`withUnsafeCurrentTask` and inheritance of actor isolation

ph1ps · February 19, 2025, 1:58pm

Just wanted to make sure, that I am not missing anything here.

There is this overload of withUnsafeCurrentTask that takes an async body and is of course async itself. However, it does not participate in actor isolation inheritance. Was this just an oversight, or deliberately left out?

github.com/swiftlang/swift

stdlib/public/Concurrency/Task.swift

63b7f05d7


      
          
            // FIXME: This retain seems pretty wrong, however if we don't we WILL crash
            //        with "destroying a task that never completed" in the task's destroy.
            //        How do we solve this properly?
            Builtin.retain(_task)
          
            return try body(UnsafeCurrentTask(_task))
          }
          
          @available(SwiftStdlib 6.0, *)
          public func withUnsafeCurrentTask<T>(body: (UnsafeCurrentTask?) async throws -> T) async rethrows -> T {
            guard let _task = _getCurrentAsyncTask() else {
              return try await body(nil)
            }
          
            // FIXME: This retain seems pretty wrong, however if we don't we WILL crash
            //        with "destroying a task that never completed" in the task's destroy.
            //        How do we solve this properly?
            Builtin.retain(_task)
          
            return try await body(UnsafeCurrentTask(_task))

If actor isolation inheritance is added, I guess the body should probably also be sending T, right?

If someone could confirm my suspicion, I'd be happy to create an issue and maybe also a PR.

ktoso · February 19, 2025, 2:18pm

I'd like to confirm what exactly do you mean with "participate in actor isolation inheritance"?

You mean adding an isolation: isolated (any Actor)? = #isolation parameter?

Adding these is a bit of a pain due to having to be careful around ABI but yeah I guess that could be done...

On the other hand... why would you need this? The unsafe API is used usually in contexts where you don't have any static current information, so I'm curious as to why you'd want that?

ph1ps · February 19, 2025, 2:33pm

Yeah, sorry, should've clarified.

Hm, what do you mean by any static current information? I am not sure if this example is representative, but this fails to compile:

@MainActor
func foo(_: UnsafeCurrentTask?) async throws {}

@MainActor
func bar() async throws {
    try await withUnsafeCurrentTask { unsafeTask in // error: Sending main actor-isolated value of type '(UnsafeCurrentTask?) async throws -> ()' with later accesses to nonisolated context risks causing data races
        try await foo(unsafeTask)
    }
}

... because the closure expression is main actor isolated.