Hi everyone, I just realized that `kotlin.random.Random` produces the same “random” values with each...

# android

[Sven Obser]

Hi everyone, I just realized that kotlin.random.Random produces the same “random” values with each app start on Android. 😨 Is this expected behavior?

Log.e { "java.util.Random().nextInt(): ${java.util.Random().nextInt()}" }
Log.e { "kotlin.random.Random.nextInt(): ${kotlin.random.Random.nextInt()}" }

Output:

java.util.Random().nextInt(): -1853486751
kotlin.random.Random.nextInt(): -351317653

java.util.Random().nextInt(): 1794921748
kotlin.random.Random.nextInt(): -351317653

java.util.Random().nextInt(): 1849140367
kotlin.random.Random.nextInt(): -351317653

[ephemient]

it's a bug in Android. Kotlin uses java.util.concurrent.ThreadLocalRandom, which Android initializes in zygote. then each app forks off zygote with the same state.

[ephemient]

https://issuetracker.google.com/issues/181797098
https://issuetracker.google.com/issues/234631055

[Sven Obser]

This sounds really bad. So the only workaround would be to use a custom val random = java.util.Random().asKotlinRandom() and supply it to the caller (e.g. listOf().shuffle(random))!?

[kevindmoore]

I always thought that Random functions always do that. I thought you had to provide a "seed" to get a truly random number (Not sure about Kotlin's version)

[ephemient]

ThreadLocalRandom.current() is supposed to be pre-seeded. the issue is that it was pre-seeded upon initialization in zygote, and every app gets a clone of the same instance

[Sven Obser]

@kevindmoore It should be the other way round. If you provide the same seed, you get the same numbers. But at least the java.util.Random() gives a a Random with a Random seed.

[ephemient]

as a short-term workaround, I think if you prevent Kotlin from using Java 8 then it'll fall back to a locally-seeded ThreadLocal<java.util.Random> to implement kotlin.random.Random.Default, e.g. with

dependencies {
    modules {
        module(kotlin("stdlib-jdk8")) {
            replacedBy(kotlin("stdlib-jdk7"), "<https://issuetracker.google.com/issues/234631055>")
        }
    }
}

in your build.gradle.kts. but that's a pretty terrible hack that could break other dependencies (as it also means dropping regex capture groups and java stream/time extensions) and won't work long-term (Kotlin is phasing out pre-Java 8 support, https://youtrack.jetbrains.com/issue/KT-45165)

[ephemient]

https://youtrack.jetbrains.com/issue/KT-52618 https://github.com/JetBrains/kotlin/commit/a2e8ed00329421a9a2997c87daa6b28c55ef9edc looks like there will be a workaround built into future versions of kotlin-stdlib

[Sven Obser]

Thanks a lot for all those links. Glad that this issue seems to get some attention!