iOS Concurrent modification exception when closing a scope

[dasdranagon]

Our iOS application occasionally crashes with ConcurrentModificationException.

Uncaught Kotlin exception: kotlin.ConcurrentModificationException
    at 0   kfun:kotlin.Exception#<init>(kotlin.String?;kotlin.Throwable?){} + 143 
    at 1   kfun:kotlin.RuntimeException#<init>(kotlin.String?;kotlin.Throwable?){} + 143 
    at 2   kfun:kotlin.ConcurrentModificationException#<init>(kotlin.String?;kotlin.Throwable?){} + 143 
    at 3   kfun:kotlin.ConcurrentModificationException#<init>(){} + 99 
    at 4   kfun:kotlin.collections.HashMap.Itr#checkForComodification(){} + 247 
    at 5   kfun:kotlin.collections.HashMap.ValuesItr#next(){}1:1 + 219 
    at 6   kfun:kotlin.collections.MutableIterator#next(){}1:0-trampoline + 99 
    at 7   kfun:co.touchlab.stately.collections.ConcurrentMutableIterator.next$lambda$1#internal + 143 
    at 8   kfun:co.touchlab.stately.collections.ConcurrentMutableIterator.$next$lambda$1$FUNCTION_REFERENCE$13.invoke#internal + 79 
    at 9   kfun:kotlin.Function0#invoke(){}1:0-trampoline + 99 
    at 10  kfun:co.touchlab.stately.concurrency.Synchronizable#runSynchronized(kotlin.Function0<0:0>){0§<kotlin.Any?>}0:0 + 355 
    at 11  kfun:co.touchlab.stately.collections.ConcurrentMutableIterator#next(){}1:0 + 271 
    at 12  kfun:kotlin.collections.Iterator#next(){}1:0-trampoline + 99 
    at 13  kfun:org.koin.core.registry.InstanceRegistry#dropScopeInstances(org.koin.core.scope.Scope){} + 459
    at 14  kfun:org.koin.core.registry.ScopeRegistry#deleteScope(org.koin.core.scope.Scope){} + 231 
    at 15  kfun:org.koin.core.scope.Scope.close$lambda$5#internal + 871 
    at 16  kfun:org.koin.core.scope.Scope.$close$lambda$5$FUNCTION_REFERENCE$4.invoke#internal + 71 
    at 17  kfun:org.koin.core.scope.Scope.$close$lambda$5$FUNCTION_REFERENCE$4.$<bridge-DNN>invoke(){}#internal + 71 
    at 18  kfun:kotlin.Function0#invoke(){}1:0-trampoline + 99 
    at 19  kfun:org.koin.mp.KoinPlatformTools#synchronized(org.koin.mp.Lockable;kotlin.Function0<0:0>){0§<kotlin.Any?>}0:0 + 375 
    at 20  kfun:org.koin.core.scope.Scope#close(){} + 223 

Methods like createAllEagerInstances and close in InstanceRegistry can cause concurrent mutation because using an iterator over a collection will cause an exception even if ConcurrentMutableMap was used. We assume that reimplementing access to _instances in a way similar to createAllEagerInstances (in the same file) should solve the problem.

Koin module and version:
koin-core:3.5.6

[arnaudgiuliani]

Do you have some code to reproduce? We can contact touchlab team for stately collection 👍
Trying local concurrent loops exercise, but nothing here to make it break

[thoutbeckers]

The behaviour is expected, if you use an iterator it will detect modification while iterating.

Since iteration works by multiple method calls a "safe" collection that wraps access to the methods with reentrant locks does not work to prevent this. Also note that Stately locks (as e.g. synchronized on JVM is) are thread based anyway, so even if you would wrap the entire iteration loop in the same reentrantlock, if you do the modification while iterating from the same thread as the iteration, you'll still get this exception.

The simple fix is to copy the values you want to an array before iterating, as done in e.g.

koin/projects/core/koin-core/src/commonMain/kotlin/org/koin/core/registry/InstanceRegistry.kt

Line 61 in c815cc6

val instances = arrayListOf(*eagerInstances.values.toTypedArray())

contrast with the method in the stacktrace above :

koin/projects/core/koin-core/src/commonMain/kotlin/org/koin/core/registry/InstanceRegistry.kt

Line 156 in c815cc6

_instances.values.filterIsInstance<ScopedInstanceFactory<*>>().forEach { factory -> factory.drop(scope) }

Similar bugfix with test in our own project:
splendo/kaluga@d936e81

[arnaudgiuliani]

Yeah the JVM magic is not helping much on iOS world
Ok, thanks @thoutbeckers

[arnaudgiuliani]

Will publish a new 4.0.2-RC2. Will be happy to have feedback 👍