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

[swift-ci]

Previous ID	SR-11647
Radar	None
Original Reporter	freak4pc (JIRA User)
Type	Bug

Environment

Swift 5.1.1 latest Docker image

Additional Detail from JIRA

Votes	0
Component/s	Foundation, libdispatch
Labels	Bug, 5.1Regression
Assignee	None
Priority	Medium

md5: 4aa6a76a0b5ee28f3059851ef77c1f74

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:
ReactiveX/RxSwift#2077 (comment)

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.

[swift-ci]

Comment by Shai Mishali (JIRA)

Wanted to note this is still happening on the last version of Swift on Linux.

[weissi]

Can confirm that this is still happening on 5.3.3. Running this in a loop gave me

Started
Started
Ended
Ended
Started
Started
Ended
Ended
bash: line 1:   136 Segmentation fault      swift test.swift
johannes:/tmp

See the segfault