swiftthreads.swift

import Foundation
class Engine {
    var fuel = 100
}
class Spaceship {
    var engine = Engine()
}
func accelerate(_ ship: Spaceship) {
    let engine = ship.engine  // Race: read
    // literally no field access
    _ = engine  // silence unused variable warning
}
func replaceEngine(_ ship: Spaceship) {
    ship.engine = Engine()  // Race: write
}
let ship = Spaceship()
let group = DispatchGroup()
let t1 = Thread {
    for _ in 0..<1_000_000 {
        accelerate(ship)
    }
    group.leave()
}
let t2 = Thread {
    for _ in 0..<1_000_000 {
        replaceEngine(ship)
    }
    group.leave()
}
group.enter()
group.enter()
t1.start()
t2.start()
group.wait()
print("Done")


// verdagon@Evans-MacBook-Pro swiftthread % swiftc -sanitize=thread main.swift -o main_tsan && ./main_tsan
// main_tsan(14565,0x1fd2d7ac0) malloc: nano zone abandoned due to inability to reserve vm space.
// ==================
// WARNING: ThreadSanitizer: data race (pid=14565)
//   Write of size 8 at 0x0001055027b0 by thread T2:
//     #0 Spaceship.engine.setter <null> (main_tsan:arm64+0x100003190)
//     #1 replaceEngine(_:) <null> (main_tsan:arm64+0x100002bc8)
//     #2 closure #2 in  <null> (main_tsan:arm64+0x100002b20)
//     #3 thunk for @escaping @callee_guaranteed @Sendable () -> () <null> (main_tsan:arm64+0x100003568)
//     #4 __NSThread__start__ <null> (Foundation:arm64+0x5451c)
// 
//   Previous read of size 8 at 0x0001055027b0 by thread T1:
//     #0 Spaceship.engine.getter <null> (main_tsan:arm64+0x10000310c)
//     #1 accelerate(_:) <null> (main_tsan:arm64+0x10000291c)
//     #2 closure #1 in  <null> (main_tsan:arm64+0x100002880)
//     #3 thunk for @escaping @callee_guaranteed @Sendable () -> () <null> (main_tsan:arm64+0x100003568)
//     #4 __NSThread__start__ <null> (Foundation:arm64+0x5451c)
// 
//   Location is heap block of size 24 at 0x0001055027a0 allocated by main thread:
//     #0 malloc <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64+0x611e4)
//     #1 swift_slowAlloc <null> (libswiftCore.dylib:arm64+0x3aa4c8)
//     #2 main <null> (main_tsan:arm64+0x100002310)
// 
//   Thread T2 (tid=456766, running) created by main thread at:
//     #0 pthread_create <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64+0x31764)
//     #1 -[NSThread startAndReturnError:] <null> (Foundation:arm64+0x7ce610)
//     #2 <null> <null> (0x0001952420e0)
// 
//   Thread T1 (tid=456765, running) created by main thread at:
//     #0 pthread_create <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64+0x31764)
//     #1 -[NSThread startAndReturnError:] <null> (Foundation:arm64+0x7ce610)
//     #2 <null> <null> (0x0001952420e0)
// 
// SUMMARY: ThreadSanitizer: data race (main_tsan:arm64+0x100003190) in Spaceship.engine.setter+0x58
// ==================
// Object 0x105504500 of class Engine deallocated with non-zero retain count 2. This object's deinit, or something called from it, may have created a strong reference to self which outlived deinit, resulting in a dangling reference.
// zsh: abort      ./main_tsan
// verdagon@Evans-MacBook-Pro swiftthread %