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Fix a race condition that caused withTimeout to not escalate the priority of the body #1780

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Fix a race condition that caused withTimeout to not escalate the priority of the body #1780

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11 changes: 8 additions & 3 deletions Sources/SwiftExtensions/AsyncUtils.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -176,9 +176,14 @@ package func withTimeout<T: Sendable>(
_ duration: Duration,
_ body: @escaping @Sendable () async throws -> T
) async throws -> T {
// Get the priority with which to launch the body task here so that we can pass the same priority as the initial
// priority to `withTaskPriorityChangedHandler`. Otherwise, we can get into a race condition where bodyTask gets
// launched with a low priority, then the priority gets elevated before we call with `withTaskPriorityChangedHandler`,
// we thus don't receive a `taskPriorityChanged` and hence never increase the priority of `bodyTask`.
let priority = Task.currentPriority
var mutableTasks: [Task<Void, Error>] = []
let stream = AsyncThrowingStream<T, Error> { continuation in
let bodyTask = Task<Void, Error> {
let bodyTask = Task<Void, Error>(priority: priority) {
do {
let result = try await body()
continuation.yield(result)
Expand All @@ -187,7 +192,7 @@ package func withTimeout<T: Sendable>(
}
}

let timeoutTask = Task {
let timeoutTask = Task(priority: priority) {
try await Task.sleep(for: duration)
continuation.yield(with: .failure(TimeoutError()))
bodyTask.cancel()
Expand All @@ -197,7 +202,7 @@ package func withTimeout<T: Sendable>(

let tasks = mutableTasks

return try await withTaskPriorityChangedHandler {
return try await withTaskPriorityChangedHandler(initialPriority: priority) {
for try await value in stream {
return value
}
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1 change: 0 additions & 1 deletion Tests/SKSupportTests/AsyncUtilsTests.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -52,7 +52,6 @@ final class AsyncUtilsTests: XCTestCase {
}

func testWithTimeoutEscalatesPriority() async throws {
try XCTSkipIf(true, "Flakey test: rdar://137640122")
let expectation = self.expectation(description: "Timeout started")
let task = Task(priority: .background) {
// We don't actually hit the timeout. It's just a large value.
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