An tool for managing actor reentrancy.

This package exposes two types: AsyncGate and AsyncRecursiveGate. These allow you to define asynchronous critical sections. Only one task can enter a critical section at a time. Unlike a traditional lock, you can safely make async calls while these gates are held.

The intended use-case for these is managing actor reentrancy.

Some other concurrency packages you might find useful are Queue and Semaphore. Gate is an independent, but extremely similar package.

Swift Package Manager:

dependencies: [
    .package(url: "https://github.com/mattmassicotte/TaskGate", branch: "main")
]

Gates are very intentionally non-Sendable. The purpose of a gate is to control tasks running concurrently within a single actor, and making them non-Sendable allows the compiler will help enforce that concept.

Note that trying to acquire an already-gated AsyncGate will deadlock your actor.

import TaskGate

actor MyActor {
  var value = 42
  let gate = AsyncGate()
  let recursiveGate = AsyncRecursiveGate()

  func hasCriticalSections() async {
    // no matter how many tasks call this method,
    // only one will be able to execute at a time
	await gate.withGate {
      self.value = await otherObject.getValue()
    }
  }

  func hasCriticalSectionsBlock() async {
    await recursiveGate.withGate {
      // acquiring this multiple times within the same task is safe
      await recursiveGate.withGate {
        self.value = await otherObject.getValue()
      }
    }
  }
}

Both of these gate types support priority escalation propagation to help avoid priority inversions. However, the API required to do this was introduced with SE-0462, which is only available in the 26 OSes and later.

It is important to note that gates cannot be used from a non-async context. Actually doing this would require some trickery, as they both only have async interfaces. But, if you find a way, perhaps via ObjC bridging, you should expect a crash.

I would love to hear from you! Issues or pull requests work great. Both a Matrix space and Discord are also available for live help, but I have a strong bias towards answering in the form of documentation.

I prefer collaboration, and would love to find ways to work together if you have a similar project.

I prefer indentation with tabs for improved accessibility. But, I'd rather you use the system you want and make a PR than hesitate because of whitespace.

By participating in this project you agree to abide by the Contributor Code of Conduct.