Calendar's CoW is broken.

[sidepelican]

Problem

Calendar's some functions is not guarded with CoW.

Calendar wraps NSCalendar and it uses _applyMutation for any mutating actions.
but next two functions on NSCalendar mutates itself in the function. This behavior slips through the CoW.

https://github.com/apple/swift-corelibs-foundation/blob/d248649df1fd102269c0ba312220a1922b4f8bec/Sources/Foundation/NSCalendar.swift#L490-L509

https://github.com/apple/swift-corelibs-foundation/blob/d248649df1fd102269c0ba312220a1922b4f8bec/Sources/Foundation/NSCalendar.swift#L756-L762

When we use these functions in another thread using copied Calendar, may cause unexpected behavior.

How to reproduce

Run this code in Linux environemnt.
Sometimes prints "bug!" or sometimes crashes.

import Foundation

func main() {
    var calendar = Calendar(identifier: .gregorian)
    calendar.timeZone = TimeZone(secondsFromGMT: 9 * 3600)!

    let now = Date()
    let expected: Int = calendar.component(.hour, from: now)

    f(calendar) // Copy the calendar here and use it in backgrount thread.

    for _ in 0...100_000 {
        let v = calendar.component(.hour, from: now)
        if v != expected {
            print("bug! expected=\(expected), actual=\(v)")
        }
    }
    print("end")
}

func f(_ calendar: Calendar) {
    let comps = DateComponents(
        timeZone: TimeZone(secondsFromGMT: 0)!,
        year: 2022, month: 9, day: 1
    )

    DispatchQueue.global().async {
        for _ in 0...100_000 {
            _ = calendar.date(from: comps)
        }
    }
}

main()

Expected

Calendar's CoW works right.
I know Calendar is not thread safe unlike the Apple platform. This issue focuses abount CoW.

[parkera]

You're right about this. Part of the issue is that ICU's ucal is itself not thread safe, so it needs to be wrapped with a lock at a higher level.

[parkera]

This will be resolved with the entirely new Calendar implementations in #5001

[sidepelican]

I have confirmed that the new implementation does not cause this problems (Swift 6.0.3)

[parkera]

Thank you for checking!