TaskRendezvous.swift

/// A type that synchronizes progress of two tasks.
///
/// After setup, two tasks should eventually call the `join` function. The
/// first task will suspend until the second task arrives and calls `join` as
/// well. Both tasks resume normally after that.
public final actor TaskRendezvous: Sendable {
    private var completion: CheckedContinuation<Void, Error>? = nil

    /// Suspends until a second task calls `join()` on this instance.
    public func join() async throws {
        return try await withCheckedThrowingContinuation { newCompletion in
            if let oldCompletion = completion {
                self.completion = nil
                oldCompletion.resume()
                newCompletion.resume()
            } else {
                self.completion = newCompletion
            }
        }
    }

    /// Lets two tasks synchronize so that both perform the closure at the same
    /// time.
    ///
    /// 1. Waits until a second task joined.
    /// 2. Performs the passed closure while the other task performs its closure.
    /// 3. Waits again for the second task to complete its closure.
    public func meet<T>(_ perform: () async throws -> T) async throws -> T {
        try await join()
        let result = try await perform()
        try await join()
        return result
    }
}