TaskMeeting.swift

// TaskMeeting
// (c) 2022, Nikolai Ruhe

/// A type that synchronizes progress of two tasks.
public final actor TaskMeeting: Sendable {
    private var completion: CheckedContinuation<Void, Error>? = nil

    /// This method synchronizes two tasks so that both perform the closure at
    /// the same time. Both tasks need to call `rendezvous`.
    ///
    /// 1. Waits until a second task calls `rendezvous`.
    /// 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 rendezvous<T>(_ perform: () async throws -> T) async throws -> T {
        try await waitForPeer()
        let result = try await perform()
        try await waitForPeer()
        return result
    }

    /// This method synchronizes the progress of two tasks, so that one task
    /// waits for the second task. The first task will suspend until the second
    /// task arrives and also calls `waitForPeer`. Both tasks resume normally
    /// after that.
    public func waitForPeer() async throws {
        return try await withCheckedThrowingContinuation { newCompletion in
            if let oldCompletion = completion {
                self.completion = nil
                oldCompletion.resume()
                newCompletion.resume()
            } else {
                self.completion = newCompletion
            }
        }
    }
}