Can't initialize actor with URL without getting a warning

MiLutz · December 14, 2021, 10:32am

Hi,

when compiling the below example with the flags -Xfrontend -warn-concurrency -Xfrontend -enable-actor-data-race-checks I get the warning "Cannot use property 'url' with a non-sendable type 'URL' across actors" in the init method and the test function.

Is the warning correct here? If yes, what would be the correct way to have an actor which has an URL as a property?

public actor ActorWithURL {
  private let url: URL
  
  init(url: URL) {
    self.url = url
  }
}

func test() {
  _ = ActorWithURL(url: URL(fileURLWithPath: "https://example.com")) 
}

Thank you, Michael

Karl · December 14, 2021, 11:58pm

Yep, pretty much nothing in Foundation is Sendable. That includes URL, Data, and even Date.

func needsSendable<T>(_ x: T) where T: Sendable {
  print("\(x) is Sendable")
}

needsSendable(URL(string: "https://example.com/")!) 
// ❌ Local function 'needsSendable' requires that 'URL' conform to 'Sendable'
needsSendable(Data(0..<5))
// ❌ Local function 'needsSendable' requires that 'Data' conform to 'Sendable'
needsSendable(Date.now)
// ❌ Local function 'needsSendable' requires that 'Date' conform to 'Sendable'

needsSendable(1.234 as Decimal)
// 🤷‍♂️ I don't know why this one type works and nothing else does, but there you go.

MiLutz · December 15, 2021, 10:00am

Hmm, maybe it is really a little bit too early to deploy -warn-concurrency and we should wait for 5.6?:

twitter.com

Doug Gregor

@dgregor79

@NachoSoto @olebegemann To be very clear, I don’t actually suggest that you use -warn-concurrency in Swift 5.5. It’s both too noisy in some cases and misses other cases. Swift 5.6 brings a model that’s designed to deal with gradual adoption of concurrency checking. https://t.co/12edRad9Ji

Ole Begemann @olebegemann

If you’re writing Swift concurrency code, add these compiler flags: -Xfrontend -warn-concurrency -Xfrontend -enable-actor-data-race-checks (in Xcode: Other Swift Flags) Warnings in Swift 5.5 identify unsafe constructs, will become errors in Swift 6. https://forums.swift.org/t/concurrency-in-swift-5-and-6/49337 …

8:27 AM - 6 Dec 2021 9 1

Karl · December 15, 2021, 10:26am

Oh, I didn't actually pass those flags when testing (I just stuck it in a unit test of a package I happened to be working on, using whatever the default settings happen to be in 5.5.2). If those flags make things even stricter, then that's an even bigger headache.

I suppose in the mean time, you'll need to add your own @unchecked Sendable conformances for anything you use from Foundation. I think that should be okay for most things (?).

MiLutz · December 15, 2021, 3:29pm

Yep, I switched the flags on, to be "prepared for the future" ;-). @unchecked Sendable works, but I hope it will be not needed in the (not so distant) future, because essentially I like the idea, that the compiler double checks my concurrent coding. Also I think it is not easy to see if @unchecked Sendable is safe to use on certain Foundation types, since we do not know the underlying implementation...