DispatchWithSafety.swift

import Foundation
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

func dispatchOnce(_ predicate: inout Bool, _ block: @noescape () throws -> Void) rethrows {
    objc_sync_enter(predicate)
    if !predicate {
        do {
            predicate = true
            try block()
        } catch let error {
            objc_sync_exit(predicate)
            thow error
        }
    }
    objc_sync_exit(predicate)
}

testDispatchOnce(dispatchOnce)