Public Thread Local Storage API in Swift

miku1958 · December 25, 2023, 7:02am

Can the TLS API used by Observation be made public? It seems like it could be used in a lot of ways in multithreading.

apple/swift/blob/main/stdlib/public/Observation/Sources/Observation/ThreadLocal.swift

//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2022 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
//
//===----------------------------------------------------------------------===//

@_silgen_name("_swift_observation_tls_get")
func _tlsGet() -> UnsafeMutableRawPointer?

@_silgen_name("_swift_observation_tls_set")
func _tlsSet(_ value: UnsafeMutableRawPointer?)

@available(SwiftStdlib 5.9, *)
struct _ThreadLocal {
  static var value: UnsafeMutableRawPointer? {

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jrose · December 25, 2023, 8:17am

Foundation already has a general API for thread-local storage, though it requires going through objects:

I think a full Swift thread-local API would use generics to preserve types that go in and out of the dictionary (I can imagine one where the keys are themselves types which conform to a protocol), but for now this is both easier and safer than the thing in Observation.

ktoso · December 25, 2023, 8:26am

If you're looking for an API right now™, you could technically just use the TaskLocal APIs because they set a thread local if there's no Task available: TaskLocal | Apple Developer Documentation

miku1958 · December 25, 2023, 8:27am

That is what I am looking for, thanks!

jrose · December 25, 2023, 8:29am

Oh? The documentation says it will use the default value:

A lookup made from the context of a synchronous function, that is not called from an asynchronous function (!), will immediately return the task-local’s default value.

Is that incorrect?

ktoso · December 25, 2023, 8:34am

That's misleading wording... Thanks for noticing, I'll send in a revamp of the docs.

Here's a test showcasing them working without any Task involved: swift/test/Concurrency/Runtime/async_task_locals_synchronous_bind.swift at main · apple/swift · GitHub

ktoso · December 25, 2023, 9:44am

Gave the rewording a shot here: [docs] TaskLocal APIs from non-Task code by ktoso · Pull Request #70622 · apple/swift · GitHub