grpc-swift/Tests/GRPCCoreTests/Streaming/Internal/BufferedStreamTests.swift

/*
 * Copyright 2023, gRPC Authors All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import XCTest

@testable import GRPCCore

@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
final class BufferedStreamTests: XCTestCase {
  // MARK: - sequenceDeinitialized

  func testSequenceDeinitialized_whenNoIterator() async throws {
    var (stream, source): (BufferedStream?, BufferedStream.Source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 5, high: 10)
    )

    let (onTerminationStream, onTerminationContinuation) = AsyncStream<Void>.makeStream()
    source.onTermination = {
      onTerminationContinuation.finish()
    }

    await withThrowingTaskGroup(of: Void.self) { group in
      group.addTask {
        while !Task.isCancelled {
          onTerminationContinuation.yield()
          try await Task.sleep(nanoseconds: 200_000_000)
        }
      }

      var onTerminationIterator = onTerminationStream.makeAsyncIterator()
      _ = await onTerminationIterator.next()

      withExtendedLifetime(stream) {}
      stream = nil

      let terminationResult: Void? = await onTerminationIterator.next()
      XCTAssertNil(terminationResult)

      do {
        _ = try { try source.write(2) }()
        XCTFail("Expected an error to be thrown")
      } catch {
        XCTAssertTrue(error is AlreadyFinishedError)
      }

      group.cancelAll()
    }
  }

  func testSequenceDeinitialized_whenIterator() async throws {
    var (stream, source): (BufferedStream?, BufferedStream.Source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 5, high: 10)
    )

    var iterator = stream?.makeAsyncIterator()

    let (onTerminationStream, onTerminationContinuation) = AsyncStream<Void>.makeStream()
    source.onTermination = {
      onTerminationContinuation.finish()
    }

    try await withThrowingTaskGroup(of: Void.self) { group in
      group.addTask {
        while !Task.isCancelled {
          onTerminationContinuation.yield()
          try await Task.sleep(nanoseconds: 200_000_000)
        }
      }

      var onTerminationIterator = onTerminationStream.makeAsyncIterator()
      _ = await onTerminationIterator.next()

      try withExtendedLifetime(stream) {
        let writeResult = try source.write(1)
        writeResult.assertIsProducerMore()
      }

      stream = nil

      do {
        let writeResult = try { try source.write(2) }()
        writeResult.assertIsProducerMore()
      } catch {
        XCTFail("Expected no error to be thrown")
      }

      let element1 = try await iterator?.next()
      XCTAssertEqual(element1, 1)
      let element2 = try await iterator?.next()
      XCTAssertEqual(element2, 2)

      group.cancelAll()
    }
  }

  func testSequenceDeinitialized_whenFinished() async throws {
    var (stream, source): (BufferedStream?, BufferedStream.Source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 5, high: 10)
    )

    let (onTerminationStream, onTerminationContinuation) = AsyncStream<Void>.makeStream()
    source.onTermination = {
      onTerminationContinuation.finish()
    }

    await withThrowingTaskGroup(of: Void.self) { group in
      group.addTask {
        while !Task.isCancelled {
          onTerminationContinuation.yield()
          try await Task.sleep(nanoseconds: 200_000_000)
        }
      }

      var onTerminationIterator = onTerminationStream.makeAsyncIterator()
      _ = await onTerminationIterator.next()

      withExtendedLifetime(stream) {
        source.finish(throwing: nil)
      }

      stream = nil

      let terminationResult: Void? = await onTerminationIterator.next()
      XCTAssertNil(terminationResult)

      do {
        _ = try { try source.write(1) }()
        XCTFail("Expected an error to be thrown")
      } catch {
        XCTAssertTrue(error is AlreadyFinishedError)
      }

      group.cancelAll()
    }
  }

  func testSequenceDeinitialized_whenStreaming_andSuspendedProducer() async throws {
    var (stream, source): (BufferedStream?, BufferedStream.Source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 1, high: 2)
    )

    _ = try { try source.write(1) }()

    do {
      try await withCheckedThrowingContinuation { continuation in
        source.write(1) { result in
          continuation.resume(with: result)
        }

        stream = nil
        _ = stream?.makeAsyncIterator()
      }
    } catch {
      XCTAssertTrue(error is AlreadyFinishedError)
    }
  }

  // MARK: - iteratorInitialized

  func testIteratorInitialized_whenInitial() async throws {
    let (stream, _) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 5, high: 10)
    )

    _ = stream.makeAsyncIterator()
  }

  func testIteratorInitialized_whenStreaming() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 5, high: 10)
    )

    try await source.write(1)

    var iterator = stream.makeAsyncIterator()
    let element = try await iterator.next()
    XCTAssertEqual(element, 1)
  }

  func testIteratorInitialized_whenSourceFinished() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 5, high: 10)
    )

    try await source.write(1)
    source.finish(throwing: nil)

    var iterator = stream.makeAsyncIterator()
    let element1 = try await iterator.next()
    XCTAssertEqual(element1, 1)
    let element2 = try await iterator.next()
    XCTAssertNil(element2)
  }

  func testIteratorInitialized_whenFinished() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 5, high: 10)
    )

    source.finish(throwing: nil)

    var iterator = stream.makeAsyncIterator()
    let element = try await iterator.next()
    XCTAssertNil(element)
  }

  // MARK: - iteratorDeinitialized

  func testIteratorDeinitialized_whenInitial() async throws {
    var (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 5, high: 10)
    )

    let (onTerminationStream, onTerminationContinuation) = AsyncStream<Void>.makeStream()
    source.onTermination = {
      onTerminationContinuation.finish()
    }

    try await withThrowingTaskGroup(of: Void.self) { group in
      group.addTask {
        while !Task.isCancelled {
          onTerminationContinuation.yield()
          try await Task.sleep(nanoseconds: 200_000_000)
        }
      }

      var onTerminationIterator = onTerminationStream.makeAsyncIterator()
      _ = await onTerminationIterator.next()

      var iterator: BufferedStream<Int>.AsyncIterator? = stream.makeAsyncIterator()
      iterator = nil
      _ = try await iterator?.next()

      let terminationResult: Void? = await onTerminationIterator.next()
      XCTAssertNil(terminationResult)

      group.cancelAll()
    }
  }

  func testIteratorDeinitialized_whenStreaming() async throws {
    var (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 5, high: 10)
    )

    let (onTerminationStream, onTerminationContinuation) = AsyncStream<Void>.makeStream()
    source.onTermination = {
      onTerminationContinuation.finish()
    }

    try await source.write(1)

    try await withThrowingTaskGroup(of: Void.self) { group in
      group.addTask {
        while !Task.isCancelled {
          onTerminationContinuation.yield()
          try await Task.sleep(nanoseconds: 200_000_000)
        }
      }

      var onTerminationIterator = onTerminationStream.makeAsyncIterator()
      _ = await onTerminationIterator.next()

      var iterator: BufferedStream<Int>.AsyncIterator? = stream.makeAsyncIterator()
      iterator = nil
      _ = try await iterator?.next()

      let terminationResult: Void? = await onTerminationIterator.next()
      XCTAssertNil(terminationResult)

      group.cancelAll()
    }
  }

  func testIteratorDeinitialized_whenSourceFinished() async throws {
    var (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 5, high: 10)
    )

    let (onTerminationStream, onTerminationContinuation) = AsyncStream<Void>.makeStream()
    source.onTermination = {
      onTerminationContinuation.finish()
    }

    try await source.write(1)
    source.finish(throwing: nil)

    try await withThrowingTaskGroup(of: Void.self) { group in
      group.addTask {
        while !Task.isCancelled {
          onTerminationContinuation.yield()
          try await Task.sleep(nanoseconds: 200_000_000)
        }
      }

      var onTerminationIterator = onTerminationStream.makeAsyncIterator()
      _ = await onTerminationIterator.next()

      var iterator: BufferedStream<Int>.AsyncIterator? = stream.makeAsyncIterator()
      iterator = nil
      _ = try await iterator?.next()

      let terminationResult: Void? = await onTerminationIterator.next()
      XCTAssertNil(terminationResult)

      group.cancelAll()
    }
  }

  func testIteratorDeinitialized_whenFinished() async throws {
    var (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 5, high: 10)
    )

    let (onTerminationStream, onTerminationContinuation) = AsyncStream<Void>.makeStream()
    source.onTermination = {
      onTerminationContinuation.finish()
    }

    source.finish(throwing: nil)

    try await withThrowingTaskGroup(of: Void.self) { group in
      group.addTask {
        while !Task.isCancelled {
          onTerminationContinuation.yield()
          try await Task.sleep(nanoseconds: 200_000_000)
        }
      }

      var onTerminationIterator = onTerminationStream.makeAsyncIterator()
      _ = await onTerminationIterator.next()

      var iterator: BufferedStream<Int>.AsyncIterator? = stream.makeAsyncIterator()
      iterator = nil
      _ = try await iterator?.next()

      let terminationResult: Void? = await onTerminationIterator.next()
      XCTAssertNil(terminationResult)

      group.cancelAll()
    }
  }

  func testIteratorDeinitialized_whenStreaming_andSuspendedProducer() async throws {
    var (stream, source): (BufferedStream?, BufferedStream.Source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 1, high: 2)
    )

    var iterator: BufferedStream<Int>.AsyncIterator? = stream?.makeAsyncIterator()
    stream = nil

    _ = try { try source.write(1) }()

    do {
      try await withCheckedThrowingContinuation { continuation in
        source.write(1) { result in
          continuation.resume(with: result)
        }

        iterator = nil
      }
    } catch {
      XCTAssertTrue(error is AlreadyFinishedError)
    }

    _ = try await iterator?.next()
  }

  // MARK: - sourceDeinitialized

  func testSourceDeinitialized_whenInitial() async throws {
    var (stream, source): (BufferedStream, BufferedStream.Source?) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 5, high: 10)
    )

    let (onTerminationStream, onTerminationContinuation) = AsyncStream<Void>.makeStream()
    source?.onTermination = {
      onTerminationContinuation.finish()
    }

    await withThrowingTaskGroup(of: Void.self) { group in
      group.addTask {
        while !Task.isCancelled {
          onTerminationContinuation.yield()
          try await Task.sleep(nanoseconds: 200_000_000)
        }
      }

      var onTerminationIterator = onTerminationStream.makeAsyncIterator()
      _ = await onTerminationIterator.next()

      source = nil

      let terminationResult: Void? = await onTerminationIterator.next()
      XCTAssertNil(terminationResult)

      group.cancelAll()
    }

    withExtendedLifetime(stream) {}
  }

  func testSourceDeinitialized_whenStreaming_andEmptyBuffer() async throws {
    var (stream, source): (BufferedStream, BufferedStream.Source?) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 5, high: 10)
    )

    let (onTerminationStream, onTerminationContinuation) = AsyncStream<Void>.makeStream()
    source?.onTermination = {
      onTerminationContinuation.finish()
    }

    try await source?.write(1)

    try await withThrowingTaskGroup(of: Void.self) { group in
      group.addTask {
        while !Task.isCancelled {
          onTerminationContinuation.yield()
          try await Task.sleep(nanoseconds: 200_000_000)
        }
      }

      var onTerminationIterator = onTerminationStream.makeAsyncIterator()
      _ = await onTerminationIterator.next()

      var iterator: BufferedStream<Int>.AsyncIterator? = stream.makeAsyncIterator()
      _ = try await iterator?.next()

      source = nil

      let terminationResult: Void? = await onTerminationIterator.next()
      XCTAssertNil(terminationResult)

      group.cancelAll()
    }
  }

  func testSourceDeinitialized_whenStreaming_andNotEmptyBuffer() async throws {
    var (stream, source): (BufferedStream, BufferedStream.Source?) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 5, high: 10)
    )

    let (onTerminationStream, onTerminationContinuation) = AsyncStream<Void>.makeStream()
    source?.onTermination = {
      onTerminationContinuation.finish()
    }

    try await source?.write(1)
    try await source?.write(2)

    try await withThrowingTaskGroup(of: Void.self) { group in
      group.addTask {
        while !Task.isCancelled {
          onTerminationContinuation.yield()
          try await Task.sleep(nanoseconds: 200_000_000)
        }
      }

      var onTerminationIterator = onTerminationStream.makeAsyncIterator()
      _ = await onTerminationIterator.next()

      var iterator: BufferedStream<Int>.AsyncIterator? = stream.makeAsyncIterator()
      _ = try await iterator?.next()

      source = nil

      _ = await onTerminationIterator.next()

      _ = try await iterator?.next()
      _ = try await iterator?.next()

      let terminationResult: Void? = await onTerminationIterator.next()
      XCTAssertNil(terminationResult)

      group.cancelAll()
    }
  }

  func testSourceDeinitialized_whenSourceFinished() async throws {
    var (stream, source): (BufferedStream, BufferedStream.Source?) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 5, high: 10)
    )

    let (onTerminationStream, onTerminationContinuation) = AsyncStream<Void>.makeStream()
    source?.onTermination = {
      onTerminationContinuation.finish()
    }

    try await source?.write(1)
    try await source?.write(2)
    source?.finish(throwing: nil)

    try await withThrowingTaskGroup(of: Void.self) { group in
      group.addTask {
        while !Task.isCancelled {
          onTerminationContinuation.yield()
          try await Task.sleep(nanoseconds: 200_000_000)
        }
      }

      var onTerminationIterator = onTerminationStream.makeAsyncIterator()
      _ = await onTerminationIterator.next()

      var iterator: BufferedStream<Int>.AsyncIterator? = stream.makeAsyncIterator()
      _ = try await iterator?.next()

      source = nil

      _ = await onTerminationIterator.next()

      _ = try await iterator?.next()
      _ = try await iterator?.next()

      let terminationResult: Void? = await onTerminationIterator.next()
      XCTAssertNil(terminationResult)

      group.cancelAll()
    }
  }

  func testSourceDeinitialized_whenFinished() async throws {
    var (stream, source): (BufferedStream, BufferedStream.Source?) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 5, high: 10)
    )

    let (onTerminationStream, onTerminationContinuation) = AsyncStream<Void>.makeStream()
    source?.onTermination = {
      onTerminationContinuation.finish()
    }

    source?.finish(throwing: nil)

    await withThrowingTaskGroup(of: Void.self) { group in
      group.addTask {
        while !Task.isCancelled {
          onTerminationContinuation.yield()
          try await Task.sleep(nanoseconds: 200_000_000)
        }
      }

      var onTerminationIterator = onTerminationStream.makeAsyncIterator()
      _ = await onTerminationIterator.next()

      _ = stream.makeAsyncIterator()

      source = nil

      _ = await onTerminationIterator.next()

      let terminationResult: Void? = await onTerminationIterator.next()
      XCTAssertNil(terminationResult)

      group.cancelAll()
    }
  }

  func testSourceDeinitialized_whenStreaming_andSuspendedProducer() async throws {
    var (stream, source): (BufferedStream, BufferedStream.Source?) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 0, high: 0)
    )
    let (producerStream, producerContinuation) = AsyncThrowingStream<Void, any Error>.makeStream()
    var iterator = stream.makeAsyncIterator()

    source?.write(1) {
      producerContinuation.yield(with: $0)
    }

    _ = try await iterator.next()
    source = nil

    do {
      try await producerStream.first { _ in true }
      XCTFail("We expected to throw here")
    } catch {
      XCTAssertTrue(error is AlreadyFinishedError)
    }
  }

  // MARK: - write

  func testWrite_whenInitial() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 2, high: 5)
    )

    try await source.write(1)

    var iterator = stream.makeAsyncIterator()
    let element = try await iterator.next()
    XCTAssertEqual(element, 1)
  }

  func testWrite_whenStreaming_andNoConsumer() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 2, high: 5)
    )

    try await source.write(1)
    try await source.write(2)

    var iterator = stream.makeAsyncIterator()
    let element1 = try await iterator.next()
    XCTAssertEqual(element1, 1)
    let element2 = try await iterator.next()
    XCTAssertEqual(element2, 2)
  }

  func testWrite_whenStreaming_andSuspendedConsumer() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 2, high: 5)
    )

    try await withThrowingTaskGroup(of: Int?.self) { group in
      group.addTask {
        return try await stream.first { _ in true }
      }

      // This is always going to be a bit racy since we need the call to next() suspend
      try await Task.sleep(nanoseconds: 500_000_000)

      try await source.write(1)
      let element = try await group.next()
      XCTAssertEqual(element, 1)
    }
  }

  func testWrite_whenStreaming_andSuspendedConsumer_andEmptySequence() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 2, high: 5)
    )

    try await withThrowingTaskGroup(of: Int?.self) { group in
      group.addTask {
        return try await stream.first { _ in true }
      }

      // This is always going to be a bit racy since we need the call to next() suspend
      try await Task.sleep(nanoseconds: 500_000_000)

      try await source.write(contentsOf: [])
      try await source.write(contentsOf: [1])
      let element = try await group.next()
      XCTAssertEqual(element, 1)
    }
  }

  // MARK: - enqueueProducer

  func testEnqueueProducer_whenStreaming_andAndCancelled() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 1, high: 2)
    )

    let (producerStream, producerSource) = AsyncThrowingStream<Void, any Error>.makeStream()

    try await source.write(1)

    let writeResult = try { try source.write(2) }()

    switch writeResult {
    case .produceMore:
      preconditionFailure()
    case .enqueueCallback(let callbackToken):
      source.cancelCallback(callbackToken: callbackToken)

      source.enqueueCallback(callbackToken: callbackToken) { result in
        producerSource.yield(with: result)
      }
    }

    do {
      _ = try await producerStream.first { _ in true }
      XCTFail("Expected an error to be thrown")
    } catch {
      XCTAssertTrue(error is CancellationError)
    }

    let element = try await stream.first { _ in true }
    XCTAssertEqual(element, 1)
  }

  func testEnqueueProducer_whenStreaming_andAndCancelled_andAsync() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 1, high: 2)
    )

    try await source.write(1)

    await withThrowingTaskGroup(of: Void.self) { group in
      group.addTask {
        try await source.write(2)
      }

      group.cancelAll()
      do {
        try await group.next()
        XCTFail("Expected an error to be thrown")
      } catch {
        XCTAssertTrue(error is CancellationError)
      }
    }

    let element = try await stream.first { _ in true }
    XCTAssertEqual(element, 1)
  }

  func testEnqueueProducer_whenStreaming_andInterleaving() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 1, high: 1)
    )
    var iterator = stream.makeAsyncIterator()

    let (producerStream, producerSource) = AsyncThrowingStream<Void, any Error>.makeStream()

    let writeResult = try { try source.write(1) }()

    switch writeResult {
    case .produceMore:
      preconditionFailure()
    case .enqueueCallback(let callbackToken):
      let element = try await iterator.next()
      XCTAssertEqual(element, 1)

      source.enqueueCallback(callbackToken: callbackToken) { result in
        producerSource.yield(with: result)
      }
    }

    do {
      _ = try await producerStream.first { _ in true }
    } catch {
      XCTFail("Expected no error to be thrown")
    }
  }

  func testEnqueueProducer_whenStreaming_andSuspending() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 1, high: 1)
    )
    var iterator = stream.makeAsyncIterator()

    let (producerStream, producerSource) = AsyncThrowingStream<Void, any Error>.makeStream()

    let writeResult = try { try source.write(1) }()

    switch writeResult {
    case .produceMore:
      preconditionFailure()
    case .enqueueCallback(let callbackToken):
      source.enqueueCallback(callbackToken: callbackToken) { result in
        producerSource.yield(with: result)
      }
    }

    let element = try await iterator.next()
    XCTAssertEqual(element, 1)

    do {
      _ = try await producerStream.first { _ in true }
    } catch {
      XCTFail("Expected no error to be thrown")
    }
  }

  func testEnqueueProducer_whenFinished() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 1, high: 1)
    )
    var iterator = stream.makeAsyncIterator()

    let (producerStream, producerSource) = AsyncThrowingStream<Void, any Error>.makeStream()

    let writeResult = try { try source.write(1) }()

    switch writeResult {
    case .produceMore:
      preconditionFailure()
    case .enqueueCallback(let callbackToken):
      source.finish(throwing: nil)

      source.enqueueCallback(callbackToken: callbackToken) { result in
        producerSource.yield(with: result)
      }
    }

    let element = try await iterator.next()
    XCTAssertEqual(element, 1)

    do {
      _ = try await producerStream.first { _ in true }
      XCTFail("Expected an error to be thrown")
    } catch {
      XCTAssertTrue(error is AlreadyFinishedError)
    }
  }

  // MARK: - cancelProducer

  func testCancelProducer_whenStreaming() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 1, high: 2)
    )

    let (producerStream, producerSource) = AsyncThrowingStream<Void, any Error>.makeStream()

    try await source.write(1)

    let writeResult = try { try source.write(2) }()

    switch writeResult {
    case .produceMore:
      preconditionFailure()
    case .enqueueCallback(let callbackToken):
      source.enqueueCallback(callbackToken: callbackToken) { result in
        producerSource.yield(with: result)
      }

      source.cancelCallback(callbackToken: callbackToken)
    }

    do {
      _ = try await producerStream.first { _ in true }
      XCTFail("Expected an error to be thrown")
    } catch {
      XCTAssertTrue(error is CancellationError)
    }

    let element = try await stream.first { _ in true }
    XCTAssertEqual(element, 1)
  }

  func testCancelProducer_whenSourceFinished() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 1, high: 2)
    )

    let (producerStream, producerSource) = AsyncThrowingStream<Void, any Error>.makeStream()

    try await source.write(1)

    let writeResult = try { try source.write(2) }()

    switch writeResult {
    case .produceMore:
      preconditionFailure()
    case .enqueueCallback(let callbackToken):
      source.enqueueCallback(callbackToken: callbackToken) { result in
        producerSource.yield(with: result)
      }

      source.finish(throwing: nil)

      source.cancelCallback(callbackToken: callbackToken)
    }

    do {
      _ = try await producerStream.first { _ in true }
      XCTFail("Expected an error to be thrown")
    } catch {
      XCTAssertTrue(error is AlreadyFinishedError)
    }

    let element = try await stream.first { _ in true }
    XCTAssertEqual(element, 1)
  }

  // MARK: - finish

  func testFinish_whenStreaming_andConsumerSuspended() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 1, high: 1)
    )

    try await withThrowingTaskGroup(of: Int?.self) { group in
      group.addTask {
        return try await stream.first { $0 == 2 }
      }

      // This is always going to be a bit racy since we need the call to next() suspend
      try await Task.sleep(nanoseconds: 500_000_000)

      source.finish(throwing: nil)
      let element = try await group.next()
      XCTAssertEqual(element, .some(nil))
    }
  }

  func testFinish_whenInitial() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 1, high: 1)
    )

    source.finish(throwing: CancellationError())

    do {
      for try await _ in stream {}
      XCTFail("Expected an error to be thrown")
    } catch {
      XCTAssertTrue(error is CancellationError)
    }

  }

  // MARK: - Backpressure

  func testBackPressure() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 2, high: 4)
    )

    let (backPressureEventStream, backPressureEventContinuation) = AsyncStream.makeStream(
      of: Void.self
    )

    try await withThrowingTaskGroup(of: Void.self) { group in
      group.addTask {
        while true {
          backPressureEventContinuation.yield(())
          try await source.write(contentsOf: [1])
        }
      }

      var backPressureEventIterator = backPressureEventStream.makeAsyncIterator()
      var iterator = stream.makeAsyncIterator()

      await backPressureEventIterator.next()
      await backPressureEventIterator.next()
      await backPressureEventIterator.next()
      await backPressureEventIterator.next()

      _ = try await iterator.next()
      _ = try await iterator.next()
      _ = try await iterator.next()

      await backPressureEventIterator.next()
      await backPressureEventIterator.next()
      await backPressureEventIterator.next()

      group.cancelAll()
    }
  }

  func testBackPressureSync() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 2, high: 4)
    )

    let (backPressureEventStream, backPressureEventContinuation) = AsyncStream.makeStream(
      of: Void.self
    )

    try await withThrowingTaskGroup(of: Void.self) { group in
      group.addTask {
        @Sendable func yield() {
          backPressureEventContinuation.yield(())
          source.write(contentsOf: [1]) { result in
            switch result {
            case .success:
              yield()

            case .failure:
              break
            }
          }
        }

        yield()
      }

      var backPressureEventIterator = backPressureEventStream.makeAsyncIterator()
      var iterator = stream.makeAsyncIterator()

      await backPressureEventIterator.next()
      await backPressureEventIterator.next()
      await backPressureEventIterator.next()
      await backPressureEventIterator.next()

      _ = try await iterator.next()
      _ = try await iterator.next()
      _ = try await iterator.next()

      await backPressureEventIterator.next()
      await backPressureEventIterator.next()
      await backPressureEventIterator.next()

      group.cancelAll()
    }
  }

  func testThrowsError() async throws {
    let (stream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 2, high: 4)
    )

    try await source.write(1)
    try await source.write(2)
    source.finish(throwing: CancellationError())

    var elements = [Int]()
    var iterator = stream.makeAsyncIterator()

    do {
      while let element = try await iterator.next() {
        elements.append(element)
      }
      XCTFail("Expected an error to be thrown")
    } catch {
      XCTAssertTrue(error is CancellationError)
      XCTAssertEqual(elements, [1, 2])
    }

    let element = try await iterator.next()
    XCTAssertNil(element)
  }

  func testAsyncSequenceWrite() async throws {
    let (stream, continuation) = AsyncStream<Int>.makeStream()
    let (backpressuredStream, source) = BufferedStream.makeStream(
      of: Int.self,
      backPressureStrategy: .watermark(low: 2, high: 4)
    )

    continuation.yield(1)
    continuation.yield(2)
    continuation.finish()

    try await source.write(contentsOf: stream)
    source.finish(throwing: nil)

    let elements = try await backpressuredStream.collect()
    XCTAssertEqual(elements, [1, 2])
  }
}

@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
extension BufferedStream.Source.WriteResult {
  func assertIsProducerMore() {
    switch self {
    case .produceMore:
      return

    case .enqueueCallback:
      XCTFail("Expected produceMore")
    }
  }

  func assertIsEnqueueCallback() {
    switch self {
    case .produceMore:
      XCTFail("Expected enqueueCallback")

    case .enqueueCallback:
      return
    }
  }
}