[SR-15686] Dispatch on Linux incorrectly inherits random thread names

[weissi]

Previous ID	SR-15686
Radar	rdar://problem/87139515
Original Reporter	@weissi
Type	Bug

Environment

all Swift versions (incl. 5.4, 5.5 and the current main snapshot (Swift version 5.6-dev (LLVM c8e0f2fe28693ea, Swift 68eb340a2b7690f))) that I've tried on Linux.

Additional Detail from JIRA

Votes	0
Component/s	libdispatch
Labels	Bug, linux
Assignee	None
Priority	Medium

md5: 908cecea496804cb997a5bfc0ef74c2b

Issue Description:

On Linux, Dispatch incorrectly inherits essentially random thread names which makes debugging certain issues very hard. Usually, threads get labelled for a good reason, so it's pretty bad if that thread name then gets duplicated onto other threads.

Consider this test program which spawns one Thread and then sets its label to "MyThread". Subsequently it dispatches some work to some DispatchGroup and prints the dispatch queue's thread name (which should either be something chosen by Dispatch (ideal) or empty or maybe the program name but certainly not "MyThread").

import Foundation

class MyThread: Thread {
    let group: DispatchGroup

    init(group: DispatchGroup) {
        self.group = group
    }

    override func main() {
        Thread.current.name = "MyThread"
        print("[on thread] thread name is", Thread.current.name ?? "n/a")

        for f in 0..<10 {
            DispatchQueue.global().async(group: self.group) {
                print("[on global queue \(f)] thread name is", Thread.current.name ?? "n/a")
            }
        }
        self.group.leave()
    }
}

let group = DispatchGroup()
let t = MyThread(group: group)
group.enter()
t.start()
group.wait()

On Darwin, this program correctly prints

$ swift test.swift
[on thread] thread name is MyThread
[on global queue 0] thread name is 
[on global queue 1] thread name is 
[on global queue 3] thread name is 
[on global queue 4] thread name is 
[on global queue 6] thread name is 
[on global queue 7] thread name is 
[on global queue 5] thread name is 
[on global queue 9] thread name is 
[on global queue 2] thread name is 
[on global queue 8] thread name is 

ie, the thread names of the DispatchQueues are empty.

On Linux however, we see

$ docker run -it --rm -v "$PWD:$PWD" -w "$PWD" swift:5.5 swift test.swift
[on thread] thread name is MyThread
[on global queue 0] thread name is MyThread
[on global queue 2] thread name is MyThread
[on global queue 3] thread name is MyThread
[on global queue 4] thread name is MyThread
[on global queue 5] thread name is MyThread
[on global queue 6] thread name is MyThread
[on global queue 8] thread name is MyThread
[on global queue 1] thread name is MyThread
[on global queue 9] thread name is MyThread
[on global queue 7] thread name is MyThread

so "MyThread" got incorrectly inherited.

This got first discovered here: apple/swift-nio#1896 (I incorrectly filed it as a SwiftNIO issue).

[weissi]

@swift-ci create