Hi! I'm running into an issue with `Mutex.withLoc...

# coroutines

[ntherning]

Hi!

I'm running into an issue with Mutex.withLock and unexpected freezing in my iOS app when upgrading my project to kotlin 1.4.10 and coroutines 1.3.9-native-mt-2. I think I have been able to make a simple reproduction case:

runBlocking(newSingleThreadContext("thread")) {
    val mutex = Mutex().apply { ensureNeverFrozen() }
    launch {
        runCatching { mutex.withLock { delay(1000) } }.exceptionOrNull()?.printStackTrace()
    }
    launch {
        runCatching { mutex.withLock { delay(1000) } }.exceptionOrNull()?.printStackTrace()
    }
}

This launches two coroutines in the same worker. Both tries to take the lock and then delays for 1 sec while holding it. As everything happens within the same worker thread I would not expect any mutability issues. But... This is what happens:

kotlin.native.concurrent.InvalidMutabilityException: mutation attempt of frozen kotlinx.coroutines.sync.MutexImpl.LockedQueue@788375b8
    at kfun:kotlin.Throwable#<init>(kotlin.String?){} + 93 (kotlin/kotlin/Throwable.kt:23:37)
    at kfun:kotlin.Exception#<init>(kotlin.String?){} + 91 (kotlin/kotlin/Exceptions.kt:23:44)
    at kfun:kotlin.RuntimeException#<init>(kotlin.String?){} + 91 (kotlin/kotlin/Exceptions.kt:34:44)
    at kfun:kotlin.native.concurrent.InvalidMutabilityException#<init>(kotlin.String){} + 91 (kotlin/kotlin/native/concurrent/Freezing.kt:22:60)
    at ThrowInvalidMutabilityException + 690 (kotlin/kotlin/native/concurrent/Internal.kt:92:11)
    at MutationCheck + 108
    at kfun:kotlinx.coroutines.sync.MutexImpl.LockedQueue.<set-owner>#internal + 102 (sync/Mutex.kt:350:19)
    at kfun:kotlinx.coroutines.sync.MutexImpl#unlock(kotlin.Any?){} + 3740 (sync/Mutex.kt:327:29)
    at kfun:MutexInvalidMutabilityTest.$doTest$lambda-2$lambda-0COROUTINE$0.invokeSuspend#internal + 2206 (MutexInvalidMutabilityTest.kt:16:61)
    ...

You can find the full code and full stack trace it in this gist:

https://gist.github.com/ntherning/a1bf85d44a5989d66677762c15587552


Is this expected behavior? Perhaps I am doing something stupid? Or a bug in MutexImpl?

[Tijl]

This is know bug fixed in Git already

[Tijl]

https://github.com/Kotlin/kotlinx.coroutines/issues/2263

[Tijl]

there is also a suggest workaround there using Semaphore(1), I’d advice using try/finally to release the semaphore (or make that into an extension function like withLock)

[ntherning]

Thanks! I will try that workaround.

[Marc Knaup]

@Tijl there is already an extension: semaphore.withPermit { … }.

[Tijl]

good point