[SR-11647] Swift 5.1 on Linux: Regression in OperationQueue concurrency

relates to:

• SR-11864 Availability warnings in dead code paths

Issue Description:

We've bumped into an interesting regression in RxSwift's OperationQueueScheduler that wraps OperationQueue, specifically under Linux. It seems concurrency is broken or hangs.

The full issue is here:
https://github.com/ReactiveX/RxSwift/pull/2077#issuecomment-544166860

I was able to reduce the issue using only OperationQueue, without Rx:

import Foundation

let condition = NSCondition()
let sema = DispatchSemaphore(value: 0)
var writtenStarted = 0
var writtenEnded = 0
var events = [String]()
var lock = NSLock()

func performLocked(_ action: () -> Void) {
    lock.lock()
    action()
    lock.unlock()
}

let concurrent = {
    performLocked {
        print("Started")
        events.append("Started")
    }

    condition.lock()
    writtenStarted += 1
    condition.signal()
    while writtenStarted < 2 {
        condition.wait()
    }
    condition.unlock()

    performLocked {
        print("Ended")
        events.append("Ended")
    }

    condition.lock()
    writtenEnded += 1
    condition.signal()
    while writtenEnded < 2 {
        condition.wait()
    }
    condition.unlock()
    sema.signal()
}

let op1 = BlockOperation(block: concurrent)
let op2 = BlockOperation(block: concurrent)

let operationQueue = OperationQueue()
operationQueue.maxConcurrentOperationCount = 8

operationQueue.addOperation(op1)
operationQueue.addOperation(op2)
sema.wait()

Running the following code on macOS (in Swift 5.1 as well) or Linux with Swift 5.0 yields:

$ swift regression.swift
Started
Started
Ended
Ended

Running it under Linux with Swift 5.1:

$ swift -v
Swift version 5.1.1 (swift-5.1.1-RELEASE)

$ swift regression.swift
Started
-> At this point the process hangs and never completes

This seems to be an open regression in Swift 5.1 under Linux, but happy for any feedback or to provide any additional information.

Start by running the embedded regression with Swift 5.1.1 on Linux and compare it with Swift 5.0 or macOS. Investigate the OperationQueue and BlockOperation interaction through Foundation and libdispatch; done means the two operations complete and print both Started and Ended lines without hanging.