Crash: Variable is not retained in await statement in Release mode.

[WarWithinMe]

Describe the bug
In Release mode, a variable might not get retained if it's being used in an await statement.
In Debug mode, the crash does not exist.

Steps To Reproduce
Minimal reproduce code:

import UIKit

class Data {
  var obj = NSObject()
}

actor Consumer {
  func consume(_ data: Data) { print(data) }
}

class View: UIView {
  var consumer = Consumer()

  var data: Data? {
    didSet {

      // 1. Crash. data is released before consumer.consume(), then consumer prints an already released object.
      //    This only happens in Release mode
      Task {
        if let data {
          await consumer.consume(data)
        }
      }

      // 2. Works fine, if data is used before await.
//      Task {
//        if let data {
//          print(data)
//          await consumer.consume(data)
//        }
//      }

      // 3. Works fine, if there's another statement before await.
//      Task {
//        if let data {
//          print("ok")
//          await consumer.consume(data)
//        }
//      }

      // 4. Works fine, if there's another task.
//      Task {
//        if let data {
//          Task {
//            await consumer.consume(data)
//          }
//        }
//      }
    }
  }
}

class ViewController: UIViewController {
  override func loadView() {
    self.view = View(frame: .init(x: 0, y: 0, width: 100, height: 100))
  }
  override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view.
    Task {
      while (true) {
        try await Task.sleep(nanoseconds: 1000)
        (view as! View).data = .init()
      }
    }
  }
}

Expected behavior
The data object shƒould stay valid until after the await statement.

Environment (please fill out the following information)

• OS: [e.g. macOS 11.0] ios 16
• Xcode Version/Tag/Branch: Xcode-14.1.0-Release.Candidate

Additional context
Tested with swiftlang-5.7.1.135.2 clang-1400.0.29.51

Minimal Project to Reproduce
Test.zip

[saagarjha]

Shorter example:

import Foundation

class Data {
	var obj = NSObject()
}

actor Consumer {
	func consume(_ data: Data) {
		print(data)
	}
}

var consumer = Consumer()

var data: Data? {
	didSet {
		Task {
			if let data {
				await consumer.consume(data)
			}
		}
	}
}

Task {
	while true {
		data = .init()
	}
}

RunLoop.main.run()

This crashes regardless of optimization level. I think there is something broken in how objects are captured across an await in an actor-isolated context.

[eeckstein]

This looks like a race condition. The global variable data is set and read without going through the consumer actor.

[saagarjha]

Ah yes, you're right. I guess my code probably has some other issue then, or the race is less obvious there…