case resumeProducer((Result<Void, Error>) -> Void)
/// Indicates that the producer should be notified about an error.
case resumeProducerWithError((Result<Void, Error>) -> Void, Error)
}
mutating func enqueueProducer(
callbackToken: Source.WriteResult.CallbackToken,
onProduceMore: @Sendable @escaping (Result<Void, Error>) -> Void
) -> EnqueueProducerAction? {
switch self._state {
case .initial:
// We need to transition to streaming before we can suspend
// This is enforced because the CallbackToken has no internal init so
// one must create it by calling `write` first.
fatalError("AsyncStream internal inconsistency")
case .streaming(var streaming):
if let index = streaming.cancelledAsyncProducers.firstIndex(of: callbackToken.id) {
// Our producer got marked as cancelled.
self._state = .modify
streaming.cancelledAsyncProducers.remove(at: index)
self._state = .streaming(streaming)
return .resumeProducerWithError(onProduceMore, CancellationError())
} else if streaming.hasOutstandingDemand {
// We hit an edge case here where we wrote but the consuming thread got interleaved
return .resumeProducer(onProduceMore)
} else {
self._state = .modify
streaming.producerContinuations.append((callbackToken.id, onProduceMore))
self._state = .streaming(streaming)
return .none
}
case .sourceFinished, .finished:
// Since we are unlocking between yielding and suspending the yield
// It can happen that the source got finished or the consumption fully finishes.
return .resumeProducerWithError(onProduceMore, AlreadyFinishedError())
case .modify:
fatalError("AsyncStream internal inconsistency")
}
}
/// Actions returned by `cancelProducer()`.
enum CancelProducerAction {
/// Indicates that the producer should be notified about cancellation.
case resumeProducerWithCancellationError((Result<Void, Error>) -> Void)
}
mutating func cancelProducer(
callbackToken: Source.WriteResult.CallbackToken
) -> CancelProducerAction? {
switch self._state {
case .initial:
// We need to transition to streaming before we can suspend
fatalError("AsyncStream internal inconsistency")
case .streaming(var streaming):
if let index = streaming.producerContinuations.firstIndex(where: {
$0.0 == callbackToken.id
}) {
// We have an enqueued producer that we need to resume now
self._state = .modify
let continuation = streaming.producerContinuations.remove(at: index).1
self._state = .streaming(streaming)
return .resumeProducerWithCancellationError(continuation)
} else {
// The task that yields was cancelled before yielding so the cancellation handler
// got invoked right away
self._state = .modify
streaming.cancelledAsyncProducers.append(callbackToken.id)
self._state = .streaming(streaming)
return .none
}
case .sourceFinished, .finished:
// Since we are unlocking between yielding and suspending the yield
// It can happen that the source got finished or the consumption fully finishes.
return .none
case .modify:
fatalError("AsyncStream internal inconsistency")
}
}
/// Actions returned by `finish()`.
enum FinishAction {
/// Indicates that `onTermination` should be called.
case callOnTermination((() -> Void)?)
/// Indicates that the consumer should be resumed with the failure, the producers
/// should be resumed with an error and `onTermination` should be called.
case resumeConsumerAndCallOnTermination(
consumerContinuation: CheckedContinuation<Element?, Error>,
failure: Error?,
onTermination: (() -> Void)?
)
/// Indicates that the producers should be resumed with an error.
case resumeProducers(
producerContinuations: [(Result<Void, Error>) -> Void]
)
}
@inlinable
mutating func finish(_ failure: Error?) -> FinishAction? {
switch self._state {
case .initial(let initial):
// Nothing was yielded nor did anybody call next
// This means we can transition to sourceFinished and store the failure
self._state = .sourceFinished(
.init(
iteratorInitialized: initial.iteratorInitialized,
buffer: .init(),
failure: failure,
onTermination: initial.onTermination
)
)
return .callOnTermination(initial.onTermination)
case .streaming(let streaming):
if let consumerContinuation = streaming.consumerContinuation {
// We have a continuation, this means our buffer must be empty
// Furthermore, we can now transition to finished
// and resume the continuation with the failure
precondition(streaming.buffer.isEmpty, "Expected an empty buffer")
precondition(
streaming.producerContinuations.isEmpty,
"Expected no suspended producers"
)
self._state = .finished(iteratorInitialized: streaming.iteratorInitialized)
return .resumeConsumerAndCallOnTermination(
consumerContinuation: consumerContinuation,
failure: failure,
onTermination: streaming.onTermination
)
} else {
self._state = .sourceFinished(
.init(
iteratorInitialized: streaming.iteratorInitialized,
buffer: streaming.buffer,
failure: failure,
onTermination: streaming.onTermination
)
)
return .resumeProducers(
producerContinuations: Array(streaming.producerContinuations.map { $0.1 })
)
}
case .sourceFinished, .finished:
// If the source has finished, finishing again has no effect.
return .none
case .modify:
fatalError("AsyncStream internal inconsistency")
}
}
/// Actions returned by `next()`.
enum NextAction {
/// Indicates that the element should be returned to the caller.
case returnElement(Element)
/// Indicates that the element should be returned to the caller and that all producers should be called.
case returnElementAndResumeProducers(Element, [(Result<Void, Error>) -> Void])
/// Indicates that the `Error` should be returned to the caller and that `onTermination` should be called.
case returnErrorAndCallOnTermination(Error?, (() -> Void)?)
/// Indicates that the `nil` should be returned to the caller.
case returnNil
/// Indicates that the `Task` of the caller should be suspended.
case suspendTask
}
mutating func next() -> NextAction {
switch self._state {
case .initial(let initial):
// We are not interacting with the back-pressure strategy here because
// we are doing this inside `next(:)`
self._state = .streaming(
.init(
backPressureStrategy: initial.backPressureStrategy,
iteratorInitialized: initial.iteratorInitialized,
onTermination: initial.onTermination,
buffer: Deque<Element>(),
consumerContinuation: nil,
producerContinuations: .init(),
cancelledAsyncProducers: .init(),
hasOutstandingDemand: false
)
)
return .suspendTask
case .streaming(var streaming):
guard streaming.consumerContinuation == nil else {
// We have multiple AsyncIterators iterating the sequence
fatalError("AsyncStream internal inconsistency")
}
self._state = .modify
if let element = streaming.buffer.popFirst() {
// We have an element to fulfil the demand right away.
let shouldProduceMore = streaming.backPressureStrategy.didConsume(
bufferDepth: streaming.buffer.count
)
streaming.hasOutstandingDemand = shouldProduceMore
if shouldProduceMore {
// There is demand and we have to resume our producers
let producers = Array(streaming.producerContinuations.map { $0.1 })
streaming.producerContinuations.removeAll()
self._state = .streaming(streaming)
return .returnElementAndResumeProducers(element, producers)
} else {
// We don't have any new demand, so we can just return the element.
self._state = .streaming(streaming)
return .returnElement(element)
}
} else {
// There is nothing in the buffer to fulfil the demand so we need to suspend.
// We are not interacting with the back-pressure strategy here because
// we are doing this inside `suspendNext`
self._state = .streaming(streaming)
return .suspendTask
}
case .sourceFinished(var sourceFinished):
// Check if we have an element left in the buffer and return it
self._state = .modify
if let element = sourceFinished.buffer.popFirst() {
self._state = .sourceFinished(sourceFinished)
return .returnElement(element)
} else {
// We are returning the queued failure now and can transition to finished
self._state = .finished(iteratorInitialized: sourceFinished.iteratorInitialized)
return .returnErrorAndCallOnTermination(
sourceFinished.failure,
sourceFinished.onTermination
)
}
case .finished:
return .returnNil
case .modify:
fatalError("AsyncStream internal inconsistency")
}
}
/// Actions returned by `suspendNext()`.
enum SuspendNextAction {
/// Indicates that the consumer should be resumed.
case resumeConsumerWithElement(CheckedContinuation<Element?, Error>, Element)
/// Indicates that the consumer and all producers should be resumed.
case resumeConsumerWithElementAndProducers(
CheckedContinuation<Element?, Error>,
Element,
[(Result<Void, Error>) -> Void]
)
/// Indicates that the consumer should be resumed with the failure and that `onTermination` should be called.
case resumeConsumerWithErrorAndCallOnTermination(
CheckedContinuation<Element?, Error>,
Error?,
(() -> Void)?
)
/// Indicates that the consumer should be resumed with `nil`.
case resumeConsumerWithNil(CheckedContinuation<Element?, Error>)
}
mutating func suspendNext(
continuation: CheckedContinuation<Element?, Error>
) -> SuspendNextAction? {
switch self._state {
case .initial:
// We need to transition to streaming before we can suspend
preconditionFailure("AsyncStream internal inconsistency")
case .streaming(var streaming):
guard streaming.consumerContinuation == nil else {
// We have multiple AsyncIterators iterating the sequence
fatalError(
"This should never happen since we only allow a single Iterator to be created"
)
}
self._state = .modify
// We have to check here again since we might have a producer interleave next and suspendNext
if let element = streaming.buffer.popFirst() {
// We have an element to fulfil the demand right away.
let shouldProduceMore = streaming.backPressureStrategy.didConsume(
bufferDepth: streaming.buffer.count
)
streaming.hasOutstandingDemand = shouldProduceMore
if shouldProduceMore {
// There is demand and we have to resume our producers
let producers = Array(streaming.producerContinuations.map { $0.1 })
streaming.producerContinuations.removeAll()
self._state = .streaming(streaming)
return .resumeConsumerWithElementAndProducers(
continuation,
element,
producers
)
} else {
// We don't have any new demand, so we can just return the element.
self._state = .streaming(streaming)
return .resumeConsumerWithElement(continuation, element)
}
} else {
// There is nothing in the buffer to fulfil the demand so we to store the continuation.
streaming.consumerContinuation = continuation
self._state = .streaming(streaming)
return .none
}
case .sourceFinished(var sourceFinished):
// Check if we have an element left in the buffer and return it
self._state = .modify
if let element = sourceFinished.buffer.popFirst() {
self._state = .sourceFinished(sourceFinished)
return .resumeConsumerWithElement(continuation, element)
} else {
// We are returning the queued failure now and can transition to finished
self._state = .finished(iteratorInitialized: sourceFinished.iteratorInitialized)
return .resumeConsumerWithErrorAndCallOnTermination(
continuation,
sourceFinished.failure,
sourceFinished.onTermination
)
}
case .finished:
return .resumeConsumerWithNil(continuation)
case .modify:
fatalError("AsyncStream internal inconsistency")
}
}
/// Actions returned by `cancelNext()`.
enum CancelNextAction {
/// Indicates that the continuation should be resumed with a cancellation error, the producers should be finished and call onTermination.
case resumeConsumerWithCancellationErrorAndCallOnTermination(
CheckedContinuation<Element?, Error>,
(() -> Void)?
)
/// Indicates that the producers should be finished and call onTermination.
case failProducersAndCallOnTermination([(Result<Void, Error>) -> Void], (() -> Void)?)
}
mutating func cancelNext() -> CancelNextAction? {
switch self._state {
case .initial:
// We need to transition to streaming before we can suspend
fatalError("AsyncStream internal inconsistency")
case .streaming(let streaming):
self._state = .finished(iteratorInitialized: streaming.iteratorInitialized)
if let consumerContinuation = streaming.consumerContinuation {
precondition(
streaming.producerContinuations.isEmpty,
"Internal inconsistency. Unexpected producer continuations."
)
return .resumeConsumerWithCancellationErrorAndCallOnTermination(
consumerContinuation,
streaming.onTermination
)
} else {
return .failProducersAndCallOnTermination(
Array(streaming.producerContinuations.map { $0.1 }),
streaming.onTermination
)
}
case .sourceFinished, .finished:
return .none
case .modify:
fatalError("AsyncStream internal inconsistency")
}
}
}
}
#endif
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