Kutex

A Kotlin Multiplatform object wrapper that provides safe access between coroutines

Normally, we have to keep a separate locking object in order to ensure safe access to the objects we want to protect. This is clumsy and can lead to errors.

• You have to ensure you always wrap every call with the locking mechanism
• You can forget to lock the object, leading to strange, hard to track down issues
• Someone else could come along and unsafely access something

Ideally, it should be difficult or impossible to access an object that should be kept behind a lock without using said lock. That's the purpose of Kutex.

Normal, unsafe object locking:

object UnsafeExample {
    val listMutex = Mutex()
    val myList = mutableListOf("some", "strings")

    suspend fun addStringToListFromRandomCoroutine(string: String) {
        listMutex.withLock {
            myList.add(string)
        }
    }

    suspend fun performCalculationsOnList() {
        listMutex.withLock {
            myList.forEach {
                delay(1000)
            }
        }
    }

    fun evil() {
        myList.add("hope you aren't iterating!")
    }
}

With Kutex:

With Kutex, you can't accidentally do anything unsafe

object SafeExample {
    val myListKutex = kutexOf(mutableListOf("some", "strings"))

    suspend fun addStringToListFromRandomCoroutine(string: String) {
        myListKutex.withLock {
            value.add(string)
        }
    }

    suspend fun performCalculationsOnList() {
        myListKutex.withLock {
            value.forEach {
                delay(1000)
            }
        }
    }

    suspend fun evil() {
        myListKutex.withLock { 
            value.add("I cannot access the underlying object in an unsafe way :'(")
        }
    }
}

Examples

Immutable

val myListKutex = kutexOf(mutableListOf("some", "strings"))

suspend fun lockExamples() {
    // Can return from the scope
    myListKutex.withLock {
        if ("someString" !in value) return
    }

    // Can assign vals outside the scope from within the scope
    val someString: String
    myListKutex.withLock {
        someString = value.first()
    }
    println(someString)

    // Can get from the Kutex's value
    val firstItem = myListKutex.withLock { value.firstOrNull() }

    // Can get information about the Kutex's value
    val isEmpty = myListKutex.withLock { value.isEmpty() }
}

fun tryLockExamples() {
    myListKutex.tryWithLock {
        if ("someString" !in value) return
    }.onAlreadyLocked {
        println("myListKutex was locked when we attempted to acquire lock")
    }

    val first = myListKutex.tryWithLock { value.first() }.getOrNull()
    if (first == null) {
        println("myListKutex was locked when we attempted to acquire lock")
    } else {
        println("First item in myListKutex is $first!")
    }

    val thing = myListKutex.tryWithLock { value.first() }
        .getOrElse { "some default string for when already locked" }
    println(thing)
}

Mutable

val countKutex = mutableKutexOf(100)

suspend fun test() {
    countKutex.withLock {
        if (value < 100) return
    }

    val count: Int
    countKutex.withLock {
        count = value
    }
    println(count)

    val currCount = countKutex.withLock { value }

    val isAtLeast100 = countKutex.withLock { value >= 100 }

    // increment count safely
    countKutex.withLock { value++ }

    // reset count to zero
    countKutex.withLock { value = 0 }
}

fun tryLockExamples() {
    countKutex.tryWithLock {
        if (value < 100) return
    }.onAlreadyLocked {
        println("myListKutex was locked when we attempted to acquire lock")
    }

    val currCount = countKutex.tryWithLock { value }.getOrNull()
    if (currCount == null) {
        println("countKutex was locked when we attempted to acquire lock")
    } else {
        println("currCount is $currCount!")
    }

    val thing = countKutex.tryWithLock { value }.getOrElse { -1 }
    println(thing)

    // Attempt to increment, doesn't matter if we can't
    countKutex.tryWithLock {
        value++
    }

    // Attempt to set value
    countKutex.tryWithLock {
        value = 0
    }.onAlreadyLocked {
        println("We couldn't acquire lock!")
    }
}

More examples and usages can be seen in the unit tests and documentation.

Contributions

Feel free to contribute. Have an issue? Create a new issue in GitHub.

FAQs

Uh... why is each extension function in its own file?

This is done primarily so that consumers of the library have better unit tests. Testing frameworks like Mockk support mocking extension functions, but they are awkward due to the way they are handled by Kotlin on the backend. For each file with top-level extensions, there secretly exists a class called <filename>Kt, and the extension functions and properties all exist as static methods within this class. In Mockk's case, mocking a static method mocks the class to which it belongs. This means that all extensions in a file will be mocked if any of them are. This can lead to good-looking tests with invalid results. For this reason, I like to put extension methods in their own files. Feel free to mock them individually with no ill-effects :)

I've cloned the project and the IDE says there are errors!

There aren't, really, and they won't stop you from building. It's a bug with how the checker is reading the OptIn annotation, unfortunately. The bug is tracked here, here, and on a few other older tickets, but it looks like they're very low priority. This is the same way that Kotlin's standard library uses the annotation. I will keep it this way (i.e., top-level declaration), since there's no need to expose that annotation to consumers of the library.